This article mainly focuses on research and optimization of defects such as easy cracking and short life span of automobile steering wheels. By using high simulation performance of Moldflow 3D technology to analyze essence of easy cracking and short life of products, find out countermeasures, provide optimization and reference for actual production of enterprises.

This product is used all over country, but service life of the product varies greatly in different regions, especially where climate changes more severely, the shorter service life, such as Lhasa, Gansu and other places have broken. Why is product easy to crack in places with severe climate change?
Causes of cracking mainly include high residual stress, insufficient strength of bond line and uneven shrinkage. For this product, there is no bond line problem, so we mainly start from two aspects of residual stress and uneven shrinkage. Targeted research and discussion, through study of interrelated factors, find key to problem, then solve root cause of product cracking by improving design and process.

This product is a plastic product on steering wheel of a car, material is ABS+PC, mold is two cavities, and product size varies. Diameter of entire steering wheel is 380MM. Due to high requirements on appearance of product, glue feeding method is one point edge feeding, as shown in Figure 1. Since size of two products is quite different, we use a sequence valve to fill two products at the same time. Metal skeleton insert is put in actual production, as shown in Figure 2. Actual product use process has been sprayed with black paint. Because metal skeleton is embedded inside product, we can only take pictures of product cracking, cannot take picture of product split into two after it is broken. Fracture area is shown in Figure 3 (red circle A white line inside).

 

1. Internal structure of original product fracture is shown in Figures 4 and 5. We can see that there are certain defects in product structure. Separate product from metal skeleton and cut product as shown in Figure 6, we can see that product cannot pass frame boss smoothly during filling process, mainly due to large angle change here, which may trap air at the root of boss during filling process.

 

2. Let’s take a look at flow condition of material flow front passing through metal frame boss. As shown in Figure 7-1, we can see trapped air at the bottom of boss. Small pink circle in Figure 7-2 indicates possible trapped position. If exhaust cannot be smoothly exhausted, roots of all bosses in product will be trapped. As a result, pressure of root gas increases rapidly, causing plastic molecules around gas to receive a great expansion force. After product is cooled, this force will be frozen in the form of residual stress. As climate changes drastically, temperature inside car changes more frequently, causing gas trapped in product to produce thermal expansion and contraction effects, residual stress inside product will be released by continuous expansion of gas, result of release will inevitably end in the form of product fracture.

 

3. Let’s take a look at volume shrinkage at the break of product. As shown in Figure 8-1, we can see that volume shrink at the break is significantly smaller than shrinkage on both sides, so molecules at the fracture will be pulled by greater shrinkage on both sides, and this pulling force will eventually be frozen in product in the form of internal stress. It can also be understood by comparing density of fracture and its sides, as shown in Figure 8-2. Release result also ends in the form of product fracture.

 

4. Revised new product is shown in Figure 9-1 and 9-2. Angle at the boss is changed from original 88 degrees to current 40 degrees. When material flow front passes here, there is no more air trapping, as shown in Figure 10. Difference in volume shrinkage between fracture and both sides is also reduced by about 0.5%, as shown in Figure 11. We have conducted high and low temperature impact tests for new products produced, there is no cracks and overall performance is much higher than original products.

 

 

Although structure of steering wheel product is relatively simple, internal air trapping of product seems to be a small problem and will not affect appearance, but there are serious quality problems in actual use. Finally, unqualified products are recycled and re-produced, which wastes a lot of manpower, material resources and seriously affects production cycle of product. Therefore, we need to predict product quality problems as early as possible in the early stage of product development, optimize product structure, instead of waiting for quality problems to occur before modifying product structure. Therefore, it is necessary to use Moldflow to analyze products in product design stage. At the same time, it has established a good image of company and won market competitiveness for company.