PMMA is often referred to as plexiglass, acrylic, etc.
Its chemical name is polymethyl methacrylate. Due to the disadvantages of PMMA such as high surface hardness, easy wiping, low impact resistance and poor molding fluidity, PMMA modifications have appeared one after another. For example, the copolymerization of methyl methacrylate with styrene and butadiene, the mixture of PMMA and PC, etc. 372 plexiglass is obtained from the copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 Plexiglas, 373 Plexiglas can be made. PMMA has poorer flow than PS and ABS. Melt viscosity is sensitive to temperature changes. During the molding process, the change of melt viscosity mainly comes 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 processing
PMMA has certain water absorption, which is 0.3-0.4%, and must be lower than 0.1% when injection molding, usually 0.04%. The presence of water will lead to bubbles, gas rays and reduced transparency in the melt. That is why drying is necessary. The drying temperature is 80-90°C and the time is 3 hours or more.
Recycled material can be used in some cases at 100%. The actual amount depends on the quality requirements and usually exceeds 30%. The recycled material should avoid pollution, otherwise it will affect the transparency and nature of the finished product.
2、The choice of
Injection Molding Machine PMMA has no special requirements for injection molding machine. Due to its high melt viscosity, it needs deeper grooves and larger diameter nozzle holes. If the strength of the product is required to be high, a screw with a large aspect ratio should be used for low-temperature plasticization. In addition, PMMA must use a dry hopper to store the material.
3、Mold and sprue design
The mold temperature can be 60℃-80℃, the diameter of the main channel should match the inner taper, the best angle is 5~7, if you want to inject products with more than 4mm, the angle should be 7, the diameter of the main channel is 8~10mm, the total length of the gate should not exceed 50mm.
For products with wall thickness less than 4mm, the runner diameter should be 6-8mm
For products with wall thickness greater than 4mm, the runner diameter should be 8-12mm
The depth of edge, fan and sheet gates should be 0.7 to 0.9t (t is the wall thickness of the product) and the diameter of needle gates should be 0.8 to 2mm; the lower the viscosity, the smaller the diameter.
The common vent depth is 0.05 to 0.07 mm and width is 6 mm
The draft angle is between 30'-1 cavity in the part of 35'-130.
4、The temperature of the melt
Can be measured by air injection: 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, multi-stage injection should be used, such as slow-fast-slow, etc. When spraying thick parts, slow speed is used.
6、Dwell time
If the temperature is 260℃, the dwell time should not exceed 10 minutes. If the temperature is 270℃, the dwell time should not exceed 8 minutes.
