ORIGINAL: Mike James We're sold... In the next week or two, I'll be making our wing molds... and will use the sand epoxy method.
Can I make a suggestion Mike? Make a test mold using a plug which is either smaller or sacrificial (ideally both) just to prove out the entire procedure from start to finish. I cant tell you how many times some of my 'great ideas' only goes 80% according to plan no matter how clear the procedure & end result is in my mind. I have some great conversation paperweights, but preserve your good plug for what you know 100%.
The video we've been referring to by Fred McClung (the glider guy) is availalbe through "Soaring Stuff"
That sounds like the one I probably saw. My recollection is rusty but as mentioned, the things I would suggest is a 2 stage shell type layup to keep weight off the plug & premixing the sand & epoxy to the consistency of thick cement vrs the sprinkling method if thats the same guy. Im sure as you get into it you will have other technique improvements.
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You're right Peter, on all counts.
I will make a test part right away, before it's time to make the wing molds.
I also think that, rather than Fred's "sprinkle" method, that the mix-it-first method makes more sense. It seems to me that the mixed material would cure more evenly.
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Ted: Thanks for the info. The FibRelease sounds similar to the Safelease product...the Safelease 30L is a wipe-on-and-forget teflon-based release liquid. I've never had much luck with PVA.
quote:
ptxman: From pics Ive seen they can extrude it out of pasta like machines at various thicknesses & press it on vertical compound surfaces like platicene.
The dough certainly has it's merits...I would imagine that it's a bit easier to work with than the sand especially on very large molds. Being able to extrude the dough to the correct thickness would be a huge timesaver on large projects.
Glass fibers added to the sand mix are still worthy of experimentation. From my understanding the glass fibers used in reinforced concrete are only 1/2" or so. You might be able to get away from the additional reinforcement you spoke of. US Composites sells 1/4" and 1/2" chopped glass in large quantities...cheap.
If you were to use some additional reinforcement why not just use a layer of heavy woven glass every 1/4" of sand / epoxy thickness? Or how about putting down a criss-cross of 12K or 24K kevlar tow? If you went back and forth (don't bother cutting the ends, just make a radius) the length of the mold spaced every 2" or so then did the same thing across the width of the mold you'd have a grid. If you did it while the preceeding sand / epoxy layer was firm but tacky it would be very easy and fast...and cheap.
I'm beginning to sound like a canary...cheap, cheap, cheap. I spend thousands annually on raw composite materials so for me cheap is good...provided that it WORKS.
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Then there's that nice, white pebbly stuff in all the golf bunkers <g>.
No experience with the sand traps...I'm usually in the woods!
Mike, I'm sold too! I began work on a canopy plug today and will hopefully be pulling molds from it by this weekend...sand molds of course. I'll post my results. Come to think of it, back in my HLG days I read something (maybe on RCSE) about 'sprinkling' sand in molds...I totally overlooked it because it sounded, well, stupid. Oh to think of the time and money I could have saved if I had only listened!
-Tom
< Message edited by TT2 -- 12/15/2004 4:30:42 AM >
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Okiedokiethen...
Ted! It's erie how you and I post at similar times. Anyway, thank you for the great information and photos here. We'll see the results of my clunky test tomorrow...May I ask, how would you describe the sand you use? Is it extremely fine? In your photos, it looks as though you didn't have any trouble getting it to cling to the vertical surfaces. In my test below, that was a major problem.
Just got back from the shop after a little test, and here are the results so far...
What is small, has a built-in draft angle, doesn't require a lot of polishing or a parting board, and is a reasonable test for this? I chose the top to a can of primer. It was placed on a piece of vacuum bagging material, which I taped down to a glass work table, and then the paint cap was not waxed, but given a coat of PVA. (photo #1)
In photo #2, I've given it a coat of tooling resin, painted black, as I would for any mold.
Photo #3 shows the sand used for this test. It's from our friends at the concrete plant accross the street from our shop, and is ordinary "filter sand". I put the pen in the photo to give you an idea of how fine it was. (not very)
< Message edited by Mike James -- 12/15/2004 3:23:40 AM >
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Continuing...
Note: I skipped the surfacing cloth on this test, but think that on my actual parts, I would add a couple of layers of cloth first, to insure that pushing this sand/epoxy around wouldn't risk gouging the tooling resin or my part. No bigee here though. (Also, I hadn't read Ted's statement about the inner cloth being part of fracture prevention... Oh well.)
Photo #4 shows something that makes perfect sense, but something I hadn't thought about... I'm using West Systems laminating resin (#105) with the slow (#206) hardener, and after stirring a bit of sand into the epoxy, the sand immediately sank to the bottom of the container. So, my solution was to add more sand, until it got to roughly the consistency of thick (and grainy!) mustard.
Photo#5 shows that after brushing it onto the plug, it separates there too. (no surprise) So, this would be great for a relatively flat surface like a wing, but is not too good for vertical surfaces. One way around this would be Fred's "sprinkle" method, and to let each coat tack in a bit, so the sand would stick to it better. But, one of the concepts of this test is to save time on steps like that, so I just plowed ahead, and added more sand, hoping to make it thick enough. It was somewhat better, but the resin still separates from the sand.
Photo #6 is a few minutes later... See the resin leaking out around the ever-enlarging perimeter of the "flange"? So, I sprinkled dry sand over it, and used a playing card to push the edges back toward the plug, creating a little ridge, or dam. At this point, about 30 minutes into the procedure, the resin was starting to tack in a bit, and the mixture was starting to get more like cookie dough. Like any layup, the longer you work on it, the more firm you can be with it, and it will tend to stay put.
< Message edited by Mike James -- 12/15/2004 3:29:11 AM >
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The last photo from tonight is at the end of the "layup" process, where I sprinkled a little more sand, and then applied a little cloth on the top and around the edges of the plug. The bits of the cloth at the edge are just there to catch any stray resin that might leak out later, but it's getting firm enough that I don't think that will happen.
Tomorrow, I'll pull the plug from this test mold, and we'll see what we've got.
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Agreed... (I emailed Ted to see if he used finer sand, and if he doesn't post here, will let you know what he says.)
In one of the intermediate batches, I did add a little Cabosil, but it didn't seem to make much difference. I'm betting it was the coarse grade of my test sand. We'll find out.
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Hey Mike
Looks like the mix is way too wet. PTXMan got the mix just right.
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A while back I had overmixed some epoxy which otherwise I would have chucked so I grabbed some clean dry sand from the kids sandbox & just started blending & increasing until it seemed wetted out but not crumbly dry.
It should be wet enough to make patties with it but not so dry that it crumbles.
Here are a few pictures. These are the female molds for the inlet ducting.
< Message edited by TGoodwin -- 12/15/2004 5:55:09 AM >
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Here is the motor access hatch mold. The entire F-16 project is now molded using the sand method including the male fuse molds that I made from my old female fuse molds. I reworked the male fuse molds and made new females. The vertical stab, inlet scoop, tail cone, canopy hatch and motor access hatch are all sand molds. The canopy and motor access hatch plus pictured here are also sand molds.
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RE: Sand / epoxy molds - 
