Defect analysis and solution of typical automobile injection molding parts

 Most of the automotive plastic parts are injection molding, which has the advantages of short molding cycle, high production efficiency and low manufacturing cost. But injection molding may produce more defects. We collected three typical defects of automotive injection molding parts and their solutions.

 1：Tiger skin pattern

 Tiger skin pattern often appears on the bumper, instrument panel, door panel, column and other large area of automobile injection parts. It is a surface defect of wavy stripes. The stripes are about perpendicular to the melt flow direction, and form different gloss engraving on the surface of injection parts. It looks like the pattern on the tiger skin, commonly known as tiger skin pattern

 The cause of tiger skin pattern：

1. The more toughening materials in the material, the easier the tiger skin phenomenon appears. During the injection molding process, the toughened material is subject to tensile and shear action, resulting in micro deformation and unstable melt flow, resulting in tiger skin. Due to the high impact resistance of polypropylene (PP) used in automobile injection molding parts, it is very easy to have tiger skin defects due to the addition of toughening components such as elastomer.

2.  The thinner the wall thickness of the injection part, or the farther the filling flow distance, the larger the flow length ratio, the more likely the melt is to produce unstable flow and tiger skin in the filling process.

 Solution：

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

 In the mold design, the gradual transition of straight gate, side gate and fan gate should be adopted as far as possible, and the latent gate and point gate with gradually reduced cross-section area should be avoided. Practice has proved that latent gate, point gate or small side gate are easy to produce tiger skin.

 In terms of injection molding process, melt temperature, mold temperature and injection speed are several important parameters affecting tiger skin pattern. Generally speaking, increasing the melt temperature and mold temperature and adjusting the injection speed are beneficial to eliminate the tiger skin. A lower injection speed is beneficial to eliminate tiger stripes, because the melt flow is more stable at low speed.

 2：Shrinkage mark

Shrinkage mark is produced on the surface of injection molded parts, which refers to the local depression formed on the surface of injection molded parts. Shrinkage mark usually occurs in the local thick wall of injection molding parts, or on the back of stiffeners, screw columns and other protrusions. On the injection molded parts of automobile, the back of screw column, reinforcing rib or buckle is easy to appear shrinkage mark.

Solution：

For the shrinkage mark on the back of the screw post, the common improvement measure is to add crater structure at the root of the screw post. Reduce the equivalent wall thickness. The smaller the equivalent wall thickness is, the less likely it is to produce shrinkage marks on the surface of the injection molded parts. When the equivalent wall thickness of the root of the screw column is close to the wall thickness of the injection molded parts, there is no risk of shrinkage marks on the injection molded parts.

When the size of stiffener is not reasonable, it is easy to form shrinkage mark on the back. If it is allowed, the stiffener is designed behind the transition surface of the intersection of two planes. Even if there is a slight shrinkage mark, the shrinkage mark can be cleverly hidden due to the covering effect.

 3：Warpage deformation

Warpage deformation refers to the large difference between the shape of injection molded parts and the designed shape, which is usually convex or concave. Cause of formation: PP material is often used in the design of auto parts injection molding parts. Compared with engineering plastics, its elastic modulus is relatively low, so the ability to resist deformation is weak. Therefore, it is necessary to ensure the strength of injection molded parts from the structural design. The front edge of automobile dashboard is easy to deform. Solution: without increasing the wall thickness, stiffeners can usually increase the strength of injection molded parts and improve the deformation of injection molded parts. However, when the stiffener design is unreasonable, it will increase the deformation of plastic injection molded parts.