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Four factors, clarify the ins and outs of burnt spot in injection molding products

May 12, 2017

Factor One: Melt rupture

When the melt is injected into a cavity with a large volume at high speed and high pressure, the melt rupture is easily produced, and the surface of the melt appears transverse fracture, and the fracture area is coarse and mixed in the surface of the plastic parts. In particular, a small amount of molten material directly injected into an easy-to-too-large cavity, the melt rupture is more serious, the rendering of the greater the paste spot.

From the above, to overcome the trapped melt rupture, to avoid the production of paste spots,

One is to pay attention to eliminating the corners of the runner, so that the flow path streamline as much as possible;

Factor two: improper molding condition control

This is also the cause of the surface of the plastic parts to produce burnt and paste spot, in particular, the size of the injection speed is very important, when the flow rate of slow injection cavity, the flow of molten flow is laminar, when the injection rate rises to a certain value, the flow state gradually become turbulent.

In general, laminar formation of the surface of the plastic parts more bright flat, turbulent conditions under the formation of plastic parts not only surface prone to paste spots, and plastic parts of the interior prone to pores. Therefore, the injection speed can not be too high, the flow should be controlled in laminar flow mode.

If the melt temperature is too high, easy to cause the melt decomposition coking, resulting in the surface of the plastic parts paste spot. General injection molding machine screw rotating number should be less than 90r/min, back pressure is less than 2mpa, so that the barrel can avoid excessive friction heat.

If the molding process due to the rotation of the screw back too long to produce excessive friction heat, can be appropriately increased screw rotational speed, extending the molding cycle, reduce the screw back pressure, improve the temperature of the barrel feeding section and the use of poor lubrication materials and other methods to overcome.

Factor 3: Mould failure

If the mold exhaust holes are demoulding agent and raw material precipitation of the solidification of blocking, the mold exhaust is not enough or the position is incorrect, as well as the mold filling speed too fast, the die too late to exhaust air insulation to produce high temperature gas will make the resin decomposition coking. In this respect, the blocking material should be eliminated, the clamping force is lowered, and the exhaust of the mould is improved.

The determination of the mould gate form and position is also very important, and the flow state of the melt and the exhaust performance of the mould should be considered in the design.

In addition, the dosage of demoulding agent can not be too much, the cavity surface to maintain a high finish.

Factor IV: Raw materials do not meet the requirements

If the moisture and volatile content of the raw materials are too high, the melting index is too large, the use of excessive lubricant will cause coke and paste spot failure.

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