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5 Common Defects in Thermoplastic Injection Moulding and How to Fix Them

19 January, 2022

Advant Medical | 5 thermoplastic injection moulding defects

Injection moulding defects can sometimes prove expensive so we’re looking at 5 of the most common, and how to prevent them. While injection moulding is very impressive with a wide range of possibilities, there are some common defects in parts that are made through this process. These factors range from material differences in batches to machine calibration changes post-repair or maintenance.

Since injection moulding defects can sometimes prove expensive, operators must take extra care to prevent these. The defects covered here can be affected by the design, but they can be caused by other factors as well. Here’s 5 of the most common injection moulding defects and how to prevent them:

1. Flash

Flash, also called “spew” or “burrs”, is a thin layer of extra material that extends beyond the geometry of the part along the split line. Flash appears when material has flowed outside of the intended flow channels and into the space between the tooling plates or at the injector pin. There must be enough clamp tonnage to make sure the tool is not forced apart, creating unnecessary material flow into that area, thus creating a flashed part. It is also critical to have the correct injection pressures, speeds, mould temperatures and barrel processing temperatures to ensure a stable process to eliminate any flash issues. Flashing can also occur more frequently as the moulds age and begin to wear.

How to prevent this?

  • • Retool or redesign the mould if plates don’t fit together properly or allow material to flow outside the channel.
  • • Increase plate clamping force to confine material flow to the channel.
  • • Adjust mould temperature, injection pressure and ventilation to improve material flow.

Image courtesy of Paulson Training

2. Voids

These injection moulding defects are characterized by tiny air bubbles or vacuum pockets. One of the main causes of voids is inadequate moulding pressure to force trapped air out of the mould cavity. Other times, the material closest to the mould wall cools too quickly, causing the material to harden and pull the material toward the outside, creating an abscess. The material itself may be especially vulnerable to voids if its density changes significantly from the molten to hardened state.

Typically, these defects are often present on the surface of the component. While quality control experts may classify these as ‘minor’ defects, the size, density, location, and volume of the air pockets could be detrimental to the structural integrity of the finished product.

How to prevent this?

  • • Increase the moulding pressure and hold times to force out trapped air pockets.
  • • Maintain a wall thickness of less than 6 mm to limit the risk of abscesses.
  • • Place gates close to the thickest parts of the mould to prevent premature cooling where the material is most vulnerable to voids.
  • • Work with materials with lower viscosity to limit the risk of air bubble formation.

Image courtesy of QualityInspection

3. Warping

Warping is deformation that can occur in injection moulded products as a result of uneven shrinkage during the cooling process. Plastic can warp during the cooling process when uneven shrinkage puts undue stress on different areas of the moulded part. As such, it causes the part to bend, fold, or twist, in part or as a whole. This is evident in a part that’s meant to lie flat but leaves a gap when laid on a flat surface.

One of the main causes for warping in injection-moulded plastic and similar materials is that cooling happens too quickly. Often excessive temperature or low thermal conductivity of the molten material can worsen the problem. Other times mould design can contribute to warping when the walls of the mould are not of uniform thickness—shrinkage increases with wall thickness.

How to prevent this?

  • • Increase the cooling time to allow for gradual and even cooling to prevent uneven stresses on the material.
  • • Decrease the temperature of the mould or material.
  • • Use materials that are less prone to shrinkage during the cooling process. (e.g. particle-filled thermoplastics shrink much less than semi-crystalline materials)
  • • Apply new design techniques to incorporate uniform wall thickness and component symmetry to ensure higher stability.

Image courtesy of Autodesk

4. Short Shot

Does the potential partner have a track record in manufacturing medical devices? How long have they been in business? What type of products have they experience in manufacturing? What are their current assets and equipment? The potential partner’s knowledge of the industry is a value add that is often overlooked but this can be of the greatest worth to your company as an experienced CM partner can apply their past knowledge and experience to new projects.

How to prevent this?

  • • Redesign the mould with wider gates and channels or switch to low viscosity material for improved flow.
  • • Increase mould temperature to prevent it from rapid cooling.
  • • Increase injection speed or pressure to ensure the molten material reaches all cavities.
  • • Add air vents or enlarge the existing ones to allow trapped air to escape.

Image courtesy of NexPCB

5. Burn Marks

Burn marks typically appear as black or rust-colored discoloration on an edge or surface of a moulded plastic part. Burn marks result from the overheating of materials as they interact with high temperatures of mould, injection, nozzle, and even trapped air. Burn marks generally don’t affect part integrity, unless the plastic is burned to the extent of degradation. In cases where the burn marks cause material degradation, it could compromise the quality of the component. Excessive injection speeds or heating of the material often lead to overheating that causes burns.

How to prevent this?

  • • Lower the melt and mould temperature to prevent overheating.
  • • Reduce the injection speed to limit the risk of trapping air inside the mould.
  • • Enlarge gas vents and gates to allow trapped air to escape the mould.
  • • Shorten the mould cycle time so that any trapped air and resin don’t have a chance to overheat.

Image courtesy of Parallel design


In this article, we have explained some of the basic defects that can prove costly when utilising the Injection Moulding process for your component. Making a new injection tool is costly. Take the above 5 Common Defects into consideration when designing a tool and choosing a material. Keep in mind as you are developing your new tool:

  • • Mould Flow
  • • Excellent tool maker
  • • Material expertise
  • • Scientific approach to validation

Choosing an Injection Moulding partner with the experience and knowledge in the area is essential to the success of your project, and avoiding these common defects.

Advant Medical’s Thermoplastic Injection Moulding Expertise

Advant Medical is a specialist provider of Precision Injection Moulding Services to the Medical Device Industry. Our injection moulding team is ready to respond to your complex moulding challenge. We provide dedicated client support which will navigate through all stages of your injection moulding project including initial design, validation, production and supply chain management. Advant Medical’s injection moulding operates from our state of the art Class 8 Cleanrooms. Our specialist moulding team and advanced technologies ensure we can support a turnkey solution in product design, packaging and assembly of your Class I, II, III medical device. Our flexible and service-oriented approach has contributed to Advant Medical being a multiple ‘Supplier of the Year’ award winner.

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