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During the injection molding process, when the molten plastic fills the mold cavity, the air inside the mold must be promptly expelled. If the air cannot be expelled smoothly, it will be compressed to form trapped air, which will affect the flow of the melt and may lead to defects such as burn marks and shrinkage cavities. This article will explain how trapped air formed and how their impact on the stability of injection molding and product quality.
What Is Trapped Air in Injection Molding
“Trapped air” refers to the situation where, during the injection molding filling process, there is still air or gas remaining in the mold cavity and has not been expelled.
Sources of Trapped Air
Trapped air may originate from several sources:
| Source | Description |
| Cavity air | Air initially present inside the mold cavity before injection |
| Moisture evaporation | The moisture in the plastic particles will turn into steam when heated |
| Material degradation | Resins that are overheated or degraded will also release gases |
| Air trapped by geometry | Air pockets caused by complex part shapes |
Air traps usually tend to accumulate at the convergence point of the melt front or in areas where structural constraints prevent the gas from escaping easily.
Common locations include:
- End-of-fill areas
- Deep ribs or bosses
- Thin sections
- Complex internal structures
If the venting from these areas is not smooth, the air will accumulate, which will subsequently cause serious production problems.
How Trapped Air Affects Injection Molding and Part Quality
When there is too much air in the mold cavity, it will directly affect the plastic filling process.
Flow Resistance
Trapped air increases resistance against the incoming melt flow. Instead of filling smoothly, the molten plastic must compress the trapped gas before advancing.
This leads to:
- Increased injection pressure
- Slower flow front movement
- Reduced filling efficiency
Gas Compression Heating
Compressed air can rapidly increase in temperature. Under extreme conditions, this temperature rise may burn the polymer or degrade the material.
Process Instability
Air entrapment often results in inconsistent production results. Even small variations in injection speed or material temperature can produce different defect patterns.
For OEM projects that require stable mass production, this instability will significantly affect product quality and output.
Common Injection Molding Defects Caused by Trapped Air
The compressed air is often the cause of various common injection molding defects.
Burn Marks
Burn marks appear as black or dark discoloration on molded parts. They occur when trapped air becomes compressed and heated to high temperatures during filling.
Typical burn mark locations include:
- Flow end areas
- Thin ribs
- Corners of cavities
Short Shots
Short shots occur when molten plastic fails to fill the entire mold cavity. Trapped air acts as a physical barrier that prevents the melt from reaching certain areas.
Surface Defects
If the gas accumulates near the surface, defects may appear on the finished product’s surface or it may become uneven.
Dimensional Instability
| Defect | Mechanism |
| Burn marks | Gas compression heating |
| Short shots | Air blocking melt flow |
| Surface defects | Gas trapped near surface |
| Dimensional variation | Uneven internal pressure |
Mold Design Factors That Lead to Air Traps
Many trapped air problems are actually caused by the design of the mold rather than the processing conditions.
Insufficient Mold Venting
If the vent channel is too shallow or improperly positioned, the gas cannot be discharged smoothly. Usually, the depth of the exhaust groove ranges from 0.01 mm to 0.05 mm, depending on the type of plastic material used.
Poor Gate Location
If the gate design is not reasonable, it will cause air to accumulate easily during the flow process, making it more difficult to achieve proper venting.
Complex Part Geometry
Parts with deep grooves, protrusions or closed structures tend to accumulate air during the filling process.
Multi-Cavity Mold Imbalance
In multi-cavity molds, differences in filling behavior can create localized air entrapment problems.
Before the mold manufacturing process begins, these issues need to be planned and designed in advance.
Process Conditions That Increase Trapped Air Problems
Although the design is important, the processing parameters also affect whether the gas can accumulate easily.
Injection Speed
Very high injection speeds may trap air because the melt front advances faster than air can escape.
Packing Pressure
Improper packing pressure can intensify gas compression inside the cavity.
Mold Temperature
Low mold temperatures can freeze the melt front prematurely, trapping air inside the cavity.
Material Drying
Moisture in plastic pellets can generate vapor during injection, increasing the amount of gas inside the mold.
Maintaining stable processing conditions can reduce these risks.
Engineering Solutions for Preventing Trapped Air
During the injection molding process, various methods are usually employed to reduce the admixture of air. For example:
Improved Mold Venting
Proper venting is the most effective solution. Vents are typically placed at:
- Parting lines
- End-of-fill regions
- Ejector pin locations
- Insert boundaries
Optimized Gate Design
Strategic gate placement can guide the melt flow in a way that pushes air toward vent locations.
Balanced Wall Thickness
Uniform wall thickness allows melt to flow more evenly, reducing the likelihood of air pockets forming.
Process Optimization
Adjusting injection speed, packing pressure, and mold temperature can help stabilize filling behavior.
These measures would be more effective if they were all taken into consideration during the early design stage.
How HingTung Helps Reduce Trapped Air Risks in Injection Molding
HingTung injection molding company can offer integrated mold manufacturing and injection molding capabilities during the molding process, which can help enhance production stability.
Large-Scale Manufacturing Infrastructure
HingTung operates a 50,000 m² modern factory with over 400 employees, providing stable production capacity for large OEM programs. The integrated production environment ensures that mold design, molding trials, and production validation can be coordinated efficiently.
Integrated Mold, Parts, and Injection Capability
The company has a dedicated mold workshop where the design, processing, assembly and testing of molds are all carried out internally. Relying on 49 high-precision CNC machines, Hongtong is able to manufacture complex molds while ensuring accuracy, thereby reducing trapped air risks.
Mature Injection Production System
HingTung operates multiple injection molding lines equipped with automated material feeding and part handling systems. The production system covers the entire manufacturing process:
- Raw material preparation
- Injection molding
- Initial inspection
Capability for Both Plastic and Silicone Products
In addition to plastic injection molding, HingTung also operates dedicated workshops for silicone molding, supporting both LSR and HTV materials.
Rapid Sampling and Assembly Support
HingTung provides rapid mold sampling services, with prototype samples delivered in as fast as 10 days. The facility also supports secondary assembly processes, enabling customers to evaluate complete product performance early in development.
Extensive OEM Industry Experience
HingTung has long-term experience supporting OEM projects across industries such as:
- Security systems
- Telecommunications equipment
- Smart home products
- Electronic devices
FAQs
1. What causes trapped air in injection molding?
When the air in the mold cavity cannot be expelled during the filling process, gas accumulation will occur. Common causes include poor exhaust, improper gate position, complex part structure, and too fast injection speed, which results in the melt advancing speed exceeding the speed at which the air can be expelled.
2. How does trapped air affect injection molding quality?
When the air is trapped inside the mold, it will affect the flow of the molten plastic and may cause burning, shrinkage cavities or surface defects. In severe cases, the compressed air will also generate internal stress, affecting the dimensional stability of the parts.
3. Where do air traps usually occur in injection molds?
Air traps usually occur at the junction of the melt fronts or at the end of the flow. Common locations include deep channels, thin-walled areas, complex cavities, and corners where gas is difficult to escape.
4. Can proper mold venting eliminate trapped air problems?
A reasonable mold venting design can reduce the accumulation of gas inside the cavity because it enables the gas to be smoothly expelled from the mold. Usually, exhaust grooves are set at the parting surface, the pin position, or the end of the filling area, which makes it easier to remove air and improves the stability of the filling process.
Conclusion
In injection molding, if air cannot be promptly expelled from the mold cavity, it will affect the plastic flow and may cause burning, shrinkage cavities or surface defects. By implementing reasonable exhaust design, optimizing product structure and maintaining stable processing conditions, these problems can be effectively reduced. For OEM projects that require stable mass production, please contact HingTung. They can help lower the risks caused by air retention through comprehensive mold design and production engineering.