Intro: [Video] Large Scale Mold Making - Part I
This is another multi part series. This is a long project for the 2007 ASME Human Powered Vehicle team from the University of Central Florida. I will be breaking up major tasks into smaller instructables as the entire process is gargantuan.
Here's a Botched together video. The fast paced part is 1 hour of work ;)
Video - Mixing Polyester Resin
Video - Tool Making
Step 1: Background
We're making an HPV! That's human powered vehicle ;) We are required to have at least 1/3 of our frontal area covered by a fairing (a fairing is just an aerodynamic shell -- like your car's outer skin). We're going for a fully enclosed (aka "sealed") fairing.
To tackle this manufacturing problem, we will be making a "female" negative mold. Similar to how casts of fish are made. To make this mold, we first need to make a positive "male" plug.
This instructable is a guide! I'm not going to instruct you on how to make the shape - there's plenty of resources on that. Instead, I'm providing rough instructions on how to start this task.
Our mentor currently works for Lockheed Martin as a model maker. When it comes to composites, this man knows his stuff. This is my third year working on this type of project, and he has provided a wealth of information and advice. If you'll be tackling a similar scale project - find yourself someone that has the experience to guide you in the right direction.
Step 2: Foam and Cutting
We're using a high density insulation foam. If you have the option, get foam that is not scored - ours is scored, and it breaks somewhat easily along the scores.
This foam is some tough stuff. Our electric carving knife has some trouble cutting through - but a jigsaw does work ;) Our preferred method? A jet of 60,000psi water.
We had 37 sections (plus 4 additional sections to create NACA ducts) machine cut on a Waterjet. We used a company in Orlando called Crossroads Unlimited. In fact, we have used them for the past three years. They have a large Waterjet (72" x 144" cutting area) capable of doing abrasives. I highly recommend them if you're in the area.
Please keep in mind the cost scale. Each sheet costs $13. Luckily more than one section could fit on one sheet.
Step 3: Dry and Dry Fitment
Our sections were soaking wet. So, time to dry. We will be gluing them together, so we need a clean and dry surface.
Once dry, check your fit to make sure you've got everything in order. We labeled each section as it went into the machine.
When assembled, admire your work and do a little mock setup. We made that wood "bikamabob" a few months ago for kinematic analysis.
Step 4: Glue
Each section uses a 2X4" cutout for indexing and alignment. This ensures that the plug goes together straight and true (sometimes with a little adjustment).
Use a foam safe glue. Since last year, 3M changed the label on their "77" spray adhesive. The only change was the word "foam" -- after a test sample (comparing last year's can and this year's), we determined it to be safe in smaller dosages. 3M 90 will melt this foam on contact. As always, test first when working with your materials (especially new ones).
Our friend from Lockheed recommended 3M 74. Not available at the local hardware stores, but I'll remember that for next year and special order it.
Before spraying, wax your alignment "dowels." This will help the sections to slide down easily and prevent the glue from sticking together. Re-wax every few sections as necessary.
Spray an even fog over the edges of both faces to be mated and then spray a little bit in the center. It is key to get a good stick on the outside edge so that when it is sanded, you get a good gradient without separation. The key is (according to our composites master) to find the line between too little and enough. Remember that too much (of this glue formula) spells melted foam.
Once an even coat of adhesive is sprayed down, align your sections and stick down. We let the glue set for a few minutes with bricks on top to ensure a good bond.
When aligning, pick an edge to follow. In our case, we wanted the leading edge of the 2X4 to line up the whole way. So before sticking down, we lined up all three of our alignment 2X4's on their leading edges - then pressed down. Once we figured this out, we got perfect results.
Step 5: Weight
When done, add weight and allow to cure.
Alternatively, use shrink wrap. Just be careful that you're not warping your foam. A few layers should do it ;)
The idea works like a piece of string on your finger. One loop is fine, but each time you wrap around your finger the pressure goes up and up.
Next up... A whole lot of sanding. Unfortunately, I won't be there for it - so I won't have an awesome video for you. But hopefully the team is taking lots of stills ;)