1. Improper control of molding conditions
The injection pressure is too low, the speed is too fast, the time and cycle are too short, the amount of material added is too much or too little, the pressure is insufficient, the cooling is uneven or insufficient, and the material temperature and mold temperature are improperly controlled, all will cause bubbles in the plastic part. .
Specifically, during high-speed injection, the gas in the mold has no time to be discharged, resulting in too much residual gas in the melt. In this regard, the injection speed should be appropriately reduced. However, if the speed drops too much and the injection pressure is too low, it will be difficult to exhaust the gas in the molten material, and bubbles, depressions and under-injection will easily 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 cooling conditions, controlling the amount of feed, etc. If the cooling conditions of the plastic part are poor, the plastic part can be slowly cooled in hot water immediately after demoulding, so that the cooling speed of the inside and outside becomes the same.
When controlling the mold temperature and melt temperature, attention should be paid to the temperature not being too high, otherwise it will cause the melt to depolymerize and decompose, produce a large amount of gas or excessive shrinkage, forming bubbles or shrinkage cavities; if the temperature is too low, it will cause the filling pressure If it is not enough, it is easy for voids and bubbles to form inside the plastic parts.
In general, the melt temperature should be controlled slightly lower and the mold temperature should be controlled slightly higher. Under such process conditions, it is neither easy to generate a large amount of gas nor shrinkage cavities. When controlling the temperature of the barrel, the temperature of the feeding section cannot be too high, otherwise it will cause backflow of material and bubbles.
2. Mold defects
If the gate position of the mold is incorrect or the gate cross-section is too small, the main channel and the branch channel are long and narrow, there are air storage dead corners in the runner or the mold is poorly vented, it will cause bubbles or vacuum. Therefore, it should first be determined whether mold defects produce bubbles and the main causes of vacuum bubbles. Then, according to the specific situation, adjust the structural parameters of the mold, especially the gate position should be set at the thick wall of the plastic part.
When selecting the gate form, since the phenomenon of vacuum holes produced by direct gates is more prominent, it should be avoided as much as possible. This is because after the pressure holding is completed, the pressure in the cavity is higher than the pressure in front of the gate. If direct gates are used at this time, If the melt is not frozen yet, the melt will flow back, causing holes to form inside the plastic part. If the gate form cannot be changed, it can be adjusted by extending the pressure holding time, increasing the feed volume, and reducing the gate taper.
The gate cross-section cannot be too small, especially when molding several plastic parts with different shapes at the same time. It must be noted that the size of each gate must be proportional to the weight of the plastic part. Otherwise, larger plastic parts are prone to bubbles.
In addition, the slender and narrow flow channels should be shortened and widened to eliminate the air storage dead corners in the flow channels and eliminate the problem of poor mold exhaust. When designing the mold, try to avoid particularly thick parts or large differences in thickness on the plastic part.
3. Raw materials do not meet the requirements for use
If the gate position of the mold is incorrect or the gate cross-section is too small, the main channel and the branch channel are long and narrow, there are air storage dead corners in the runner or the mold is poorly vented, it will cause bubbles or vacuum. Therefore, it should first be determined whether mold defects produce bubbles and the main causes of vacuum bubbles. Then, according to the specific situation, adjust the structural parameters of the mold, especially the gate position should be set at the thick wall of the plastic part.
When selecting the gate form, since the phenomenon of vacuum holes produced by direct gates is more prominent, it should be avoided as much as possible. This is because after the pressure holding is completed, the pressure in the cavity is higher than the pressure in front of the gate. If direct gates are used at this time, If the melt is not frozen yet, the melt will flow back, causing holes to form inside the plastic part. If the gate form cannot be changed, it can be adjusted by extending the pressure holding time, increasing the feed volume, and reducing the gate taper.
The gate cross-section cannot be too small, especially when molding several plastic parts with different shapes at the same time. It must be noted that the size of each gate must be proportional to the weight of the plastic part. Otherwise, larger plastic parts are prone to bubbles.
In addition, the slender and narrow flow channels should be shortened and widened to eliminate the air storage dead corners in the flow channels and eliminate the problem of poor mold exhaust. When designing the mold, try to avoid particularly thick parts or large differences in thickness on the plastic part.
1. According to the causes of bubbles, the solutions include the following aspects:
(1) When the wall thickness of the product is large, the outer surface cools faster than the central part. Therefore, as the cooling proceeds, the resin in the central part shrinks and expands toward the surface, causing insufficient filling of the central part. This situation is called a vacuum bubble. The main solutions are:
According to the wall thickness, determine the reasonable gate and runner size. Generally, the gate height should be 50% to 60% of the wall thickness of the product.
Until the gate is sealed, a certain amount of additional injection material remains.
The injection time should be slightly longer than the gate sealing time.
Reduce the injection speed and increase the injection pressure,
Use materials with high melt viscosity grades.
(2) The main solutions to bubbles caused by the generation of volatile gases are:
Pre-dry thoroughly.
Reduce resin temperature to avoid decomposition gases.
(3) Bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and mold and increasing the injection speed.
