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Thinning of automobile plastic bumper

Time: 2021-03-04 Hits: 77

The car bumper is one of the relatively large trim parts on the car. It has three main functions: safety, functionality and decoration.

 

automobile plastic bumper


There are three main ways to reduce the weight of automobile bumpers: material lightweight, structural optimization design, and manufacturing process innovation. Material lightweight generally refers to replacing the original material with a material with a lower density under certain conditions, such as replacing steel with plastic; the structural optimization design of the lightweight bumper mainly includes thin-wall technology; the new manufacturing process includes micro-foaming New technologies such as materials and gas-assisted molding.

 

Material selection of plastic bumper

 

Plastics are widely used in the automotive industry because of their light weight, good performance, simple manufacturing, corrosion resistance, impact resistance, and greater design freedom, and they account for an increasing proportion of automotive materials. The amount of plastic used in a car has become one of the standards for measuring the development level of a country's automobile industry. At present, the plastic used in the production of a car in developed countries has reached 200kg, accounting for about 20% of the mass of the entire car.

 

The application of plastics in our country's automobile industry is relatively late. In economic cars, the amount of plastics is only 50-60kg, medium- and high-end cars have 60-80kg, and some cars can reach 100kg. In the production of medium-sized trucks in my country, each car Use about 50kg of plastic. The amount of plastic used in each car only accounts for 5% to 10% of the car's weight.

 

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 kind of general-purpose plastic with relatively good performance, but PP itself has poor low temperature performance and impact resistance, is not abrasion resistant, easy to age, and has poor dimensional stability. Therefore, modified PP is usually used for automobile bumpers. material. At present, polypropylene special materials for automobile bumpers are usually made of PP as the main material, and a certain proportion of rubber or elastomer, inorganic filler, masterbatch, additives and other materials are mixed and processed.

 

The problems caused by the thinning of bumpers and their solutions


The thinning of the bumper is likely to cause warpage deformation, which is the result of internal stress release. The thin-walled bumper will generate internal stress due to various reasons in all stages of injection molding.

 

Generally, it mainly includes orientation stress, thermal stress and demolding stress. Orientation stress is the internal attraction caused by the fibers, macromolecular chains or chain segments in the melt oriented along a certain direction and insufficient relaxation. The degree of orientation is related to the thickness of the product, melt temperature, mold temperature, injection pressure, and pressure holding time. The greater 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 holding 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, and the faster cooling rate of the melt in the region close to the mold cavity results in uneven distribution of mechanical internal stress.

 

The demolding stress is mainly caused by insufficient strength and rigidity of the mold, elastic deformation under the action of injection pressure and ejection force, and unreasonable distribution and arrangement of ejector rods, resulting in uneven force during ejection of the product.

 

The thin wall of the bumper will also have the problem of difficult demoulding. Due to the small wall thickness gauge and small shrinkage, the product adheres tightly to the mold; due to the relatively high injection speed, the holding pressure time Control is more difficult; thinner wall thickness and stiffeners are also easily damaged during demolding. The normal opening of the mold requires that the injection machine can provide sufficient mold opening force, and the mold opening force should be able to overcome the resistance during mold opening.

 

The resistance to be overcome during the mold opening process is as follows:

 

First of all, it is necessary to overcome the direct mold opening force. When the mold is opened, the plastic will have a certain adhesion force parallel to the mold opening direction. This is due to insufficient cooling of the plastic parts when the mold is cooled, and the elastic expansion of the cavity is not completely restored. The size of this adhesion is related to the properties of the plastic, the surface quality of the mold, and the degree of demolding.

 

In addition, it is necessary to overcome the indirect mold opening resistance, that is, to overcome the pulling resistance of the motorized side core pulling process when the mold is opened.

 

It is also necessary to overcome the frictional resistance generated by the movement of the movable template and movable template of the mold.

 

Finally, the participating pressure of the cavity needs to be overcome. The pressure of the cavity may not be equal to the atmospheric pressure when the mold is opened, and the pressure in the cavity is not equal to the external pressure.

 

In order to solve the above two major problems, it is necessary to appropriately improve the design of the mold. Choose the right mold material to improve the thermal strength and wear resistance of the mold. Reasonable mold structure design and manufacturing, appropriately increase the thickness of the push plate and the middle backing plate, improve the rigidity of the mold, and reduce the elastic deformation of the mold.

 

Improve the manufacturing and matching accuracy of the core-pulling mechanism and motion system, reduce the surface roughness of the cavity, core and punch assembly, and reduce the demolding force. Since products require higher design and matching accuracy after the product is thinned, interlocking devices are usually set to prevent the relative displacement of the mold core and the mold cavity.

 

Reasonably design the gating system, and the runner design should make the plastic part transition from a thicker area to a thinner area during the injection process. Also need to have enough exhaust ports. In the injection process, the internal stress of the plastic parts should be minimized, the injection speed should be increased, and the cooling rate should be reduced. Therefore, it is necessary to increase the melt temperature and mold temperature in order to relax the orientation. It is also necessary to select a reasonable injection pressure, pressure holding time and cooling rate.