What are the defects of injection molding?

1、 Cracking is a common defect of plastic products, which is mainly caused by stress deformation. It mainly includes residual stress, external stress and stress deformation caused by external environment.
（1） Cracking caused by residual stress
The residual stress is mainly caused by the following three conditions, namely, overfilling, demoulding and metal inlay. As the crack caused by overfilling, the solution can be mainly in the following aspects:
(1) Since the pressure loss of the straight gate is the smallest, if the crack is mainly generated near the straight gate, the multi-point distribution gate, side gate and handle gate can be considered.
(2) On the premise of ensuring that the resin does not decompose and deteriorate, properly increasing the resin temperature can reduce the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
(3) Generally, when the mold temperature is low, it is easy to produce stress, and the temperature should be appropriately increased. However, when the injection speed is higher, even if the mold temperature is lower, the stress generation can be reduced.
(4) Too long injection and pressure holding time will also produce stress, so it is better to shorten it properly or carry out Th pressure holding switching.
(5) Non-crystallic resins, such as AS resin, ABS resin, PMMA resin, etc., are more likely to produce residual stress than crystalline resins, such as polyethylene, polyformaldehyde, etc., which should be noted.
During demoulding and ejection, due to the small demoulding slope, rough mold glue and punch, the ejection force is too large, resulting in stress, and sometimes even whitening or cracking around the ejection rod. The cause can be determined by carefully observing the location of cracking.
When inserting metal parts during injection molding, stress is most likely to occur, and it is easy to crack after a period of time, which is extremely harmful. This is mainly due to the great difference in the thermal expansion coefficient of metal and resin, which causes stress, and over time, the stress exceeds the strength of the gradually deteriorated resin material and causes cracks.
In order to prevent the resulting cracking, as an experience, the wall thickness of 7 "and the outer diameter of embedded metal parts
Universal polystyrene is basically not suitable for adding inserts, and the insert has the least impact on nylon. Due to the small thermal expansion coefficient of glass fiber reinforced resin material, it is more suitable for embedded parts.
In addition, preheating the metal insert before forming also has a good effect.
（2） Cracking caused by external stress
The external stress here is mainly caused by stress concentration due to unreasonable design, especially at sharp corners. It can be seen from Figure 2-2 that R/7 "- 0.5~0.7 can be taken.
（3） Cracking caused by external environment
Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials will lead to deterioration of physical properties and cracking.
2、 The main reasons for insufficient filling are as follows:
Insufficient resin capacity.
Insufficient pressure in the cavity.
Insufficient resin fluidity.
The exhaust effect is not good.
As improvement measures, we can mainly start from the following aspects:
1) The injection time is extended to prevent the resin countercurrent before the curing of the gate from filling the mold cavity due to the short molding cycle.
2) Increase injection speed.
3) Increase mold temperature.
4) Increase the resin temperature.
5) Increase injection pressure.
6) Enlarge the gate size. Generally, the height of the gate shall be equal to 1/2~1/3 of the wall thickness of the product.
7) The gate is set at the place with the largest wall thickness of the product.
8) Set exhaust slot (average depth 0.03mm, width 3-mm) or exhaust rod. It is more important for smaller workpieces.
9) There is a certain buffer distance (about smm) between the screw and the injection nozzle.
10) Select materials with low viscosity grade.
11) Add lubricant.
3、 The causes of such defects in wrinkled and pitted faces are essentially the same as that of insufficient filling, but the degree is different. Therefore, the solution is basically the same as above. Especially for resins with poor fluidity (such as polyformaldehyde, PMMA resin, polycarbonate and PP resin), it is more necessary to pay attention to properly increase the gate and appropriate injection time.
4、 The cause of shrinkage pit and shrinkage pit is the same as that of insufficient filling. In principle, it can be solved by excessive filling, but there is a risk of stress. In design, the wall thickness should be uniform, and the wall thickness of stiffeners, convex columns and other places should be reduced as much as possible.
5、 The treatment of overflow should focus on the improvement of the mold. In terms of molding conditions, we can start with reducing liquidity. Specifically, the following methods can be used:
1) Reduce the injection pressure.
2) Reduce the resin temperature.
4) Select materials with high viscosity grade.
5) Lower the mold temperature.
6) Grind the mold surface where the overflow occurs.
7) Use harder die steel.
8) Improve the clamping force.
9) Adjust the joint surface and other parts of the accurate mold.
10) Add mold support column to increase rigidity.
Ll) Determine the size of different exhaust grooves according to different materials.
6、 Weld mark The weld mark is caused by the cooling of the front part of the molten resin from different directions and the incomplete fusion at the junction. Generally, it mainly affects the appearance, painting and electroplating. In serious cases, it will affect the strength of products (especially in the case of fiber reinforced resin). The following items can be referred to for improvement:
l) Adjust molding conditions to improve liquidity. For example, increase the resin temperature, mold temperature, injection pressure and speed.
2) It is also beneficial to exhaust by adding an exhaust slot and setting a push rod at the place where the weld line is generated.
3) Minimize the use of release agent.
4) Set up the process overflow and use it as the place where the weld line is generated, and then cut off and remove it after forming.
5) If it only affects the appearance, it can change the position of the burn four to change the position of the weld line. Or, the part where the weld line is produced shall be treated as a dark glossy surface for modification.
7、 Burns are caused by different reasons such as machinery, mold or molding conditions, and the solutions are also different.
1) For mechanical reasons, for example, the material barrel is overheated due to abnormal conditions, which causes the resin to decompose at high temperature, burn and inject into the product, or the resin is stagnant due to the nozzle, screw thread, check valve and other parts in the material barrel, which causes the resin to decompose and discolor, and then bring into the product, with black and brown burn marks in the product. At this time, the nozzle, screw and barrel should be cleaned.
2) The main reason for the mold is poor exhaust. This kind of burn usually occurs in a fixed place, which is easily different from the first case. At this time, it is necessary to take measures such as adding exhaust slots and anti-exhaust rods.
3) In terms of forming conditions, when the back pressure is more than 300MPa, the material barrel will be overheated, causing burns. When the screw speed is too high, overheating will also occur, which is generally within the range of 40~90r/min. When there is no exhaust slot or the exhaust slot is small, too high injection speed will cause overheating gas burn.
8、 Silver thread Silver thread is mainly caused by the hygroscopicity of the material. Therefore, it should be dried at a temperature 10~15C lower than the thermal deformation temperature of the resin. For the PMMA tree wax series with higher requirements, it needs to be dried for 4~6h under the condition of about 75t). In particular, when using the automatic drying hopper, it is necessary to select a reasonable capacity according to the molding cycle (molding volume) and drying time. It is also necessary to turn on the machine for drying several hours before the injection.
'In addition, too long stagnation time of material in the silo will also produce silver lines. When different kinds of materials are mixed, such as polystyrene. It should not be mixed with ABS resin, AS resin, polypropylene and polystyrene.
9、 Jet striae Jet striae are traces that bend like a snake from the gate along the flow direction. It is caused by the high injection rate of resin from the gate. Therefore, it is an optional measure to expand the cross section of the fourth burner or reduce the injection speed. In addition, increasing the mold temperature can also slow down the cooling rate of the resin in contact with the cavity surface, which also has a good effect on preventing the formation of surface hardening skin at the initial stage of filling.
10、 Warping and deformation The warping and deformation of injection products are very difficult problems. It should be solved mainly from the mold design, while the adjustment effect of forming conditions is very limited. The causes and solutions of warpage and deformation can refer to the following items:
1) When the deformation is caused by the residual stress caused by the molding conditions, the stress can be eliminated by reducing the injection pressure, increasing the mold and making the mold temperature uniform, increasing the resin temperature or adopting the annealing method.
2) When the stress deformation is caused by poor demoulding, it can be solved by increasing the number or area of push rods and setting the demoulding slope.
3) If the cooling method is not suitable and the cooling is uneven or the cooling time is insufficient, the cooling method can be adjusted and the cooling time can be extended. For example, the cooling circuit can be set as close to the deformation as possible.
4) For the deformation caused by forming shrinkage, it is necessary to modify the mold design. The most important thing is to keep the wall thickness of the product consistent. Sometimes, as a last resort, we have to measure the deformation of the product and trim the mold in the opposite direction for correction.
For resins with large shrinkage, the deformation of crystalline resins (such as polyformaldehyde, nylon, polypropylene, polyethylene and PET resins) is greater than that of non-crystalline resins (such as PMMA resin, PVC, polystyrene, ABS resin and AS resin). In addition, due to the fiber orientation of glass fiber reinforced resin, the deformation is also large.
11、 According to the causes of bubbles, the solutions are as follows:
1) When the wall thickness of the product is large, the cooling speed of its outer surface is faster than that of the center part. Therefore, with the cooling process, the resin in the center part shrinks and expands to the surface, resulting in insufficient filling in the center part. This condition is called a vacuum bubble. The main solutions are:
a) Determine reasonable gate and gate size according to wall thickness. Generally, the gate height shall be 50%~60% of the product wall thickness.
b) Until the gate is sealed, a certain amount of supplementary injection material is left.
C) The injection time should be slightly longer than the gate sealing time.
d) Reduce the injection speed and increase the injection pressure,
e) Use materials with high melting viscosity grade.
2) The main solutions to the bubbles caused by the generation of volatile gases are as follows:
a) Pre-dry fully.
b) Reduce the resin temperature to avoid decomposition gas.
3) Bubbles caused by poor fluidity can be solved by increasing the temperature of resin and mold and increasing the injection speed.
12、 Albinism
Whitening mainly occurs in the introduction of ABS resin products. Poor demoulding effect is the main reason. It can be improved by reducing the injection pressure, increasing the release angle, increasing the number or area of push rods, and reducing the surface roughness value of the mold. Of course, spraying release agent is also a method, but it should be careful not to have adverse effects on subsequent processes, such as hot stamping, painting, etc.