The 5 Most Common Defects in Plastic Injection Molded Products & Their Causes

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Plastic injection molding companies can produce durable, high-strength, high-quality components. The process can be used for prototyping new designs and small-volume and high-volume production runs. Prototype injection molding can also shorten the design phase of new products or drive down the per-unit costs of established products by testing new part revisions. When non-experienced design and tooling engineers, or injection molding technicians don’t handle the process, the results can often cause an increase in plastic molded product defects, leading to defective or scrap product, unexpected additional tooling costs, or dissatisfied end-users. 

So, what are these defects that occur during the injection molding process? Let’s take a look at the five most common ones.

1. Burn Marks

Black or rust-colored blotches or unfilled features on plastic injection molded parts are commonly referred to as burn marks. In most cases, minor burn marks don’t impact the structural integrity of the finished plastic part. However, if those burn marks or discolorations are more prominent and continue unabated, they lead to material degradation. Most importantly, burn marks affect the aesthetics of products, making parts that require a nice appearance unusable. 


There is one primary cause for burn marks and several other leading contributors. The one main reason that burn marks occur is from trapped air and gases in the mold cavity that cannot vent out of the mold as the plastic is injected into the cavity. The trapped air becomes so compressed that it superheats and burns the plastic material that it touched. As long as air and gases can be vented out of the mold cavity completely burn marks will not occur. The fix for burn marks is to design proper venting into the mold’s design for any part features that are suspected to trap air or gases during the injection of plastic into the mold.

Now that you know the actual cause, and fix, we should also point out some leading contributors to burn marks that are found in the injection molding process. First, some conditions include molding at a higher temperature than the plastic resin is made for, or molding with plastic that hasn’t been dried properly. Both elevated material temperatures and wet material will lead to greater off-gassing of the plastic, which will amplify an already poor venting condition in the mold. Another process condition that can amplify poor venting conditions is injecting the molten plastic too fast into the mold’s cavity. Filling a mold fast is actually a desirable condition most of the time to reduce molding cycle times and achieve nice surface finishes. Unfortunately, a poorly designed venting system in a mold can prevent a technician from proper processing.

Main Cause: trapped air and gases in the mold cavity that cannot vent out of the mold as the plastic is injected into the cavity

Contributing Conditions:

  • Molding at too high of plastic temperatures
  • Molding with wet material
  • Injecting/Filling the mold faster than the venting design will allow

2. Flow Lines

You can quickly identify plastic flow lines on your injection molded parts as they appear as rings, light streaks, wavy lines, or uneven patterns on the surface of plastic injection molded products. Flow lines do not impact the plastic part’s structural integrity. However, flow lines are issues that customers can easily spot. It gives the appearance of a poorly molded component and for parts that require a nice appearance flow lines can be a big problem.


Flow lines are often caused when the molten plastic in the mold cavity encounters changes in speed or cavity pressure. As plastic flows into all of the part features within the mold it can encounter these speed and cavity pressure changes as it travels into pockets or ribs, around corners and hole features…etc. Some contributing causes are poorly designed gate locations or gate type used, incorrect plastic or mold temperatures being used, or incorrect fill speeds.

Main Cause: molten plastic in the mold cavity encounters changes in speed or cavity pressure as the mold fills

Contributing Causes:

  • Poorly designed or chosen gate type or location
  • Incorrect temperatures being used for plastic and mold
  • Too fast or too slow cavity fill speed

3. Warping

Warping occurs during the cooling process after the plastic injection molded part is ejected from the mold’s cavity. Warping can lead to uneven surfaces that appear as if the region has been slightly bent or deformed. This defect is easily spotted when the injection molded product is measured, assembled, or when the warped part is laid flat on a surface. 


The primary causes of warping on plastic molded parts are non-uniform cooling rates across the part, or non-uniform shrinking of the plastic across the part. The part design can be the root cause of warping if the walls of the part are designed as having varying thicknesses.  Also, some plastic materials shrink in different amounts according to the direction they flow into the mold cavity. These different shrink amounts cause stress in the part which warps the part’s surfaces.

Main Causes: non-uniform cooling rates and non-uniform shrink rates

Contributing Causes:

  • Non-uniform part wall thickness
  • Choice of plastic with non-uniform shrink values

4. Sink Marks

Sink marks are noticeable depressions on the surface area of a plastic part. They are often circular, where the material drops or recesses. These depressions are often easy to spot. Sometimes, it requires manipulation of the part where you tilt or move it while looking at the part surface in different lighting conditions. When you see a depression on a plastic molded part, it’s probably a sink mark.


Sink marks occur because the wall thickness under the sink is greater than the intersecting wall thicknesses. In this condition, after the part is molded the greater mass of plastic will shrink a greater amount thus leaving a sunken area or sink mark. Like other appearance defects, they are very undesirable on plastic molded parts that need to look nice. Fortunately, sink marks are very predictable, and plastic part designers can avoid them by making sure their part designs prevent them.

Main Cause: a mass of plastic at intersecting walls on plastic part designs shrinks more than the surrounding areas

Fix: design intersecting wall thicknesses to be 66% of the nominal appearance wall thickness

  • High Resin Molding Temperature
  • Obstruction or Poor Design of Sprue’s Orifice
  • Rapid Cooling
  • Low Injection Molding Pressure

5. Weld Lines

Lastly, weld lines often appear as streaks or lines on the surface of a plastic molded part. Weld lines are also often called knit lines on plastic molded parts. This issue occurs when two or more plastic flow fronts converge at a specific point on a part when the mold cavity is filled. They are referred to as weld lines because the two flow fronts try to “weld” or “knit” together under injection pressure. If molding conditions are not ideal there is a visible line where the weld occurred.


Again, molding temperatures and injection fill speeds play a role here. Usually molding at too low of plastic or mold temperatures for a given resin can cause weld lines. In addition, choosing a poor gate location to inject the plastic into the mold can cause weld lines. There are software tools available to plastic part and mold designers that can predict the location of potential weld lines. These predictive mold flow softwares can help designers choose the proper gate locations to avoid these defects before the mold is made.

Main Causes: molding with temperatures that are too low, poor choice of gating location

Prevent Defects in Plastic Injection Molded Products With Help From MSI Mold

The all-important step to avoiding or reducing the impact of these defects is to ensure you’re working with a well-established, professional plastic injection molding company. That’s where MSI Mold can help.

Founded in 2005, we’re a USA plastic injection molding company that offers comprehensive injection molding services to clients in many markets. Our capabilities include plastic injection mold design, prototyping, custom injection molding, assembly, and more. As a quality-focused company, we continuously improve our processes to reduce waste and ensure defect-free plastic injection molded products. Contact us today to learn more about our capabilities or to request a free design/project consultation.