Acrylic injection molding process
The chemical name of acrylic is polymethyl methacrylate (PMMA for short). Due to the shortcomings of PMMA surface hardness is not high, easy to rub hair, low impact resistance, poor molding fluidity and other shortcomings, the modification of PMMA has appeared one after another. Such as the copolymerization of methyl methacrylate and styrene, butadiene, the blending of PMMA and PC, etc.
The fluidity of PMMA is worse than that of PS and ABS, and the melt viscosity is more sensitive to temperature changes. During 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. Handling of Plastics
PMMA has a certain degree of water absorption, and its water absorption rate reaches 0.3-0.4%, while the humidity of injection molding must be below 0.1%, usually 0.04%. The presence of water causes bubbles, gas lines, and reduced transparency in the melt. So it needs to be dried. The drying temperature is 80-90°C, and the drying time is more than 3 hours. In some cases, 100% recycled materials can be used. The actual amount depends on the quality requirements, usually more than 30%. The recycled materials should avoid pollution, otherwise it will affect the transparency and the properties of the finished product.
2. Selection of injection molding machine
PMMA has no special requirements for injection molding machines. Because of its high melt viscosity, a deeper screw groove and a larger diameter nozzle hole are required. 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 be stored in a dry hopper.
3. Mold and gate design
The mold temperature can be 60°C-80°C. The diameter of the sprue should match the inner taper. The best angle is 5° to 7°. If you want to inject products with 4mm or more, the angle should be 7°, and the diameter of the sprue should be 8 to 7°. 10mm, the overall length of the gate should not exceed 50mm. For products with wall thickness less than 4mm, the flow channel diameter should be 6-8mm
For products with a wall thickness greater than 4mm, the diameter of the runner should be 8-12mm. The depth of the diagonal, fan-shaped and vertical sheet gates should be 0.7 to 0.9t (t is the thickness of the product wall), and the diameter of the needle gate should be 0.8 to 2mm; low viscosity should choose a smaller size.
The common vent holes are within 0.05mm deep, 6mm wide, and the demoulding slope is between 30′-1° and the cavity part is between 35′-1°30°.
4. Melt temperature
It can be measured by air injection method: ranging from 210°C to 270°C, depending on the information provided by the supplier.
Withdraw the rear seat, make the nozzle of the injection molding machine leave the main channel bushing, and then manually carry out plasticizing injection molding, which is hollow injection molding.
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5. Injection temperature
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, use slow speed.
6. Residence time
If the temperature is 260°C, the residence time should not exceed 10 minutes; if the temperature is 270°C, the residence time should not exceed 8 minutes
