Thermosetting plastic injection molding uses a screw or a plunger to pass the polymer through a heated barrel (120-260 °F) to reduce viscosity and then inject into a heated mold (300-450 °F). 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.
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% of glass fiber weight, because this damage to the fiber is small, and higher strength can be obtained. .
Another process that was first used in the formation of thermoset plastics is compression moulding and transfer moulding. Compared to them, the advantages and disadvantages of injection moulding (Injection Moulding) are as follows:
The injection molding method is superior to the compression molding method in that it has a faster molding cycle (2 to 3 times) process automation; less product changes; lower labor costs; and higher production capacity.
The disadvantages of the injection molding method over the compression molding method are: higher equipment and mold investment; the compression molding method can obtain higher product strength and better surface finish.
The advantages of die casting are generally between injection molding and compression molding.
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/in2, 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.
The main markets for thermoset materials produced by injection molding include:
Automotive industry: engine components, headlight reflectors and brakes.
Electrical industry: circuit breakers, switch housings and coil formers.
Household appliances: bread oven board, coffee maker base, motor commutator, motor housing and waste handler housing.
Other: power tool housing, lamp housing, gas flow meter and tableware.
Commercial technical information
US phenolic plastic injection molding production has made significant progress in the past five years, with approximately 55% of molded phenolic plastic products being produced by injection molding. Polyester monolithic molding compounds account for about 35% of their applications by injection molding.
In order to obtain high quality uniform products, it is currently necessary to increase the accuracy of equipment control and machine operation. The price of the machine is 500-1000 US dollars per ton according to the clamping device capacity, depending on the size and ancillary equipment. For example, a 200-ton clamping unit with an injection capacity of 30 in3 is priced at approximately $190,000.