2012
04.29

Fixing a major mistake

After sawing the halves of my pod mold apart, the first thing I did was to fit the two halves together to see just how much clearance there was (should be right about 0.010″).  Unfortunately, this is what I got:

The two halves should fit fully together with no gap, so I started trying to figure out what went wrong.  As it turned out, this was simply a result of picking precisely the wrong surfaces to be machined, and I wound up with the exact inverse of what I wanted.  I was guessing that I’d have to simply recut the molds (I say ’simply’, but it would be kind of a pain), until I realized that I should be able to simply make molds from the molds, thus flipping the surfaces back to how they’re supposed to be by turning the male half into a female half and vise versa.  Copying molds in this way is not uncommon for composite work – a male master or ‘plug’ might have a number of molds pulled from it, with each mold being used to create dozens or perhaps hundreds of parts before it starts getting warped or damaged.  If another mold is needed, you simply cast a new one from the master.

The first step was to polish the surfaces (which I would have had to do without a screw-up anyhow), for which I wet sanded each half with 400 grit sandpaper to eliminate the ridges left from machining (I could have done a little better, as there are some ridges left, but I think it will certainly be good enough).  After that, I applied a coat of Partall #2 and buffed it off. After applying and buffing 4 more coats (to ensure that there are no missed spots – if the epoxy adheres to the mold surface itself, you have a ruined mold), I was ready to apply a coat of PVA. I diluted the PVA about halfway with distilled water and then brushed it on the surfaces with an acid brush, then set the parts aside to dry (you can see the green tint of the PVA pooling in low spots on the Corian mold halves).

Next I needed to dam up the sides of each half, so I used some sheet foam and hot glue, taking care to leave no gaps between the Corian and foam (lest epoxy leak out the side).

Then it was time to mix up some tooling resin, which is basically just epoxy mixed with graphite powder to provide a nice dark surface (which makes it easier to see when you have fiberglass properly wet out against the mold surface).  I used a little bit of West 404 filler as well to thicken the mix a little.

I carefully brushed the tooling coat over the entire mold surface and up the sides of the walls.  I then poured the remaining resin into each half, and took a blurry photo.

I tossed the halves into the oven for an hour on a very low temperature to help the epoxy ‘kick’ and start polymerizing (which is what actually causes it to harden).  With the tooling coat thickened, I was ready to fill in the remainder.  Since epoxy is expensive (and epoxy fillers aren’t cheap, though certainly less costly than the epoxy itself), I figured I’d try using aquarium gravel as an aggregate filler since it comes in relatively small bags for just a few bucks.  I mixed in a bit of 404 filler as well, though my mixes were a bit unbalanced – one half was gravel poor, while the other had an abundance.

The aquarium gravel turned out to be not as strong as I’d like (it chips and breaks easily), so perhaps I’ll just use sand as a filler in the future.  After letting the halves cure for a day, I used a utility knife to slice away the foam dams and a pick to dig out the hot glue.  In retrospect, I should have really used plasticine clay, as the hot glue was difficult to remove.  Separating the Corian master from the epoxy mold was simple, if brutal – just whack the block on a hard surface a few times until the epoxy half pops away (this was how I discovered that the aquarium gravel isn’t the greatest in a structural sense).

When I had a look at the molded halves, I was amazed at the level of detail captured by the resin – not only the minute milling ridges in the Corian and smoother areas where I had wet-sanded were visible, but even the edge of the puddle of dried PVA could be clearly seen.  I did a little more wet sanding on these halves, then applied 5 coats of wax, and then a coat of PVA (this time standing the mold halves up on end to keep the PVA from puddling).

Meanwhile, I realized I had neglected another part of the sailplane that will probably take a bit of wear from landings – the tail skid.  While there is a strip of plywood embedded, I’m sure large gouges in the foam will result the first time I miss a grassy landing strip and plow through a gravel driveway.  Rather than haul the fuse back over to Frankie’s studio to digitize the tail, I thought I’d try making a flexible mold with some OOMOO 25 silicone rubber that was past its shelf life – best to put it to use than throw it out.

I brushed it on the tail skid and threw in a few strips of fiberglass to give it a little more strength (a technique I recall seeing in a book long ago where strips of burlap were used to strengthen and stiffen a latex mold).  The silicone started setting up quickly, so I blobbed the remainder on and covered it with another piece of fiberglass.

Once cured, the silicone popped right off the tail skid.

I then made a plug in the silicone mold, using more aquarium gravel for bulk.

Just as with the foam, the silicone separated very easily from the cured epoxy.  Note that the texture of the original foam is perfectly captured.  The plug was thoroughly waxed, brushed with PVA, and set aside to dry.

Tonight I finally attempted using both the wing pod and tail skid molds to make actual parts.  I used a layer of 3oz and a layer of 1.4oz glass for the wing pod and mashed the mold halves together (I should clamp them together, but they seemed to be sticking together quite well on their own).  The 3oz glass wasn’t draping well over the tail skid plug, so I abandoned that weight and went with a layer of 1.4oz. and a layer of 0.75oz.  Trying to fit the silicone mold over the fiberglass covered plug was a bit tricky, as the shape doesn’t ‘key’ together as well as it could, but I finally called it good enough and set it aside to cure.  In 2 or 3 days I’ll see if all this work has actually yielded anything useful when I crack open the molds.

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  1. can’t wait to see the results!