Analysis of Bubbles Generated in the Plastic Product Manufacturing Process
Causes of bubbles in plastic product and solutions
Bubbles often appear at the final seams or thicker sections of plastic products. Initially, when the product is inside the mold, the surface cools down first, leaving the central portion still in a hot state.
As the central portion cools down, it contracts, but since the plastic surface has already cooled, the contraction can only happen internally, leading to the formation of bubbles. Many thick parts may initially come out of the mold without bubbles, but after several seconds, bubbles may appear in the central portion.
There are several scenarios for bubbles, each with different manifestations and causes, which are explained as follows:
1. Thicker products exhibit surface swelling and bulging with bubbles visible when cut.
The possible reasons for this issue may include:
- Injection speed is too fast, trapping air that cannot be expelled, forming
- Incorrect material dosage, whether excessive or insufficient, along with high injection pressure, can result in this issue. Adjusting the material dosage appropriately can improve this condition.
- Inadequate cooling time, especially for thick-walled products. This can be addressed by external cooling using cold water and cold molds.
- Presence of decomposition lines in the product due to excessively high temperatures. If the issue is not severe, you can lower the temperature as a remedy. If the entire barrel is decomposed, disassembly and cleaning are necessary.
- Small runner gates and poor venting in the mold can also contribute to this problem.
2.Solid products exhibit voids and show a blackening phenomenon.
When molding solid products, it’s essential to maintain low injection pressure and a slow injection speed. Otherwise, air can be trapped within the product at high pressure, causing friction-induced burning, resulting in black voids within the product. Therefore, reducing injection pressure and injection speed appropriately can eliminate the voids. Additionally, it’s beneficial to check if the material dosage is excessive and reduce it as needed to improve this issue.
For plastic products with varying thicknesses, small bubbles may appear in thicker sections or large black bubbles may form within very thick areas.
When molding several different-shaped products simultaneously, it’s crucial to ensure that the size of each runner gate matches the product’s weight. Otherwise, thicker products are prone to developing bubbles. Venting in the mold should also be considered, as products that frequently stick to the front mold make venting more challenging, increasing the likelihood of the aforementioned issue.
Modifications to the mold, an extension of holding and cooling times, or an adjustment of thick-section pressure can help improve this condition. Most importantly, when designing products, it is advisable to avoid creating excessively thick sections.
