Many of you already know that Quantum, like most solar cars, is made up of a lightweight yet extremely strong carbon fiber composite. However, it takes many steps to turn rolls of frozen carbon into a championship winning solar car. This Technical Thursday’s article, written by Mechanical Engineer AJ Trublowski, sheds light on the composite fabrication (aka “layup”) process. AJ is a sophomore member of both the Engineering and Strategy Divisions. He was at Roush every single day of the Spring Break Roush Week, which makes him especially knowledgeable about this process.
Composite Basics:
Before we get into the gory details of layup procedures, let’s start at the beginning. What is a composite? A composite is made up of two or more materials that have been combined for greater strength. In our case, the composite in question is a carbon fiber fabric coated with epoxy resin. The individual carbon strands give the material superior strength, while the epoxy hardens to hold the carbon in the desired shape. Epoxy resin can come pre-injected into the carbon fiber. This is called pre-preg carbon which is used in “dry layups”. Epoxy resin can also be spread manually on the carbon during the layup process (this sort of carbon is used in a “wet layup”). Most of the layups for Quantum are dry layups, while layups for electrical enclosures such as the battery box are wet layups that use Kevlar instead of carbon fiber. Pre-preg carbon (which is used in a dry layup) must be stored in the freezer to prevent the resin from hardening, and must be cured at high temperatures. Wet layups do not need heat to cure.
The team uses two types of pre-preg carbon fiber: normal weave and unidirectional or “uni” weave. Normal weave consists of threads of carbon woven together similar to normal fabric. Since carbon strands go in both the “x” and “y” directions, weave is strong in all directions and is used as the base material for our parts. Unidirectional weave consists of a high density of carbon strands that all go the same way, which makes it extremely strong in that particular direction. Uni is used as a supplement to weave in order to gain stiffness or strength at key points on the car, such as the suspension mounts.
Carbon uni (left) and weave (right) .
While carbon has exceptional tensile strength, it is not particularly stiff. We need to add another component to our composite in order to gain axial stiffness. This material is called core. Core comes in large honeycomb sheets that can be made out of anything from paper to Kevlar. When honeycomb core is sandwiched between two sheets of carbon fiber, the final product is strong in nearly every direction, but still very lightweight. Nearly every part of Quantum is made of this carbon/core sandwich.
Honeycomb core used in Quantum.
The Layup Process:
Now that we’ve covered the basics, it is time to talk about the layup. The first step is to prepare the mold (or other flat surface if a mold is not being used). After sanding out imperfections and cleaning with a solvent such as denatured alcohol, we apply a releasing agent such as Frekote. This chemical prevents the finished part from sticking to the mold.
Once the surface has been prepped, the next step is to apply the carbon and core. These materials are placed on the surface, and special care is taken to ensure that everything sticks properly to the mold. This is easier said than done, especially if the mold contains complex curves or fillets, since it can be difficult to get the carbon to stick without bridging over corners or folding on itself. Any errors in placement can drastically affect the quality of the part produced, and sometimes it can ruin the layup entirely.
After the carbon and core have been laid properly, it is time to vacuum bag the layup. Before the part can be put in a vacuum bag, the layup must be covered in several other layers that help ensure a good vacuum. These layers, collectively known as consumables, include peel ply, perforated film, and breather. Peel ply is the first layer that is placed down, and it is used to create a rough surface that will make it easier to attach additional carbon pieces during later layups. Breather and perforated film work together to make it easy to evacuate the air from the vacuum bag. Breather is a fluffy material that allows air to continue to flow out when a partial vacuum is attained, while perforated film covers the carbon to prevent the breather from sticking. Once the consumables are placed, the whole layup is put in a vacuum bag and the air is sucked out. Putting the part under pressure is necessary to hold the carbon and core together.
One of the vacuum bags from Roush Week. Bags are much larger than the layup so they can adequately cover and put pressure on all key features of the part.
The final stage of the process is curing the layup. For wet layups, simply leaving it under vacuum for several hours will be enough to produce a finished part. For dry layups, like for example the solar car chassis, the part must be cured in an oven or an autoclave. An autoclave is basically a pressurized oven. While ovens contain vacuum hoses to keep the part under pressure, an autoclave exerts greater pressure than would be attainable with a vacuum bag alone.
Santosh the Engineering Director and a campus autoclave.
After a part is cured, it is finally ready to be attached to the car. This may require additional layups to attach different carbon bodies together or additional bodywork if the part is an exposed body panel that will be painted.
While fairly straightforward, extreme attention to detail is required during the layup process as well as in the rest of the manufacturing phase to ensure that every aspect of Quantum is as the engineers had intended it to be.
AJ grips an imaginary steering wheel as he models the new UMsolar car driver’s seat at Roush.
