When plastic melt fills the mold cavity, cavities formed by multiple melt fronts or gas trapped in the melt at the filling end due to gas not being able to escape result in air bubbles on the plastic part, also known as air cavitation, as shown in Figure 3-30.
Bubbles are different from bulges and vacuum bubbles (shrinkage cavities). Bubbles refer to tiny air bubbles present within the plastic part; while vacuum bubbles are voids where gas has been expelled. These voids are created when the melt cools and solidifies due to uneven shrinkage, and no gas is present inside the void.
During injection molding, if the material is not sufficiently dried, the injection speed is too fast, air is trapped in the melt, the mold has poor venting, or the plastic has poor thermal stability, tiny bubbles may appear inside the plastic part (visible in transparent plastic parts, as shown in Figure 3-31). When tiny bubbles are present inside the plastic part, the surface often exhibits silver streaks (material inclusions). Bubbles in transparent parts affect the appearance quality and are also considered a sign of poor material quality, reducing the part's strength.
The main reason for the formation of air bubbles is that the flowing melt is diverted due to various obstacles in the structure of the plastic part or the design of the mold, and they meet at a certain position, causing gas to be trapped in the cavity. If it is not removed in time, or if gas is continuously generated, the trapped area will not be filled, or it will burn and result in underfilling.
Table 3-7 Factors Causing Air Bubbles
| Category | Main Causes of Air Bubbles |
|---|---|
| Spinning Process | ① Excessive injection pressure ② Too high melt temperature ③ Melt retention time too long |
| Mold Design | ① Gate position or size improper ② Exhaust at parting line or ejector pin insufficient |
| Molding Conditions | ① Too large pressure loss in runner system ② Contains excessive moisture or volatile components in raw material ③ Too fast material temperature rise, causing partial carbonization |
| Molding Equipment | ① Screw tip check valve has large wear ② Temperature control unstable |
Table 3-8 Causes and Solutions for Silver Streaks (Flow Marks/Stress Whitening)
| Cause Analysis | Solution |
|---|---|
| ① Melt temperature or barrel temperature too high | ① Lower barrel temperature and melt temperature |
| ② Raw material not sufficiently dried | ② Dry the raw material thoroughly |
| ③ Screw speed or injection speed too fast | ③ Reduce screw speed or injection speed |
| ④ Mold temperature too low | ④ Increase mold temperature appropriately, improve exhaust performance |
| ⑤ Too much residual pressure, melt stays too long in the barrel | ⑤ Reduce back pressure and residual pressure in the barrel |
| ⑥ Gate size or runner too small or improperly shaped | ⑥ Reduce gate and runner resistance, change gate shape, make the cavity fill in a radial pattern |
| ⑦ Poor thermal stability of plastic coloring agent | ⑦ Use heat-resistant coloring agent or masterbatch |
| ⑧ Air trapped inside the melt | ⑧ Lower the lower section temperature of the barrel, improve degassing performance |
Real-world examples
The plastic part shown in the picture below has several grid holes.The mold over these grid holes creates significant flow resistance to the melt, causing it to become trapped and resulting in air entrapment. This air entrapment defect is caused by the gate location and the structure of the plastic part. The solution is to use a sequence valve to control the melt filling sequence, thereby avoiding air entrapment.