PMMA is commonly known as plexiglass, acrylic and so on. The chemical name is polymethyl methacrylate. Due to the disadvantages of low surface hardness, easy rubbing, low impact resistance, and poor molding flow properties of PMMA plastic raw materials, the modification of PMMA has appeared one after another. Such as the copolymerization of methyl methacrylate with styrene and butadiene, the blending of PMMA and PC, etc. 372 plexiglass is obtained by the copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 plexiglass, 373 plexiglass can be obtained.

The flow of PMMA is worse than that of PS and ABS, and the melt viscosity is more sensitive to changes in temperature. During the molding process, the melt viscosity is mainly changed from the injection temperature. PMMA is an amorphous polymer with a melting temperature greater than 160°C and a decomposition temperature of 270°C.

1. Treatment of plastics

PMMA has a certain water absorption, and its water absorption rate is 0.3-0.4%, and the injection molding temperature must be below 0.1%, usually 0.04%. The presence of water makes the melt appear bubbles, gas streaks, and reduce transparency. So it needs to be dried. The drying temperature is 80-90 ℃, and the time is more than 3 hours.

In some cases, 100% of the recycled material can be used. The actual amount depends on the quality requirements, usually it can exceed 30%. The recycled material should avoid pollution, otherwise it will affect the transparency and the properties of the finished product.

2. Selection of injection molding machine

PMMA has no special requirements for injection molding machines. Because of its high melt viscosity, a deep screw groove and a larger diameter nozzle hole are required. If the strength of the product is required to be high, a screw with a larger aspect ratio should be used for low-temperature plasticization. In addition, PMMA must be stored in a dry hopper.

3. Mold and gate design

The mold-ken temperature can be 60℃-80℃. The diameter of the sprue should match the inner taper. The best angle is 5° to 7°. If you want to inject 4mm or more products, the angle should be 7°, and the diameter of the sprue should be 8°. To 10mm, the overall length of the gate should not exceed 50mm.

For products with wall thickness less than 4mm, the diameter of the runner should be 6-8mm

For products with wall thickness greater than 4mm, the diameter of the runner should be 8-12mm

The depth of the diagonal, fan-shaped and vertical-shaped gates should be 0.7 to 0.9t (t is the wall thickness of the product), and the diameter of the needle gate should be 0.8 to 2mm; for low viscosity, a smaller size should be used.

Common vent holes are 0.05 to 0.07mm deep and 6mm wide

The demolding slope is between 30′-1° and 35′-1°30° in the cavity part.

4. Melting temperature

It can be measured by air injection method: ranging from 210°C to 270°C, depending on the information provided by the supplier.

5. Injection temperature

Rapid injection can be used, but to avoid high internal stress, multi-stage injection should be used, such as slow-fast-slow, etc. When injecting thick parts, use slow speed.

6. Residence time

If the temperature is 260°C, the residence time should not exceed 10 minutes at most, and if the temperature is 270°C, the residence time should not exceed 8 minutes.