TT2

My Feedback: (13)

The microballoon / glass mixture has cured to a point where I thought I'd start evaluating it. The resulting density figured (very loosely) to be around 21 lb / cu ft...much less than the dough and less than one-fourth that of the sand matrix. The sample was MUCH harder than expected...full thumbnail force would just barely dent it. The stiffness was quite acceptable and would be further enhanced by sandwiching this mixture between layers of glass cloth as the majority have recommended. The resistance to breakage was far better than expected...the test piece was 2 5/8" diameter and averaged about 0.20" thick. The piece could be snapped in half by hand but only when considerable force was applied...nearly all I could give it and I’m no girly-man. Understand to that the sample had probably not reached a 'full' cure yet.

The strength of this 'dough' is probably overkill as is but could certainly be increased by additional amounts of chopped glass.

The surface that was against the coffee-can lid ‘mold’ was glossy with very few voids, and certainly no voids larger than one or two thousandths of an inch. Cross-sections of the sample showed hardly any void content. I didn’t go to extremes when I was packing the mixture onto the mold and would probably take more time on a production piece.

I have rethought the Q-Cells and believe that glass (not phenolic) microballoons should be used since they are the hardest of the hollow spheres.

I would think that the dimensional stability would be the same as the sand matrix molds even over time. The advantage here is that much lighter framework, parting boards, etc. could be used, and the mold wouldn't have any tendency to sag under it's own weight over time…which may be a serious consideration with larger sand molds. I'm sure a 'drop test' of the microballoon / chopped glass mold would far outperform the sand mold...

Bottom line is that I’m lovin’ the microballoon / chopped glass / epoxy and will be using it on my upcoming canopy, cowling, and wheel pant molds for the 30% 50cc Edge 540 that I’m developing. I’m fully convinced that it will produce extremely strong, yet lightweight, molds…far superior to using Coremat as a thickening layer. When you really think about it, substituting the microballoons for the sand makes perfect sense...they're made from silica just like grains of sand, but they're smaller in diameter (which seems to be desirable) and HOLLOW! Though I haven't experimented with the sand I would guess that placement of the microballoon / chopped glass onto the mold would be a bit easier...not quite as nice as the apparently supreme handling qualities of the RT2C dough though...

My question to all of you is this: Has anyone heard of using microballoons during moldmaking before? I’ve been playing around with composites for over a decade and haven’t heard anything good or bad about the subject. What seems the most curious is that chopped glass and microballoons are as common in any composite shop as epoxy! I’m just wondering if someone has tried this method and it proved to result in poor quality molds over time. Just being paranoid I guess…everyone thinks I’m paranoid…

UPDATE: I mixed up another batch in an attempt to arrive at somewhat accurate mixture ratios for those of you who want to try it. By volume: 1 part resin, 1/3 parts chopped glass, 2.5 parts microballoons. This results in an easy handling mixture. I added a layer of 6 oz glass to the mold (coffee can lid) face and spread out the mixture, then backed it up with another layer of glass. I found that after the dough is placed in a fairly consistent thickness on the mold, applying the top layer of glass makes it very easy to smooth the underlying dough out without any of it sticking to the gloves. Also, the top layer of glass becomes wet-out without any additional resin (though an additional application of resin may be desirable to smooth out the outside of the mold).

-Tom

Mike James

Microballoons and epoxy are plenty stable enough.

Personally, I'm going with the sand. I don't care about the weight issue, and it's a LOT cheaper than microballoons or the other materials you mentioned. The savings in time, glass cloth and epoxy are the main advantages of the sand/epoxy method for me.

TT2

My Feedback: (13)

Mike, I was a bit over zealous after my first microballoon experiment. After I figured out what the mix ratios actually were I found the microballoon method to be more expensive than originally thought. The sand method is still very attractive...it's sooooo cheap, and the hardness of the mold can't be beat...for 'soft' tooling that is. I still need to get some sand!

-Tom

heli001

My Feedback: (5)

Blasting beads may be the answer....a bit lighter than sand, uniform round shape and very cheap. I will check on the pricing as compared to "play ground or blasting" sand today. If I remember correctly, it is roughly 1 dollar more than a standard 90 pound bag of sand.

TT2

My Feedback: (13)

Excellent idea! I'm guessing, thought, that the mix ratio would be similar to the microballoons.

Assuming that the mix ratio of sand is indeed 10:1 as we have presumed then something here doesn't make sense. Since the smaller particles (microballoons or blasting beads) can be more tightly compacted (leaving less annular space between the 'balls' for the epoxy) then shouldn't the amount of epoxy be LESS than if using the larger grains of sand? Hmmm...maybe the ratio of the sand isn't anywhere near 10:1...maybe it's closer to 4:1...which would ultimately increase the cost of the resulting mixture.

Could it be that the larger grains of sand are carrying with them lots of air when the stuff is mixed together?

This riddle is killing me! I'm going to go rob some sand from the neighbor kid's sand box RIGHT NOW! I suppose I can 'dry' it in the oven.

HenrikJohansson

I have made almost all my molds with sand, and it works very well. Naturally they get very heavy though.
Don't put all your trust in the sand, lay down a couple of layers of glass as well.

Don't be so obsessive about the mix ratio, just add sand until it is easy to work with and leave it there....

The smaller grain size you have, the more epoxy it will take due to the larger overall surface that needs to be covered with epoxy.
When you use !QUOT!normal!QUOT! sand all the voids don't get filled with epoxy they get filled with air. if you use finer sand, the air voids will be less.

TT2

My Feedback: (13)

I realize that I'm beating this subject to death, and it may sound like paralysis by analysis, but there is a reason behind my fixation on which matrix to use. I’ve got some very large molds coming up in the not-so-distant future...around 30 sq. ft…and I am trying to balance all the factors before I make a final decision on what mold thickener to use. I had considered a grid of divinycel panels separated by about ¼� glass / resin (gridlines) but I’m worried about delamination when handling the molds and deformation over time…so my quest for enlightenment continues…

Bill Jennings

My Feedback: (85)

What a great thread. I would like to thank all you blokes for the information that you have made available. It has been a great learning curve for me. Thank you all.
Regards.
Bill.

macro-RCU

My Feedback: (1)

What about Polyester resin and sand... may be sand can stop shrink effect...

Magne

Hello.
The use of fillers (such as sand) in production of wing molds has been used extensively by the glider people for years.
For pictures of typical wing molds see this thread: http://www.rcgroups.com/forums/showt...ht=raketenwurm
These molds are back-filled to greater thicknesses, to form solid mold "blocks". (Similar to what you would have if the female mold was machined from a solid block of material.)
It is however very interesting (and new to me), how this well established method is being adapted here for use in the manufacture of shell molds, e.g. for fuselages, canopies etc. where filling up the entire thickness to make a block would require too much material.
I like the idea of using microballoons and chopped glass.
Thank you for all your development and testing work.
Regards,
Magne

HenrikJohansson

I will also make some relative large molds soon. I will use sand, as I've made in the past, but this time I'll add some chopped glass to add strength to the sand matrix. Weight is a concern though. I've seen sand molds which takes to people to carry...

I've made a small mold once (spinner) with microbaloons as distance material. It got very light (no surprise). The downside is that it takes a LOT of microbaloons to mix even a small cup of paste...

Making molds with polyester is a bad idea, they don't last long and they don't hold the shape very well. I't is cheap though but thats it...
I have no experience of polyester molds of my own, I herd this from a frend who makes more molds than i do.

buttuh

Going from epoxy to polyester is a big step the wrong direction.

boater

My Feedback: (2)

Just a thought-- would bean bag chair filler ( polystyrene foam beads) have any application here, not alone, but maybe as a weight modifier?? Oooorrr maybe crushed (not too crushed)volcanic porous stone??

TT2

My Feedback: (13)

Rudy, I don't know about the EPS beads...I think there would be a problem with inconsistencies in the final mix. The volcanic rock might work great if you can find it in small enough rocks...something smaller than 'pea gravel'. Anything is worth experimenting with!!!

I've let another chopped glass and microballoon test piece fully cure. This one is about 7" in diameter, 1/4" thickness (max.), one layer of 6 oz glass both sides...applied with no additional resin. The piece turned out very light, incredibly stiff. I tried to break it by placing the center of it at the edge of my workbench and pushing on it with all of my 175 lbs...my feet were coming off the floor. I couldn't get it to break! The surface did not dent either. Wow!

I'm still going to try the sand method since it's undeniably economical but I'm pretty sure that the microballoon / chopped glass is the method I'll be using for most of my small to medium sized molds. An added benefit to the microballoon / chopped glass is that it can be rolled out with a rolling pin (like grandma's bisquits) to an even thickness then slapped onto the mold in one large-ish sheet. This makes it a bit easier to keep the thickness consistent and takes very little time. I have yet to experiment with the RT2C dough...it should arrive within a few days.

-Tom

winship

HIGH HEAT TOOLING DOUGH (Resin Services)

The listing below is a a Resin Services product and is intended for a high temp tool although it can be used at room temp. The epoxy already has the dough addditives to make it a dough consistency. All that is required is to mix part A and B.


HHTD-"A"

RATIO: 100pbw to 25pbwHARDENER: HHTD-"B"GENERAL DESCRIPTION: HHTD A & B is a lightweight, color coded, high heat, two component epoxy system. When mixed together it forms a soft dough-like consistency that gels at room temperature. Then post cure.
RECOMMENDED USE: Apply two high temperature epoxy gel coats (HSC-905 & HT-9234), then 2-4 layers of fiberglass cloth (6 oz. TO 10 oz.) with high temperature laminating resin (HTR-350 & HT-9234). Then apply HHTD A & B to about 1/4" to 3/4" thick, making sure resin and hardener are mixed thoroughly. Then 2-4 layers of fiberglass cloth with high temperature laminating resin. This application will yield a very lightweight sandwich mold.

TYPICAL PHYSICAL PROPERTIES


Ratio by Weight 100pbw to 25pbw
Ratio by Volume 4.3 to 1
Pot Life 140 minutes
Shore "D" At 72ºF 60
Shore "D" At 350ºF 55
Density Mixed 4.27 W.P.G.
Shelf Life Resin/Hardener 1 Year



HHTD PART A & B

COVERAGE: One unit at 20 pounds, covers approximately 15 square feet at 1/2" thick.
Upon completion the tool should be allowed to cure at ambient temperature. Apply oven cure according to the following schedule:

1 hour at 100º
1 hour at 150º
1 hour at 200º
1 hour at 250º
1 hour at 300º
2 hours at 350º
1 hour at 250º
1 hour at 200º
1 hour at 150º

After oven has cooled to ambient temperature, tool can be removed.

DT-081 is a product from Freeman Supply and is a great filler to create your own tooling dough. It looks like sand but holds the resin better. This product can be mixed to a dough that can be formed into a ball. This product takes on the properties of the resin used and will work with high temp resins. One advantage of this product is the indefinite shelf life. Although these products are more costly than sand they are designed for the composite industry. I am not knocking the sand for filler but thought I would add another approach.
Dan Winship

http://www.freemansupply.com/RenDT081DT082Liqui.htm

Ren DT-081/DT-082 Liquid Tooling Resin Fillers

DT-081
Low density filler for use with RP-6426 or Epoxy Laminating Resins as a lightweight back-up material.

DT-082
High density filler for use with RP-6426-1, RP-6470 or Laminating Resins as a back-up material.

stumax

Hi guys, I don't come here that often but sand/epoxy moulds are one of my favourite inventions (I don't think I was the first to do it, but nobody told me about it). Mike, I have moulds that are 15 years old and as good as the day I made them. I have glass moulds that are as old and look it. For wing moulds, there is no better way, but you do need to use a solid, stable base board (1/2" alum plate is OK) for all the bolt holes etc. BTW, if you use a baseboard like that you don't need any glass layers. For fuse moulds I have actually gone back to glass. I find for producion moulds it is helpful to be able to give them a slight twist to eject the part. Also, the weight is a problem, as is the durability in a production environment (they don't like being dropped). However, unless you cnc machine your moulds from alum or steel I doubt that you'll find a more stable mould material. I use a mix of 7:1 by weight. It comes out a bit like brown sugar - sort of dry, but it will pack together nicely. If your plugs are up to it (like a solid wing plug) vac bagging it will give super results, otherwise you really do need to make a compactor block from a lump of steel about 2"sq by 4" long, and use the 2"sq face to compact the mix onto the plug. Obviously you don't use a 1/16 skinned balsa fuse for a plug. Ream holes in the parting board to accept steel dowel pins, so that the gelcoat forms the dowel holes as you ain't gonna be able to ream them later. Drill holes for joining the mould halves with a masonary drill bit, normal drill bits get rounded off real quick. A better method may be to mould in threaded inserts. I have tried mixing milled glass fibres with the mix and it hinders the addition of enough sand. The fibres prevent the mix from being mixed, if you know what I mean. I mix with a 2" wide paddle until it gets to hard, then I used my hands. You probably should wear gloves, BTW, as it's quite abrasive! Oh, the sand. Well, we have quite a bit of it lying around here in Sydney, but every beach has it's own type of sand. Ocean beaches tend to have coarser sand than the ones inside the harbour. I prefer the finer sand, and it goes without saying that it must be very dry - stick it in the oven if necessary!
I hope some of this helps you guys!

Mike James

Great info, Stu. Thanks! I have been wondering if these crack or distort with age, so it's great to hear that you have 15 year old molds in great shape. We'll be doing our next wing molds using this method, and I'll post photos. (a week or two)

TT2

My Feedback: (13)

Thanks Stu!

ptxman

My Feedback: (2)

Thanks for the info Stu.

So what do you do in the case of sand matrix WING molds to assist removing the part from the mold? I would think bagged hollow core wing structures would be even more delicate that fuselage type components. ie what would make removing a wing layup any easier than a fuse?

Do you recommend jackscrew type threaded inserts into the mold flange to initially part the 2 mold halves (after joining), or do you chanfer one of the edges, or?


I dont understand. Do mean your profile board (splitter board) is made of 1/2" aluminum? How does a commoner cut the silouette of the fuse profile out of that material? Why does it need to be so robust relative to plug made from only solid wood for example?


I see what you are saying, Ive seen that technique in sand casting, but is this really necessary over just moderatly compacting the sand matrix together by hand pressure or whatever? Did you have problems when this vac or hard compacting step was ommitted? Do you use any form of secondary reinforcement similar to the 'rebar' type methods I was wondering out loud about up-thread?

You didnt mention if you put the sand in 1 shot against the tooling gel or a 2-stage method, say 1/2" first layer, then fill to box line).

Do you post cure parts in your sand matrix molds?

stumax

No worries, guys!

I find that using Freekote mould release makes releasing quite easy for simple shapes like wings. I use jacking screws to part the mould, and the wing kinda just fall out. Fuselages can be a pain due to deep draws etc. The backing board for wing moulds is what each half of the wing mould is attached to, so there's a backing board for the top and for the bottom halves of the mould. This allows the sand/epoxy mix to occupy only the area of the wing plus about 6mm for a flange at the LE and 3mm flange at the TE. The TE flange allows me to get a plastic knife under the TE of the wing to help it out of the mould if necessary, without scratching the actual wing surface. I usually leave this extra bit on until the model is ready for the top coat of paint then sand it off - protects against hangar rash whilst building. The compacting is necessary if you go for the really dry mix (which is cheaper on epoxy and more stable). If you use a little more epoxy then it will settle by itself. No reinforcing is necessary for wing moulds as they have the alum baseboard to keep it all together. For fuse moulds I use a thin second coat of gelcoat with a layer of 2oz glass , then a layer of 6oz glass, then the epoxy/sand then another layer of 6oz glass with a smoothing layer of 2oz glass. BTW, I post cure at 85 degC in the moulds - I might add I use a tooling epoxy whihc is post cured at 120degC and good for 150 degC when cured.

Happy new year to all!

Stu.

ptxman

My Feedback: (2)

Is this schematically what you are talking about? ie what you call an aluminum 'board' is a strip integrated into the sand matrix of the top/bot female molds?

When it comes to vac bagging the glass/core sandwich into the molds, what method do you use to seal the bag? Im guessing that trying to manipulate these heavy clunky molds into the open ended tube style bagging films would be a royal pain. Do you use sealant tape around the perimeter of the mold flange so the bag itself is basically as sheet? Ive seen some pics of (cnc milled) grooves that look like they capture the bag film with a o-ring type rubber material - kind of analagous to the ACP type plastic 'C' clasp over a matching rod.

ptxman

My Feedback: (2)

Another mold detail question. When it comes to the trailing edge of the female mold, can you describe your technique. Do you attempt to have a defined TE thickness 'ridge' or do you let the TE run out to a theoretical airfoil x=100,y=0 type feather edge? Ive heard different schools of thought. Personally I think a tiny ridge would help define the wing outline after the part is pulled. But its not uncommon to see 0.020" thick (or less) TE's on racing/high performance wings, which means a mold depression of only 0.010" on each top/bot bold half. Thats pretty skinny. Ive heard others go to a 100,0 type feather edge & then let the outer glass cloth extend right over the mold flange & the top/bot cores get bonded together in this manner. The logic here is the cloth is only say 2 thou, which enlarges the airfoil by 4 thou, but that is either 'close enough' or contemplated in the original airfoil shape. Your thoughts..?

ptxman

My Feedback: (2)

Which Frekote part#? Are you able to pre-spraypaint into the mold successfully with this stuff? Any special technique?

Mike James

Peter,

The trailing edges on the tail parts I've made so far have a thickness of 1/16th inch, which I consider close enough. Also, it's the best I could do, and still see a clear imprinted trailing edge in the mold. To keep them crisp and straight, I'm using a method learned from the Fred McClung video, for the trailing edges.

When the first mold half is made, a balsa stick is epoxied to the parting board, right up against the trailing edge of the plug. When the second half is made, the stick is left in the first half, and another stick is epoxied right on top of it, also up against the trailing edge. Only after the molds are separated for the first time, are these sticks chiseled out of their positions, leaving a "trough" at the trailing edge of each mold half. It's handy to have the trough in both mold halves, in case the wing sticks to one half or the other, on separation. It serves several purposes...

At the end of the mold, it serves as a place for the vacuum fitting to go. Along it's length, it serves to collect excess resin from vacuum bagging. Trailing edges can be trimmed after the vacuum bagging step, before joining the molds, by running a sharp chisel along the trough. That step also removes the collected excess resin. Finally, when the molds are separated, it allows a sharp chisel to be run along the trough, both cleaning out the ramaining excess resin, and trimming the trailing edge at the same time. I hijacked your graphic, and added (black) indicators of where the troughs should go. It's helpful if the balsa stick has a slight draft angle, but not critical, as it comes out quite easily.

PS> Since my next wing molds will be made using the sand/epoxy method, and therefore won't flex, I'm using your suggestion of having several jackscrews around the perimeter, to aid in mold separation.

gachezem

My Feedback: (20)

Mike just a suggestion, I dont do molds, but have been following the subject , 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, also is pretty cheap, 25lb bag is $17.00. maybe somebody will give them a try..Gary