Week 9. Composites.


1. An instrument case. Vacuum molding around a one-piece foam core.

A. The Plan.

I would like to build a two-piece case for my treble viol. I'll start by making a ~ four inch deep bottom shell, then make a top that is less deep but more strongly arched.

B. 3D machining a foam core.

I designed a part in Solidworks that will be the foam core for the case bottom. I defined the outline, created an extrusion, added fillets to the perimeter, and used Solidworks' dome feature to make a gentle (0.5") arch in what will be the back of the case.

Above are pictures of the rough cut, the finish cut (0.5 diameter end mill, 15% stepover), and the finished part with some of the 0.050" support layer still hanging on. Things learned in machining this part on the shopbot using the shopbot 3D software:

C. Assembled mold.

I cut out a straight-sided layer of the foamboard with a bandsaw and sanded to match the perimeter of the first part. The two parts will be taped or glued together to form a mold. The assembled mold was not available for photographing at the time of this writing.

D. Plans for molding. Sketches of fabric cutting.

The sketch below shows my plan for cutting fabric to make four layers of material around the shell of the case.

I decided that I needed to get some practice with a small composite project first before tackling this rather large one. The 3D machining was valuable learning. The demonstration on composite layup inspired many thoughts on alternative ways to make molds and squeeze composite layers, one of which I will expore for a smaller project. More later on the viol case!

2. An egg carton. A two-piece "Great Stuff" mold.

A. The idea and the plan.

Here is a smaller project. I will make a four-egg egg carton. Since I was at home and had already spent happy hours at MIT I decided to make my composite with locally obtained materials and tools. Instead of machined foam I use "Great Stuff", a polyurethane(?) closed-cell foam used to insulate and fill cracks in houses. This foam sprays from a can, expands about 2x and cures

B. Making the Foam mold.

The bottom part of the carton did not distort as much. Below I show the mold after curing the foam, and the process of cutting away the excess and freeing the carton.

C. Molding the composite.



After a 24-hour cure, the sandwich comes out of the mold. The polyester batting is mostly saturated with epoxy, but not much excess has come out. The layers need to be teased apart - the hardest part is removing the peel-ply from the composite. The array of tools below do the trick, with one crack made in the bottom part of the carton (repaired with epoxy).

The parts and the original.


3. Notes in class 11/5/13

Notes on composites