1) Improper control of molding conditions
Many 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 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 not properly controlled, which will cause plastic parts Bubbles are generated inside.
Especially during high-speed injection, the gas in the mold is too late to be discharged, resulting in too much gas remaining in the melt. For this, the injection speed should be appropriately reduced. However, if the speed drops too much and the injection pressure is too low, it is difficult to exhaust the gas in the melt, and it is easy to produce bubbles, dents and undershot. Therefore, special care should be taken when adjusting the injection speed and pressure.
In addition, by adjusting the injection and holding pressure time, improving the cooling conditions, controlling the amount of feed and other methods to avoid the generation of bubbles and vacuum bubbles. If the cooling condition of the plastic part is poor, the plastic part can be put into hot water and slowly cooled immediately after demolding, so that the cooling rate of the inside and outside tends to be the same.
When controlling the mold temperature and melt temperature of the cnc turning part, pay attention to the temperature not too high, otherwise it will cause the melt to depolymerize and decompose, produce a lot of gas or excessive shrinkage, forming bubbles or shrinkage holes; if the temperature is too low, it will cause Insufficient filling and compaction can easily produce voids in the plastic parts and form bubbles.
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 not easy to produce a large amount of gas, and it is not easy to produce shrinkage holes. When controlling the temperature of the barrel, the temperature of the feeding section should not be too high, otherwise it will cause backflow and return of the material and cause bubbles.
2) Mold defects
If the gate position of the cnc machine parts mold is incorrect or the gate section is too small, the main runner and branch runner are long and narrow, there are dead corners in the runner or the mold exhaust is poor, it will cause bubbles or vacuum.
Therefore, first determine whether the mold defect produces bubbles and the main reason for 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 type, since the vacuum hole produced by the direct gate is more prominent, it should be avoided as much as possible. This is because the pressure in the cavity is higher than the pressure in front of the gate after the pressure holding is over. The molten material at the place has not been frozen, and the phenomenon of molten material backflow will occur, causing holes to form inside the plastic part.
In the case that the gate form cannot be changed, it can be adjusted by extending the holding time, increasing the amount of feed, and reducing the taper of the gate.
The gate cross-section should not be too small, especially when forming several plastic parts with different shapes at the same time, it must be noted that the size of each gate should be proportional to the weight of cnc plastic machining, otherwise, larger plastic parts are likely to produce bubbles.
In addition, the slender and narrow runners should be shortened and widened to eliminate dead corners in the runners, and eliminate the failure of poor mold exhaust. When designing the mold, try to avoid any extra-thick parts on the shape of the plastic part or too large a difference in thickness.
3) The raw materials do not meet the requirements for use
If the moisture or volatile content in the molding raw material exceeds the standard, the material particles are too small or uneven in size, resulting in too much air mixed in the feeding process, the shrinkage of the raw material is too large, and the melt index of the melt is too large or too small. Too much recycled material content will affect the production of bubbles and vacuum bubbles in plastic parts.
In this regard, it should be solved by pre-drying raw materials, sieving out fine materials, replacing resins, and reducing the amount of recycled materials.