ME-542 Manufacturing with Non-metallic Materials

Process for Making the Suntrakker Solar Car Windshield
By Team No Doug: Carl Tedesco, Beronica Escobar and Sipand Zarrabi

The design approach for making the windshield was to keep the process simple. The process began with designing and drafting the windshield. The second step was making the mold. The final step was to thermo-vacuum the heated acrylic sheet to the mold.

TOOLING (Mold Process) 

Materials: wood boards, pliable wood strips, foam, resin, fiber glass sheets, plastic sheets, Bordo ^TM(Polyester based body filler) 

Tools: electric saw, straight edge, air gun, sanders, rough sand paper, 600 grit sand paper, spatulas, ceramic board, latex gloves 

We chose to make a male mold because it would require less work, time, and attention to precision. The male mold dimensions are 1.5 by 2 by 4 foot. To-scale print outs of the design were used to make the tooling for the windshield. Print outs consisted of the skeletal frame, the width being the ribs separated about 4 inches apart and the length being the spine cutting along the middle of the ribs. The print-outs were pasted to wood boards. We used electric hand saws, and band saws to cut boards following the pattern. The ribs and spine boards were assembled to a base board. 

The mold was closed by nailing thin pliable wood strips lengthwise. To make the mold solid, liquid foam was injected and allowed to cure. Later, excess dry foam was manually sanded down to the wood strips. 

To prepare for the resin and fiber glass coat, the mold was blown of dust and placed on a table covered by clean plastic sheets. The floor was also covered by plastic sheet to protect the floor from spilled resin and allow us to clean up easily. The mold was raised with blocks to prevent the resin from adhering to the plastic sheets. 

Fiber glass was rolled out and cut a few inches longer around the mold. Since the fiber glass was not wide enough, we overlapped the sheets by about 4 inches. Paying close attention to the directions for making the resin, we wore latex gloves and used hard plastic spatulas to apply the resin quickly and efficiently. We began the application at the top making sure to saturate the fiber glass until the mold had shown through. The mold was allowed to cure for a day. 

  A coat of Bordo was applied thoroughly over the mold until the fiber glass no longer showed through. This had to be applied quickly and efficiently before the body filler cured due to the catalyst and ambient temperature. This was allowed to cure a day before sanding. The sanding technique used here was a straight and perpendicular motion rather than a circular motion. The mold was air blown of dust, Bordo reapplied to fill some uneven areas, and sanded.  The mold was painted black to reflect heat to the acrylic sheet during the thermo-vacuum process.  To ensure a glassy and smooth surface, the mold was sanded with wet 600 grit sand-paper.

WINDSHIELD MATERIAL 

From conversations with Dr. Burns and other Suntrekker team members, we had the choice of using polycarbonate or acrylic sheet for the windshield. Polycarbonate would have been preferred but it was revealed to us that it requires additional measures in preparation for its use. Polycarbonate has the tendency to absorb moisture form the air and would have to be pre-heated to dry it. Since time is of the essence, acrylic was selected to be the windshield material. The cost of 49 x 76 x 1/8 inch acrylic sheet was $50.00. Four sheets of acrylic were bought because the thermo-vacuum forming process is done by eye-balling the event. 

THERMO-VACUUM PROCESS 

Materials: 1/4 inch thick and 4 x 6 foot PVC sheet, several sheets of acrylic, adhesive putty, high temperature plastic sheets 

Tools: as many C-clamps as you can find, drill with large drill bits, scissors, 3 heat shields, one 4 inch x 4 x 6 foot wood box with many finely drilled holes 

 
Now that the mold was done, team No Doug made a visit to the art department's first floor to get dimensions for thermo-vacuum box and familiarize ourselves with thermo-vacuum machine. Since the machine was locked we had to contact an art department technician at 594-5959. A technician by the name Oray helped by demonstrating to us how to operate the machine. He also made sure the machine had its proper fluids and was functioning before the day we planned to use it. To give you an idea of how long this process took each step was lettered and each major step was timed.

Results 

The first test was allowed to sag about 6 inches and the heat shields were not used. Thus, first test did not pass because the acrylic sheet was not heated long enough or uniformly. Also, the perimeter resisted to the vacuum and one side pulled away from the clamps. The second test did not completely pass but faired better because we used the heat shields and allowed the acrylic to sag approximtely 8 inches in 12 minutes. So, the shape of the second test was relatively good but the visibility was poor (blurry) and pitted from the cold water contact. 

Suggestions for Better Results 

• Put aluminum foil on shields. 
• Improve the mold by making it air-tight. 
• Decrease the length and width of wood box to increase vacuum pressure along mold perimeter. 

Additional Comments: 

• Have more material than you expect to need. There isn't anything more distressing than not having enough material or resin to complete the job right. 
• Be prepared to spend more time than you think necessary for the task. (Murphy's Laws are at work) 
• Keep a log of design measurements, material, contact people, new ideas or suggestions that come your way.