2012
05.06

Successful pod covers

The other day I finally cracked open the molds to see how things had turned out with my wing pod and tail skid.

A few air bubbles here, and a lot of weave texture showing through – this is no substitute for vacuum bagging, but will it at least be sufficient?

Oh noes!  Only two lightweight fiberglass layers were apparently not enough – while flexible enough to be removed from the core, the molded part just wasn’t strong enough to hold up to the tugging and pulling needed to free it.  On the plus side, the fact that it was able to be fully removed means that the waxing and PVA application was sufficient to keep from ruining the plug, so this was promising for the wing pod.

My ‘kinetic separation method’ for breaking the mold halves apart (whack it on the floor a few times) is still far from ideal, as another corner broke off of the mold.  I now see why people who know what they’re doing build in screwdriver slots so that the halves can be separated in a less destructive manner.  With the satisfying sound of PVA breaking away from a surface, the halves popped free.  The part was still adhered to the female mold half (as evidenced by the black center), but a little careful pulling finally extracted it.

Finally, a completed, intact part!  Now, would it fit…

Almost perfect!  There’s a tiny bit of side-to-side slop, but I’d say this is well more than “good enough”.  Now, to make 3 more!

3 comments so far

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  1. YESSSSSS!

    • Yeah, 3D scanning FTW! The material is really thin, but I think it will be just fine for preventing minor scuffs and dings. At any rate, I now have another fabrication method in my arsenal!

  2. Try pressure instead of vacuum. A page I did on making a two part silicone mold. http://partsbyemc.com/pub/mold-making.htm

    A paint pot and cheap air compressor can be bought from an import store like Harbor Freight for around $100 on one of their frequent sales.

    RTV silicones and urethane resins are amazing materials. When making a mold from a part with a glossy surface you want to make certain there are no fingerprints on the item or your mold and castings will duplicate the fingerprints.

    When I started in this business circa 2004, I followed the “standard practices” and made several molds that didn’t provide the quality I needed. I’d bought a Gast rotary vane vacuum pump and I still had molds with subsurface bubbles that would collapse or rupture under pressure, making bumps, spikes and little balls on the castings. Another issue with not casting the silicone under pressure is it’s extremely difficult to get the silicone into tight inside corners. No problem at all with pressure!

    Another ‘baby’ I tossed out with the vacuum bathwater was using boards or anything board-like for mold forms. I’d made a couple of sets of L boards to clamp together, don’t use them anymore. Shiny surface cardboard and hot glue is all I use for most mold forms now. Easily cuts to any shape and size, can be found at the right price, FREE! and can easily be laminated with hot glue and/or coated with resin to make sturdier mold forms that are reusable. Clear packing tape also works well for surfacing cardboard for mold forms. Until I found that the silicone wouldn’t stick to the shiny cardboard I was covering plain cardboard with packing tape.

    My casting work is a business, not a hobby, so anything that saves time, money and materials is something I’ll use and traditional methods can go buh-bye.

    A book I highly recommend to anyone doing any kind of molding and casting is The Prop Builder’s Molding & Casting Handbook by Thurston James. It’s old and many of the materials he shows the use of are considered “obsolete” (especially sugar glass) but the techniques translate easily to newer materials. But what really makes it a useful book is everything in it *still works* and is very affordable. There’s even a chapter on making a mold from a frozen fish.