Defects And Solutions For Injection Molding

Cracking is a common defect of plastic products, and the main reason 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 in 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 and other relatively crystalline resins such as polyethylene, polyoxymethylene, etc., tend to cause residual stress, which should be noted.

(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.

2. Insufficient filling

The main reasons for insufficient filling are as follows:

(1) The resin capacity is insufficient.

(2) Insufficient pressure in the cavity.

(3) The resin has insufficient fluidity.

(4) 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 1/2~1/3 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 smm) or the exhaust rod. It is more important for smaller workpieces.

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

(10) Select materials with low viscosity grades.

(11) Add a lubricant.

3. Wrinkle and pockmark

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. In particular, it is more necessary to pay attention to appropriately increasing the gate and the proper injection time for resins with poor fluidity (such as polyoxymethylene, PMMA resin, polycarbonate, and PP resin).

4. Shrink pit

The reason for the shrinkage pit is also the same as the filling deficiency. In principle, it can be solved by excess filling, but there is a danger of stress. The design should pay attention to the uniform wall thickness, and the ribs and studs should be reduced as much as possible. Wall thickness.

5. Overflow

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) Select materials with high viscosity grades.

(4) Reduce the mold temperature.

(5) Grinding the mold surface where the overflow occurs.

(6) Use a 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 exhaust slots according to different materials.

6. Weld marks

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:

(1) Adjust molding conditions to improve liquidity. For example, increase the resin temperature, increase the mold temperature, increase the injection pressure and speed.

(2) Adding an exhaust groove, and providing a push 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.

7. Burns

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

(1) Mechanical reasons, for example, due to overheating of the barrel due to abnormal conditions, pyrolysis of the resin, injection into the product after burns, or stagnant resin due to nozzles and screw threads, check valves, etc. in the material. After disintegrating and discoloring, it is brought into the product, and there is a dark brown burn mark 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 and burns are caused. 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 burnt of the overheated gas.

8. Silver wire

The silver wire is mainly caused by the hygroscopicity of the material. Therefore, it should generally be dried under conditions of 10 to 15 C lower than the heat distortion temperature of the resin. For the more demanding PMMA tree wax series, it needs to be dried for 4~6h 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 material will produce 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.

9. Jet flow 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.

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 lowering the injection pressure, increasing the mold and making the mold temperature uniform, increasing the resin temperature, or annealing.

(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.

A resin with a large shrinkage ratio is generally a crystalline resin (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin) than a non-crystalline resin (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin). And AS resin, etc.) have large deformation. In addition, since the glass fiber reinforced resin has fiber orientation, the deformation is also large.

10. Bubbles

According to the cause of the bubble, the solution to the problem 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 being shrunk, so that the center portion is insufficiently filled. This condition is called a vacuum bubble. The main solutions are:

1) Determine the reasonable gate and runner size according to the wall thickness. Generally, the height of the gate should be 50% to 60% of the wall thickness of the product.

2) Until the gate is sealed, a certain amount of supplementary injection material is left.

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

4) Reduce the injection speed and increase the injection pressure.

5) Use a material with a high melt viscosity grade.

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

1) Fully pre-drying.

2) Lower the resin temperature to avoid decomposition gas.

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

11. 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 used to reduce the injection pressure, increase the draft angle, increase the number or area of the push rod, and reduce the surface roughness value of the mold. Of course, the spray release agent is also a method, but care should be taken not for the subsequent process. Such as hot stamping, painting, etc. have adverse effects.


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