Compression molding is a common process used for both thermoplastic and thermoset stock shape materials. Compression molding is accomplished by placing the plastic material (can be a granular or pelletized form) in a mold cavity to be formed by heat and pressure.
The process is someone similar to making waffles. The heat and pressure force the materials into all areas of the mold. The heat and pressure cycle of the process will harden the material and then it can be removed.
Typically, thermosetting compounds like polyesters, phenolics, melamines and other resin systems are compression molded using alternating layers of different reinforcement materials to create a final product. However, there are various thermoplastics that are commercially compression molded as well.
The Process of Compression Moulding
Compression molding requires a mold made up of two parts- a cavity (female) part of the mold and a plunger (male) part of the mold. The molding process requires a press with a heated platen (upper and lower). Because the mold parts are attached to the upper and lower platens, the parts are designed to align when assembled. This alignment attribute forms the basis of the compression molding technique.
It is worth mentioning that before commencing any molding operation, the positioning of the mold parts relative to the upper and lower platen of the press must be determined. When molding compounds are used, the cavity part of the mold is usually attached to the lower platen of the press, while the plunger part is mounted on the upper platen.
The process of compression molding is quite different for thermoplastics. Contrary to the thermoset plastics molding, the mold needs to be cooled at the end of the molding operation for thermoplastics. As plastics become hardened and cannot flow after molding, the thermosets need not be cooled after moulding.
The moulding process can be as quick as three minutes to complete and as long as two hours: it all depends on the type of plastic used and the size of the charge. Even with the numerous stages or processes highlighted, it is essential to note that the curing or cooling stage consumes most of the time in the moulding operation, and three types of moulds are used for compression Moulding:
Semi Positive Mould
In the positive mould, all the mould material is charged into the mould cavity, which is usually very deep. As there is very little escape for the plastic material, the smallest possible volume is formed when the plunger compresses the material. By altering the weight of plastic material, the thickness of the moulded parts increases or decreases proportionately. Hence, for multi-cavity moulds, a variation of the part thickness or density may occur if one cavity is charged more than the other.
This is the most used mould type. It is simple to construct and holds the thickness and density of parts within close limits. These attributes are possible in flash compression moulding because of a unique feature called the pinch-off line. The pinch-off line allows excess material to escape the mould cavity as a flash, thereby allowing the material to be compressed to a density that is proportionate to the force applied.
The semi-positive mould is just as popular as the flash mould. Besides, it combines features of the positive mould and the flash mould to produce even better-moulded parts. It has a large cavity that allows the material to be trapped positively, and it has sidewalls that allow excess material to escape the cavity as a flash.
Thermoset Compression Moulding
Compression moulding offers many advantages during the production process and to the properties of its components. Compressed parts have a higher durability than parts manufactured using the injection moulding process. They do not have a visible injection gate, very low shrink, and they have a higher surface quality. It is very easy to change the material and colour. Therefore the compression moulding is ideal for small batch sizes.
In combination with pressure and temperatures of 130 to 180° C, thermosets react as a chemical process, thus generating the high forces of resistance typical for this material. Thermoset materials can be processed in the form of granulates, powders, tablets, SMC mats or BMC compounds. The production cycle time strongly depends on the handling (insertion, screwing into the thread, etc.) as well as on the curing time.