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Table of Contents
1.Introduction |
| 2.Types of Air Bubble Defects and How to Identify Them |
| 3.Main Causes of Air Bubbles in Injection Molding |
4.Effective solutions for preventing injection molding bubbles |
5.Case Studies: Solving Air Bubble Defects in Injection Molding |
6. Conclusion |
Injection molding is one of the most widely used manufacturing processes across industries such as automotive, medical, electronics, consumer goods, and packaging. With high efficiency, repeatability, and compatibility with a wide range of plastic materials and complex geometries, it is the go-to method for producing plastic components at scale. However, even with advanced equipment and optimized processes, injection molding defects remain inevitable—among them, air bubbles in injection molding are particularly common and troublesome.
These air pockets, visible as voids, surface blisters, or internal holes, can significantly compromise both the appearance and functionality of the part. Especially in sectors like automotive and medical, even minor defects may result in product failure, safety concerns, or customer rejection. Understanding the causes and implementing proper solutions to avoid gas-related molding issues is essential to ensure part quality, minimize scrap, and optimize production costs.
As a plastic injection mold manufacturer with 23 years of industry experience, Alpine Mold is committed to helping clients eliminate common molding issues, with air bubble control being one of our core technical strengths.
In injection molding, air bubbles are not a singular phenomenon but rather a series of injection molding defects triggered by multiple factors. Proper identification of each defect type is the first step in diagnosing the root cause and applying targeted solutions. Below are the four most common types of air bubbles in injection molding, along with how to recognize them:
Surface bubbles are among the most visible and easy-to-detect injection molding defects. They typically present as tiny blisters, dimples, waves, or areas of reduced gloss on the part’s exterior. In severe cases, they can cause surface distortion, resulting in poor aesthetics or even rejection—especially in high-gloss or transparent plastic parts.
The main causes of surface bubbles are related to raw material moisture, injection speed, and mold design for air release. If the plastic material is not properly dried and retains excessive moisture, the water will vaporize during injection, forming gas bubbles. Similarly, high injection speeds can cause molten resin to trap air that fails to escape in time. Additionally, poor gas venting design, especially in cavity ends or thin-wall sections, can cause air accumulation. Improper gate positioning may also lead to trapped air along the melt path.
To solve these issues, manufacturers should focus on three key areas: proper resin drying, optimizing the mold’s venting system, and fine-tuning injection parameters to ensure smooth and complete filling without air entrapment.
Internal voids are hidden defects that are not visible to the naked eye but can severely compromise a product's structural strength, airtightness, and long-term reliability. These defects are typically found at the center of the part or in thick-wall sections, and they often coexist with other issues such as sink marks or incomplete fill.
The main causes of internal voids include insufficient holding pressure or too short a packing time, which prevents the molten plastic from fully compensating for shrinkage during cooling. If the cooling rate is too fast, the surface may solidify before the inner areas are fully filled, leading to trapped air. In addition, a restricted or poorly designed flow path may prevent air from escaping, and high-viscosity materials make it harder for gas to vent properly.
These types of air bubbles in injection molding often require advanced inspection methods—such as ultrasonic testing, CT scans, or cross-section analysis—to detect. Effective prevention involves comprehensive optimization of process parameters, gate design, and material flow characteristics.
Vacuum voids are a specific type of injection molding defect caused by negative pressure forming inside the mold during the packing or cooling stage. These voids are usually smooth and regular in shape, and they are often misidentified as shrink marks. They are most common in parts with varying wall thicknesses or localized thick-wall areas, particularly when using high-melting or crystalline plastics such as POM or PA.
The primary causes include insufficient packing pressure or short holding time, which prevents adequate compensation for material shrinkage; poor mold filling in the end-of-flow areas; and uneven mold temperature settings that create inconsistent cooling rates. In thick sections, the core may cool more slowly and contract significantly, forming hollow zones if not properly packed.
To address vacuum voids, solutions include increasing packing pressure, extending holding time, adjusting mold temperatures, and improving the cooling system design. These measures help maintain sufficient back pressure during the cooling phase to avoid void formation.
Air traps occur when molten resin encloses air during the injection process and that air is unable to escape, resulting in trapped gas within the cavity. These gas venting problems in molds are common in complex part geometries, long flow paths, or designs with multiple runners and gates. The defect usually appears at the leading edge of the melt flow.
Contributing factors include poor or misplaced venting channels, insufficient gate quantity or improper gate location, and high injection speeds that quickly seal the cavity before air can be expelled. Mold cavities with sharp corners, dead zones, or enclosed features also hinder effective gas evacuation.
To eliminate air traps, manufacturers should improve the mold design for air release by adding vent grooves, air channels, or ejector pin vents. Adjusting the gate and runner layout also helps to facilitate better air displacement. In production, lowering the injection speed or refining the mold structure can quickly reduce the incidence of air entrapment and enhance molding stability.
Multiple factors can cause air bubbles in injection molding, typically involving materials, mold structure, process parameters, and product design. Key contributors include:
·Moisture-sensitive resins (e.g., ABS, PC, PA) not properly dried.
·Poor-quality regrind or uneven material blending.
·Improper material storage in humid environments.
Recommendation: Use dehumidifying dryers, control regrind ratios, and store materials in sealed containers.
·Inadequate venting, especially at flow ends or dead corners.
·Long runners or improperly located gates causing air entrapment.
·Poor machining precision limiting vent gaps.
Recommendation: Optimize mold design for air release, increase vent locations, and improve tooling precision.
·Excessive injection speed causing air entrapment.
·Low packing pressure or short packing time leading to vacuum voids.
·Incorrect barrel or mold temperatures affecting material flow and moisture evaporation.
Recommendation: Adjust speed, pressure, and temperature settings based on resin type and part geometry.
·Thick wall sections cool slowly and shrink significantly, creating voids.
·Sharp corners or enclosed cavities restrict airflow.
·Uneven wall thickness causes stress concentration and inconsistent cooling.
Recommendation: Redesign with uniform wall thickness, use radii instead of sharp corners, and facilitate better venting through geometry.
To effectively eliminate air bubbles in injection molding and reduce the occurrence of related injection molding defects, manufacturers must take a systematic approach that encompasses raw material management, mold design, process parameter optimization, and product structure. Below are practical strategies and implementation points for each aspect:
Moisture in raw materials is one of the most common causes of bubble defects. To prevent water vapor from generating gas during the injection process, hygroscopic plastics such as PA, PC, and PET must be thoroughly dried using a dehumidifying dryer. It’s critical to maintain moisture content within the recommended processing limits. In addition, the storage of such materials must be tightly controlled. Use sealed packaging, desiccants, or humidity-controlled storage cabinets to prevent reabsorption of moisture.
During production, the time between opening the packaging and feeding the material into the machine should be minimized to avoid prolonged exposure to air. Proper raw material handling reduces the risk of air bubbles in injection molding and stabilizes product quality.
A well-designed mold structure is fundamental to preventing air entrapment. First and foremost, the mold must incorporate an effective gas venting system. Vent grooves or air release holes should be strategically placed at the end of flow paths, sharp corners, and thin-wall sections to ensure trapped air can escape smoothly.
Next, the runner and sub-runner layouts should be optimized to avoid turbulence, backflow, or trapped pockets of air during cavity filling. Gate locations and quantity must be carefully determined to provide a smooth flow path for the molten plastic. For parts with complex geometries or long flow paths, an optimized mold design for air release is even more critical to eliminate gas venting problems in molds.
Properly setting injection molding parameters is key to improving filling quality and minimizing the risk of injection molding defects such as voids and bubbles. Increasing injection speed or pressure—within safe process limits—can reduce melt hesitation and avoid air entrapment.
Additionally, extending packing time helps ensure the melt continues to fill any shrinkage voids that develop during cooling. Barrel and mold temperatures should be fine-tuned based on the specific plastic material: if too high, they may cause degradation and off-gassing; if too low, they can increase melt viscosity and lead to poor flowability—both scenarios can trigger air bubbles in injection molding.
Product geometry plays a critical role in air bubble formation. Areas with excessive wall thickness can cool unevenly and experience greater internal shrinkage, often leading to vacuum voids or internal gas pockets. Sudden changes in wall thickness and poorly designed transitions may also generate stress and restrict cooling efficiency.
To address these issues, product designers should minimize unnecessary thick sections and avoid abrupt dimensional changes. Ensure gradual transitions in wall thickness and use radii instead of sharp edges where possible. Cooling channel design must also be optimized to promote consistent and efficient cooling across the entire part. These design improvements help reduce internal stress and mitigate injection molding defects related to poor heat dissipation and trapped gases.
Air bubbles in injection molding are a frequent challenge in production, affecting both appearance and structural reliability. The following two real-world case studies offer an in-depth analysis of root causes and demonstrate how adjustments to process parameters and tooling can effectively eliminate these injection molding defects, helping manufacturers develop targeted solutions.
A client commissioned Alpine Mold to manufacture a dashboard bracket for an automotive interior assembly. During final assembly, repeated issues of poor fit, clip deformation, and warping at joint surfaces were observed. Initial suspicion fell on dimensional deviation in the mold, but CT scanning and cross-sectional analysis revealed central vacuum voids within the product, measuring approximately 3–5mm in diameter. These voids were primarily located at rib intersections and thick-walled zones. Such internal defects not only weakened the structure but also posed a risk of stress cracking during use.
The part was a classic thick-wall injection-molded component with substantial wall thickness in key structural areas. After injection, the molten plastic underwent significant shrinkage during cooling. If the packing time was insufficient or if the ability to compensate for shrinkage was inadequate, voids were likely to form in the center of thick sections. In this case, the mold design for air release was relatively conservative, with only basic venting features along the parting line. During high-speed injection, trapped air could not be evacuated efficiently, further contributing to the formation of internal air bubbles.
·Increased mold temperature from 60°C to 70°C to slow cooling and promote better material compensation.
·Extended the packing time by 2 seconds to ensure complete filling during the cooling phase.
·Added auxiliary venting grooves at the rib bases to improve gas evacuation paths.
After these adjustments, the internal voids were fully eliminated, and structural integrity significantly improved. Both ultrasonic inspection and destructive testing confirmed the absence of delamination or hollow zones. The final product met all consistency requirements set by the automotive OEM. First-pass yield increased from 82% to 98.6%, and the rework rate dropped sharply.
A client engaged Alpine Mold to produce the housing for a high-end medical device using transparent PC (polycarbonate) material. During initial mold trials, the product surface displayed numerous tiny air bubbles, particularly in thin-wall areas and around reinforcing ribs. The client had strict optical appearance standards, and even the smallest defect could lead to full-batch rejection, risking delivery delays.
Polycarbonate is a highly hygroscopic engineering plastic. If not thoroughly dried, residual moisture will vaporize at high temperatures during injection, resulting in surface-level air bubbles in injection molding. In this case, the client's initial process parameters included a relatively short packing time. Combined with the large, complex surface area of the part, this led to inadequate shrinkage compensation during cooling, making the bubbles more visible.
Additionally, the mold featured a high-gloss finish, which magnified the visibility of even the smallest surface imperfections due to light reflection—raising the standard for cosmetic acceptability even further.
·Increased drying temperature from 90°C to 110°C and extended drying time from 2 to 4 hours.
·Lengthened the packing time by 1.5 seconds to improve fill stability and reduce residual stress.
·Adjusted the barrel temperature profile to achieve a more uniform melt flow.
Post-optimization, the molded parts showed no visible surface air bubbles, fully meeting the client's medical-grade visual inspection standards. The products passed 100% of high-intensity light tests. The yield rate exceeded 99%, and the client’s satisfaction significantly increased, with additional orders confirming the long-term effectiveness of the improvements.
Case Number | Application Area | Defect Type | Cause Analysis | Solution | Improvement Effect |
Case 1 | Automotive Interior Part | Vacuum voids | Thick-walled area shrinkage + poor venting + insufficient pressure maintenance | Increase mould temperature, extend holding pressure, add venting channels, and optimise gate location. | Internal bubbles eliminated, strength improved, pass rate increased to 98.6% |
Case 2 | Medical Device Housing | Surface bubbles | Raw material moisture content + insufficient pressure maintenance + high gloss reflection on mold surface magnifies defects | Enhanced resin drying, extended packing time, optimized melt temperature profile, and recommended matte mold surface treatment. | Product appearance met high medical-grade standards. No visible injection molding defects were detected under light inspection. Yield rate reached 99%, and customer satisfaction increased significantly. |
Air bubble defects in injection molding not only compromise the visual appearance of plastic parts but also reduce their structural integrity and sealing performance. This can result in functional failures or even product returns from customers. To effectively eliminate such injection molding defects, manufacturers must take a systematic approach—starting with raw material handling, followed by optimized mold design and precise process control.
At Alpine Mold, with 23 years of experience as a professional plastic injection mold manufacturer, we’ve built extensive frontline expertise in identifying, preventing, and resolving issues such as air bubbles in injection molding. We support our clients throughout the entire production cycle—from mold development and DFM analysis to trial runs and mass production—with consistent technical guidance.
If you're looking for a one-stop partner capable of delivering high-quality molds and reliable injection molding solutions, we’re here to help. Our team is committed to professionalism, efficiency, and genuine support—ensuring your project is in expert hands.
Contact us today to get customized mold design and injection molding services tailored to your needs.
