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Warpage Issue in 92mm HDPE Cap – 6 Cavity Hot Runner Mold 1

R

ruwanmadu

Industrial
Joined
May 17, 2025
Messages
4
We have recently fabricated a 6-cavity hot runner injection mold for a 92mm HDPE plastic cap. The injection point is located at the center of the cap. We have conducted initial sampling, but we are experiencing an issue with the part.

Problem:
The cap is warping on the top surface after molding. The deformation appears to be consistent across all cavities and is affecting the flatness and overall quality of the cap.

Key Details:
  • Resin: HDPE
  • Mold Type: 6 cavity hot runner
  • Gate location: Center of the cap (direct injection)
  • Warpage: On the top side of the cap
Has anyone faced a similar issue with large diameter HDPE caps?
What factors should we investigate first—material behavior, gate design, cooling layout, or processing parameters?

Any suggestions or advice would be greatly appreciated.

1747495669695.png
 

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Last edited:
Guess, is there adequate time to cool?
 
Nonuniform results are usually from nonuniform conditions.

Ensure the cavities are of uniform temperature at the start of the process and can remove heat uniformly from the part after injection. This might be done with a mold analysis Finite Element Model.

Other causes:

Plastic is uniformly too cold when injected, injection pressure is too low, post injection time is too short before ejection, the gate is too small to provide sufficient flow rate to keep the plastic at a uniform temperature during injection, the part is being removed too soon/while still hot, barrel temp is too low and the plastic is injected with varying temperatures during the process.

Since any of them can cause this same result, pick the one that is cheapest to deal with and work to either prove it is the problem or that it can not be the problem. By this method if the answer is expensive then there is evidence that the less expensive options don't work. Change the process variables by +10% and then -10% (or some suitable range for temperature) and see if the result is better or worse.
 
3DDave said:
Nonuniform results are usually from nonuniform conditions.

Ensure the cavities are of uniform temperature at the start of the process and can remove heat uniformly from the part after injection. This might be done with a mold analysis Finite Element Model.

Other causes:

Plastic is uniformly too cold when injected, injection pressure is too low, post injection time is too short before ejection, the gate is too small to provide sufficient flow rate to keep the plastic at a uniform temperature during injection, the part is being removed too soon/while still hot, barrel temp is too low and the plastic is injected with varying temperatures during the process.

Since any of them can cause this same result, pick the one that is cheapest to deal with and work to either prove it is the problem or that it can not be the problem. By this method if the answer is expensive then there is evidence that the less expensive options don't work. Change the process variables by +10% and then -10% (or some suitable range for temperature) and see if the result is better or worse.
Click to expand...
Thank you for your advise
 
The webpage is most likely due to the top of the cap being much warmer than the rest is the cap upon ejection from the mold. Thus it cools and shrinks more. Orientation of the polymer due to flow is also a factor. Longer cooling time and perhaps warmer mold temperature should help. It will increase mold cycle time, however.

Try dunking a warped cap in hot (boiling?) water and cooling on a flat surface.
 
Ya exactly. Let it sit in die longer, run test with a timer . Try different times . Inspect dimensional control.is the cavity larger than it needs be?
 
Compositepro said:
The webpage is most likely due to the top of the cap being much warmer than the rest is the cap upon ejection from the mold. Thus it cools and shrinks more. Orientation of the polymer due to flow is also a factor. Longer cooling time and perhaps warmer mold temperature should help. It will increase mold cycle time, however.

Try dunking a warped cap in hot (boiling?) water and cooling on a flat surface.
Click to expand...
Thanks you
 
What's your injection pressure, fill time, and melt temps? Transfer at 90-95% fill?
What's the wall thickness of top surface vs. sides? That could contribute to warp if there's a large enough difference.
The external ridges + internal threads (assumption based on the sink/bleed-through) on the sidewalls could be also be strengthening that geometry, which may be why you're only seeing warp on top. These are all guesses without seeing full part design, tool design (adequate cooling), or processing sheet.
 
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