Analysis and Solutions for Bubble Generation in Plastic Products

Bubble and vacuum bubble defects refer to defects in PPSU medical injection molding products caused by the slowest cooling at the center of the product wall thickness and rapid cooling on the surface. The rapidly shrinking surface will pull the material over, causing uneven shrinkage of the product volume and causing voids in the thickness part. Bubbles are formed when the water or gas in the plastic forms bubbles. Bubbles can be divided into water bubbles and vacuum bubbles. Bubbles on transparent products can usually be observed directly, while bubbles on opaque products may not be easy to find and can only be identified by cutting or other means. The presence of bubbles will cause the product to be incompletely filled and the surface to be uneven, affecting product quality.

Analysis and elimination methods of bubble and vacuum bubble defects

1. Improper control of molding conditions

– A variety of process parameters have a direct impact on the generation of bubbles and vacuum bubbles. If the injection pressure is too low, the injection speed is too fast, the injection time and cycle are too short, the amount of feed is inappropriate, the pressure holding is insufficient, the cooling is uneven or insufficient, and the material temperature and mold temperature are improperly controlled, bubbles will be generated in the plastic part. Especially during high-speed injection, the gas in the mold cannot be discharged in time, resulting in too much residual gas in the melt. In this regard, the injection speed should be appropriately reduced, but if the speed is reduced too much and the injection pressure is too low, it will be difficult to exhaust the gas in the melt, and bubbles, dents and under-injection are likely to occur. Therefore, special caution should be taken when adjusting the injection speed and pressure.
– In addition, bubbles and vacuum bubbles can be avoided by adjusting the injection and holding time, improving the cooling conditions, and controlling the amount of feed. If the cooling conditions of the plastic part are poor, the plastic part can be placed in hot water for slow cooling immediately after demolding to make the cooling speed inside and outside consistent.
– When controlling the mold temperature and melt temperature, it should be noted that the temperature should not be too high to avoid causing the melt to decompose, generate a large amount of gas or shrink excessively, and form bubbles or shrinkage holes; if the temperature is too low, it will lead to insufficient filling compaction, and it is easy to generate gaps inside the plastic part, forming bubbles. In general, the melt temperature should be slightly lower than the standard, and the mold temperature should be slightly higher than the standard to avoid the generation of a large amount of gas and the appearance of shrinkage holes.
– At the same time, when controlling the barrel temperature, the temperature of the feeding section should not be too high to avoid backflow and bubbles.

2. Mold defects

– Incorrect gate position of the mold or too small gate cross section, too long and narrow main channel and branch channel, dead corner of air storage in the flow channel or poor mold exhaust will lead to the generation of bubbles or vacuum bubbles. Therefore, first confirm whether the mold defect is the main cause of bubbles and vacuum bubbles. According to the specific situation, the structural parameters of the mold should be adjusted, especially the gate position should be set at the thick wall of the plastic part.
– When selecting the gate form, try to avoid using direct gates, because they are easy to produce vacuum holes. After the pressure holding is over, the pressure in the cavity is often higher than the pressure in front of the gate. If the molten material at the direct gate has not frozen at this time, the molten material will flow back, resulting in holes in the plastic part. If the gate form cannot be changed, it can be adjusted by extending the pressure holding time, increasing the feed amount, and reducing the gate taper.
– The gate cross section should not be too small, especially when molding multiple plastic parts of different shapes at the same time, it should be noted that the size of each gate should be proportional to the weight of the plastic part, otherwise larger plastic parts are prone to bubbles. In addition, the slender and narrow flow channel should be shortened and widened to eliminate the dead corner of air storage in the flow channel to ensure good mold exhaust. When designing the mold, try to avoid the situation where there are extra thick parts or the thickness difference is too large on the plastic part.

3. The raw materials do not meet the use requirements

– If the moisture or volatile content in the molding raw materials exceeds the standard, the particles are too small or uneven in size, resulting in too much air mixed in the feeding process, the shrinkage rate of the raw materials is too large, the melt index of the molten material is too large or too small, or the content of recycled materials is too much, it will affect the generation of bubbles and vacuum bubbles in the plastic parts. In this regard, methods such as pre-drying the raw materials, screening out fine materials, replacing resins, and reducing the amount of recycled materials should be adopted to solve the problem.

Through the above analysis and solutions, the defects of bubbles and vacuum bubbles in PPSU medical injection molding products can be effectively reduced, and the overall quality of the products can be improved.