How to eliminate air wrinkles and bubbles

How to eliminate wrinkles and bubbles

1.1 bubbles mainly form "vacuum bubbles" because water and other gases in the resin cannot be discharged or because of insufficient mold filling, and the condensing surface condenses too quickly. According to the origin of bubbles, through cooperation with Siemens, they can be divided into the following:

(1) when the wall thickness of the product is large, the cooling speed of its outer surface is faster than that of the central part. Therefore, with the progress of cooling, the resin edge of the central part shrinks and expands to the surface, resulting in insufficient filling of the central part. This condition is called vacuum bubble. The solution is to determine the reasonable gate and runner size according to the wall thickness. Generally, the gate height should be 50% - 60% of the product wall thickness; Until the gate is sealed, a certain amount of supplementary injection material is left; The injection time should be slightly longer than the gate sealing time; Reduce the injection rate and increase the injection pressure; Materials with high melting viscosity grade are used

(2) for bubbles caused by the generation of volatile gases, the solution is to fully pre dry the raw materials and use the injection molding machine with drying hopper for production; Reduce the melt temperature to avoid the generation of decomposition gas

(3) bubbles caused by poor fluidity can be solved by increasing the temperature of resin and mold and increasing the injection rate

1.2 crazing

crazing is mainly caused by the moisture absorption of the resin. Therefore, generally, the resin should be dried at a temperature 10~15 ℃ lower than its thermal deformation temperature. Especially when using the automatic drying hopper, it is necessary to select a reasonable capacity according to the molding cycle (molding amount) and drying time, and it is also necessary to start the machine to dry the material several hours before the injection. In addition, too long resin stagnation time in the barrel will also produce crazing. Materials of different types and brands should not be mixed

secondly, in the process of mold filling and condensation, due to the anisotropy of internal stress, the stress generated in the vertical direction will make the resin flow oriented, and the non flow oriented part will produce different refractive index and appear flash lines. When it expands, the product may appear cracks. In addition to paying attention to the injection molding process and mold, it is best to anneal the product. For example, PC material can be heated to above 160 ℃ for 3~5min, and then cooled naturally

1.3 poor surface gloss

due to the large roughness of the mold surface or premature condensation, the resin cannot copy the state of the mold surface, all of which make the product surface slightly uneven, thus losing gloss. The solution is to polish the mold forming surface; Extend the cooling time; Appropriately increase the melt temperature, injection pressure, injection rate and mold temperature

1.4 ripple

ripple is the development of a large amount of resources and environment consumption. The old road is unsustainable. It refers to the dense ripple formed with the gate as the center. The main reason is that the melt viscosity is too large, and the front material has been condensed in the mold cavity. Later, the material broke through this condensation surface, causing the surface to show ripple. The solution is mainly to improve the fluidity and adjust the molding conditions, such as increasing the resin temperature, mold temperature, injection pressure and injection rate; Add exhaust slots; Minimize the use of release agent; If it does not affect the appearance, you can change the position of the gate to change the position of the vibration pattern, or treat the part where the vibration pattern is generated as a dark glossy surface, and so on

1.5 whitening and fog halo

are mainly caused by the dust in the air falling into the resin or the resin has too much water content. The main solution is to remove resin impurities and pollution; The resin is fully dried and produced by an injection molding machine with a drying hopper; Accurately control melt temperature and production cycle; Increase injection pressure and mold temperature

1.6 white smoke and black spots

are mainly formed by the decomposition or metamorphism of plastic melt in the barrel due to local overheating. The main solution is to remove resin impurities and pollution; Try to reduce the melt temperature; Try to shorten the retention time of resin in the barrel

1.7 the main reasons for wrinkle and pitting are insufficient resin capacity, insufficient pressure in the mold cavity, poor resin fluidity and poor exhaust effect. It can be improved mainly from the following aspects:

(1) extend the injection time to prevent the reverse flow of resin before gate curing due to too short molding cycle, which makes it difficult to fill the mold cavity

(2) increase the injection rate, mold temperature, resin temperature and injection pressure

(3) expand the gate size. Generally, the height of gate shall be 1/3~1/2 of the product wall thickness

(4) the gate is set at the place with the largest wall thickness of the product

(5) set exhaust slots (average depth 0.03mm, width 3~5mm) or exhaust rods. It is more important for smaller parts

(6) there is a certain buffer distance between the screw and the injection nozzle

(7) choose low viscosity resin

1.8 shrinkage pit

the cause of shrinkage pit is the same as wrinkle and pitting. In principle, it can be solved by excess filling, but there is a risk of stress. Therefore, attention should be paid to the uniformity of wall thickness in the design, and the wall thickness of stiffeners, bosses, etc. should be reduced as much as possible

1.9 burns

according to the burns caused by different reasons such as machinery, mold or molding conditions, the solutions adopted are also different

(1) reasons for injection molding machine. Due to abnormal conditions, the barrel is overheated, so that the resin is decomposed at high temperature and burned for injection molding, or because the nozzle in the barrel and the above are the uses and functional characteristics of the anchor fatigue testing machine introduced by our technicians, the screw thread, check valve and other parts of the screw cause resin stagnation, improve the production and management of the enterprise and product quality, and bring it into the products after decomposition and discoloration, so that the products have dark brown burn marks. Clean the nozzle, screw and barrel

(2) die cause. Mainly due to poor exhaust. This kind of burn usually occurs in fixed parts, which is easy to distinguish from the first case. Pay attention to measures such as adding exhaust slots and exhaust rods

(3) reasons for forming conditions. If the back pressure is too high, part of the barrel will overheat and cause burns. If the screw speed is too high, overheating will also occur, generally 40~90r/min is the best. If there is no exhaust slot or the exhaust slot is small, too high injection rate will also cause overheating gas burns

1.10 weld marks

weld marks are caused by the cooling of the front part of the molten resin from different directions and the incomplete fusion at the joint. Generally, it mainly affects the appearance, and in serious cases, it affects the strength of the product. The solution is mainly to improve the fluidity and adjust the molding conditions, such as increasing the resin temperature, mold temperature, injection pressure and injection rate; Adding an exhaust slot and setting a push rod at the place where the weld line is produced is also conducive to exhaust; Minimize the use of release agent; If only the appearance is affected, the position of the gate can be changed to change the position of the weld line, or the position where the weld line is produced can be treated as a dark glossy surface and modified

1.11 warpage, deformation

warpage and deformation should be solved mainly from the aspect of mold design, and the adjustment effect of forming conditions is very limited. The causes and solutions of warpage and deformation mainly include:

1) when the deformation is caused by residual stress caused by molding conditions, the stress can be eliminated by reducing the injection pressure, increasing the mold temperature and making the mold temperature uniform, increasing the resin temperature, or annealing

(2) when stress deformation is caused by poor demoulding, it can be solved by increasing the number or area of push rods and setting demoulding slope

(3) when the cooling is uneven or the cooling time is insufficient due to improper cooling method, the cooling method can be adjusted and the cooling time can be extended. For example, set the cooling circuit close to the deformation as much as possible

(4) for the deformation caused by forming shrinkage, the design of the die must be corrected. The most important thing is to make the wall thickness of products consistent. Sometimes, as a last resort, the deformation of the product can only be measured, and the mold can be trimmed in the opposite direction for correction