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week ten: composites
This week's processes were new, both to me and How To Make. After shrinking 'Make Something Big' from two weeks to one, we got the chance to make molds and lay up composites in them. I made a paddle for my kayak, and did some testing for its composite skin. The materials provided were foam for tooling, linen as the fiber, and West Systems Epoxy as the matrix.
After doing some research on what features differentiate the various types of paddles, I wrote some code to generate meshes for them. The basic set of parameters I used were width, length, thickness, angle of assymetry (the point of the paddle), dihedral angle (each half of the blade is angled for smooth tracking), and shaft diameter. Below, the left picture shows a highly assymetrical paddle, and the right shows a large dihedral angle.
I was dissatisfied with the results from lofting and meshing in Rhino, so I computed the lofted meshes directly. The code is linked at the bottom.
To avoid double sided milling with thin foam pieces, I chose a symmetric paddle (zero dihedral angle) and split it into two parts. By milling each seperately, we can avoid the need to register and refixture a piece. The video at the bottom shows some of the milling process.
With the two halves milled, I glued them together with wood glue to make a full foam core. This core will remain inside the composite to guard against buckling. Outside this mode, the composite skin should be extremely strong and stiff for its weight.
After running a shopbot job that was calculated at 56 minutes (and probably took a bit longer) at 10% stepover on the finishing pass, I used a coarser stepover of 20% for the second paddle. This cut the calculated time to 34 minutes. This surface finish of the foam showed visible rastering lines (below right), but this will be completely hidden by the layup.
I used the laser cutter to cut out my linen panels. I cut two layers of linen at once on the epilog setting speed to 50%, power to 60%, and frequency 500. In my experience, it's better to blast through fabric on the laser with aggressive settings, as any uncut threads can cause unravelling of panels, particularly with natural fibers like linen. The cutfile I used is a silhouette of the geometry generated by my code, offset by an inch on all sides.
Now is the fun part: layup. I used two layers on linen on each side of both paddles and an additional strip around the shafts of the paddles. This was a lot of linen to wet out and get in the bag. After curing, the paddles weren't quite as stiff as I wanted, so I applied another whole sheet to each face, as well as a narrow strip along the length of each face. Big thanks to Tom for helping me with the second layup and to Will for recording it.
After removing most of the flashing, I started digging out the foam from the shaft section so I could fit the composite over the shaft I had (conduit I found in the basement...). At this point I noticed some sections of linen that weren't completely saturated with epoxy (right). In light of this and the fact that I wanted the paddles to be stiffer, I did the second layup described above.
I uploaded some footage from milling and the second layup to youtube: