Solution to Warpage Deformation in Injection Molded Parts

Warpage deformation is a common defect in injection molded parts that occurs when the shape of the part deviates from the shape of the mold cavity during the cooling process. Warpage can cause problems such as dimensional instability, poor appearance, and reduced functionality of the part. Therefore, it is important to understand the causes and solutions of warpage deformation in injection molding.

Warpage Deformation in Injection Molded Parts

What Causes Warpage Deformation in Injection Molding?

Warpage deformation is caused by uneven shrinkage of the plastic material during the cooling process. Shrinkage is the reduction in volume of the plastic material as it changes from a molten state to a solid state. Shrinkage depends on several factors, such as:

Material properties: Different types of plastic materials have different shrinkage rates and coefficients of thermal expansion. For example, amorphous materials (such as polystyrene) have lower shrinkage rates than crystalline materials (such as polyethylene). Materials with high coefficients of thermal expansion tend to warp more than materials with low coefficients of thermal expansion.

• Mold design: The mold design affects the flow and cooling of the plastic material in the mold cavity. Factors such as gate location, type, and number; wall thickness and shape; cooling channel layout and temperature; and ejection system design can influence the shrinkage and warpage of the part. For example, a single central gate can cause more warpage than multiple pin-point gates or a film gate for flat parts. A thin wall can cool faster than a thick wall, resulting in differential shrinkage and warpage. A uniform cooling channel layout and temperature can prevent uneven cooling and warpage. A balanced ejection system can avoid excessive or uneven force on the part during ejection.
• Part design: Complex part geometries with varying wall thicknesses lead to uneven cooling and shrinkage, resulting in warpage. Abrupt changes in part cross-section, holes, and ribs also increase warpage.
• Processing parameters: The processing parameters affect the temperature, pressure, and time of the plastic material in the injection molding machine. Factors such as barrel temperature, injection pressure, injection speed, holding pressure, holding time, cooling time, and screw speed can influence the shrinkage and warpage of the part. For example, a low barrel temperature can cause under-heated material that will solidify before filling the mold cavity completely. A low injection pressure or holding time can cause insufficient packing of the material in the mold cavity. A long cooling time can increase the residence time of the material in the barrel, leading to thermal degradation and increased shrinkage.

How to Solve Warpage Deformation in Injection Molding?

The solutions for warpage deformation in injection molding depend on identifying and controlling the factors that cause uneven shrinkage of the plastic material during the cooling process. Some general solutions are:

• Select a suitable material: Choose a material that has a low shrinkage rate and a low coefficient of thermal expansion for the part design and application. Consult with the material supplier for data on shrinkage and warpage behavior of different materials.
• Optimize the mold design: Design a mold that facilitates uniform flow and cooling of the plastic material in the mold cavity. Use appropriate gate location, type, and number for the part geometry and material type. Use uniform wall thickness and avoid sharp corners or abrupt changes in cross-section. Use conformal cooling channels that follow the shape of the part and maintain a consistent mold temperature. Use adequate and well-distributed ejector pins that minimize deformation during ejection.
• Improved part design: Use uniform wall thicknesses whenever possible. Minimize abrupt changes in cross-section. Use ribs and gussets to strengthen parts. Optimize hole placement and add fillets to hole edges.
• Adjust the processing parameters: Adjust the processing parameters to achieve optimal temperature, pressure, and time conditions for the plastic material in the injection molding machine. Use a high barrel temperature that ensures homogeneous melt temperature throughout the shot size. Use a high injection pressure and holding time that ensure complete filling and packing of the mold cavity. Use a short cooling time that reduces residence time and thermal degradation of the material.
• Advanced solutions: Use computer simulations to analyze shrinkage and warpage before molding actual parts. Adjust part and mold designs accordingly. Apply localized heating elements for in-mold pre-expansion of areas prone to high warpage.

Conclusion

Warpage deformation is a common defect in injection molded parts that occurs when the shape of the part deviates from the shape of the mold cavity during the cooling process. Warpage deformation is caused by uneven shrinkage of the plastic material during the cooling process. Shrinkage depends on several factors, such as material properties, mold design, and processing parameters. The solutions for warpage deformation in injection molding depend on identifying and controlling these factors that cause uneven shrinkage of the plastic material during the cooling process.