How do you design mold vents to prevent gas traps and burn marks?

1 What are gas traps and burn marks?

Gas traps and burn marks are common molding defects that occur when the air or other gases inside the mold cavity are not able to escape during the injection process. Gas traps can create internal voids, bubbles, or blisters in the molded part, while burn marks can cause discoloration, degradation, or scorching on the surface of the part. Both defects can weaken the mechanical properties, reduce the dimensional accuracy, and impair the visual appeal of the part.

2 Why is mold venting important?

Mold venting is the process of creating channels or openings in the mold that allow the air or other gases to escape from the mold cavity during the injection process. Mold venting is important because it helps to fill the mold cavity completely and evenly, reduce the injection pressure and cycle time, prevent gas traps and burn marks, and improve the quality and consistency of the molded parts. Mold venting also helps to prevent short shots, flash, sink marks, and other molding defects that can result from excessive or uneven pressure in the mold cavity.

3 How to design mold vents?

Mold vents can be designed in various forms to suit the shape, size, and complexity of the mold and the part. Edge venting, which involves making a gap or groove along the parting line or edge of the cavity or core, is a simple and effective method for venting. Pin venting, which involves creating small holes or pins in the cavity or core, is useful for venting specific areas or features of the part. Blade venting, which involves creating thin slots or blades in the cavity or core, provides more venting area and less resistance. Vacuum venting, which involves creating a vacuum system that sucks the air or gas out of the mold cavity during injection process, is effective for large or complex parts. However, it requires more equipment and cost, and may affect cooling and shrinkage of the part.

4 What factors to consider when designing mold vents?

When designing mold vents, there are several factors to consider, such as location, size, shape, and type. The location should be chosen based on the flow direction and pattern of the molten plastic, and the geometry and features of the part. The size should be determined based on the type and viscosity of the plastic material, as well as the injection speed and pressure. The shape should match the shape and contour of the part, while avoiding sharp edges or corners that can cause stress concentration or flow restriction. The type should suit the requirements and specifications of the mold and the part, providing optimal venting performance and efficiency.

5 How to test and optimize mold vents?

Mold venting is not a one-time process, but a continuous process that requires testing and optimization. Visual inspection can be done to check the appearance and quality of molded parts for any signs of gas traps or burn marks. Pressure measurement can also be used to measure the pressure inside the mold cavity during injection, and determine the adequacy and effectiveness of the mold venting. Additionally, trial and error can be used to experiment with different variables and parameters of mold venting, such as location, size, shape, and type of vent, to find the optimal combination and configuration to prevent gas traps and burn marks.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?