Thermoplastics have been used in body panels for more than 25 years. Thermoplastics are far superior to metals in corrosion resistance, weight loss, and impact resistance. However, due to the significant differences in linear thermal expansion coefficients, it may not be possible to achieve the desired accuracy of automotive engineers for panel gap tolerances.
Despite the extensive efforts of material suppliers in market and application development, the penetration of thermoplastics on body panels has never been achieved by the paradigm shifts that thermoplastics have made on instrument panels or front and rear bumper panels. In instrument panel and bumper panel applications, thermoplastics are far superior to metal materials in terms of design and design freedom. So far, for cost reasons, the use of thermoplastics in body panels has not been able to break through the limitations of small batches (less than 50,000 per year).
Currently, automotive manufacturers use the electrophoretic coating process to achieve the corrosion protection required for the body and frame. This is a relatively high temperature process. After the electrophoretic coating, the vehicle enters the painting line and the paint is hardened by the paint oven. Conventional steel does not cause any thermal deformation problems during these processes.
The thermoplastic material used for the body panels is divided into three different heat resistant categories depending on the stage in which the body panels are applied to the finishing process. Among them, high temperature engineering thermoplastics are generally based on nylon compounds and are suitable for panels that require electrophoretic coating. Engineering thermoplastics are generally based on PC/PBT blends and are suitable for panels that require a varnished paint oven (in exceptional cases, they can also be designed for low temperature varnish painters, such as panels used in general Saturn models). The last thermoplastic is not painted in the paint oven and is still attached to the body after the painting process. In order to reduce the difference in paint color, automakers try to avoid painting parts outside the production line.
The cost of thermoplastics increases with thermal deformation, so the materials used for electrophoretic coating are the most expensive thermoplastics in automotive applications, and the cost of thermoplastics for varnish paint furnaces is reduced and the cost is lowest. It is a thermoplastic used for painting outside the production line.
Although the price of the metal raw materials itself is relatively low, the overall capital cost is not low due to the need to invest a lot of money to purchase the various steel tools needed to build the body panels. Although thermoplastics are relatively expensive in terms of raw materials, only one injection molding tool is required in the production of parts. Therefore, its economy depends on the ratio of the cost curve of a single part to the cost of capital. Usually, this break-even point is about 50,000 units per year. In the past 20 years, steel producers have improved corrosion resistance and reduced the weight of body panels through steel thinning. In terms of the requirements for tools, the original 6-7 tools were reduced to 4-5, which reduced the cost of capital and improved the competitiveness of steel panels.
As automakers, especially Asian automakers, start to produce low-priced cars for only $2,500-$5,000, this seems to have a large potential for plastics. Tata Motors originally planned to use thermoplastics in the body panels of the Nano, but it was not cost-competitive.
In fact, the use of existing technology has the opportunity to produce more competitive thermoplastic body panels and change the break-even point.
First, engineering thermoplastics are too costly for a wide range of high volume applications. PC/PBT has been used in bumper panels since the early 1980s, and TPO has been successfully replaced by cost advantages. The same model will also be reproduced on the body panel, which will reduce the cost of raw materials by 40% or more. The universal Saturn model with a low-temperature varnish paint furnace still has a higher cost than the metal used for high-volume automobile production. To increase the cost competitiveness of thermoplastic solutions, it is necessary to reduce the cost of raw materials.
Second, the applicable technology has been developed for the "painting" technology of ordinary models. Coating technologies based on PVDF and co-extruded thermoplastic vulcanization/PP systems, such as Schulman's Invision technology, have provided non-painting solutions that meet the quality requirements of body panels. But Schulman lacks sufficient funds to implement this seemingly promising technology. In addition, for such low-priced cars priced at only $2,500, molding thermoplastic panels into color panels offers potential quality/price advantages, giving consumers new and new options.
Although the cost of coatings and co-extruded structures is high, given the high cost of the paint line and the considerable potential for reductions, it is hoped that new options will be brought to Asian automakers who are building new plants. It is not realistic for mature Western automakers to consider unpainting production because their funds have already been invested, and they are not planning to produce cars that cater to the lower-income mass market in Asia. Newly built factories for Asian countries can take advantage of non-painting solutions and reduce capital investment.
Third, the level of materials available for thermoforming has been developed on PP and is now used in bumper panels and door panels in North American cars. There are a variety of rotary thermoforming machines that can achieve the productivity of a single injection molding machine, but the capital investment is relatively low. In addition, the use of single-sided molds, usually made of aluminum (although steel may be more desirable for such applications), can be cost-effective compared to injection molding (double-sided molds).
Fourth, the corresponding fastening technology has been developed through laser welding attachments or using Velcro straps (proven on thermoplastic front bumper panels). This type of fastening technology minimizes the limitations of thermoforming in the inability to easily produce 3-D depth geometries and is also easier to assemble.
Fifth, the use of PP-based technology can help improve the car's reproducibility. For front and rear bumper panels, instrument panels, sealing systems and certain under-the-hood components, when the amount of PP used in the vehicle reaches a certain critical amount of material, the vehicle dealer will be encouraged to regenerate the scrapped car.
So, since there are such attractive advantages in both the economy and the environment, why can't thermoplastics be fully applied in automobiles? In fact, it has gradually been applied. The Daimler Smart model is a good example. The first to use PC/PBT is now extended to PP (still injection molding). As for the application of thermoplastics on individual components, there are many examples. However, thermoplastics have not taken a leap forward in automotive applications. One of the main reasons why no substantial breakthroughs have been made (such as the use of metal sheets in projects such as Tata Nano) is the lack of large PP suppliers to drive the development of this application. PP producers are reluctant to invest money and make commitments to support OEMs, who believe that the risk of switching to new materials is too great. This change can only be achieved in Asian markets driven by Asian OEMs, as cars produced using such technologies are primarily marketed in Asia, and Western automakers lack the incentive to change their traditional processes to produce such low cost, while quality requirements may Also lower products.
Can thermoplastics be able to take advantage of the performance advantages of a busy urban environment, and will cars that are both cost-competitive and have a "green" advantage not become a new market?
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