Air traps, formed at the end of melt filling phase during injection molding, are defects in injection molded parts. During this phase, air within mold runner system and mold cavity is pushed forward by melt, ultimately forming high-pressure gas at the end of melt filling process. This prevents melt from further filling these trapped areas and completely replicating molded part's cavity contour. Many people may mistakenly believe this is a short shot problem rather than air traps because they experience similar defects in their molded parts.
Air traps generally manifest in two forms. As shown in Figure 1, the first type forms at the ends, edges, and corners of molded part; second type forms within molded part, as shown in Figure 2.

 

Figure 1: Type 1 Air Traps - Air trapped at the ends, edges, and corners of molded part

 

Figure 2: Type 2 Air Traps - Air trapped in the center of molded part

In injection molding process, each time mold opens, injection system ejects material from female mold and ejects part from male mold together with part. This air fills mold cavity. When mold begins next cycle, air in mold is sealed within cavity. At the start of filling, air in cavity is pushed forward and compressed, eventually concentrating in a certain area of cavity, generating sufficiently high air pressure that hinders further melt filling. This trapped air is ultimately trapped in this space.
For Type 1 air trapping, this air is pushed and compressed, ultimately becoming trapped at the ends, edges, or corners of molded part.
For Type 2 air trapping, it is a poor melt filling pattern, commonly known as racetrack effect. This effect occurs when melt flows faster in thicker areas of cavity than in thinner areas. Ultimately, melt from different directions traps air in interior of molded part. Figure 2 shows second type of trapped air. This is caused by a racetrack effect created by uneven part thickness design, resulting in air being trapped within molded part.

Based on root cause analysis above, it can be inferred that any remedial measure that helps expel trapped air from mold will help resolve trapped air problem.
With this in mind, by considering how to expel trapped air from mold or prevent it from being trapped, corrective/preventive solutions for venting issues can be developed by analyzing five aspects of injection molding engineering system: process conditions, mold design/ manufacturing/ structure, part design, plastic materials, and equipment.

<1> Run multiple short-shot tests (e.g., filling at 20%, 40%, 50%, 60%, ... 90%) and collect samples to determine which type of trapped air is likely in molded part. Is it Type 1, Type 2, or both?
<2> Reduce injection speed.
When encountering trapped air, simply adjusting process conditions is not enough to resolve it. Mold also needs a good venting system. Lower injection speeds result in longer melt filling times, giving trapped air more time to escape mold through venting system.

An effective venting system is likely the most effective way to release trapped air in mold cavity and reduce risk of air entrapment. Mold designers should design such venting systems, specifying and standardizing them on relevant mold drawings, rather than leaving it up to mold designer to create venting channels only after air entrapment occurs.
Advanced simulation technology allows for prediction of melt filling patterns within mold part cavities and locations of air entrapment on molded part. Venting channels can be designed and machined based on predicted air entrapment locations without waiting for short-shot test results mentioned earlier.
Mold designers can design venting channels in three ways based on location of trapped air.
<1> When trapped air is located at the edge of molded part surface, as shown in top example in Figure 1-1
Design venting channels on parting line surface of mold core and core molds, as well as near air entrapment location.
<2> When air is trapped in deep ribs of molded part, as in example at the bottom of Figure 1, design an insert at this location and design venting channels within insert.
<3> When air is trapped due to runway effect, as in example in Figure 2, design an ejector pin at air trap location and design venting channels.
In addition to designing runway channels at air trap location, it is recommended to design adequate venting channels along melt filling path. This allows some trapped air to escape from cavity early, reducing risk of air trapping. This also allows for use of high injection speeds without air trapping.
It is crucial to ensure that venting system is an open channel so that air exhausted from cavity can completely exit mold through channel. Maintaining a clean venting system prevents plastic material powder or residue from clogging venting channels after long-term production.

Regardless of gate design and location, there will be melt fill edges and edges at the end of filling phase; therefore, air trapping, as shown in Figure 1, is unavoidable. However, Type 2 air entrapment, as shown in Figure 2, can be avoided through proper part design.
Type 2 air entrapment is caused by uneven wall thickness in part, leading to a racetrack effect. The best solution is to achieve a uniform thickness distribution throughout part, eliminating racetrack effect.

If multiple plastic material options are available, it is best to select one that produces the least amount of residual powder and sediment during injection molding process. This residual powder and sediment can form deposits in venting slots shortly after production begins, clogging venting system and rendering it ineffective.

Injection molding machine may not be able to help solve air trap problem. However, if injection molding machine is equipped with an integrated vacuum pump system that extracts trapped air from mold before melt injection begins, it can be significantly helpful. In this case, a mold venting system becomes unnecessary. Instead, mold parting surface should be sealed, creating a completely enclosed cavity. When vacuum pump system is activated, only trapped air is extracted, while no ambient air is drawn in.