Thermosetting plastic injection molding uses a screw or a plunger to pass the polymer through a heated barrel to reduce the viscosity, and then injects it into a heated mold. Once the material fills the mold, it is kept under pressure. At this point chemical crosslinks occur, making the polymer stiffer. A hard (i.e. solidified) part is ejected from the mold while hot, it cannot be reshaped or remelted.
Thermosetting plastic injection molding has a hydraulically driven mold clamping device and an injection device for conveying materials. Most thermosetting plastics are used in granular or sheet form, and can be fed into the screw injection device by a gravity hopper. When processing polyester BMC, it is like a "bread dough", using a feed piston to force the material into the thread groove. Injection molding of thermosetting plastics The polymers processed by this process are arranged according to their dosage; phenolic plastics, polyester integral molding compounds, melamine, epoxy resins, urea-formaldehyde plastics, vinyl ester polymers and phthalic dienes Propyl ester. Most thermosetting plastics contain a large amount of fillers to reduce costs or improve their low shrinkage properties, increase strength or special properties. Commonly used fillers include glass fibers, mineral fibers, clay, wood fibers and carbon black. These fillers can be very abrasive and generate high viscosities that must be overcome by processing equipment.
Both thermoplastics and thermosets decrease in viscosity when heated. However, the viscosity of thermosets increases with time and temperature due to chemical crosslinking reactions. The combined result of these effects is a U-shaped curve of viscosity with time and temperature. Filling the mold is done in the lowest viscosity region, which is the purpose of thermoset injection molding, because the pressure required to form the material into the mold shape is the lowest. This also helps to minimize damage to the fibers in the polymer.
The thermosetting plastic injection molding process uses a screw to flow the material through a heated barrel, and the barrel is circulated with water or oil in a jacket around the barrel. The screw can be designed according to different types of each material, slightly compressed to remove air and heat the material to obtain low viscosity. Most thermoset materials flow reasonably well here.
The operation of making the material enter the mold is to stop the rotation of the screw and push the screw forward at high speed with hydraulic pressure, so that the plasticized low-viscosity material is pressed into the mold. This fast flow requires filling the mold cavity in 0.5 seconds, and the pressure needs to reach 193MPa. Once the membrane cavity is filled, the high-speed flow of the material generates greater frictional heat to accelerate the chemical reaction. Once the mold cavity is filled, the injection pressure will drop to the holding pressure of 34.5~68.9MPa. This holding pressure is maintained on the material for 5-10 seconds, then the pressure is released, and then the next cycle of plasticizing begins.
This material is kept in the hot mold until hardened, and then the clamping device is opened to eject the product. The thermosetting plastic injection molded product can be slightly uncured and a little soft when it is just ejected, and the final curing is completed within 1 to 2 minutes after being taken out by using the heat retained inside the product. The entire production cycle of thermosetting products is 10 to 120 seconds, depending on the product thickness and the type of raw material. Many different and specialized techniques have been employed to improve the quality and reproducibility of the articles. Since some thermoset polymers generate gas when heated, there is often a venting operation after the mold is partially filled. During this step, the mold is opened slightly to allow the gas to escape, and then snapped shut.
BMCs made of glass fibers, fillers and polyester unsaturated resins can be injection molded on machines equipped with additional specialized equipment. A piston feeder is attached to the barrel for forced feeding, which can then be operated in two different ways. One with a traditional reciprocating screw that pushes the material forward while mixing and heating. This requires a check valve at the end of the screw. Prevent material from flowing back onto the screw flight, because the viscosity of the material is very low. Another way is to use a plunger or piston to press the material into the mold cavity. The plunger is often used for materials containing more than 22% glass fiber by weight, because it does less damage to the fiber and can also obtain higher strength.