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Top 5 defects in injection molding

Plastic molded components produced by injection molding generate a variety of defects that require advanced vision systems to inspect in real time.  The key is to understand each defect , categorize it and to derive a traegy to find, measure and elimate it. This article looks at the top 5 defects common in injection molding.

Injection molding defects

Defects in injection molded products can generally be attributed to issues in one of the three following areas:

  • The raw materials fed into the mold
  • The mold, or
  • The injection molding process (such as the settings of the machine)

pl1.pngHere are five common injection molding defects to watch out for when inspecting injection-molded products, what the causes are and how you can avoid them:

Flow marks

Flow marks appear as a wavy pattern generally on narrower sections of the molded component. Molten plastic, for example, cools very quickly during the injection process and flow marks are evident when the injection speed is too slow. The plastic becomes partially solid and gummy while still filling up the mold, causing the wave pattern to appear. To avoid this, the manufacturer can increase the:

  • Injection speed
  • Nozzle diameter
  • Cylinder temperature, and/or
  • Injection pressure

For the sake of product inspection, flow marks might be considered “minor” injection molding defects if subtle and not on a prominent part of the item. You might consider a flow mark to be a “major” defect if it affects functionality of your product or obscures a logo.

Short shot

A short shot is an incomplete component which is caused by the flow freezing off before the mold is completely filled during the injection process. There are a few possible reasons why injection might leave a void in the mold, such as:

  • Flow restrictions, usually due to complex or frozen flow channels
  • Hesitation during injection, or
  • Inadequate venting causing a buildup of air pressure against the flow

You might see short shot in your plastic display in the form of incomplete compartments in plastic shelves. A simple example is short or missing prongs on a plastic fork. Short shots are a typical major defect in injection-molded products. 

Addressing short shots in manufacturing will depend on the cause. If the cause is related to fill rate or hesitation, raising the injection pressure, speed or temperature are possible remedies. If there is flow restriction, you may need to clear frozen flow channels or redesign the mold. And inadequate venting is best addressed by adding air vents near the end of the affected channel(s).

Burn marks

Burn marks in the plastic appear as black or dark red discoloration when the material burns during injection. Commonly considered a minor defect, a burn mark during the injection process might be caused by one or more of the following:

  • Overheating due to trapped air
  • Excessive injection speed, and
  • Excessive melt temperature

If you notice burn marks in your plastic components, there are a few possible corrective actions to take in the manufacturing process. Shortening the cycle time, lowering the temperature and/or slowing down the injection speed might prevent further burning. Trapped air can be resolved by ensuring adequate gas vents and gate sizes.


Flash is an excess of filler material that can appear as a thin lip or protrusion at the edge of a component. This excess material appears because material has flowed outside of the intended flow channels between the plates. Flash is generally subtle but might be considered a major defect if particularly obvious on a product. Some common causes of flash include:


  • Poorly fitting or designed molding plates
  • Insufficient clamp force, and
  • Improper ventilation

Molds should be checked to see that their plates fit together properly and only allow material to flow in the channels. Clamping force may need to be increased, since this force keeps the material from flowing outside the flow channels. Lastly, air vents may need to be added to prevent air pressure from forming and resisting the flow.

Unless the flash is so significant that tooling needs to be reworked, trimming the components is usually enough to ensure these are not evident on the finished product. If unacceptable flash is appearing on your products, you can ask your supplier to make sure it’s trimmed before shipping.


Discoloration is an injection molding defect that shows unspecified streaking or coloring in an injection-molded product. Discoloration is usually due to one of two causes:

  • Improper mixing of the master batch, the additive used for coloring material, or
  • Impurities introduced to the material during the molding process


If the master batch is not evenly mixed, you might see a streak of coloring in the end product. And you can have impurities introduced to a mold if the hopper, material feed area or mold plates of a machine are not cleaned properly prior to production. To avoid this issue, ensure that the manufacturer is properly cleaning the injection molding machine prior to producing your items.

Depending on severity, discoloration could constitute either a minor or a major defect. And there is no way to remove this defect through rework of the finished product.


Now that you have an idea of some of the common injection molding defects to look out for, you’ll be in a better position to assess the quality of your own products. You’re better prepared to review an inspection report of your product or inspect the product yourself. 

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Topics: Machine Vision