The car bumper is one of the larger accessories in the car. It has three main functions: safety, functionality and decoration.
There are three main ways to reduce the weight of automotive bumpers: lightweight materials, structural optimization, and manufacturing process innovation. Light weight of materials generally refers to replacing the original materials with materials with lower density under certain conditions, such as plastic-made steel; the structural optimization design of lightweight bumper is mainly thin-walled; the new manufacturing process has micro-foaming. New technologies such as materials and gas-assisted molding.
Plastics are widely used in the automotive industry because of their light weight, good performance, simple manufacturing, corrosion resistance, impact resistance, and large degree of freedom in design, and they are increasingly used in automotive materials. The amount of plastic used in a car has become one of the criteria for measuring the level of development of a country’s automotive industry. At present, the plastic used in the production of a car in developed countries has reached 200kg, accounting for about 20% of the total vehicle quality.
Plastics are used in China’s automobile industry relatively late. In economical cars, the amount of plastics is only 50~60kg, for medium and high-class cars, 60~80kg, and some cars can reach 100kg. When manufacturing medium-sized trucks in China, every car Use about 50kg of plastic. The plastic consumption of each car is only 5% to 10% of the weight of the car.
The material of the bumper usually has the following requirements: good impact resistance and good weather resistance. Good paint adhesion, good fluidity, good processing performance and low price.
Accordingly, PP materials are undoubtedly the most cost-effective choice. PP material is a general-purpose plastic with excellent performance, but PP itself has poor low-temperature performance and impact resistance, is not wear-resistant, easy to age and has poor dimensional stability. Therefore, modified PP is usually used for automobile bumper production. material. At present, special materials for polypropylene automobile bumpers are usually made of PP, and a certain proportion of rubber or elastomer, inorganic filler, masterbatch, auxiliary materials and other materials are mixed and processed.
Problems caused by thin wall of bumper and solutions
The thinning of the bumper is easy to cause warping deformation, and the warping deformation is the result of the release of internal stress. Thin-walled bumpers generate internal stresses for various reasons during various stages of injection molding.
Generally, it mainly includes orientation stress, thermal stress, and mold release stress. The orientation stress is an internal attraction caused by fibers, macromolecular chains or segments in the melt oriented in a certain direction and insufficient relaxation. The degree of orientation is related to the thickness of the product, the melt temperature, the mold temperature, the injection pressure, and the dwell time. The larger the thickness, the lower the degree of orientation; the higher the melt temperature, the lower the degree of orientation; the higher the mold temperature, the lower the degree of orientation; the higher the injection pressure, the higher the degree of orientation; the longer the dwell time, the greater the degree of orientation .
The thermal stress is due to the higher temperature of the melt and the lower temperature of the mold to form a larger temperature difference. The cooling of the melt near the cavity of the mold is faster and the mechanical internal stress is unevenly distributed.
The demoulding stress is mainly caused by the lack of strength and rigidity of the mold, the elastic deformation under the action of the injection pressure and the ejection force, and the uneven distribution of the force when the product is ejected.
The thinning of the bumper also has the problem of difficulty in demolding. Because the wall thickness gauge is small and has a small amount of shrinkage, the product adheres tightly to the mold; because the injection speed is relatively high, the dwell time is maintained. Control is difficult; relatively thin wall thicknesses and ribs are also susceptible to damage during demolding. Normal opening of the mold requires the injection machine to provide sufficient mold opening force, and the mold opening force should be able to overcome the resistance when opening the mold.
Post time: Apr-23-2023