Cracking is a common defect of plastic products. It is mainly caused by cracking at places where stress is easy to concentrate or weld lines, or cracking after painting for a period of time. Main reason for this is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by external environment.

Residual stress is mainly caused by following three conditions, namely overfilling, demolding, and metal inserts. As a crack generated in the case of overfilling, solution can be mainly started from following aspects:
1) Since pressure loss of gate is the smallest. If crack is mainly generated near gate, consider using a multi-point distribution gate, side gate and shank gate.
2) Under the premise of ensuring that resin does not decompose or degrade, appropriately increasing resin temperature can lower melt viscosity, improve fluidity, and also reduce injection pressure to reduce stress.
3) Under normal circumstances, stress is easy to occur when mold temperature is low, and temperature should be appropriately increased. However, when injection speed is high, even if mold temperature is lower, stress can be reduced.
4) If injection and holding time are too long, stress will also be generated, and it will be better to shorten it appropriately or to perform Th holding pressure switching.
5) Non-crystalline resins such as AS resin, ABS resin, PMMA resin which are more likely to cause residual stress than crystalline resins such as polyethylene, polyoxymethylene, etc. It should be noted.
When demoulding, drafting force is too large, stress is generated due to small draft of mold demoulding, mold cavity and punch are rough, and sometimes whitening or cracking occurs even around push rod. Just look carefully at the location of crack and determine its’ cause.
When metal parts are embedded during injection moulding, stress is most likely to occur, and it is easy to generate cracks after a certain period of time, which is extremely harmful. This is mainly due to fact that thermal expansion coefficients of metal and resin are in great difference to generate stress, and over time, stress exceeds strength of gradually degraded resin material to cause cracks. In order to prevent resulting cracks, as a rule of thumb, wall thickness and outer diameter of embedded metal part are generally not suitable for inserting insert, and insert has the least influence on nylon. Resin material has a small coefficient of thermal expansion and is suitable for an insert.
In addition, preheating metal insert before molding also has a good effect.

External stress here is mainly due to unreasonable design of stress concentration, especially at sharp corners.

Degradation of water caused by chemicals and moisture absorption, excessive use of recycled materials can degrade physical properties and cause cracks.

i. Insufficient resin capacity.
Ii. Insufficient pressure in cavity.
Iii. Insufficient resin fluidity.
Iv. Bad exhaust effect.

1) Lengthen injection time to prevent resin from flowing back before gate is solidified due to short molding cycle, which makes it difficult to fill cavity.
2) Increase injection speed.
3) Increase mold temperature.
4) Increase resin temperature.
5) Increase injection pressure.
6) Expand gate size. Generally, height of gate should be equal to 1/2 to 1/3 of wall thickness of product.
7) Gate is placed where wall thickness of product is the largest.
8) Set exhaust groove (average depth 0.03mm, width 3 to 5mm) or exhaust rod. It is more important for smaller workpieces.
9) A certain (about 5 mm) buffer distance is left between screw and injection nozzle.
10) Use low viscosity grade materials.
11) Add lubricant.

Cause of this defect is essentially same as filling, but to a different extent. Therefore, solution is basically same as above method. Especially for resins with poor fluidity(such as polyoxymethylene, PMMA resin, polycarbonate and PP resin), it is necessary to pay attention to increasing gate and proper injection time.

 

Reason for shrinkage pit is same as insufficient filling. In principle, it can be solved by excess filling, but there is a danger of stress. Wall thickness should be designed to be uniform. Wall thickness of reinforcing ribs and studs should be reduced as much as possible.

Most of them occur in joint position of mold, such as split surface of movable mold ( front mold) and static mold (rear mold), sliding part of slider, gap of insert, hole of ejector pin and so on. Overflow is largely due to failure of mold or machine clamping force.
Focus of processing on overflow should be mainly on the improvement of mold. In terms of molding conditions, it is possible to reduce fluidity. Specifically following methods can be used:
1) Reduce injection pressure.
2) Lower resin temperature.
3) Use high viscosity grade materials.
4) Reduce mold temperature.
5) Grinding mold surface where overflow occurs.
6) Use harder mold steel.
7) Improve clamping force.
8) Accurately adjust joint surface of mold and other parts.
9) Increase mold support column to increase rigidity.
10) Determine size of different exhaust slots according to different materials.

Weld line is caused by front end portion of molten resin from different directions being cooled and not being completely fused at the joint. Under normal circumstances, it mainly affects appearance, coating and plating. In severe cases, it has an effect on the strength of product (especially in the case of fiber reinforced resins). Can be improved by referring to following items:
l) Adjust molding conditions to improve liquidity such as increasing resin temperature, mold temperature, injection pressure and speed.
2) Adding a venting groove, providing a push-out rod at the place where weld line is generated is also advantageous for exhausting.
3) Minimize use of release agents.
4) Set process flash as place where weld line is formed, cut and remove after molding.
5) If only appearance is affected, melting position can be changed to change position of weld line. Alternatively, portion where weld mark is generated may be treated as a dark glossy surface.

 

Generally, so-called burn mark includes discoloration caused by plastic degradation on the surface of product and blackening of filling end of product; gas grain refers to streaks caused by gas impinging on material due to poor mold discharge or insufficient molding conditions, and occurs mostly at nozzle position. Poor exhaust causes a high degree of compression of gas and burns in the product.
Solutions adopted are different depending on burn caused by mechanical, mold or molding conditions.
1) Mechanical reasons. For example, due to abnormal conditions, barrel is overheated, resin is pyrolyzed, burned, and injected into product, or resin is stagnant due to nozzle in the barrel, screw thread, check valve, etc. It is disintegrated and brought into product. At this time, nozzle, screw and barrel should be cleaned.
2) Mold reasons is mainly due to poor exhaust. This type of burn usually occurs in a fixed place and is easily distinguished from the first case. At this time, measures such as adding a venting groove to exhaust rod should be taken.
3) In terms of molding conditions, when back pressure is above 300 MPa, barrel is partially overheated, causing burns. When screw speed is too high, overheating will also occur, generally in the range of 40 to 90 r/min. When there is no venting groove or venting groove is too small,  injection speed is too high, which may cause air marks and hot gas burns.

 

It is characterized by a long silver wire on the surface of product, opening direction is along flow direction. Where product is not completely filled, front end of fluid is rough. Silver wire is mainly caused by hygroscopicity of material. Therefore, it should generally be dried at a temperature 10 to 15℃ lower than heat distortion temperature of resin. For more demanding PMMA resin series, it needs to be dried for 4-6 hours under condition of about 75t. Especially when using automatic drying hopper, it is necessary to select a reasonable capacity according to molding cycle (forming amount) and drying time, and it is also necessary to start baking a few hours ago before injection.
In addition, too long a stagnation time of material in the barrel will also produce a silver wire. When mixing different kinds of materials, such as polystyrene and ABS resin, AS resin, polypropylene and polystyrene, etc., it is not suitable to mix.

 

Spray pattern is a trace that curves like a meander from gate along flow direction. It is caused by high injection speed of resin starting from gate.
Therefore, expanding cross section of melt or lowering injection speed is an optional measure. Further, by increasing mold temperature, cooling rate of resin in contact with surface of cavity can be slowed down, which also has a good effect in preventing formation of surface-hardened skin at initial stage of filling.

Deformation can be divided into two phenomena: warping and twisting. Deformation of parallel side is called warping, and deformation of diagonal direction is called twisting. Warpage and deformation of injection product is a very difficult problem. Main problem should be solved from mold design, adjustment effect of molding conditions is very limited. Causes and solutions for warpage and deformation can be referred to following:
1) When deformation is caused by residual stress caused by molding conditions, stress can be relieved by reducing injection pressure, increasing and making mold temperature uniform, and increasing resin temperature or by annealing.
2) When stress is deformed due to poor mold demoulding, it can be solved by increasing number or area of push rod and setting draft angle.
3) Since cooling method is not suitable to make cooling uneven or cooling time is insufficient, , adjust cooling method and cooling time. For example, a cooling circuit can be placed as close as possible to deformation.
4) For deformation caused by molding shrinkage, it is necessary to correct mold design. Among them, the most important thing is to make wall thickness of product consistent. Sometimes, in the case of a last resort, mold must be trimmed in opposite direction by measuring deformation of product. For resin having a large shrinkage ratio, generally crystalline resins (such as polyacetal, nylon, polypropylene, polyethylene, and PET resins) is more deformed than amorphous resin (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin, or AS resin). In addition, since glass fiber reinforced resin has fiber orientation, deformation is also large.

 

According to cause of bubble, solution is as follows:
1) When wall thickness of product is large, outer surface is cooled faster than center portion. Therefore, as cooling progresses, resin at center portion is expanded toward surface while shrinking, causing insufficient filling at center portion, which is called a vacuum bubble. Main solutions are:
Reasonable gate and runner size are determined according to wall thickness. Generally, height of gate should be 50% to 60% of wall thickness of product.
Until gate is sealed, a certain amount of supplementary injection material is left.
Injection time should be slightly longer than gate sealing time.·
Reduce injection speed and increase injection pressure.
Material with a high melt viscosity grade is used.·
2) Bubbles caused by generation of volatile gases are mainly solved by:
Fully pre-drying.
Reduce resin temperature to avoid decomposition gas.
3) Bubbles caused by poor fluidity can be solved by increasing temperature of resin and mold, increasing injection speed.

Whitening phenomenon mainly occurs in eject part of ABS resin products. Poor demould effect is main reason. It can be improved by reducing injection pressure, increasing draft angle, increasing number or area of push rod, and reducing surface roughness of mold. Of course, spray release agent is also a method, but should not adversely affect subsequent processes such as hot stamping, painting, and so on.