Polymethyl methacrylate, a polymer obtained by polymerizing acrylic acid and its esters, is collectively referred to as an acrylic resin, and the corresponding plastics are collectively referred to as polyacrylic plastics, of which polymethyl methacrylate is the most widely used. Polymethyl methacrylate is abbreviated as
PMMA , commonly known as plexiglass. It is the most excellent synthetic material and the price is suitable.
Application:
PMMA is soluble in organic solvents such as phenol, anisole, etc., and can form a good film by spin coating. It has good dielectric properties and can be used as a dielectric layer of an organic field effect transistor (OFET) or organic thin film transistor (OTFT). .
PMMA resin is a non-toxic and environmentally friendly material that can be used to produce tableware, sanitary ware, etc., with good chemical stability and weather resistance. PMMA resin is not easy to produce sharp fragments when broken. The United States, Japan and other countries and regions have made mandatory provisions in the law. PMMA resin must be used for glass for primary and secondary schools and kindergartens. The pace of urban construction has been accelerated throughout the country, and street signs, advertising light boxes, etc. have appeared in large numbers, and a considerable part of the materials used are PMMA resins. The outdoor color building materials of the Beijing Olympics project also use a lot of green PMMA resin.
Due to the shortcomings of PMMA surface hardness, easy wiping, low impact resistance and poor molding flow properties, PMMA modification has appeared one after another. For example, copolymerization of methyl methacrylate with styrene and butadiene, blending of PMMA with PC, and the like. 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 be made into 373 plexiglass. The flow of PMMA is worse than that of PS and ABS. The melt viscosity is sensitive to temperature changes. In 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, plastic processing
PMMA has a certain water absorption, its water absorption rate is 0.3-0.4%, and injection molding must be below 0.1%, usually 0.04%. The presence of moisture causes bubbles, gas ray, reduced transparency, and the like in the melt. So dry it. The drying temperature is 80-90 ° C and the time is 3 hours or more.
Recycled materials can be used 100% in some cases. The actual amount depends on the quality requirements, usually over 30%. Recycled materials should avoid contamination and otherwise affect the transparency and the nature of the finished product.
2, injection molding machine selection
PMMA has no special requirements for injection molding machines. Because of its high melt viscosity, deeper grooves and larger diameter nozzle holes are required. If the strength of the product is required to be high, the low temperature plasticization is carried out using a screw having a large aspect ratio. In addition, PMMA must use a dry hopper to store material.
3, mold and gate design
The mold temperature can be 60 ° C -80 ° C, the diameter of the main channel should match the inner taper, the best angle is 5 to 7, if you want to injection 4mm or more products, the angle should be 7, the mainstream diameter is 8 to 10mm, The overall length of the gate should not exceed 50mm.
For products with a wall thickness of less than 4 mm, the runner diameter should be 6-8 mm. For products with a wall thickness greater than 4 mm, the runner diameter should be 8-12 mm. The edge, sector and tab gate depth 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 2 mm; the lower viscosity should be a smaller size. A common vent has a depth of 0.05 to 0.07 mm and a 6 mm wide draft angle between the 30'-1 cavity portions 35'-130.
4, the temperature of the melt
It can be measured by the air injection method: 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 spraying thick parts, slow speed is used.
6, staying time
If the temperature is 260 ° C, the residence time is no more than 10 minutes. If the temperature is 270 ° C, the residence time should not exceed 8 minutes.
