Mold cavity compression

The injection blow molding process involves two stages. In the first stage, a plastic part called a preform is made, using a traditional injection molding process. The preform is a precise shape, with precise wall thicknesses. This preform is then transferred to a blow molding station, where it is placed in another mold and held in place by specific features of the preform that mate with the blow mold cavity. Compressed air is then injected into the center of the part, forcing the unsupported walls of the preform tube to stretch and expand outward, until they hit the walls of the second mold. Once the material cools and solidifies, the mold is opened and the part is removed. 

Molding, compression Process of shaping plastic articles by placing material in a confining mold cavity and applying pressure and usually heat. 

Regardless of the variant of LCM the process can be subdivided into a number of steps. The first step is preforming, which means that dry reinforcement is tailored to the shape of the mold. This can be done in many different ways depending on the complexity of the geometry and the requirements on mechanical performance of the part. The preform is then placed in a mold cavity that is subsequently closed. The preform is usually compressed slightly by the mold. The next step is resin injection into the mold cavity until the preform is fully impregnated. The final step is in-mold cure (i.e., curing inside the mold until the part is sufficiently stiff to be demolded). A controlled postcure is sometimes performed to ensure that optimum properties are obtained. 

The time required to cure the materials is dependent entirely on the method of molding, mold temperature, and material temperature when introduced into the mold cavity. There are now four basic methods of molding thermoset molding compounds (1) Compression, (2) Transfer, (3) Injection, and (4) Extrusion—with the method most commonly used as rated. Very small quantities are processed by extrusion. All methods of molding may be done automatically or semi-automatically. In most cases, the injection method is practically all done automatically. 

Figure 6.81 Schematic diagram of a disc-shaped compression mold cavity.

Compression molding method called coining is used for parts that are too complicated to be produced by machining. In coining, a sintered molding is heated to the melting point, then it is quickly pressed into a mold cavity and held under pressure until it solidifies.4... 

Blow molding uses compressed air to blow and expand a hot plastic tube ( rubbery melt ) in a female mold cavity, until the plastic conforms to the walls of the mold. This has developed into a major way to produce plastic bottles, and also more specialized hollow shapes such as fuel tanks, seat backs, tricycles, surfboards, and so on. The leading material is HDPE for milk, water, and household chemicals. Second largest is PET for carbonated beverages. A number of other thermoplastics are blow molded in smaller amounts for more specialized uses. 

Transfer Molding. This preheats the molding powder in the upper part of the mold. Then a plunger pushes the fluid material down into the (lower) mold cavity and holds it there until cure is complete. This gives more fluid flow than compression molding, and avoids damage to delicate molds or molded parts. 

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

Either crystalline and amorphous types (Chapter 1) can be used with flow molding because the plastic is melted prior to forming. The forming temperature is usually lower than for injection molding or extrusion. Plastics need not be trimmed, as the composite is compression-molded to completely fill the mold cavity (Chapter 14). Very important, the flow molding process permits more complex parts to be formed than solid-state forming. The process cycle time is usually about 1 min. 

Three general types of molds are used for CM. In the positive mold (Figure 14.3a) all the material is trapped in the mold cavity. The pressure applied compresses the material into the smallest possible volume. Any variation in the weight of the charge will result in a variation in part thickness. In multicavity molds, if one cavity has more material than the others, it will receive proportionately greater pressure. Multiple cavities, therefore, can result in density variations between parts if loading is not done with some degree of precision control.1 278 284... 

Preheating is usually carried out at 150 to 300F (66 to 149C) followed by quick transfer to a mold cavity. The actual heat depends on the material, the heater capability, and the speed of transfer. Circular preforms are normally used with dielectric heaters so they can be rotated to obtain uniform heating. Pills of compressed compound are used to produce preforms to facilitate handling, reduce the bulk factor in the cavity, and control the uniformity of charges for mold loading. Preforms can also be the shape of the mold cavity. 

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