Authored By: SDI Plastics

Injection moulding is one of the most important and adaptable methods of producing components and products on a mass scale. It is a process that is quick, works with a wide range of polymers, and produces a finished product that is detailed, durable and flexible, depending on the requirements.

The plastic injection moulding process is highly technical and is often a complex process involved to produce the perfect part. There’s a lot of room for error, which can range from minor defects to major issues that could affect your product’s performance, desired design result, and safety.

This article highlights five common injection moulding issues and how to fix them.

Let’s look at the most common injection moulding quality problems, what causes them, and what you can do to avoid them.

1. Flow Lines

Flow lines are visible streaks or lines that appear on the surface of a moulded part. They are caused by the uneven cooling of the material during the moulding process, which results in areas of the part cooling faster than others and are often the result of variations in the cooling speed of the plastic as it flows in different directions through the mould. They also can occur when you’re creating pieces of differing thicknesses, as the thinner areas cool before thicker areas are fully filled, causing flow lines.

To prevent flow lines there are a number of factors to consider. The temperature of the molten plastic can be increased, the temperature of the mould raised, or injection speed and pressure can be reduced.

2. Sink Lines

Sink marks are depressions or dimples that appear on the surface of a moulded part. They are portions of the plastic part recessed from the remainder of the piece due to uneven cooling while in the mould. 

When the melt front enters the mould cavity, it touches the mould wall and begins to cool, contracting and pulling away from the mould. They are caused by the shrinkage of the material as it cools, which can result in uneven cooling and deformation of the surface. These flaws are aesthetic, but they are nevertheless unappealing.

Some of the most common sink mark fixes are:

• Increase the holding pressure to ensure the item remains in good contact with the mould as it cools.
• Increase the cooling time to ensure that the part solidifies in the correct location. 
• Create a function with thinner walls to reduce the cooling time required.

3. Warping

Plastic part warping (or warpage) is the distortion of a moulded part, which can occur when the material cools and shrinks unevenly. As different areas of the plastic cool at different rates, they begin to shrink, introducing uneven stresses, bending moments, and torsion into the part, resulting in deformations that were not anticipated when the feature was built. This can be caused by a number of factors, including uneven cooling, excessive packing pressure, and incorrect mould design. This can also result in some precision parts being out of tolerance, resulting in scrap and rework.

Some of the most common warping fixes are:

• Check that the part is cooling uniformly to shrink equally and not introduce any unequal strains.
• If feasible, design the portion with symmetry to stabilise the cooling.
• Lessen the temperature of the melted plastic to reduce the requirement for cooling.
• Select plastic materials that are less likely to shrink and deform. Noting that semi-crystalline materials are generally more prone to warping.

4. Air Traps

Air traps involve air that is caught inside the mould cavity. The formation of air traps in the completed product is one of the most problematic faults in injection moulding. When the air previously in the mould has no exit and is absorbed into the finished product, these air bubbles can cause structural and cosmetic flaws. However, if the air in the mould becomes too compressed and too heated, it can potentially burst, harming both the mould and the finished product.

Some of the most popular air trap fixes are:

• Reduce the cycle duration to prevent trapped air from compressing and igniting.
• To drive trapped air out of the cavity, increasing the back pressure setting will make the melt denser and help remove gases and minimise trapped air volume.
• Choose a substance with a lower viscosity to limit the development of air bubbles.

5. Short Shots

Short shots occur when the material does not completely fill the mould cavity, resulting in incomplete or underfilled parts. This can be caused by a number of factors, including incorrect material viscosity, improper venting, and inadequate injection pressure. These flaws might result in the mould having to be recut or rebuilt, as well as costly project delays and substantial financial and productivity losses.

To fix this issue, the injection pressure can be increased, the material viscosity can be adjusted, and the mould venting can be improved.

• Select a less viscous plastic with higher flowability. This plastic will fill the hardest-to-reach cavities.
• Increase mould or melt temperature so as to increase flowability.
• Account for gas generation by designing the mould so that gas is not trapped within the mould and is properly vented.
• Increase the material feed in the moulding machine or switch to a machine that has a higher material feed in the event that the maximum material feed has been reached.

Conclusion

It’s important to note that these are just a few of the common injection moulding issues that may arise during production, and there are many other issues that can also arise during this process.

It is always best to consult with an experienced injection moulding professional and technical team to diagnose and fix any issues that may occur during the manufacturing process.

With many years of experience we know that customers don’t want to be left in the dark; we like our customers to develop an understanding of the design and manufacturing processes that go into their products and the possible challenges that may arise.

If you would like to work with a trusted partner for your injection moulding requirements, please do hesitate to contact the team at SDI Plastics.