TT2

My Feedback: (13)

I saw an article on an EDF F-16 (thanks for the links Mike!) that mentioned the usage of sand and epoxy molds. After the plug / parting tray(s) are finished, you lay down a thick gelcoat, then a layer of 6 oz cloth (though I'd use surfacing veil before this as a print-blocker), then a mixture of good ol' sand and epoxy, then a couple layers of 6 oz cloth with a thick gelcoat. Sounds way too easy.

I've experienced problems using mold thickening materials (like Coremat) because they seem 'soft' even when fully wet out / fully cured out. I would think the sand / epoxy would be hard as a rock...since that's essentially what it is. Cheap too.

I guess the sand / epoxy gets mixed to the consistency of peanut butter (or slightly thicker) then molded by 'hand' (literally) to approx 3/8" thick.

Anyone try this?

-Tom

Marco Vergara

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Tom,
I made two wing molds with this technique it is faster than traditional lay-up fiberglass molds they are hard as a rock and cheap but heavy that is why you parting board must be strong.

Marco

Mike James

I saw this technique in a video by Fred McClung, and did a little research on it. thinking I would try it on my next small mold. I've heard that the approximate ratio by volume is 1 part epoxy/10 parts sand. Does that sound about right, Marco?

buttuh

I use a tooling dough, but never tried the sand trick. Tooling dough is a mixture of polyurethane foam, chopped fiberglass and epoxy resin.

Mike James

Interesting...

How about some photos? What's the cost of this stuff? And, most importantly, are you happy with the results?

TT2

My Feedback: (13)

Tooling dough huh? Do you make your own? How is the hardness?

I'm wondering about application of the mixture (either sand or the stuff that Evan is using). I suppose you could go about it like frosting a cake. What works for you?

-Tom

buttuh

The dough is made by PTM&W for room temp as well as for hi-temp application. Here's how I do my molds 99% of the time.

Tooling surface coat
4oz cloth
10oz cloth
1/4" - 1/2" thick of the tooling dough
10oz cloth

It's polyurethane foam so it's light. I like it because I can build up the mold's thickness without wasting a lot of resin/fiberglass/time. It's extremely strong as well and it's a soft dough that can be worked into hard to reach areas. I also like it because I do build a mold in one sitting and be done with it. Overall I save a lot of time and money so I can't complain.

Marco Vergara

Yes Mike, you are right. I use this techinique after I saw the same video.

Marco

TGoodwin

The sand method is great. Just keep in mind that the sand does not really add any strength. The cloth sand cloth "sandwich" (pun intended) is what gives it strength. Just like any other core material. It functions like an "I" beam. I usually use silica sand purchased in 100 lb bags at the local hardware store and use a drill and a grout mixer to mix it up. Molds build very fast this way. I have never had any print through problems but the tooling gel coat is fairly thick.

Thanks Ted

Mike James

I'm not worried about strength so much. It's not like my molds are pulling 30 Gs or anything.

Curious though...
Assuming you've built traditional molds, and then tried the sand/epoxy method, how would you say that it affects the amount of epoxy used per mold? It seems to me like you would use less, but I'd like to hear from you guys who have actualy done it.

Thanks.

TGoodwin

I guess it would be the same or less resin. I am not really sure. I have now started using two or more layers of 6oz cloth top and bottom on the sand molds. They really do not come out that much heavier than a standard mold as long as you dont go crazy and build it up too thick. There are no problems with getting the sand to wet out properly and there are no air pockets as the wet sand is packed down onto the surface and then rolled out to level the outer surface. I will be doing all of my molds this way in the future. I bet if you were to mix chopped glass fibers into the sand it would be much stronger.

A mold maker here in town told me that my molds would delaminate using this method due to the dis-similar materials used? I thought glass was made from silica? I have not had any problems yet.

Thanks Ted

Mike James

Longevity concerns me too. Has anyone reading this thread used this method on molds that are, say, 10 years or more old? If so, how are they holding up?

ptxman

My Feedback: (2)

Some random comments...
Ive played with small test pieces to the degree I think the sand method has merit. The sand is cheap, dimensionally stable & it conforms well. Unfortunately I didnt write down how much resin was used per unit of sand, but it struck me as quite a bit less than what would be involved in a typical cloth/mat layup. It kind of make sense because you are just coating the sand grains & that is forming the bond, they naturally settling & compact so a sloppy resin rich mixture is not required (or desired). 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. I compacted it slightly & left it in a graduated plastic cup to cure. I was very surpised at how strong the cured chunk was & even whacked at it with a hammer just to see. One my summer jobs was working for a tile setter & this sand/adhesive is very common practice for setting large pieces of tile & marble. They use different bases of adhesive but the principle is the same.

The way I would approach it on a typical RC mold would be:
- extra rigid profile board & supports
- same gelcoat & glass layer on the plug as normal
- lay in base coat of sand/epoxy matrix say 1/4" uniform thickness, that way you are not stressing the plug or profile board assembly too much with weight
- the idea here is when this first layer is cured, it acts as a structural shell for the bulk (heavier) backup fill layer. The sand will already have a nice granular finish so there should be no bad bonding issues
- maybe between these 2 layers it would be a good idea to lay in some reinforcing rods just like a concrete process. That will give the mold strength.
- install a block & tackle system in your shop to move the finished mold around (thats probably the only downside on large molds, HEAVY!)

Mike James

Thanks a lot, Peter.

Your input is always valued and appreciated!

TT2

My Feedback: (13)

Thanks guys, keep the info coming!

I spoke to PTM&W about the RT2C tooling dough. I've got a sample on the way. Seems rather expensive though...

OK, I ran the numbers. The tooling dough that buttah was talking about is about $250 for a five gallon kit, 40 lbs of material.

Density: 60 lbs per cubic foot
Cost: $50.00 per gallon


Figuring sand to cost $2.48 for a 60 lb. bag and have a density of 100 lbs per cubic foot (may be slightly higher) and figuring epoxy to cost $52 per gallon (I buy it in five gallon drums) and weigh 8 lbs per gallon, and figuring that the sand to epoxy ratio is 10:1, we end up with... (drumroll please, Mike!)

Density: 96 lbs per cubic foot
Cost: $5.22 per gallon

Geez...is that right? Feel free to check my math. If all is correct then it's a no-brainer. I can't imagine the dough being worth the additional...OK I'll say it...DOUGH...roughly ten times the price of the sand / epoxy. Sure the sand / epoxy is about 38% heavier but my molds aren't usually very large.

I totally agree with Ted about adding some 1/4" chopped glass fibers to the sand / epoxy mixture. I would think that would yield a very nice bulking paste / dough (whatever you want to call it) that would QUICKLY produce seriously rigid and durable, yet inexpensive, molds.

Off to the lumber yard....

Ted, that was some outstanding work you did on the F-16 project! The gloss you achieved on the fuse plugs was incredible. I've got some Meguiar's 'final cut' cleaner (liquid polishing compound)...is that what you used before waxing?

-Tom

Mike James

Drum roll.....................Cymbal crash!

I think I see some sand in my future...
(And, regardless of strength, I like Fred McClung's idea of putting a layer or two of cloth on the outside, just for handling purposes around the shop... Otherwise, it's kind of like a big block of Perma-Grit.)

This thread is great... A way to maintain our "high-tech" approach, and save some money without sacrificing quality.

TGoodwin

Thanks Tom. I blocked and filled until I could not see any imperfections. Then I shot it with two coats of gloss black plastikote(spray can). Let it sit for a couple days. Then I wet sanded with 1000 and 1200. Polished with 3M fine cut and applied Meguier's glaze. The glaze is tricky to apply on non-catilyzed paint. If you are not carefull you can really mess up the plug with it. I no longer use it in making plugs.

Ted

TGoodwin

Mike I really want to stress that both the inner and outer layer of glass is required for fracture resistance and to help to resist warping. Leave them out at your own risk. I usually put a wet coat of resin on the outside.

I dont think the rods are required unless it is a very large mold.

TGoodwin

Here is another set.

TT2

My Feedback: (13)

Nice work Ted! Is that the System 2000 'epoxy surface coat' you are using? Does it retain a high gloss? Do you degas it before application? I haven't used it yet but will probably order some soon. I like the FiberGlast products...I use the System 2000 resin anytime I'm not needing high-temp or doing vacuum infusion...good stuff.

In the third pic of post #18 it looks like you are beginning to apply the 'wet' sand to the mold without wetting out the glass underneath. Does the epoxy / sand mixture wet the glass out enough? Does the glass tend to shift when applying the sand...or is the gelcoat underneath still tacky...???

-Tom

davidfee

Sand also works for propeller molds. You'd be amazed how much sand you can add... the sand keeps settling leaving a wet upper surface.

-David

TGoodwin

I hand mix it and I do not degas it. I have never had any reason to. Just make sure you use three cups to mix in. Mix it up, put it into a new cup and new mix stick, mix it up, etc.. I usually mix in black pigment like Mike says. As far as retaining the gloss of the plug yes it does and it polishes up nice with some fine cut compound. Here are a couple shots of one of my molds. One half has been cleaned and the other is just how it released from the plug. Here is also a shot of the resulting part as it looked when released from the mold. I no longer use PVA . I have been using a release from Fiberglast called fibrelease. It seems to work well but I have only used it on a couple of molds and layups so far.



It depends. I like to add the glass when the tooling resin is still tacky. If I am not able to do that then I usually sprinkle some sand onto the tacky surface to give the next layer somthing to bite into. If you wet the glass out first it always seems too wet and the sand mix creeps and shifts. I would wet out any corners but otherwise just mix the sand a little wetter to allow for this. I usually dont have to wet out the top layer either but add a flow coat to the outside to smooth it out.

Hey nice molds David. How hard would it be to do some small scale carbon gear of the F16 like you have done on the propellor? Think it would be strong enough?

Thanks Ted

davidfee

Hey Ted, thanks! The props from that mold were used by the USA F5B team at the electric world champs in York this summer, so I'm a little proud of it.

For the landing gear, I'd be tempted to mold carbon over a music wire gear. I guess I really don't have a good feel for the scale of the F16 gear. If they are kinda stubby, then all carbon should work fine. You might have a problem with them being too stiff though.

Nice work, guys!
-David

ptxman

My Feedback: (2)

The math is right & that is the attractive part of it. Ive always wondered why more guys havent developed sand matrix molds more. I think it usually comes down to follow the leader, preconceptions about how it might not work or weight. For bigger applications like wings & fuses, I can see weight becoming a real pain just moving molds here & there, storage & debonding & lifting the halves after layup.

Dough is an engineered material & has applications & capabilities that sand wont be able to achieve. From pics Ive seen they can extrude it out of pasta like machines at various thicknesses & press it on vertical compound surfaces like platicene. Ive also heard of additives like metal powder & ceramic for heat cured tooling. But like anything, use the method & materials that best suit the application & I think it has potential for RC use as David's nice molds illustrated.

The good thing about low resin concentration coating the sand grains is a stable mold from a shrinkage standpoint. The less resin in there & the more stable the filler material, the less that mold should ever distort over time or under elevated temp postcuring.

I saw a video at some point of a glider? guy using it, but the method struck me as a bit labour intensive. He wetted out epoxy, springled some sand etc. I would mix it up in a slurry with a grout propeller mixer & get the consistency right first, then apply it. If I wanted to control thickness for the first layer I would put some simple 'thumbtack-like' reference height markers down on teh gel coat layer to guide the trowelling process.


Well, it wouldnt hurt, but my guess is the chopped glass wouldnt do much either unless the fibers were long. Im not a chemist but my understanding is fiberglass comes from actual glass, which comes from silica which comes from ... sand! I know you can buy different grades/types of sand, some of it is finer grain stuff probably made up of all kinds of native rocks, other is more pure silica sand. Then there's that nice, white pebbly stuff in all the golf bunkers <g>.

I visualize a long skinny sand/epoxy mold as like a ceramic ornament. It seems very & strong but probably only in compression & unfortunately also brittle. "Amorphous" is that the word Im after? If it was hanging over the table it might be fragile enough to snap under its own weight. I can forsee this being an issue when the mold halves are wedged apart after part layup. I would integrate some threaded inserts along the mold flange so when it comes time to part the mold, you just give the bolts a slight turn in succession & jack the top up evenly.

Somebody told me that the carbon fiber rods we use in RC actually have an industrial application where they twist them in bundles like re-bar for high stress concrete pillars & such. I think something like this belongs in the sand/epoxy mold to tie the matrix together, maybe a criss cross of piano wire or something. (Im talking > 1m fuselages & wings, not smaller stuff). I feel a project coming on this winter!

Mike James

We're sold...

In the next week or two, I'll be making our wing molds for the "Eliminator 2", ( http://www.nextcraft.com/j47_sportjet01.html ) and will use the sand epoxy method. We're creating MDF frames around the wings now, which will be locked in and supported by welded (rounded rectangular) tubing. Then, we'll do the sand/epoxy method, and I'll post photos here. Seems like a great idea, and after all this recent input, I'm convinced that it will be fine.

The video we've been referring to by Fred McClung (the glider guy) is availalbe through "Soaring Stuff" at http://www.soaringstuff.com/videos.html and is called "Building Hollow Composite Aircraft Structures". Fred is a retired aircraft engineer from Hughes Aircraft, and makes some very nice glider wings in the video, from the plugs, all the way through to the actual parts.