Defects of Automobile Injection Parts and Solutions -Tiger skin patterns

Tiger skin patterns often appear on automobile injection parts with large areas such as bumpers, dashboards, door panels and pillars. They are a kind of wavy stripe surface defects. The stripes are approximately perpendicular to the direction of melt flow and are formed on the surface of the injection molded parts. Engraved with different gloss, it looks like the pattern on tiger skin, commonly known as tiger skin pattern.

Tiger skin pattern is easy to produce on injection parts with thin wall thickness and large process. It has the following characteristics: (1) The surface of the injection part has periodically changing light and dark stripes; (2) The stripes are roughly perpendicular to the melt The flow direction of (3) The stripes are divided into bright and dark areas, the gloss of the bright area is high, and the gloss of the dark area is poor; (4) If the appearance of the injection molded part is a bright area, the back is a dark area, two Those appear alternately.

 

the reason:


1. The more toughening materials in the material, the easier the tiger skin phenomenon will appear. The toughened material is stretched and sheared during the injection molding process, causing a small amount of deformation, which causes unstable flow of the melt, which leads to tiger skin patterns. Tiger skin patterns rarely occur in materials with poor toughness. Reinforced materials, non-toughened nylon, polybutylene terephthalate, and other materials rarely have tiger skin patterns during the molding process. Polypropylene (PP) used in automotive injection molded parts is required to have high impact resistance. Toughening ingredients such as elastomers are added, which is very prone to tiger skin defects.


2. The thinner the wall thickness of the injection molded part, or the farther the filling flow distance is, the greater the flow length ratio is. The melt is more likely to produce unstable flow during the mold filling process, and the easier it is to produce tiger skin patterns. Properly increasing the wall thickness of the injection molded parts, or shortening the filling distance of the single gate, can reduce the mold filling resistance of the melt, ensure the stability of the melt flow, and help eliminate tiger stripes. However, with the increasing requirements for lightweight vehicles, thinning is the development trend of automobile injection parts, so it is not realistic to eliminate tiger skin by increasing the wall thickness.

 

Solution:


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


    In the design of the mold, the straight transition gate, side gate and fan gate with gradual transition should be used as much as possible, avoiding the use of latent gates and point gates with gradually decreasing cross-sectional area. Practice has proved that latent gates, point gates or small side gates are 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 tiger skin pattern. In general, increasing the melt temperature and mold temperature, and adjusting the injection speed are beneficial to eliminate tiger skin lines. The use of a lower injection speed is beneficial to eliminate tiger stripes, because the melt flow is more stable at low speeds.