I. Cracking

Cracking is a common defect of plastic products, which is mainly caused by stress and deformation. There are mainly residual stress, external stress and stress deformation caused by external environment.

(-) Cracking caused by residual stress

Residual stress is mainly caused by the following three situations: overcharging, demoulding and pushing out, and metal inlaying. Because cracks are produced under the condition of over-filling, the solutions can mainly start from the following aspects:

(1) Because the pressure loss of sprue is the smallest, if cracks mainly occur near sprue, multi-point distribution gate, side gate and handle gate can be considered.

(2) Under 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 reduce the injection pressure to reduce the stress.

(3) Under normal circumstances, stress is easy to occur when the mold temperature is low, so the temperature should be raised appropriately. However, when the injection speed is high, the stress can be reduced even if the mold temperature is low.

(4) Injection and holding time is too long, which will also produce stress. It is best to shorten the holding time appropriately or switch the holding time for the th time.

(5) Amorphous resins, such AS As resin, ABS resin, PMMA resin and other crystalline resins, such as polyethylene, polyoxymethylene, etc., are prone to generate residual stress, which should be paid attention to.

When demoulding and pushing out, because the demoulding inclination is small, the mold glue and punch are rough, the thrust is too large, resulting in stress, and sometimes even whitening or cracking around the push rod. As long as the location of the crack is carefully observed, the cause can be determined.

When metal parts are embedded in 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 thermal expansion coefficient between metal and resin, which leads to stress. As time goes by, the stress exceeds the strength of the deteriorating resin material, which leads to cracks. In order to prevent the resulting cracks, according to experience, the wall thickness is 7 "and the outer diameter is embedded metal parts.

General polystyrene is basically not suitable for inlay, and inlay has the least influence on nylon. Because the thermal expansion coefficient of glass fiber reinforced resin material is small, it is more suitable for embedded parts

In addition, preheating the metal insert before molding also has a good effect.

(2) Cracking caused by external stress

The external stress here is mainly caused by unreasonable design, especially at sharp corners. As can be seen from Figure 2-2, R/7 "is 0.5 ~ 0.7.

(3) cracks caused by external environment

Water degradation caused by chemicals, moisture absorption and excessive use of recycled materials will worsen physical properties and produce cracks.

Second, insufficient filling.

The main reasons for insufficient filling are as follows:

Insufficient resin capacity.

Insufficient pressure in the cavity.

The fluidity of the resin is insufficient.

Exhaust effect is not good.

As improvement measures, we can mainly start from the following aspects:

1) lengthen the injection time to prevent the resin from flowing back before the gate is solidified, and it is difficult to fill the cavity because the molding cycle is too short.

2) improve the injection speed.

3) increase the mold temperature.

4) increase the resin temperature.

5) Increase the injection pressure.

6) Enlarge the gate size. In general, the gate height should be equal to 1/2 ~ L/3 of the product wall thickness.

7) The gate is set at the place with the largest wall thickness of the product.

8) Exhaust slots (with an average depth of 0.03mm and a width of 3 SMM) or exhaust rods shall be provided. This is more important for small workpieces.

9) There is a certain buffer distance (about smm) between the screw and the injection nozzle.

10) Choose materials with low viscosity grade.

1 1) Add lubricant.

Third, wrinkles and pits.

The cause of this defect is essentially the same as that of insufficient filling, but the degree is different. So the solution is basically the same as the above method. Especially resins with poor fluidity (such as polyoxymethylene, PMMA resin, polycarbonate, PP resin, etc.). ), more attention should be paid to properly increasing the gate and proper injection time.

Fourth, shrink the pit

The cause of shrinkage is the same as that of underfilling. In principle, it can be solved by overfilling, but there is a danger of pressure. When designing, attention should be paid to uniform wall thickness, and the wall thickness of reinforcing ribs and convex columns should be reduced as much as possible.

Verbs (short for verb) are rampant.

The treatment of overflow should focus on the improvement of mold. In terms of molding conditions, we can start with reducing fluidity. Specifically, the following methods can be adopted:

1) Reduce the injection pressure.

2) reduce the resin temperature.

4) Choose materials with high viscosity grade.

5) Reduce the mold temperature.

6) Polishing the mold surface with flash.

7) Use hard die steel.

8) Improve the clamping force.

9) Accurately adjust the joint surface and other parts of the mold.

10) increase the mold support column to increase the rigidity.

Ll) Determine the size of different exhaust slots according to different materials.

Six, welding marks

Weld marks are caused by the cooling of the front part of molten resin from different directions and the incomplete fusion of the joint.

Yes Generally speaking, it mainly affects the appearance, and has an impact on painting and electroplating. In severe cases, it will affect the strength of the product.

(Especially in the case of fiber reinforced resin, it is especially serious). You can refer to the following items to improve:

L) Adjust molding conditions to improve fluidity. For example, increase the resin temperature, increase the mold temperature, and increase the injection pressure and speed.

Degree and so on.

2) It is also beneficial to exhaust gas by adding an exhaust groove and setting an ejector pin at the place where weld marks occur.

3) Minimize the use of release agent.

4) Set the process flash as the place where weld marks are generated, and then cut it off after molding.

5) If it only affects the appearance, you can change the position of burning four to change the position of welding marks. Or treat the part where the weld mark is generated into a dark smooth surface, etc. , and modify it.

Seven, burns

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

1) mechanical reasons, such as overheating of the barrel due to abnormal conditions, so that the resin is decomposed at high temperature and injected into the product after combustion.

Or because of the nozzle, thread, check valve and other parts in the bill of materials, the resin stagnates and is brought into the product after decomposition and discoloration, and there is dark brown burn scars 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 and is easily associated with the first kind.

Things are different. At this time, measures such as adding exhaust slots and anti-exhaust rods should be taken.

3) In terms of molding conditions, when the back pressure is above 300MPa, the barrel will overheat and burn. Screw speed

Too high will also cause overheating, generally in the range of 40 ~ 90r/min is better. When there is no exhaust slot or the exhaust slot is small, too high injection speed will cause overheating gas to burn.

Eight, silver line

The silver wire is mainly caused by the hygroscopicity of the material. Therefore, the temperature should be lower than the thermal deformation temperature of the resin 10 ~ 15c.

Drying under certain conditions. For PMMA wax series with high requirements, it needs to be dried at about 75t for 4 ~ 6h. 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 start the machine to dry the materials several hours before the injection.

In addition, the materials in the material list will also produce silver lines if they stay too long. When different kinds of materials are mixed, such as polystyrene.

. And ABS resin, because resin, polypropylene and polystyrene are not suitable for mixing.

Nine, spray flow mode

Spray pattern is a trace that bends like a snake from the gate along the flow direction. This is because the injection speed of resin from the gate is too fast. Therefore, expanding the cross section of sintering furnace or reducing the injection speed is an optional measure. In addition, increasing the mold temperature can also slow down the cooling rate of the resin in contact with the cavity surface, which also plays a good role in preventing the formation of hardened skin at the initial stage of filling.

+,warping, deformation

Warpage and deformation of injection molded products are very difficult problems. It should be mainly solved from the aspect of mold design, but the adjustment of molding conditions is very limited. The causes and solutions of warping deformation can refer to the following items:

1) When the deformation is caused by the residual stress caused by molding conditions, the stress can be relieved by reducing the injection pressure, improving 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 mode is inappropriate, uneven or insufficient, the cooling mode can be adjusted to extend the cooling time. For example, the cooling circuit can be set as close to the deformation as possible.

4) For the deformation caused by molding shrinkage, the mold design must be modified. Among them, the most important thing is to make the wall thickness of the product consistent. Sometimes, if necessary, we have to measure the deformation of the product, trim the mold in the opposite direction and correct it. Resins with large shrinkage are generally crystalline resins (such AS polyoxymethylene, nylon, polypropylene, polyethylene and PET resins) with larger deformation than amorphous resins (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin and AS resin). In addition, because the glass fiber reinforced resin has fiber orientation, the deformation is also large.

XI。 bubble

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, with the progress of cooling, the resin in the central part shrinks and expands to the surface, resulting in insufficient filling in the central part. This condition is called a vacuum bubble. The main solutions are:

A) According to the wall thickness, determine the reasonable gate and runner size. Generally, the gate height should be 50% ~ 60% of the wall thickness of the product.

B) Until the gate is sealed, there is a certain amount of supplementary injection.

C) The injection time should be slightly longer than the gate sealing time.

D) reducing the injection speed and increasing the injection pressure,

E) Use materials with high melt viscosity grade.

2) The methods to solve the bubbles generated by volatile gases mainly include:

A) Fully pre-dried.

B) Reduce the temperature of the resin 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 speed.

Twelve. albinism

Whitening mainly occurs in the push-out part of ABS resin products. Poor demoulding effect is the main reason. It can be improved by reducing the injection pressure, increasing the demoulding slope, increasing the number or area of push rods and reducing the surface roughness of the die. Of course, spraying release agent is also a method, but care should be taken not to cause adverse effects on subsequent processes, such as hot stamping and painting.