Plastic Molding Defects

1. Plastic Molding Defects
Qutaibah Oudat
University of Cincinnati
Material Use
or Storage
Defect
Molding
Defects
Mold Design
or
Maintenance

3. A. Molding defects Process
1. Flow lines
Flow lines appear as a wavy pattern often of a slightly different color than the around area and generally on narrower
parts of the molded component (Heaney, 2018).
• Common remedies for flow lines in injection-molded products:
A. Increase the injection speed, pressure and material temperature to ensure the material fills the mold
before cooling.
B. Round the corners of the mold where wall thickness increases to help keep flow rate consistent and
prevent flow lines.
C. Relocate mold gates to create more distance between them and the mold coolant to help prevent the
material from cooling too early during flow.
D. Increase the nozzle diameter to raise flow speed and prevent early cooling.
2. Burn marks
Burn marks typically seem as black on an edge, exterior, or surface of a molded plastic part. Generally, the "Burn
marks" do not harm parts integrity, unless the plastic is burned to the degree of degradation (Heaney, 2018).
• Common remedies for Burn marks in injection-molded products:
A. Lower the melt and mold temperature to prevent overheating.
B. Reduce the injection speed to limit the risk of trapping air inside the mold.
C. Enlarge gas vents and gates to allow trapped air to escape the mold.
D. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat.

4. 3. Warping
Warping is deformation that can happen in injection molded products when various parts of a component
shrink unevenly (Mohan et al., 2017).
• Common ways to prevent warping in your molded parts:
A. Ensure the cooling process is gradual and long enough to prevent uneven stresses on the material.
B. Lower the temperature of the material or mold.
C. Try switching to a material that shrinks less during cooling (e.g. particle-filled thermoplastics shrink
much less than semi-crystalline materials or unfilled grades).
D. Redesign the mold with uniform wall thickness and part symmetry to ensure greater stability in
the part during cooling.
4. Vacuum voids / air pockets
Vacuum voids, or air pockets, are cornered air bubbles that surface in a finished molded component.
Typically, Quality control professionals consider the vacuum voids as a “minor” defect (Antczak, 1984).
• Common ways to prevent Vacuum voids in your molded parts:
A. Raise the injection pressure to force out trapped air pockets .
B. Choose a grade of material with lower viscosity to limit the risk of air bubbles forming .
C. Place gates close to the thickest parts of the mold to prevent premature cooling where the
material is most vulnerable to voids.

5. 5. Sink marks
Sink marks are small holes or cavities in an otherwise flat and consistent surface of a molded part. These
can happen when the inside part of a molded component recoils, pulling material from the outside inward
(Shen et al., 2007).
• Common ways to prevent the Sink-marks in your molded parts:
A. Increase holding pressure and time to allow the material near the part’s surface to cool.
B. Increase cooling time to limit shrinkage.
C. Design your mold with thinner component walls to allow for faster cooling near the surface.
6. Weld-lines
Weld-lines can arise on the surface of a molded part where the molten material has gathered after
splitting off into two or more directions in a mold (Jaworski, 2018). The hair-like weld line is the result
of weak material bonding, which lowers the strength of the parts of the final product (Jaworski, 2018).
• Common ways to prevent the Sink-marks in your molded parts:
A. Increase material temperature to prevent partial solidification.
B. Raise injection speed and pressure to limit cooling before the material has filled the mold.
C. Redesign the mold to eliminate partitions.
D. Switch to a material with a lower melting temperature or viscosity to allow faster flow and
prevent early cooling.

6. 7. Jetting
Jetting regards to a sort of deformation in a molded component that can happen when there’s an initial “jet”
of molten material injected into the mold cavity that begins to harden before the hole/cavity is filled
(Pugalendhi et al., 2020). Jetting often looks like a squiggly line in the surface of the finished component, and
it is leading from the initial port of injection (Pugalendhi et al., 2020). This can weaken the product part.
• Common ways to prevent the Jetting in your molded parts:
A. Reducing injection pressure to prevent rapid squirting of the material into the mold cavity.
B. Increasing material and mold temperature to keep the initial jet of material from solidifying early.
C. Designing the mold with the injection gate located such that the material is directed across the
mold, rather than lengthwise.
B. Defects that are related to material use or storage
1. Discoloration
Discoloration happens when a molded part is a different color than what is planned (Takahata et al., 2017).
Oftentimes, the discoloration is limited to a specific or particular area or a few stripes of strange color on
the molded part. This defect impacts the appearance of the model without lessening its strength or power
(Takahata et al., 2017).
• Common ways to prevent the Discoloration in your molded parts:
A. Ensure that workers properly clean the hopper, nozzle and mold between production runs to
eliminate any residual pellets or base material.
B. Consider using a purging compound to remove excess color from the machine.
C. Ensure you or your supplier is using a color agent with adequate thermal stability.
D. Ensure that the masterbatch is evenly mixed for consistent color output.

7. 2. Delamination
Delamination is a defect characterized by a flaking surface layer. It regarded as a moderately serious defect because
it diminishes the strength of the component (Peterson et al., 2018).
• Common ways to prevent the delamination in your molded parts:
A. Increase the mold temperature or pre-dry the material properly if excess moisture is an issue.
B. Ensure workers are properly storing and handling the resin pellets or base material to prevent
contamination.
C. Consider redesigning the mold with a focus on the injection nozzle to limit your dependence on release
agents.
C. Defects that are related to a poor mold design or maintenance
1. Short-shot
A short-shot happens when the flow of molten material does not completely fill the holes in a mold.
Consequently, the formed component will be incomplete after cooling (Kazmer, 2016). The short-shot might
look like an incomplete section in plastic shelves of a show or missing prongs on a plastic fork (Kazmer, 2016.
• Common ways to prevent the short-shot in your molded parts:
A. Redesign the mold with wider channels or gates for better flow.
B. Increase injection speed or pressure or choose a thinner base material to improve flow.
C. Increase mold temperature to prevent material from cooling too rapidly.
D. Add additional air vents or enlarge existing vents in the mold to allow trapped air to escape.

8. 2. Flash
Flash, also called “spew” or “burrs”, is an overflow of the molding material that seems as a thin lip or
bulge at the side of the component (Kazmer, 2016). Flash emerges because the material has flowed
outside of the designed flow channels and into the space between the tooling plates or at the injector pin
(Kazmer, 2016).
• Common ways to prevent the Flash in your molded parts:
A. Retool or redesign the mold if plates don’t fit together properly or allow material to flow outside
the channel.
B. Increase plate clamping force to confine material flow to the channel.
C. Adjust mold temperature, injection pressure and ventilation to improve material flow.
Conclusion
Mainly, manufacturing using injection molding requires an important upfront
investment in tooling. As such, it’s exceptionally important to design your mold
appropriate the first time, rather than begetting to start over after encountering
serious defects. Defects related to the molding process or material mind to be
easier and less costly to be solved. In addition, regardless of the cause, defects in
molded products can hurt your bottom line considerably.

9. References
1. Antczak, T. (1984). Vacuum injection molding process. In: Google Patents.
2. Heaney, D. F. (2018). Handbook of metal injection molding: Woodhead Publishing.
3. Jaworski, M. J. (2018). Numerical and Experimental Evaluation of Weld Line Strength Prediction in
Injection Molded Parts. University of Massachusetts Lowell,
4. Kazmer, D. O. (2016). Injection mold design engineering: Carl Hanser Verlag GmbH Co KG.
5. Lyashenko, V., Matarneh, R., & Sotnik, S. (2018). Defects of Casting Plastic Products: Causes,
Recurrence, Synthesis and Ways of Elimination. International Journal of Modern Engineering
Research (IJMER), 8(2), 1-11.
6. Mohan, M., Ansari, M., & Shanks, R. A. (2017). Review on the effects of process parameters on
strength, shrinkage, and warpage of injection molding plastic component. Polymer-Plastics
Technology and Engineering, 56(1), 1-12.
7. Peterson, K. W., Peterson, B. W., & Crawford, M. L. (2018). Low delamination mold release. In:
Google Patents.
8. Pugalendhi, A., Ranganathan, R., & Ganesan, S. (2020). Impact of process parameters on
mechanical behaviour in multi-material jetting. Materials Today: Proceedings.