Page 1

VERLAG MODERNE INDUSTRIE

Hotmelt Molding

Low-pressure injection molding with hotmelt adhesives

Henkel, mikkelsen, OptiMel, U. Kolb



verlag moderne industrie

Hotmelt Molding Low-pressure injection molding with hotmelt adhesives Olaf MĂźndelein


This book was produced with the technical collaboration of Henkel AG & Co. KGaA, mikkelsen electronics as, OptiMel Schmelzgußtechnik GmbH & Co. KG and U. Kolb Werkzeug Vertriebsgesellschaft mbH. The author’s thanks for collaboration on this book go to: Bettina Becker, Rüdiger Butterbach, Dr. Uwe Franken, Jürgen A. Haberl, Thorsten Kaselow, Uwe Kolb, Dr. Siegfried Kopannia, Ulrike Müßigbrodt, Theo Pinto-Fernandes, Paul Ranft, Anna Remisch, Brian James Rothwell, Achim Schöneweiß, Christian Schulz, Thomas Stein and Marcel Ugoagwu. Translation: Kocarek Übersetzungen, Essen The present work was carefully compiled. All information, notes and recommendations are based on skills and experience acquired in practice. However, the author and publisher accept no liability for the content, in particular with regard to the correctness and completeness of the information made available. The assertion of claims of any kind is excluded. Due to diverse substrate materials and working conditions, users are generally advised to carry out comprehensive tests and to seek technical advice from adhesives manufacturers or professionally qualified service providers.

© 2009 All rights reserved with Süddeutscher Verlag onpact GmbH, 81677 Munich www.sv-onpact.de First published in Germany in the series Die Bibliothek der Technik Original title: Hotmelt Moulding © 2009 by Süddeutscher Verlag onpact GmbH Figures and tables: Figs. 1, 6, 7, 16–20, 22, 23, 26–28, 31, 32–34 and 36 OptiMel Schmelzgußtechnik GmbH & Co. KG, Iserlohn; Figs. 8, 11, 25, 29 and 30 U. Kolb Werkzeug Vertriebsgesellschaft mbH, Waldenbuch; all others Henkel AG & Co. KGaA, Düsseldorf Typesetting: abavo GmbH, 86807 Buchloe Printing and binding: Sellier Druck GmbH, 85354 Freising Printed in Germany 889094 ISBN 978-3-937889-94-8


Contents Introduction

4

From adhesive to hotmelt molding

6

Basic concepts......................................................................................... Classification of the process ................................................................... Typical applications ................................................................................

6 9 12

Common hotmelt adhesives

19

Non-reactive polyamide-based hotmelt adhesives.................................. Non-reactive polyolefin-based hotmelt adhesives .................................. Post-crosslinking polyamide-based hotmelt adhesives........................... Post-crosslinking polyurethane-based hotmelt adhesives....................... Criteria for material selection .................................................................

20 25 26 27 29

Parts design

33

Design guidelines .................................................................................... Tolerances ............................................................................................... Color and printability ..............................................................................

33 41 43

Molds, machines, and plants

45

Mold design ............................................................................................ Machine technology ................................................................................ Facility concepts .....................................................................................

45 49 53

Hotmelt molding in practice

55

Machine settings and production sequence............................................. Troubleshooting ...................................................................................... Machine maintenance .............................................................................

55 58 61

From the idea to the part

62

Technical terms and abbreviations

66

Bibliography

69

The companies behind this book

70


4

Introduction

Protection of electronics from environmental influences

Hotmelt molding

Whether in industrial surroundings, in vehicles or in the household, electronics is our constant companion and indispensable. With increasing distribution, the demands on its reliability have also increased. Electronics must function in any situation and under all circumstances. For this reason, more and more electronic components are today protected from environmental influences. Traditionally, casings made of plastic or metal offer good protection. Sealants and adhesives, with which casings are sealed or cemented, are now being widely used to ensure total seal. When casings are filled with a polyurethaneor epoxy-resin-based casting compound, there is no need to use a casing cover to protect the electronics. Butyl sealants are ideal to seal kink protection bushings for cables. These die cut pieces or strips are placed between the individual conductors before they are brought together in the bushings. Due to the economic pressure to reduce production costs and to optimize production another process to protect electronics has gained in significance, particularly since the 1990s: hotmelt molding. During this process, the electronic component is inserted into a potting mold and covered with a hotmelt adhesive (Fig. 1). Short cycle times and very good stability of the employed hot melts as well as very few emissions of volatile organic compounds (VOC) are only a few advantages of this method. Primarily applied in the area of automotive electronics, it gains more and more importance in electronics as well. Hotmelt Molding shields electronic components effectively from environmental influences without the additional need of a


Introduction 5 Fig. 1: Uncast boards (top) and boards cast with black hotmelt (bottom, with sprue)

casing and makes a quick and reliable production possible. This book pursues the objective of demonstrating the scope and limitations of hotmelt molding. It provides an overview of the various hotmelt adhesives, optimum parts design, and the latest state of mold and machine technology. In addition there are numerous practical examples, which engineers, designers, and manufacturers will find invaluable.


6

From adhesive to hotmelt molding Basic concepts Adhesive In the DIN EN 923 standard, an adhesive is defined as a “non-metallic material which can join substrates through surface adhesion (by adhesive forces) and internal stability (by cohesive forces)”. The German DIN 16920 standard also assumes in its definition that the substrate’s structure is not “essentially” altered by the adhesive. Hotmelt

Cohesion and surface tension

Hotmelt adhesive A hotmelt adhesive is a solvent-free adhesive which is a solid at room temperature. It is applied to the bonding surface in the hot and liquid state. After cooling, it is bonded to the substrate(s). Cohesion Pertaining to adhesives, cohesion is understood to be the effect of forces of attraction between the adhesive’s molecules, which produce its cohesive strength (Fig. 2). Cohesive forces are responsible for the viscosity of the liquid adhesive and the internal stability of the cold adhesive. The greater the cohesion, the higher the surface tension. This results in poor wetting of the substrates. On the other hand, a high cohesive force is required to withstand a high mechanical load. The cohesion of a hotmelt adhesive depends among other things on the following material properties: molecular weight, crystal structure, melting point and molecular orientation.


Basic concepts 7

Substrate 1

Adhesive

Cohesion

Adhesion Adhesion is the effect of forces of attraction between the adhesive layer and the surfaces of the substrates (see Fig. 2). The better the wetting of the surfaces of the substrates, the stronger the adhesion. Adhesive forces generally have a very narrow range on the order of 0.1 to 0.5 nanometers (1 nm = 10 –9 m). They can therefore only be effective on properly treated and cleaned substrate surfaces, which increases the surface energy and as a result achieves a better bond. The exact processes during adhesion are still not fully understood. The current concept of the adhesive’s mechanical bonding with the rough substrate surface is only an inadequate explanation. Intermolecular forces in the boundary layer between adhesive layer and substrate also contribute towards the adhesion. There are just as many different theories on adhesion as there are different forces exercised in adhesive bonds, but their detailed description would be beyond the scope of this book.

Adhesion

Substrate 2

Fig. 2: Cohesion and adhesion

Adhesion and wetting


8 From adhesive to hotmelt molding

Wetting and adhesive force

Wetting With adhesives, wetting is understood to be the tendency of the adhesive layer to distribute itself uniformly over the material surface. The better the wetting, the more molecules contribute effectively to the adhesion. The adhesive force is thereby increased. A measure of wetting is the contact angle (Fig. 3): The smaller the contact angle, the better the wetting.

Fig. 3: The contact angle is a measure for wetting. Contact angle

Adhesive

Substrate

Wetting also depends on the temperature. Since the surface tension of liquid adhesives decreases with increasing temperature, it follows that wetting increases with higher adhesive temperature. For hotmelt adhesives, the temperature of the substrate to be bonded is also important. The warmer the substrate, the less heat it extracts from the adhesive. The adhesive then has more time for wetting and can thereby form more adhesion bridges. The substrate’s thermal conductivity also has an effect on wetting. Materials with high thermal conductivity (i.e. metals) should be preheated before bonding so that they do not cool the hotmelt adhesive too quickly (heat


Classification of the process 9 sink effect). This would cause an increase in viscosity and a detrimental effect on the bond strength. Open time Open time is characterized as the maximum allowed time interval from application of the adhesive onto the substrate to joining and fixing of the parts to ensure good adhesion. Only when the substrates are joined within the open time optimum bonding can occur. The following factors determine the open time: • • • • • •

application temperature applied quantity substrate temperature substrate thermal conductivity ambient temperature air movement on the application area (e.g. draft) • application speed • application method • adhesive composition. The open time of hotmelt adhesives is generally very short in comparison to that of twocomponent casting compounds.

Short open time

Classification of the process The hotmelt molding process can be technically classified between plastic injection molding and casting with two-component casting compounds (Fig. 4). As opposed to these two processes, hotmelt molding exhibits essential application-related advantages. The pressure with which the hotmelt adhesive is injected into the mold cavity (potting mold) is usually between 5 and 40 bar for hotmelt molding. This is considerably below the 100-to-1000-bar injection pressures used during conventional plastic injection molding.

Comparison to injection molding


10 From adhesive to hotmelt molding

Cycle time

Abb. 4: Classification of the hotmelt molding process

Injection molding

Hotmelt molding 2K casting

Pressure

Comparison to 2K casting

Fig. 5: Casting electronics with a two-component resin

Hotmelt molding is a low-pressure molding process. The low injection pressure prevents damage to any electronic components. In contrast to casting with two-component compounds (Fig. 5), the advantages are the cost savings, as no casings are required, and also faster processing. When using casting compounds, the electronic components must be enclosed in a metal or plastic casing before casting. If it is possible to axe even only one single plastic part, calculations show that the total costs for hotmelt molding are lower than those for casting. Although the material costs for hotmelt adhesives are often three times higher than for a two-component casting compound, the process is much faster, thus achieving a significant reduction in WIP (work in progress). Casting compounds consist of two components, resin (A component) and hardener (B component), which must cross-link after casting. This usually takes


Classification of the process 11 several hours at room temperature, or the time can be shortened by the application of heat. For hotmelt molding, the components are placed into a mold without a casing. The form is closed and the components are covered with a hotmelt adhesive. The form reopens after cooling and the components can be removed. In general, the cycle time is between 10 and 50 seconds. At most, the cycle can last a few minutes for very large components. The following prerequisites must be fulfilled for the hotmelt adhesive to permanently seal the component: • compliance with the working temperature (service temperature) • complete coating of the component. The minimum application temperature must be at least 20 to 40°C above its softening point to achieve maximum adhesion to the component. For polyamide-based hotmelt adhesives (see Common hotmelt adhesives, p. 19 ff.), the required injection temperatures lie in the range of 180 to 240°C, for example. Temperatures directly on the surface of the coated component, however, are measured at just around 130°C. As soon as the hotmelt adhesive makes contact with the component, it cools and acts as a thermal insulator. This is the only explanation for the fact that PVC cables which are stable only up to 80°C or soldered joints are not damaged. Most of the heat introduced by the hotmelt adhesive dissipates quickly as the mold is made of metal which is a good heat conductor. Components that project out of the casting must be completely coated so that the hotmelt adhesive can shrink onto the components (see Design guidelines, p. 33 ff.). This is needed to

Prerequisites for permanent sealing


12 From adhesive to hotmelt molding

Adhesive or plastic?

Suitability of the process

ensure good bonding. This is required, because the applied hotmelt adhesives do not bond as well to the component surfaces a, e.g., twocomponent casting compounds. Hotmelts may basically not bond so well so that component demolding is not too difficult. If a specialist in plastic injection molding and a specialist in pressure-sensitive adhesives are asked for an assessment of whether a hotmelt adhesive for hotmelt molding involves a plastic or an adhesive, the answer will turn out differently: For the injection molding specialist, an adhesive is involved because the finished component does not fall unaided from the potting mold. For the adhesives expert on the other hand, a plastic is involved because the hotmelt adheres less well than a pressure-sensitive adhesive. The ordinary user must know that the adhesive force of a hotmelt adhesive for hotmelt molding lies somewhere in between that of a plastic and that of a pressuresensitive adhesive. In summary, it can be said: The hotmelt molding process lends itself to situations where at least one of the three subsequently named aspects is relevant for the planned application: • low injection pressure − direct coating of sensitive parts such as electronic components and cheaper potting molds • adhesive force of the hotmelt − sealing of components • injection molding process − short cycle times and saving of plastic casings.

Typical applications Connectors and switches The connector casting is the oldest application of hotmelt molding. In the process, the connector together with the cable is coated with


Typical applications 13 hotmelt adhesive (Fig. 6). The hotmelt adhesive protects the connector’s contact area from environmental influences such as water or oil and provides strain relief for the cable. This also improves the aesthetics significantly. Fig. 6: A truck connector cast with black hotmelt adhesive

The process is more reliable than sealing with heat-shrink tubing, where the seal quality depends significantly on the person executing it. For hotmelt molding, the machine operator needs only place the parts into the potting mold, inject the hotmelt and remove them again. Then the process runs automatically. Compared to casting with two-component compounds, the advantage is the processing speed. While two-component casting compounds must cross-link before further processing – the hardening process can last several minutes or even hours depending on the temperature – with hot melts, it is only necessary

Process reliability

Rapid processing


14 From adhesive to hotmelt molding to wait a few seconds or minutes until the component has cooled down to ambient temperature. Bushings Bushings made of hotmelt adhesive are often used in cable harnesses. Cable bushings made of hotmelt adhesive (Fig. 7) offer the following advantages: • cord grip of the cable • simple positioning of the bushing even on complicated cable harnesses. Comparison with the rubber grommet

A cord grip for a rubber bushing can only be produced with additional effort. As opposed to a rubber bushing, there is the additional advantage with cable bushings that the adhesive can be injected in a targeted manner directly onto

Fig. 7: Cable bushings

the cable and also adheres to the cable. A rubber bushing must first of all be manufactured and then drawn onto the cable. This can be difficult, above all with complicated cable harnesses with many connection points. In addi-


Typical applications 15 tion, the connection between the hotmelt adhesive and the cable is watertight in longitudinal direction. A rubber bushing made of EPDM, which is sealed with a butyl sealing material, is in fact also watertight in longitudinal direction but must be produced in two stages, and placement of the butyl strip is done manually as a rule. The advantage of rubber bushings lies in their elasticity. They can also seal a casing. Hotmelt adhesive, on the other hand, exhibits plastic behavior after cooling. Even if the hotmelt bushings were given an allowance, it would not permanently seal the casing as the material becomes soft at elevated temperatures and a gap would eventually appear between the hotmelt bushing and the casing. If a cable and casing are to be sealed with a hotmelt adhesive, both parts must be coated in the same process. Electronic components All the advantages of hotmelt molding can be made use of during the overmolding of electronic components such as controls and sensors: The electronics are directly coated (Fig. 8), but not damaged, because a low-pressure process is involved. They are protected from environmental influences because the applied material involves an adhesive. In many cases, a plastic casing can be dispensed with because the hotmelt adhesive replaces its function. As opposed to casting with two-component casting compounds, advantages also result from the speed of processing. In comparison to plastic injection molding, the advantages lie quite clearly with the low injection pressures of hotmelt molding machines. As a rule, electronic components are not damaged at an injection pressure of between 5 and 40 bar.

Casting of boards ‌


16 From adhesive to hotmelt molding

Fig. 8: Circuit board for a personal identification system

‌ and additional parts

LED electronics

It is not just the electronics components that are potted. There are many parts associated with the electronics that are potted with a hotmelt adhesive in a final step. Such parts can be connectors, cables, switches, casing parts, and similar products. In some cases it is even necessary to coat additional plastic parts. For example, if a hotmelt casing must feature a collet for screws, it should be implemented using plastic inserts. Hotmelt adhesives cannot indefinitely withstand forces transmitted through screws or comparable fastening elements, because the adhesive softens at elevated temperatures and it would flow away under the screws in time. The casting of electronics for light emitting diodes (abbreviated LED) is now used widely. Light emitting diodes are being increasingly employed in areas such as façade illumination, in which they must be effectively protected from environmental influences. Hotmelts protect the electronic circuit board and


Typical applications 17 Fig. 9: Signal electronics

light emitting diodes and form the common casing (Fig. 9). Furthermore, the hotmelt adhesive provides a cord grip for the cable. In most cases, there is no hotmelt adhesive over the light emitting diodes, but instead a transparent plastic sheet. The light emitting diodes would indeed be visible under the amber colored hotmelt, but the hotmelt adhesive would scatter the radiated light and create a loss of intensity. Keeping the light emitting diodes free of the potting material is usually difficult due to the positional tolerances. Hotmelts for these kinds of applications are predominantly white or grey colored (see Color and printability, p. 43 f.). Prototyping Hotmelt molding can be an alternative to plastic injection molding for the fabrication of prototypes and small batches. Due to the lower injection pressures, aluminum potting molds can be employed which can be economically

Economical molds and machines


18 From adhesive to hotmelt molding manufactured. Correspondingly small hotmeltmolding machines are available. Even a hotmelt gun for manual application suffices to fill the potting mold. If the prototypes exhibit many of the features of commercially available plastics, polyamide-based hotmelts are a good choice (see Common hotmelt adhesives, p. 19 ff.). These can achieve hardness values of up to Shore D 50 and therefore outstrip the other hotmelts used.

Protection from high injection pressure

Pre-potting of electronics before injection molding If electronic parts which cannot withstand the pressure during injection molding are to be coated with plastic, pre-potting by hotmelt molding is suitable. In the process, the electronics are first coated with a hard hotmelt adhesive. After that, they can be encased in plastic with the normal injection molding process. Pre-potting protects the electronics from damage inflicted by the higher injection pressure.


19

Common hotmelt adhesives There are non-reactive and reactive hotmelt adhesives. Non-reactive hotmelt adhesives do not cross-link but undergo a change in condition as the result of a physical process. These kinds of hotmelt adhesives are heated to above the softening point before application and harden after application as their temperature drops to ambient temperature. Reactive hotmelt adhesives − also called post-crosslinking hotmelt adhesives − react chemically with moisture from the air during curing; for this reason they can no longer be melted after curing. The following can be used as the chemical basis for non-reactive hotmelt adhesives: • polyamides (abbreviated PA) • polyolefins such as PP, PE and PB. • ethylene copolymers such as EVA, EA and EEA • styrene-polyolefin-styrene block copolymers such as SIS, SBS, SEBS and SEPS • polyesters such as PET. Table 1 shows a comparison between important properties of these adhesive families. Polyamide-based hotmelt adhesives cover a wide range of application temperatures. Adhesive family

Polyamides Polyolefins Ethylen copolymers Block copolymers Polyesters

Softening point acc. to ASTM E 28 in °C 90 –180 80 –160 80 –130 80 –130 90 –190

Non-reactive hotmelts

Table 1: Characteristics of non-reactive hotmelt adhesives: the range of information results from the diverse characteristics of specific products in the respective adhesive family.

Chemical resistance aqueous media weak good good good weak

organic media good weak medium weak good

Cold flexibility up to … °C – 50 – 35 – 10 – 60 –35


20 Common hotmelt adhesives Reactive hotmelts

The following are appropriate as the chemical basis for reactive hotmelt adhesives: • polyamides • polyurethanes (abbreviated PU or PUR) • polyolefins. Non-reactive polyamide-based hotmelt adhesives are the most widely used adhesives in hotmelt molding. Moreover, non-reactive polyolefins as well as post-crosslinking polyamides and polyurethanes are also used.

Non-reactive polyamide-based hotmelt adhesives

Fig. 10: Molecular structure of polyamide 6 (left) and polyamide hotmelt adhesive (right)

Non-reactive polyamide-based hotmelt adhesives are dimeric fatty-acid-based polyamides. The fatty acids are obtained from renewable raw materials such as soya, rapeseed or sunflower seeds. The properties of the polyamide hotmelts can be tailored by means of different raw material combinations. Non-reactive polyamide-based hotmelt adhesives can be easily processed. In the solid state, they differ from polyamide materials in their grid structure (Fig. 10). In contrast to polyamide materials such as PA 6, which can form higher crystalline and very compact structures, dimeric fatty-acid-based poly-


Non-reactive polyamide-based hotmelt adhesives 21 amides have a complex and primarily amorphous structure with few crystalline parts. Polyamide materials are consequently more stable and heat-resistant than dimeric fattyacid-based polyamides. Instead, hotmelt adhesives in the solid state are very flexible even at low temperatures. In comparison to PA 6, they have the following general properties that are tuned to their function as adhesives:

Structure and properties

• • • • •

lower cohesion lower viscosity lower moisture intake lower temperature creep resistance adhesive force based on polymer formulation • broad softening temperature range. In comparison to other hotmelt adhesive systems, non-reactive polyamide-based hotmelts are very stable, meaning that they have at least a material-grade character. During hotmelt molding, these adhesive systems do in fact assume material functions, i.e. the adhesive is not located as a thin layer between two substrates, but rather in part determines the external shape. Casings made from a thermoplastic synthetic can be completely replaced by these adhesive systems. Another essential aspect besides the mechanical stability are the adhesive properties. Watertight systems can be created through adhesive connection to the substrate to be sealed such as cable insulation materials, casing materials or circuit boards. Dimeric fatty-acid-based polyamides are predominantly of polar configuration and therefore absorb moisture. However, due to the non-polar fatty acid portion, the moisture intake is normally less than for polyamide materials. At 20°C and storage in water, it amounts to approximately 1% within 24 hours. The ab-

Adhesive force


22 Common hotmelt adhesives

Table 2: Electrical characteristics of various nonreactive polyamide hotmelt adhesives

sorption is reversible. Damp material can therefore be dried. Polyamide hotmelt adhesives have no exact melting point, but more of a relatively wide softening range. The glass transition also occurs within a wider temperature range. Since hotmelt adhesives contain no metallic fillers, they act as thermal and electrical insulators (Table 2). Tests have shown that the heat conduction behavior can be marginally improved through the addition of additives. To obtain an electrically conductive material, however, the filling level would have to be so high that the material could no longer be worked due to its considerably poor flow behavior.

Property

Breakdown resistance1 (IEC 93) in Vcm (after 24 h at 23°C and 50% RH, 100 V)

PA PA PA PA hotmelt hotmelt hotmelt hotmelt adhesive 1 adhesive 1 adhesive 2 adhesive 2 (amber) (black) (amber) (black) 1.0 ∙ 1012

0.6 ∙ 1012

1.7 ∙ 1013

2.4 ∙ 1013

Dielectric strength2 (IEC 243-1) in kV/mm

>14

>15

>16

>16

Dielectric constant (IEC 250) at 50 Hz 1 kHz 1 MHz

6.5 6.2 3.8

6.8 6.3 3.8

4.9 4.5 3.0

5.1 4.7 3.1

0.144 0.128 0.044

0.156 0.129 0.048

0.041 0.052 –

0.039 0.057 –

Dielectric loss factor (IEC 250) at 50 Hz 1 kHz 1 MHz 1

With regard to the listed values and allowing for deviations in the individual measurements, no appreciable differences between the amber and the black hotmelt adhesives can be detected.

2

The measured values are very dependent on the state of the specimen, meaning, for example, that small bubbles in the specimen can lead to a discharge. For this reason, the values presented here are to be considered minimum values.


Non-reactive polyamide-based hotmelt adhesives 23 According to the Underwriters Laboratories’ vertical burning test, polyamide-based hotmelt adhesives are self-extinguishing. Most standard materials attain the UL94 V-0 classification. The relative temperature index (RTI) determined in accordance with the Underwriters Laboratories’ specifications is a measure of the ageing stability of plastics at elevated temperatures. Because of the high cost of the test and the long period (two to three years) needed for testing, not all hotmelt adhesives are subjected to this test. Most non-reactive polyamide hotmelts are assigned an RTI which is based on knowledge and experience with similar products and represents an absolute minimum. It lies at around 65°C. RTIs established within the scope of long-term tests on high-end products, on the other hand, achieve 95°C. For many applications, the stability against ultraviolet radiation (UV) is significant. Nonreactive polyamide hotmelts equipped with UV stabilizers are also available. These are suitable for exterior applications such as solar products (Fig. 11).

Combustibility

RTI

Fig. 11: Cable bushing for a solar facility


24 Common hotmelt adhesives No declarable substances

Delivery containers and shelf life

Non-reactive polyamide hotmelts fulfill directive 2002/95/EC for limiting the use of hazardous materials in electrical and electronic devices (RoHS) and directive 2002/96/EC concerning outdated electrical and electronic devices (WEEE). They contain no materials that are listed in the Global Automotive Declarable Substance List (GADSL). Besides this, they meet the Food and Drug Administration’s specifications for adhesives (FDA 21 CFR 175.105). Non-reactive polyamide-based hotmelt adhesives are delivered in granules (Fig. 12) in moisture-proof sacks. Most products can be stored for up to two years in unopened packaging. Opened bags should be rolled up and closed tightly for further storage so that no moisture can get in.

Fig. 12: Amber-colored and black hotmelt adhesive pellets

If the granules become moist, this would manifest itself by bubble formation in the melting device. In this case the material must be dried


Non-reactive polyolefin-based hotmelt adhesives 25 before processing. As a rule, about three to five hours in commercially available hot air ovens with the temperature at 40°C will suffice. Drying devices or ovens that work with room air are inappropriate.

Non-reactive polyolefin-based hotmelt adhesives Non-reactive polyolefin-based hotmelt adhesives distinguish themselves particularly through the following properties: • very low hygroscopic effect (0.1%) • adhesion to non-polar plastics such as PE and PP. Non-reactive polyolefin-based hotmelt adhesives are more resistant to alcohol and less resistant to oils than polyamide hotmelts. They exhibit an extremely high resistance to alkalis and acids. There are products which can be used at temperatures of up to 100°C. Because of their electrolyte resistance, they are used for bonding the separator side seams in batteries, for example. Due to their low hygroscopic property, they are moreover used to insulate underfloor heating contacts. It is also possible to apply them in combination with polyamide hotmelts. This makes sense when a PE cable is to be sealed and a temperature stability of over 100°C is required, for example. A cable of this kind is first coated with polyolefin hotmelt and subsequently with polyamide hotmelt. Only a few polyolefin hotmelts are available as granules. These hotmelt adhesives are mostly delivered as 1-kg blocks (Fig. 13).

Chemical resistance


26 Common hotmelt adhesives Fig. 13: Non-reactive polyolefin hotmelt adhesives

Post-crosslinking polyamide-based hotmelt adhesives

Dimensionally stable to over 200°C

Post-crosslinking polyamide hotmelts are characterized by their extremely high temperature stability, which is achieved through a chemical reaction with air moisture. These kinds of hotmelt adhesives remain dimensionally stable even at temperatures greater than 200°C. They can even survive a reflow soldering process during which short-term temperatures of 270°C are reached. The hotmelt does not, however, achieve its final strength and temperature stability until after a delay. The duration of the post-crosslinking reaction depends on the casting thickness (Fig. 14). With reactive polyamide hotmelts, potted parts can be stored, transported, and reprocessed immediately after casting. The requisite reaction time, however, must elapse before their intended application. In order to allow the necessary air moisture to reach the casting, the parts must not be hermetically packed. They must be stored with sufficient humidity in the surroundings.


Post-crosslinking polyurethane-based hotmelt adhesives 27 4.0

Reaction depth in mm

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

2

4

6

8

10

12

14

Time in days

Reactive polyamide hotmelts are delivered in 2-kg blocks hermetically packed in aluminum. Reaction with air moisture sets in as soon as the packaging is opened. Special melting devices such as tank facilities with cover seals that work with dehumidified air or nitrogen as a protective gas, or bag melting equipment (see Melting device, p. 49 ff.), however, ensure that reactive polyamide hotmelts do not behave like non-reactive hotmelts during melting and working. Reaction with the air moisture does not then start until after casting. Compared to the post-crosslinking polyurethane hotmelt adhesives described below, post-crosslinking polyamide hotmelts have the advantage that they react all the way through independently of the casting thickness.

Post-crosslinking polyurethanebased hotmelt adhesives Post-crosslinking polyurethane hotmelt adhesives likewise achieve their final strength and

Fig. 14: Reaction time of reactive polyamide hotmelt adhesives

Processing


28 Common hotmelt adhesives temperature stability only after the reaction with water molecules from air moisture. They distinguish themselves through the following properties: • • • • Suitable for special applications

low injection temperature of 130 to 180°C good chemical resistance very good adhesive force extremely good low-temperature behavior.

As they are temperature stable to approx. 80°C and only cure through up to a casting thickness of 1 mm, post-crosslinking polyurethane-based hotmelt adhesives are very seldom used for hotmelt molding (Fig. 15). Depending on the layer thickness, the cooling-down time can be longer than that for polyamide hotmelts.

1.2

Reaction depth in mm

1.0 0.8 0.6 0.4 0.2 0.0

0

Fig. 15: Reaction time of reactive polyurethane hotmelt adhesives

2

4

6 Time in days

8

10

12

Polyurethane hotmelt adhesives are also delivered in 2-kg blocks hermetically packed in aluminum foil. The special melting devices already mentioned in connection with postcrosslinking polyamide hotmelts are suitable for their processing.


Criteria for material selection 29

Criteria for material selection Primarily, there are three questions that present themselves to the user during material selection: • What temperature range must the material tolerate? • To which substrate materials must the hotmelt adhesive adhere? • To which media must the hotmelt adhesive be resistant? The following remarks are limited to non-reactive polyamide-based hotmelt adhesives because these products are currently most frequently used for hotmelt molding. Different hotmelt products are suitable depending on the application temperature range (Table 3). Basically, the following applies: The higher the required application temperature, the harder the applied materials are and the lower the bond strength. The upper temperature limit, above which it is difficult to seal electronic parts with non-reactive polyamide hotmelt adhesives, is about 150°C. Property

Application temperature range

PA hotmelt adhesive 1

PA hotmelt adhesive 2

PA hotmelt adhesive 3

– 40 to +100

– 40 to +130

– 20 to +150

Shore hardness at room temperature

A 77

A 90

D 42

Adhesive force spectrum

broad

limited

narrow

Application temperature in °C

The application temperature given on the data sheet is just a standard value and must be tested on the part. Non-reactive polyamide hotmelt adhesives become softer with increasing temperature (Table 4). The possible temperature application range can be adjusted up-

Table 3: Important characteristics of non-reactive polyamide hotmelt adhesives


30 Common hotmelt adhesives Temperature

Shore hardness PA hotmelt adhesive 1

PA hotmelt adhesive 2

PA hotmelt adhesive 3

–40°C

A 85

D 40

D 55

20°C

A 77

A 90

D 42

60°C

A 67

A 86

D 37

80°C

A 58

A 80

D 33

100°C

A 50

A 74

A 90

120°C

A 36

A 68

A 85

140°C

A 48

A 75

Table 4: Changes in the Shore hardness of nonreactive polyamide hotmelt adhesives from table 3 with temperature

Table 5: Adhesion of nonreactive polyamide hotmelt adhesives from table 3 to various materials

ward or downward depending on the mechanical load. Non-reactive polyamide-based hotmelt adhesives adhere well to polar surfaces (Table 5), but certainly not as well as two-component polyurethanes or epoxy-resin-based casting compounds. They do not adhere well to metal surfaces. This is not at all desirable either, because too good an adhesive force would make it difficult to de-mold. Rapid adhesive cooling is the cause of the poor adhesion. The metal substrate conducts so much heat away from the hotmelt adhesive so quickly that complete wetting can no longer occur. Good adhesive

Material

Adhesive force PA hotmelt adhesive 1

PA hotmelt adhesive 2

PA hotmelt adhesive 3

PVC

good

good

moderate

PA 6.6

good

moderate

moderate

PC

good

moderate

moderate

ABS

good

moderate

moderate

PU

good

good

moderate

PE

moderate

none

none

good

good

moderate

PE (corona treatment) Steel

none

none

none

Steel (preheated)

good

good

moderate


Criteria for material selection 31 force is achieved if the temperature difference between the hotmelt adhesive and the metal surface is reduced. To this end, the metal substrate must be heated to between 100 and 120°C. If the hotmelt is in principle suitable due to the temperature requirements but still does not adhere adequately, then the substrate’s surface can be activated by pretreatment. Corona treatment, plasma treatment and the application of a bonding agent (primer) are processes that assist wetting. As an alternative to pretreatment, the part can be potted with a soft hotmelt adhesive with good adhesive properties in a first step, so that it can be coated with a hard hotmelt adhesive as a second step. Experience shows that hotmelt adhesives adhere well to one another. The adhesive force of a hotmelt adhesive on plastic can be increased using the above-mentioned corona or plasma treatment. During corona treatment, the plastic surface is briefly exposed to an electrical charge. The adhesive can bind itself to the resulting dipole moments. During plasma treatment, the plastic surface is activated with oxygen. In the process, thin organic layers are ablated in some cases. To ensure that the hotmelt adhesives can fully develop their adhesive force, it is important that the parts being coated are clean. In particular, the plastic parts must be free from release agent residues. If the parts are placed manually into the mold, they must not be touched on the surfaces where adhesion is to occur. Hand creams in particular can impede adhesive force build-up. If touching the part in the adhesion region cannot be avoided, the machine operator should wear gloves. The chemical resistance of polyamide hotmelt adhesives depends on contact duration, the temperature during contact, and on the me-

Improvement of the adhesive force

Chemical resistance


32 Common hotmelt adhesives Chemical resistance

Conditions/medium

Brief contact1

Long contact2

Oil Brake fluid Diesel fuel Gasoline Methanol Oil Brake fluid Diesel fuel Gasoline Methanol

PA hotmelt adhesive 1 excellent good good good good good moderate moderate unsuitable unsuitable

PA hotmelt adhesive 2 excellent good good good good good good good unsuitable unsuitable

1 Adhesive

is covered with a small amount of fluid at room temperature.

2 Adhesive

is immersed in the fluid at room temperature for 24 h.

Table 6: Chemical resistance of non-reactive polyamide hotmelt adhesives from table 3

PA hotmelt adhesive 3 excellent good good good good good good good unsuitable unsuitable

dium (Table 6). Non-reactive polyamide-based hotmelt adhesives exhibit good oil resistance. With respect to alcohol, however, the resistance is low. Since there are countless resistance tests, the hotmelt manufacturer’s data can serve only as a guide. The user is ultimately responsible for sufficiently testing the parts himself.


33

Parts design Design guidelines Hotmelt adhesives do not adhere as well as two-component materials. As soon as different ambient temperatures – brought about, for example by a temperature shock test – act on the part, the hotmelt adhesive can come off in places (Fig. 16). The flaking results from uneven temperature propagation in the part and the hotmelt. The heating and cooling leads to alternating mechanical stresses in the boundary layer which strains the bond and ultimately damages it irreversibly.

Adhesive force ‌

Fig. 16: Incorrect connector design: casting (translucent yellow) directly after overmolding (above) and after a temperature shock (below)


34 Parts design Fig. 17: Correct connector design: the cable is completely enclosed by hotmelt adhesive (translucent yellow).

‌ plus contraction

Complete coating

It is only under extremely favorable conditions such as moderate and uniform application temperatures or low impermeability demands on the part (e.g. dust-tightness) that it is possible for the adhesive force of the hotmelt adhesive on the substrate to be sufficient to permanently seal the part. Apart from these kinds of exceptional cases, advantage must also be taken of the hotmelt’s contraction as well as the adhesive force to permanently ensure impermeability. The heat shrinkage of the hotmelt on the part assumes that the hotmelt completely surrounds the part (Fig. 17). The coating of the parts is therefore probably the most important prerequisite for successful hotmelt molding. The sealing effect may indeed be present without coating, but is usually not permanent. Circumferential heat shrink surfaces If the construction does not offer sufficient space to completely surround the part with the hotmelt, the designer must provide appropriate heat shrink surfaces on the part (Fig. 18). In practice, heat shrink surfaces are often missing because the designer tries to use a parts design intended for a two-component


Design guidelines 35 Fig. 18: If connector and casting (translucent yellow) are to have the same diameter, the connector will be equipped with an internal heat shrink surface (bright yellow).

polyurethane or epoxy-resin-based casting compound without modifying it for hotmelt molding. Often, for example, a plastic casing in which the electronics are positioned is used for the casting. This casing is then filled with the casting compound. Due to the two-component casting compound’s better adhesive force, this system remains impermeable even after a temperature shock. From a purely production-oriented perspective, the same casing could be filled with a hotmelt without problem, with a shorter cycle time. At first, the system would indeed be impermeable. However, because the circumferential heat shrink surfaces are missing, the hotmelt would come off the casing after a couple of temperature cycles (Fig. 19). To solve this problem, it should first of all be checked whether the plastic casing is at all necessary when using a hotmelt. If not, there can be a significant cost saving. If it is possible to completely coat the electronics, the hotmelt’s contraction will adequately take care of sealing. In the event that a casing really is required, there are various solutions: If enough room exists, the hotmelt should overlap the upper edge of the casing so that it can heat-

Casing


36 Parts design Fig. 19: Sectional views of casings without heat shrink surfaces: directly after the casting (above) and after one or more temperature cycles (below)

Boards

shrink onto the casing. If this constructed space is not available, then, depending on the casing wall thickness, an inner or outer heat shrink surface must be provided (Fig. 20). The flow gap must exhibit a thickness of at least 0.5 mm and a height of at least 1 mm. In the same way, an appropriate solution for hotmelt molding is often not initially developed for electronic circuit boards. Other than with protective lacquers or casting compounds, they cannot be simply be potted on one side with a hotmelt because mechanical stresses would arise which would possibly deform the board. The following design solutions are available: First, it should be checked whether


Design guidelines 37 Fig. 20: Casings in section: with external heat shrink surface (above) and with internal heat shrink surface (below)

the board can be potted on both sides, because this would be the simplest solution. It might even be possible to pot both sides of the board one after the other. The seam along which the second casting meets the first is highly stable. It is essential during complete casting of boards that distribution of the hotmelt adhesive is totally uniform on both sides of the board. If the electronic board can only be potted on one side due to lack of space, first check whether it can accommodate the hotmelt’s shrinkage stress without deforming. No potting mold is needed for this; it suffices to pot it manually. If the board warps during this test, it is not suitable for single-sided casting. If it does not


38 Parts design warp, it must be tested whether sufficient adhesive force builds up on the board. This can be determined using a spot test. If the board has also passed this test, then the designer must give consideration to the circumferential heat shrink surface. If the board is at least 1 mm thick, the four lateral surfaces can be used. Radii instead of sharp edges Where the design allows it, radii instead of sharp edges should be provided on the part’s surface around the corners, because radii facilitate de-molding. If a sharp edge cannot be dispensed with, the mold must feature an air vent at the edge. Draft angles As in plastic injection molding, draft angles facilitate de-molding of the part. The larger the draft angles are, the easier it is to demold. Prevention of shrinkage cavities and pockmarks

No hotmelt material accumulations As a rule, hotmelt adhesives do not behave as critically during material accumulations as other materials of the same chemical basis. Material accumulation should nevertheless be avoided to eliminate the formation of shrinkage cavities or pockmarks. If the part’s design leads to material accumulation, at least the sprue should be in the accumulation area. Even better would be to fabricate the part in two steps (Fig. 21). Material accumulations are problematic on boards, for example, where several large parts are arranged on the top side and a connector on the underside. In this rare case, the large parts cause an accumulation of hotmelt adhesive on the top side. Unequal tensile forces caused by shrinkage on the top and underside


Design guidelines 39 Fig. 21: USB stick: blank (below), pre-molded (center) and overmolded (above)

of the board can lead to the hotmelt adhesive being pulled away from the connector. This can be prevented if the large parts are coated first and the rest of the board together with the connector is coated in a second step. Alternatively, the connector can be provided with a slot in which the hotmelt adhesive can get a grip. Kink protection on cables For each cable that leads out of a part to be coated, a kink protection should be provided in the design which relocates the cable’s kink point to outside the sealed area (Fig. 22). Without the kink protection, every cable motion leading to a change of direction in the vicinity of the part would stress the sealed area and ultimately lead to the hotmelt adhesive coming off the cable. This could in turn lead to impermeability problems.


40 Parts design Fig. 22: Cable bushing with kink protection

Mold with slider

Approximate values

Undercuts It is possible to realize an undercut on a part in hotmelt molding. However, a slider is necessary on the mold. Forced de-molding using a knockout would destroy the undercut because the hotmelt adhesive would undergo plastic deformation. Flow gap and flow path The flow gap as well as the flow path depend on the size of the part, the employed hotmelt adhesive, the injection temperature and the mold temperature. That is the reason why only approximate values can be given. In most cases, flow gaps of 1 to 2 mm in width suffice. Flow gaps may only be smaller when the corresponding flow path is very short (1 to 2 mm). For a flow gap of less than 0.5 mm in width, neat filling can no longer be guaranteed. For parts such as metal parts which extract a lot of heat from the hotmelt adhesive, the flow gaps must be dimensioned wider or the parts must be pre-heated before application of the hotmelt.


Tolerances 41

Tolerances The following aspects must be considered with regard to part tolerances: • the shrinkage of the hotmelt adhesive • dimensional and positional tolerances of the inserts. Shrinkage of the hotmelt adhesive Most hotmelts have no fixed percentage of shrinkage; the shrinkage varies within a certain percentage range. With polyamide hotmelt adhesive for example, contraction lies in the range of 7 to 9%. A shrinkage of 1 to 2% is obtained with the hotmelt molding machine by increasing the dwell time. With a diameter of 10 mm in the potting mold, the ejected parts consequently exhibit a diameter of between only 9.8 and 9.9 mm. The designer must take this 1 to 2% shrinkage into consideration. Inserts can reduce the shrinkage. In the following example, an insert with a diameter of 5 mm is placed centrically in the same potting mold. With a hotmelt shrinkage of 1%, the part’s diameter amounts to:

Reduced shrinkage through inserts

D1 = 2 ⋅ 2.5 mm ⋅ 0. 99 + 5 mm = 2 ⋅ 2.475 mm + 5 mm = 9.95 mm

With a hotmelt shrinkage of 2% the part’s diameter amounts to: D2 = 2 ⋅ 2.5 mm ⋅ 0. 98 + 5 mm = 2 ⋅ 2.45 mm + 5 mm = 9.90 mm

While the diameter of the part varies between 9.8 and 9.9 mm without the insert, it amounts to between 9.90 and 9.95 mm with the insert. Tolerances of the inserts As a rule, hotmelt molding involves coating inserts such as cables, connectors and boards with hotmelt adhesive. The dimensional and


42 Parts design Dimensional and positional tolerances

Fig. 23: Mold with mating connector

positional tolerances of these inserts must be considered in the potting mold. The operator should nevertheless be able to position the inserts easily, and burrs or even the leakage of hotmelt adhesive from the potting mold should be avoided. At injection pressures common for hotmelt molding, the hotmelt adhesive flows into gaps that are wider than 0.05 mm. If the tolerances of the inserts are so great that broader gaps can develop, the designer must solve this problem. In the case of flexible inserts such as cables, he can expand the smallest dimension and crimp the larger diameters. For connectors, the gap between the connector’s casing and the metal contact can be too large. In such cases, the gap must either be sealed beforehand or a mating connector must be provided in the mold (Fig. 23). The mating connector’s task is to keep the hotmelt adhesive away from the connector area. For very large insert tolerances, it is necessary to


Color and printability 43 equip the potting mold with spring-loaded seals or sliders.

Color and printability Most non-reactive hotmelt adhesives are usually available as black or amber-colored granules (see Fig. 12, p. 24). Amber is the basic color. Black is currently the most used color. Commercially available varieties of carbon black are mixed in for black coloration. But other colors are also possible. If large quantities are ordered, colored granules or blocks can be obtained directly from the manufacturer. Otherwise, the user can pigment amber-colored granules himself with a colored master batch (Fig. 24) or with colored pastes. The master batch must be blended according to the manufacturer’s specifications. The chemical basis of the master batch must be compatible with that of the employed hotmelts. It is important to know that the color variations of the

Basic colors

Details on pigmentation

Fig. 24: Colored master batches


44 Parts design basic materials are carried over into the end result. Therefore, a color with a particular RAL number, for example, can only be achieved within certain limits. Color guarantees are not advised. Fig. 25: Printed motor control board for subfractional- and fractional-horsepower motors

Pad or laser printing

Hotmelt adhesives can be printed after application (Fig. 25). Pad and laser printing are appropriate processes.


45

Molds, machines, and plants Mold design Potting molds used for hotmelt molding (Fig. 26) bear great similarity to injection molding forms. There are, however, a few fundamental differences. Water temperature control connection

Cavities Sprue runner Lower part

Upper part

Air vent Since as a rule, potting molds for hotmelt molding open vertically, they consist of an upper and a lower form halve. Due to gravity, the hotmelt adhesive generally fills the lower form half first and then the upper. An air vent on the mold parting surface is thus insufficient; the upper form half must be separately vented. In the rare exceptional case in which the upper form half fills first because, for example, the hotmelt stream is diverted through an insert, an air vent must also be allowed for in the lower half of the form.

Fig. 26: Design of a potting mold for hotmelt molding


46 Molds, machines, and plants Fig. 27: Label insert serves to simultaneously vent the cavity.

Implementation

An air vent can be achieved through insert mold parting surfaces. Further possibilities are knockouts, date stamps and label inserts (Fig. 27). The air vents’ separation gaps should not be greater than 0.05 mm. Otherwise, they could fill with hotmelt adhesive and clog up. The air vent should always be placed at the highest spot in the cavity. An air vent must also be provided wherever there are sharp edges. Injection point and sprue The injection point should be located wherever the greatest material accumulation or the narrowest flow gap is. The size of the injection point depends on the casting volume for the cavity. Common diameters are:

Diameter of the injection point

• 1 to 2 mm for a casting volume up to 20 cm3 per cavity • 3 to 4 mm for a casting volume between 20 and 50 cm3 per cavity. The sprue distributes the hotmelt in the cavities. It should be as short as possible. The length of the sprue depends on the total volume of all cavities. Common sprue diameters are:


Mold design 47 • 5 mm for a total casting volume up to 40 cm3 • 7 mm for a total casting volume over 40 cm3. These values are intended as a guide. The optimum sprue diameters and positions can be determined on a prototype mold. The total casting volume and the sprue volume yield the injection volume, i.e. the volume that must be conveyed through the machine’s injector for the injection process. Number of cavities The maximum number of cavities in a potting mold is dictated by the closing force of the machine and the projected areas of the cavities including the sprue. The following sample calculation assumes a closing force for the hotmelt molding machine of FM = 10 000 N and an attainable injection pressure of p = 40 bar ≈ 400 N/cm2. Further, the projected area of a cavity amounts to AC = 4.2 cm2 and the projected area of the sprue AS = 3.5 cm2. From the relationship between closing force and applied injection pressure, a maximal projected area of: Amax =

10 000 N = 25 cm 2 N 400 cm 2

is derived. Minus the projected area of the sprue, there remains for the cavities a projected area of: Amax − AS = 25 cm 2 − 3.5 cm 2 = 21.5 cm 2

The number of cavities results from these equations:

Recommended sprue diameter

Injection volume


48 Molds, machines, and plants nC =

21. 5 cm 2 21.5 cm 2 = = 5.1 AC 4.2 cm 2

Hence, up to five cavities are possible in the potting mold. De-molding the casting

Aluminum ‌

‌ and steel

Knockouts Since hotmelt adhesives adhere to the potting mold more strongly than plastics, it makes sense to use one or more knockouts to completely demold the casting. Ideally, the knockouts are attached to the inserts and raise them. If this is not possible and they must instead be attached to the casting, then the knockout should have as large an area as possible in order not to damage the casting. This danger exists because hotmelt adhesives are generally softer than plastics of the same chemical base. If the part cannot be completely de-molded this way, the de-molding of the rest of the part must be done by the machine operator or a robotic gripper. Potting mold material Aluminum potting molds are suitable for hotmelt molding. This material dissipates heat thereby reducing the cooling time. Because most hotmelt adhesives are non-abrasive, aluminum makes for a good mold life. The areas of the potting mold which come into contact with inserts such as connectors, boards or cables are ideally fabricated from steel. In prinPotting mold material

Table 7: De-molding forces for various potting mold materials

De-molding force in N/cm2

Aluminum (3.4365)

17

Steel

48

Copper

39

Bronze Titanium

31 (de-molding not possible)


Machine technology 49 ciple, potting molds can also be completely fabricated from steel. The de-molding force would then, however, be greater than that of aluminum (Table 7). Surface of the potting molds Practical experience has shown that potting molds with slightly roughened surfaces can be de-molded easier than those with polished surfaces. This is because hotmelt adhesives do not wet roughened surfaces as well as polished ones. Prerequisite is, however, that the hotmelt adhesive must not be able to flow into gaps that are wider than 0.05 mm. If this prerequisite is met, structures on slightly roughened surfaces will neither be completely filled nor completely wetted by hotmelt. The roughness may increase the effective surface, but the de-molding force is smaller than that for a polished area. If a roughened surface exhibits gaps that are wider than 0.05 mm, then the hotmelt adhesive can get a grip in the gaps and the de-molding force is significantly greater. Experience shows that the smallest de-molding forces are achieved with sand-blasted surfaces (blasting agent: glass beads with a diameter of 200 ¾m). This applies for potting molds that are cooled to a temperature of 20 to 30°C.

Sandblasted surfaces

Machine technology The melting device is the principal part of a hotmelt molding machine. The machine is equipped with a suitable locking device for the mold (Fig. 28). Melting device In the case of smaller devices, they are generally air-pressure melting devices. The material

Air-pressure melting devices


50 Molds, machines, and plants Fig. 28: Design of a hotmelt molding machine with front view (top), rear view (center) and detail view (bottom)

Control panel Application head Mold holder

C-frame with closing cylinder

Melting device

Hose

Injector

tanks of these devices mostly have only one heating zone in which the hotmelt adhesive is melted. To inject the hotmelt adhesive into the potting mold, the material tank is charged with compressed air. The injector opens under the increased pressure. These kinds of devices achieve an injection pressure of between 1 and 6 bar. This suffices for simple potting molds with up to two cavities, but they are also used


Machine technology 51 Fig. 29: Manual spray gun for the application of hotmelt adhesives

for experiments and small batches. They are available as hand devices (Fig. 29) and stationary systems. Melting devices with a gear pump are the ones used most frequently. They come in various sizes. Key figures are the material tank’s size, the melting volume, and the gear pump’s delivery rate. The material tanks of these melting devices have one or two heating zones in which the hotmelt adhesive is melted. The gear pump conveys the fluid hotmelt adhesive into a heated tube or a heated hose. From there, the hotmelt goes into a heated application head. The application head is usually moved up to the mold and then opened pneumatically. The injection pressure builds up between the gear pump and the application head. It is generally regulated by a pneumatic bypass to a value between 10 and 40 bar. If the pressure exceeds the maximum pressure set on the bypass valve, the valve opens and the excessive pressure is vented. There is a special melting device for reactive hotmelt adhesives which has a sealed tank cover. The tank is charged with dehumidified air or nitrogen to keep moist air away. Post-crosslinking polyamide or polyurethanebased hotmelt adhesive delivered in 1-to-2-kg

Melting devices with gear pump

Devices for reactive hotmelts


52 Molds, machines, and plants Bag melting equipment

bags can best be processed with bag melting equipment. The hotmelt adhesive has a cylindrical form like a thick candle. The bags are slit open cross-wise on one side and are loaded into the bag melting equipment. The equipment has a heated plate. A plunger presses the bag against the heated plate, the adhesive melts there and is ultimately conveyed into a hose, which leads to the application head. As soon as the bag is empty, it can be removed from the machine. Locking device The locking device (Fig. 30) is tasked with closing the mold, holding it shut during the injection, and opening it after the part has cooled down. There are various designs for the lock-

Fig. 30: Hotmelt molding machine: mold locking system (left) and melting device (right)

ing device. For simple hotmelt molding machines, the operator must open and close the mold with a knee lever. For larger lot sizes, machines that close and open the mold via pneumatic or hydraulic cylinders are more suitable.


Facility concepts 53 Due to the low injection pressures, the closing forces of hotmelt molding machines are smaller than those for injection molding machines. For this reason, pneumatic closing cylinders are preferred. A closing force of between 5 and 10 kN suffices for most applications. Following the trend toward the coating Fig. 31: Hotmelt molding machine with a closing force of 8 t

of even larger electronic components, manufacturers are increasingly bringing machines with greater closing forces onto the market. These feature hydraulic closing cylinders (Fig. 31).

Facility concepts Sliding table facilities generally work with two mold lower parts and a common mold upper part. The mold lower parts are mounted on the sliding table, the mold upper part on the closing cylinder. While hotmelt is injected into

Sliding table facilities


54 Molds, machines, and plants

Fig. 32: Sliding table facility (left) and double station (right)

Round table facilities

the closed mold, the machine operator can remove finished parts from the open mold lower part and reload it. Double stations, featuring two complete molds, are even more effective (Fig. 32). Four to eight mold lower parts with one to two mold upper parts or three or four complete molds can be mounted on a round table facility (carousel-type machine). This kind of facility enables more rapid fabrication than sliding table facilities. Often, inserts and parts are automatically introduced and removed. Round table facilities can also be integrated into assembly lines. This increases productivity. Many equipment manufacturers build their facilities to be upwardly compatible, i.e., an entry-level user with a small facility can continue using his potting mold after upgrading to a larger facility. The mold dimensions for all facility sizes are the same.


55

Hotmelt molding in practice Machine settings and production sequence The machine settings required for fabrication are illustrated in the following text using the example of a melting device with gear pump. As a rule, these kinds of hotmelt molding machines have four temperature zones (Fig. 33): • • • •

in the material tank: melting in the material tank: pre-melting in the hose in the application head. Fig. 33: Machine parameters

To ensure that the hotmelt adhesive can completely develop its adhesive properties, the temperatures set in these zones should be around 20 to 40°C above the respective melting point. The melt in the material tank generally has the lowest temperature. The hose temperature is then selected 5 to 10°C higher. Depending on the injection volume, the tem-

Temperature settings


56 Hotmelt molding in practice

Injection speed

perature at the application head is set to be the same as the temperature in the hose or around 5°C higher. The higher the set temperature, the lower the viscosity of the hotmelt adhesive. The injection rate is regulated by the rotational speed of the gear pump, the injection pressure via the bypass (see Melting device, p. 49 ff.). There are machines which can execute a freely selectable speed-pressure profile (Fig. 34). It is thereby possible, for example, to fill the potting mold first at high

Fig. 34: Parameter settings on a hotmelt molding machine

Time settings

speed and low pressure and, during dwell pressure time, to work at lower speed and high pressure. The injection and cooling-down time are usually set on the hotmelt molding machine. A few machines also regulate the dwell pressure time and the stop time. For other machines, both these times are included in the injection time. The form is filled during the injection time. This occurs within a few seconds. Once the form is filled, the dwell pressure time, which is intended to reduce shrinkage, begins. This time interval is gen-


Machine settings and production sequence 57 erally somewhat longer, since the material flow is reduced. No more material flows during the stop time. The injector remains on the form for another one to two seconds longer so that nothing can flow back from the sprue. The cooling-down time then begins. This lasts until the parts are stable and just warm enough that they can be removed. In 90% of all cases, the cycle time (the sum of the injection, dwell pressure, stop and coolingdown times) lies in the range of 10 to 50 seconds. The potting molds are cooled with water during molding of the parts. It has been proven that the parts are most easily de-molded at a potting mold temperature between 20 and 30°C. During fabrication, a certain amount of liquid hotmelt adhesive always remains in the material tank. If the tank is not filled with protective gas, there is constantly air in it. Contact with atmospheric oxygen leads in time to the formation of non-soluble particles – the hotmelt cracks. In the case of continuous fabrication, during which fresh material is continually added (Fig. 35) and melted material quickly used up, oxygen contact generally does not present a problem. But if the material remains fused for longer than eight hours, then insoluble particles will begin to clog the machine. To prevent this, most machines can be operated in sleep mode. If the powered up machine is not used for two hours, for example, the melting device’s temperature falls to a preselected value. This temperature should lie below the hotmelt’s softening point. When the machine is used again, the device is reheated. Since, however, the reheating does not have to start from room temperature, but starts from a considerably higher temperature of, for exam-

Fig. 35: Filling the melting device

Sleep mode


58 Hotmelt molding in practice ple, 100°C, the time until the machine is ready for operation is not nearly as long as it is during machine start-up. At the end of production, the material in the tank should be used up so that during the next production start, fresh material can be introduced. Material remaining in the tank during scheduled breaks in production absorbs moisture, which causes foam build-up and a reduction in viscosity. This can adversely affect the set casting parameters.

Troubleshooting The most frequent faults in hotmelt molded parts include: • • • • •

Air or vacuum bubble

bubble(s) in the casting warping of the part pockmarks no or incomplete filling of a cavity shrinkage of the hotmelt adhesive from the part.

Bubble(s) in the casting Bubbles in the casting can have various causes. To be able to remedy this fault, it must first be determined whether air bubbles or vacuum bubbles (shrinkage cavities) are involved. There are two ways to test this. One way is to place the part into an oven and heat it to a temperature between 10 and 20°C below the hotmelt’s softening point. Air bubbles are recognizable in that the expanding air causes them to bulge. If vacuum bubbles are involved, the adhesive skin is drawn inward because the soft hotmelt adhesive can no longer withstand the vacuum. In the second method, the bubble can be cut open under water. In the case of an air bubble, the escaping air rises in the water.


Troubleshooting 59 An air bubble is generally caused by a lack of or insufficient air venting of the mold’s cavity. A fill test can be used to determine in which area of the cavity air is trapped. Once located, an air vent must be placed at this position. A vacuum bubble generally results from an incorrect layout of the sprue or a material accumulation. To prevent their formation, the injection point should be set as closely as possible to the position of the bubble. If this does not suffice, the injection point, and also the sprue diameter if necessary, should be enlarged. In individual cases, a higher injection temperature can also be helpful. Warping of the part Part warping is mostly caused by a mass imbalance on opposing areas of a two-dimensional part. On the side where the greater quantity of adhesive cools, greater tensile stress arises from shrinkage than on the back side. The tensile stress difference is compensated for the part warping. Warping can only be prevented by applying equal masses of adhesive to both sides of the part.

Frequent cause: mass imbalance

Pockmarks Pockmarks have the same cause as vacuum bubbles, namely an incorrect layout of the sprue or a material accumulation. Accordingly, the fault can be corrected by moving the sprue position and enlarging the sprue and injection point diameters (Fig. 36), by applying design measures to prevent material accumulation, or by increasing the injection temperature. No or incomplete filling of a cavity In most cases, unsuccessful or partly successful filling of a cavity is caused by incorrect air venting of the potting mold. In rare cases, the

Frequent cause: missing air vent


60 Hotmelt molding in practice Fig. 36: Sprue runner and injection point

Injection point Sprue runner

coated part extracts too much heat from the adhesive. This can be established by not inserting the part: If, in spite of this, the cavity is not filled, an air vent is definitely lacking. To remedy this, a small ventilation channel from the cavity to the outside of the potting mold can be installed. Shrinkage of the hotmelt adhesive from the part Shrinkage of the hotmelt from the part is also caused by material accumulation. If on a circuit board with a connector, for example, the connector is only coated with 1 mm of the material and the board with 5 mm of the material, then the force created by shrinkage in the thicker area will pull the material off the connector. This can be prevented in that both parts are coated with the same thickness. If this is not possible, the connector must be provided with a circumferential slot in which the hotmelt adhesive can get a grip.


Machine maintenance 61

Machine maintenance Hotmelt molding machines are low maintenance. The machine’s manufacturer prescribes the optimum maintenance intervals and maintenance scope. Some manufacturers offer maintenance as a service. The melting device should be serviced at least once a year. Among other things, the scope of this annual servicing includes changing the seals and filters and cleaning the device. It is also recommended, however, that the melting device be cleaned after each material change, whether to a hotmelt adhesive of another chemical basis or to a product of the same chemical basis from a different manufacturer. Adhesive manufacturers offer effective cleaning agents matched to the employed hotmelt. Details about them are given in the data sheets on the hotmelt adhesives.

Cleaning the melting device


62

From the idea to the part Although introduced in the 1980s, hotmelt molding is still a new process for many users. It always involves a complex system of parts and casting materials which must be chemically compatible. For a simple overmolding of connectors, the system consists of only a connector, a cable, and the hotmelt. In many cases, the connectors are combined with an electronic board, or the system consists of several different connectors, cables, and boards. Meanwhile, there are many firms in the market that have specialized in hotmelt molding and in parts design, and which are glad to advise on material selection as well on mold and machine configuration. Recommendations are subsequently made regarding how first-time users can develop their idea for a hotmelt molding application to optimum production in a step-by-step manner. Selection of hotmelt adhesive

Requirements portfolio The most important requirements for hotmelt adhesive are: • the application temperature range of the parts • the adhesive force on the employed materials • the chemical resistance. Supplementary information facilitates product selection. Will the parts, for example, be statically or dynamically stressed in the application temperature range? Can impact loading occur at low temperatures?


From the idea to the part 63 Unnecessary safety reserves in the application temperature range restrict the selection of possible adhesive systems. If, for instance, the end customer permits a temperature range of –40 to +85°C, there is no point in the user extending the range to –50 and +100°C. The more stringent the requirements the more expensive the solution. In the worst case, the application cannot be realized at all. The adhesive force of the hotmelt depends not only on the material type, but also, under certain circumstances, on the supply source. If, for example, a connector made of the same type of material is purchased from two different manufacturers, the hotmelt’s adhesive force can be completely different. The adhesive force on both connectors should therefore be tested. If mold release agents are used during manufacture of the parts’, then the parts must be cleaned before casting. A better method is to prohibit the use of mold release agents by the manufacturer right from the very beginning. If a cord grip is required on a cable, the acceptable tensile force must be known because it has an influence on the length of the casting. With regard to the chemical resistance, it is important to know how long, and at what temperature, the contact will occur. Most hotmelts are not affected by short contact at room temperature. During longer contact at higher temperatures, however, they can possibly approach their limits. Knowledge of the following requirements is important for the layout of the molds and machines: • • • • •

the injection volume the lot size to be produced the number of layers the number of working days the production run time.

Requirements for molds and machines


64 From the idea to the part If all requirements are known, it is possible to make a fairly good estimate of with what probability and within what time frame the planned application can be implemented. An overview of the requisite tests can also be prepared. Pilot tests The spot test is a first simple pilot test on parts that are to be sealed. To this end, a dab of hotmelt is applied to the parts and removed again a little later. The spot test provides a subjective impression of the adhesive force. The parts’ design must be brought into line with the demands of hotmelt molding.

Prototype mold

Prototype and test phase Sufficient time and budget must be planned for the prototype and test phase because it constitutes the most important phase of the whole project. It is recommended that a prototype mold be made. A prototype mold is a potting mold with only one cavity. In most cases, it is constructed so that enough space is available for additional cavities. This way, the mold can also be used for production after conclusion of the prototype and test phase. The prototype mold serves primarily to produce prototypes. With it, however, the parameters for production are also determined and optimized. To determine whether the system of material(s) and hotmelt adhesives harmonizes, the customer should test the first parts produced with the prototype mold. Important tests are, among others: • • • •

impermeability tests temperature cycle tests resistance tests vibration tests.


From the idea to the part 65 Layout of the fabrication After successful completion of the prototype and test phase, the parts design, hotmelt adhesive, injection pressure and cycle times are known. Given the injection pressure and cycle times, it can be calculated how many molds with how many cavities are required, what tank volumes the melting device and what closing force the hotmelt molding machines must have. With this information, the most suitable facility for the lot size to be produced can be chosen. Here too, it is advisable to draw upon the experience of a hotmelt molding specialist. While the standard mold and hotmelt molding machine are under construction, production can begin with the prototype mold. Most hotmelt molding specialists offer a prototype production or rental machines.

Machine selection


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Technical terms and abbreviations Adhesion bridge Region in which molecular groups of the substrate and of the adhesive interact, e.g. via van der Waals forces. Cavity Hollow area in the potting mold into which the hotmelt adhesive is injected. The designation “nest” is often used. Contraction Free, i.e. mold-independent contraction of the hotmelt adhesive during cool down. Cooling-down time Part of the ↑cycle time: The mold is still closed and no more material is conveyed. Cycle time Sum of ↑injection time, ↑dwell pressure time, ↑stop time and ↑cooling-down time. Dimer A d. is composed of two similar monomers. Dwell pressure time Part of the ↑cycle time: During the d.p.t., material is still being injected into the already filled form to compensate for ↑contraction of the material and consequently to minimize ↑shrinkage. EA

Ethylene acrylate copolymer.

EEA EPDM rubber. EVA

Ethylene ethyl acrylate copolymer. Ethylene propylene diene monomer Ethylene vinyl acetate.

Flow gap The f.g. determines the thickness of the hotmelt adhesive on the part. Flow path bridged.

Length of the flow gap to be


Technical terms and abbreviations 67 Glass transition Transition of a polymer from the hard elastic state (glass-like brittle) into the rubber elastic state (flexible). Injection time Part of the ↑cycle time: During the i.t., the hotmelt adhesive is injected into the form. Master batch M.b. is characterized as granules of the same chemical basis as the base material. It is blended in as pigmentation or to alter properties. Accordingly, its pigment or additive content is higher than in the end product. In comparison to pastes, powders or liquid additives such as colored inks, the use of a m.b. increases process stability at very good plasticity. Non-polar Property of chemical bonds which consist of two similar or different elements with equal or similar electronegativity. PA

Polyamide.

PA 6 Polyamide 6; polyamide from caprolactam. PB

Polybutylene.

PE

Polyethylene.

PET

Polyethylene terephthalate.

Polar Property of chemical bonds which consist of two elements with significantly different electronegativity. Polycondensation Polymerization reaction where simply constructed molecules such as water are split off. PP

Polypropylene.

PU

(also PUR) Polyurethane.

PVC

Polyvinyl chloride.

RAL number Color code for characterizing paints and lacquers.


68 Technical terms and abbreviations SBS Styrene-polyolefin-styrene block polymer with the polyolefin butadiene. SEBS Styrene-polyolefin-styrene block polymer with the polyolefin ethylene butylene. SEPS Styrene-polyolefin-styrene block polymer with the polyolefin ethylene propylene. Shrinkage Diminution of the part created with hotmelt adhesive during cool down after molding. The s. is generally less than the ↑contraction, because during the filling process of the cavity, material is always backfilled (↑dwell pressure time). Shrinkage cavity Shrinkage-induced vacuum bubble. SIS Styrene-polyolefin-styrene block polymer with the polyolefin isoprene. Spot test Subjective adhesive force test, during which hotmelt adhesive is applied to the substrate and manually removed after a defined time. Stop time Part of the ↑cycle time: During the s.t., the application head remains with the injector on the mold. The injector is closed, i.e., no material is conveyed. Temperature shock test Bonded joints are brought from a very low (e.g. – 40°C) to a very high temperature (e.g. +100°C) within a few seconds and back to the original temperature. This procedure can be repeated. UL94 V-0 Classification of the combustibility of plastics pursuant to UL94 (Tests for Flammability of Plastic Materials for Parts in Devices and Applications) of Underwriters Laboratories, USA.


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Bibliography Becker, Bettina; Schöneweiß, Achim: Moulding Technology for Electronic Components. In: ATZautotechnology 7 (2007), No. 3, p. 34−37. Gruber, Werner: Hightech-Industrieklebstoffe: Grundlagen und industrielle Anwendungen. Landsberg: verlag moderne industrie: 2000 (Die Bibliothek der Technik, Volume 206). Habenicht, Gerd: Kleben – Grundlagen, Technologie, Anwendungen. 5. ed. Berlin: Springer, 2006. Hund, Matthias C.; Grünewald, Norbert: Additives and masterbatches. In: Kunststoffe 93 (2003), No. 7, p. 38−39. Krebs, Michael: Neue monomerreduzierte Polyurethan-Hotmelts: Nicht als Gefahrstoff kennzeichnungspflichtig. In: adhäsion KLEBEN & DICHTEN 48 (2004), No. 1, p. 15–19. Röthemeyer, Fritz; Sommer, Franz: Kautschuktechnologie: Werkstoffe – Verarbeitung – Produkte. 12. ed. Munich: Carl Hanser, 2006. p. 122–136. Wichelhaus, J.: Thermoplastic Polyamide Adhesives. In: European Adhesives and Sealants 5 (June 1988), p. 26 –29.


The companies behind this book Henkel AG & Co. KGaA HenkelstraĂ&#x;e 67 40191 DĂźsseldorf, Germany Tel.: +49 211 797-9339 Fax: +49 211 798-19339 E-mail: assembly@henkel.com Internet: www.henkel-elektrische-industrie.de Henkel has one of the most multifaceted ranges of adhesive and sealing materials in the world as well as surface treatments for industrial applications. When developing efficient solutions, our specialists consider all relevant aspects to ensure you will be successful. You profit not only from our range of products, but also from professional advice and support. Furthermore, you profit from technology for process optimization and implementation of innovative design options. Regardless of whether it involves filling and cementing cable constructions or laminating or smart cards, Henkel technologies enable rapid, safe, and cost-efficient production.

mikkelsen electronics as Havremarken 3–4 3520 Farum, Denmark Tel.: +45 4434 0300 Fax: +45 4434 0310 E-mail: info@mikkelsen-electronics.com Internet: www.mikkelsen-electronics.com Long-standing experience in component sales, cable harnessing, and hotmelt molding puts us in the position to offer a multitude of individual and system solutions. We have worked in the hotmelt molding field for more than twenty years. We sell not just the adhesives, potting molds, and machines, but we also fabricate them ourselves. Our specialists have great experience and are not afraid of any challenge. In the hotmelt molding area, we offer: Parts development including 3-D drawings, potting molds, facilities, hotmelt adhesives, prototype fabrication, toll manufacturing of small and medium quantities, as well as customer services in connection with hotmelt molding facilities.


OptiMel Schmelzgußtechnik GmbH & Co. KG Almeloer Straße 9 58638 Iserlohn, Germany Tel.: +49 2371 1597-0 Fax: +49 2371 1597-50 E-mail: info@optimel.de Internet: www.optimel.de

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OptiMel Schmelzgußtechnik GmbH & Co. KG, the Henkel KGaA subsidiary founded in 1995 and today independent, is a leading supplier of low-pressure casting technology with a broad range of standard and special customer specific solutions. Our systems are in use worldwide. With an export quota of approx. 60%, we are an internationally experienced and acknowledged supplier for technically challenging system solutions. A high degree of customer orientation paired with flexibility and a high quality standard distinguishes our working methods. Our employees are glad to support you in any way they can. This naturally also holds true after installation of our equipment.

U. Kolb Werkzeug Vertriebsgesellschaft mbH Neuer Weg 32 71111 Waldenbuch, Germany Tel.: +49 7157 7371-0 Fax: +49 7157 72901 E-mail: info@u-kolb-gmbh.de Internet: www.u-kolb-gmbh.de U. Kolb Werkzeug Vertriebsgesellschaft mbH has been active worldwide for more than twenty years in connector assembly, cable, and hotmelt processing. Our customer-oriented corporate philosophy includes conscientious recording of project data and precise implementation of customer wishes, in the machinery as well as in the construction and mold areas. Our broad customer base includes almost all areas of the electrical, electronic, and automotive industry. Applications range from simple cable bushings and electronic castings to special applications in the solar technology area. Our employees advice is competent and goal-oriented. They process all inquiries and tasks smoothly.


ISBN 978-3-937889-94-8