Injection Molding Defects and How to Control Them

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Injection molding defects affect part performance, function and aesthetics. Part and injection mold design, Thermoplastic material and Injection Molding Process affects molding defects in injection moulded part. Moulding defects can be minimized by following Injection Molding Design Guidelines. This article covers various injection molding defects and how to control them during design and manufacturing?

Short Shot / Short Molding

Short shot defect in injection molding appears if sufficient material is not injected into the mold. In other words if the injection mold is not filled completely.

Causes of Short shot Defect during Injection Moulding
  • Low Temperature of Molten Plastic or injection Mold results in cooling and solidification of molten plastic. 
  • Resistance in the path of molten plastic Flow.
  • Complex flow channels in injection mold.
  • Bad Runner design
  • Inadequate venting results in air traps.
  • Inadequate molten plastic supply to mold cavity.
How to Avoid Short Shots Defect during injection molding

Change in Part Design

  • Increase wall thickness or provide rib in the section where molten plastic is solidifying quickly.
  • Selection of low viscous plastic with higher flow-ability.

Changes in Molding Conditions

  • Increase injection plastic temperature and pressure.
  • Reduce mold clamping Force.
  • Increase core and cavity Temperature

Change in Injection Mold Design

  • Increase Gate and Runner Size.
  • Change gate position.
  • Avoid Air Traps.
  • Provide effective Air Vents where air is trapping.

Flow Marks / Lines

Flow marks creates a striped pattern around the gates during molten plastic flow inside the Injection Mold. These Flow marks/lines are off-toned in color.

Causes of Flow Lines Injection Molding Defect
  • Early material freezing near gate and Low material melting & mold temperature causes the cold material to run inside cavity. This partially solidified material takes the shape of flow lines on top part surface.
  • Plastic flows through varying wall thickness section with low injection speed can cause the plastic to solidify at different speeds. As a result, Flow Lines are created.
How to Avoid Flow Lines Injection Molding Defect

Change in Part Design

  • Maintain uniform wall thickness at maximum places.
  • Avoid any sudden Change in material wall thickness. This helps in avoiding any sudden change in direction and flow of material.

Change in Molding Conditions

  • Increase molten plastic temperature during injection moulding.
  • Increase injection speeds and pressure to ensure the cavities are filled properly.

Change in Mold Design

  • Optimize the runner system and locate gates near the thin walls.

Sink Marks and Voids

Sink Marks

Sink Marks are small craters or depressions that are developed in thicker section of the injection molded parts. Sink marks are not generally accepted in product aesthetics surfaces.

Whereas Voids are pockets of trapped air inside or close to the surface of an injection molded part.

Causes of Sink Marks and Voids Injection Molding Defects
Sink marks defects are caused due to thermal contraction of plastic during cooling. After the material on the outside has cooled and solidified, the core material starts to cool. Shrinkage of inner material pulls the outer surface inward. This results in a sink mark. If the outer surface is rigid, deformation of the skin may be replaced by formation of a void in the inner section.
    How to Avoid Sink Marks and Voids Injection Molding Defects

    Change in Part Design

    • Avoid Thicker Sections in Part Design. It ensures faster cooling. As a result Sink marks are reduced.
    • Select a material with reduced volumetric shrinkage. 

    Change in Molding Conditions

    • Increase dwelling time and pressure.
    • Lower Mold temperatures and increased holding pressure & holding time allows more adequate cooling and curing.
    • Reduce Nozzle Temperature.

    Change in Injection Mold Design

    • Change gate position towards thick wall section.
    • Increase cooling efficiency.
    • Provide effective Air vents near air trapping section.


    It is the deformation due to uneven shrinkage in the different areas of injection molded part. Warpage results in a twisted, uneven or bent part shape.

    Causes Of Warpage Defect

    Warpage is caused by non-uniform cooling of the mold material. Different cooling rates in different mold sections cause the plastic to cool differently. Therefore internal stresses are created. Releasing internal stresses, lead to warpage.

      How to Avoid Warpage Injection Moulding Defect

      Change in Part Design

      • Ensure part uniform wall thickness. That results in the plastic flows in a single direction.

      Change in Molding Conditions

      • Ensure cooling time is sufficiently long and that it is slow enough to avoid the development of residual stresses being locked into the part.


      This injection molding defect occurs when molten plastic escapes from the mold cavity. Generally this happens through the parting line or ejector pin locations. 

      Flashes cools and remains attached to the finished product.

        Causes of Flashes Injection Molding Defect
        • Flash can occur when the mold is not clamped together with enough force. Clamping force should be strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold.
        • Improper Venting
        • Very old or damaged injection molds can also cause flashes.
        • Over packing of plastic.
        • Excessive injection pressure can also force the plastic out through least resistance.
        How to Avoid Flashes Injection Molding Defect

        Change in Molding Conditions

        • Increase the clamp pressure.
        • Avoid Over-packing.
        • Ensure that the mold is properly maintained and cleaned.
        • Adopt optimal molding conditions like injection speed, injection pressure, mold temperature.

        Change in Mold Design

        • Verify mold parting line matching.
        • Sometime Very Old and damaged mold also results in creating parting lines.

        Knit / Weld Lines

        Weld / Knit lines are actually more like a plane than a line that appears in a part where molten plastics meet each other as they flow from two different parts of the Injection Mold. Molded part is weak in weld line section.

        Causes Of Knit / Weld Lines

        Weld lines are caused by the inadequate bonding of two or more molten material flow fronts due to partial solidification of the molten plastic.

        How to Avoid Knit / Weld Lines Injection Molding Defect

        Change in Part Design:

        • Change part thickness .
        • Use less viscous or lower melting temperature injection molding plastic.

        Change in Injection Moulding Conditions

        • Raise the temperature of the mold or molten plastic.
        • Increase the injection speed.

        Change in Mold Design

        • Changing gate position to make a flow pattern to be a single source flow. 

        Surface Delamination

        Thin surface layers appear on the part due to a contaminant material. These layers appear like coatings and can usually be peeled off.

        Causes of Surface Delamination Injection molding Defect
        • Reduce Shear Stress.
        • Excessive use of mold releasing agent.
        • Moisture in material.
        • Foreign unwanted material.
        • Incompatible material mixture.
        How to Avoid Delamination Injection Molding Defect

        Change in Part Design

        • Reduce shear stress by avoiding sharp corners in part design.

        Change in molding Conditions

        • Pre-dry the plastic properly before molding.
        • Increase the mold temperature.
        • Reduce the use of regrind material.

        Change in Mould Design

        • Reduce the use of mold releasing agents.

        Burn Marks

        Burn marks are discolorations, small, dark or black spots that appear on injection molded part surfaces.

        Causes of Burn Marks
        • Degradation of the plastic material due to excessive heating or by fast injection speeds.
        • Overheating of trapped air.
        How to Avoid Burn Marks Injection Molding Defect

        Change in Part Design:

        • Eliminate air traps by moving them in vented area.
        • Switch to a less viscous plastic. This will ensure less gas is trapped as air is able to escape more rapidly.

        Change in Injection Molding Conditions

        • Reduce injection speeds
        • Reduce mold and melt temperatures.

        Change in Mold Design

        • Optimize gas venting and degassing.


        Jetting refers to a molding defect. Where molten plastic fails to stick to the mold surface due to the speed of injection. Jetting leads to part weakness, surface blemishes, and other internal defects in injection molded parts.

        Causes Of Jetting during Injection Molding

        Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold. When the plastic comes in contact with the mold walls, it is rapidly cooled and the viscosity is increased. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the finished product.

        Remedies For Jetting Injection Molding Defect

        Change in Injection Molding Conditions

        • Increase mold and melt temperatures.

        Change in Mold Design

        • Locate the gate at the thickest part of the molding.
        • Optimize gate design to ensure adequate contact between the molten plastic and the mold. 


        To sum up, Part Design, material, injection mold design and molding conditions affects the quality of injection molded part. By controlling above parameters injection molding defects can be reduced.

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