PMMA is commonly known as organic glass, acrylic, etc. Its chemical name is polymethyl methacrylate. Due to the shortcomings of PMMA such as low surface hardness, easy to scratch, low impact resistance, and poor molding flowability, PMMA modifications have appeared one after another. Such as the copolymerization of methyl methacrylate with styrene and butadiene, and the blending of PMMA with PC. 372 organic glass is obtained by copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 organic glass, 373 organic glass can be obtained. The flowability 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 by the injection temperature. PMMA is an amorphous polymer with a melting temperature greater than 160¡ãC and a decomposition temperature of 270¡ãC.

Molding process

1. Plastic treatment

PMMA has a certain water absorption, and its water absorption rate is 0.3-0.4%, and the injection molding must be at a temperature below 0.1%, usually 0.04%. The presence of water causes bubbles and gas lines in the melt, and the transparency is reduced. Therefore, it is necessary to dry it. The drying temperature is 80-90¡æ, and the time is more than 3 hours. In some cases, recycled materials can be used 100%. The actual amount depends on the quality requirements, usually more than 30%. Recycled materials should avoid contamination, otherwise it will affect the transparency and 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, it requires a deeper screw groove and a larger diameter nozzle hole. If the strength requirements of the product are high, a screw with a larger 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¡æ-80¡æ. The diameter of the main channel should match the inner taper. The best angle is 5¡ã to 7¡ã. If you want to inject 4mm or more products, the angle should be 7¡ã, the main channel diameter is 8 to 10mm, and the overall length of the gate should not exceed 50mm. For products with a wall thickness of less than 4mm, the flow channel diameter should be 6-8mm

For products with a wall thickness of more than 4mm, the flow channel diameter should be 8-12mm

The depth of the opposite side, fan and vertical sheet gates should be 0.7 to 0.9t (t is the wall thickness of the product), and the diameter of the needle gate should be 0.8 to 2mm; a smaller size should be used for low viscosity. Common vents are 0.05 to 0.07mm deep and 6mm wide

The demoulding slope is between 30¡ä-1¡ã and 35¡ä-1¡ã30¡ã for the cavity part.

4. Melt temperature

It can be measured by air injection method: ranging from 210¡æ-270¡æ, depending on the information provided by the supplier.

5. Injection temperature

Fast injection can be used, but to avoid high internal stress, multi-stage injection is recommended, such as slow-fast-slow, etc. When injecting thick parts, slow speed is used.

6. Residence time

If the temperature is 260¡æ, the residence time shall not exceed 10 minutes at most. If the temperature is 270¡æ, the residence time shall not exceed 8 minutes.

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