How can you prevent warpage in injection molding?

1 Understand the causes of warpage

Warpage is caused by uneven shrinkage of the plastic material during the cooling stage of injection molding, which is due to different temperatures, pressures, and orientations in different regions. Factors that influence the shrinkage and warpage of the part include material properties, such as semi-crystalline or fiber-reinforced materials being more prone to warpage; mold design, with uniform wall thickness, smooth transitions, adequate vents, and minimal undercuts or ribs; and process parameters, such as injection speed, pressure, temperature, cooling time, and holding time needing to be optimized for balanced filling and cooling.

2 Choose the right material

When selecting a material for injection molding, it is important to consider the properties that affect the shrinkage and warpage of the plastic. The shrinkage rate should be low or have a predictable behavior, the thermal expansion coefficient should be low or similar to that of the mold material, and the viscosity should be low or have a narrow range. High viscosity or wide variations in viscosity can cause uneven filling and cooling, which can lead to warpage. Choosing the right material is an essential first step in preventing warpage.

3 Design the mold properly

In order to prevent warpage in injection molding, proper mold design is essential. Factors such as wall thickness, transitions, vents, undercuts and ribs should be taken into consideration. Wall thickness should be uniform or gradually changing to avoid differential cooling and shrinkage. Transitions should be smooth or generous radii to avoid stress concentration and orientation effects. Vents should be adequate or have a vacuum system to avoid trapped air, gas, or moisture. Undercuts and ribs should be minimal as they can cause difficulty in ejecting the part, increase cooling time, and create orientation effects.

4 Optimize the process parameters

The final step to prevent warpage in injection molding is to optimize the process parameters. You should consider the injection speed, pressure, temperature, cooling time, and holding time that affect the filling, packing, and cooling of the cavity. Optimizing the injection speed ensures complete and consistent filling of the cavity, while optimizing the pressure ensures adequate and balanced packing. Temperature should be optimized to ensure optimal viscosity and flow of the material, while cooling time should be optimized for sufficient and uniform cooling of the part. Lastly, holding time should be optimized to ensure adequate compensation for shrinkage. By optimizing these parameters, you can avoid short shots, sink marks, flash, thermal degradation, burning, premature ejection, residual stress, overpacking, underpacking, and warpage.

5 Here’s what else to consider

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