PMMA is commonly known as plexiglass, acrylic and so on. The chemical name is polymethyl methacrylate. Due to the shortcomings of PMMA surface hardness, easy scratching, low impact resistance, poor molding fluidity, etc., 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 copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 plexiglass, it can become 373 plexiglass.
The flow of PMMA is worse than that of PS and ABS, and the melt viscosity is more sensitive to temperature changes. During the plastic inject molding, 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. Plastic machining
PMMA has a certain degree of water absorption, with a water absorption rate of 0.3-0.4%, and injection molding must be at a temperature below 0.1%, usually 0.04%. The presence of water causes bubbles, gas lines, and reduced transparency in the melt. So it needs to be dried. The drying temperature is 80-90℃, and the time is more than 3 hours. Recycled materials can be used 100% in some cases. The actual amount depends on the quality requirements, usually over 30%. Recycled materials 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 deeper screw groove and a larger diameter nozzle hole are required. If the strength of the product is higher, a screw with a larger length to diameter 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 temperature can be 60℃-80℃. The diameter of the main runner should match the inner taper. The best angle is 5-7. If you want to inject products with 4mm or more, the angle should be 7, and the diameter of the runner should reach 8-10mm. The overall length of the gate should not exceed 50mm.
For uhmw machining with a wall thickness of less than 4mm, the runner diameter should be 6-8m, for products with a wall thickness of more than 4mm, the runner diameter should be 8-12mm, and the gate depth of diagonal, fan and tab should be 0.7 to 0.9t (t is the thickness of the product wall), the diameter of the needle gate should be 0.8 to 2mm; the smaller size should be used for low viscosity. Common vent holes are 0.05 to 0.07mm deep and 6mm wide. The demolding slope is between 30′-1 and 35′-130 in the cavity.
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
Fast injection can be used, but to avoid high internal stress, it is advisable to use multi-stage injection, such as slow-fast-slow, etc. When injecting thick parts, use slow speed.
6. Detention time
If the temperature is 260°C, the residence time cannot exceed 10 minutes at most, and if the temperature is 270°C, the residence time cannot exceed 8 minutes.
