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What's the reason and solution for bending and warping issue in plastic injection molding?


What is warpage?

Warpage in plastic injection molding is when the intended shape of the molded part is distorted during or after the cooling process. Mold warping can cause the part to fold, bend, twist or bow. Warpage is one of the most common quality issues in plastic injection molding.

Before you determine what is causing molding warpage you need to know:

  • How much the part warps

  • What direction the warpage tends to occur

  • Whether it cause problem for the assembling with other components

What's the reason and solution for warping, bending, and twisting of injection molded parts?

1. Uneven or insufficient cooling 

The countermeasure is to fully cool in the mold cavity, or you can lower the mold temperature and extend the cooling time.
Or change the path of cooling water, the position of cooling water channel or additional cooling tip hole, choose the right cooling way, water cooling, or air cooling and other ways.

2. Mold release resistance

You can improve the polish of the mold, making it easy to release, and sometimes by using a release agent.
The most fundamental method is to grind the core, reduce the mold release resistance, or increase the mold draft angle, and add the ejector rod in the part that is not easy to eject, etc., while changing the ejector method is more important.

3. Incorrect Material Selection

Resins with large shrinkage, generally crystalline resins (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin, etc.) than non-crystalline resins (such as PMMA resin, polyethylene, polystyrene, ABS resin and AS resin, etc.) deformation is large, in addition, due to the glass fiber reinforced resin with fiber matching, deformation is also large.

4. Inadequate Residence Time

If there is not enough injection pressure the plastic material will cool and solidify before the mold is properly packed.
If there is inadequate mold injection hold time, the packing process is minimized.

5. Barrel Mold or Nozzle Temperature 

Adjust barrel mold and nozzle to suitable temperature  based on the resin supplier’s recommendations and adjust accordingly.

6. Inadequate Gate Size

Mold gate size and shape should be optimized based on the resin supplier’s data. Usually, the best solution for mold warpage is to increase the gate size as much as possible.

7. Gate Location

A wrong gate location can force flow back in the direction of the gate, causing welding lines or air entrapment, or uneven cooling can cause differing shrink rates, leading to warpage.
Sometimes, changing the gate location to a proper location, problem can be solved. For example, this part, firstly, the gate was located at (see below picture OLD), and it warps a lot, but after moving its gate location to (see below picture NEW), it does not warp. 

gate location.jpg

8. Part design itself

Product design itself can also be an issue that causes mold warpage. The thickness and weight of the product should be even at every place. Add ribs to increase the warpage resistance of the part. The transition area and the corner of the thick and thin parts should be smooth enough.

9. Mold Steel Material  

If mold steel too soft or too thin, the pressure inside of the mold can not hold the internal stress that the part warps. Mold Steel should be harder and thicker.

10. Good Venting for the mold

11. If necessary, the parts that are easily warped and deformed can be shaped in the mold or secondary process after demolding.

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