Background Information:

Workpiece: rear wheel eyebrow/front wheel eyebrow

Mold: one out of two, L+R

Project name: AS33

Material grade: ASAK6041

Problem description:

The tiger skin pattern appeared in the trial stage of the front/rear two sets of wheel eyebrows of the automobile, which could not be solved effectively, which affected the progress of the project. This time, the melt flow index is again upgraded to the client-side matching process for verification.

Research on the mechanism of tiger skin pattern:

In injection molding, the workpiece is thin, the cavity space is small, the mold temperature is low, the material temperature is low, the structure is complex, and the melt flow is long, which will cause the frozen layer at the front of the melt to become thicker and larger, and the flow resistance will increase, a faster rate of fire will also cause increased resistance. There will be a slowdown or stagnation of the melt front, where the filled areas have poor gloss. With the application of injection pressure, the subsequent hot melt colloid continues to pour in. When the melt accumulates to a certain level, it breaks through the resistance ahead and advances in a jumping manner, at this time, the appearance and gloss are better. In this way, the deceleration and jumping cycle will form a difference in gloss on the surface of the product, and the light and dark will alternate, thus forming a tiger skin pattern (see Figure 2).

After multi-dimensional process verification, using high temperature, high pressure, low speed, gates on both sides but not participating in the molding injection process, the tiger skin pattern is significantly improved and basically eliminated.

Left image: Before improvement               Right image: After improvement

Conclusions:

1. Using materials with a melting index of 9 to verify different processes, the improvement of the tiger skin pattern is limited, and the surface of the product will be foggy;

2. Under the original process state, when the melting index is 20 and 24 respectively, the tiger skin pattern on the rear wheel eyebrows is the same;

3. Under the original process state, when the melting index is 20 and 24 respectively, the tiger skin pattern of the front wheel eyebrow is improved;

4. When the melt index is 24, high temperature, high pressure and low speed are used for the rear wheel eyebrows, and gates on both sides but not participating in the molding injection process, the tiger skin pattern is significantly improved and basically eliminated;

5. When the melt index is 24, high temperature, high pressure and high speed are used for the front wheel eyebrows, and gates on both sides but not participating in the molding injection process, the tiger skin pattern is significantly improved and basically eliminated;

6. Under the same process, because of the difference in the gate width of the left/right wheel eyebrow and the difference in the skin pattern on the back, there are also differences in the state of the tiger skin pattern;

7. The front wheel eyebrow is produced under the conditions of high temperature, high pressure and high speed, and the product quality is unstable, and five injection molding defects will occur. The final adjustment is medium and high temperature, high pressure, medium speed/high speed/low speed process, other defects are eliminated, production is stable, but the tiger skin pattern is basically unchanged.

8. The temperature of the rear wheel eyebrow mold should not be too high. When the temperature exceeds 80 °C, there will be defects on the surface of the product caused by poor cooling and solidification effect and deformation of the mold;

9. This mold trial is based on different melting index, different molds, different injection molding processes, and different gate widths, so as to draw different conclusions about the state of the back skin pattern, which provides sufficient experience for solving the problem of product tiger skin pattern in the future.