Manufacturing a Carbon-Fiber Wheel

Step-by-Step

A Process Designed by Team Super G

Team members:

Kurt Clemmons
David Cain
David Krebs
Jimmie MacLean
Jeff Craycraft
Greg Wood

For: ME 4182 ,Dr. Lipkin

On: December 3rd 1998

Executive Summary

The goal of team Super G was to design a process to manufacture a carbon-fiber wheel for the Georgia Tech Motorsports car. To achieve this goal Super G researched many design considerations for the process. Super G raised over $800 to fund this project with support from the Georgia Tech staff and the business community. Super G also gained certification on several manufacturing machines to complete the various phases of production.

After experimentation and calculations, including pairwise comparison, Super G selected the best design techniques. The wheel fabrication process uses a CNC lathe to machine two molds from 16inch aluminum slugs. Since the wheel is comprised of two non-symmetric halves, the process requires two molds, or four mold halves. Given the wheel specs the CNC machine was programmed to machine two male plugs and two female cavities. The final mold design was a basic tapered male plug and female cavity. A hand lay-up was used to place 16 layers of carbon fiber on the male half. After cutting 5 slits in a square piece the fiber was hand pressed over the plug. Each layer was aligned so the fibers are 45 degrees from the previous layer. After pressing the parts the two wheel halves are attached with an eight bolt circle and RTV sealant (similar to caulk) around the edges. Future work would include investigating better mold release chemicals for this part/mold combination.

Table of Contents

List of Figures ---------------------------------------------------------i

Introduction (Problem Statement) --------------------------------1

Process Design Description ---------------------------------------2

Considerations / System Descriptions / Discussion ------3-6

Conclusions / Future Work --------------------------------------6-7

Appendices

Mold Design Considerations ------------------------------------A1

Project Budget -------------------------------------------------------A2

Functional Flow Chart ---------------------------------------------A3

List of Figures:

Figure 1 -------------------Exploding Diagram of Wheel Assembly

Figure 2 -------------------Final Mold Design

Figure 3. -------------------Project Plan for Production (timeline)

Appendix 1 ---------------Mold Considerations

Appendix 2 ---------------Project Budget

Appendix 3 ---------------Functional Flow Chart

Problem Statement:

The specific goals of Team Super G are to engineer a method of producing carbon fiber wheels and fabricate the required tooling to produce a working prototype. The most important aspect of the creation of the prototype is the functioning of the mold. A mold must be capable of forming the composite material into the shape of the wheel. Heat and pressure must be applied in order for the carbon fiber to have the required strength to withstand the loading of the racecar. This project also presents many other challenges from an engineering standpoint. Certain questions must be answered, such as choice of materials to minimize cost, availability of facilities, testing of the manufacturing processes, and overall effectiveness of the process design. Team Super G feels very confident that these questions have been answered and a successful process design has been achieved.

In the motorsports industry, one of the most crucial elements in design of the vehicle is the weight. If the overall weight of a racecar is reduced, the performance characteristics of vehicle can be greatly enhanced. For example, a car weighing 2000lbs with a 200hp engine will be able to outperform a vehicle weighing 3000lbs with the same 200hp engine if all other variables remain the same. Georgia Tech Motorsports has been one of the top teams in the Formula SAE competition for over ten years. Part of this success comes from focusing on aspects such as decreasing weight to increase the racecar’s performance. The team is constantly searching for new alternatives and technologies to decrease weight. In 1996 Georgia Tech Motorsports switched from using 13-inch diameter wheels to a lighter configuration utilizing 8-inch wheels. This change resulted in a net weight loss of over 25 lbs. This was significant considering the car only weighs around 430 lbs. Georgia Tech Motorsports is interested in returning to the 13-inch wheel configuration for 1998-99 car because of the beneficial handling improvements gained from using the larger wheels. However, the team would also like to reduce the weight of such a set up. In the past, the 13-inch wheels were bought and were made of forged steel. This design was deemed too heavy and alternative materials were considered. Carbon fiber was determined to be a viable alternative to steel because of its large strength to weight ratio.

Team Super G has taken on the task of producing such a wheel for use on the Georgia Tech Motorsports racecar. This carbon fiber wheel will fill the need that Georgia Tech Motorsports has for a car with handling qualities of a 13-inch wheel, while decreasing the total weight of the car. The wheel must be able to withstand the loads encountered under the strenuous conditions of auto racing. These include lateral cornering loads, pressure exerted from pneumatic tires, as well as impact loading from imperfection of the racetrack.

Process Design Description:

The idea of creating a process to manufacture a carbon fiber wheel was broken down into several sub-functions (see appendix 3). The process has four major functions. The major functions are designing the mold, fabricating the mold, casting the wheel, and finishing the wheel. These four functions were broken down into further sub-functions to show the various tasks involved in creating the wheel. This break down is shown in Figure3.

The first step in the process is to design a mold that can be used in a repeatable process. In order to design the mold, the team must first determine all of the specifications for the wheel that is to be produced. Using the wheel specifications, the mold specifications are determined. These specifications must also account for all of the forces needed to successfully press the wheel. From the mold specifications, a detailed AutoCAD drawing of the mold is created.

After the drawing is completed, a mold can then be fabricated. In order to make the mold, proper materials must first be chosen to minimize cost and time. Once these materials have been chosen and obtained, then using the specifications and drawings of the mold, computer code must be written. This code can then be used in the final step, which is machining the mold. Since the machining process could be very expensive, great care must be taken to prepare and test the tools.

Next, the wheel parts may be cast. To cast the wheel parts, the mold must be properly set up. Then the fiber is to be laid up within the mold. The fiber is then pressed under certain temperature and pressure and is allowed time to cure.

After the wheel parts have been pressed, the final step can be executed. The final step is finishing and assembling the wheel parts. Any excess material that may have flowed out during pressing is trimmed. Then the separate pieces of the wheel are assembled so the holes for any necessary bolts may be drilled (see Figure 1).

Subsystem – Mold Design

Designing the mold can be broken down into three problems. The wheel specifications must be obtained, the mold specifications must be set accordingly, and the mold must be drawn using a CAD program. The specifications for the wheel are readily available because the wheel has already been designed. Several calculations are needed in order to make the mold with proper allowances for wheel thickness and turning radii. Also, the forces placed on the carbon fiber by the mold must be analyzed to insure that forces are applied equally in all directions. If not, the wheel may have a weak spot where the carbon fiber does not set and cure correctly. Once the mold design has been finalized that design must be drawn on the computer using a CAD program. This is necessary so that the mold can be entered in computer code that the CNC machine can read. Figure 2 illustrates the mold design chosen. This was found to adequately apply pressure to all surfaces for uniform part thickness.

After considering the cost and time of machining other design from the alternatives, the above design was the most feasible option. Super G has access to all of the necessary machines to machine this design. Expert faculty and machinists agree that this mold design is suitable for the purposes expressed. The hydrostatic force of the fluid was proven suitable to apply pressure on the vertical faces.

Subsystem – Fabricating the Mold

To fabricate the mold, we must first determine the material used, and then determine the machine(s) used. We must also design a method for mounting the slug on the lathe. A pairwise comparison determined that aluminum was the most viable option for machining the molds. Super G also determined that a CNC lathe was the best machine for fabricating the mold. Super G designed a mounting flange that allowed the part to hold steady on the CNC spindle. Once the design (Figure 2) has been entered into the CNC machine a test run will be preformed on a block of foam. This will save money in the event that there is an error in the milling process. After analyzing the required functions of the mold and considering cost and weight the mold material selected was aluminum. Finally Super G obtained machine time and the parts were machined. Since Super G has access to a CNC lathe and an expert instructor this was an attractive option. Since we obtained sponsorship for the aluminum this was the most attractive option. Though steel would have also served the purpose, its weight would make it incredibly difficult to transport and mount for machining.

Subsystem – Initial Pressing of the Wheel

This is the first step in the process of producing a finished working wheel. The mold must first be coated so that the wheel does not adhere to the mold. The next problem is to lay-up the fibers in such a manner as to minimize the seams and maximize the strength of the fibers (fibers are strongest in tension).

The mold was coated with 6 layers of "Freekote" mold release and a knock out plate was machined out of the female cavity. This assures us that the part would detach from the cavity. The optimal lay-up technique was as follows: 16 layers of fiber were placed over the male half. Cutting 5 slits in each layer allowed us to stretch the segments down and line up the seams. Each additional layer was rotated 45degrees from the previous and spare fibers were inserted in any resulting gaps. After the lay-up, the part is ready to be pressed.

For best results the molds should be preheated. Also the "Freekote" should be baked on the molds. The press will ramp the temperature cycle to 250F and then to 350F, for about an hour and a half, at a constant pressure of 2.5 tons. The curing process will then begin while the wheel is held under pressure and the part should be cooled under the press until it can be separated.

Subsystem – Wheel Finishing Process

Finishing decisions include: how to remove the flashing? How to attach the wheel halves? And how can we protect the carbon fiber finish? Since the molds are aluminum we must only use brass instruments to trim flashing. Once detached, it was determined that the two halves should be attached using an ten-bolt circle. This was determined safe for holding the pressure of the tire. Also a protective flange will be attached to each wheel-half to eliminate damage to the carbon fiber (see exploding diagram figure 1).

All other loads will be supported by the lugs attaching the wheel to the hub. Once this is done the tire may be placed over the wheel halves and they may be mated together.

Conclusion / Future Work:

Team Super G did not design a Georgia Tech Motorsports car wheel. Team Super G did not research the composite material required to create the carbon-fiber wheel. What Team Super G did do is take the wheel specifications and design a process for manufacturing a carbon-fiber wheel. An outline of the project plan appears in timeline form in figure 3.

The motorsports industry, and in particular, Georgia Tech Motorsports team has expressed a need to make their race cars lighter. To make a car lighter would certainly make the car faster, but the strength of the parts cannot be compromised. The introduction of a carbon-fiber wheel would result in a lighter, stronger wheel and ultimately a faster car.

Super G researched many design considerations for the process, and raised the necessary funds to produce a prototype carbon-fiber wheel.

This required certification on several manufacturing machines. Design considerations included what material to use in the mold? What are the dimensions of the mold? What type of fiber lay-up is optimal? And what machines should be used in the machining process?

After experimentation and calculations, including pairwise comparison, Super G selected the best design techniques. The wheel fabrication process uses a CNC lathe to machine molds from aluminum slugs. Since the wheel is comprised of two non-symmetric halves, the process requires two molds, or four mold halves. Given the wheel specs the CNC machine was programmed to machine two male halves and two female cavities.

After machining the mold parts it was necessary to lay-up the fibers by hand to ensure the loads would be supported. Tests concluded that the optimal lay-up was 16 layers of fiber rotating each layer 45 degrees. This allows the weakest seams to be axially. The most efficient lay-up technique involves cutting five slits outward from the center of the male half. Then each section was stretched down to cover the plug and the slits were lined up as closely as possible. Spare strips of fiber were placed between any outstanding gaps.

The molds were coated with teflon and several layers of mold release before pressing. A knockout plate was machined out of the female cavity, as analysis showed the part would stick to the cavity after separating the two halves. The molds were pressed with 2.5 tons at temperatures up to 350 F. This process allows the epoxy in the composite to flow throughout the layers. The carbon fiber was cured and released producing a solid lightweight part.

The flashing was trimmed by hand with brass instruments and the parts were refinished on the lathe. The two wheel halves were attached with a specified ten bolt circle (see figure 1) and sealed at the edges with RTV. This circle was determined to hold the necessary loads presented by the tire pressure. When the wheels are bolted to the hub, loads will be supported by four lug nuts.

The process of manufacturing was successful, however we observed plenty of room for improvement. Future work would increase efficiency in production and quality of the finished part. Super G would suggest investigating a teflon coated aluminum material for the mold parts. It would also be helpful to line the entire cavity with teflon before pressing the mold. This would minimize the greatest problem of detaching the part from the female cavity.

With the finished molds the GT Motorsports team can repeat the Super G process to produce the remaining wheels for the racecar. The Super G process was proven feasible, cost effective, and repeatable (see budget of the project in appendix 2). The finished prototype was a success. Given the rigorous design requirements the wheel can safely perform its function.

An outline of the design process and considerations exists in appendix 3 (Functional Flow Chart). With the see engineering analysis and expert advise, the process was satisfactory. A "good part" of carbon-fiber should ring like a bell when struck with ametal object such as a class ring. Though the first part did not pass this test, later trials arrived at the optimal technique (so far), outlined above.

Appendix 1

Appendix 2

Project Budget:

Expenses:

Part Name

Description / Purpose

Vendor

Part #

Price

Foam

Tooling foam used for proof machining

Renshape

 

$300.00

Aluminum

Outer Wheel Half Mold (Male)

Tull Metals

 

$422.82

Aluminum

Outer Wheel Half Mold (Female)

Tull Metals

 

$422.82

Aluminum

Inner Wheel Half Mold (Male)

Tull Metals

 

$512.50

Aluminum

Inner Wheel Half Mold (Female)

Tull Metals

 

$512.50

Lathe Tool

1" Boring Bar for CNC Lathe

Rutland Tool

2027-7816

$216.00

Lathe Inserts

TiN Coated Turning Inserts CNMG-431

Rutland Tool

2046-9431

$41.00

Carbon-Fiber

Graphite / Thermoset Prepreg weave

Cytec-Fiberite

T300/934

$2000.00

Shipping

Above Prepreg shipped in dry ice

Ground Carrier

 

$200.00

TOTAL: $4,627.64

Contributions:

Sponsor

Description

Price / Value

Cytec-Fiberite

Donating approximately 55lbs. of carbon-fiber thermoset woven prepreg

$2000.00

GT Motorsports

Donating Tooling foam required for proof machining

$300.00

Tull Metals

Discount on aluminum mold material (15%)

$280.59

GT Motorsports

CASH contribution to the project

$1200.00

TOTAL: $3,780.59

Appendix 3