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Injection molding of thermosetting plastics
- Aug 13, 2018 -

Thermosetting plastic injection molding uses a screw or plunger to pass the polymer through a heated barrel (120-260 degrees F) to reduce viscosity, which is then injected into a heated mold (300-450 degrees F). Once the material is filled with mold, it will keep pressure on it. Chemical crosslinking is then made to harden the polymer. Hard (i.e. solidified) products can be ejected from the mold while they are hot, and can not be molded or re-melted.

The injection molding equipment has a hydraulic drive clamping device for closing the mold and an injection device capable of conveying materials. Most thermosetting plastics are used in granular or flaky forms and can be fed into a screw injection device by a gravity hopper. When processing polyester monolithic molding (BMC), it is like a "bread ball", using a feed piston to press the material into the threaded groove.

The processed polymers used in this process are phenolic plastics, polyester monolithic moulding plastics, melamine, epoxy resin, urea-formaldehyde plastics, vinyl ester polymers and diallyl phthalate (DAP).

Most thermosetting plastics contain a large number of fillers (up to 70% by weight) to reduce costs or improve their low shrinkage properties and increase strength or special properties. Commonly used 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.

Process process

Thermoplastics and thermosets will reduce viscosity when heated. However, the viscosity of thermosetting plastics increases with time and temperature because of chemical crosslinking. The combined effect of these effects is that the viscosity assumes a U type curve with time and temperature. Filling the mold in the lowest viscosity region is the goal of thermosetting injection molding, where the pressure required to shape the material into the mold is the lowest. This also helps to minimize the damage to fibers in polymers.

The injection molding process uses a screw to make the material flow through the heated barrel, and the barrel is circulated in the jacket around the barrel with water or oil. Screw can be designed according to different types of materials, slightly pressurized to remove air and heat the material to obtain low viscosity. Most thermosetting materials flow well here.

The operation of making the material into the mold is to stop the screw rotation and push the screw forward at high speed by hydraulic pressure, so that the plasticized low viscosity material is pressed into the mold. This rapid flow requires filling the mold cavity in 0.5 seconds, with a pressure of 193MPa. Once the film cavity is filled, the high-speed flow of material produces a greater mot of heat to accelerate the chemical reaction. Once the cavity is filled, the injection pressure will drop to 34.5 - 68.9MPa.  This pressure is maintained on the material for 5-10 seconds, then unloaded, and then the next cycle of plasticization begins.

The material is kept in a hot mold until it hardens, then the clamping device is opened and the product is ejected. The product may be slightly unsolidified and a little softer when it is just ejected. The final solidification can be completed within 1 or 2 minutes after removal by using the heat retained in 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 have been adopted to improve the quality and reproducibility of products. 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 opens slightly to allow the gas to escape, and then closes immediately to refill the remaining material.

Injection molding provides higher strength, better dimensional control, and improved surface condition (appearance) because of the use of a mold with a telescopic cavity and core, the mold can be opened during the injection process 1/8-l/2 in, and then quickly compacted, as if the mold were closed.

An integral molded plastic made of glass fibers, fillers and polyester unsaturated resins can be molded by installing additional specialized equipment on the machine. A piston feeder is connected to the barrel to force the feeding, and can then be operated in two different ways. The utility model relates to a reciprocating screw which pushes the material forward, mixes and heats simultaneously. This requires a stop 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. Another 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% weight of glass fiber, because the damage to the fiber is small and the strength is high.

Comparing with the compression moulding and transfer moulding, the injection moulding has the following advantages and disadvantages:

The advantages of injection molding over compression molding are: faster molding cycle (2-3 times) process automation; less product changes; lower labor costs; high production capacity.

The disadvantages of injection molding method compared with compression molding method are: higher investment in equipment and mold; compression molding method can obtain higher product strength and better surface finish.

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

equipment

Important factors for selecting equipment for thermosetting plastics injection molding include: clamping device capability and injection molding capability; control system and barrel temperature.

The selection of the clamping device with closing pressure of tons shall be based on the projection molding area of the product and the flow channel. The required tonnage can range from 1.5 to 5 t/in 2, depending on the complexity of the molded product and the raw materials used. The device size is between 30~3 000t and most common devices are between 100 and 600t. The thickness of the steel plate and rigidity of the machine are of great importance. The bending deformation is produced as little as possible during the injection process, resulting in difficulty in removing the overflow material.

The injection capacity of the machine needs to be analyzed according to the maximum injection pressure required for filling the mold and the volume of material in the cavity and runner system. The required injection pressure ranges from 96.5 MPa required for polyester monolithic molding to 207 MPa required for some special phenolic plastics. The injection capacity of a machine is often marked by its theoretical volume (the area of a 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 the equipment can produce. When the equipment is marked by the polystyrene production capacity, the difference in density between the thermosetting plastic and the workpiece weight gauge must be taken into account in determining the production capacity.

At present, the popular control system is computer control, which can choose injection speed and load of clamping device. Procedures for process operation, movement of side die core into the die, working cycle of ejector and control of barrel and die temperature. The method of adjusting and recording the successive feeding of a particular mold and material is extremely valuable. Because there are lots of variables in the process.

The temperature of the barrel is controlled by hot water flowing through the jacket of the drum. Temperature control of dies is most commonly carried out by means of plug-in heaters, but steam or circulating hot oil can also be used.

Highly controllable mold temperature is the most important factor for obtaining uniform products.

Common equipment options include: feeders for monolithic moulds, rapid die replacement systems, hydraulic fluid storage tanks for rapid injection, side die cores for sliding moulds connected to hydraulic systems, mechanical hand-type parts removal systems, and air-jet devices (to remove spills from each molding cycle).

Due to the low viscosity of the polymer, it flows into the parting line to form a thin film, so thermosetting plastic products often need to be trimmed to remove the flying edges. Flying edge removal of molded products is often done by rolling the products or passing them through a device in which high-speed plastic particles knock out the brittle flying edge layer.

application

The main markets for thermosetting materials produced by injection molding include:

Automotive Industry: engine parts, headlight reflectors and manufactured products.

Electrical industry: circuit breaker, switch housing and coil rack.

Household appliances: bread oven board, coffee maker base, motor rectifier, motor housing and garbage disposal machine housing.

Others: electric tool housing, lamp housing, gas flowmeter and tableware.