The phenomena and solutions of various defects in injection molding refer to the phenomena and corresponding solutions of cracking, insufficient filling, wrinkles and pockmarks, shrinkage pits and overflow in plastic products during injection molding.

Basic introduction

1. Cracking

Cracking is a common defect in plastic products. The main reason for its occurrence is due to stress deformation. There are mainly residual stress, external stress and stress deformation caused by the external environment.

(I) Cracks caused by residual stress

Residual stress is mainly caused by the following three situations, namely, overfilling, demolding and metal inlays. As cracks caused by overfilling, their solutions can mainly start from the following aspects:

(1) Since the pressure loss of the direct gate is the smallest, if the cracks are mainly generated near the direct gate, you can consider using multi-point distribution gates, side gates and handle gates.

(2) Under the premise of ensuring that the resin does not decompose or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity, improve fluidity, and also reduce the injection pressure to reduce stress.

(3) In general, stress is easily generated 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 generation of stress can be reduced.

(4) If the injection and holding time is too long, stress will also be generated. It is better to shorten it appropriately or switch to the th holding time.

(5) Non-crystalline resins such as AS resin, ABS resin, PMMA resin, etc. are more likely to generate residual stress than crystalline resins such as polyethylene and polyoxymethylene, so attention should be paid.

When demolding and pushing out, due to the small demolding slope, the mold type glue and the rough punch, the pushing force 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 during injection molding, stress is most likely to be generated, and cracks are likely to occur after a period of time, which is extremely harmful. This is mainly due to the great difference in the thermal expansion coefficients of metal and resin, which produces stress, and as time goes by, the stress exceeds the strength of the gradually deteriorating resin material and cracks are generated. To prevent the resulting cracks, as a rule of thumb, the wall thickness of 7" and the outer diameter of the embedded metal part

General-purpose polystyrene is basically not suitable for inserts, and inserts have the least impact on nylon. Since the thermal expansion coefficient of glass fiber reinforced resin materials is small, they are more suitable for inserts.

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

(II) Cracks caused by external stress

The external stress here is mainly caused by stress concentration due to unreasonable design, especially at sharp corners. As shown in Figure 2-2, R/7" can be taken as 0.5~0.7.

(III) Cracks caused by external environment

Water degradation caused by chemicals and moisture absorption, as well as excessive use of recycled materials will deteriorate physical properties and cause cracks.

II. Insufficient filling

The main reasons for insufficient filling are as follows:

Insufficient resin capacity. Insufficient pressure in the cavity. Insufficient resin fluidity. Poor exhaust effect.

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 solidifies and making it difficult to fill the cavity due to the short molding cycle.

2) Increase the injection speed.

3) Increase the mold temperature.

4) Increase the resin temperature.

5) Increase the injection pressure.

6) Enlarge 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 set at the maximum wall thickness of the product.

8) Set exhaust grooves (average depth 0.03mm, width 3~smm) or exhaust rods. It is more important for smaller workpieces.

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

10) Select materials with low viscosity grades.

11) Add lubricants.

3. Wrinkles and pitting

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

4. Shrinkage

The cause of shrinkage is the same as insufficient filling. In principle, it can be solved by overfilling, but there is a risk of stress. Attention should be paid to uniform wall thickness in design, and the wall thickness of reinforcing ribs, convex columns, etc. should be reduced as much as possible.

5. Overflow

For overflow treatment, the focus should be on improving the mold. As for molding conditions, we can start with reducing fluidity.

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