When it comes to producing high-quality plastic parts, it is essenioal to avoiding defects like warping and sink marks. These imperfections not only affect the appearance of the final product but can also compromise its performance and functionality.
Fortunately, many of these issues can be minimized or even avoided entirely with proper planning and design. From material selection and part geometry to mold design and cooling time, several key factors play a role. Just as important is choosing a plastic injection molding company with the experience and expertise to guide you through these decisions and deliver consistent, precise results every time. Read on to learn more.
How to reduce sink marks and warping in injection molding?
Here’s a closer look at what causes these defects and how to minimize them for a better-quality end product.
What causes sink marks?
Sink marks typically occur when the outer surface of the plastic part cools and solidifies faster than the inner material. This happens most often in areas with thick sections, where the internal material continues to cool and shrink, pulling the surface inward and creating visible indentations.
Common causes include:
- Uneven wall thickness
- Insufficient packing pressure
- Inadequate cooling time
- Poor material selection
How to reduce sink marks
Design with uniform wall thickness
The number one rule in injection molding design is to aim for consistent wall thickness. Thick sections cool unevenly and are more prone to shrinkage. Where thickness is necessary (such as at ribs or bosses), coring out the section can help maintain uniformity.
Optimize gate placement and size
The gate is the point where molten plastic enters the mold. If it’s too small or poorly located, it won’t allow adequate packing pressure to reach thicker sections, leading to sink marks. Strategic gate design helps maintain consistent pressure throughout the part.
Use proper packing pressure and time
After the mold cavity is filled, the packing stage ensures that enough material is pushed in to compensate for shrinkage. Insufficient packing pressure or time can cause the inner material to collapse inward, so it’s important to make sure these parameters are optimized for your part.
Extend cooling time if needed
Rushing the cooling phase can leave inner material soft, allowing sink marks to form. Slightly increasing cooling time allows the entire section to solidify uniformly and reduces the risk of surface defects.
What causes warping?
Warping happens when different areas of the part cool and shrink at different rates, causing the part to twist or bend out of shape. This can be caused by:
- Uneven cooling across the mold
- Improper fiber orientation in filled materials
- Residual stress due to poor design or processing
- Non-uniform wall thickness (again!)
How to reduce warping
Use balanced mold cooling
Cooling channels must be evenly distributed throughout the mold. Uneven mold temperature leads to inconsistent shrinkage and internal stress, which causes parts to warp.
Watch material selection
Some materials are more prone to warping than others. For example, fiber-reinforced plastics can warp due to fiber alignment during flow. Choosing the right resin and understanding how it behaves is crucial.
Design for symmetry and consistency
Asymmetrical designs or abrupt changes in wall thickness invite trouble. It’s best to keep geometry simple and balanced where possible to avoid pulling the part out of shape during cooling.
Optimize ejection
Ejecting the part too soon or with uneven force can cause distortion. Ensure that ejection pins are placed symmetrically and that the part is fully cooled before removal.
Partnering with the right experts
Even with the best design and material choices, execution matters. Choosing a company with deep technical experience ensures your part is manufactured with the precision and process control needed to avoid these costly defects. A knowledgeable partner can offer insights during the design phase, troubleshoot mold issues, and fine-tune processing parameters, ultimately saving you time, money, and frustration.
