Cut From A Different Cloth
From the earliest days of our work developing the Tesla Roadster’s body, we realized we had several major challenges on our hands. We had to achieve a low level of aerodynamic drag to increase efficiency, and we had to keep our mass down in order to maintain a high power-to-weight ratio and achieve maximum acceleration. Equally important was our imperative to create a body style for the Tesla Roadster that made people desperately want the car - irrespective of its efficiency or level of performance.
Selecting the right body material and manufacturing process was crucial to achieving our goal. We ultimately chose carbon fiber, a material you will see in few cars available at the Tesla Roadster’s price point. Each Tesla Roadster that rolls off the production line sports a skin made from lightweight carbon fiber/epoxy composite that took two years of design, prototyping, test, redesign, retest, meetings and arguments to develop.
So what’s so special about carbon fiber?
Carbon fiber is light, right? Well, a pound of carbon fiber weighs exactly the same as a pound of steel; so, no, not really. The advantage carbon fiber has is that it’s very strong for its weight. Therefore, we could use carbon fiber to achieve the same level of strength with less mass. Depending on how it’s processed, for example, a carbon fiber-reinforced plastic part can replace an equivalent steel part using less than 30 percent of the original part’s mass.
Carbon fiber on its own isn’t much use, though. It’s like a very thin fishing line, it is only strong in tension (when you try to break it by pulling it along its length). So, to make a panel that is strong in all directions, carbon fiber is typically woven into cloth (to give it strength in two directions) and then the carbon fiber cloth is encapsulated in plastic. In our case, it is encapsulated in epoxy resin – it has a higher specific strength than the alternatives. The epoxy is strong in compression but relatively weak in tension, so the two materials act together to produce a panel strong in tension and compression.
Carbon fiber parts that you see on some cars, especially aftermarket products, are produced using carbon fiber cloth pre-impregnated with resin (abbreviated to ‘prepreg’) that is heated and pressed against a former in a pressurized oven called an autoclave. The very high temperature and pressure squeeze the air out of the cloth and force the resin to flow around the fiber and create a consolidated molded panel.
This can produce very lightweight and very stiff components, but with a couple of drawbacks. First, the cost of producing the parts is very high because they need a long time to fully cure in the autoclave and the process isn’t cheap. (If you think the Tesla Roadster is expensive now, you should consider how much it would cost if we added several thousand dollars worth of autoclaved carbon panels.) Second, there aren’t many manufacturers with enough autoclave space to produce a whole set of body panels at the rate we need.
An alternative to using an autoclave is to “vac-bag” the parts. This is a similar and less expensive approach that doesn’t use an autoclave but the drawback is that, as the name suggests, you can only apply atmospheric pressure to the parts (by creating a partial vacuum in a bag that surrounds the mold with prepreg carbon loaded onto it). The pressure isn’t high enough to fully consolidate the resin into the fiber so the final panel’s surface isn’t of a consistently high enough level of quality for a car like the Tesla Roadster.
The process we ultimately adopted for our body panels is Resin Transfer Molding (RTM), which uses what’s called a “closed mold.” Two huge blocks of steel are machined and polished so that when they’re nested together there’s a gap between them of less than 2mm representing the shape of the part we want. We lay carbon fiber mat (and some additional material we discuss in more detail below) against the concave surface of the tool, bring the other half of the tool into place to create the cavity, and then inject resin to fill the gap. This technique allows us to control thickness (which keeps weight down), reduce processing time, and maintain a very good level of surface quality. An additional advantage of using a closed-mold tool is that we can vary the thickness of the part in key areas to integrate features that add strength or provide a location for mounting hinges, etc.
For body panels what we need most is high strength in bending (which is really creating tension on one surface of the part and compression on the other) so that we can make thin, lightweight parts that can withstand the loads seen during a car’s lifetime (car washes, car parking lot contact, aerodynamic loads, etc.). To achieve high specific bending stiffness, we need to get the carbon as close to the surface as possible. So our panels are actually a sandwich made from two layers of carbon separated by a middle layer of glass and polypropylene that presses the carbon against the face of the tool and keeps it close to the surface of the panel. To create a smooth surface ready to paint we spray the inside of the tool with a special paint primer that then adheres to the resin and comes out of the tool on the part.
So, in the end, our carbon fiber body panels are made from a layer of primer, a layer of carbon, a layer of glass, a layer of polypropylene, another layer of glass and another layer of carbon, all encapsulated in epoxy resin and all in a space about as thick as a couple of credit cards. This allowed us to maintain a bending stiffness similar to that of a regular steel body panel and lose about 50 pounds from the weight of the body panels compared to ‘lightweight’ glass fiber composites.
Now it hasn’t been easy to get where we are. The Tesla Motors body engineering team (based in the UK) and I have spent the last two years working hard with the styling studio at Lotus, aerodynamic experts from the UK’s Motor Industry Research Association (MIRA), our body system supplier, and the manufacturing engineering team at Tesla Motors to arrive at a solution that satisfies all of our requirements.
A significant hurdle along the way was encountered when we learned that global supplies of the particular carbon fiber cloth we’d chosen dried up due to demand from aerospace (the new Airbus and Boeing superjumbo planes both make extensive use of carbon fiber) and defense manufacturers. Our supply chain team rose to the occasion, generating so much excitement for the Tesla Motors vision that a new supplier allocated the needed material to us.
It took a good deal of sweat, ingenuity, and persistence on our part, but we finally developed the perfect body material to suit the Tesla Roadster. We’re pleased with the final result, and we think our customers will be too.