The cause of shrinkage of injection molded parts and how to solve them

I believe that many mold manufacturers have encountered a plastic defect-shrinkage on injection molded parts. Few processors really know how to solve the shrinkage issue technically.In most case, mold makers just increase injection pressure,use longer injection time,or open larger injection gate, only annoyed to find that the issue is still there. Among the commonly used raw materials, the shrinkage problem of modified PC and PP are the most difficult to handle due to fast cooling rate.

Through 20 years, we have made plenty of plastic components with different kinds of resins for our global customers and has mastered some unconventional molding techniques to improve the shrinkage problem on plastic molded parts with thick wall. Basically, the plastic shrinkage defect could be contributed to 3 aspects, molding machine, plastic mold, and plastic material.

Injection molding machine

Cause:
1: The injection pressure in the cavity is too low.
2: Insufficient plastic in the cavity.
3: The mold temperature is too high.
4: Unusual injection molding conditions.
5: The nozzle hole is too small.
6: The mold cavity filling speed is too slow.
7: The gate in the mold is not condensed and closed.
8: Difference in melt temperature within the injection volume.
Remedy:
1.Gradually increase the injection pressure and check the size of the molded part at each shot until slight flash of the molded part occurs.
2. Increase the amount of injection valume .
3. Reduce the mold temperature.
4. Incorrect injection molding conditions (temperature, pressure, etc.) Samples for are checked before injection molding process gets stable.
5. Increase the diameter of the nozzle opening.
6. Increase the injection speed or use multiple stages of filling speed.
7. Increase the screw forward time and measure the sample as each increase.
8. Reduce the screw rotation speed so that the screw goe back to orgin just before the cooling is completed.Increase back pressure.

Mold

Cause:

1.The gate is too small or improperly designed.
2. The runner system is not properly designed.
3. Insufficient mold water cooling

Remedy:

1.Increase the gate size.
1. Use runner of round or trapezoid shape.Increase the runner size to suit the viscosity of the plastic or the flow characteristics of the plastic.
3. Measure the surface temperature at different points in the cavity and calculate the actual plastic shrinkage.If there are significant differences, redesign the water cooling channels.

Plastic
Cause:
The flowability of plastic is too low.

Remedy:
Use lower viscosity plastic

I hope that our experience can help you. For solutions to other plastic plastic defect, please click the below links.

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