moldmaking with Hydrocal/gypsum cement?

[chlee]

Has anybody tried making fuselage molds with Hydrocal or other gypsum cements? I know that Bill Hayman's DVD describes such mold-making -- before I invest in that DVD, I'd like to ask about the pros and cons of that technique. I've made glass/resin molds before; they're quite durable and accurate but a bit messy to construct -- are there significant advantages in using gypsum cement?

Chung

[davidfee]

My impression after having used both types of molds (epoxy and plaster) is that I prefer epoxy. With the plaster molds (even high quality stuff) you must seal the surface well and the edges will be prone to damage and chipping.

-David

[SoarNeck]

Personally, I don't see the benefit. Most of the work goes into making the plug and a tightly-fitting splitter plate. The actual layup of an epoxy glass mold isn't be more than 3 hours in 0.25-0.5 sittings, and the ease of using the finished mold is much greater (plus you can paint the parts etc).

I bought the video, but I don't plan to use the technique ever.

[chlee]

Thanks for your advice, gents. I also read some posts on this topic in the HLG forum. I've concluded that I should just stick with epoxy molds, and this won't be too much more expensive if I get a gallon of cheap Aeropoxy and use sand or sawdust to build up the mold thickness.

cheers,
Chung

[bgriggs]

I have made molds for vacuum forming with Hydrocal. It works quite nicely on small parts. Larger parts require some thought. A good alternative to Hydrocal is "Durham's Rock hard water putty". This stuff is available at local hardware stores in small cans. Cheap to experiment with.


Bill

[soholingo]

Personally, I don't see the benefit. Most of the work goes into making the plug and a tightly-fitting splitter plate. The actual layup of an epoxy glass mold isn't be more than 3 hours in 0.25-0.5 sittings, and the ease of using the finished mold is much greater (plus you can paint the parts etc).

I bought the video, but I don't plan to use the technique ever.

My thoughts exactly... If I had read they were using hydrocal I wouldn't have bought the dvd. However there is supposed to be a video coming out soon on how to do the epoxy molding....

[Sled Driver]

When I make an epoxy fiberglass mold, I frequently make the plug out of foam, then glass over the foam, use fillers, then filling primers and paint – all with a lot of sanding and rubbing. And I do all this time consuming fine finishing work because I know that any imperfection will be in every part produced from the mold.

What Terry Luckenbach’s method allows is one to go from a sanded foam shape directly to a negative mold without all the time consuming work on the plug – because the Hydrocal negative can be easily smoothed after pulling the plug and before sealing with epoxy. The time required to produce the mold from the plug is also significantly less.

So, is it the way to produce a hundred parts you are going to sell? No. Is it a good way to generate a few, say less than a dozen, parts for that unique project, or to prototype for a new mold design? I think that it is worth further consideration because of the time savings – even if you have to do a little more finishing work on each part.

I intend to make a set of Hydrocal fuse and nacelle molds for an electric design called a Sportwin and possibly a lead-sled sloper. I might make a three or four sets of parts for each, so it definitely isn’t worth building nice epoxy fiberglass plugs and molds those quantities of parts.

Hasta,

Jay Decker
Kennewick, WA

[mrittinger]

Hmmm, I designed that ship, and it would be much more effort to make a molded fuselage than just build the wood one , my opinion.

Mark

[Kenneth Paine]

I got my copy of Bill Hayman's DVD today. Excellent delivery service for I live in the UK and ordered it over the weekend.

Whilst personally I prefer traditional epoxy moulds, it struck me that the hydrocal way is a good way of making a "pre-plug" for insurance purposes. I have read many horror stories of plugs sticking to the new mould during the mould manufacturing process causing the whole project to be trashed and re-started.

With the hysdrocal method you can choose make a end user part as in the DVD or use the moulded component as a base from which to achieve really high standard finish before using it to build a traditional epoxy glass mould. If during the layup of the final epoxy mould it gets stuck to the plug with dire consequences, no problem! Just lay a new piece in your hydrocal moulds. Better than carving and finishing pine jelutong or blue foam all over again.

Kenneth

[Joedy]

I don't know what all of the fuss is about when making epoxy molds. Instead of spending hours and hours sanding a perfect plug, why not spend your time wisely?

It is easier to polish the mold rather than the plug. I usually take my poplar plugs to 600 grit and then put about 7 layers of Meguires Professional (doesn't stink as much as the Meguires Mold Release wax). The minor wood grain lines (even after all of that wax) will show up on the mold surface, but these are very quickly sanded down with wet/dry 2000 grit. I end up getting quicker molds and without all of the work associated with a perfect fuse.

On a HLG fuse, it takes me about 20 minutes of wet sanding the mold halves and an application of DuPont No.7 to bring out a mirror-perfect finish on the mold halves. In fact, my rule of thumb for mold surface finishes is that you must be able to see your reflection in it. (Keep in mind that I add graphite powder into the gel coat so this makes it easy to judge this.)

I have found that I spend less making a finished mold by focusing on the mold itself rather than focusing on perfecting the plug. It takes me less time to polish the mold than it does sanding, filling, priming, etc a plug.

But as they say, Your Milage May Vary.

-joedy

[DLD]

I'm sorry Joedy, I have to disagree. Having made dozens of molds, I believe it is always better to spend your time on the plug. When I finish my molds, hardly anything other than waxing has to be done. If your plug has the mirror finish, your mold will also, with no polishing. If you have wing fillets it can be very difficult to polish them in the mold, but no problem on the plug.

As far as making the molds, if you want/need cheap and quick, try polyester resin. Personally I hate the stuff, but I have made a mold and produced a part from that same mold on the same day. It's cure time can be accelerated by adding more MEKP, so you can build up the mold very quickly. It's not as durable as epoxy, but with some care you could get many parts out of it. Also, epoxy and polyester don't tend to stick to each other very well, so the epoxy parts come out of the mold easily.

Good Luck,

David Layne

[charlieuk]

would it be posible to use epoxey for the first few layers then build it up with poleyester. im planing on doing a verey large mold to only pull a small number from and it would cost a huge amount if all done in epox. i have done a number of molds in poleyester some worked some not so good.the problem was the first layer of resin was cracking of after a while so i was planing on puting micro fibers in to ti it all togeter, or is there a proper way??

[DLD]

Charlieuk,

I wouldn't recommend that, since the two really don't stick together very well. Go with one or the other. If money is an issue, use polyester for all of it. It will do fine as long as you aren't trying to produce them in quantity.

David Layne

[TGoodwin]

So, is it the way to produce a hundred parts you are going to sell? No. Is it a good way to generate a few, say less than a dozen, parts for that unique project, or to prototype for a new mold design? I think that it is worth further consideration because of the time savings – even if you have to do a little more finishing work on each part.

I intend to make a set of Hydrocal fuse and nacelle molds for an electric design called a Sportwin and possibly a lead-sled sloper. I might make a three or four sets of parts for each, so it definitely isn’t worth building nice epoxy fiberglass plugs and molds those quantities of parts.


I agree. Hydrocal molds are OK for taking a quick splash but not for any kind of production.

would it be posible to use epoxey for the first few layers then build it up with poleyester. im planing on doing a verey large mold to only pull a small number from and it would cost a huge amount if all done in epox. i have done a number of molds in poleyester some worked some not so good.the problem was the first layer of resin was cracking of after a while so i was planing on puting micro fibers in to ti it all togeter, or is there a proper way??

Adtek sells a tooling resin that is designed for use with plaster based products. I think they call it hydrophobic tooling resin. You can use that as your surface coat and then let it tack then mix some more but this time mix in some dry hydrocal plaster. Sort of a peanut butter coat and apply that then apply the hydrocal plaster. The molds will be heavy and they will last a bit longer but by the time you do all of that you could have made a nice set of epoxy molds. Save the plaster for taking quick one off splashes and make some quality resin molds.

Just my opinion.

Ted

[charlieuk]

thanks il probably use poleyester for the mold the first time as its so much cheaper as the fuz i want to do is 4.3meters long and im only planing on taking maybe 10 at the most.by then il probably be able to aford anuff for a epoxey mold.il just have to try and ind what is the best resin and aditives to use for the mold gel coat.i think if i used the plaster they would end up to hevey to lift,il make a stell frame to suport them i think.

thanks

[briandlg]

Charlie, I'm curious what you're making. You can't go saying that you're going to make a 4.3 meter fuse and then not tell us what it is! :)

[charlieuk]

a very lage scale glider. ow ok its a 1/2 scale duo discus.
im still in the proces of doing the drawings for the plug so the funs oly just starting but i like to plan ahead.

[briandlg]

Wow. Big project. Please keep us all updated on your progress!

[charlieuk]

im shore i will have a fair few questions to post during the build however im only doing the fuz my frend is going to bag the wing.

[Tony D.]

What section are you going to use on your Discus charlie?

Will the wings be wood sheeted or glass?



Tony D.

[charlieuk]

i think i will use the same section as on the rosenthal 1/3 scale duo which is from root out E68/E67/E66.

it will probably be a foam core with carbon composite spar with scale joiner.it will the have more cabon and glass under a layer of vanear(dont know how to spell it) which will all be baged down. thats the plan so far,it will probably change a bit on the way.

[Tony D.]

Are you going to use plywood veneer?

If so do you plan to make the wings 4- piece?

I have been looking for a source to buy .9mm birch ply in 1.5 or 2 metre lenghts and if you have a source I would appreciate a link to same.

Thanks.


Tony D.

[charlieuk]

i will probably be using obeche veneer.i do want to try and find some thin ply to have a play with.il let u know if i have any luck.the wing will be 4 parts as its span will be 10m

[OzDragonFlyer]

Simple question...

Is hydrocal and other gypsum related products machineable without chipping?

[greyhound flyer]

Not really. They can be sanded with drywall abrasive (open screen cloth) or ground with a dremel/rotary, but sawing or routing will chip them.

Simple question...

Is hydrocal and other gypsum related products machineable without chipping?

[OzDragonFlyer]

thanks gf.. there's also stuff I found called Ultracal which apparent;y is a bit tougher than hydrocal. Just curious if a plug could be machined and handfinished from this stuff rather than using tooling board, it sure would be a cheap alternative.

[greyhound flyer]

Hi Oz,

The best of gypsum cements is called hydroSTONE. It's a US Gypsum product, like hydrocal et al. It has a 5000 psi compressive strenght, IIRC.

Gypsum makes wonderful plugs. It cures quickly and can be sanded with a belt or disc sander and then polished to a good shine. You can bond wood, etc., for subassemblies like wing fillets or scoops. It also takes epoxy well after its surface is plasticized with mold soap. When making a larger plug, you should use a piece of rebar in the middle for support.

And most importantly--DON'T DROP IT. :D

thanks gf.. there's also stuff I found called Ultracal which apparent;y is a bit tougher than hydrocal. Just curious if a plug could be machined and handfinished from this stuff rather than using tooling board, it sure would be a cheap alternative.

[OzDragonFlyer]

great, thanks for the info.. hydrostone might even make a good backing material for molds.

Edit:

Having just used 2 litres of epoxy resin for a sand mix for backing a gelcoated part with a large surface area, I think hydrocal could possibly be used in conjuction.

Lay down a thin sand/epoxy mix backing over the partially cured gelcoat, allow to cure and then pour hydrocal mix on top. The sand should provide a good mechanical bond for the hydrocal.

Anyone tried this? Would save $$ on epoxy and the molds would be ready to use without any further finishing/sealing.

[OzDragonFlyer]

*** Don't do it this way! ***

After laying up a large wing mold with epoxy/sand, I decided to add a 3/4" layer of hydrocal to strengthen it.

It warped the mold badly in ALL directions making it totally unusable.

The hydrocal bonded well to the sand, however, in setting it expands slightly. 2 different substrates, 2 different expansion coeffecients.

Expensive experiment. Pity as the mold turned out flawless with sharp edges and not a bubble in sight :(

Back to the drawing (or building) board it seems.

[GaryO]

Anyone have experience with a urethane casting product, thickened with aluminum powder?

[KevinSharbonda]

Smooth-on...use #1508
http://www.smooth-on.com/liqplas.htm

http://www.smooth-on.com/PDF/1508.pdf

Use it a lot and love it! .0007 shrinkage,
Hard, shines well and is still sandable etc. Quick potting.

[Gary Warner]

Thanks for the smooth-on link. Turns out that one of their distributors is here in Dallas. I need to go pay them a visit.

Gary
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