During the injection molding process in Dalian, the generation of bubbles (pores or air pockets) can affect the appearance and performance of the product, such as strength and sealing. Bubbles usually originate from moisture and volatile substances in the raw materials, or from air entrainment during the molding process, or from poor flow of the melt. The following are key methods to avoid bubbles, explained in dimensions from raw material processing, process adjustment, mold optimization to equipment maintenance:

1、 Raw material pretreatment: eliminate moisture and volatile substances

Dry raw materials

Many hygroscopic plastics, such as nylon PA, polycarbonate PC, ABS, polyester PET, etc., need to be thoroughly dried before molding, otherwise moisture will vaporize at high temperatures and form bubbles.

Drying method: Control temperature and time according to material characteristics. For example:

PA6/PA66: Dry at 100-120 ℃ for 4-8 hours;

PC: Dry at 120-130 ℃ for 3-4 hours (using a hot air circulation dryer);

ABS: Dry at 80~90 ℃ for 2-3 hours.

Testing standard: Use a moisture content tester to ensure that the moisture content of the raw material is below 0.02% (different materials have different requirements, please refer to the supplier's specifications).

Avoid degradation of raw materials

Excessive heating of plastics at high temperatures can decompose and produce gases (such as PVC and POM, which are easily degradable). It is necessary to control the temperature of the material barrel not to exceed the thermal stability limit of the material, in order to avoid excessive residence time of the melt (such as the idle time of the screw should not exceed 30 seconds).

Regularly clean the material barrel and screw to prevent the accumulation and release of gas from carbonized materials.

2、 Process parameter adjustment: controlling melt flow and exhaust

Injection speed and pressure

Low speed injection: High speed injection can easily cause turbulent flow of the melt into the air, especially for thin-walled products or complex flow channels. Segmented injection (slow first and then fast) should be used to reduce air entrainment.

Holding pressure and time: Adequate holding pressure can compact the melt and eliminate vacuum bubbles caused by cooling shrinkage. The holding pressure is usually 60% to 80% of the injection pressure, and the holding time is adjusted according to the thickness of the product (thick walled parts need to extend the holding pressure).

Tube and mold temperature

Barrel temperature: Increasing the barrel temperature appropriately can reduce the viscosity of the melt and decrease flow resistance, but it is necessary to avoid overheating and degradation. For example, if the temperature of the PC barrel is too high, silver wire like bubbles will be generated.

Mold temperature: Low temperature molds will cause rapid solidification of the melt surface, preventing internal gases from escaping. Raising the mold temperature (such as ABS mold temperature of 50-80 ℃) can improve the fluidity of the melt and prolong the exhaust time.

Screw speed and back pressure

Low speed: If the screw speed is too high, it will intensify shear heating and air entrapment. It is recommended to control the speed between 50-100r/min (adjusted according to the material).

Back pressure regulation: Increasing the back pressure appropriately (5-15MPa) can reduce the gas content in the molten material at the front end of the screw and improve plasticization uniformity, but excessive back pressure can lead to degradation.

3、 Mold design and exhaust optimization: ensure gas exhaust

Add exhaust system

At the filling position of the mold cavity (such as the parting surface, the top of the rib, and the bottom of the deep cavity), an exhaust groove is opened, with a depth of usually 0.02~0.05mm (to avoid melt overflow) and a width of 5~10mm.

For deep cavities or complex molds, inserts, needle gaps, or sintered metals (porous materials) can be used to assist with exhaust.

Optimize runner and gate design

Channel cross-section: Increase the channel diameter (such as circular channel diameter ≥ 8mm) to reduce the flow resistance of the melt and avoid turbulent gas entrapment caused by too fast flow.

Gate position and size:

The sprue should be located at the wall thickness to avoid melt spraying (such as fan-shaped sprues and hidden sprues can reduce spraying);

Thin walled products use wide and thin gates, while thick walled products use point gates or direct gates to ensure balanced filling of the melt.

Mold surface treatment

The surface roughness Ra of the mold should be ≤ 0.8 μ m to avoid air retention due to surface roughness. Polishing or coating treatment (such as nitriding treatment) can be applied to deep cavity molds to improve surface smoothness.

4、 Equipment maintenance and operation details

Check the sealing of the equipment

The connection between the hopper and the barrel, as well as the gap between the screw and the barrel, may suck in air. It is necessary to regularly check the wear of the sealing ring and replace it in a timely manner.

For high viscosity materials such as PC and PMMA, ensure that there is no leakage at the connection between the barrel and nozzle to prevent air from entering the mold with the melt.

Prevent melt retention

Before stopping the machine, rinse the material cylinder with cleaning material (such as PE) to prevent residual material from carbonizing. For thermosetting plastic injection molding machines, the material cylinder should be thoroughly cleaned after shutdown to avoid cross-linking and solidification.

Manual exhaust operation

During the trial molding stage, "under injection" (intentionally not filling the mold cavity) can be used to observe the position of bubbles, adjust the position of the exhaust groove or process parameters.

If bubbles are found during production, the mold can be briefly opened ("air injection") to discharge the gas in the mold cavity before continuing production.

5、 Handling techniques for special materials

Fiber reinforced materials (such as GF reinforced PA): The interface between fibers and resin is prone to air entrapment, so it is necessary to increase the back pressure to 10-20MPa and increase the depth of the mold exhaust groove (such as 0.05mm).

Transparent materials (such as PMMA, PC): Bubbles can cause a decrease in light transmittance, and strict control of dryness (moisture content<0.01%) is required. Multi stage injection (low-speed mold filling+high-speed pressure holding) should be used.

Foamed plastics: It is necessary to distinguish between process types (physical foaming/chemical foaming), and by precisely controlling the decomposition temperature and injection speed of foaming agents, avoid uneven foaming and the generation of large bubbles.

To avoid injection molding bubbles, it is necessary to control the entire process from "raw material drying → process matching → mold exhaust → equipment maintenance". The core logic is to reduce gas sources (drying, anti degradation), promote gas exhaust (optimizing flow and exhaust), and compact the melt (maintaining pressure and cooling). In actual production, problems can be gradually investigated through first piece inspection (observing the interior of the cut product) and adjusting the trial mold parameters (such as gradually reducing the injection speed) until the bubbles are completely eliminated.