In injection molding, bubbles are not a common defect in general small workshops. Bubbles mainly occur in non-large transparent products; small transparent products are not prone to bubbles, while non-transparent products typically do not have specific requirements. Today, we will focus on bubbles in injection molding as we continue our exploration of molding defects.
If you would like to learn about other injection molding defects, click the link below for more information.
| Understand Different Injection Molding Defects | ||||
|---|---|---|---|---|
| Flash | Short Shot | Sink Mark | Warpage/Deformation | Burn Mark |
| Splay Mark/Silver Streak | Dark Spot/Black Speck | Flow Mark | Bubble | Weld Line |
| Color Difference/Uneven Color | Ejector Pin Mark | |||
What Are Bubbles in Injection Molded Products?
Bubbles form when gases in the mold cavity are trapped during the flow of the molten plastic. If these gases are not properly vented, they remain trapped, forming bubbles or resulting in voids where the cavity is not fully filled. This affects the subsequent molding effect and quality of the product.
For technicians, addressing bubbles in products often requires more than just reducing speed and melt temperature. Adjusting transparent products is one of the most challenging defects in injection molding.
Methods to Distinguish Between Bubbles and Vacuum Voids:
Bubbles and vacuum voids are different. Vacuum voids form due to uneven cooling and shrinking after the molding process is completed, often due to uneven wall thickness. Inside, there is actually no air, just sink marks. Bubbles typically occur at weld lines or at the end of the filling process due to excessive gases in the mold that are not vented during filling, particularly in larger products, resulting in large bubbles that produce a popping sound when the product is broken.
In summary, most bubbles are present when the mold opens. If there are no bubbles immediately after demolding, but bubbles appear after the product cools for a while, those are vacuum voids (sink marks). Vacuum voids often occur at thicker sections, usually appearing singly. Bubbles appear at the ends of products or at junction areas, often in multiples. Therefore, clearly understanding the type of bubbles can significantly help during the machine adjustment process.
Causes of Bubbles in Plastic Products and Solutions:
Excessive air incorporation during the screw plasticizing process.
Cause Analysis:
When the feed rate is too fast or the back pressure is too low during the screw plasticizing process, the material prematurely enters the plasticizing section of the barrel, trapping excessive air. During the metering stage, air mixes with the melt and cannot be vented through the gate and nozzle gaps. During injection filling, both gas and melt are injected into the mold cavity, forming bubbles.
Solutions:
1. Reduce the screw rotation speed.
2. Increase the back pressure according to processing standards.
Excessive moisture in the melt.
Cause Analysis:
1. Improperly stored or highly hygroscopic materials may contain too much moisture. If not sufficiently dried before molding, high-temperature hydrolysis occurs in the barrel, trapping gases in the melt.
2. Poor thermal stability of the plastic, use of loose-structured recycled materials, or incorporation of air inside the material particles.
3. Use of recycled materials exceeding the process standard ratio, generally not more than 20% of the raw material ratio.
Solutions:
1. Check if the barrel drying system is functioning properly and dry the materials thoroughly according to processing standards.
2. Appropriately reduce the barrel temperature.
3. Reduce the injection speed.
4. Increase the back pressure.
Material thermal degradation
Cause Analysis:
1. Excessively high barrel temperature settings (uncontrolled heating devices) leading to thermal degradation of the material.
2. Melt staying too long in the barrel, resulting in thermal degradation.
3. Excessive shear heat from too fast injection speeds during injection filling, typically occurring near the gate.
4. Excessive back pressure causing friction heat during screw rotation, leading to thermal degradation.
Solutions:
1. Appropriately reduce the barrel temperature.
2. Minimize unplanned shutdowns and shorten the molding cycle time. Generally, the melt should not stay in the barrel for more than five minutes. The barrel should be emptied before resuming injection.
3. Re-adjust the process parameters, reducing injection speed and pressure.
4. Reduce the back pressure.
Poor mold venting
Cause Analysis:
1. Incomplete mold venting, lack of necessary venting slots on the parting line, or blocked and deformed venting channels. Deep sections of the product lack necessary inserts and venting pins, causing the melt to accumulate and not vent during filling.
2. Bubble formation at the end of filling, particularly at corners (e.g., sliders).
3. Gas-venting type hot runners, excessive hot runner temperatures causing thermal decomposition and bubble formation.
4. Poor surface finish of the mold, creating high friction during melt filling, leading to material thermal decomposition.
5. Improperly chosen gate location or too small gate size, causing localized trapped gas bubbles due to poor mold venting.
Solutions:
1. Based on the location of the bubbles, add or enlarge venting slots to improve mold venting.
2. Improve mold structure, avoid sharp angles, and adopt multi-stage injection methods to control injection pressure and speed in sections, reducing pressure and speed at bubble-prone areas.
3. Lower the temperature of the hot runner heating coils, increase the back pressure to reduce the amount of gas drawn into the barrel, and increase the fill volume.
Improper injection molding conditions.
Cause Analysis:
1. The injection molding speed is too fast, preventing timely venting of gases in the mold and trapping them in the molten plastic, resulting in trapped gas bubbles.
2. Excessive barrel temperature, enhancing the flowability of the material beyond its actual flow properties.
3. Excessive back pressure, raising the melt temperature and thereby enhancing its flowability.
4. Excessive clamping pressure, locking the mold too tightly, causing gas accumulation and inability to vent.
Solutions:
1. Increase venting depth and adopt multi-stage injection, reducing injection pressure and speed at the bubble formation areas.
2. Set the temperature according to the material’s processing standards and detect the actual melt temperature if necessary, to reduce the potential for thermal decomposition of the melt.
3. Excessive back pressure can cause thermal degradation of the melt and bubble formation, while too low back pressure can also lead to bubble formation due to air incorporation. Set the back pressure value according to the material’s processing standards.
4. Reducing clamping pressure can significantly solve mold gas trapping issues, but it may also lead to other processing defects such as burns and flash.
Conclusion
Bubbles are a unique defect in transparent injection molded products. Since many plastic materials can exist in a transparent form, bubble issues are also common in injection molding factories. If you have any cases related to injection molding bubbles to share, please contact me at my email: firstmold88@gmail.com
