Factors That Affect Plastic Product Shrinkage in Injection Molding

Plastic shrinkage is one of the most important factors that influences the dimensional accuracy of injection molded parts. Shrinkage occurs as the molten plastic cools and solidifies inside the mold cavity. If shrinkage is not controlled properly, the final product may suffer from dimensional deviation, warpage, sink marks, or poor assembly performance.

The shrinkage rate of plastic products is affected by molding conditions, mold structure, part geometry, and material properties. Understanding these factors helps manufacturers improve dimensional stability and reduce molding defects.

1. Effect of Molding Process on Shrinkage

Injection molding parameters have a direct influence on the shrinkage behavior of plastic products.

• Increasing injection pressure generally reduces shrinkage because more material is packed into the cavity.
• Higher holding pressure and longer holding time also reduce shrinkage by compensating for material contraction during cooling.
• Increasing melt temperature may slightly reduce shrinkage because the material flows more easily into the cavity.
• Higher mold temperature usually increases shrinkage because the part cools more slowly.
• Longer cooling time inside the mold can reduce shrinkage and improve dimensional stability.
• Higher injection speed has only a small influence on shrinkage but may slightly increase the shrinkage rate in some cases.

In many molded products, most shrinkage occurs during the first two days after molding, then gradually becomes stable after about one week.

2. Effect of Mold Structure on Shrinkage

Mold design also plays an important role in controlling shrinkage.

• Larger gate sizes usually reduce shrinkage because they allow more packing pressure to enter the cavity.
• Shrinkage varies depending on the gate direction. Shrinkage parallel to the gate may differ from shrinkage perpendicular to the gate.
• Areas close to the gate generally have lower shrinkage than areas farther away from the gate.
• Plastic parts that are mechanically restricted by the mold tend to have lower shrinkage than unrestricted areas.

3. Effect of Part Geometry on Shrinkage

The shape and wall thickness of the plastic part can significantly affect shrinkage behavior.

• Thick-wall plastic parts usually have greater shrinkage than thin-wall parts because they contain more material and cool more slowly.
• Plastic parts with metal inserts generally have lower shrinkage than parts without inserts.
• Complex shapes often shrink differently compared with simple geometric shapes.
• Shrinkage in the thickness direction is usually greater than shrinkage in the length direction.
• Long and narrow parts tend to have lower shrinkage along their length.
• Inner holes often show greater shrinkage compared with outer dimensions.

4. Effect of Material Properties on Shrinkage

Different plastic materials have different shrinkage characteristics.

• Crystalline plastics generally have higher shrinkage than amorphous plastics.
• Materials with better flow properties often have lower molding shrinkage.
• Adding fillers such as glass fiber, talc, or mineral additives can significantly reduce shrinkage.
• Even different batches of the same material may show slight differences in shrinkage performance.

Conclusion

Plastic product shrinkage is influenced by many factors, including molding conditions, mold structure, product design, and material properties. Careful control of these variables can improve dimensional accuracy, reduce defects, and help manufacturers achieve more stable production results.