Injection Molding Defects and How to Control Them

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Injection molding defects affects performance, function and aesthetics of a part. Part design, injection mold design, Thermoplastic material and Injection Molding Process causes defects in injection molded parts. Injection Molding defects can be minimized by following Injection Molding Design Guidelines. This article covers various injection molding defects and how to control molding defects in 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.

Short shot defect in injection molding appears if sufficient material is not injected into the mold.

Causes of Short shot Defect during Injection Molding
  • Low temperature of molten plastic and injection mold during injection molding results in early cooling and solidification of molten plastic. 
  • Any resistance in the path of molten plastic flow results in short shot defects.
  • 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

If we can avoid above conditions, Short shot injection molding defects can be avoided in injection molded parts. We need to consider following points to control short shot defects..

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 in air trapping section.

Flow Marks / Lines

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

Flow marks create a striped pattern around the gates during molten plastic flow inside the Injection Mold.

Causes of Flow Lines Injection Molding Defect
  • Early material freezing near gate, Low material and mold temperature causes the cold material to run inside cavity. This partially solidified material takes the shape of flow lines on top part surface.
  • When plastic flows through variable wall thickness sections with low injection speed. It results in the solidification of plastic at different speeds. As a result, Flow Lines are created.
How to Avoid Flow Lines Injection Molding Defect

Following points are considered to avoid flow lines in injection molded parts.

Change in Part Design
  • Maintain uniform wall thickness.
  • Gradual change in wall thickness is recommended. Because it avoids any sudden change in material flow direction.
Change in Molding Conditions
  • Increase molten plastic temperature during injection molding.
  • 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 are small craters or depressions that are developed in thicker section of the injection molded parts.

Sink Marks are small craters or depressions that are developed in thicker section of the injection molded parts. They are not acceptable in part aesthetics surfaces.

Whereas Voids are pockets of trapped air inside or close to the surface of an injection molded part. Voids are not visible from outer surface.

Causes of Sink Marks and Voids Injection Molding Defects

Sink marks defects are caused due to thermal contraction of plastic during cooling.

During cooling cycle in injection molding. After outside material has cooled and solidified. Core material starts cooling. As a result of cooling in core material, shrinkage occurs in inner section. Shrinkage in inner section pulls the outer surface inward. This results in a sink mark.

If the outer surface is rigid, deformation of the skin may cause formation of a void in the inner section.

How to Avoid Sink Marks and Voids Injection Molding Defects

Following points can be considered to avoid shrinkage in injection molded parts.

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, increased holding pressure and 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 the air trapping section.

Warpage

Deformation due to uneven shrinkage in the different sections of injection molded part is known as warpage. Warpage defect in injection molded part, results in a twisted, uneven or bent part shape.

deformation due to uneven shrinkage in the different areas of injection molded part

Causes Of Warpage Defect

Warpage is observed in injection molded parts due to 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

Following points are considered to avoid warpage in injection molded parts.

Change in Part Design

  • Ensure uniform wall thickness. It helps in the flow of molten plastic 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.

Flash

Flashes occurs when molten plastic escapes from the mold cavity.

Flashes are observed in injection molded parts if molten plastic escapes from the mold cavity. This phenomenon occurs where two or more parts of injection mold meets. For example flash occurs at the parting line or ejector pin locations. 

After ejection, flashes remains attached to the finished product. And they have impact on product aesthetics.

Causes of Flashes in Injection Molded Parts
  • 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
  • With time injection molds stats causing flashes in injection molded parts.
  • 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.
  • Sometimes very old and damaged mold also results in flashes. Therefore it can be an indication of manufacturing new injection mold.

Knit / Weld Lines

Weld Lines appears in the same section of injection molded part, where flow of molten plastics meets with each other.

Weld / Knit lines are a section in the injection molded part. It appears in the same section of injection molded part, where flow of molten plastics meets with each other. Strength of the injection molded part is comparatively less in the weld line section.

Weld Lines also has an impact on product aesthetics.

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

Following points are considered to avoid weld lines.

Change in Part Design
  • Change part thickness.
  • Use less viscous or lower melting temperature injection molding plastic.
  • Avoid Cutouts in part design in critical areas.
Change in Injection Moulding Conditions
  • Increase 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 is known as surface delamination. These layers appear like coatings and can usually be peeled off.

Thin surface layers appear on the part due to a contaminant material is known as surface delamination.

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

Following points are considered to avoid surface delamination in injection molded parts.

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

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.

How to avoid Jetting in Injection Molding Parts

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. 

Conclusion

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

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