Common Cosmetic Defects and How to Fix Them

Not only does your product have to be functional, but to a certain extent, it must look good, too. Injection molding is an excellent way to create plastic parts in bulk, but there are many problems that can cause cosmetic defects ranging from minor to major. Here are some of the most common cosmetic defects and how to avoid or fix them.

What Are Cosmetic Standards for Plastic Parts?

There are 5 different grade levels of cosmetic standards for plastic parts:

  • Grade 1: Clear transparent or highly polished
  • Grade 2: Low-grade polish, clear transparent or non-textured & textured
  • Grade 3: Low-grade polish, non-textured
  • Grade 4: Painted parts
  • Grade 5: Ink stamped, silk-screened, printed, or other decorated parts

Each grade has its own standards and allowances for defects, with Grade 1 having the highest standards and lowest allowance for defects.

Potential Injection Molding Defects and How to Avoid Them

Now, let’s talk about the most common defects and how you can fix or avoid them by keeping things in mind as you design your mold and run your first few batches of parts.


Flash happens when some molten plastic escapes from the mold cavity, protrudes over a part’s edge, and creates a small flap of thin extra plastic on the edge of the part.


Flash is most commonly caused by:

  • Worn-out molds
  • A low-cost or poorly-designed mold
  • The mold isn’t clamped together with enough force to withstand the molten plastic flowing through it
  • Excessive injection pressure


Depending on what is causing the flash, possible fixes include:

  • Weld and recut parting lines
  • Build molds with good straight locks
  • Ensure molds are properly cleaned and maintained, and replace them when they wear out
  • Make sure the mold is well-designed and well-made
  • Increase the clamp pressure
  • Decrease the injection speed

Sink Marks

A sink mark is a small depression or crater that may appear in thicker areas of your part when shrinkage occurs in the inner parts of the finished product.


Common causes of sink marks include:

  • Poor part design where thick wall sections exist in cosmetic areas
  • Low pressure in the injection molding cavity
  • Too-high temperatures at the tool’s gate
  • Thick areas of a part cooling slower than thinner areas


To avoid or fix sink marks:

  • Increase injection pressure
  • Lower mold temperature, increase holding time, or raise holding pressure
  • Ensure your part has a proper wall thickness, which is usually at least 40-60 percent of the part’s thickest section.

Flow Lines

Also called flow marks, flow lines are discolored lines, streaks, or waves near the gate part of the mold, where the molten plastic is injected. While they don’t affect functionality, flow lines may impact a part’s aesthetic appeal.


Flow lines can be caused by:

  • Variations in the plastic’s cooling rate as it flows through the mold
  • Lack of uniform wall thickness
  • Low pressure or slow injection speed


To help prevent flow lines, keep these things in mind while designing your mold:

  • Avoid sharp corners – round out corners when possible
  • Ensure all walls are the same thickness
  • Locate the gate in an area with thin walls

If you already have a mold and are experiencing flow lines, try these fixes:

  • Increase the injection speed
  • Adjust the temperature and pressure of the molten plastic so it can completely fill the mold before solidifying
  • Increase the nozzle size
  • Apply a lubricant to the surface

Knit Lines

Also known as weld lines, knit lines resemble cracks or hairline fractures on the surface of a plastic part, usually at the edge of a hole or indent or where two melt fronts converge. While usually a cosmetic issue, knit lines can create failing points on areas that receive stress.


Knit lines are caused by inadequate bonding between two or more flow fronts as a result of partial solidification of the molten plastic.


When designing your mold, keep these things in mind to help prevent flow lines:

  • Incorporate a single gate rather than multiple
  • Choose a material with low viscosity
  • Plan on drilling holes into parts rather than incorporating them into your mold design

If you’re currently having problems with weld lines, you can try:

  • Increasing the material temperature
  • Increasing the injection pressure and speed

You can learn more about avoiding knit lines here.


Voids are air bubbles trapped in or near the surface of an injection-molded part. While voids are primarily a cosmetic problem, multiple bubbles can weaken the part.


The most common cause of voids is insufficient molding pressure or parts with very thick wall section. They may also be caused by a mold with two halves that don’t align properly.


There are several ways to avoid or fix vacuum voids, including:

  • Increase injection pressure
  • Choose a material with low viscosity
  • Locate gates near the thickest part of the mold
  • Avoid sharp corners in your mold design
  • Make sure the mold parts are perfectly aligned
  • Design parts with thinner wall sections


Discoloration is when a product is a different color than it should be and is typically only apparent in one part of the product or as a few streaks on the surface of the part. Discoloration is a cosmetic issue that does not affect a part’s functionality.


Typical causes of discoloration include:

  • Residual material stuck in nozzle or mold
  • Leftover plastic pellets from a previous run in the machine
  • Coloring agent didn’t mix well with the raw material
  • Too much heat


To help avoid discoloration:

  • Regularly purge the machine to remove residual material or excess color
  • Thoroughly clean all parts of the injection molding machine before each new batch
  • Ensure your raw material is properly mixed with the coloring agent
  • Lower melt temperature


Warping is a deformation caused by uneven cooling and shrinking in different areas of the mold, resulting in a part that is uneven, twisted, or bent.


Warping is typically caused by different cooling rates in various parts of the mold, causing internal stresses. Glass filled materials have a higher propensity for warp as well.


The best ways to avoid warping are to:

  • Use materials that don’t shrink too much while cooling
  • Make the cooling process gradual
  • Design your mold with uniform wall thickness

Short Shots

Short shots are where the molten plastic doesn’t fill the entire mold cavity, resulting in an area with no plastic. Since they are incomplete, short shots are typically unusable since the product’s functionality is impaired.


Many different things can cause short shots, including:

  • Blocked gates
  • Material that’s too viscous
  • Mold isn’t at a high enough temperature
  • Not enough pressure during injection
  • Trapped air


There are many ways to prevent or fix short shots, including:

  • Choose a less viscous plastic
  • Increase the mold or melt temperature
  • Use larger gates
  • Increase the number of air vents
  • Adjust injection speed or pressure


Jetting is a type of deformation where the plastic solidifies in a way that shows the wavy folds of the jet stream on the surface of the part because the molten plastic failed to stick to the mold surface.


Jetting is caused by a jet of molten plastic entering the mold and cooling quicker than the rest of the material, usually as a result of high injection pressure or low melt temperature.


To avoid or prevent jetting:

  • Increase melt and mold temperatures
  • Reduce injection pressure
  • Increase the gate size

Burn Marks

Burn marks are discolorations (usually rust-colored or black) on the surface of injection molded parts. They’re usually harmless as long as the burn hasn’t degraded the plastic.


Burn marks are typically caused by overheated resin or trapped air bubbles due to excessive heating or injection speeds that are too fast.


The best ways to fix burn marks are to:

  • Reduce injection speeds
  • Lower melt and mold temperatures
  • Enlarge gas vents

Surface Delamination

Surface delamination is when thin surface layers start to peel away from the surface of the part. It may cause a part to weaken and lose functionality.


Surface delamination is usually caused by contamination of the raw materials with particles of a foreign substance. The contaminant can’t properly bond with the plastic, which causes the flaking. Delamination can be dangerous if the product is a crucial safety component. Another cause of delamination is that the product hasn’t dried properly and has moisture on its surface.


To prevent delamination:

  • Reduce or eliminate dependence on mold release agents by focusing on the ejection mechanism in the mold design
  • Ensure the material is dry before loading it into the hopper
  • Carefully handle and store raw materials to avoid contamination
  • Redesign the nozzle to help ensure contaminants can’t easily enter the mold
  • Increase mold temperature
  • Avoid using sharp corners in your mold design

Learn More or Get a Quote Now

Ready to get a quote? Want to learn more about how Universal Plastic Mold can help you avoid cosmetic problems with your parts? Click here or call 1-888-893-1587 today.

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