Thermoset injection molding utilizes a screw or a plunger to pass polymer through a heated barrel (120-260℃) to reduce viscosity and then inject it into a heated mold (300-450℃). Once material fills mold, it is pressurized. At this point, chemical crosslinking occurs, which hardens polymer. A hard (ie cured) product is ejected from mold while it is still hot, it cannot be reshaped or remelted.
Injection molding equipment has a hydraulically driven clamping device for closing mold and an injection device capable of conveying materials. Most thermosetting plastics are used in granular or flake form, which can be fed into a screw injection unit from a gravity hopper. When processing polyester BMC, it acts like a "bread dough" with a feed piston forcing material into threaded groove.
Processed polymers using this process are (in order of amount); phenolic, polyester BMC, melamine, epoxy, urea-formaldehyde, vinyl ester polymers, and diallyl phthalate (DAP).
Most thermosets contain large amounts of fillers (up to 70% by weight) to reduce cost or improve their low shrinkage properties, increase strength or special properties. Common fillers include glass fibers, mineral fibers, clay, wood fibers and carbon black. These fillers can be very abrasive and produce high viscosity which must be overcome by processing equipment.

 

Both thermoplastics and thermosets will reduce viscosity when heated. However, viscosity of thermosets increases with time and temperature due to chemical cross-linking reactions. Combined result of these effects is a U-shaped curve of viscosity with time and temperature. Filling mold is done in the lowest viscosity region. This is purpose of thermoset injection molding because pressure required to shape material into shape of mold is minimal. This also helps to minimize fiber damage in the polymer.
Injection molding process uses a screw to move material through a heated barrel, which circulates water or oil in a jacket around barrel. Screw can be designed for different types of each material, slightly compressed to remove air and heat material to obtain low viscosity. Most thermosets flow quite well here.
Operation of making material enter mold is to stop rotation of screw and push screw forward at a high speed with hydraulic pressure, so that plasticized low-viscosity material is pressed into mold. This fast flow requires filling cavity in 0.5 seconds, and pressure needs to reach 193MPa. Once film cavity is filled, high-speed flow of material generates greater frictional heat to accelerate chemical reaction. Once cavity is filled, injection pressure will drop to holding pressure of 34.5-68.9MPa. This holding pressure is maintained on material for 5-10 seconds, then pressure is released, and next cycle of plasticizing stage begins.
This material is held in hot mold until it hardens, then mold clamping device is opened to eject product. Article may be lightly uncured and somewhat soft when first ejected, with final curing being accomplished within 1 or 2 minutes of removal using heat retained inside article. The entire production cycle of thermosetting products is 10-120 seconds, depending on thickness of product and type of raw materials.
Many different and specialized techniques are used to improve quality and reproducibility of product. Given that some thermoset polymers generate gas when heated, there is often a degassing operation after mold is partially filled. During this step, mold is opened slightly to allow gas to escape, then closed quickly to reinject remaining material.
Injection molding provides higher strength, better dimensional control, and improved surface condition (appearance) due to use of molds with retractable cavities and cores. During injection process, molds It can be opened 1/8-l/2 in, then quickly compacted, as if mold is closed.
BMCs made of glass fibers, fillers and polyester unsaturated resins can be injection molded with additional specialized equipment on the machine. A piston feeder is attached to barrel for forced feeding, which can then be operated in two different ways. One with a traditional reciprocating screw that pushes material forward while mixing and heating. This requires a stop valve at the end of screw. Prevent material from flowing back onto screw flight because viscosity of material is very low. Another way is to use a plunger or piston to press material into mold cavity. Plunger is often used for materials containing more than 22% glass fiber by weight, because it does less damage to fiber and can also obtain higher strength.
Another process method that was first used in thermosetting plastic molding is compression moulding and transfer moulding. Compared with them, advantages and disadvantages of injection moulding are as follows:
Advantages of injection molding over compression molding are: faster molding cycle (2 to 3 times) process automation; less product changes; lower labor costs; high production capacity.
Disadvantages of injection molding compared to compression molding are: higher investment in equipment and molds; compression molding can obtain higher product strength and better surface finish.
Advantages of die casting are generally between injection molding and compression molding.

 

Important factors in selecting equipment for thermoset injection molding include: clamping device capability and injection molding capability; control system and barrel temperature.
Selection of clamping device with closing pressure in tons should be based on projected molding area of product and runner. Required tonnage can vary from 1.5 to 5t/in2, depending on complexity of molded product and raw materials used. Size of equipment is between 30 and 3 000t, most common equipment is between 100 and 600t. Thickness of steel plate and rigidity of machine are of the utmost importance. Make bending deformation as little as possible during injection, which makes it difficult to remove flash.
Injection capacity of machine needs to be analyzed according to maximum injection pressure required to fill mold and material volume in cavity and runner system. Required injection pressure ranges from 96.5 MPa for polyester BMCs up to 207 MPa for some specialty phenolics. Injection capacity of a machine is often indicated by theoretical volume (area of screw or piston injected multiplied by its stroke).
In general, capacity of equipment is determined by 85% of product volume that equipment can produce. When equipment is marked with polystyrene capacity, difference in density from thermoset plastics must be taken into account when determining part weight capacity.
Current popular control system is computer control, which can select injection speed, load of clamping device, program of process operation, movement of side core to moving into mold, working cycle of ejector, control of barrel and mold temperature. Method of setting and recording sequential feeding of a specific mold and specific materials is extremely valuable. Because there are a lot of variables in process.
Barrel temperature is controlled by hot water flowing through jacket covering barrel. Mold temperature control is most commonly done with plug-in heaters, but steam or circulating hot oil can also be used.

Common equipment options include: feeders for bulk molding compound, quick-change mold systems, hydraulic fluid accumulators for rapid injection, side cores attached to hydraulic systems for mold sliding, robotic pick-up systems and air jets (to remove flash produced during each molding cycle).
Due to low viscosity of polymer, it flows into parting line to form a thin film, so finished thermoset plastics are often trimmed to remove flash. Flash removal of molded articles is often accomplished by tumbling articles or passing them through a device in which high-velocity pellets knock off brittle layer of flash.

Main markets for thermosets produced by injection molding include:
Automotive industry: engine parts, headlight reflectors and brake products.
Electrical industry: circuit breakers, switch housings and coil formers.
Household appliances: bread oven boards, bases for coffee brewers, motor commutators, motor housings and garbage disposal housings.
Others: power tool housings, lamp housings, gas flow meters and tableware.