During the process of injection molding, due to the poor exhausting condition of molds, the water vapor in the melt, the air in cavity, the volatile gas from volatile substance in raw materials as well as the material decomposition gas either remain inside the products to form air bubble or are stored in between the melt and surface wall of cavity to form air cavitation which causes inner concave on the surface of products.
Sometimes, at parts with thick walls, gas may also exist in the shrinkage porosity caused by insufficient shrinkage compensation. The occurrence of air cavitation and air bubble may further result in inadequate filling in the cavity and insufficient dwell, and may even lead to the rising of gas temperature due to the compression of melt, thereby occurring such phenomenon as degradation, discoloration or scorching of plastics around the air cavitation or air bubble.
As for injection raw material. If the raw material contains too much water, additive of decomposition gas and recycled materials may occur therein and thereby forming gas sources. Hence, the raw material of plastics should be strictly dried and barrels with exhausting function should be applied to the injection machine; preferably reduce or replace the additives easy to decompose, and reduce the use of remnant materials or recycled materials.
In the field of injection molding, cavitation is a problem that cannot be ignored. Cavitation refers to the process in which the material in the injection mold begins to solidify on the surface. Due to the relatively insufficient volume of the material inside the mold, a vacuum hole is formed. This phenomenon often occurs at the thickness change of the product and the injection port material part, which has a particularly significant impact on the appearance quality of transparent products. For opaque products, although the existence of cavitation does not affect its use function, the cavitation inside the product is still not ideal. Especially when cavitation is caused by moisture and volatiles, these pores tend to spread to every corner of the product, and the shape is usually relatively small.
1. Analysis of the causes of cavitation in injection molded products
Insufficient pressure in the injection mold: This is one of the important causes of cavitation. When the pressure in the mold is not enough to fully fill the mold cavity, it is easy for the material to be incompletely solidified, thus forming cavitation.
Water content and volatile substances in the material: If the injection molding material contains too much water or volatile substances, these components will be converted into gas during the injection molding process, and then cavitation will be formed in the product.
Excessive or uneven product thickness: Excessive or uneven product thickness may result in the material not being evenly filled during the curing process, resulting in cavitation.
2. Solutions to cavitation of injection molded products
Adjust injection molding parameters: Increase the injection pressure during injection molding and extend the holding time to ensure that the material can fully fill the mold cavity. At the same time, for plastic materials containing moisture and volatile substances, they must be fully dried and the temperature of the heating barrel must be appropriately reduced to reduce gas generation.
Optimize the design of injection ports and gates: Appropriately expanding the size of injection ports and gates can improve the flow properties of materials and reduce the occurrence of cavitation.
Eliminate over-thick parts of products: By optimizing the design, eliminate over-thick parts in the products, so that the materials can be evenly filled during the curing process, thereby avoiding the occurrence of cavitation.
3. The impact of material differences on cavitation
It should be noted that there are differences in the cavitation tendency of different materials. In particular, crystalline materials and occasions with thicker products are more prone to cavitation and other problems. Therefore, these factors should be fully considered when selecting materials and designing products.
4. Precautions for cavitation of injection molded products
Finally, it is worth mentioning that injection molded products should be allowed to cool slowly after being taken out. Too fast cooling speed may lead to uneven stress distribution inside the product, thereby increasing the risk of defects such as cavitation. Therefore, during the injection molding process, the cooling speed should be reasonably controlled to ensure the quality stability of the product.
Bubble defects in injection molding can significantly compromise the quality and integrity of plastic parts, leading to unsightly surface imperfections and weakened structural performance. These molding defects, caused by factors like inadequate material drying, improper injection parameters, and insufficient mold venting, can be costly and time-consuming to rectify. Understanding the root causes and implementing effective prevention strategies is essential for maintaining consistent production quality. This article delves into the origins of bubble defects, practical troubleshooting methods, and proven techniques to eliminate and prevent them, ensuring flawless results in plastic injection molding.
What Are Bubble Defect in Injection Molding?
Bubble defects in injection molding occur when trapped air or gas forms pockets within the molded plastic parts. These air bubbles can degrade the part’s performance, structural integrity, and aesthetics, making the molded part prone to cracking and other mechanical failures. They can either appear on the surface as visible depressions or remain hidden within the structure, compromising the strength of the part. Surface bubbles often appear as small depressions or pimples on the outer surface, while internal bubbles may be harder to detect without specialized tools.
Materials like polypropylene (PP), acrylonitrile butadiene styrene (ABS), and polyethylene (PE) are more susceptible to bubble defects, especially if the injection molding process is not optimized, due to their lower viscosity and flow characteristics.
How to Prevent Bubble Defects in Injection Molding?
Bubble defects in injection molding can significantly affect product quality and structural integrity. To eliminate or minimize these defects, it’s essential to address material handling, processing parameters, and mold design.
Material Handling and Preparation
Use a dehumidifying dryer or desiccant dryer to maintain low humidity levels, typically below 0.02% moisture content for sensitive materials, especially when working with hygroscopic materials such as nylon, ABS, and polycarbonate. Follow the manufacturer’s recommended drying time and temperature for each resin to achieve optimal results.
Optimizing Processing Parameters
Adjusting Injection Speed and Pressure for Optimal Filling: Adjust the injection speed to ensure smooth, laminar flow without turbulence. Typical settings range from 20 to 100 mm/s, depending on the material and part geometry. Likewise, pressure should be adjusted to achieve uniform filling without causing excessive material stress.
Controlling Melt Temperature to Reduce Gas Formation: Typically, ABS should be processed at 210-250°C, while polypropylene requires 160-200°C. Consistently monitoring and maintaining temperature stability across the entire shot ensures homogeneous flow and minimizes gas formation that could lead to bubbles.
Balancing Cooling Time and Maintaining Uniform Cooling Rates: Maintain uniform mold temperatures—preferably within ±2°C across the cavity. Cooling channels should be evenly distributed to avoid hotspots that might slow solidification and trap gas pockets. Use temperature controllers to maintain stability throughout the cycle.
Mold Design and Maintenance.
Incorporating Effective Venting Systems: Venting channels should be strategically placed at critical points where trapped air is likely to accumulate, such as the end of the flow path. Vents typically measure around 0.01 to 0.05 mm in depth to allow gas to escape without causing material leakage.
Mold Surface Treatment and Maintenance Practices: Applying nano-coatings or chrome plating on mold surfaces reduces friction and enhances material flow, minimizing turbulence and air entrapment. Additionally, maintaining a smooth and polished cavity surface prevents small imperfections from causing air pockets. Regularly clean mold surfaces to remove residues that could obstruct venting or cause flow inconsistencies.
Regular Inspection and Maintenance: Routine inspection of vent channels and cooling systems, clean clogged vents and inspect cooling lines, address any signs of wear or damage promptly.
This injection molding defect appears as black or rust-colored discoloration on the molded part’s surface or edge. Like flow lines, burn marks do not usually compromise the item’s integrity, but they may become a problem once the product is burnt to the point of degradation.
Burn marks are most commonly caused by the overheating of trapped air bubbles or resin in the mold during the injection. This happens due to either high injection rates or excessive heating of the material itself.
The most important prevention method for this defect is to reduce the melt and mold temperature, as well as the injection speed, of the molding tool. Moreover, manufacturers can avoid burn marks by adding exhaust systems and expanding the gas vents so that trapped air can escape easily during the low-pressure injection. Last but not least, they might try to shorten the molding cycle time (the injection and cooling time) so any trapped air or resin does not overheat.
Burn marks (also known as gas burns or dieseling) are black or brown discolorations that appear on molded plastic parts, typically near the end of the flow or in areas where air is trapped. They are caused by high-temperature degradation of the resin or combustion of compressed air in the mold cavity.
Even if minor in appearance, burn marks can:
Compromise product aesthetics. Cause structural brittleness. Lead to rejected parts or customer complaints.
Main Causes of Burn Marks
1. Trapped Air: Air not vented properly gets compressed during high-speed injection. Local temperatures exceed 300°C, igniting air or degrading resin.
2. Excessive Injection Speed: High-speed filling compresses air rapidly. Creates localized overheating and combustion.
3. Overheating of Melt: Barrel temperature or screw RPM too high. Degradation of plastic begins inside the injection molding machine.
Common Locations and Appearance
Burn marks tend to occur in: End-of-fill zones where the melt stagnates. Ribs, bosses, enclosed shapes with limited venting. Corners or dead zones without proper gas release. Weld lines where two melt fronts meet.
Appearance: Black or dark brown streaks. Surface dullness, blistering, or cracking. In some cases, part weakness or failure.
4 Effective Strategies to Prevent Burn Marks
1. Conduct Mold Flow Analysis
Use mold flow simulation software during mold design to identify: Trapped air locations. Weld line positions. Pressure and shear zones. Early simulation allows you to adjust venting and runner layout before steel cutting.
2. Standardize Machine Settings with a Parameter Database
Create a database of optimized injection settings by material: Injection pressure & speed curves. Back pressure, screw RPM. Barrel zone temperatures. This improves repeatability across batches and machines.
3. Overheating of Melt
Barrel temperature or screw RPM too high. Degradation of plastic begins inside the injection molding machine.
5. Strengthen Machine & Material Maintenance
Implement a preventive maintenance program: Regular screw & barrel purging. Heater band calibration. Thermocouple inspections. Raw material storage and inspection. Every rejected part due to burn marks incurs a hidden cost—in raw material waste, machine downtime, and, most critically, lost customer confidence. Burn marks may seem like a minor surface issue, but they often signal deeper process flaws.
By proactively addressing mold design, machine parameters, material handling, and preventive maintenance, manufacturers can significantly reduce defect rates, improve production consistency, and protect their reputation in competitive markets.
A burn mark is a sort of serious surface defect on injection-molded parts. In some cases, it can be difficult to reduce the burn marks by traditional methods. In this study, external gas-assisted injection molding (EGAIM) was introduced to reduce the burn marks, as EGAIM has been reported to reduce the holding pressure. The parts with different severities of burn marks were produced by EGAIM and conventional injection molding (CIM) with the same molding parameters but different gas parameters. The burn marks were quantified by an image processing method and the quantitative method was introduced to discuss the influence of the gas parameters on burn marks. The results show that the burn marks can be eliminated by EGAIM without changing the structure of the part or the mold, and the severity of the burn marks changed from 4.98% with CIM to 0% with EGAIM. Additionally, the gas delay time is the most important gas parameter affecting the burn marks.
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.
Design is one of the most important factors in avoiding part defects. It’s your earliest opportunity to avoid mistakes that can be costly both in regard to time and budget later on. Good design takes into account objectives including part function, aesthetic, manufacturability and assembly. Working with a knowledgeable design engineer and involving your injection molder early will help you find solutions to meet the needs of your specific part.
There are a number of important design elements to consider to ensure costly part mistakes are avoided:
Wall thickness:
Wall thickness is one of the most important factors with part design. The first rule of thumb is to determine the minimum wall thickness that will meet your design requirements. It is always good practice to work with your injection molder / design engineer to check thickness specifications for the material(s) you are considering for your part. Typical wall thickness ranges from .04 – .150 for most resins.
Important wall-thickness facts:
Thinner walls require easier flowing plastics
Longer flow lengths (distance from nozzle to the furthest corner of the part) may require thicker walls
Radius:
Sharp corners or angles can impede the flow of material. These abrupt transitions can cause the cavity to not fill or pack properly, creating a part with defects. Material flowing across a sharp corner creates stress in the plastic which can contribute to warp and dimensional instability.
Smooth corners that have a curve versus an angle are important in the injection molding process. The radius should be consistent on the inside and outside of the wall creating a uniform thickness. By incorporating this design element, the material will be able to flow through the cavity evenly.
Gate location:
The location where the molten plastic material flows into the mold part cavity called a gate. Every injection-molded part has at least one gate, and some have several gates. The placement of the gate can help ensure the cavity fills properly; however, it is best to have a uniform wall. Uniform wall thickness helps the mold fill and cool properly. In unavoidable situations, having a proper gate location can be a deciding factor in the success of a part. It is recommended that parts be designed with the gate in a location at which the melt enters the thickest section of the cavity to then flows out of a narrower region.
Draft:
Draft is an angle incorporated into the wall of a mold and the shape of the plastic part so the opening of the cavity is wider than its base. A plastic drinking cup is a good example of draft – it is smaller at the base than at the mouth so that the cup will come out of the mold. Draft is essential for injection molding.
Plastic heavily relies on mold draft in the removal of the part from the mold. When a part is designed without appropriate draft, removal of plastic parts is essentially impossible.
A design with sufficient draft is always considered to be a good practice. 1.5 degrees for a depth of 0.25mm is usually recommended by design engineers. General guidelines suggest that a draft angle of 0.5 degrees is recommended for core and 1.0 degree for cavity.
Surface textures also influence draft requirements. The more depth in a texture the more draft it requires. It is a good practice to determine the surface finish / texture requirements prior to starting you part design.
Ribs:
Ribs are used to help reinforce the overall part strength and support dimensional components of the design. Depending on the material used, ribs should be no greater than 2/3 of the wall thickness. Greater width could cause issues with the design and sinking may occur. To avoid this problem, a designer can often core out some material to reduce the shrinking. In addition, ribs cannot be too tall or too thin.
The height recommendations are generally no more than 3x the wall thickness. The corners should include radii and the height should include a draft (.5 to 1.5 degrees). The draft angle allows the part to be ejected from the mold.
Mold Flow Analysis:
When working with an experienced part designer and injection molder, mold flow analysis should be conducted before tooling production begins. Mold flow software can be used to evaluate the design to make sure it will produce the most consistent and highest quality parts from each cavity of the tool.
A virtual model of the mold is created and, using the known data and characteristics of the chosen material, the software is able to predict how the material will flow into the mold and its cavities. Different data points can be assessed, including pressure, fill time and melt temperature. Doing so allows for optimization of the process before tool production ever begins.