Bubble injection molding is a common defect that occurs when gas or air is trapped inside the plastic part during the injection molding process. Bubbles can affect the appearance, performance, and quality of the molded part, and can lead to customer dissatisfaction and product failure.
In this blog post, we will explain what causes bubbles in injection molding, how to identify them, and how to prevent them from happening. We will also provide some tips and best practices for troubleshooting bubbles in injection molding.
Table of Content
- What are bubbles in injection molding?
- How to identify bubbles in injection molding?
- What causes bubbles in injection molding?
- How to prevent bubbles in injection molding?
- How to troubleshoot bubbles in injection molding?
What are bubbles in injection molding?
Bubbles are round or oval-shaped protrusions that form on the surface or inside the plastic part during injection molding. They are caused by gas or air that is trapped in the molten plastic as it flows into the mold cavity.
Bubbles can vary in size, shape, location, and number depending on the type and severity of the defect. Bubbles can be classified into two main categories: gas bubbles and vacuum voids.
Gas bubbles are formed when gas or air is compressed and trapped inside the plastic part as it solidifies. Gas bubbles can be caused by high melt temperature, low back pressure, high decompression, venting issues, moisture content, material contamination, unmelts, or venturi effect.
Vacuum voids are formed when plastic shrinks and creates a vacuum inside the part as it cools down. Vacuum voids can be caused by thick or uneven wall sections, insufficient packing pressure, low melt temperature, high mold temperature, or material degradation.
How to identify bubbles in injection molding?
Bubbles can be detected by visual inspection or by using various testing methods such as ultrasonic testing, x-ray testing, or destructive testing.
Visual inspection involves examining the part surface for any signs of bubbles or blisters. Visual inspection can be done manually or with the help of automated vision systems. However, visual inspection may not be able to detect small or internal bubbles that are hidden from view.
Ultrasonic testing involves sending sound waves through the part and measuring the reflections or echoes. Ultrasonic testing can detect internal bubbles by identifying changes in sound velocity or attenuation. Ultrasonic testing can be done with handheld devices or with automated systems.
X-ray testing involves exposing the part to x-rays and capturing images of its internal structure. X-ray testing can detect internal bubbles by identifying changes in density or contrast. X-ray testing can be done with radiographic film or with digital detectors.
Destructive testing involves cutting, heating, or breaking the part to expose its internal structure. Destructive testing can detect internal bubbles by observing their shape and size. Destructive testing can be done with simple tools or with specialized equipment.
What causes bubbles in injection molding?
- High melt temperature: If the melt temperature is too high, it can cause gas generation from material degradation, moisture evaporation, or volatile release. High melt temperature can also reduce the viscosity of the plastic and make it more difficult to vent out the gas.
- Low back pressure: If the back pressure is too low, it can cause insufficient compression of the melt and allow gas to remain trapped inside. Low back pressure can also cause poor mixing of the material and result in unmelts or contamination.
- High decompression: If the decompression is too high, it can cause sudden expansion of the gas and create bubbles inside the part. High decompression can also cause material drooling from the nozzle and result in cold slugs or contamination.
- Venting issues: If the mold vents are insufficient, blocked, or poorly located, they can prevent gas from escaping from the mold cavity during injection. Venting issues can also cause excessive pressure buildup and flash formation.
- Moisture content: If the material has high moisture content, it can cause steam generation during heating and create bubbles inside
the part. Moisture content can be affected by material storage, drying, or regrind conditions.
- Material contamination: If the material is contaminated by foreign particles, dust, or moisture, it can cause gas generation or unmelts during heating and create bubbles inside the part. Material contamination can be caused by poor material handling, cleaning, or recycling practices.
- Unmelts: If the material is not fully melted or mixed, it can cause solid particles or lumps to remain in the melt and create bubbles inside the part. Unmelts can be caused by low screw speed, low back pressure, low barrel temperature, or poor screw design.
- Venturi effect: If the melt flow is too fast or turbulent, it can cause air entrainment or suction due to the venturi effect. Venturi effect can be caused by high injection speed, high injection pressure, narrow gates, or sharp corners.
How to prevent bubbles in injection molding?
- Melt temperature: Adjust the melt temperature to the optimal range for the material and part geometry. Avoid overheating or underheating the material and monitor the temperature profile along the barrel and nozzle.
- Back pressure: Adjust the back pressure to the optimal level for the material and part geometry. Avoid using too low or too high back pressure and monitor the pressure profile along the screw and nozzle.
- Decompression: Adjust the decompression to the optimal level for the material and part geometry. Avoid using too high or too low decompression and monitor the decompression profile along the screw and nozzle.
- Venting: Design adequate and proper venting for the mold cavity and runner system. Avoid venting near gates, corners, or thick sections and ensure venting is clear of flash, dirt, or grease.
- Moisture content: Control the moisture content of the material to the recommended level for the material type and grade. Use proper drying equipment and methods and avoid exposing material to humid conditions.
- Material contamination: Prevent material contamination by using clean and dry material and regrind. Use proper material handling, cleaning, and recycling practices and avoid mixing different materials or grades.
- Unmelts: Prevent unmelts by ensuring complete melting and mixing of the material. Use appropriate screw speed, back pressure, barrel temperature, and screw design for the material type and grade.
- Venturi effect: Prevent venturi effect by controlling the melt flow rate and direction. Use appropriate injection speed, injection pressure, gate size, and mold design for the part geometry and wall thickness.
How to troubleshoot bubbles in injection molding?
Bubbles in injection molding can be troubleshooted by following a systematic approach that involves identifying, analyzing, testing, and solving the problem. Some of the troubleshooting steps are:
- Identify: Identify whether the bubble is a gas bubble or a vacuum void by using visual inspection or testing methods. Identify where, when, how often, and how many bubbles occur on the part.
- Analyze: Analyze the possible causes of bubbles based on the bubble type, location, frequency, and number. Analyze the molding process parameters, mold design features, machine performance indicators, and material properties that may be related to bubbles.
- Test: Test different hypotheses of bubble causes by using short shots or trial runs. Test different combinations of process parameters, mold design modifications, machine adjustments, or material changes that may eliminate bubbles.
- Solve: Solve the bubble problem by implementing the most effective solution based on test results. Solve any secondary problems that may arise from changing process parameters, mold design features, machine performance indicators, or material properties.
Bubble injection molding is a common defect that can affect the appearance, performance, and quality of plastic parts. Bubbles can be caused by various factors related to gas or air trapping inside the part during injection molding. Bubbles can be prevented by optimizing the molding process, mold design, machine performance, and material properties. Bubbles can be troubleshooted by following a systematic approach that involves identifying, analyzing, testing, and solving the problem. We hope this blog post has helped you understand what bubble injection molding is and how to prevent it.