Solving the Injection Molding Shrinkage with Effective Ways

Before starting the injection molding process, manufacturers must estimate the shrink rate. If they fail to do so, the geometry, appearance, and performance of the parts can be compromised, leading to costly and time-consuming production delays. Despite careful calculations, injection molding shrinkage may still occur due to the complexity of the process. So, what can be done when shrinkage happens? This article will explore the causes of shrinkage and ways to address it effectively.

What is Injection Molding Shrinkage?

Injection molding shrinkage refers to the contraction of polymers during the cooling phase of the molding process. As molten plastic material is injected into a mold and cools, it solidifies and shrinks. This can result in a smaller part than intended and may lead to warpage—uneven shaping due to inconsistent cooling.

Shrinkage rates vary depending on the material used, the design, and processing conditions. Manufacturers often account for shrinkage by oversizing the mold cavity or adding extra material, which is trimmed after molding. Despite these precautions, addressing shrinkage issues during design and tooling is critical to ensuring that the final product meets required specifications.

What Causes Injection Molding Shrinkage?

Several factors influence shrinkage during the molding process:

1. Plastic Composition: Materials like high-density polyethylene (HDPE) exhibit more shrinkage due to their crystalline structure, which causes polymers to contract as they cool.

2. Molecular Weight: Higher molecular weight polymers shrink less than those with lower molecular weight, as the stronger intermolecular forces reduce shrinkage.

3. Additives: Fillers and other additives can alter shrinkage behavior. For example, fillers may increase the material's stiffness, resulting in more shrinkage during cooling.

4. Stress Level: High pressure and temperature during the molding process can create stress in the material, leading to shrinkage and potential defects like brittleness.

5. Processing Conditions: Parameters such as mold temperature, cooling time, and injection speed impact shrinkage. Inconsistent cooling, especially from hot spots in the mold, can lead to uneven shrinkage.

   

How to Minimize Injection Molding Shrinkage

While shrinkage can’t always be eliminated, several strategies can help reduce its occurrence:

1. Optimize Cooling: Ensuring uniform cooling through mold design (e.g., cooling channels or plates) helps prevent uneven shrinkage. Mold temperature controllers can also ensure consistent cooling, avoiding hot spots that lead to warpage.

2. Adjust Wall Thickness: Thinner walls reduce cooling time and shrinkage, but there must be a balance. If the walls are too thin, the part may become fragile. Optimizing wall thickness and part strength is crucial for consistent results.

3. Reduce Plasticizing Temperature: Lowering the plasticizing temperature can decrease shrinkage by reducing the molten plastic's viscosity. However, care must be taken not to reduce the temperature too much, as this can lead to defects like flash or incomplete mold filling.

4. Increase Injection Pressure Speed: A higher injection pressure fills the mold cavity faster, leading to more uniform parts and reduced shrinkage. This must be done carefully, as too much pressure can cause other issues like flow lines or burn marks.

5. Improve Back Pressure: By increasing back pressure, the material fills the mold more evenly, reducing shrinkage. However, too much back pressure can cause defects like sink marks or voids, so it’s essential to find the optimal setting.

Conclusion

Injection molding shrinkage is a common but manageable issue. It can be minimized through thoughtful design and process optimization, such as controlling cooling effects, adjusting wall thickness, and refining processing conditions. Manufacturers must carefully calculate shrinkage rates during the design and tooling stages to prevent defects in the final product. For those needing assistance with their plastic injection molding projects, professional expertise can help ensure the success of their production runs.

AAA MOULD, with over 15 years of experience, is your go-to manufacturer for custom plastic injection molds and moldings. We deliver high-quality solutions across automotive, medical, and industrial sectors.

Contact us today to discuss your injection molding needs and discover how our expertise can benefit your project!