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Cause analysis and solution of cracking of plastic shell products

Release time:2021-11-12

Engineering plastics are widely used in manufacturing industrial parts and various shells because of their excellent properties - high strength, heat resistance, impact resistance, aging resistance and so on. But the cracking of plastic products is also a headache, which is caused by mold, molding process, plastic material, environmental stress and so on. Today, we will discuss the causes and solutions of plastic products cracking.

1、 Analysis of cracking causes and suggestions for improvement

Different cracking causes will lead to different types of cracking. According to the cracking conditions encountered in the actual development process, there are usually four kinds of cracking phenomena of plastic products:

1. Demoulding cracking, when plastic products come out of the mold, they crack directly, and the causes and consequences of this kind of cracking are relatively easy to predict;

2. Reliability test cracking, such as local cracking or cracks after the completion of drop test, environmental test and twist test;

3. The product cracks during assembly;

4. Application cracking, the cracking of products after being placed for a period of time or during use, is often difficult to predict, and the consequences may be devastating.

The above cracking conditions have internal and external causes, and the same reasons; In view of the above problems, we comprehensively analyze the main factors affecting product cracking from the following aspects;

2、 Influence of product design

1. The product wall thickness is too large, and local shrinkage is serious;

2. During the design, the partial bone position of the product is too thin to bear large impact force;

3. The design strength of the whole machine is too weak and there are too few stiffeners;

4. There are too many sharp corners inside the product, and the glue doesn't go smoothly during injection molding. It is better to use fillet transition to reduce the risk of cracking due to stress concentration;

3、 Influence of mold design

1. The rubber inlet is too large;

2. The gate design and distribution are unreasonable, which is easy to form fusion lines, and the product strength near the fusion lines is weak;

3. The design of the cooling system is unreasonable, and local supercooling or overheating causes stress contraction;

4. Insufficient demoulding slope;

5. Poor mold cavity design (internal stress concentration);

6. Insufficient exhaust;

7. The ejection of the product is unbalanced, and it is easy to crack due to local stress during ejection;

8. The surface of the mold is too rough.

4、 Influence of injection molding process

1. Excessive material measurement.

2. The resin temperature is too low.

3. Mold temperature is too low.

4. If the injection pressure is too high or too low, it will affect the product strength. If the pressure is too high, the product structure near the gate will become brittle due to high stress. If the pressure is too low, the strength of the joint line at the end of the product flow will become weak, and the product is also easy to crack. It is suggested to adjust it according to the mold flow analysis data.

5. The injection holding pressure is too high.

6. The shooting time is too long.

7. Ejection speed is too fast or too slow.

5、 Influence of plastic raw materials

1. This material contains volatiles;

2. Mix and match of different materials;

3. The material is not dry;

4. Excessive nozzle material is added;

5. The surface technology in surface treatment will corrode the products, and cracks will appear after a long time;

There are many factors that affect the cracking of injection molded products, which should be comprehensively analyzed and considered. Many factors restrict each other. Product design should be reviewed in advance together with mold design, so as to make reasonable tradeoffs, solve problems and improve quality.

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