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PMMA plastic injection molding process analysis

Mar 23, 2019

PMMA plastic injection molding process analysis

PMMA is commonly known as plexiglass, acrylic, etc. The chemical name is polymethyl methacrylate. 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. 372 plexiglass is obtained by copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 plexiglass, it can be made into 373 plexiglass.
The fluidity 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 of more than 160 ° C and a decomposition temperature of 270 ° C.

PMMA is a non-toxic and environmentally friendly material. The biggest feature is high transparency and 92% transmittance. It is the best light transmissive in plastics, and its UV transmittance is also 75%. At the same time, PMMA materials are also available. Has good chemical stability, and weather resistance.

PMMA acrylic materials are often used as acrylic sheets, acrylic plastic pellets, acrylic light boxes, signboards, acrylic bathtubs, etc. The automotive sector is mainly automotive taillights, signal lights, instrument panels, etc., pharmaceutical industry (storage containers), industrial applications (videos, lighting) Scatterers), buttons for electronic products (especially transparent), consumer goods (beverage cups, stationery, etc.).

1, plastic processing

PMMA has a certain water absorption, its water absorption rate is 0.3-0.4%, and the 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%. Recycling materials should avoid contamination or affect transparency and finished product properties.
2, 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, it is necessary to perform low temperature plasticization with a screw having a large aspect ratio. In addition, PMMA must use a dry hopper to store material.
3, 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 mainstream diameter is 8 to 10mm, pouring The overall length of the mouth 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 vents are 0.05 to 0.07 mm deep and 6 mm wide

    The draft angle is between 30'-1 cavity portions 35'-130.
4, the temperature of the melt

    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.
5, 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.
6, staying 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.

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