Cracking is a common defect of plastic products. It is mainly caused by cracking in places where stress is easy to concentrate or weld, or cracking after painting for a period of time. The main cause is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by the external environment.

(1) Crack caused by residual stress

Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. As a crack generated in the case of excessive filling, the solution can be mainly started from the following aspects:

1) Since the pressure loss of the sprue is the smallest, if the crack is mainly generated near the sprue, it may be considered to use a multi-point distribution point gate, a side gate and a shank gate.

2) Under the premise of ensuring that the resin does not decompose or deteriorate, the proper increase of the resin temperature can lower the melt viscosity and improve the fluidity, and at the same time, the injection pressure can be lowered to reduce the stress.

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

4) If the injection and holding time are too long, stress will also be generated, and the effect of appropriately shortening or performing Th holding pressure switching is better.

5) Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., are more likely to cause residual stress than crystalline resins such as polyethylene, polyoxymethylene, etc., and should be noted.

When the demoulding is introduced, the ejection force is too large, the stress is generated due to the small draft of the mold release, the mold type rubber and the convex mold are rough, and sometimes whitening or cracking occurs even around the push rod. Just look carefully at the location of the crack and determine the cause.

When metal parts are embedded at the same time as injection molding, 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 the fact that the thermal expansion coefficients of the metal and the resin differ greatly in stress, and over time, the stress exceeds the strength of the gradually degraded resin material to cause cracks. In order to prevent the resulting cracks, as a rule of thumb, the wall thickness of 7" and the outer diameter of the embedded metal part of the general-purpose polystyrene is basically not suitable for the insert, and the insert has the least influence on the nylon. The resin material has a small coefficient of thermal expansion and is suitable for an insert.

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

(2) Crack caused by external stress

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

(3) Cracks caused by the external environment

Chemicals, water degradation due to moisture absorption, and excessive use of recycled materials can degrade physical properties and cause cracks.

Second, insufficient filling or lack of glue

The main reasons for insufficient filling are as follows:

i. Insufficient resin capacity.

Ii. Insufficient pressure in the cavity.

Iii. Insufficient fluidity of the resin.

Iv. The exhaust effect is not good.

As an improvement measure, 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 due to the short molding cycle, and it is difficult to fill the cavity.

2) Increase the injection speed.

3) Increase the mold temperature.

4) Increase the resin temperature.

5) Increase the injection pressure.

6) Expand the gate size. Generally, the height of the gate should be equal to 1/2 to 1/3 of the wall thickness of the product.

7) The gate is placed at the maximum wall thickness of the product.

8) Set the exhaust groove (average depth 0.03mm, width 3 to 5mm) or the exhaust rod. It is more important for smaller workpieces.

9) A certain (about 5 mm) buffer distance is left between the screw and the injection nozzle.

10) Use low viscosity grade materials.

11) Add lubricant.

Third, wrinkles and pockmarks

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

Fourth, shrink pit

The cause of shrinkage is the same as that of filling. In principle, it can be solved by excess filling, but there is a danger of stress. The wall thickness should be designed evenly. The walls of reinforcing ribs and studs should be reduced as much as possible. thick.

Five, weld line

The weld line is caused by the front end portion of the molten resin from different directions being cooled and not being completely fused at the joint. Under normal circumstances, it mainly affects the appearance and affects painting and plating. In severe cases, it has an effect on the strength of the product (especially in the case of fiber reinforced resins). Can be improved by referring to the following items:

l) Adjust molding conditions to improve liquidity. Such as increasing the resin temperature, increasing the mold temperature, increasing the injection pressure and speed.

2) Adding a venting groove, and providing a push-out rod at the place where the weld line is generated is also advantageous for exhausting.

3) Minimize the use of release agents.

4) Set the process flash as the place where the weld line is produced, and then cut and remove after molding.

5) If only the appearance is affected, the four positions can be changed to change the position of the weld line. Alternatively, the portion where the weld mark is produced may be treated as a dark glossy surface or the like.

Six, burns

Generally, the so-called charring includes the discoloration caused by plastic degradation on the surface of the product and the blackening of the filling end of the product; the gas grain refers to the streaks caused by the gas impinging on the rubber due to poor mold discharge or insufficient molding conditions, which occurs mostly in the The position of the nozzle, the poor exhaust gas causes the gas to be highly compressed and heated, which will also cause the product to burn.

The solutions adopted are different depending on the burn caused by mechanical, mold or molding conditions.

1) Mechanical reasons. For example, due to abnormal conditions, the barrel is overheated, the resin is pyrolyzed, burned, and injected into the product, or the resin is stagnant due to the nozzle and the screw thread of the barrel, the check valve, etc. The product has dark brown burn marks in the product. At this time, the nozzle, screw and barrel should be cleaned.

2) The reason for the mold 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, attention should be paid to measures such as adding a venting groove to the exhaust rod.

3) In terms of molding conditions, when the back pressure is above 300 MPa, the barrel is partially overheated, causing burns. When the screw speed is too high, overheating will also occur, which is generally in the range of 40 to 90 r/min. When there is no venting groove or the venting groove is too small, the injection speed is too high, which may cause air slicks and hot gas burns.

Seven, overflow edge (flash)

Most of them occur in the split position of the mold, such as the split surface of the movable mold (male mold, front mold) and static mold (mother mold, rear mold), the sliding part of the slider, the gap of the insert, the ejector Porosity and so on. The overflow is largely due to the failure of the mold or machine clamping force.

The focus of processing on the overflow should be mainly on the improvement of the mold. In terms of molding conditions, it is possible to reduce the fluidity. Specifically, the following methods can be used:

1) Reduce the injection pressure.

2) Lower the resin temperature.

3) Use high viscosity grade materials.

4) Reduce the mold temperature.

5) Grinding the mold surface where the overflow occurs.

6) Use harder mold steel.

7) Improve the clamping force.

8) Adjust the joint surface of the accurate mold and so on.

9) Increase the mold support column to increase rigidity.

10) Determine the size of the different vents according to different materials.

Eight, silver line

It appears that the surface of the product has a long silver wire opening direction along the flow direction. Where the product is not completely filled, the front end of the fluid is rough. The silver wire is mainly caused by the hygroscopicity of the material. Therefore, it should generally be dried at a temperature lower than the heat distortion temperature of the resin by 10 to 15 °C. For the more demanding PMMA tree wax series, it needs to be dried for 4-6 hours under the condition of about 75t. Especially when using the automatic drying hopper, it is necessary to select a reasonable capacity according to the molding cycle (forming amount) and the drying time, and it is also necessary to start the baking before a few hours before the start of the injection. In addition, too long a material stagnation time 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.

Nine, spray pattern

The jet pattern is a trace of a meandering curve from the gate along the flow direction. It is caused by the excessive injection speed of the resin from the gate.

Therefore, expanding the cross section of the burnt or lowering the injection speed is an optional measure. In addition, by increasing the temperature of the mold, the cooling rate of the resin in contact with the surface of the cavity can be slowed down, which also has a good effect in preventing the formation of the surface-hardened skin at the initial stage of filling.

X. Bubbles

According to the cause of the bubble, the solution is as follows:

1) When the wall thickness of the product is large, the outer surface is cooled faster than the center portion. Therefore, as the cooling progresses, the resin at the center portion is expanded toward the surface while shrinking, causing insufficient filling at the center portion. It is called a vacuum bubble. The main solutions are:

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

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 speed and increase the injection pressure.

A material with a high melt viscosity grade is used.

2) The bubbles caused by the generation of volatile gases are solved by:

Fully pre-drying.

Reduce the resin temperature to avoid decomposition gases.

3) Bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and the mold and increasing the injection speed.

Eleven, warping, deformation

The deformation can be divided into two phenomena: warping and twisting. The deformation of the parallel side is called warping, and the deformation of the diagonal direction is called twisting. The warpage and deformation of the injection product is a very difficult problem. Mainly from the mold design, the adjustment effect of the molding conditions is very limited. The causes and solutions for warpage and deformation can be referred to the following:

1) When deformation is caused by residual stress caused by molding conditions, stress can be relieved by reducing the injection pressure, increasing the mold and making the mold temperature uniform, increasing the resin temperature, or using an annealing method.

2) When the stress is deformed due to poor mold release, it can be solved by increasing the number or area of ​​the push rod and setting the draft angle.

3) Since the cooling method is not suitable, and the cooling is not uniform or the cooling time is insufficient, the cooling method and the cooling time can be adjusted. For example, a cooling circuit can be placed as close as possible to the deformation.

4) For the deformation caused by the molding shrinkage, it is necessary to correct the design of the mold. Among them, the most important thing is to make the wall thickness of the product consistent. Sometimes, in the case of a last resort, the mold must be trimmed in the opposite direction by measuring the deformation of the product. Resins with a large shrinkage ratio are generally crystalline resins (such as polyacetal, nylon, polypropylene, polyethylene, and PET resins) than non-crystalline resins (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin, and The deformation of AS resin or the like is large. In addition, since the glass fiber reinforced resin has fiber orientation, the deformation is also large.

Twelve, whitening

The whitening phenomenon mainly occurs in the introduction part of ABS resin products. The poor release effect is the main reason. It can be improved by reducing the injection pressure, increasing the draft angle, increasing the number or area of ​​the push rod, and reducing the surface roughness of the mold. Of course, the spray release agent is also

It is a method, but care should be taken not to adversely affect subsequent processes such as hot stamping and painting.