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The most common cause of black streak is the material residue in the injection moulding machine barrel.Residue remains in the plunger injection moulding machine.For valve gated injection moulding machine, the valve-regulated part also tends to remain residue.The air is compressed to a high temperature in the cylinder, with the high temperature gas in the gate, so it will be easy to be black streak.
Black Streaks
Residual gas inside the injection mould is compressed into high temperature state and burn the material.
Insufficient air discharging will cause burn mark.
Insufficient air discharging will also cause material shortage.
Burn Marks
In the nozzle and side slip, cooled resin flow into the mould cavity in a semi-cured state. After contact with the injection mould, the resin immediately cured, and then into a deep cavity formed flow mark.Different thickness of the plastic parts can also cause flow mark.
Flow Marks
Using only one sprue (gate), without confluence the material from two directions, in this way the molded parts generally do not have weld line. If you want to avoid weld lines, you can only change the position of the sprue in the injection mould so that the welding mark will not occur in obvious places.
Change the location of the sprue in injection mould
Same reason with reticular fracture.
If not well demolded, the release pin will apply excessive force resulting in molded plastics whitening.
Whitening refers to stress whitening. We can see from the stress yield curve and crack white perception that it indeed to the energy expended. Just add a little energy will crack after whitening. (Figure A)
ABS material yield curve
Fracture looks like crack, but the essence is different, that the fracture is not like a void defects, coupled with stress in the polymer due to the parallel arrangement direction of itself. If heat the product, it can return to the state without fracture. In this way, we can distinguish the reticular fracture and crack.
Polystyrene has high impact resistance. The rubber particles dispersed in the polystyrene stress concentration point, majority cause tiny fracture. This is because the absorption of deformation energy. ABS is very subtle, so the fracture is very tiny, and has superior impact resistance.
Injection molded parts crackThe residual stress within the product. Moulding conditions should be set towards to elimination of stress. That is to increase cylinder and mould temperature, and when the flow ability of the material becomes a good state, the injection pressure should not be too high.

Pre-heated metal insert can ease the residual stress during injection moulding. In the same principle, annealing after forming the insert is also a method. When setting the moulding condition, reduce the injection pressure is the most effective. If the ejector pins are balanced, release more force when demoulding.
Reason of molded parts deformation (warping, bending, small wrinkles) is the same to cracks. That is the residual stress within the product. moulding conditions should be set towards to elimination of stress. That is to increase cylinder and mould temperature, and when the flow ability of the material becomes a good state, the injection pressure should not be too high.
Eliminate Molded Parts Deformation
To reduce the residual stress, the injection mould should be annealed. Heat distortion temperature below 10℃, heated over 2 hours to eliminate the effect of internal stress. However, due to the high cost, this method is not widely used.

If the cooling water holes of the mould cannot uniformly cool the product, and it also have residual stress, which means that the cooling water hole cannot be too shallow.
Molded parts flash Injection mould parting line, sliding surface, release pin, pad and so on, constitute a variety of slot gap between the moulds. The inflow material will cause flash. The flash looks like wedge-shaped blocks. It is gradually trend to be bigger.

The general principle to eliminate plastic parts flashes is to fix the injection mould. If the flash is caused by improper configuration, we should reduce cylinder and mould temperature, and reduce injection speed, in order to reduce material flow ability. However, it should be noted that cause a residual stress in the plastic parts.

It should be noted that add too much pressure on the injection mould will produce flashes. Foreign matters between the parting surfaces will cause damage to the mould and produce flash.Injection mould material commonly used materials such as S50C or pre-hardening HRC30 special steel, in this way, the parting surface is difficult to produce flash.
Material contains trace amounts of water, in addition to causing silver streak, wave current pattern, whitening, bubbles and other undesirable phenomena will also occur. It also has bad effect on the flow ability and thermal deterioration of material.
Silver Streak
If the temperature gradient is too large, the flowing material will be easy to cut.
If gas evolution is not well in the injection mould, it can also occur silver streak.
Injection moulding defects generally decide by mould shape and material liquidity. When defect is due to the setting of moulding material, injection moulding condition setting should be set towards the right direction, namely to improve the mould and barrel temperature, injection pressure and speed.
Spiral flow mould
If the cavity exhaust edge is not well designed, the air in the mould will be difficult to be discharged. In addition to cause moulding defect, it will also cause burn mark and fusion mark.
In the production process, sink mark is the highest frequency of occurrence of undesirable phenomena in plastic injection mould. The early cooling part surface will be hard first, at the same time, the internal will produce bubbles. The sink mark is the concave surface in the direction of contraction of the bubble of the slow cooling part.
Avoid sink marks
Materials with large shrinkage are also easy to produce sink mark. When you want to change the moulding conditions to eliminate sink mark, you should set conditions to a low contraction direction. That is to reduce plastic mould and cylinder temperature, increase injection pressure. It should be noted that this may cause internal residual stress.

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