The Disadvantages of Compression Molding

By Lewis R. Humphries

Compression molding is the process of applying heat to a base plastic resin, using hydraulic pressure to create a basic shape. This occurs when the heat melts the resin and subsequent pressure forces the malleable liquid into a particular mold. This process is conducted in a compression mold and is traditionally used for the molding of thermoset materials. These form the initial resin or liquid, but their properties dictate that, once set and cured through the compression molding process, they cannot be reset.

Unsuited to Complex Designs

While there are three basic types of molds used in compression molding (flash, positive and semi-positive), none of these lend themselves to complex or intricate part moulding. Because of the complexity of the processes involved, the molds need to remain simple to function. This causes limitations to the products that a mold can create. The molds are historically geared towards simple designs and mass production.

Low Cost Effectiveness

Because of the resolute nature of the material typically used for compression molding, it is not a particularly cost-effective process. The main issue is that thermoset material is rigid when set and therefore cannot be melted down in the case of rejected or unsatisfactory parts. In this instance, any slight defect in the molded object cannot be repaired or reprocessed, and this factor is directly responsible for waste of money, material, time and labor.

Damage

Certain components within the compression mold are vulnerable and susceptible to breakage when the finished item is removed from the equipment. The ejector pin is one such component, located in the base of the mould. After the hydraulic press in the mold has compressed the plastic, it will eject the finished product, and this is repeated each time the process is undertaken. Such repetitive use can lead to wear and ultimately to component failure.

Process Duration

The compression molding process cycle can take between one and six minutes, depending on the product and the finish required. Typically, the cycle will take between three and four minutes, and this makes it more time consuming than injection molding. The average injection molding cycle is approximately two minutes, and in the case of mass production of a single item, time can be a significant factor.