Compression molding is a very important molding process for molding thermoset plastics rather than thermoplastics. Thermoset plastics cannot be remolded or reformed after the initial heating and forming of the plastic from its original state. A thermoplastic is a plastic polymer that can be reheated and reformed after the first instance in which the material was molded.
Examples of Compression Molded Products
- Pallets
- Plug Housings
- Grommets
- Phone Keypad
- Small components
- Small Housings
- Rubber Bumper
Process of Compression Molding
The compression molding process uses a combination of heat pressure and time to ensure that cross-linking agents within the resin can form between the long chain polymer molecules. This is where the difference in process and material properties becomes apparent. Given that the resin used is a thermoset it cannot be reformed or remolded once the process has occurred.
The first stage of the process begins with a preformed charge of thermosetting resin known as slug. It is placed between two mold halves; one half is a female cavity and the other is a male form. It is important the mold slug is weighed accurately as this will help reduce any excess produced which can result in additional part flash and wasted resin. Flash is the term used to describe the excess plastic that forms along the parting line of the finished piece. The concern with having excess flash is additional labor is required to remove the resin adding cost to the part. Once the slug is placed in between the 2 mold halves, the mold is heated to a temperature which allows the cross links to form within the resin, the mold is then closed onto the preform and the pressure and force pushes out any excess resin. The mold halves are then held tightly under pressure until the required temperature is reached. This ensures the material is cured and all of the cross-links are formed. Once the mold is opened the product can be ejected while it is hot as it does not require a cooling cycle like other processes. The process can then begin again with a new preformed slug.
Advantages and Disadvantages
- The advantage of compression molding is that relatively complex parts can be molded over long production runs.
- The disadvantages of the process is that heavy machinery is required such as hydraulic rams that provide the force required so that the cross-linking reaction can occur. Maintenance costs can quickly build up which are passed onto product costs. The startup costs are relatively higher than rotational molding however less expensive than those used in injection molding.