Injection Molding Technology of Polymethyl Methacrylate (PMMA)
PMMA is commonly known as polymethyl methacrylate, acrylic and so on and its chemical name is polymethyl methacrylate. PMMA has some defects such as low surface hardness, easy to snag, low impact resistance, poor molding liquidity performance, so PMMA’s modification appears one after another such as copolymerization between methyl methacrylate and styrene and butadiene, a blend of PMMA and PC, etc. 372 PMMA is the copolymer of methyl methacrylate and styrene. If you add a small amount of nitrile rubber (about 50%) in the 372 PMMA, it will become 373 PMMA.
The fluidity of PMMA is poorer than that of PS, ABS and its melt viscosity is rather sensitive to the change of temperature. In molding, it changes the melt viscosity mainly from the injection temperature. PMMA is an amorphous polymer, the melt temperature is above 160 Celsius degree and the decomposition temperature can reach 270 Celsius degree.
- Processing of plastic
PMMA has a certain degree of water absorption of 0.3-0.4%. The injection molding moisture must be less than 0.1%. Typically, it is 0.04%. The presence of moisture makes the melt bubbles, gas lines, and low transparency. So it should be dried. Dying temperature is 80-90 Celsius degree and drying time is 3 hours or above. In some cases, the utilization of recycled materials can reach 100%. The actual quantity depends on the requirements for quality. Usually, it can reach 30%, you should make sure there is no pollution of recycled materials, or it will influence the transparency and the nature of the finished products.
- Selection of injection molding machine
The PMMA material doesn’t have special requirements for the injection molding machine. Due to the great melt viscosity, it requires the grooves deeper and nozzle hole a larger diameter. If you need higher strength requirements of products, you should use a larger aspect ratio of the screw to implement low-temperature plastics. In addition, PMMA must use drying hopper to store material.
- Design of mold and gate
Mold temperature can be 60 -80 Celsius degree, The diameter of the sprue should match up with the internal taper. The optimum angle is 5 to 7, if the products of injecting molding are 4mm or more, the angle should be 7, the diameter of the mainstream channel is 8 to 10 mm, the overall length of the gate should not exceed 50 mm.
For the products with wall thickness less than 4mm, the diameter of the flow channel should be 6-8mm. For the wall thickness is more than 4mm, the diameter of the flow channel should be 8-12mm. Depth of gon, fan-shaped and tabs-shaped gate should be 0.7 to 0.9t(t is the wall thickness of product), the diameter of pin gate should be 0.8to 2mm; The product with lower viscosity should choose the smaller size. The common venting hole’s depth is 0.05 to 0.07mm, width is 6mm, the stripping slope 30′-1, cavity portions are between 35′-1 and 30.
- Melt temperature
You can use the air injection method to measure temperature ranging from 210-270 Celsius degree, which depends on the information provided by the supplier.
- Injection temperature
You can use rapid injection to avoid generating a high degree of internal stress. It’s suitable to use multi-level injection, such as slow – fast – slow, etc. When injecting the thick parts, you should use the slow speed.
- Dwell time
If the temperature is 260 Celsius degree, the dwell time is within 10 minutes. If the temperature is 270 Celsius degree, the dwell time is within 8 minutes.
Sincere Tech has special injection molding machines for PMMA injection molding service, each year we produce millions of PMMA plastic molding parts for our customers. If you are looking for a PMMA injection molding service or any other customer molding service, welcome to send us an email.