10 common injection molding defects — what causes each one, how to identify it, and how to prevent it through design and process control.
Injection molding defects cost time and money. Some are immediately visible; others only show up in service under load, temperature cycling, or chemical exposure.
Understanding the root cause of each defect is the first step to preventing it. The cause is almost always traceable to part design, mold design, material handling, or process parameters. This guide covers 10 of the most common injection molding defects, including their root causes and potential fixes.
Key Troubleshooting Principle
Effective defect correction begins by determining whether the problem originates in the part design, mold design, material, or molding process. Process adjustments may temporarily reduce a defect, but they cannot permanently correct an underlying design or tooling problem.
Sink Marks
Sink marks are depressions on the part surface caused by localized volumetric shrinkage in thick sections during cooling. The surface skin solidifies first, while the core continues to shrink and pulls the outer surface inward.
Common causes: Thick walls or isolated thick sections, bosses or ribs thicker than 65% of the nominal wall, insufficient packing pressure, or premature gate freeze.
Potential fixes: Design ribs to approximately 50–65% of the nominal wall thickness, increase packing pressure and holding time, move the gate closer to thick sections, or increase mold temperature.
Warpage
Warpage is dimensional distortion that occurs during cooling or after the part is ejected. It is caused by differential shrinkage, with different areas or sides of the part shrinking at different rates.
Common causes: Non-uniform wall thickness, imbalanced mold cooling, semi-crystalline materials with high anisotropic shrinkage such as polypropylene or nylon, an excessive mold-temperature difference between the core and cavity sides, or ejecting the part before it has cooled sufficiently.
Potential fixes: Maintain uniform wall thickness, balance the cooling-channel design, use symmetrical part geometry, extend cooling time before ejection, and maintain balanced mold temperatures.
Flash
Flash consists of thin plastic fins that form along the mold parting line, around ejector pins, or near vent locations. It occurs when molten plastic enters gaps in the mold that should remain closed.
Common causes: Insufficient clamp force, worn or damaged parting surfaces, excessive injection pressure, contamination preventing the mold from closing completely, or inadequate venting that causes excessive cavity pressure.
Potential fixes: Verify that the machine provides adequate clamp tonnage, inspect and repair mold parting surfaces, reduce injection pressure or speed, clean mold-sealing surfaces, and adjust the molding process.
Short Shots
A short shot is an incomplete molded part caused by the plastic failing to fill the entire cavity. It is effectively the opposite of flash: the material freezes, encounters trapped air, or runs out before the cavity is full.
Common causes: Insufficient injection pressure or speed, material that is too cold or viscous, inadequate venting that traps air, an undersized gate, or an insufficient shot-size setting.
Potential fixes: Increase injection pressure and speed, raise the melt temperature, add or enlarge mold vents, increase the gate size, and verify the shot-size setting.
Knit Lines and Weld Lines
Knit lines, also called weld lines, appear where two separate melt-flow fronts meet and fuse. The polymer chains at the meeting point are generally less entangled than those in the surrounding material, making the line both a cosmetic blemish and a potential structural weak point.
Common causes: Melt fronts separating around a hole or core and then rejoining, multiple gates producing intersecting flow fronts, or a low melt temperature when the fronts meet.
Potential fixes: Reposition the gate so the weld line does not occur in a structurally critical area, increase melt temperature, increase injection speed, redesign the part to reduce divided flow, reposition holes or cores, or add overflow wells at the weld-line location.
Splay and Silver Streaks
Splay appears as silver or white streaks on the part surface, usually oriented in the direction of material flow. It is caused by moisture, steam, trapped gas, or material-degradation products escaping ahead of the melt front and becoming trapped near the surface.
Common causes: Inadequately dried material, excessive melt temperature causing resin degradation, material contamination, or excessive shear caused by undersized gates and runners.
Potential fixes: Dry the resin according to the material supplier’s specifications, verify moisture content before processing, reduce barrel temperature, clean the hopper and barrel, and enlarge restrictive gates or runners.
Flow Marks
Flow marks are visible ripples, rings, or wavy patterns on the molded surface. They develop when the leading edge of the plastic flow front begins to cool or freeze before being pushed farther through the cavity by incoming material.
Common causes: Low injection speed, low melt temperature, a cold mold, an undersized gate that causes jetting, or significant viscosity changes as the material moves through the cavity.
Potential fixes: Increase injection speed, raise the melt temperature, increase mold temperature, enlarge the gate, or reposition the gate to reduce the required flow length.
Burn Marks
Burn marks are areas of discoloration, typically dark brown or black, that commonly appear near the last location in the cavity to fill. They are usually caused by trapped air or gas overheating as it is rapidly compressed.
Common causes: Inadequate venting near the end of fill, excessive injection speed that compresses trapped air through the diesel effect, blocked vents, or material degradation caused by excessive barrel temperature.
Potential fixes: Add or enlarge vents near the burn location, reduce injection speed near the end of fill, verify that barrel temperatures are not excessive, and inspect the mold for blocked or contaminated vents.
Delamination
Delamination appears as thin surface layers that peel or flake away from the molded part, revealing a separate layer beneath. The result is a mechanically weak surface that may deteriorate during assembly or service.
Common causes: Resin contamination with an incompatible material, excessive moisture in the melt, or excessive injection speed that creates high shear and layered flow near the surface.
Potential fixes: Identify and eliminate the source of contamination, purge the molding machine thoroughly, clean the hopper and dryer, dry the material to specification, and reduce injection speed.
Voids
Voids are internal cavities or vacuum pockets that form within thick sections of a molded part. Unlike sink marks, which pull the outer surface inward, voids form when the solidified outer skin is strong enough to resist deformation while the interior material continues to shrink away from it.
Common causes: Excessive wall thickness, insufficient packing pressure to compensate for internal shrinkage, or premature gate freeze that prevents additional material from entering the cavity during packing.
Potential fixes: Core out thick sections to reduce wall thickness, increase packing pressure and holding time, increase gate size to delay freeze-off, increase melt temperature where appropriate, or add overflow wells.
Design Problems Cannot Always Be Processed Away
Defects associated with excessive wall thickness, abrupt geometry changes, poor gate placement, or inadequate venting may persist even after extensive process optimization. Addressing these risks during design for manufacturability can reduce troubleshooting time, scrap, and tooling modifications after production begins.
Frequently Asked Questions
Are injection molding defects caused by design or process?
Injection molding defects can be caused by both design and process conditions, and identifying the correct source is critical to finding an effective solution.
Design-related problems, such as thick walls that create sink marks or insufficient draft that interferes with ejection, generally cannot be permanently corrected through process adjustments alone. Process-related defects caused by incorrect temperature, speed, pressure, cooling time, or material preparation may be corrected without modifying the mold.
A systematic troubleshooting approach evaluates four primary areas: part design, mold design, material condition, and process parameters.
What is the most common injection molding defect?
Sink marks and warpage are among the most common defects encountered in production injection molding. Both are frequently associated with non-uniform wall thickness, uneven shrinkage, or inadequate packing and cooling.
Splay is one of the most common process-induced defects and is frequently caused by resin that has not been dried adequately before molding.
