Let’s dive in- we’re going to break it down into 3 basic steps about how thermoset plastics are formed
Step 1…What they are
Step 2…How they are formed
Step 3…What they are formed into
What is a thermoset plastic?
Thermoset is a plastic polymer that starts out as a soft solid or viscous liquid. It becomes irreversibly hardened when cured due to heating, subjected to high pressure (also inducing heat), or mixed with a catalyst. These processes cause a chemical change that “Set” a permanent chemical bond.
What types of thermoset plastics are there?
Some common types of thermoset plastics are polyurethane, polyureas, polyester, and vinyl ester. Other types of thermoplastics include bismaleimide (BMI), epoxy, fluoropolymers, phenolic, polyvinylidene fluoride (PVDF), and silicone. Many of these resins can be mixed with fibers to improve structure and stability. Examples of common fibers used include; Glass, Cotton, Carbon, Graphite, Boron, or an aramid like Kevlar®.
How are they formed?
Basic processes for forming thermoplastics are compression molding, transfer molding, and injection molding.
- Compression molding – This process can use a preheated polymer however; it is not required for all materials. It is placed in a preheated mold cavity. The mold is then closed and compressed using a hydraulic or electric press to form the plastic along the inner contours of the mold. Heating the mold improves the flow of material and also starts the chemical bonding process.
- Transfer molding – This process can use a preheated polymer however; it is not required for all materials. The material is placed in a transfer pot, usually residing either above the upper mold half or as part of it. The mold is closed to seal and a transfer ram extends forcing the material from the pot into the mold cavity. Force is increased on the transfer ram to displace the polymer along the inner contours of the mold. Heating the mold improves the flow of material and also starts the chemical bonding process. In some cases, the transfer pot itself is heated.
- Injection molding – Using a screw, bulk polymer is fed, heated, and injected into a mold. This process nearly liquifies plastic resin, reducing its viscosity for easy displacement into a single or multiple cavities within a mold. Since most thermoset compounds are not free flowing a stuffer is required as opposed to a gravity fed hopper. The stuffer feeds material at a constant rate into the screw within the injection barrel of the press.
Some additional sub-processing equipment
Thermoset Plasticator – Is a simple device that works much like the injection part of an injection press. There is a bulk feed system that holds and feeds the plastic, and a screw that heats, compresses, and homogenizes the polymer. Once heated and homogenized, the screw reciprocates to extrude a length of material, a knife then cuts to separate. The cut extrudite is then carried or transferred to either a compression molding machine (CMM) or transfer molding machine (TMM).
Preformer – A preformer transforms thermoset compound into formed and sized pucks. The system is designed to use either bulk molding compound (BMC), loaf molding compound (LMC), or sheet molding compound (SMC). The material is fed into an orifice where it is compressed into a desired dimension using interchangeable tooling. The final part or “Puck” can either be dumped to bulk or directly transferred to the next process. Preforms are generally used in transfer molding & straight compression molding operations. They allow for greater control and accuracy of the material. Another advantage in using preforms is they tend to be easier to manage than bulk compound.
Stuffer – A stuffer is generally fitted to the barrel of an Injection Molding Machine (IMM). Since some thermoset compounds are not free flowing a stuffer is required as opposed to a gravity fed hopper. The stuffer feeds material at a constant rate directly into the screw of the IMM.
What are some common products made of thermoplastics?
Silicone
- Toys
- Baby pacifiers
- Toothbrush handles
- Baking trays
Epoxy
- Electronics
- Electrical components
- Insulators
- Hulls for watercraft
- Airplane parts
Polyurethane
- Seals and gaskets
- Bushings
- Instrument cases and bezels
- Structural components
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