The most popular and important material utilized in compression molding is thermoset composites. Thermosets consist of alkyd resins, melamine, polyester, polyurethane, Bakelite, and urea-formaldehyde. The selection of thermoset materials depends on several factors, including application, cost, and characteristics. For instance, epoxy resin meets the design specifications of high-strength applications, vinyl ester resin is used for corrosion resistance, and polyester resin is typically chosen based on both performance and cost considerations.

Types thermoset used

Common thermosets used in the compression molding process include epoxy, phenolic and unsaturated polyester resins, unsaturated polyester, vinyl ester, polyurethane, and anaerobic adhesives.

• Sheet molding compound

Often utilized for bigger pieces requiring higher mechanical strength, sheet molding compound (SMC) is a fiber-reinforced thermoset material composed of polymer resin, inert fillers, fibers, catalysts, colors, stabilizers, release agents, and thickeners. The percentage of glass reinforcement ranges from 10 to 60%, and the fiber length is marginally longer than that of bulk molding compound (BMC), measuring between ½ and 1 inch.

To prevent auto-adhesion, the material is encased on both the top and bottom with a polyethylene or nylon plastic film during the continuous in-line manufacturing process used to create SMCs. The top film is added to the process, rolled to a predetermined thickness, and allowed to develop for 48 hours. The paste is then evenly spread onto the bottom film, and the chopped fibers are added to the paste in a random pattern.

SMC is perfect for structural and painting applications that demand strength and stiffness but not an appearance-critical aesthetic. It also fulfills the needs of applications that generate appearance-critical parts with a Class A finish of custom plastic parts.

• Bulk molding compound

Thermoset plastic resin, known as bulk molding compound (BMC), is comparable to SMC but is made differently when the fibers and resin are mixed. Though they combine to create a viscous putty, the ingredients are still inert fillers, fiber reinforcement, catalysts, stabilizers, and pigments. Short glass fibers with lengths ranging from 1/32 inch to ½ inch and a glass reinforcement measure of 10 to 30 percent are used to greatly reinforce the material.

BMCs are perfect for applications requiring precision in detail, dimension, and performance because they will achieve close dimensional control, flame, and track resistance, electrical insulation, corrosion, and stain resistance, as well as enhanced mechanical properties, minimized shrink capacity, and color stability. It is also capable of withstanding water-based paint and powder coating. 

• Composite substances

The compression molding method also makes use of thermosets in composites, such as laminated plastics. A unique type of polymer-matrix composite known as laminated plastics is made up of layers of fiber-reinforced materials that are soaked with thermosetting resins (usually phenolics) and then fused under pressure and heat. The material is cut into sheets, stacked, and compressed using high pressure and predetermined temperatures (270° to 350°F) after it has been impregnated and allowed to dry. 

High mechanical strength, dimensional stability, stiffness, a high strength-to-weight ratio, and good resistance to moisture, heat, and electrical current are all features of laminated thermosets. It is important that fibers be completely wetted with resin throughout the creation of these multilayer composites, and that any extra resin be extracted for perfect custom plastic parts.

To prevent porosity in the material, the laminate must be consolidated to thickness and any trapped air, moisture, and solvent evacuated. To make sure the material is neither over nor under-cured, the curing period is crucial. Laminated composite materials find their use in switchboard panels and terminal boards, gaskets, washers, gears, gas/generator/television insulation, and other products requiring fast feeding and cutting rates.