Thermosetting plastic injection moulding uses a screw or plunger to pass the polymer through a heated drum (120 ~ 260°F) to reduce viscosity, and then into a heated mold (300 ~ 450°F). Once the material is filled to the injection mold, the pressure is maintained. Chemical crosslinking occurs, hardening the polymer. Hard (that is, solidified) products can be ejected from the mold while hot and cannot be reformed or remelted.

The injection molding equipment has a hydraulically driven mold closing device for closing the mold and an injection device for conveying materials. Most thermosetting plastics are used in granular or sheet form and can be fed into a screw injection device by a gravity hopper. When processing polyester mono-molded plastics (BMC), which act as "bread balls", a feed piston is used to press the material into the threaded grooves.

The processing of polymers using this process is (in order of the size of its dosage); Phenolic plastics, polyester monolithic molded plastics, melamine, epoxy resins, urea-formaldehyde plastics, vinyl ester polymers and diallyl phthalate (DAP).

Most thermosetting plastics 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 fiber, mineral fiber, clay, wood fiber, and carbon black. These fillers can be very abrasive and produce high viscosity which must be overcome by processing equipment.

The technological process

Both thermoplastics and thermosets will reduce viscosity when heated. However, the viscosity of thermosetting plastics increases with time and temperature due to chemical crosslinking reactions. The combined result of these actions is that the viscosity follows a U-shaped curve with time and temperature. This is the purpose of thermosetting injection molding, where the pressure required for injection molding of the material into the mold shape is at its lowest. This also helps to minimise damage to the fibres in the polymer.

The injection molding process uses a screw to pass the material through a heated cylinder, which is circulated in a jacket around the cylinder with water or oil. The screw can be designed for a different type of each material, compressed slightly to remove air and heat the material for low viscosity. Most thermosetting materials flow fairly well here.

The operation to make the material into the mold is to stop the screw rotation and push the screw forward at high speed with hydraulic pressure, so that the plasticized low viscosity material is pressed into the mold. This rapid flow requires the cavity to be filled in 0.5 seconds at a pressure of 193MPa. Once the membrane cavity is filled, the high speed flow of the material produces a greater heat of inspection to accelerate the chemical reaction.

Once the cavity is filled, the injection pressure is reduced to a holding pressure of 34.5 -- 68.9MPa. This holding pressure is maintained on the material for 5-10 seconds, after which the pressure is relieved and the next periodic plasticizing phase begins.

The material is held in a hot die until it hardens, and then the closing mechanism is opened to eject the product. The product can be slightly uncured and slightly soft when ejected, and the final cure can be completed within 1 or 2 minutes after removal using the heat retained inside the product. The entire production cycle for thermosetting products is 10-120 seconds, depending on the thickness of the product and the type of raw material.

Many different and specialized techniques are used to improve the quality and reproducibility of products. Since some thermosetting polymers produce gas when heated, there is often a bleed operation after the mold is partially filled. In this step, the mold is opened slightly to allow the gas to escape, and then quickly closed to re-fill the remaining material.

Injection moulding provides higher strength, better dimensional control, and improved surface condition (appearance) due to the use of a mold with a retractable membrane cavity and core. The mold can be opened 1/8 -- L / 2-in. during injection and then pressed quickly as if the mold were closed.

Monolithic molded plastics made from fiberglass, filler and polyester unsaturated resins can be injected with additional specialized equipment on the machine. A piston feeder is connected to the barrel to force the feed and can then be operated in two different ways. One has a conventional reciprocating screw that pushes the material forward while mixing and heating it. This requires a check valve at the end of the screw. Prevent the material from flowing back to the screw thread because the viscosity of the material is very low. The other way is to use the plunger or piston to press the material into the mold cavity. The plunger is often used for materials containing more than 22% of the weight of the glass fiber, because it does less damage to the fiber and can also get higher strength.

Compared with compression moulding and transfer moulding, another process method which was first used in thermosetting plastic molding, Injection moulding has the following advantages and disadvantages:

Injection molding method is superior to pressure molding method: faster molding cycle (2 ~ 3 times) process automation; Product change less; Lower labor costs; High productivity.

The disadvantages of injection molding method are as follows: higher equipment and mold investment; Pressure-plastic method can obtain higher product strength and better surface finish.

The advantages of die casting are generally between injection molding and compression molding.