Acrylic chemical name polymethyl methacrylate (PMMA for short), due to PMMA surface hardness is not high, easy to scuff, low impact resistance, poor molding fluidity and other shortcomings, PMMA modification of the material appeared one after another, successfully solved this defect.
PMMA fluidity than PS, ABS poor, melt viscosity is sensitive to changes in temperature, in the molding process, mainly from the injection temperature to change the melt viscosity. PMMA is an amorphous polymer, melting temperature greater than 160 ℃, decomposition temperature up to 270 ℃.
Treatment of plastic raw materials
PMMA has a certain degree of water absorption, its water absorption rate of 0.3% -0.4%, and injection molding must be below 0.1% moisture, usually 0.04%. The presence of moisture is likely to make the melt bubbles, vapor lines, reduced transparency, etc., so the drying process. Drying temperature 80-90 ℃, time for more than 3H. Recycled material can be used 100% in some cases, the actual amount depends on the quality requirements, usually more than 30%, recycled material to avoid contamination, otherwise it will affect the transparency and the quality of the finished product.
Selection of injection molding machine
PMMA injection molding machine has no special requirements, because of its melt viscosity, the need for deeper screw grooves and a larger diameter of the injection nozzle hole. If the strength requirements of acrylic injection molded products are higher, it is necessary to use a larger length-to-diameter ratio of the screw to implement low-temperature plasticization. In addition, PMMA must be used to dry hopper storage material.
Mold and gate design
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 4MM or more products, the angle should be 7 ℃, the diameter of the main runner up to 8-10mm, the overall length of the gate should not be more than 50mm. for the wall thickness of less than 4mm of the product, the diameter of the runner should be 6-8mm.
For products with wall thickness greater than 4mm, the runner diameter should be 8-12mm for edge-shaped, fan-shaped and pendant-shaped gates, the depth should be 0.7-0.9T, the diameter of the pin-shaped gate should be 0.8-2mm; low-viscosity should be selected with a smaller size.
The common venting holes are 0.05 or less in depth and 6mm in width, the demolding slope is 30′-1°, and the cavity part is between 35′-1°30′.
Melt temperature
It can be measured by the air-to-air injection method: it varies from 210°C to 270°C, depending on the information provided by the supplier.
Plasticizing injection is done manually by withdrawing the backseat so that the nozzle of the injection molding machine leaves the main flow path bushing, which is called counter-air injection.
Injection speed
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, slow speed is used.
Retention time
If the temperature is 260℃, the retention time should not exceed 10 minutes at most, if the temperature is 270℃, the retention time should not exceed 8 minutes.