Processing of Injection Molded Thermosetting Plastics
Thermosetting plastic injection molding uses a screw or a plunger to push the polymer through a heated barrel (120-260°F) to reduce viscosity and then inject it into a heated mold (300-450°F). Once the material fills the mold, it is pressurized. At this time, chemical crosslinking occurs, causing the polymer to harden. The hard (i.e., cured) product can be ejected from the mold while hot, and it cannot be remolded or remelted.
Injection molding equipment has a hydraulically driven clamping device for closing the mold and an injection device that can convey the material. Most thermosetting plastics are used in granular or flake form, which can be fed into the screw injection device by a gravity hopper. When processing polyester bulk molding compound (BMC), it is like a "bread dough" and a feed piston is used to press the material into the thread groove.
Polymers processed by this process are (in order of amount); phenolics, polyester BMCs, melamine, epoxies, urea-formaldehyde plastics, vinyl ester polymers and diallyl phthalate (DAP).
Most thermosets contain large amounts of fillers (up to 70% by weight) to reduce cost or improve low shrinkage, strength or special properties. Common fillers include glass fibers, mineral fibers, clays, wood fibers and carbon black. These fillers can be very abrasive and produce high viscosities that must be overcome by the processing equipment.
Processing
Both thermoplastics and thermosets will decrease in viscosity when heated. However, the viscosity of thermosets increases with time and temperature because of chemical crosslinking reactions. The combined result of these effects is a U-shaped curve of viscosity over time and temperature. It is the goal of thermoset injection molding to complete the mold filling operation in the lowest viscosity area 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 injection molding process uses a screw to move the material through a heated barrel, which circulates water or oil in a jacket around the barrel. The screw can be designed for each type of material and slightly compressed to remove air and heat the material to achieve low viscosity. Most thermosetting materials flow quite well here.
The operation of getting the material into the mold is to stop the screw rotation and push the screw forward at high speed with hydraulic pressure to force the plasticized low-viscosity material into the mold. This rapid flow requires filling the mold cavity in 0.5 seconds and the pressure must reach 193MPa. Once the mold 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 a 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 the next cycle plasticization phase begins.
The material is kept in the hot mold until it hardens, and then the clamping device is opened to eject the product. The product may be slightly uncured and a little soft when it is just ejected. The final curing is completed within 1 or 2 minutes after removal using the heat retained inside the product. The entire production cycle of thermosetting products is 10-120 seconds, depending on the thickness of the product and the type of raw materials.
Many different and specialized technologies are used to improve the quality and reproducibility of the product. In view of the fact that some thermosetting polymers produce gas when heated, there is often a deflation operation after the mold is partially filled. In this step, the mold is slightly opened to allow the gas to escape, and then quickly closed to inject the remaining material.
Injection molding provides higher strength, better dimensional control, and improved surface condition (appearance), which is obtained by using a mold with a telescopic membrane cavity and membrane core. The mold can be opened 1/8-1/2 in during the injection process and then quickly compressed, just like the mold is closed.
Integral molding compounds made of glass fiber, filler and polyester unsaturated resin can be equipped with additional specialized equipment on the machine to complete injection molding. A piston feeder is connected to the barrel to force feed, which can then be operated in two different ways. One has a traditional 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 onto the screw threads 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 is less damaging to the fiber and can also obtain higher strength.
Another process method that was first used for thermosetting plastic molding is compression molding and transfer molding. Compared with them, the advantages and disadvantages of injection molding are as follows:
The advantages of injection molding over compression molding are: faster molding cycle (2 to 3 times) process automation; fewer product changes; lower labor costs; high production capacity.
The disadvantages of injection molding compared to compression molding are: higher equipment and mold investment; compression molding can obtain higher product strength and better surface finish.
The advantages of die casting are generally between injection molding and compression molding.
Equipment
Important factors in selecting equipment for thermosetting plastic injection molding include: mold clamping device capacity and injection molding capacity; control system and barrel temperature.
The selection of the mold clamping device with closing pressure in tons should be based on the determined projected molding area of the product and the flow channel. The required tonnage can be 1.5 to 5t/in2, depending on the complexity of the molded product and the raw materials used. The size of the equipment is between 30 and 3,000t, and most common equipment is between 100 and 600t. The thickness of the steel plate and the rigidity of the machine are very important. Minimize bending deformation during injection, which makes it difficult to remove the overflow.
The injection capacity of the machine needs to be analyzed based on the maximum injection pressure required to fill the mold and the volume of the material in the cavity and flow channel system. The required injection pressure ranges from 96.5 MPa for polyester integral molding compounds to 207 MPa for some specialty phenolic plastics. The injection capacity of the machine is often indicated by theoretical volume (the area of the screw or piston injected multiplied by its stroke).
In general, the capacity of the equipment is determined by 85% of the volume of the product that the equipment can produce. When the equipment is indicated by polystyrene production capacity, the difference in density between it and thermosetting plastics must be taken into account when determining the production capacity by part weight.
The current popular control system is computer control, which can select injection speed, clamping device load. The process operation program, the movement of the side mold core to the mold, the working cycle of the ejector device and the control of barrel and mold temperature. The method of setting and recording the feeding of a specific mold and specific raw materials in sequence is extremely valuable. Because there are a large number of variables in the process.
The barrel temperature is controlled by hot water flowing through the jacket covering the barrel. The mold temperature control is most commonly carried out by plug-in heaters, but it can also be carried out by steam or circulating hot oil.
Highly controllable mold temperature is the most important to obtain uniform products.
Common equipment options include feeders for integral molding compounds, quick-change mold systems, hydraulic fluid reservoirs for rapid injection, side cores connected to the hydraulic system for mold sliding, robotic pick-up systems, and air jets to remove flash generated during each molding cycle.
Due to the low viscosity of the polymer, it flows into a thin film on the parting line, so the finished thermoset plastics often need to be trimmed to remove flash. The flash removal of molded parts is often done by rolling the parts or passing them through a device where high-speed plastic pellets knock off the brittle flash layer.
Applications
Major markets for thermoset materials produced by injection molding include:
Automotive industry: engine parts, headlight reflectors and brake parts.
Electrical industry: circuit breakers, switch housings and coil formers.
Household appliances: bread oven plates, coffee maker bases, motor commutators, motor housings and garbage disposal housings.
Others: power tool housings, lamp housings, gas flow meters and tableware.
