Causes and countermeasures of bubble defects in injection parts
In the injection molding products injection molding products are air bubbles is a lot of people in the problems in production and processing, in general, some of the reasons for such problems, but if cannot be handled timely, will not only make plastic molding bad, also may produce more defects, so, when in injection molding products are air bubbles, must be timely solve. The appearance of bubbles will make the product filling dissatisfaction, uneven surface. Vacuum bubble, this bubble is mainly caused by the shrinkage of plastic products, plastic products with thick wall in cooling, because the cooling rate of the surface is faster than the cooling rate of the middle, this will lead to the lack of glue in the center of plastic products, forming a vacuum. The solution to this vacuum bubble can be improved by reducing the injection speed, increasing the injection pressure, increasing the feed, and using materials with high melt viscosity grade.
The appearance of bubbles will make the product filling dissatisfaction, uneven surface. Vacuum bubble, this bubble is mainly caused by the shrinkage of plastic products, plastic products with thick wall in cooling, because the cooling rate of the surface is faster than the cooling rate of the middle, this will lead to the lack of glue in the center of plastic products, forming a vacuum. The solution to this vacuum bubble can be improved by reducing the injection speed, increasing the injection pressure, increasing the feed, and using materials with high melt viscosity grade.
Bubble defects often occur. Bubbles can be divided into two types: bubble and vacuum bubble. Generally speaking, bubbles occurring on transparent products can be directly observed, while bubbles occurring on opaque products are sometimes invisible from the outside and can only be found by cutting them open or by other means. Bubbles are usually produced because the injection speed is too fast for the plastic flow to force all the air in the mold cavity out of the exhaust tank, and the air mixes into the plastic to form bubbles. The vacuum bubble is due to the slow cooling of the wall thickness center of the product, the rapid surface cooling and shrinkage tend to pull the material over, and the uneven shrinkage of the volume caused by the thickness of the cavity; The raw material has moisture and air in it when it gets hot and cold, and it quickly becomes a bubble.
1. Improper control of forming conditions
Many process parameters have a direct effect on the formation of bubbles and vacuum bubbles. Setting too low injection pressure, too fast injection speed, too short injection time and cycle, too much or too little feeding amount, insufficient pressure retention, uneven cooling or insufficient cooling, and improper control of material temperature and mold temperature will cause bubbles in the plastic parts. Especially in high-speed injection, the gas in the mold can not be discharged, resulting in too much residual gas in the melt. For this, the injection speed should be appropriately reduced. If the speed drops too much and the injection pressure is small, it is difficult to exhaust the gas in the melt, and bubbles, depressions and underinjection are easy to occur. Therefore, special care should be taken when adjusting the injection speed and pressure. In addition, bubbles and vacuum bubbles can be avoided by adjusting injection and pressure holding time, improving cooling conditions and controlling the amount of feeding. If the cooling condition of the plastic parts is poor, the plastic parts can be immediately put into the hot water to slow cooling, so that the internal and external cooling rate tends to be consistent. When controlling the mold temperature and the melt temperature, it should be noted that the temperature should not be too high, otherwise it will cause the melt degradation and decomposition, produce a large amount of gas or excessive shrinkage, and form bubbles or shrinkage holes; If the temperature is too low, it will cause insufficient filling material compaction, and the plastic parts are easy to produce gaps and form bubbles. In general, the melt temperature should be controlled slightly lower, mold temperature control slightly higher. Under such process conditions, it is not easy to produce a large amount of gas, and it is not easy to produce shrinkage cavity. When controlling the temperature of the cylinder, the temperature of the feeding section should not be too high, otherwise it will produce reflux and return material and cause bubbles.
2. Mold defects
If the sprue position of the mold is not correct or the sprue section is too small, the main channel and the diverter are long and narrow, the dead Angle of gas storage in the runner or the mold exhaust is not good, it will cause bubbles or vacuum. Therefore, it is necessary to first determine whether the mold defect is the main cause of bubble and vacuum bubble. Then, according to the specific situation, adjust the structural parameters of the mold, especially the sprue position should be set in the thick wall of the plastic parts. When selecting the sprue form, it should be avoided as far as possible because the phenomenon of vacuum hole in the direct sprue is relatively prominent. This is because the pressure in the cavity is higher than the pressure in front of the sprue after the end of pressure holding. If the melt at the direct sprue has not frozen at this time, the molten backflow phenomenon will occur, resulting in the formation of holes in the plastic parts. Under the condition that the sprue form cannot be changed, it can be adjusted by extending the holding time, increasing the feed quantity and reducing the sprue taper. The gate section can not be too small, especially when forming several different shapes of plastic parts at the same time, we must pay attention to the size of the gate to be proportional to the weight of the plastic parts, otherwise, the larger plastic parts are easy to produce bubbles. In addition, the slender and narrow flow channel should be shortened and widened to eliminate the dead Angle of gas storage in the flow channel and eliminate the fault of poor exhaust of the mold. Mold design, should try to avoid plastic parts on the body has a particularly thick part or thickness is too large.
3. Raw materials do not meet the requirements
If the moisture or volatile content of molding raw materials exceed the standard, the grain is too fine or uneven in size, resulting in the feeding process mixed with air too much, the shrinkage rate of raw materials is too large, the melt index of the melt is too large or too small, the content of recycled materials is too much, will affect the plastic parts to produce bubbles and vacuum bubbles. To solve this problem, pre-drying raw materials, screening and removing fine materials, replacing resin and reducing the amount of recycled materials should be adopted.
Air bubbles often occur at the final joints or thicker parts of plastic products. The product in the model is first surface cooling, when the outer surface has cooled, the central part is still in a hot state. The central part contracts as it cools, whereas the plastic surface is already cooled, so bubbles can only form from the inside itself. There are many thick parts in the mold when there is no bubble, but after a few seconds, there is a bubble in the center part.
There are many reasons for injection molding products to have bubbles. When such a situation occurs, it should be eliminated one by one. In addition, it should also find a way to solve it.