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Injection Moulding Defects

Causes of injection moulding defects
  • Injection moulding machine insufficient pressure
  • Low injection pressure
  • Material lack of liquidity
  • Poor cavity air discharging
  • Gate unbalance (under high quantity production per mould)
Associate knowledge about injection moulding defects
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.

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.

Troubleshooting & Solutions
Immediately: Increase mould and barrel temperature, increase injection pressure, install air discharging groove on parting surface (depth 0.02~0.04mm, width 5~10mm).
Short-term: Expand the gate, increase the flow boundary. Under high output per mould occasions, expand the mould cavity gate of moulding defect, as well as change the configuration of the flow boundary, install gas evolution pin, improve mould smooth finish.
Long-term: Avoid different designs of plastic parts thickness, install additional gates for thicker plastic parts, understand the using occasion of plastic parts, if appropriate, make sure to use material with good flow ability.

Differences in material
If the mould temperature is not high, and is using low flow ability material such as PC, PPO or PETP, the mould temperature must be increased to over 80 degrees (using a mould temperature conditioner).

So called material flow ability, generally refers to the material quantity flow in the injection hole measured at certain temperature and pressure within a certain period of time. Flow property indicates has a melt index of the material. For example: polystyrene melt index is 2160±10 degree of load under 190 degrees and 10 minutes out of the flow through the orifice of weight.

As the measurement method of resin flow ability, using spiral flow (vortex) mould can measure the length of the results of the resin filled.
Spiral flow mould

1) Injection dwell time is not related to filling length, but too short will cause insufficient filling (moulding defect).
2) Not related to the retention time on the charging barrel.
3) If the injection speed is too slow, the filling will become insufficient. However, it has little effect above a certain speed.
4) The mould temperature is better to be higher, the impact will be smaller.
5) If there is a slight surplus of screw plasticizing stroke, then stuffing length becomes longer.
6) Injection pressure and filled length is substantially linear relationship.
7) In the appropriate field of moulding material temperature, the temperature of the stuffing length is substantially the same linear scale.
8) The thickness of the cavity with stuffing length substantially linearly proportional.

Filling maximum length (L) and cavity thickness (T) specific value L/T is the moulding material flow ability criteria.
Material L/T specific value
PVC (hard) 150-100
PVC (soft) 240-160
PC 150-100
PA6 320-200
PS 200-220
PP 280-160
PE 280-200

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