Carbon fiber reinforced polymer (CFRP) composites are made by curing a carbon fiber/matrix laminate around a tool which represents the desired geometry of the part. Uncured lamina may be either dry cloth or pre-impregnated (prepreg) unidirectional fibers or cloth with thermoset matrix. Curing can be achieved by applying hardening agents to resin (in a wet layup) or by baking the CFRP laminate (from a prepreg layup) in a composites oven.
To understand the philosophy of the 2020 manufacturing, the 2017-2019 manufacturing processes should be discussed. Typically, to create a master geometry, CNC material removal machines are used to take advantage of their accuracy and repeatability in 3D space. Additionally, CNC machines allow locating holes to be drilled into the master geometry. The male master geometry for the 2017-2019 was machined into foam using Zodiac Aerospace’s (now Safran) 5th axis router. A durable female tool was then created. The tool was created in a wet layup using high temperature resin and dry carbon fiber cloth. These materials were chosen to withstand the elevated temperatures necessary to cure thermoset prepreg mats, and to closely match the coefficient of thermal expansion for thermoset prepreg mats. The female tool was used to cure prepreg parts, which are more uniform and repeatable than wet-laid parts.
For the 2017-2019 seasons, the CPFSAE chassis were manufactured from prepreg in two halves. Once the two parts were cured and trimmed, they were bonded together with a microballon-resin adhesive and sealed with a wet layup strap joint. Upon weighing the chassis halves before and after applying the strap joint, it was found that the strap joint added 8 lbs. to the total chassis weight.
Objectives
Aside from yielding a usable part for the CPFSAE team, the MCD team’s objectives were to eliminate the strap joint, increase modularity of the chassis tooling, withstand elevated temperatures seen in prepreg curing cycles, and create durable tooling. Removal of the strap joint reduced chassis weight, which aligns with the team’s performance goals. The 2017-2020 tooling was not designed for modularity, so the entire chassis geometry required a re-design if the platform needed tuning. While it was not in the scope of the project, future CPFSAE teams could manufacture new tools that interface with the ones created during the 2020 season. For instance, if powertrain packaging direction changes, two new rear tools might be manufactured that mate with the front tools. The tools were manufactured using resin that would not re- enter the plastic state at elevated temperatures. The tooling was designed to yield 50 parts, allowing the CPFSAE team to make many season’s worth of chassis from one investment of capital. Additionally, this report details the design and modelling process used to make the tooling so that future teams have a solid foundation for future redesigns.
Manufacturing Concept
The MCD team manufactured the chassis utilizing 4 female molds that created a single CFRP chassis, with no requirement for a strap joint. The master geometry was created by machining a foam plug similar to 2017 senior project team Carbon Fiber Monocoque Chassis Platform for Formula SAE and Formula SAE Electric Race Cars. The durable female tool was wet laid over the foam and was able to produce thermoset prepreg parts. Drill bushings were built into the durable tooling for positioning critical components such as the suspension or engine mounts. Unlike the 2017 project, the molds bolted together so the entire closed chassis was laid up inside the tool, and there was no need for a strap joint. The last notable difference in
75 Master
Geometry
geometry; Include locating holes and datums 2 Prime Foam White High
Build Primer
Fills voids & prevent over-sanding 3 Guide Coat Blue Paint Acts as a guide coat to show low spots 4 Sand 180-240 Grit
Sandpaper
Creates a smooth surface on master geometry 5 Body Fill Axson APF-7 Fills voids, scratches, over-sanded areas
6 Seal REN RP802
PVC Lacquer Sealer
Acts as final seal of the foam tool
7 Final Sand 800-1500 Grit Sandpaper
Creates a smooth surface on master geometry Female
Mold
1 Mold Release PTM&W PA0828
Creates a barrier between foam male and CFRP female
2 Place Datums Brittle Resin Pins and Steel Drill Bushings
References subsystem parts and jigs
3 Surface Coat PTM&W PT1995
Creates smooth, hard surface on female mold. Apply thinly (0.030” thick maximum) 4 Wet Layup Carbon +
PTM&W PT2520, Aluminum Core
Creates female mold structure
5 Cure Peel-Ply,
Breather, Vacuum Bag
CFRP cures tightly against foam tool
6 Drill Holes - Flange holes drilled for tool assembly 7 Trim - Access holes trimmed; Front bulkhead cut
out
8 Repair - Voids filled with PT1995 or high
temperature body filler; Re-sanded to 1500 where necessary
9 Post Cure ¼” Bolts, Support Structure
Post cured female tooling in oven at 250℉ while assembled and supported via. “eggcrate” Male Chassis 1 Install Boss Inserts Ultem Printed Bosses, Plasticine
Affixed boss inserts using silicon; Filleted any gaps using plasticine
2 Mold Release Frekote 710 Creates barrier between female tool and male chassis
76
Table 10: Chassis Manufacturing Steps Continued Male
Chassis
3 Outer Skin Prepreg Applied outer skin per layup schedule 4 Pad-up Prepreg Applied outer skin pad-ups according to
location
5 Film Adhesive Film Adhesive Film adhesive applied all over
6 Core Core + Core
splice
Core applied per layup schedule 7 Repeat Steps 5-2 - Use reverse order (ex. #5, #4, …)
8 Cure Peel-Ply,
Breather, Vacuum Bag
CFRP cures tightly against female tool
9 Drill Holes - Drilled out locating holes while in assembled female tool
10 Repair - Delamination addressed using structural adhesive; Sanded as needed
Many of the products were procured through sponsorships. Safran donated one of the largest production costs, their manufacturing time and most of the consumables needed for the foam tools. Another large donation was the raw foam from General Polymers. The PTM&W products were purchased using a collegiate discount with MESFAC funding. The dry carbon was donated by Chomarat, and the prepreg carbon was donated by TenCate and Toray. Finally, the core was donated by Hexcel.