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Eliminating Injection Molded Parts Fracture

I: Causes of molded parts stress fracture
Stress fracture is often caused by residual internal stress of injection molded parts under the effect of solvents, oils or drugs.

Associate knowledge about stress fracture
  • Same reason with deformation and crack.
  • We consider the stress fracturing effect may be due to the solvent, oil and medicine, which can decrease injection molded parts binding force. (Figure A)
  • Detect the stress fracture by added with additional force. The simplest method is to hang the injection molded parts with weight of 20% stress intensity, then use solvents, oils, medicines coating for observation.
  • Because of medicine is mixture, so cannot be generalized. Small amount of anti-rust oil or cutting oil will cause stress fracture.
  • All the solvents, oils and pharmaceutical items have an impact on the injection molded parts, so the used containers must be carefully pre-inspection, cleaning, without foreign matters.
Fracture

Solutions
Immediate: Degrease mould parts and screws after the installation of the heat treatment.
Long-term: Use fracture-resistant material.

Differences in the materials
Polystyrene material has lowest GP value, HI followed. AS, ABS have relatively high value.
For poly class material, polyethylene (PE) is weak for solvent, while PP is very strong.

References
It is impossible to get no stress force molded parts under tiny control of injection moulding condition.
Attention: Drug-resistant high-performance data is under stress-free injection molded parts. The data has significant difference under stress effect.
Stress fracture can be measured by the residual stress, the polystyrene can be soaked in kerosene to detect residual stress position.

II: Causes of molded parts reticular fracture
Residual stress (over-filling)
Force when demoulding
Excessive load

Associate knowledge about reticular fracture
  • 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.
  • It is impossible for fracture to become crack, but at the same time, due to the density of the fracture portions are only 50% to 60% of other parts, that is to say there is a very fine gap mixed in there should take into account how added when a very large force, this gap has a tendency to expand and become the gap.
  • Reticular fractures is the same phenomenon with whitening. Due to the transparent material that can see the strip light, this is usually referred to as reticular fracture.
Solutions
Instant: Decrease injection pressure
Short-term: Improve the smoothness of the mould cavity
Long-term: Demoulding slope should be sufficient, and the release pin should be balance arranged.

Difference of materials
GP polystyrene was obtained reticular fracture length reaches 1 to obtain the number of small 2mm, HIPS we observed very little bleaching.

Reference
Reticular fracture with crack available after the ink is applied to distinguish.

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