Manufacturing¶
Cielo¶
The 2017-2018 design cycle was the first in which the team made a significant attempt to optimize the aerodynamics of the exterior shell of our car. It was also the first design cycle in which we actually had 5-axis CNC’d molds of something we designed in CAD. In the 2016-2017 cycle, very crude molds were made with plywood and insulation foam.
In this design cycle we landed a mold sponsorship with Marine Concepts in Sarasota, Florida. Unfortunately, Marine Concepts ran on a tight schedule and required final CAD earlier than expected. The body team had gone through several different iterations of body designs and had found one which was relatively aerodynamic. When trying to surface this design in conjunction with the final mechanical chassis/suspension CAD, Solidworks constantly failed to build the proper surfaces. With the Marine Concepts deadline fast approaching, a much worse design had to be submitted as it was the only one Solidworks didn’t crash in trying to surface.
Our mold was a negative mold milled out of EPS foam and done by our sponsor with their 5-axis CNC machine. Once the mold was constructed, our team made regular trips to Tavares to complete the rest of the manufacturing process, albeit with the aid of our sponsor since we were unfamiliar with many of these processes. After mold construction, we performed a wet layup of fiberglass over the EPS foam to have a non-porous surface to be able to vacuum-bag off of. We then sprayed ITW spray-core, a type of tooling resin, over our mold to give it rigidity and a smoother surface finish.
We then sanded down the mold surface, starting with 60 grit and using successively finer sandpaper up to 400 grit. We then sprayed a final layer of Duratec Primer over our mold and followed the same process for sanding down the surface.
To make our body shell, we used a resin infusion process with the help of our sponsor. This is a type of composite manufacturing process that takes plenty of prior experience and additional expenses to perform; it is not recommended for future in-house manufactured body shells until we have enough collective experience as a team. An outline of the resin infusion process can be found here.
Our laminate sequence was a sandwich core structure with one ply of carbon fiber on each side of a layer of 3mm. Lantor Coremat Xi. We used a 2x2 twill weave for the outside layer of carbon fiber for optimal surface finish and used a unidirectional weave for the interior carbon fiber layer to minimize costs. We used Inkamesh as our flowmedia and implemented scribe lines to differentiate between our part and flange. Multiple resin ports were used to account for the size of our layup.
Car Three¶
The construction of Car 3’s body shell for the 2019-2020 cycle has been repeatedly delayed for multiple reasons. Our team has had to transition from manufacturing a body shell with a sponsor who guided us through the resin infusion process while sponsoring all necessary materials for mold construction and a 5-axis CNC service) to manufacturing our body shell entirely in-house while maintaining the quality of our previous design. This presented multiple challenges to the team: (1) planning the manufacturing and surface preparation of our mold (2) creating our own composites manufacturing process that would work with our resources, (3) setting up a training protocol ensured that leadership and team members had sufficient hands-on experience with composites. Manufacturing was further complicated due to the Covid-19 pandemic, which resulted in the team’s access to our shop being delayed until October 2020. Our team has met our design challenges through a combination of research and hands-on experience; we are expecting to begin manufacturing our body shell in the Spring 2020 semester.
We are making our molds out of XPS Foamular 250, which is economical while having sufficient density yield an acceptable surface finish from CNC milling with a local sponsor. We decided to construct a straight to negative mold by milling a cavity out of stacked foam sheets to ensure an optimal surface finish while avoiding the expense and logistics involved in the conventional process of pulling a fiberglass negative mold from a positive plug. Composite material is then laid up inside the mold to make the final shell. A vacuum-bagging system will be used to draw out excess resin and improve the composite’s strength to weight ratio.
To ensure proper mold design , it was important for us to understand how our CAD design would be translated through a CNC program and how milled into the foam and how to ensure proper draft angles. A negative draft angle will make manufacturing extremely hard, if not impossible. We are currently finalizing our mold design with our CNC sponsor and expect to have our mold done by the end of the spring 2020 semester.
We used our limited time in the shop in the Fall 2020 semester to start manufacturing our ribbing components. We plan to waterjet our CAD-designed profiles out of vacuum-bagged sandwich core carbon fiber panels. We have been experimenting with the amount of resin used in our layups in order to determine the optimal carbon fiber to resin ratio for our body shell.