Injection molding defects are common issues that occur during the molding process, affecting the quality and functionality of the final product. Addressing these defects requires optimizing process parameters like temperature, pressure, and material flow, as well as ensuring proper mold design and maintenance. Here are some of the most common defects in injection molding, along with their causes and solutions.
1. Warping (Deformation)
- Cause: Uneven cooling or excessive shrinkage of the material. Different sections of the part may cool at different rates, leading to internal stresses that warp the product.
- Solution: Optimize cooling rates by using uniform mold temperatures, and adjust injection speeds or material flow to reduce internal stress. Use materials with low shrinkage.
Source: Sybridge Technologies
2. Sink Marks
- Cause: Occurs when the inner sections of a part cool and contract faster than the outer layers, creating small depressions on the surface. This often happens with thick sections.
- Solution: Reduce wall thickness in problematic areas, increase holding pressure, and extend the cooling time. Ensure even material distribution and consistent wall thickness.
Source: Sybridge Technologies
3. Flow Lines
- Cause: Visible streaks or patterns appear on the surface of the molded part, caused by improper flow of the molten material within the mold. This often happens due to slow injection speeds, or low mold or material temperature.
- Solution: Increase the injection speed and pressure, adjust mold and material temperatures, and ensure proper gate placement to improve material flow.
Source: Sybridge Technologies
4. Flash
- Cause: Excess molten material leaks out of the mold cavity, forming a thin layer or “flash” along the parting line. This usually happens if the mold is not clamped tightly or the injection pressure is too high.
- Solution: Ensure proper clamping force, reduce injection pressure, and check for wear or damage on the mold surface that could allow material leakage.
Source: WayKen Rapid Manufacturing
5. Short Shots
- Cause: Incomplete filling of the mold cavity results in a part that is missing sections or has incomplete areas. This is often caused by insufficient injection pressure, a clogged gate, or inadequate material flow.
- Solution: Increase injection pressure, ensure the material is properly melted, and check for blockages or obstructions in the mold’s gates or runners.
Source: Sybridge Technologies
6. Burn Marks
- Cause: Dark or blackened spots on the molded part caused by trapped air overheating and burning the material. This often occurs when air cannot escape from the mold cavity.
- Solution: Add proper venting to the mold, reduce injection speed to prevent air from being trapped, and adjust mold temperature to avoid material degradation.
7. Jetting
- Cause: A snake-like pattern on the surface of the part caused by the molten material “jetting” into the mold rather than flowing smoothly. This happens when the material enters the mold too quickly.
- Solution: Reduce injection speed, optimize gate design, and increase mold temperature to help the material flow more smoothly.
Source: Sybridge Technologies
8. Weld Lines
- Cause: Visible lines where two flow fronts of molten material meet but don’t fully bond. This usually happens due to improper material flow, insufficient injection pressure, or low mold temperature.
- Solution: Increase mold temperature, adjust injection pressure and speed, and improve material flow by optimizing gate placement and design.
Source: Prototool
9. Voids
- Cause: Voids are typically caused by material shrinkage during cooling or insufficient packing pressure in thick sections of a part. They occur when the inner layers of a part contract and create empty spaces.
- Appearance: Voids are internal and appear as empty pockets or hollow spaces within the part, often invisible from the outside unless the part is transparent or is cut open.
- Solution: To reduce voids, increase packing pressure, adjust cooling time, and use materials with lower shrinkage rates.
Source: Prototool
10. Bubbles (Blisters)
- Cause: Bubbles are caused by trapped air, moisture in the material, or decomposition of the polymer due to excessive heat. They form when gas or air gets trapped inside the molten material or is released during heating.
- Appearance: Bubbles are generally visible on the surface or near the surface of the part and appear as raised, round blisters or small pockets of air.
- Solution: Properly dry the material before molding, reduce injection speed to prevent air from being trapped, and ensure the material doesn’t degrade by using appropriate temperatures.
Voids and bubbles are similar defects in injection molding but differ in their causes and characteristics. While both voids and bubbles are internal defects that create empty spaces, voids are caused by material shrinkage, while bubbles are caused by trapped air or gas.
11. Delamination
- Cause: Thin layers of the molded part peel or flake off, usually due to contamination of the material or improper material selection. It can also occur when incompatible materials are used together.
- Solution: Ensure that the material is pure and free from contamination, and avoid using incompatible materials during the molding process.
Source: Sybridge Technologies
12. Discoloration (Color Streaking)
- Cause: This defect results in inconsistent color or unwanted streaks on the surface of the molded part. It can be caused by contamination of the raw material with foreign particles, improper mixing of color additives, or degradation of the polymer due to excessive heat or shear.
- Solution: Ensure that the material is clean and properly mixed with colorants. Use appropriate temperatures to avoid overheating, clean the machine and mold thoroughly between production runs, and check for any remaining residue from previous molding cycles that could affect the color.
Source: BOYI Technology
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