Case Study: How to use Moldflow to solve problem of easy cracking of steering wheel plastic parts?

Time:2019-12-16 10:09:10 / Popularity: / Source:

First, introduction:

This paper is mainly about research and optimization of defects such as easy cracking and short life of steering wheel of automobile. By using high simulation performance of Moldflow 3D technology, essence of product is easy to crack and life is short, solution is found to provide optimization and reference for actual production of enterprise.

Second, fracture analysis overview:

This product is used all over the country, but service life of product varies greatly in different regions, especially the more severe climate change, the shorter life expectancy, such as Lhasa, Gansu and other places. Why are products prone to cracking in places with severe climate change?
Causes of cracking mainly include high residual stress, insufficient strength of bonding line, and uneven shrinkage. There is no problem with bonding line for this product. Therefore, we mainly start from two aspects of residual stress and uneven shrinkage, conduct targeted research and discussion. By studying interrelated factors, we can find key to problem, then solve root cause of product cracking by improving design and process.

Third, case description:

This product is a plastic product on steering wheel of car. Material is ABS+PC. Mold is two pieces, and product size ranges. Diameter of entire steering wheel is 380mm. Due to high appearance requirements of product, injection solution is pin-point filling, as shown in Figure 1. Due to large difference in size between two products, we use a sequence valve to allow both products to be filled at the same time. Metal frame inserts are placed in actual production, as shown in Figure 2. Actual product has been sprayed with black paint. Since metal skeleton is embedded in the product, we can only capture cracked picture of product and can't take a picture of product after it breaks. Broken area is shown in Figure 3 (a white line in red circle).

Fourth, case analysis and improvement plan:

1. Internal structure of original product breakage is shown in Figures 4 and 5. We can see that product structure has certain defects, separating product from metal skeleton and cutting product as shown in Figure 6. We can see that product cannot pass through skeleton boss during filling process, mainly because angle here varies greatly, and root of boss may be trapped during filling process.
2. Let us look at flow condition of flow front when flowing through metal skeleton boss. As shown in Figure 7-1, we can see that there is a trapped air at the bottom of boss. Small pink circle in Figure 7-2 shows a small trapped position and can not be vented smoothly. Roots of all bosses in the production will be trapped. Pressure of root gas is rapidly increased, causing plastic molecules around gas to be subjected to a large expansion force. After product is cooled, force will be frozen in the form of residual stress. Because temperature change in the car is more frequent in places where climate change is more severe, gas trapped in the product has effect of thermal expansion and contraction. Residual stress inside product will be released by continuous expansion of gas over time, and release result will end in the form of product fracture.
3. Let us look at volume shrinkage at break area of product. As shown in Figure 8-1, we can see that volume shrinkage at break area is significantly smaller than shrinkage on both sides, so molecules at break area will be subjected to a greater contraction of two sides, which will eventually freeze in the form of internal stress. It can also be understood by comparing density of fracture to its sides, as shown in Figure 8-2. Result of release is also terminated in the form of a broken product.
4. Modified new product is shown in Figures 9-1 and 9-2. At the boss, angle is changed from original 88 degrees to current 40 degrees, and there is no more air trapping at the front of stream, as shown in Figure 10. Difference in volumetric contraction between fracture and sides is also reduced by about 0.5%, as shown in Figure 11. New products produced have been tested for high and low temperature, no cracks and overall performance is much higher than original products.

Fifth, conclusion:

Although structure of steering wheel is relatively simple, internal gas trap seems to be a small problem without influence on appearance. However, in actual use, serious quality problems have occurred. Finally, unqualified products are recycled, re-production, a lot of manpower and material resources are wasted, which seriously affects production cycle of products. Therefore, we need to predict quality of product as early as possible in the early stage of product development, optimize product structure, instead of waiting until quality problem really occurs, then we think that we need to modify product structure. Therefore, it is necessary to use Moldflow for product analysis in product design stage. At the same time, it has also established a good image of company and won market competitiveness for company.

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