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Processing Of Thermosetting Plastics For Injection Molding

2008/8/9 10:10:00 41735

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Thermosetting plastic injection uses a screw or a plunger to reduce the viscosity of the polymer through a heated cylinder (120~260 degree F), and then injects 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 (solidified) products can be ejection from the mold when heated, and it can no longer be formed or remelted.

The injection molding equipment has a hydraulic driving clamping device with a closed mold, and an injection device capable of conveying materials.

Most thermosetting plastics are used in granular or flake, and can be fed into the screw injector by gravity hopper.

When processing polyester molding compounds (BMC), it is like "bread bunk", and a material piston is used to press materials into threaded grooves.

The polymers processed by this process are (according to their size), phenolic plastics, polyester molding compounds, melamine, epoxy resin, urea formaldehyde plastics, vinyl ester polymers and diphenyl two diphenacrylate (DAP).

Most thermosetting plastics contain a large amount of filler (up to 70% wt%) to reduce cost or improve its low shrinkage performance, increase strength or special properties.

Commonly used fillers include glass fiber, mineral fiber, clay, wood fiber and carbon black.

These fillers may be very abrasive and produce high viscosity, which must be overcome by processing equipment.

Technological process

Thermoplastics and thermosets will reduce viscosity when heated.

However, the viscosity of thermosetting plastics increases with time and temperature because of chemical crosslinking reaction.

The combined effect of these effects is that the viscosity assumes a U type curve with time and temperature.

It is the purpose of thermosetting injection molding to complete the filling mold operation in the lowest viscosity area, because the pressure required for molding the material into the mold shape is the lowest at this time.

This also helps to minimize the damage to fibers in polymers.

In the injection molding process, a screw is used to make the material flow through the heated cylinder, and the cylinder is circulated in the jacket around the cylinder by water or oil.

The screw can be designed according to the different types of materials, slightly compressed to remove air and heat the material to obtain low viscosity.

Most thermosetting materials flow well here.

The operation of 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 can be pressed into the mold.

This rapid flow requires the filling of the cavity in 0.5 seconds. The pressure needs to be 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.

The holding pressure is maintained on the material for 5 to 10 seconds, followed by pressure relief, and then begins the next cycle of plasticization.

The material is kept in the hot mold until it hardens, then the mold closing device is opened and the product is ejecting.

The product can be lightly solidified and soft when it is just ejecting. It will finish the final curing within 1 minutes or 2 minutes after the product is removed.

The whole production cycle of thermosetting products is 10 to 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 during heating, there is usually an outgassing operation after the mold is partially filled.

In this step, the die is slightly opened to allow the gas to escape and then shut down immediately to recharge the remaining material.

The injection molding provides higher strength, better size control, and improves the surface condition (appearance). This is because the mold with a telescopic membrane cavity and membrane core is obtained. The mold can open 1 / 8 - L / 2 in in the injection process, and then rapidly compress as if the mold were closed.

Molded plastic made of glass fiber, filler and polyester unsaturated resin can be installed on the machine with additional specialized equipment to complete the injection molding.

A piston feeder is connected to the barrel to force the feed, and then it can be operated in two different ways.

A traditional reciprocating screw is used to push the material forward and mix and heat.

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 press the material into the mold cavity with plungers or pistons. The plunger is often used for materials containing more than 22% of the weight of the glass fiber, because the damage to the fiber is small and high strength can be obtained.