AIM Processing Small Plastic Parts Blog

Injection molding is one of the most common methods used in manufacturing plastic components. It allows for the production of intricate and highly detailed parts at high volumes. However, like any other manufacturing process, it presents several challenges. Two of the most prevalent issues encountered are warping and shrinkage. These defects can affect the overall quality of the molded part and even lead to part rejection if not properly managed. Understanding how to prevent warping and shrinkage in injection mold design is essential to maintaining high-quality production standards.

Understanding Warping and Shrinkage

Before diving into preventive measures, let’s break down the issues of warping and shrinkage.

• Shrinkage refers to the reduction in the size of the molded part as it cools. As plastic cools from its molten state, it contracts. This shrinkage can lead to dimensional inaccuracies, causing parts to be smaller than expected.
• Warping occurs when the cooling is uneven. Different areas of the part cool and shrink at different rates, leading to internal stresses that cause the part to twist or deform.

Both issues are influenced by material properties, mold design and processing parameters. While shrinkage is inevitable to some degree, controlling it and minimizing warping is crucial.

Key Factors Leading to Warping and Shrinkage

Several factors contribute to warping and shrinkage in molded parts, including:

1. Material Selection
   • Different materials have different shrinkage rates. Semi-crystalline materials like polyethylene and polypropylene tend to shrink more than amorphous materials like polystyrene or polycarbonate.
2. Mold Design
   • The design of the mold plays a critical role in how the part cools and solidifies. Poorly designed molds with uneven wall thickness, inadequate cooling channels or improper gate placement can exacerbate warping and shrinkage issues.
3. Processing Parameters
   • Temperature and pressure settings during molding directly impact how the material cools and contracts. Incorrect injection speed, packing pressure and cooling rates can lead to uneven cooling, resulting in warping and excessive shrinkage.
4. Part Geometry
   • Thin-walled parts, large flat sections or parts with varying wall thicknesses are more prone to warping. Complex geometries can also make it harder for the part to cool evenly.

Best Practices to Avoid Warping and Shrinkage in Injection Mold Design

Now that we understand the root causes of warping and shrinkage, here are some strategies to mitigate these issues.

1. Optimize Part Design
   • Uniform Wall Thickness: Ensure that the part design incorporates uniform wall thickness. Variations in thickness lead to uneven cooling rates, which increases the risk of warping.
   • Avoid Sharp Corners: Sharp corners concentrate stress and can lead to warping. Use radiused corners to distribute stress more evenly.
   • Ribbing and Gussets: To add strength without increasing wall thickness, consider using ribs or gussets. This reinforces the part without causing uneven shrinkage.
2. Control Material Flow and Cooling
   • Gate Placement: Proper gate placement ensures uniform material flow. Placing gates near thick sections or ensuring that material fills symmetrically will help reduce the likelihood of shrinkage and warping.
   • Cooling Channels: Use strategically placed cooling channels to maintain uniform cooling across the mold. Efficient cooling prevents one part of the mold from cooling faster than the other, which can lead to warping.
   • Cooling Time: Increasing cooling time may prevent warping. Giving the part sufficient time to cool before ejection allows it to solidify properly, reducing the chance of deformation.
3. Material Selection
   • Low-Shrinkage Materials: Select materials with low shrinkage rates, particularly for precision parts. Amorphous polymers like polycarbonate or polystyrene tend to exhibit less shrinkage and are less prone to warping compared to semi-crystalline materials.
   • Glass-Filled Resins: Reinforced materials, such as glass-filled polymers, reduce shrinkage and warping. The filler helps stabilize the material as it cools and solidifies.
4. Fine-Tune Processing Parameters
   • Injection Pressure and Speed: Adjust the injection pressure and speed to optimize material flow and packing. High pressure ensures the mold is fully filled, reducing the risk of voids or areas of underfilled material, which can cause warping.
   • Packing and Holding Pressure: Holding pressure should be maintained long enough to compensate for the material's shrinkage during cooling. This ensures that any excess material remains in the mold to fill the space caused by shrinkage.
   • Cooling Rate: Control the cooling rate to ensure uniform cooling. If the part cools too quickly, it may warp. Balancing the cooling rate with proper cycle times is key to reducing internal stresses.
5. Optimize Mold Design
   • Use Shrinkage Compensation: When designing the mold, factor in shrinkage compensation. Mold cavities are often oversized to account for the expected shrinkage of the material after cooling.
   • Venting: Proper venting in the mold allows trapped air and gases to escape, ensuring uniform material filling. Poor venting can cause localized shrinkage.
   • Multiple Gates: If the part is large or has complex geometry, consider using multiple gates to fill the cavity evenly. This reduces the likelihood of one area cooling faster than another.

Avoiding warping and shrinkage in injection mold design requires a comprehensive understanding of material properties, mold design and processing conditions. By adhering to best practices such as maintaining uniform wall thickness, optimizing gate placement and controlling cooling rates, manufacturers can minimize these defects and ensure the production of high-quality parts. While shrinkage cannot be eliminated entirely, careful planning and attention to detail can help mitigate its effects and reduce the risk of warping. Proper collaboration between design engineers, material scientists and process technicians is essential to achieving optimal results.

As you can see, there are many factors involved in controlling warpage in your finished parts. Let AIM Processing's experience with over 2,000 parts help you reach your goal. Contact AIM Processing to learn more and get started.