Causes and Solutions for Tiger Skin Patterns in the Plastic Injection Molding Automotive Parts

Most plastic injection molding automotive parts adopt the craft of injection molding, with short molding cycle, high production efficiency and low manufacturing cost. However, automotive injection molding may produce some defects, such as tiger skin patterns.

1. Defects of plastic injection molding automotive parts: tiger skin pattern

Tiger skin pattern often appears on large-area plastic injection molding automotive parts such as bumpers, dashboards, door panels, and pillars. It is a surface defect of wavy stripes. The stripes are approximately perpendicular to the direction of melt flow and are formed on the surface of the injection molding. The stamp marks with different gloss look like patterns on tiger skin, commonly known as tiger skin pattern.

Tiger skin patterns are easy to produce on injection moldings with thin walls and complex processes.

2. Reasons for tiger skin patterns on plastic injection molding automotive parts:

(1) The more toughening materials in the material, the more likely the tiger skin pattern will appear. The toughening material is stretched and sheared during the injection molding process of the automobile, which produces a small amount of deformation and makes the melt flow unstable and causes the tiger skin pattern. Materials with poor toughness rarely show tiger skin patterns. For example, reinforced materials, non-toughened nylon, polybutylene terephthalate and other materials will barely have tiger skin patterns in the molding process. Polypropylene (PP) used in plastic injection molding automotive parts is required to have high impact resistance and is added with toughening components such as elastomers, which is very prone to have tiger skin defects.

(2) The thinner the wall thickness of automotive injection molding parts, or the farther the filling flow distance, the larger the flow length ratio, the more likely the melt is to produce unstable flow during the filling process, and the more likely it is to produce tiger skin patterns. Appropriately increasing the wall thickness of the injection molding or shortening the filling distance of a single pouring gate can reduce the mold filling resistance of the melt, ensure the stability of the melt flow, and help eliminate tiger skin patterns. However, as the requirements for lightweight automobiles are becoming higher and higher, thin wall is the development trend of plastic injection molding automotive parts, so it is not realistic to eliminate tiger skin patterns by increasing wall thickness.

The solution to the tiger skin pattern of plastic injection molding automotive parts:

In the design of plastic injection molding automotive parts, increasing the diameter of the runner and expanding the thickness and width of the pouring gate are effective measures to eliminate tiger skin patterns. The gate thickness is preferably 0.7 to 0.8 times the wall thickness. The purpose of this is to reduce the pressure loss of the melt in the runner and gate, and reduce the effect of mold expansion when entering the cavity.

In the mold design, the direct gate, side gate and fan gate with gradual transition should be adopted as far as possible, and the use of latent gate and point gate with gradually reduced cross-sectional area should be avoided. Practice has proved that latent gates, point gates or very small side gates are all prone to tiger skin patterns.

In terms of injection molding process, melt temperature, mold temperature and injection speed are several important parameters that affect the emergence of tiger skin pattern. Generally speaking, increasing the melt temperature and mold temperature, and adjusting the injection speed will help eliminate tiger skin patterns. The use of a lower injection speed is beneficial to eliminate tiger skin patterns, because the melt flow is more stable at low speeds.