Common Problems and Corresponding Solutions of Injection Molding for Bumpers

1. The application of plastic parts in automobiles is becoming more and more extensive

With the rapid development of the automotive industry, plastic parts are used more and more widely in automobiles. The application of automotive plastic parts has significant advantages in terms of reducing the quality of cars, saving fuel, promoting environmental protection, and recyclability. Most automotive plastic injection molding parts adopt injection molding processes, such as bumpers in BMW cars. However, tiger skin patterns, poor surface replication, sink marks, weld lines, warping deformation, etc. are also common defects in automotive injection molded parts. These defects are not only caused by materials, but also have a lot to do with the structural design, mold design and molding process. Here are some common problems and solutions for bumper injection molding.

2. Common problems and corresponding solutions of injection molding for bumpers

(1) Pressure line

There are obvious pressure lines around the bumper fog lamps, which affect the appearance and surface quality of the product. Because the bumper is part of the outer surface of the car, it has strict requirements on the apparent quality, and the pressure line will have a serious impact on its apparent quality.

Possible cause analysis and improvement measures:

a. Mold: a gate G5 is set near the hole around the fog lamp. When this gate is opened, due to the influence of the hole, the pressure on both sides of the hole will reach a balanced pressure line again.

b. Solution: try to reduce the pressure difference around the weld line, or make the pressure difference insufficient to push the solidified melt to move.

(2) Tiger skin pattern

Tiger skin pattern is a surface defect of wavy stripes, which is approximately perpendicular to the direction of melt flow, forming markings with different gloss on the surface of the part, which looks like the pattern on the skin of a tiger, commonly known as tiger skin pattern.

How to improve tiger skin pattern?

a. As for mold design, increasing the diameter of the runner and expanding the thickness and width of the gate are all effective measures to eliminate the occurrence of tiger skin patterns. The thickness of the gate is preferably 0.7 to 0.8 times that of the wall thickness, the purpose of which is to reduce the pressure loss of the melt in the runner and gate, and reduce the expansion effect when entering the cavity.

b. In the injection molding process, increasing the melt temperature and the mold temperature, and adjusting the injection speed will help eliminate the occurrence of tiger skin lines. Under normal circumstances, using a lower rate of fire is beneficial to eliminate tiger skin patterns, because the melt flow is more stable at a low speed.

(3) Material flower

The flower-like cold material near the car bumper not only affects the apparent quality of the part, but also affects its mechanical properties. The car bumper is a safety device that absorbs and relieves external impact and protects the front and rear of the body. Generally, it is better to choose PP with better mechanical strength.

Possible cause analysis and improvement measures:

As for injection molding processing, some needle valve gates may not be sealed tightly, there is residual cold material on the gate, and the air pressure for closing the needle valve is very low, which cannot play the role of sealing glue. The air pressure of the needle can be increased by using an air pressure booster, and the cold material will disappear when there is no leakage around the hot gate.

(4) Peeling

Because the surface area of automobile bumper parts is large and the defect rate of it is low, it is rather difficult to find out the peeling defects, but this defect seriously affects the surface quality of the parts.

Possible cause analysis and improvement measures:

a. Forming process: due to the manual pickup during the production process, when the pickup takes a long time, each hot nozzle will have different degrees of salivation. After shortening the pickup time, it was found that the salivation phenomenon of the hot mouth was effectively improved. Therefore, it is possible that the cold material produced by the drool is washed into the cavity, which causes the occurrence of delamination and peeling.

b. Solution: increase the injection distance of the injection molding machine so that the hot nozzle will no longer produces salivation.