Car bumper is one of the larger car accessories, it has three main functions of safety, functionality and decoration.
There are mainly three ways to reduce the weight of the car bumpers: lightweight materials, structural optimization design and manufacturing process innovation.Lightweight material generally refers to under certain conditions, the material with a lower density to replace the original material, such as the plastic substitution steel ; Structural optimization of lightweight bumper design are mainly thin-walled technology; manufacturing of new technology micro-foam materials and gas-assisted molding and other new technologies.
Plastic bumper material selection
Plastic is widely used in the automobile industry due to its light weight, good performance, simple manufacture, corrosion resistance, impact resistance and large design freedom, and its share in automobile materials is increasing. The amount of plastic used in a car has become one of the criteria for measuring the level of development of a country's automobile industry. At present, the amount of plastic used in producing a car in developed countries has reached 200 kg, accounting for about 20% of the total mass of a car.
Plastic in China's automobile industry in the late application of the amount of plastic in the economy car only 50 ~ 60kg, 60 to 80kg in the high-class cars, some cars can reach 100kg, China in the manufacture of medium-duty trucks, each car About 50kg of plastic.Each car uses only about 5% to 10% of the weight of the car.
Bumper materials usually have the following requirements: good impact resistance, good weatherability, good paint adhesion, good fluidity, good processability and low cost.
Accordingly, PP material is undoubtedly the most cost-effective choice.PPP material is a relatively good performance of the common plastic, but its low-temperature PP performance and impact resistance is relatively poor, not wear-resistant, easy to aging and dimensional stability Relatively poor, it is usually used to make the modified PP car bumper production of materials.At present, polypropylene bumper special materials are usually PP-based materials, and add a certain percentage of rubber or elastomer, inorganic filler, masterbatch, additives After mixing and processing of other materials.
Problems Caused by Thinning of Bumper and Their Solutions
Thinner bumper easily lead to warp deformation, warpage is the result of internal stress relief thin walled bumper in all stages of injection molding will be due to a variety of reasons, internal stress.
Generally include orientation stress, thermal stress and release stress.Oriented stress is the melt in the fiber, macromolecule chains or segments along a certain direction orientation, caused by the lack of relaxation of internal gravitation.The degree of orientation and product thickness, Melt temperature, mold temperature, injection pressure and dwell time.The higher the thickness, the lower the orientation degree, the higher the melt temperature, the lower the orientation degree, the higher the mold temperature, the lower the orientation degree.The higher the injection pressure, The higher the degree of orientation; the longer the dwell time, the greater the degree of orientation.
Thermal stress is due to the higher temperature of the melt and mold temperature is lower to form a larger temperature difference, the melt near the mold cavity area faster cooling and uneven distribution of mechanical stress.
Mold release stress is mainly due to the lack of strength and rigidity of the mold, the injection pressure and ejection force under the effect of elastic deformation and ejector rod arrangement is not reasonable when the product is pushed out of the uneven force generated.
Thin-walled bumper will have the problem of demoulding, due to the smaller wall thickness and a smaller amount of shrinkage, making the product tightly adheres to the mold; due to the injection speed is relatively high, leaving dwell time Control is more difficult; thinner wall thickness and tendons are also easy to damage in the demolding mold normally open need injection machine can provide enough mold opening force, mold opening force should be able to overcome the mold opening resistance.
There are several types of resistance that need to be overcome during mold opening:
The first is the need to overcome the direct mold opening force. When the mold is opened, the plastic will have some adhesion in the direction parallel to the mold opening. This is due to the insufficient cooling of the plastic parts and the incomplete expansion of the elastic expansion of the mold when the mold is cooled. The size of the adhesion and the nature of the plastic, the mold surface quality, demoulding slope and so on.
In addition, it also needs to overcome the indirect mold opening resistance, that is to overcome the pull-out resistance of the core pulling process during the mold opening.
But also to overcome the mold movement template and activities such as the movement of the body friction generated by the resistance.
Finally need to overcome the pressure involved in the cavity, the mold cavity pressure may not equal atmospheric pressure, cavity pressure and external pressure are not equal.
In order to solve the above two problems, it is necessary to properly improve the mold design.Select the appropriate mold material to improve the mold's thermal strength and wear resistance.Model mold design and manufacturing reasonable, increase the thickness of the push plate and the middle plate, Improve the rigidity of the mold, reduce the elastic deformation of the mold.
Improve the core pulling mechanism, the movement of the system manufacturing and coordination accuracy, reducing the cavity, core and punch components of the surface roughness and reduce the release force due to thinning of the product after the higher design and coordination accuracy, usually Set the interlock to prevent the mold core and cavity from relative displacement.
Reasonable design of the pouring system, the flow channel design should make the plastic parts in the injection process from thick to thin area over the area also need to have enough exhaust port in the injection process should minimize the internal stress plastic parts, Increase the injection speed, reduce the cooling rate.Therefore, need to increase the melt temperature and mold temperature, so that the orientation of the relaxation.Also need to choose a reasonable injection pressure, dwell time and cooling rate.