Common Causes and Solutions for Burn Marks (Trapped Gas) and Poor Venting in Injection Molding

In the injection molding process, burn marks (trapped gas) and poor venting are common and challenging issues. These problems not only affect product quality but can also lead to lower production efficiency and increased costs.

I. Causes of Burn Marks and Poor Venting

1. Mold Design
The structural design of the mold has a significant impact on the venting performance during injection molding. If the parting line is not properly selected, gases may not be discharged smoothly. For example, if the parting line is located at the flow end of the molten plastic, and there are no adequate venting channels, gases tend to get trapped in the cavity.

In addition, the design of mold cores and inserts is also critical. For complex-shaped cores and inserts, if no proper venting solutions are applied, gases will accumulate during injection, causing burn marks and trapped gas defects.

2. Injection Molding Process Parameters
Improper process parameter settings are another major cause of these problems. If the injection speed is too fast, the molten plastic flows rapidly into the cavity, leaving insufficient time for the trapped gas to escape. This gas gets compressed in the cavity, causing the temperature to rise sharply, which eventually results in burn marks.

Excessive injection pressure also makes it difficult for gases to escape, further worsening the problem. Additionally, excessively long holding time or excessive holding pressure can trap gas inside the molded part, affecting product quality.

3. Material Properties
Different plastic materials have varying flow characteristics and gas release behaviors. Some plastics with poor flowability require higher pressure and speed during molding, increasing the risk of trapped gas.

Furthermore, certain plastics decompose and release gases during processing. If these gases are not properly vented, they contribute to burn marks and poor venting. For example, materials containing moisture or impurities may release steam or other gases at high temperatures.

4. Inadequate Mold Venting System
An effective mold venting system is crucial for solving venting problems. If the venting slots are too small, gases cannot escape smoothly. Generally, venting slot depth should be around 0.02 - 0.05mm, and width should be 3 - 6mm. If these dimensions fall outside the recommended range, effective venting will be difficult to achieve.

The position of the venting slots is also important. If they are not placed in areas where gas tends to accumulate, they will fail to work efficiently. Additionally, if the mold surface is too rough, gas flow will encounter higher resistance, further hindering effective venting.

II. Solutions and Improvements

1. Optimize Mold Design
Re-evaluate the parting line design to ensure it is located where gas can be easily vented. Use mold flow simulation software to predict the plastic flow path and potential gas accumulation areas. This analysis can guide optimization of the parting line.

For complex cores and inserts, design dedicated venting structures, such as adding venting holes to the core or using porous steel, to allow gases to escape smoothly.

2. Adjust Process Parameters
Reduce injection speed appropriately to ensure the molten plastic fills the cavity smoothly, allowing sufficient time for gases to escape. Adjust injection pressure to a reasonable level to avoid trapping gas.

Through process trials, determine the optimal holding time and holding pressure to minimize defects caused by trapped gas during the holding stage.

3. Material Pre-treatment
Thoroughly dry raw materials to remove moisture and ensure they meet the required moisture content before processing. Use high-quality materials with minimal impurities to reduce the generation of gas from the source.

4. Improve the Mold Venting System
Design venting slots based on the actual mold structure, optimizing their size and position. Increase the number and depth of venting slots if necessary to ensure smooth venting.

Regularly clean the venting slots to prevent blockages caused by plastic residue or oil contamination. For areas where conventional venting slots are insufficient, consider using vacuum venting technology by installing a mold vacuum system.