Injection Molding Defect Troubleshooting Matrix

Identifying the root cause of an injection molding defect requires analyzing three distinct variables: the machine process parameters, the mold tooling design, and the original part CAD. This matrix provides our engineering team's standard operating procedure for diagnosing and resolving the four most common molding defects.

1. Short Shots (Incomplete Fill)

A short shot occurs when the molten plastic fails to completely fill the mold cavity, resulting in an incomplete part with missing features, typically at the end of the flow path.

• Process Causes: Injection pressure too low, injection speed too slow, melt temperature too low, or inadequate shot size (V-P switchover occurs too early).
• Tooling Causes: Inadequate venting (trapped air creates backpressure that stops flow), gates too small, or runners too restrictive.
• Design Causes: Wall thickness too thin for the selected material's flow length, or sudden transitions from thick to thin walls that freeze off prematurely.
• Solutions: Increase injection speed to utilize shear thinning; increase melt/mold temperature; add vents at the end of fill; increase gate size.

2. Sink Marks

Sink marks are localized depressions or dimples on the surface of a molded part. They occur when the inner core of a thick section shrinks as it cools, pulling the outer skin inward.

• Process Causes: Packing pressure too low, packing time too short (gate freezes before cavity is fully packed), or cooling time too short.
• Tooling Causes: Gate located on a thin section (freezes before thick section is packed), or gate too small to allow sufficient packing volume.
• Design Causes: The most common cause. Ribs or bosses that are too thick relative to the nominal wall. Rule of thumb: Rib base thickness should not exceed 60% of the nominal wall thickness.
• Solutions: Increase pack pressure and time; core out thick sections in CAD; relocate gate to the thickest part of the geometry.

3. Flash (Burrs)

Flash is excess plastic that escapes the mold cavity and forms a thin web along the parting line, ejector pins, or slide interfaces.

• Process Causes: Injection pressure too high, clamp tonnage too low for the projected area of the part, or melt temperature too high (viscosity too low).
• Tooling Causes: Worn parting lines, inadequate support pillars causing mold plate deflection under pressure, or poorly fitted inserts/slides.
• Design Causes: Projected area of the part exceeds the clamp tonnage capacity of the selected molding machine.
• Solutions: Decrease injection pressure; increase clamp tonnage; verify parting line seal with spotting bluing; rebuild worn tool surfaces.

4. Warpage

Warpage is the dimensional distortion of a part after it is ejected from the mold. It is caused by differential shrinkage—different areas of the part shrinking at different rates or in different directions.

• Process Causes: Uneven mold temperatures (core vs. cavity), cooling time too short, or excessive packing pressure causing high residual stress.
• Tooling Causes: Poor cooling channel layout resulting in hot spots; inadequate ejection system causing mechanical distortion during part removal.
• Design Causes: Asymmetrical geometry, uneven wall thicknesses, or lack of structural ribs to resist shrinkage forces. Highly crystalline materials like HDPE and Nylon are highly susceptible to warpage.
• Solutions: Optimize cooling channel design for uniform temperature distribution; core out thick sections to maintain uniform wall thickness; use fixtures to hold parts flat during post-mold cooling.