Why uneven shrinkage of injection molded parts!

If the plastic parts shrink unevenly, the internal stress distribution is often uneven. If this stress exceeds the rigidity of the plastic part, it will cause warpage and deformation  and the dimensional instability of the plastic part.

What is warpage?

When internal stress (due to uneven shrinkage) > material stiffness , warpage occurs

If the plastic parts shrink unevenly, the internal stress distribution is often uneven. If this stress exceeds the rigidity of the plastic part, it will cause warpage and deformation of the plastic part ,and affect the dimensional stability of the plastic part.

Four reasons for uneven shrinkage:

1. different melting temperature: the shrinkage is larger if the melting temperature is high.

2. Different cooling rates (crystalline):

Reason: fast cooling speed < slow cooling speed (molecules with slow cooling speed have more time, so the shrinkage is large)

3. Melting flows in different directions (amorphous): in short, the direction along which the material flows has large tensile internal stress and therefore large shrinkage. During the filling process, molecular chains are aligned due to flow alignment.

The aligned polymer chains are stretched differently in the flow direction and the vertical flow direction, resulting in different shrinkage behaviors. It is called directional contractility.

Generally , the contraction rate in the flow direction is higher than that in the vertical flow. This is because the plastic polymer chains in the flow direction are stretched seriously, and there is a greater tendency to restore the unstretched state.

Differential shrinkage caused by flow alignment often results in warpage of plastic parts.

Therefore, if the molecular alignment can be scattered, the uniformity of shrinkage will be facilitated and the warping deformation caused by directional shrinkage will be reduced.

4. Different sizes: in simple terms, the larger the part size is, the larger the shrinkage will be.

Warping due to design:

Wall thickness

For areas with thick wall, it is difficult to cool and maintain the pressure. The cooling time required is longer and the pressure maintaining  is poorer. After demoulding, it still maintains local high temperature and continues to cool. Therefore, in places where local wall thickness is relatively bigger, such as ribs, local shrinkage leads to dents in plastic parts easily.

Therefore, for plastic parts with wall thickness changes, the choice of the gate position at a thicker place can help maintain the holding pressure. Even if the workpiece solidifies, the holding pressure can still be smoothly transferred to remove the shrinkage.

Wall Thickness Difference

Uniform thickness of plastic parts will improve shrinkage. If the thickness of the plastic parts is unevenly distributed, it should be considered whether the shrinkage difference caused by different cooling and pressure holding effects will cause warping deformationand stress concentration in the transition area of  the plastic parts.

Internal stress concentration in the transition area  will cause short-term or long-term warping problems and reduce the mechanical properties of plastic parts. Ribs can be introduced into plastic parts to reinforce structural strength and reduce shrinkage.

The contact between rib and plastic wall should be large enough to alleviate stress concentration and overcome flow resistance. However, there is a sink problem. Generally , the sink size is also affected by the shrinkage characteristics of the plastic. Considering the strength and rigidity, if the thickness of the plastic part can be reduced by cut out, shrinkage can be reduced.

Injection Molding and Deformation

The injection molding setting mainly involve injection and holding time, cooling time, injection speed and mold temperature.

  • 1. Injection and Pressure Holding Time

The total time of injection and pressure holding should be set to be longer than the gate closing time. If the closing time is shorter than that of the gate, the deformation will increases.

  • 2. Cooling time

In general, extending the cooling time will reduce the deformation.

  • 3. Injection speed

Depending on the shape of the molded product, sometimes the deformation is small if the injection speed is fast, whereas the deformation is small if the injection speed is slow. In actual molding, the minimum deformation condition should be found by changing the injection speed.

  • 4. Mold temperature

The molded product with low mold temperature has small deformation. However, if the  temperature of the molded product is high, the problem of post-shrinkage deformation or dimensional change will sometimes occur. Mold temperature should be determined according to these factors.

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