Jialixin PP teach you, slove injection molding 12 kinds of defects
Jialixin PP teach you, save injection molding 12 kinds of defects
Jialixin PP-- a brand dedicated to the study of modified PP
In the process of injection molding, it is inevitable to encounter all kinds of problems, jialixin collected and sorted out 12 kinds of problems that may occur in the process of injection molding and solutions, hope to be of some help to you:
1、crack
Cracking is a common defect of plastic products. The main reason is stress deformation. The main factors are residual stress, external stress and external environment caused the tress deformation caused.
（一）Cracking caused by residual stress
Residual stress is mainly caused by the following three conditions: overfilling, stripping and metal inlays.As a crack caused by overfilling, its solution can be mainly in the following aspects:
（1）Since the pressure loss of the sprue is the least, if the cracks are mainly generated near the sprue, the multi-point distribution sprue, side sprue and handle sprue can be considered.
（2）Under the premise that the resin does not decompose and deteriorate, appropriately increasing the temperature of the resin can reduce the melt viscosity, improve the fluidity, and also reduce the injection pressure, in order to reduce the stress. （3）In general, the low mold temperature is easy to produce stress, should be appropriate to increase the temperature.But when the injection speed is higher, even if the mold temperature is lower, the stress can be reduced.
（4）Injection and holding pressure for too long will also produce stress. It is better to shorten it appropriately or switch holding pressure for Th time.
（5）Non-crystalline resin, such AS AS resin, ABS resin, PMMA resin than crystalline resin such AS polyethylene, polyformaldehyde and other easy to generate residual stress, should be paid attention to.
When demoulding is rolled out, because demoulding slope is small, mould type glue and punch is coarse, make push out force is too big, produce stress, produce albinism or rupture phenomenon even around push out rod sometimes.As long as you carefully observe the location of the cracks, you can determine the cause.
In the injection molding at the same time embedded in the metal parts, the most easy to produce stress, and easy to after a period of time to produce cracks, great harm.This is mainly due to the large difference in thermal expansion coefficient between metal and resin, and with the passage of time, the stress exceeds the strength of gradually deteriorating resin materials and crack.To prevent the resulting cracking, as a rule of thumb, the wall thickness is 7" with the outer diameter of the embedded metal piece.
General purpose polystyrene is basically not suitable for insert, and insert has the least effect on nylon.Because the coefficient of thermal expansion of glass fiber reinforced resin is small, it is more suitable for embedding.
In addition, preheating the metal insert before forming also has a good effect. （二）Cracking caused by external stress
The external stress here is mainly caused by unreasonable design and stress concentration, especially at the sharp corner.According to figure 2-2, R / 7" can be taken as 0.5 to 0.7.
（三）Cracking caused by external environment
Chemical drugs, moisture absorption caused by water degradation, and the excessive use of recycled materials will make the physical deterioration, resulting in cracking.
2、Insufficient filling
The main reasons for insufficient filling are as follows:
i. Insufficient resin capacity.
ii. Insufficient pressure in the cavity.
iii. Insufficient fluidity of resin.
iv. Exhaust effect is not good.
As improvement measures, we can proceed from the following aspects:
1）Longer injection time, to prevent the molding cycle is too short, cause before the gate curing resin reflux and difficult to fill the cavity.
2）Increase injection speed.
3）Raise mold temperature.
4）Raise resin temperature.
5）Increase injection pressure.
6）Enlarge gate size.General gate height should be equal to the product wall thickness of 1/2 ~ l / 3.
7）The gate is arranged at the maximum wall thickness of the product.
8）Set the exhaust tank (average depth 0.03mm, width 3 ~ SMM) or exhaust rod.This is more important for smaller artifacts.
9）A buffer distance (about SMM) is left between the screw and the injection nozzle.
10）Choose materials of low viscosity grade。
11）Add lubricant.
3、Wrinkles and pockmarks
The cause of this defect is essentially the same as insufficient filling, but the degree is different.Therefore, the solution is basically the same as above.Especially for the poor mobility of resin (such as polyformaldehyde, PMMA resin, polycarbonate and PP resin) more need to pay attention to increase the gate and appropriate injection time.
4、Shrinkage hole
The reason of shrinkage pit is the same as that of insufficient filling. In principle, it can be solved by excessive filling, but there will be the risk of stress generation. It is necessary to pay attention to the uniformity of wall thickness in design, and reduce the wall thickness of reinforcing rib and convex column as far as possible.
5、Overflow edge
The treatment of overflow edge should mainly focus on the improvement of the mold.In the molding conditions, it can be in the reduction of liquidity.Specifically, the following methods can be adopted:
1）Lower injection pressure。
2）Lowering resin temperature。
4）Choose materials of high viscosity grade。
5）Lower mold temperature。
6）The die surface where the grinding overflow occurs。
7）Use hard die steel。
8）Increase clamping force。
9）Adjust the joint surface of the mould accurately。
10）Add die support column to increase rigidity。
11）According to different materials to determine the size of different exhaust tank。
6、Weld mark
The weld mark is caused by the front part of the melt resin from different directions being cooled and failing to completely fuse at the junction.Under normal circumstances, the main impact on the appearance of coating, electroplating impact.In serious cases, the strength of the product is affected.(especially in fiber-reinforced resins).Please refer to the following items for improvement:
1）Adjust molding condition to improve fluidity.For example, improve resin temperature, mold temperature, injection pressure and speed.
2）The addition of exhaust groove and the setting of push rod in the production of weld line are also beneficial to exhaust。
3）Minimize the use of release agent。
4）Set up the process overflow and take it as the place where the weld mark is generated, cut off and remove it after forming。
5）If only the appearance is affected, the position of the fourth position can be changed to change the position of the weld mark.Or the place that produces weld mark is processed for dark burnish surface, give decorate。
7、burns
Depending on the cause of the burn, such as the machine, the mold or the molding condition, the solution may be difference.
1）Mechanical reasons, for example, due to abnormal conditions caused material cylinder overheating, resin decomposition at high temperature, burn injection into the product, or because of the nozzle and screw screw, check valve and other parts of resin stagnation, decomposition discoloration into the product, in the product with black brown burn scar.At this time, the nozzle, screw and barrel should be cleaned。
2）Mold cause, mainly because of poor exhaust caused by.This burn usually occurs in a fixed place and is easily distinguished from the first case.At this time should pay attention to take measures such as gas sump counter exhaust rod。
3）In terms of molding conditions, the back pressure above 300MPa will cause overheating of the cylinder and cause burns.When the screw speed is too high, it will also produce overheating, generally within the range of 40 ~ 90r/min.Excessive injection speed may cause overheating gas burns in the absence of an exhaust vent or when the exhaust vent is small。
8、Silver wire
The silver wire is mainly caused by the hygroscopicity of the material.Therefore, generally should be in the resin thermal deformation temperature lower than the condition of 10 ~ 15C drying.For PP wax series with higher requirements, it needs to be dried for 4 ~ 6h under the condition of about 75t.Especially in the use of automatic drying hopper, need to according to the molding cycle (molding volume) and drying time to choose a reasonable capacity, should also be a few hours before the start of injection boot drying materials。
Additional, material is short inside material stagnant time is too long also can produce silver line.When different kinds of materials are mixed, such as polystyrene.And ABS resin, AS resin, polypropylene and polystyrene are not suitable for mixing.
9、Jet lines
Jet lines from the gate along the direction of the flow, curved like a snake traces.It is caused by the excessive injection rate of resin from the gate.Therefore, enlarging the cross section of burning fourth or lowering the injection speed are optional measures.In addition, increasing the mold temperature can also slow down the cooling rate of the resin in contact with the surface of the mold cavity, which has a good effect on preventing the formation of surface hardened skin in the early stage of filling。
10、Air bubbles
According to the causes of bubbles, the countermeasures are as follows：
1）When the wall thickness of the product is large, the cooling speed of the outer surface is faster than that of the central part. Therefore, as the cooling progresses, the resin in the central part contracts and expands towards the surface, resulting in insufficient filling in the central part.This situation is called a vacuum bubble.There are major solutions：
a）According to the wall thickness, determine the reasonable gate, gate size.General gate height should be the product wall thickness of 50% ~ 60%。
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 injection speed and increase injection pressure，
e）Adopt the material with high melting viscosity grade。
2）As a result of the production of volatile gas bubbles, the solution is mainly：
a）Predry thoroughly。
b）Reduce the resin temperature to avoid the production of decomposition gas。
3）The bubble caused by poor fluidity can be solved by raising the temperature of resin and mold and increasing the injection speed。
11、An albino
The albinism phenomenon mainly occurs in ABS resin products.Poor demoulding effect is the main reason.Can be used to reduce the injection pressure, increase the degree of release, increase the number or area of the push rod, reduce the surface roughness of the mold and other methods to improve, of course, spray release agent is also a method, but should be careful not to the subsequent process, such as hot stamping, coating and other adverse effects.
12、Buckling deformation
Warping and deformation of injection products is a very difficult problem.The adjustment effect of forming condition is very limited.Warpage, deformation causes and solutions can refer to the following：
1）When deformation is caused by residual stress caused by molding conditions, the stress can be eliminated by reducing injection pressure, increasing the mold temperature and even the resin temperature, or by annealing。
2）When the stress deformation is caused by the bad demoulding, it can be solved by increasing the number or area of the push rod and setting the demoulding slope.
3）Due to improper cooling method, uneven cooling or insufficient cooling time, the cooling method can be adjusted and the cooling time can be extended.For example, a cooling loop can be placed as close to the deformation as possible.
4）For the deformation caused by forming shrinkage, it is necessary to modify the mold design.Among them, the most important is to pay attention to make the wall thickness of products consistent.Sometimes, in the case of no choice, had to measure the deformation of the product, according to the opposite direction to trim the mold, to be corrected.Shrinkage of larger resin, ~ generally is crystalline resin (such AS polyformaldehyde, nylon, polypropylene, polyethylene and PET resin) than non-crystalline resin (such AS PMMA resin, polyvinyl chloride, polystyrene, ABS resin and AS resin) deformation.In addition, glass fiber reinforced resin has fiber orientation, deformation is also large.
