Finding The Right Process: Molding Process Selection
Engineering thermosets, unlike engineering thermoplastics, are reacted and cured during molding. Hold time and temperature are critical control parameters to achieve a consistent degree of cure. Phenolics evolve a small amount of water (hexa-cured molding compounds also evolve a small amount of ammonia) as part of their curing process. Adequate venting is important for these materials.
Depending on the part and its applications, different molding processes can be considered:
Lowest warpage, minimal shear stress on melt as it flows into cavities
- Low processing and post shrinkage
- Low warpage of molded part
- Minimal damage of reinforcing fibers
- Cycle times depend on cure time of the thickest wall section
Highest automation, highest consistency
- Shortest cycle times, compared to compression or transfer molding
- Highest degree of automation
- Tooling more complex
- High melt shear during injection
More automated and consistent than compression molding.
- Shorter cycle times, compared to compression molding
- Minimal flash (cavity is filled with the mold closed)
- Low shear
- low warp
- Higher degree of warpage than compression molding (due to anisotropy)
Compression molding is the simplest process. A set amount of molding compound, often preheated or preplasticized, is charged into the heated mold while it is open. The mold is then closed squeezing the softened material to fill out the cavity. After enough time is allowed for the molding compound to cure, the part can be removed from the mold.
Cure time is dependent on the reactivity of the molding compound, its prior treatment, the mold temperature and the wall thickness of the part being molded.
The figure contains a schematic of the compression process showing two different style tools. Both are viable options for molding compounds; flash, or overflow, type molds are not recommended. Semipositive molds have been especially successful since they compress the molding compounds during cure and produce a consistent part despite variations in the material charged.
Compression molding may be a manual, semiautomatic or fully automatic process.
Transfer molding, also known as “pot and piston” molding, is a process in between compression molding and injection molding. Pelletized and preheated or preplasticized molding compound is pushed from the heated pot by a plunger (piston) into the mold cavity via a runner and gate system where the material cures.
Three plate molds with the plunger working from the top can be built into conventional presses. Single split molds, easier from the point of view of process technology – the plunger works from below or horizontally from the side — require a press with two hydraulic plungers.
Transfer systems with screw preplastification and plunger feeding are an obvious choice for molding parts which are subjected to stress and have tight dimensional tolerances. This method can accommodate more reactive molding compounds which results in relatively short cycle times and accurate shot weights.
Due to its high degree of automation and efficiency, injection molding is the most widely used process today. A no or low compression plasticating screw designed for free flowing (or pourable) thermosets, melts and homogenizes the molding compound then injects it into the mold cavity tool via the sprue and runner system. The process is self-contained and machine controlled, which leads to greater part-to-part consistency in the molding cycle.
This process is a hybrid that combines the quality benefits of compression molding with the efficiency and consistency of injection molding.
This is accomplished by injecting a fixed amount of plasticated molding compound into the mold while it is held partially open, followed by closing the mold to compress the material and fill out the part cavity.