Sink marks, warping, and short shots can ruin a plastic part. This guide explains how draft angles, gate placement, and uniform wall thickness prevent defects and improve part quality with Sundar Industries.
Injection molding is a highly efficient process, but it is also sensitive to design details. Many common part defects — such as sink marks, warpage, and short shots — can be traced back to design choices that impede the flow of molten plastic or cause uneven cooling.
At Sundar Industries, our tooling engineers have extensive experience in optimizing part designs for injection molding. Here are key design principles we recommend to avoid common defects:
Maintain Uniform Wall Thickness
Varying wall thickness is the leading cause of warpage and sink marks in injection-molded parts. When a thick section cools more slowly than a thin section, the part contracts unevenly, causing deformation. Design your parts with consistent wall thickness wherever possible.
Add Draft Angles
Draft is the slight taper applied to vertical walls to allow the part to eject from the mold smoothly. Without adequate draft (typically 0.5° to 2°), parts can stick to the mold or experience surface damage during ejection.
Design Proper Ribs
Ribs add stiffness without increasing wall thickness. However, ribs that are too thick can cause sink marks on the opposite surface. A good rule of thumb: rib thickness should be 40-60% of the nominal wall thickness.
Gate Placement Matters
The gate is where molten plastic enters the mold cavity. A poorly placed gate can cause weld lines, air traps, and flow imbalances. Position gates in thicker sections and away from critical aesthetic surfaces to ensure smooth filling and minimal defects.
Add Radii to Sharp Corners
Sharp corners create stress concentration points and impede plastic flow. Adding a generous radius (at least 0.5mm) distributes stress and improves mold filling.
Choose the Right Material
Different polymers flow differently. High-viscosity materials require larger gates and runners. Sundar Industries’ engineering team can help you select a material that matches your part design and performance requirements.
By following these guidelines, you can significantly improve the quality of your molded parts and reduce the risk of defects. Sundar Industries provides comprehensive DFM feedback for all injection molding projects to help you get it right the first time.
