Posts: 189
Joined: 1/14/2003 From: Sydney, AUSTRALIA Status: offline
When I say baseboard, I mean it forms the base on which the epoxy/sand is attached, like the pic below. The LE flange is wide enough for small steel dowel pins to be used fo alignment. To help the sand/epoxy to adhere to the baseboard I fill it with 1/2" or so holes which are countersunk from the otherside. For TE's, I just allow the uppper and lower edges of the layup to touch and define the TE thickness. As the glass here is only 0.1mm thick I end up with a very sharp TE every time. Not so practical for large scale models, however, but for them I would define the TE thickness by having a block on the baseboard which atops the mould closing up too much. I find it's better to keep the TE flash on the wing most of the way during construction, then sand it off with a long straight sanding plank.
Gary, I prefer sand, it has several advantages: it's free and I get to check out the "talent" at the beach whilst getting it!
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Stu Maxwell - stumax@optusnet.com.au StumaxAircraft - www.stumaxaircraft.com
Posts: 133
Joined: 9/27/2004 From: Hoffman Estates, IL, USA Status: offline
This may be a stupid question but, why don't you just use microballoons instead of sand. Its much lighter, cost is about $10/cu ft and mixed with epoxy about 1.5 or 2 to 1 (balloons to epoxy) makes something like cake frosting. Somewhere above 1.25 parts baloons it will not run or slump any more. It cures rock hard but can be shaped easily. I used it a couple of days ago to stick a 5 inch diameter fiberglass tube to a flat steel plate and was very happy with the results. Not exactly your use but similar. the fit between the tube and plate was very sloppy.
I didn't go above 2-1 but i would think you could go about as high with balloons as you could with sand if you want to keep the cost of resin down.
aside from the weight the only problem I would worry about with sand is water. The sand better be dry. Which means it better be stored inside.
And you're right about support. You've basically reinvented concrete here. An aggregate in a matrix. And as anybody knows, a long, unreinforced span of concrete is gonna fail.
ORIGINAL: gachezem and nobody has mentioned useing beads for the bead blasting machines in place of sand, it is lighter and might be stable enough to do the job..Gary
What are they made of do you know? I always assumed they were some sort of nylony/plastic shot with a specific shape, but Ive never actually seen them.
Posts: 58
Joined: 6/8/2002 From: Keizer, OR, USA Status: offline
beads for bead blasting R just glass beads very much like micro ballons, they R light and will shatter if U use over 100 lbs of pressure for blasting with them, would be a cheap way to go if U need something much lighter than sand, they dont seem to take on moisture, but may need to be stored in a dry area, or use a desicant to keep them dry, or if in doubt put in a warm oven, I think they would be lighter than a standard lay up for making molds..............If U need more INfo I can look it up for U, or just use Google for a search?? ............Gary
Posts: 39
Joined: 12/19/2002 From: Lebanon, IN, USA Status: offline
I do Composites full time in the racing field and would like to make some comments. #1- Reinforcements within the core will add very little strength to a sandwich mold. The reinforcement is too close to the center axis of the mold. It will add some tensile strength but virtually no stiffness. You have to realize that the concept of a sandwich construction is the strength comes from the outer (surface) plies. All the core does is supply buckling support for these surface plies. If you build a sandwich with 1/4" core it will have x amount of stifness. Build an identical sandwich with 1/2" core and it will have 7x amount of stiffness. I have built quite a few cored molds because it is a very fast way to build a large mold. I have used sand, ADTech room temp dough, Vantico DT081 (I think that is the number), and Polyurethane sanding dust. Tha foam dust is nice because I get it free from a guy that cuts buns into sheets. I feel the easiest to use is the Vantico product mixed at 111% ballons to 100% resin by weight. This stuff has a high tack level that makes it very easy to use. The ADTech dough is nice also but it is expensive. The foam dust I just used last week at a ratio of 3 to 1 dust /epoxy by weight. It worked very well but is a little harder to apply. But Its CHEAP! and light. #2 If you're using your own mixture or concoction. Use a resin with a long pot life. This will prevent exotherm in the large thicknesses of resin. #3 I believe anything will work as a core material (sawdust) as long as epoxy will bond to it. #4 Just remember,,,, the mold "strenght" comes from the facing plies, NOT the core. If you want more strenght add more plies. The mold I made with foam dust was a seat mold. It had 3 mold surface plies of 9oz Bid glass, 3/16" to 1/4" foam dust/epoxy, and 2 plies 9oz Bid glass. It was room temp free air cured. You could park a car on it! #5 "Dough" type cored molds offer detail reproduction, (ie. tight sharp corners and indentations) that is very easy to achieve.
Posts: 39
Joined: 12/19/2002 From: Lebanon, IN, USA Status: offline
One comment I forgot to make. I was not able to finish the foam dust seat mold in one setting. I had all three surface plies and about half the "dough" on and had to leave it. I covered the entire mold with about 1/4" of plain foam dust. In the morning I swept off the dust, blew the remaining off with an air hose and had what amounted to a peel-ply surface ready to finish the mold.
Posts: 73
Joined: 1/1/2005 From: Bellflower, CA, USA Status: offline
Well your reply looks like it never got replied and since i know a little about Freekote's, i'll try ... Ive used freekote 77 and i think its 21, but 77 is a non bake application, i think. you can buy it in a gallon and in a spray can. do a search on yahoo or google using freekote and ride it on out the the site...it should be able to answer all your questions...and yes it works great for almost any type of mold release situation.
quote:
ORIGINAL: ptxman
quote:
ORIGINAL: I find that using Freekote mould release makes releasing quite easy for simple shapes like wings. Stu.
Which Frekote part#? Are you able to pre-spraypaint into the mold successfully with this stuff? Any special technique?
Posts: 189
Joined: 1/14/2003 From: Sydney, AUSTRALIA Status: offline
Sorry, I missed that question! The one I use is Freekote770. I spend a couple of hours with a new mould applying as many coats as I have the patience for, leaving them in the sun until it has completely dried between each coat. Someone once said you can buff it up once it's on and dried but I'm not sure if that's true - I'd be a little wary of doing that. You can paint in the mould with it, but be careful as it's Teflon, so spray light coats of a fast drying lacquer, ie nothing that will sit wet on the mould surface as it will fisheye. If you want a colour coat you still should spray a light coat of primer before laying up as the slight porosity of the prime aids the bond & stops the colour coat scratching off. BTW, it's also a good idea to spray a coat of white over the whole colour coat, then spray the primer, as this will bring out the true colour, otherwise the colours will dull a little from the grey primer. Like any paint in the mould technique, you still really need to at least clearcoat & polish the finished product if you can live with visible seams (which is why I only prime in the mould, as I can't live with visible seams).
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Stu Maxwell - stumax@optusnet.com.au StumaxAircraft - www.stumaxaircraft.com
Posts: 2438
Joined: 1/19/2002 From: Anchorage,
AK, USA Status: offline
Got the first half of both wing molds done last night, and am waiting for them to cure.
I used a typical process:
First, the plugs were waxed and PVA coated. Then a surface coat of #1099 surfacing resin, from Fibreglast (pigmented black) was applied. When that had tacked in, I put a fillet of epoxy/Cabosil into all the sharp corners. When that had tacked in, I put on 2 layers of 2 oz. cloth. After a couple of hours, I started with the sand epoxy mixture. I didn't get too "deep" about the exact mix ratio... Just mixed sand and epoxy in a big plastic tub with gloved hands, until it felt like it was as dry as I could make it without it falling apart. I put a layer of sand/epoxy about 1/4" thick on the whole wing surface, followed by two final layers of 6 oz. cloth.
It looks like a good method. The only potential trouble I see is that during the curing process, some of the epoxy leached away from the surfacing cloth, and into the sand below. (You can see some of the white spots in the photo) After this is cured, and I can inspect it more carefully, I'll decide if more cloth, etc., is needed. I can report better on the results tomorrow, and even better in a few days, when both halves are done and separated.
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Mike James RC Design and Building - www.nextcraft.com
ORIGINAL: The only potential trouble I see is that during the curing process, some of the epoxy leached away from the surfacing cloth, and into the sand below. (You can see some of the white spots in the photo)
Look forward to the progress. Re the cloth, I guess maybe a better way might be put the cloth down at the same time as the final sand 'fillup' layer, that way any resin leaching will go down & saturate the cloth & the 1/4" sand layer will be cured & act as a permeability barrier. next time.... theres always a next time in this business... <g>
Posts: 2438
Joined: 1/19/2002 From: Anchorage,
AK, USA Status: offline
Hi Peter,
That's what I did...
The sand had only sat for about 20 minutes when I added the final layers of cloth, and so I'm a bit confused about what apparently was a void or dry spot in the sand. (It wasn't obvious, when I was tamping it down, and I couldn't see a dip in the surface visually. It was only after I applied the second layer of cloth that I started to see what you see in the photo. Looks like I might have needed a little more resin in that part of the sand.
Next time...
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Mike James RC Design and Building - www.nextcraft.com
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RE: Sand / epoxy molds - 
