Bubble
Injection product defects

Bubble refers to slow cooling of center of wall thickness of product, rapid cooling and shrinkage of surface pulls material, so that volume shrinks unevenly during molding and creates a cavity; in addition, moisture and air generated when raw material is heated and decomposed will form bubbles inside product.
This kind of defect is usually manifested by inclusion of many very tiny bubbles in finished product, which will also occur with silver streaks. For precision plastic products with waterproof performance requirements, even a small air bubble will cause return of the entire product and customer complaints. All precision parts and high-quality parts do not accept this appearance defect.

 

 

 

 

▲ Picture 1 Defect picture of product with bubbles

Use methods of man, machine, material, method, and environment to analyze, classify and summarize all factors that affect product, then analyze it step by step according to product development process.

 

▲ Figure 2 Cause analysis
For product design, plastic materials, and mold reasons, problems can generally be found through observation. Practitioners can easily make a preliminary judgment when they see product. After making a preliminary judgment, then adjusting injection molding process parameters, there will be a clearer idea, so as to find specific cause of injection product defect.

 

▲ Figure 3 Analysis process

Thickness of product is poorly designed and partly too thick.

1 Raw materials are mixed with other plastics or pellets mixed with a large amount of powder, which will easily entrain air when melting.
2 Regrind is used, structure of material is loose, and amount of air retained in micropores is large.
3 Raw materials contain volatile solvents or liquid additives, such as dye auxiliary white oil, lubricant silicone oil, plasticizer dibutyl phthalate, stabilizer, antistatic agent, etc. Additives are excessively used or mixed unevenly, enter cavity in an aggregate state, forming bubbles.
4 Plastic has not been dried or absorbed moisture from atmosphere.
5 Some brands of plastic cannot withstand higher temperature or longer heating time.

1 Design defects, such as poor gate position, too small gate, small runner, unreasonable mold cooling system, too large difference in mold temperature, etc., which cause discontinuous flow of melt in mold cavity and block air passage.
2 Insufficient exhaust and blockage of mold parting surface.
3 Surface of mold is rough and friction resistance is large, causing local overheating and decomposing passing plastic.

1 Material temperature is too high, causing decomposition.
2 Injection pressure is small and pressure holding time is short, so that melt does not adhere to surface of cavity.
3 If injection speed is too fast, molten plastic will be decomposed by shearing to produce gas; if injection speed is too slow, cavity cannot be filled in time, resulting in insufficient surface density of product and producing bubbles.
4 Insufficient amount of material, too large feeding cushion, too low material temperature or too low mold temperature will affect flow and molding pressure of melt and generate bubbles.
5 Use multi-stage injection to reduce bubbles, that is, medium-speed injection fills runner→slow speed to fill gate→fast injection→low pressure and slow speed to fill mold, so that gas in mold can be removed in time in each section.
6 When screw is pre-molded, back pressure is too low and speed is too high, so that screw returns too fast, air is easily pushed to front end of barrel along with material.

Nozzle hole is too small; material is drooling or drawing at nozzle; there are obstacles or burrs in barrel or nozzle; friction heat is generated when high-speed material flows to decompose material.

Try to design products with uniform wall thickness.

1 Use pure raw materials.
2 Reduce or do not use recycled materials.
3 Reduce use of additives.
4 Bake according to standard time of material.
5 Replace materials or add additives to improve heat resistance.

1 Adjust position of gate and size of runner to minimize difference in mold temperature.
2 Fully exhaust.
3 According to customer needs and product appearance requirements, reduce surface roughness.

1 Reduce material temperature.
2 Increase injection pressure and extend holding time.
3 Adjust injection speed according to product structure.
4 Increase amount of material.
5 Use multiple injections.
6 Increase back pressure and reduce screw speed.

Use a nozzle with a large aperture or check if there is a foreign body blocking nozzle mouth.

 

Product material is LCP, average wall thickness is 0.30mm, and mold adopts direct gate side feeding method. Product has a small size, with a wall thickness of only 0.2mm at the thinnest position, uses high-temperature resistant engineering plastics to meet customer performance requirements. After injection molding, product needs to be baked at a high temperature.

After plastic product is filled, during baking process, plastic layer of outer package is separated from inner parts, and there are cracks and blisters at welding line.

Judging from product drawings and samples, cracking and blistering are at the thinnest point. Product has no cracks and bubbles before baking. After baking, product cracked or blistered at weld line. It can be analyzed from this that bubbles are produced because wall thickness of product is too thin to withstand gas released by internal structure after high temperature.

For product defects, combined process debugging and verification of high-speed injection, low-speed injection, high material temperature, low material temperature, high mold temperature, low mold temperature can not be solved, and there is no obvious improvement effect.

Product design defects caused unchangeable final product defects. Communicate with development department personnel. On the premise of meeting size requirements of product drawings, add 0.02mm at the thinnest part of 0.2mm, problem of product cracking and foaming can be solved during trial production again, which ensures smooth production and production stability of product.