Main reason is that water vapor and other gases in resin cannot be discharged or because of insufficient mold filling, condensing surface condenses too quickly to form a "vacuum bubble". According to causes of bubbles, it can be divided into following types:
01 When wall thickness of the product is large, cooling rate of outer surface is faster than that of central part. Therefore, as cooling progresses, resin in central part shrinks and expands toward surface, causing insufficient filling of central part. This situation is called a vacuum bubble.
Solution is to determine a reasonable gate and runner size according to wall thickness. Generally, height of gate should be 50% to 60% of wall thickness of product; until gate is sealed, there is a certain amount of supplementary injection material; injection time should be slightly longer than gate sealing time; reduce injection rate and increase injection pressure ; use materials with high melt viscosity grades.

 

02 Solution to bubbles caused by generation of volatile gases is to fully pre-dry raw materials and use injection molding machine with drying hopper to produce; reduce melt temperature to avoid generation of decomposition gases.
03 Air bubbles caused by poor fluidity can be solved by increasing temperature of resin and mold, and increasing injection rate.

Silver streak is mainly caused by moisture absorption of resin. Therefore, resin should generally be dried under condition of 10~15℃ lower than its heat distortion temperature. Especially when using an automatic drying hopper, a reasonable capacity should be selected according to molding cycle (molding volume) and drying time, material should be turned on and dried a few hours before start of injection.
In addition, too long resin stagnation time in barrel will also produce silver streaks. Different types and grades of materials should not be mixed.
Secondly, during mold filling and condensation process, affected by internal stress anisotropy, stress generated in vertical direction will cause resin to flow orient, non-flow orientation part will produce different refractive indexes and flash silk lines. When it expands, it may cause cracks in product.
In addition to paying attention to injection molding process and mold, it is best to anneal product. For example, PC material can be heated to above 160℃ for 3~5min, and then cooled naturally.

Due to large surface roughness of mold, or premature condensation, resin cannot copy state of mold surface, all of which make surface of product produce small irregularities and lose its luster.
Solution is to polish molding surface of mold; extend cooling time; appropriately increase melt temperature, injection pressure, injection rate, and mold temperature.

Shock marks are dense ripples formed around the gate. Main cause is that melt viscosity is too high. Material at the front end has been condensed in cavity. Later, material breaks through condensation surface, and shock lines appear on the surface.
Solution is mainly to improve fluidity and adjust molding conditions, such as increasing resin temperature, mold temperature, injection pressure and injection rate, etc.; add exhaust grooves; minimize use of mold release agents; if it does not affect appearance, you can change gate position to change location of shock marks, or treat parts produced by shock marks as a dark glossy surface for modification.

It is mainly caused by dust falling into resin in air or too much water content in resin.
Main solution is to remove resin impurities and pollution; fully dry resin and use injection molding machine with drying hopper to produce; accurately control melt temperature and production cycle; increase injection pressure and mold temperature.

It is mainly caused by dust falling into resin in the air or too much water content in the resin.
Main solution is to remove resin impurities and pollution; fully dry resin and use injection molding machine with drying hopper to produce; accurately control melt temperature and production cycle; increase injection pressure and mold temperature.

 

Main reasons are insufficient resin capacity, insufficient pressure in cavity, poor resin fluidity, and poor exhaust effect. Mainly from following aspects to improve
1 Extend injection time to prevent resin from flowing backwards before gate is cured due to short molding cycle, and it is difficult to fill cavity.
2 Increase injection rate, mold temperature, resin temperature, and injection pressure.
3 Expand gate size. Generally, height of gate should be 1/3~1/2 of wall thickness of product.
4 Gate is set at the largest wall thickness of product.
5 Set up exhaust groove (average depth 0.03mm, width 3~5mm) or exhaust rod. It is more important for smaller parts.
6 There is a certain buffer distance between screw and injection nozzle.
7 Choose low-viscosity grade resin.

Reason for shrinkage is same as that of wrinkles and pockmarks. In principle, it can be solved by excess filling, but there is a risk of stress. Therefore, wall thickness should be uniform in design, and wall thickness of stiffeners, convex columns, etc. should be reduced as much as possible.

According to burn caused by different reasons such as machinery, mold or molding conditions, solutions adopted are also different.

Barrel is overheated due to abnormal conditions, causing resin to be decomposed at high temperature and burned after injection molding, or resin stagnation, decomposition and discoloration caused by nozzle and screw threads in barrel, check valve and other parts of barrel are brought into product, causing product to have dark brown burn marks. Nozzle, screw and barrel should be cleaned.

Mainly due to poor exhaust. This kind of burn usually occurs in a fixed part, which is easy to distinguish from the first situation. Attention should be paid to add exhaust grooves and exhaust rods and other measures.

If back pressure is too high, barrel will overheat and cause burns. Screw speed is too high, it will also produce overheating, generally 40~90r/min is better. When exhaust groove is not provided or exhaust groove is small, injection rate is too high and it will cause burns of overheated gas.

Weld mark is caused by front end of molten resin from different directions being cooled and not completely fused at joint. Under normal circumstances, it mainly affects appearance, and when it is severe, it affects strength of product.
Main solution is to improve fluidity and adjust molding conditions, such as increasing resin temperature, mold temperature, injection pressure and injection rate, etc.; adding an exhaust groove and setting a push rod at the place where weld mark is generated is also conducive to exhaust; minimize use of release agents;
If only appearance is affected,  gate position can be changed to change position of weld mark, or part where weld mark is generated can be treated as a dark glossy surface for modification.

Warpage and deformation should be solved mainly from aspect of mold design, and adjustment effect of molding conditions is very limited. Main causes and solutions for warpage and deformation are:
(1) When deformation is caused by residual stress caused by molding conditions, stress can be eliminated by reducing injection pressure, increasing mold temperature and making mold temperature uniform, increasing resin temperature or using an annealing method.
(2) When poor demolding causes stress and deformation, it can be solved by increasing number or area of push rods and setting demolding slope.
(3) If cooling method is not suitable, cooling is uneven or cooling time is insufficient, cooling method can be adjusted and cooling time can be extended. For example, set cooling circuit as close to deformed place as possible.
(4) For deformation caused by molding shrinkage, design of mold must be revised. The most important thing is to make product wall thickness consistent. Sometimes, in unavoidable circumstances, it can only be corrected by measuring deformation of product and trimming mold in opposite direction.
In summary, phenomenon of inferior waste produced in production process of transparent plastic parts is manifold. Only by finding reasons for defects and implementing various solutions can we produce qualified products and make application of transparent plastic parts more extensive.