Plastic Molding

Plastic Molding Here the usual methods in the shaping of plastic parts are exposed, whether in thermorigid or thermoplastic plastics, accompanied by explanatory videos.

In the manufacture of plastic material parts, various types of molds are used to delimit the plastic mass, while it hardens and preserves the desired shape. These molds are mounted on a press that will open and close the mold, apply high pressure if necessary, and facilitate loading of the mold by external means.

The plastic material is held in the mold under pressure while it hardens sufficiently so that its shape is retained after removal.

Steam, hot water, oil or electricity are used to heat the molds. The type of heating to be used in a given job is determined by the means available and by the nature of the job itself.

In some cases, the molds must be cooled by circulating water or another cooling liquid. In order to keep the temperature of the molds constant, equipment equipped for this purpose is available.

Plastic compounds differ greatly from one another and lend themselves to a wide variety of molding methods. Each material is better adapted to one of the methods although many can be manufactured by several of them. The material to be molded is in powder, granular form, although for some there is a preliminary preforming operation before use.

COMPRESSION It is a process used with thermosetting plastics, since there are faster processes for thermoplastics: It consists of depositing certain quantities of the material in a granulated state or preformed in tablets in the lower cavity of the metal mold; It is continuously heated by steam or electricity.

The bottom mold is raised and comes into contact with the top half which is also heated, pressure is applied by a hydraulic cylinder, causing plastic to flow into the mold cavity and forming the part. After compression, the part solidifies, it is removed from the die. Compression pressures can vary between 140 to 700 Kg/cm, depending on the size and configuration of the part. Mold temperatures vary from 60 to 120° C., also depending on the filling material.

Some thermoplastic materials are processed by compression, for the rapid heating and cooling cycle of the mold increases the difficulty in using such materials, unless the mold is sufficiently cooled before ejection, there is a danger of warping of the part.

The advantages of the process are: 

Little waste of material.

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Low cost of finishing and obtaining large parts.

The disadvantages of the process are:

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The difficulty of obtaining intricate shapes (edges, small holes).

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The difficulty of obtaining tolerances closer than +0.13 mm. and -0.13mm.

TRANSFER This process is used more with thermosetting plastics when intricate or highly accurate parts are required.

Using this method, the preformed and preheated material is placed in the transfer cavity and as soon as it is sufficiently softened, it is forced by the hydraulic piston to penetrate into the mold cavity where it finishes polymerizing or curing.

Molding is expensive, but tighter tolerances and more uniform density can be maintained, time is generally shorter for thick sections, and thick and thin sections and inserts can be molded with fewer problems than other molds.

The pressures used are 50 to 100% higher than for compression molding and this is one of the reasons that complex shapes can be obtained. Dimensional accuracy is good and production speed is faster than compression molding.

The main advantages are:

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Very complex parts can be molded with long and complicated inserts.

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Parts that have both thick and thin sections can be molded at the same time.

The disadvantages of this method are:

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The molds are more expensive than for compression molding.

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There is greater waste of the material, since the leftovers cannot be processed.

INJECTION Injection molding is the most widely used process for the manufacture of thermoplastic components. In this process, molten plastic is forced into the cavity of the metal die that has been machined into the shape of the desired product.

When the plastic has solidified sufficiently, the die is opened and the part is removed. The raw plastic material is placed in the form of pellets in the hopper of the machine. It then enters the heater where it melts. The molten plastic is then pushed into the die cavity by direct application of hydraulic or mechanical pressure.

High-capacity injection molding machines can exert several hundred tons of pressure and can be used to make large one-piece plastic parts. Examples include auto body components such as assemblies, hoods, fenders, bumpers, and grills.

The injection process can be summarized in five steps: Step 1: The mold parts are closed.

Step 2: The piston moves forward and pushes the material into the heating cylinder, at the same time it injects the plasticized material into the mold.

Step 3: The piston remains in this position for some time maintaining pressure through the nozzle. During this time the material is cooling and solidifying enough to maintain the shape of the mold.

Step 4: The piston moves back, but the mold remains closed, a new amount of material falls from the feed hopper.

Step 5: The mold opens at the same time as it expels the molded parts through the action of the rams.

The advantages of this process are:

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Savings in material, manufacturing space and production time.

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Accuracy of shape and dimensions of the injected parts.

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Possibility of forming holes and inserting elements of other materials with which the production is completed.

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Smooth and clean surface of the injected parts.

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Good resistance properties.

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Fast production of large number of parts.

The disadvantages of the process are:

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Not recommended for low production due to high tooling cost.

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Resins can solidify before filling the mold when dealing with very thin sections.

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Complicated parts increase tooling cost.

EXTRUSION In this process, granular material is fed into the hopper, then forced through a heating chamber by means of a spiral thread. In the chamber the material becomes a dense and viscous mass, in which form it is passed through the die. Upon leaving the matrix, it is cooled by means of air, water or by contact with a cold surface, gradually hardening as it is discharged onto the conveyor. Long tubes, rods, molded sections, and many special sections are easily produced in this form.

BLOWN In this method a softened plastic cylinder is used which is placed inside the mold, once the mold is closed a current of air or steam is blown through one end causing the material to expand and fill the mold cavity. This method allows to obtain containers of small and large volumes such as bottles.