Acrylic Chemistry is called polymethyl methacrylate (referred to as PMMA). 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.
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.
PMMA has a certain water absorption, its water absorption rate is 0.3-0.4%, and injection molding must be below 0.1% humidity, 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.
- 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.
- 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 main channel 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 depth of the edge, sector and tab gates should be 0.7 to 0.9 t (t is the wall thickness of the product), the diameter of the needle gate should be 0.8 to 2 mm; the lower viscosity should be smaller.
Common venting holes are 0.05 to 0.07 mm deep, 6 mm wide, and the draft angle is 30′-1° between the cavity portions 35′-1°30°.
- Melt temperature
It can be measured by the air-to-air injection method: from 210 ° C to 270 ° C depending on the information provided by the supplier. The rear seat is withdrawn, the injection molding machine nozzle is separated from the main channel bushing, and then plasticized injection molding is performed, which is the air injection molding.
- 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.
- Residence 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.