3. Uses of Plastic in an Automobile

3. Uses of Plastic in an Automobile

1. Abstract:

Over time, automobiles have changed dramatically from their first inception. The focus of this report was on the replacement of traditional metal parts with plastic parts. The reason for this change can be attributed to the gas shortage of the 70’s. Engineers knew that a lighter weight car was needed to gain more miles per gallon of gas. The bumper, for example, is a part that has achieved weight reduction of 2.5 pounds while eliminating 13 metal parts. Another example would be the engine manifold where 5 pounds are now saved as well as increasing the horsepower by 33%8. There are many parts that have made the change but this paper will focus only on the fuel tank, engine and interior/exterior of the automobile. In some cases plastic has become more prevalent than metal. Plastic frees engineers from the design constraints imposed by metal. There are environment benefits from the more fuel-efficient vehicle, due to plastics lighter weight. However, there are no cars made completely of plastic so metal has it’s own advantages. Therefore the sections to follow will discuss each materials characteristics in manufacturing application, and how industry and consumer both benefit.

2. Introduction:

Through history, cars were typically made completely from steel. However, over time, cars have evolved into a composite of materials. The reason for this evolution can be blamed on the increase in the price of oil during the decade of the 70’s. Society looked for a more efficient car in terms of mileage per gallon of gas. Engineers looked toward plastic due to its lightweight. Plastic provides an average weight savings of 400 pounds. With 15 million cars manufactured each year, this translates to energy savings of 5.25 million gallons of gas per year and 10.5 billion pounds less carbon dioxide in the air8. The question then arose “Why not make a car completely out of plastic?” The answer is an easy one, to expensive. Therefore a compromise had to be made depending on the different characteristics of the materials. This leads to the fact that the automobile is now a composite of materials. Better gas mileage helps us manage our natural resources such as gasoline, while reducing emissions released into the atmosphere. Some areas where the use of plastics has become more proficient are exterior/interior, electrical, fuel, engine, chassis and power train. The first known use of plastic in an automobile, aside from tires, was the bumper. Nowadays the majority of plastic bumpers are made of thermoplastic olefins, polyesters, polypropylene or blends of these compounds with glass fibers to increase strength.

3. Uses of plastic in an Automobile:

3.1 Fuel Tank:

As changes in weight and cost savings drive the performance criteria for automotive materials, equipment manufacturers are taking a hard look at the historically terne-coated steel used for gas tanks5. Thus, we will compare steel and plastic for gas tank uses according to competitive analysis and performance attributes. Throughout history, terne-coated steel has been used for automotive gas tanks. However, several issues must be met regarding the changing performance criteria. This exploration proves to be a threat to the application of steel products. Many characteristics can be taken into account for the material change such as permeability, weight, packaging, safety, and cost. Even though the use of plastic fuel tanks has increased in the marketplace, a comparative analysis of the various plastic and steel alternatives indicates that steel remains a cost-effective material that meets all of the required performance criteria5.

3.1.1 Plastic Fuel Tanks

Plastic fuel tanks are made from high density polyethylene(HDPE), a strong, lightweight material which has allowed manufacturers to substantially lower the net weight of the automobile. Since the mid-1980s, automakers have been displacing coated-steel fuel tanks with plastic ones. During the early 1990’s, approximately 2.7-3 million cars and trucks built in North America used nonmetallic tanks. At this time it represented 22-25% of the market, compared to 16% in the late 1980’s. Experts dealing with automotive designs predict plastic tanks will capture 60% of the North American market by the end of 2001. This can be considered as a worst-case scenario for the steel industry if it fails to provide a cost-effective steel alternative that meets all of the performance criteria.

3.1.2 Performance Attributes

3.1.2.a Manufacturability

Terne-plate holds a materials cost advantage over high-density polyethylene. The cost of the material is not the only driving force. Consideration must also include the cost of the tank and its reliability within the fuel system of the vehicle. This system is composed of the tank, filler tube, and level control to name a few5. All of these components must function properly as any unforeseen corrosion can easily contaminate the fuel delivery system and cause costly repairs. Metal tanks cost structure indicate a lower cost per piece versus plastic ones3.

3.1.2.b Design Features

Plastic tanks have the ability to meet packaging constraints with complex shapes, and design engineers have greater flexibility in the car design and styling without having to worry about fitting the gas tank. The average gas tank for a compact automobile can boast weight savings of up to 30% versus a similar steel tank. However, the weight advantage of plastics has diminished due to new permeability requirements5.

3.1.2.c Safety

One critical part of the performance criteria of the tank is its ability to meet crash requirements. Generally, plastic tanks are considered safer in crashes because they are seamless and, thus, not prone to failures in the seam areas. Also, plastic tanks deform and have some ability to rebound back to shape. When steel tanks absorb energy and deform, the pressure within the tank is inversely related to the volume. As the pressure in the tank increases the volume decreases. This makes them vulnerable at welded or clamped areas where failure can potentially occur. At the same time, the tank must withstand extreme in-tank temperatures in North America. The high point (79°C) temperature exceeds the boiling point of the alcohol fuels while the extreme cold introduces potential cracking problems12. Plastic, with its insulating properties, slows heat transfer to the fuel when compared to a steel tank. Also, plastics cannot be considered a source for sparks12. In the case of an under-car fire, plastic tanks will hold back the rise in fuel temperature. However, this is not a permanent solution as the tank will soften, sag, and eventually release the fuel. A steel tank does not sag in a fire; however, the fuel temperature may rise rapidly, perhaps resulting in over pressurization and release of fuel through a mechanical fitting.

3.1.2.d Corrosion

Corrosion is a well-known concern on both the inside and outside surfaces of tanks. The outside surfaces and supporting structure are exposed to road chemicals, salt, mud, and gravel. The corrosion issue is critical with zinc-coated products that replace terne-coated plates because of their nature, which puts an even higher demand on the barrier film for both the inside and outside surfaces. In contrast, the HDPE gas tanks are inert to the corrosive environments inside and outside the tank.

3.1.2.e Recyclability

This is the hardest obstacle to overcome for a plastic part. Despite progress in recycling, the propagation of plastics in automotive applications faces some problems, such as5:

1)The absence of a plastics recycling infrastructure.

2) A typical passenger cars steel and iron parts are recoverable.

3)The molding process for plastic fuel tanks. This process results in 35% of plastic material ending as waste.

4)The lack of technology that dismantlers can use to quickly collect various plastics.

5)Cost. Recycled plastics are not cost competitive with newer plastics.

As a result, automotive-design engineers must not only meet customer, design, styling, cost, weight, and regulatory needs but also environmental criteria. All material suppliers must show that their product is not only lighter and cost effective but also recyclable.

3.1.3 Tank Materials and Manufacturers

3.1.3.a Manufacturers

Chrysler made the decision to outsource plastic tanks and they remain committed to this decision. The listed advantages of plastic, according to Chrysler, are lack of corrosion, easier packaging and lower weight.

Ford called for a switch in 1997 to zinc-nickel-coated steel from terne-coated steel tanks in all models. In some models they will also switch from plastic to zinc-coated tanks. However, they will continue to use plastic tanks in certain models.

General Motors at this time has an ongoing corrosion test program to see if Plastic would be better than metal in fuel tanks5.

3.1.3.b Competitive Analysis

There are two aspects to compare between steel and plastic fuel tanks; production volume and the ability to recycle the material. Plastic is much cheaper when it comes to production volume while steel is cheaper to recycle. The difference in these characteristics will be the driving influence in plastic versus metal fuel tanks.

3.2 Exterior

For the last several years there has been enormous expansion in the awareness and attempt in the development of innovative polymers for automotive body exteriors. The challenge, which confronts the automotive car companies of today, focuses around cost reduction, improved durability and quality while concurrently providing a vehicle, which is pleasurable to drive and stylishly in appearance.

Technologies for automobile exteriors comprise an extensive continuum. Thermoplastic polymers, alloys and Reaction Injection Molded (RIM) thermosets are primary candidates for vertical panels. Thermoset and thermoplastic composites vie for horizontal components and uni-body verticals that carry structural loads. Applicants for composite fabrication technology includes SMC compression molding, Structural RIM for thermosets, and high-pressure flow molding or stamping for thermoplastics2. Last but not least, coating systems establish a sizeable portion of assembly plants and vehicular costs; nonetheless contribute significantly to style and appearance.

The sections that follow concentrate on both, elementary and marketable issues of materials implementation, and processing, therefore, all overall industrial progress in the technologies listed above.

Plastic body panels have been considered cost acceptable with that of steel parts. The fairly low tooling cost of plastics compared to steel offsets the elevated material cost of plastic. This is only true in lower volume applications. One advantage for using plastics in exterior body panels is its low capital investment for plastic tooling changes in comparison to steel. Exterior plastic panels may be changed frequently to alter and update vehicle styling, while a homogeneous vehicle stage is employed to minimize production fee. Changes in materials and vehicle subsystem technologies and source represent the greatest cost reduction opportunities for U.S. automakers. Materials and subassemblies currently account for over 50 percent of total vehicle cost and further affect assembly costs that represent another 30 percent of production cost4.

Steel does have performance deficiencies. U.S. produced steel has suffered from wavering value that has resulted in inadequate vehicle fit and finish and the need for over designed, and more expensive tools. Further, poor corrosion resistance has increased life cycle costs for automobiles. On the other hand, companies are helping to improve consistency in sheet production and the use of electrogalvanization and surface treatment technologies are improving steel’s corrosion resistance.

SMC can withstand exposure in paint ovens designed for steel and have begun to meet one-minute cycle targets, which match vehicle build rates, due to technology development on the part of the SMC fabrication community.

Reaction injection molded polyurethane-type systems and injection-molded thermoplastics have had smaller quantity of industrial applications, but offer the potential for damage resistance in low-impact collision4.

3.2.1 Thermoplastic Composites:

Techno polymers are a combination of GE Plastics own proficiency in resin technology, and PPG’s venerable work with fiberglass and composite technology. Thus, after the combination of these two technologies, a high-strength industrial thermoplastic composite is produced. Thermoplastic composites represent two extremes. One, the high-end consist of exotic polymer matrices with specialized reinforcement systems, usually found in the aerospace industry. The second is sheet-molding compounds, used mainly in industrial applications. The advantages of using thermoplastic as opposed to thermoset-based composites are as followed4:

1)No hand cutting/weighing

2)No controlled storage

3)No hot molds

4)Thermoplastic recyclability

5)Greater than 50% reduction in cycle time

6)Minimal deflashing

7)No post-mold curing step

Not only do thermoplastics enjoy a high modulus, they additionally have exceptional collision resistance. Currently this knowledge is used in a multiplicity of applications. The present day consumer desires a safer and more fuel-efficient automobile, thus the automotive industry demands lighter, stronger materials in which to make automobiles. Application of this technology includes structural roofs whose load bearing eliminates the roof rack, subsequently the creation of an aerodynamic storage compartment. Second application, is for the integrated lighting housings and locking platforms for the hood or tailgate. Third application, totally integrated dashboard platforms that incorporate knee-bar support beams, steering column and pedal support, and heating and ventilation housing. A final application, are for back panels of truck cabs.

The potential advantage of a composite consisting of chopped nature of glass, approximately 60-70%, the glass will flow more effectively and fill bosses and ribs7. His technology is applied in the development of a thermoplastic composite for horizontal automotive panels.

Saturn trail blazed the use of thermoplastic systems in body panels with the introduction of the industry’s foremost quality, high production thermoplastic door panel a decade ago. Daimler/Chrysler also has a firm dedication to designing and building plastic-bodied cars. The use of thermoplastics saves 20 to 50 percent in net weight and 50 to 70 percent in production time.

Developing new material technologies continues to make thermoplastic systems trendy and profitable. Molded in color panels, are extremely attractive quality, because of an effective elimination of manufacturing time and cost.

Thermoplastic polyesters are one of the most recycled materials in the world. This provides numerous advantages, both promoting recycled product to the consumer and the savings that result from in-company recycling of needless material.

Still, most plastic-based body panels rely on sheet molding compounds (SMC), a thermoset poyester sheet. Manufactures find production costs for SMC based panels are, for the most part, lesser than production cost for steel and aluminum6.

RIM is being used in production of automobile bumpers and fascias as well as body panels. RIM technology is lighter than SMC, with slight compromises in structural rigidity.

Transparent polycarbonate plastics have revolutionized lighting systems, for automobiles. These plastics are designed to resist high temperatures, are also durable, and can be molded into almost any desired shape. Thus, glass headlight lenses on automobiles have been, for the most part, replaced by this improved technology. In the future, both head and tail light systems will integrate this plastic light emitting diode brake-light systems1. Whereby an effortlessly reachable, single light source is used to provide exterior lighting for the automobile by means of acrylic fiber-optic wires. This amalgamation will eradicate the requirement for high-heat resistant plastics in automobile lighting systems.

Nylons, polystyrene, polycarbonates, polypropylene, polyesters, and urethanes are the most commonly used plastics for the visual components and trim of the automobile.

Using plastic instead of metal in the manufacturing of wheel covers, and then coating the plastic with a metallic like polish, manufactures will spend a fraction of the cost, compared to real metal production, while making the plastic appear like metal alloy.

Plastics represent one of the greatest challenges for coating manufactures. Plastic substrates are unique in that they represent a multiplicity of surfaces each with its own adhesion characteristics and physical properties. One characteristic property being the rigid thermoplastics, for example, nylon, polycarbonate, and polyester, blends. The second characteristic is, thermoplastic elastomers, urethane, styrenic, and polyolefin. The final plastic physical characteristic is, thermosets, such as, polyester (SMC) and urethane (RIM). The cost for painting during assembly can account for up 45% of the total cost of a new assembly plant4.

Plastic automobiles create a number of solutions to the problem of surface appearance. The first being an in-mold coating system, which first primes the surface of the SMC part, and second adds different pigments while the part is still in the process of molding. An alternate solution is integral coloring, which is how most plastics are presently colored. This process is used primarily for sport utility vehicles, where surface appearance is not a top priority. Traditional surface coating techniques such as, spraying, dipping, or vapor permeation curing, are available by companies familiar in painting plastics.