PROCESSING OF PLASTICS DEPARTMENT OF MECHANICAL ENGINEERING KHON KAEN UNIVERSITY
INTRODUCTION Plastics can be molded, cast, shaped, formed, machined, and joined into many shapes with relative ease and with few or no additional operations required. Because the properties of plastic parts and components are greatly influenced by the method of manufacture and the processing parameters, their proper control is essential for good part quality. Plastics are usually shipped to manufacturing plants as pellets or powders and are melted just before the shaping process. Plastics are also available as sheet, plate, rod, and tubing, which may then be formed into a variety of products. Liquid plastics are often used to make reinforced-plastic parts. A brief summary of plastic properties are shown next.
Schematic illustration of polymer chains. (a) Linear structure; thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as polyethylene. (c) Crosslinked structure; many rubbers and elastomers have this structure. Vulcanization of rubber produces this structure. (d) Network structure, which is basically highly crosslinked; examples include thermosetting plastics such as epoxies and phenolics.
Specific volume of polymers as a function of temperature. Amorphous polymers, such as acrylic and polycarbonate, have a glass-transition temperature, Tg, but do not have a specific melting point, Tm. Partly crystalline polymers, such as polyethylene and nylons, contract sharply at their melting points during cooling.
Effect of temperature on the stress-strain curve for cellulose acetate, a thermoplastic. Note the large drop in strength and increase in ductility with a relatively small increase in temperature.
EXTRUSION In extrusion, raw thermoplastic materials in the form of pellets, granules, or powder are placed into a hopper and fed into the extruder barrel. The barrel is equipped with a screw that blends and conveys the pellets down the barrel. The internal friction and shear stresses from the mechanical action of the screw, along with heaters around the extruderâ€™s barrel, heats the pellets and liquefies them. Once the extruded product exits the die, it is cooled, either by air or by passing it through a water-filled channel. Controlling the rate and uniformity of cooling is important for minimizing product shrinkage and distortion. The extruded product can also be drawn by a puller after it has cooled; the extruded product is then coiled or cut into desired length. Because the material being extruded is still soft as it leaves the die and the pressure is relieved, the cross section of the extruded product is different than the shape of the die opening; this effect is known as die swell. Thus, the diameter of a round extruded part is larger than the die opening. Considerable experience is required to design proper dies for extruding complex cross sections of specific shape and dimensions. Modern software can assist in the design of extrusion dies. To understand the mechanics of polymer extrusion, the schematic illustration of a typical extruder is shown next on the top left image, the geometry of the pumping section of an extruder screw is illustrated on the top right image, the extruder and die characteristics are shown on the bottom right image. Typical characteristics of extrusion process are long, uniform, solid or hollow, simple or complex cross sections; wide range of dimensional tolerances; high production rates; low tooling cost. Page 4
The flow rate of plastic out of the extruder, or drag flow, is given by Qd = Where
H D N
π 2 HD 2 N sin θ cos θ 2
is the channel depth is the screw diameter is the shaft speed is the flight angle
The die characteristic is the expression relating flow to the pressure drop across the die and in general form is written as
Where Qdie p K
is the flow through the die is the pressure at the die inlet is a function of die geometry
One closed-form solution for extruding solid round cross sections is given by
πDd4 K= 128ηld Where Dd η ld
is the die opening diameter is the viscosity of plastic is the die land
Qdie = Kp Page 5
INJECTION MOLDING In injection molding, the pellets, or granules, are fed into a heated cylinder, where they are melted. The melt is then forced into a split-die chamber, either by a hydraulic plunger or by the rotating-screw of an extruder. As the pressure builds up at the mold entrance, the rotating screw starts to move backward, under pressure, to a predetermined distance, thus controlling the volume of material to be injected. The screw then stops rotating and is pushed forward hydraulically, forcing the molten plastic into the mold cavity. After the part is set or cured, the molds are opened, and the part is ejected. The molds are then closed, and the process is repeated automatically. Top left figure: Injection molding with (a) a plunger and (b) a reciprocating rotating screw. Bottom left fiture: Illustration of mold features for injection molding. (a) Twoplate mold, with important features identified Top middle figure: Illustration of mold features for injection molding. (b) injection molding of four parts, showing details and the volume of material involved. Top right figure: Types of molds used in injection molding. (a) Two-plate mold, (b) three-plate mold, and (c) hot-runner mold. Bottom right figure: Products made by insert injection molding. Metallic components are embedded in these parts during molding. Page 6
BLOW MOLDING Blow molding is a modified combination of extrusion and injection molding processes. In extrusion blow molding, a tube is extruded and clamped into a mold with a cavity much larger than the tubeâ€™s diameter, and then blown outward to fill the mold cavity. Corrugated pipe and tubing are made by continuous blow molding, where by the pipe or tube is extruded horizontally and blown into moving molds. In injection blow molding, a short tubular piece (parison) is first injection molded. The dies are then opened, and the parison is transferred to a blow-molding die. Hot air is injected into the parison, which expands and fills the mold cavity. Typical products made by this process include plastic beverage bottles and hollow containers. Multilayer blow molding involves the use of coextruded tubes, or parisons, thus allowing the use of multilayer structures. Typical examples of multilayer structures include plastic packageing for food and beverages, with characteristics such as odor and permeation barrier, taste and aroma protection, resistance to scuffing, printing capability, and ability to be filled with hot fluids. Other applications include the cosmetics and pharmaceutical industries.
Schematic illustrations of (a) the blow-molding process for making plastic beverage bottles, (b) the injectionblow-molding process, and (c) a three-station injectionblow-molding machine for making plastic bottles.
ROTATIONAL MOLDING Most thermoplastics and some thermosets can be formed into large hollow parts by rotational molding. Typical parts made include trash cans, boat hulls, buckets, housings, toys, carrying cases, and footballs. The thin-walled metal mold is made of two pieces (split female mold), designed to be rotated about two perpendicular axes. A premeasured quantity of powdered plastic is placed inside a warm mold. The mold is then heated, usually in a large oven, while it rotates about the two axes. This action tumbles the powder against the mold, where the heat fuses the powder, and cross-linking occurs after the part is formed in the mold by continued heating.
The rotational molding (rotomolding or rotocasting) process. Trash cans, buckets, carousel horses and plastic footballs can be made by this process.
THERMOFORMING Thermoforming is a family of processes for forming thermoplastic sheet or film over a mold with the application of heat and pressure or vacuum. A sheet is first heated in an oven to the sag (softening) point, but not to the melting point. It is then removed from the oven, placed over a mold, and forced against the mold by the application of a vacuum. Since the mold is usually at room temperature, the shape of the plastic is set upon contacting the mold. Because of the low strength of the material formed, the pressure differential caused by the vacuum is sufficient for forming, although air pressure or mechanical means are also applied for some parts.
Various thermoforming processes for thermoplastic sheet. These processes are commonly used in making advertising signs, cookie and candy trays, panels for shower stalls, and packaging.
COMPRESSION MOLDING In compression molding, a preshaped charge of material, a premeasured volume of powder, or a viscous mixture of liquid resin and filler material is placed directly in a heated mold cavity. Molding is done under pressure with a plug or the upper half of the die. The flash is removed by trimming or some other means. Typical parts made by this method include dishes, handles, container caps, fittings, electrical and electronic components, washing-machine agitators, and housings. Elastomers and fiber-reinforced parts with long chopped fibers are also formed exclusively by this process. Compression molding is used mainly with thermosetting plastics, with the original material in a partially polymerized state. Cross-linking is completed in the heated die. The thicker the part, the longer it will take to cure. Three types of compression molds are available: (1) flash type for shallow or flat parts, (2) positive for high-density parts, and (3) semipositive for highquality production. Undercuts in parts are not recommended; however, dies can be designed to be opened sideways to allow easy removal of the part.
Types of compression molding, a process similar to forging: (a) positive, (b) semipositive, and (c) flash. The flash in part (c) is trimmed off. (d) Die design for making a compression-molded part with undercuts. Such designs also are used in other molding and shaping operations.
TRANSFER MOLDING Transfer molding represents a further development of the compression molding process. The uncured thermosetting material is placed in a heated transfer pot or chamber. After the material reaches the proper temperature, it is injected into heated, closed molds. Depending on the type of machine used, a ram, a plunger, or a rotating screw feeder forces the material to flow through the narrow channels into the mold cavity. Because of internal friction this flow generates internal heat, which raises the temperature of the material and homogenizes it. Curing then takes place by cross-linking. Typical parts made by transfer molding include electrical and electronic components and rubber and silicone parts.
Sequence of operations in transfer molding of thermosetting plastics. This process is particularly suitable for making intricate parts with varying wall thicknesses.
CASTING Some thermoplastics, such as nylons and acrylics, and thermosetting plastics, such as epoxies, phenolics, polyurethanes, and polyester, can be cast in rigid or flexible molds into a variety of shapes. Typical parts cast include large gears, bearings, wheels, thick sheets, and components that require resistance to abrasive wear. In casting thermoplastics, a mixture of monomer, catalyst, and various additives is heated and poured into the mold. The part is formed after polymerization takes place at ambient pressure.
Schematic illustration of (a) casting, (b) potting, and (c) encapsulation of plastics.
PROCESSING OF REINFORCED PLASTICS Because of their unique structure and the characteristics of their individual components, reinforced plastics require special methods to shape them into useful products. In order to obtain good bonding between the fibers and the polymer matrix, as well as to protect the fibers during subsequent processing steps, the fibers are first surface treated by impregnation (sizing). When impregnation is done as a separate step, the resulting partially cured sheets are referred to by prepegs. In the manufacture of prepegs, the continuous fibers are aligned and subjected to surface treatment to enhance their adhesion to the polymer matrix. They are then coated by being dipped in a resin bath and made into a sheet or tape. Finally, individual pieces of the sheet are assembled into laminated structures, such as the horizontal stabilizer for the F-14 fighter aircraft. Special computer-controlled tape-laying machines have been developed for this purpose. Typical products made include flat or corrugated architectural paneling, panels for construction and electric insulation, and structural components of aircraft that require good retention of properties and fatigue strength under various environmental conditions.
Manufacturing process for producing reinforced-plastic sheets. The sheet is still viscous at this stage and can later be shaped into various products.
CONTACT MOLDING This process is used in making products with high surfacearea-to-thickness ratios, such as swimming pools, boats, tub and shower units, and housings. It uses a single male or female mold, made of of materials such as reinforced plastics, wood, or plaster. The contact molding process is a wet method, in that the reinforcement is impregnated with the resin at the time of molding. The simplest method is called hand layup. The materials are placed and formed in the mold by hand, and the squeezing action expels any air and compacts the part.
Manual methods of processing reinforced plastics: (a) hand lay-up and (b) spray-up. These methods are also called open-mold processing. (c) A boat hull made by these processes.
FILAMENT WINDING Filament winding is an important process whereby the resin and fibers are combined at the time of curing. Symmetric parts, such as pipes and storage tanks, as well as axisymmetric parts are produced by this method. The reinforcing filament, tape, or roving is wrapped continuously around a rotating mandrel or form. The reinforcements are impregnated by passing through a polymer bath. The products made by filament winding are very strong because of their highly reinforced structure.
(a) Schematic illustration of the filament-winding process. (b) Fiberglass being wound over aluminum liners for slide-raft inflation vessels for the Boeing 767 aircraft.
PULTRUSION Parts with high length-to-cross-sectional area ratios and various constant profiles, such as rods, structural profiles, and tubing, are made by the pultrusion process. Typical products made by this process include golf club shafts, drive shafts, and structural members such as ladders, walkways, and handrails. In this process, the continuous reinforcement (roving or fabric) is pulled through a thermosetting-polymer bath, and then through a long heated steel die. The product is cured during its travel through the die and then cut into desired lengths. The most common material used in pultrusion is polyester with glass reinforcements.
(a) Schematic illustration of the pultrusion process. (b) Examples of parts made by pultrusion.
REFERENCES - S. Kalpakjian, S. R. S. (2003). Manufacturing Engineering and Technology. New Jersey, Pearson Prentice Hall. - Groover, M. P. (2010). Principles of Modern Manufacturing: Materials, Processes, and Systems, John Wiley & Sons Ltd.