In injection molding, bubbles are not a very common defect for general small processing workshops. Bubbles mainly appear in non-large transparent products. Small transparent products are not easy to produce bubbles, while non-transparent products are usually not required.
Products that usually produce bubbles: products made of transparent materials, large transparent products (surface appearance products), and surface of thin-walled products. Since appearance of bubbles will seriously affect quality and appearance of products, for technicians, when bubbles appear in products, it is often not possible to solve them by reducing speed and melt temperature. Debugging of transparent products is also one of the most difficult defects in injection molding.

Bubbles are different from vacuum bubbles (shrinkage pits). Vacuum bubbles are formed because of uneven shrinkage caused by uneven wall thickness of product after injection molding filling is completed and cooled. In fact, there is no air inside product, that is, shrinkage pits. Bubbles usually appear at intersection of weld marks or at the end of product filling. This is because there is too much gas in mold and it cannot be emptied during filling. Especially for large products, large bubbles will be formed. After product is broken, there will be a bang~bang~ explosion sound.
In summary, most of products are bubbles when mold is opened. If there are no bubbles after product is taken out, and bubbles appear after a period of stagnation, they are vacuum bubbles (shrinkage pits). Vacuum bubbles usually appear in thicker position of ribs and exist in a single form. Bubbles appear at the end of product or intersection area and exist in multiple forms. Therefore, it is very helpful to identify what kind of bubbles are in process of adjusting machine.

Cause analysis:
a) During screw plasticization process, when storage speed is too fast or storage back pressure is too low, raw material enters plasticization section of barrel too early, causing it to enter too much air. In metering section, gas and molten material are mixed together, a large amount of air cannot be discharged in gap between gate sleeve and nozzle. During injection filling, gas and melt are injected into mold cavity at the same time, and bubbles are formed in product.
Solution:
b) Screw rotates too fast, reduce screw speed.
c) Back pressure is too low, select to increase back pressure according to process standard.

 

Cause:
a) For raw materials that are improperly stored and have high hygroscopicity, if moisture content of raw material is too high, it is not fully dried before injection molding, high-temperature hydrolysis occurs in barrel, and generated gas is wrapped in melt.
b) Thermal stability of plastic is poor, added raw material is loose in structure of recycled material, and air is wrapped inside pellets.
c) Recycled material added exceeds process standard ratio, generally not exceeding 20% of raw material ratio.
Solution:
a) Check whether barrel drying system is normal, and fully dry raw material according to process.
b) Appropriately reduce barrel temperature.
c) Appropriately reduce injection speed.
d) Appropriately increase storage back pressure.

Cause analysis:
a) Barrel temperature is set too high (heating device is not controlled), causing thermal degradation of material.
b) Thermal degradation caused by long residence time of melt in barrel.
c) During injection filling, shear heat generated by injection speed is too fast, causing thermal degradation of material, which usually occurs near gate.
d) Friction heat generated by screw rotation due to excessive back pressure of storage material causes thermal degradation.
Solution:
a) Appropriately reduce barrel temperature.
b) Reduce abnormal shutdowns and shorten molding cycle time. Generally, melt should not stop in barrel for more than five minutes, and injection molding can only be carried out after melt in barrel is emptied.
c) Re-adjust process parameters, reduce injection speed and injection pressure.
d) Reduce back pressure of storage material.

Cause analysis:
a) Incomplete mold exhaust, parting surface lacks necessary exhaust groove, or exhaust channel is blocked and deformed, deep bone position of product does not have necessary inserts and exhaust needles, resulting in melt gathering and being unable to be discharged during filling.
b) Bubble position is generated at confluence of filling end, and there is a corner (slider).
c) Valve hot runner, hot runner temperature is too high and produces bubbles caused by thermal decomposition.
d) Surface finish of mold is poor. When molten material is filled into mold cavity, friction is large, resulting in thermal decomposition of material.
e) Gate position is unreasonable or gate is too small, resulting in local trapped air bubbles caused by poor mold exhaust.
Solution:
a) According to location where bubbles are generated, add or enlarge exhaust groove to improve exhaust of mold.
b) Improve mold structure, avoid sharp corners, adopt a multi-stage injection method to control injection pressure and speed in stages, appropriately reduce injection pressure and speed at location where bubbles are generated.
c) Reduce temperature of hot runner heating coil, increase back pressure of material storage to reduce gas involved in barrel and increase filling amount.

Cause analysis:
a) Injection molding speed is too fast, gas in mold is not discharged in time, and it remains in melt of mold, resulting in trapped gas bubbles.
b) Barrel temperature is too high, material fluidity is enhanced, and material fluidity exceeds original actual fluidity.
c) Back pressure of material storage is too large, melt temperature becomes higher, resulting in increased fluidity.
d) Shrinking pressure is too large, mold is locked too tightly, and gas accumulation cannot be emptied.
Solution:
a) Increase exhaust depth, it is advisable to use multi-stage injection, reduce injection pressure and speed at location where bubbles are generated.
b) Set temperature according to material process, detect actual melt temperature when necessary, and reduce possibility of thermal decomposition of melt.
c) Too much back pressure will cause melt to thermally degrade and produce bubbles, while too little back pressure will also easily produce bubbles due to entrainment of air. Back pressure value should be set according to material process.
d.) Reducing clamping pressure can obviously solve problem of mold air entrapment, but it is also very easy to cause other process defects, burning and burrs.

Cause analysis:
a) Melt injection speed is too fast.
b) Mold exhaust is poor.
Solution:
a) Re-adjust process, use segmented injection, medium speed filling, and low speed to fill product to exhaust gas.
b) Mold temperature is too high, lower temperature of hot runner temperature control box.
c) Remove slider to increase exhaust at slider and add exhaust grooves.

 

Cause analysis:
a) There is air in melt.
b) Front melt injection speed is too fast.
c) Material temperature is too high (material decomposition or edge of decomposition).
d) Melt viscosity is too high.
Solution:
a) Reduce melt injection speed to fill front slowly.
b) Mold temperature is too high, lower temperature of hot runner temperature control box.
c) Empty melt in runner when starting machine to avoid thermal decomposition of melt caused by shutdown.
d) Increase back pressure appropriately to compact material in barrel evenly, and exhaust air in glue to increase density of sol.
e) Polish spherical surface of air needle to prevent air needle from sticking to glue.