What are the causes of bubbles in injection molded products?
The gas in the bubble is very thin and belongs to the vacuum bubble. Many process parameters have a direct impact on the generation of bubbles and vacuum bubbles; generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem.
There are many reasons for vacuum bubbles, such as: injection pressure is too low, injection speed is too fast, injection time and cycle are too short, too much or too little feeding, insufficient holding pressure, uneven cooling or insufficient cooling, as well as material temperature and mold temperature Improper control, etc. In summary, there are three types of situations:
(1) Improper control of molding conditions
1. The injection speed is too low or the injection pressure is too low: during high-speed injection, if the injection speed is too low or the injection pressure is too low, the gas in the mold is too late to be discharged, resulting in excessive gas remaining in the melt, and injection molded products are easy Defects such as bubbles, depressions, and short shots are generated.
Solution: The flow direction speed should be appropriately reduced, and special care should be taken when adjusting the injection speed and pressure.
2. The mold temperature or the melt temperature is too low, and the barrel temperature is too high:
(1) When controlling the mold temperature and the melt temperature, pay attention to the temperature not being too high, otherwise it will cause the degradation of the melt; if the temperature is too low, it will cause insufficient filling and compaction, and voids are likely to occur inside the plastic parts. bubble.
(2) When controlling the temperature of the barrel, the temperature of the feeding section should not be too high, otherwise it will produce backflow and cause bubbles.
Solution: Under normal circumstances, control the melt temperature to be slightly lower and the mold temperature to be 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 holes.
(2) Mold defects
1. Improper gate position of the mold or too small gate section: If the gate position of the mold is incorrect or the gate section is too small, the main runner and the runner are long and narrow, there is a dead angle in the runner or the mold exhaust is poor , Will cause bubbles and vacuum bubbles.
Solution: First determine whether the mold defect is the main cause of the bubble or vacuum bubble. Then, according to the specific situation, adjust the structural parameters of the mold, especially the position of the gate should be set at the thick wall of the plastic part.
2. The pressure in the mold cavity is too high and the melt flows backwards: When selecting the gate type, direct gate should be used. After the pressure holding is over, the pressure in the cavity is higher than the pressure in front of the gate. If the melt at the mouth is not solidified, the melt will flow backwards, which will cause holes and vacuum bubbles in the plastic parts.
(1) Since the vacuum bubble generated by the direct gate is more prominent, the direct gate form should be avoided as far as possible;
(2) In the case that the gate type cannot be changed, it can be adjusted by extending the holding time, increasing the material supply, and reducing the taper of the gate.
3. The mold has poor exhaust due to the narrow runner and large thickness gap.
(1) The slender and narrow runner should be shortened and widened, the dead angle of the gas storage in the runner should be eliminated, and the failure of poor mold exhaust should be eliminated.
(2) When designing plastic parts, try to avoid parts with too large thickness gap on the shape of the plastic parts.
(3) The raw materials do not meet the requirements for use
If the moisture content in the molding material exceeds the standard, the material particles are too fine or uneven in size, it will cause bubbles and vacuum bubbles in the plastic parts.
Solution: It should be solved by pre-drying raw materials and screening fine materials.
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