Difficulties in Injection Molding of Automobile Thin-walled Plastic Bumper

Automobile bumper is one of the larger decorative parts in automobile. It has three main functions: safety, functionality and decoration.

There are three main ways to lighten the weight of automobile bumper: material lightening, structural optimization design and manufacturing process innovation. Material lightweight generally refers to replacing the original materials with materials with low density under certain conditions. For example, replace steel with plastic (automotive plastic mould). The structural optimization design of lightweight bumper mainly includes thin-walled technology. New manufacturing processes include micro foaming materials, gas assisted molding and other new technologies.

I. Material selection of bumper for injection moulding car parts

Plastics are widely used in the automotive industry because of their light weight, good performance, simple manufacture, corrosion resistance, impact resistance and large design freedom. It accounts for an increasing proportion in injection molding automotive parts materials. The amount of plastic used in a car has become one of the standards to measure the development level of a country's automobile industry. At present, the plastic used in the production of a vehicle in developed countries has reached 200kg, accounting for about 20% of the vehicle mass.

Bumper materials usually have the following requirements: good impact resistance, good weather resistance, good paint adhesion, good fluidity, good processing performance and low price.

Therefore, PP materials are undoubtedly the best cost-effective choice in injection molding auto parts. PP material is a kind of general plastic with excellent properties. However, PP itself has has the characteristics of poor low-temperature performance and impact resistance, no wear resistance, easy aging, and poor dimensional stability. Therefore, modified PP is usually used as automobile bumper production material. At present, polypropylene special materials for automobile bumpers are usually made of PP and mixed with a certain proportion of rubber or elastomer, inorganic filler, color masterbatch, additives and other materials.

II. Problems caused by thin wall of bumper in injection moulding car parts

The thin-walled bumper is easy to cause warping deformation, which is the result of internal stress release. The thin-walled bumper will produce internal stress due to many reasons in each stage of plastic injection molding.

1. Plastic injection molding generally includes orientation stress, thermal stress and demoulding stress. Orientation stress is the internal attraction caused by insufficient relaxation of fibers, macromolecular chains or segments oriented in a certain direction in the melt. The degree of orientation is related to the thickness of the product, melt temperature, mold temperature, injection pressure and holding time. The greater the thickness, the lower the orientation. The higher the melt temperature, the lower the orientation. The higher the mold temperature, the lower the orientation. The higher the injection pressure, the higher the orientation. The longer the holding time, the greater the orientation.

2. In the process of plastic injection molding, if the melt temperature is high while the mold temperature is low, a large temperature difference will be formed. Therefore, thermal stress is the mechanical internal stress with uneven distribution due to the rapid cooling rate of the melt near the die cavity.

3. Demoulding stress is mainly caused by insufficient strength and stiffness of automobile injection mold, elastic deformation under the action of injection pressure and jacking force, and uneven stress during product ejection when ejector rods are distributed and arranged unreasonably.