Thermosetting  plastic injection uses a screw or a plunger to pass the polymer through  a heated barrel (120-260F) to reduce viscosity and then inject into a  heated mold (300-450F). Once the material is filled with the mold, it is held in place. Chemical cross-linking occurs at this point to harden the polymer. The hard (i.e., cured) article can be ejected from the mold while it is hot, and it cannot be reshaped or remelted.
The  injection molding apparatus has a hydraulically driven clamping device  for closing the mold and an injection device for conveying the material.  Most thermoset plastics are used in particulate or flake form and can be fed into a screw injection unit by a gravity hopper. When  processing a polyester monolithic molding compound (BMC), it is like a  "bread", which uses a feed piston to press the material into the thread  groove. Processed  polymers using this process are (depending on the amount); phenolic  plastics, polyester monoliths, melamine, epoxy, urea-formaldehyde, vinyl  ester polymers and diallyl phthalate (DAP). Most  thermoset plastics contain large amounts of filler (up to 70% by  weight) to reduce cost or increase their low shrinkage properties,  increasing 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 viscosities that must be overcome by processing equipment.
crafting process
Thermoplastics and thermosets will reduce viscosity when heated. However,  the viscosity of thermosetting plastics increases with time and  temperature because of the occurrence of chemical crosslinking  reactions. The combined result of these effects is that the viscosity exhibits a U-shaped curve with time and temperature. The  operation of filling the mold in the lowest viscosity region is the  purpose of thermosetting injection molding because the pressure required  to form the material into the mold shape is the lowest at this time. This also helps to minimize fiber damage in the polymer. The  injection molding process utilizes a screw to cause the material to  flow through the heated barrel, which is circulated as water or oil in a  jacket around the barrel. The  screw can be designed for each type of material, with a slight  compression to remove air and heat the material to achieve a low  viscosity. Most thermoset materials flow fairly well here. The  operation of entering the material into the mold is to stop the  rotation of the screw and hydraulically push the screw forward at a high  speed to press the plasticized low-viscosity material into the mold. This rapid flow requires filling the cavity in 0.5 seconds with a pressure of 193 MPa. The  high velocity flow of the material as soon as the membrane cavity is  filled creates a greater heat of friction to accelerate the chemical  reaction. Once the cavity is filled, the injection pressure will drop to a holding pressure of 34.5-68.9 MPa. This  holding pressure is maintained on the material for 5-10 seconds, then  the pressure is released, and then the plasticizing stage of the next  cycle begins. This material is held in a hot mold until it hardens, and then the mold clamping device is opened to eject the product. The  product may be slightly uncured and somewhat soft when it is ejected,  and the final curing is accomplished using the heat retained within the  article within 1 minute or 2 minutes after removal. The  entire production cycle of a thermoset article is 10 to 120 seconds,  depending on the thickness of the article and the type of raw material.
Many different and specialized techniques have been employed to improve the quality and reproducibility of the product. In  view of the fact that some thermoset polymers generate gas upon  heating, 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 shut down, and the remaining material is reinjected. Injection  molding provides higher strength, better dimensional control, and  improved surface condition (appearance) due to the use of a mold with a  telescopic membrane cavity and a membrane core. It can be turned on 1/8 - l / 2 in, and then quickly pressed, like the mold is closed. The  integral molding compound made of glass fiber, filler and polyester  unsaturated resin can be equipped with additional special equipment on  the machine to complete injection molding. A piston feeder is attached to the barrel to force the feed, which can then be operated in two different ways. A conventional reciprocating screw that pushes material forward while mixing and heating. This requires a stop valve at the end of the screw. Prevent material from flowing back to the screw threads because the viscosity of the material is very low. Another  way is to use a plunger or a piston to press the material into the mold  cavity. The plunger is often used for materials containing more than  22% by weight of the glass fiber, because the damage to the fiber is  small, and higher strength can be obtained. .
Device Selection
Important  factors in selecting equipment for injection molding of thermoset  plastics include: clamping device capability and injection molding  capability; control system and barrel temperature. Closing  pressure in tons of the clamping device, the choice should be based on  the determination of the product and the flow path to project the  molding area. The required tonnage can be from 1.5 to 5 t/in 2 depending on the complexity of the molded article and the raw materials used. The size of the equipment is between 30 and 3,000 tons, and most common equipment is between 100 and 600 tons. The thickness of the steel plate and the rigidity of the machine are of the utmost importance. The bending deformation is caused as little as possible during the injection, which makes the removal of the flash difficult.
The  injection capacity of the machine needs to be analyzed according to the  maximum injection pressure required to fill the mold and the volume of  the material in the cavity and runner system. The  required injection pressure is from 96.5 MPa required for polyester  monolithic moldings to 207 MPa required for some specialty phenolic  plastics. The  injection capacity of a machine is often marked by a theoretical volume  (the area of the screw or piston injection multiplied by its stroke).  In general, the  capacity of the equipment is determined by 85% of the volume of the  product that can be produced by the equipment. When  the equipment is marked by polystyrene production capacity, the  difference in density between the thermoset and the thermoset must be  taken into account when determining the production capacity of the part  weight.
The  current popular control system is computer controlled, and the  injection speed and the load of the clamping device can be selected. Process  procedures, side cores to movement into the mold, duty cycle of the  ejector and control of barrel and mold temperatures. The  method of setting and recording the sequential feeding of a specific  mold and a specific raw material is extremely valuable. Because there are a lot of variables in the process. The barrel temperature is controlled by hot water flowing through the jacket of the coated barrel. Mold temperature control is most commonly done with plug-in heaters, but it can also be done with steam or circulating hot oil. Highly controllable mold temperatures are the most important for obtaining a uniform product. Common  equipment options include: feeder for integral molding compound, quick  change mold system, hydraulic fluid storage cylinder for rapid  injection, side core for mold sliding, hydraulic system, robotic pick-up  system And an air jet device (removing the flash generated during each molding cycle). Due  to the low viscosity of the polymer, it flows into the parting line to  form a thin film. Therefore, the finished thermosetting plastic often  needs to be trimmed to remove the flash. The  flash removal of molded articles is often accomplished by rolling the  articles or passing them through a device in which high speed plastic  particles knock out the brittle flash layer.