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What Causes Bubbles in Plastic Injection Molding?

2024-02-20
Uncover the reasons behind bubbles in injection molding. Learn to control conditions, fix mold defects, and choose the right materials for flawless products.

In the world of production and processing, the presence of bubbles in injection-molded products can directly impact the plastic injection molding process if not addressed promptly. But why do these bubbles appear in the first place?

 

When we talk about the bubble in an injection-molded part, we're referring to a round protrusion on the plastic's surface caused by air trapped inside. These bubbles aren't picky about where they show up; they can occur in thick walls or any other part of the product, and they come in various sizes and shapes. Let's uncover the three main reasons behind the appearance of bubbles and vacuum bubbles in injection-molded parts.

 

Reasons Cause Bubbles in Plastic Injection Molding

 

1. Improper Control of Injection Molding Conditions

 

Many factors in the injection molding process directly influence bubble formation. If the injection molding pressure is too low, the injection speed is too fast, the injection time is too short, or the material amount is not just right, it can lead to the generation of bubbles inside the plastic molded parts. Uneven or insufficient cooling, along with improper control of material and mold temperatures, can contribute to bubble troubles.

 

Especially during high-speed injection molding, inadequate gas discharge from the mold can leave excess gas in the melt, resulting in bubble formation. Adjusting the injection molding speed and mold pressure is crucial. Careful attention to parameters such as injection and holding pressure time, improving cooling conditions, and controlling material feed can prevent the occurrence of bubbles.

 

2. Injection Mold Defects

 

Issues with the injection mold itself can also be a culprit. Incorrect gate positions or too small gate sections, long and narrow runners, dead corners, or poor exhaust in the mold can cause bubbles or vacuum bubbles. Adjusting the injection mold's structural parameters, especially placing the gate at the thick wall of the plastic molded part, is essential.

 

When selecting a gate type, it's wise to avoid direct gates as they can cause prominent vacuum holes. If stuck with a particular gate form, adjustments can be made by extending holding time, increasing feed amount, and reducing the taper of the gate. Ensuring the gate section isn't too small, particularly when molding different shapes simultaneously, is vital to avoid air bubbles in larger plastic parts.

 

3. Raw Materials Falling Short of Requirements

 

If the injection molding raw material has excess moisture or volatile content, uneven particle sizes, or a significant amount of recycled material, it can introduce too much air during the feeding process, leading to bubbles in the plastic molded parts. Addressing this involves pre-drying raw materials, sieving out fine materials, substituting resins, and minimizing the use of recycled materials.

 

How to Avoid Bubbles in Plastic Injection Molding?

 

In the realm of injection molding, time is precious, and a minor hiccup like bubbling can quickly become a costly affair. The primary culprits behind bubbles are gas pockets or vacuum voids, both stemming from issues related to airflow.

 

To tackle this concern effectively, it's crucial to pinpoint the type of bubble you're dealing with. Speed is key because the faster you identify the bubble type, the sooner you can diagnose and resolve production slowdowns.

 

To distinguish between gas pockets and vacuum voids, a straightforward heat test can be employed. Simply use a hot air gun or a small lighter to gently warm the bubbled area until it softens. If the bubble expands, you're dealing with a gas pocket; if it collapses, it's a vacuum void.

 

Gas pockets often result from jetting, a front-flow problem. Non-vented core pins, causing venting issues, might be the culprit. Excessive decompression, linked to bubbling, can also occur. Moisture in the molding system is another potential troublemaker, leading to erratic airflow.

 

Material flow may also be a straightforward solution. While in the process, observe the melt flow pattern by running short shots. If the plastic jets around itself, creating a pocket, poor melt flow pattern is likely causing the bubbles.

 

Adjusting the gate location may be necessary to eliminate air trapping or encourage more uniform filling if melt flow is the issue.

 

When examining the purging of a regular shot, check if the problem stems from the barrel or screw. General-purpose screws with dimensions of 18:1 L/D or lower may be responsible for bubbles or blisters. Options include raising backpressure to 1000 to 1500 psi melt pressure or pulling a vacuum on the mold just before injection to remove air.

 

If troubles persist after addressing these issues, it might be time to disassemble. With hot runners, air might get sucked between plates due to the Venturi effect, pocketing in the melt and causing bubbling. Disassemble the machine, use a bluing agent near the drops (careful not to obstruct the flow path), and if the bluing agent shows up in the part, you've found the source.

 

As airflow is the root cause, a proactive routine maintenance plan, ensuring all heated parts vent properly, is key to keeping bubbles at bay. Follow this with periodic machine maintenance and regular random part inspections for smooth, trouble-free operations.

 

Conclusion


In conclusion, understanding and managing these factors are crucial for preventing the appearance of bubbles in plastic injection molding. By controlling injection molding conditions, addressing mold defects, and ensuring raw materials meet standards, manufacturers can enhance the quality of their molded products, keeping bubbles at bay.

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