Burn Marks in Injection Molding: Causes And Solutions

1. Introduction

Injection molding is a widely used manufacturing process for producing high-quality plastic parts. However, one common issue that manufacturers face is the occurrence of burn marks. Burn marks refer to brownish or black spots that can appear on injection molded plastic parts. These undesirable marks indicate that the material has been overheated and decomposed in localized areas. Burn marks can detrimentally affect both aesthetics and performance. This blog post takes a closer look at the common causes of burn marks and potential solution.

2. What is Burn marks in Injection Molding?

Burn marks are the result of thermal degradation of the plastic material during the injection molding process. They can manifest as dark or black streaks, often accompanied by a burnt odor. These marks can vary in size and intensity, ranging from minor discolorations to severe blemishes that compromise the quality of the part.

3. What are the Causes of Burn Marks?

There are several root causes that can lead to the overheating that creates burn marks in injection molding:

High melt temperature:

Excessive barrel temperatures or molten plastic that is too hot can lead to burn marks. When the plastic material is heated beyond its optimal temperature range, it can undergo thermal degradation and result in discoloration and burn marks on the molded part's surface.

Long residence time:

If the plastic material remains in the barrel for an extended period, it can degrade and discolor, leading to burn marks. This can occur if the injection molding cycle is too long or if there are delays in the process.

Insufficient cooling:

Inadequate cooling can cause localized overheating in hot spots within the mold, resulting in burn marks. Insufficient cooling can be caused by factors such as inadequate cooling channel design, improper water flow rates or temperatures, or blockages in the cooling system.

Unbalanced filling:

Uneven flow fronts during the injection molding process can lead to shear heating in slower areas. This uneven flow can cause localized overheating and result in burn marks.

Inadequate venting:

Inadequate venting can trap gases during the injection process, creating dielectric heat spots. These heat spots can cause localized overheating and burn marks on the molded parts.

Solutions for Preventing Burn Marks in injection Molding

Optimize process parameters:

To prevent burn marks, it is important to optimize process parameters such as temperature settings and cycle times. Reduce barrel temperatures and melt temperatures to prevent excessive heating of the plastic material. Optimize the injection speed and cooling time to achieve a balance between filling the mold and allowing sufficient cooling. By fine-tuning these parameters, you can reduce the risk of overheating and burn marks.

Improve mold cooling:

Upgrading the cooling design of the mold is crucial for effective heat dissipation and preventing burn marks. Consider adding more cooling channels or optimizing their layout to ensure even and efficient cooling throughout the mold. Keep the water temperatures low to enhance cooling effectiveness. Adequate cooling will help maintain the temperature within the desired range and prevent localized overheating and burn marks.

Use venting:

Adding vents to the mold can help release trapped gases and prevent the formation of burn marks. Proper venting design and placement allow for the smooth escape of air or gases during the injection process. This helps to minimize the risk of localized overheating and burn marks. Ensure that the vents are appropriately sized and strategically placed to optimize their effectiveness.

Balance runners:

Uneven flow paths can lead to shear heating in slower areas, resulting in burn marks. Balancing the runners ensures that the plastic material flows uniformly into all cavities, reducing the risk of localized overheating. Adjust the dimensions and design of the runners to achieve balanced flow and prevent burn marks.

Change gate location:

The location of the gate, where the molten plastic enters the mold cavity, can impact the flow pattern and the risk of burn marks. Redirecting the gate location away from problem areas can help minimize shear heating and reduce the likelihood of burn marks. Work with mold designers to optimize gate placement for balanced flow and heat distribution.

Adjust clamping force:

Excessive clamping force can contribute to shear heating and the formation of burn marks. By adjusting the clamping force to a lighter level, you can alleviate the stress and reduce the risk of overheating. However, ensure that the clamping force is still sufficient for proper mold closure and part quality.

Use insulated runners:

Insulated runners help minimize heat loss from the molten plastic to the mold, reducing the risk of localized overheating and burn marks. Insulated runners can be made from materials with low thermal conductivity or by incorporating insulation materials into the runner design. This helps to maintain the desired temperature during the injection molding process.

Conclusion

Burn marks are a common injection molding defect, but they can be prevented through proper processing and an optimized mold design. Eliminating the root causes like excessive heat, uneven filling, and poor venting will result in better looking parts free of burn marks. Addressing any issues early on can help avoid more extensive mold rework down the road.

Alpine Mold is a professional injection molding company that specializes in producing high-quality plastic parts. With our expertise in mold design and processing, we ensure that burn marks are minimized or eliminated altogether. Contact us today to discuss your injection molding needs and let us help you achieve flawless, burn mark-free parts.