How to achieve thin-wall automotive plastic bumpers?

Automotive bumper is one of the larger trim parts on the car, which has three main functions: safety, functionality and decoration.

There are three main ways to achieve light-weighting of automotive bumpers: material lightweight, structure optimization design and manufacturing process innovation. Material lightweight generally refers to the replacement of the original material with a lower density material under certain conditions, such as plastic instead of steel; the structure optimization design of bumper lightweight mainly includes thin-wall technology; the new manufacturing process includes new technologies such as micro-foaming materials and gas-assisted molding.


 

Plastic bumper material selection

Plastics are widely used in the automotive industry because of their lightweight, good performance, simple manufacturing, corrosion resistance, impact resistance, and greater freedom of design, and are taking up a larger and larger proportion of automotive materials. The amount of plastic used in a car has become one of the criteria to measure 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 mass of the car.

Plastic in China's automotive industry is relatively late, the amount of plastic in the economy car is only 50 ~ 60kg, 60 ~ 80kg in the senior car, some cars can reach 100kg, China in the production and manufacture of medium-sized cargo vehicles, each car with about 50kg of plastic. The amount of plastic used in each car is only 5%~10% of the weight of the car.

The material of bumper usually has the following requirements: good impact resistance, good weather resistance. Good paint adhesion, good flowability, good processability, and low price.

PP is a general-purpose plastic with good performance, but PP itself has poor low-temperature performance and impact resistance, is not wear-resistant, easy to age and has poor dimensional stability, so it is usually made of modified PP for car bumpers. At present, polypropylene bumper material is usually made of PP as the main material, and a certain proportion of rubber or elastomer, inorganic fillers, masterbatches, additives and other materials are added through mixing and processing.
 

Problems caused by the thin-walled bumper and measures to solve them

Thin-walled bumpers are prone to warpage, which is the result of internal stress release. Warpage is the result of the release of internal stresses in the injection molding process.

These include orientation stresses, thermal stresses, and demolding stresses. Orientation stress is the internal gravitational force caused by the orientation of fibers, macromolecular chains or chain segments in the melt along a certain direction with insufficient relaxation. The degree of orientation is related to the thickness of the product, melt temperature, mold temperature, injection pressure, and holding time. The higher 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 higher the orientation.

Thermal stress is due to the high temperature of the melt and the low temperature of the mold and the formation of a large temperature difference, the cooling rate of the melt in the area near the mold cavity faster and unevenly distributed mechanical internal stress.

The release stress is mainly due to the insufficient strength and stiffness of the mold, the elastic deformation under the action of injection pressure and ejecting force and the uneven force when the ejecting bar is not arranged properly to make the product ejected.

The thin-walled bumper also has the problem of difficult mold release, because the wall thickness table is small and has a small shrinkage, making the product cling to the mold; because the injection speed is relatively high, which makes it difficult to control the holding time; the thin wall thickness and reinforcement bars are also easy to damage during mold release. The normal opening of the mold requires the injection machine to provide sufficient opening force, which should be able to overcome the resistance during mold opening.
 

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 a certain adhesion force in the direction parallel to the mold opening, which is due to the insufficient cooling of the plastic part when the mold is cooled and the elastic expansion of the cavity is not fully recovered. The size of this adhesion force is related to the nature of the plastic, the surface quality of the mold, and the slope of the mold release.

It is also necessary to overcome the indirect mold opening resistance, i.e., to overcome the extraction resistance of the motorized side core extraction process when opening the mold.

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

Finally, it is also necessary to overcome the pressure of cavity participation. The pressure of the cavity during mold opening may not be equal to the atmospheric pressure, and the pressure inside the cavity is not equal to the external pressure.

To solve the above two major problems, the design of the mold needs to be improved appropriately. Choose suitable mold material to improve the thermal strength as well as wear resistance of the mold. Reasonable mold structure design and manufacturing, appropriate increase the thickness of the push plate and the middle cushion plate, improve the mold stiffness, reduce the elastic deformation of the mold.

Improve the manufacturing and fitting accuracy of core extraction mechanism and motion system, reduce the surface roughness of cavities, cores and convex mold components, and reduce the demoulding force. As the products require higher design and fitting accuracy after thin-walled, interlocking devices are usually set to prevent the relative displacement of core and cavity.

The flow path should be designed in such a way that the plastic part will move from a thicker to a thinner area during the injection process. Adequate exhaust ports are also required. The injection process should minimize the internal stress of the plastic part, increase the injection speed and reduce the cooling rate. Therefore, the melt temperature and mold temperature need to be increased to allow relaxation of the orientation. A reasonable injection pressure, holding time and cooling rate also need to be selected.