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Sep 09, 2010, 08:23 AM
Thermal Wrangler
DrFragnasty's Avatar
Thread OP

Fun with dental plaster


Spent an easy 2 hrs mixing and pouring 2 molds for the "rude shape" with Bruce and Stephen.

Punctuated with choc-coated Scotch Finger bikkies and cups of tea.

Easy process compared to resins; 3 to 1 ratio of plaster to water, mix well and pour into waxed/ taped molds.

Should know tomorrow (althoug it goes off very quickly....you could probably pop the molds after 2 hrs in mild temps).

Brand name of plaster is "stonecast" from memory although any old dental plaster will do ($20 Aus/US a 20Kg bag).

Inherent strength tested with sample pieces poured into a plastic cup; very hard to break; it should be fine for small parts, might need bracing for wings/ fuselages.

Chris.
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Sep 09, 2010, 06:41 PM
Twisted and Confused
flyonline's Avatar
Chris

where did you get this stuff? Is it readily available or do you have to order it from a specialised shop?

Steve
Sep 09, 2010, 08:56 PM
Thermal Wrangler
DrFragnasty's Avatar
Thread OP

hydrastone


Try Dalchem

Based in Melb./ Cheltenham.

Look under gypsum/plaster and Hydrastone TB is the stuff we used.

Chris.
Sep 09, 2010, 09:07 PM
Detail Freak
target's Avatar
Hi, Chris-

I'm very interested in the finish you will get from the part pulled from these molds.

Is there a reason other than cost as to why you didn't use fabric, resin and filler? Just curious, as I have a bunch of Hydrocal here in my shop.

R,
Target
Sep 09, 2010, 09:24 PM
Just fly it!
wyowindworks's Avatar

Plastic Face Plaster


Looking good, Chris! The dental plaster sound interesting. Does the mold surface need to be sealed?

There are some great materials out there for doing plastic faced plaster (PFP) molds. Adtech makes an epoxy surface coat that is hydrophobic, meaning that it cures in the presence of water. Essentially, the surface coat in brushed on, allowed to go tacky, then a second coat is applied and the plaster can then be immediately pour on top of it. This results in a mold surface that replicates the plug surface, is sealed, able to handle temps about 200*F, quickly constructed, stable, and more resilient than plaster alone.

I've kept my distance from plaster molds, but this systems has me reconsidering the process for smaller molds (large molds would be too heavy for my taste). I'm also curious to see how durable the dental plaster is.

Adam
Last edited by wyowindworks; Sep 09, 2010 at 09:44 PM.
Sep 09, 2010, 11:21 PM
Detail Freak
target's Avatar
Quote:
Originally Posted by wyowindworks
This results in a mold surface that replicates the plug surface, is sealed, able to handle temps about 200*F, quickly constructed, stable, and more resilient than plaster alone.
Adam
Now that sounds downright inviting!

R,
Target
Sep 10, 2010, 03:53 AM
mmmmm....BEER
Hi Target,

The decision to go with this product was primarily driven by cost and time. Its quick, simple and easy cleanup etc. At the moment we are just experimenting to see how it goes.

As for the quality of the mold surfaces........we will have to wait and see. Some of the test samples Bruce has done look very promising.

Even if it doesn't have the same quality of finish as a conventional epoxy/tooling gel mold, if its acceptable, the ease and cost of producing the molds makes it a very attractive alternative. Especially if you want to produce more than one set of molds

cheers

Steve



Quote:
Originally Posted by target
Hi, Chris-

I'm very interested in the finish you will get from the part pulled from these molds.

Is there a reason other than cost as to why you didn't use fabric, resin and filler? Just curious, as I have a bunch of Hydrocal here in my shop.

R,
Target
Sep 10, 2010, 04:14 AM
Thermal Wrangler
DrFragnasty's Avatar
Thread OP

plastered


Mold released OK. Plugs had minor water damage that will need a little sanding and re-finishing to remedy. The Everdure might not have been enough to seal it from water which is a tiny moelcule as opposed to the long-chain structure of epoxies which Everdure can resist.

The molds had no pinholes what-so-ever. The mold edges were almost as crisp as a tooling-gel.

Thanks for the heads-up on the surface resin Adam. We'll definately have a look at it.

Chris.
Sep 14, 2010, 07:02 AM
Thermal Wrangler
DrFragnasty's Avatar
Thread OP

Dressmaking, alchemy and surgery


FYI

Making molded gliders is a cross between dressmaking, alchemy and surgery.

Anyhoo, all the fabrics are cut. We need a bigger cutting board.

Carbon trimmed to LE planform, TE with a 15mm surplus overhang.

3/4oz cloth with 15 mm overhang all around.

Hairspray is vital for keeping fabric from distorting. Fabrics all rolled onto cardboard tubes and ready to nroll onto the mold.

Lay-up the fabircs and balsa skins tomorrow night.

Chris.
Sep 14, 2010, 04:23 PM
Life begins at transition
I've seen hairspray used a few times for holding the cloth together, and assumed the epoxy would dissolve it in a similar way to 77. I've also read though about it being able to be used as PVA, i.e. a barrier to epoxy.

Is using it to keep the cloth tame going to affect the epoxy/carbon bond?
Sep 14, 2010, 06:04 PM
Twisted and Confused
flyonline's Avatar
I've used it successfully a number of times without problems on glass ranging from 3/4oz to 5oz. Not tried carbon or kevlar though.

You only need the lightest of mists to hold it all together.
Sep 14, 2010, 07:18 PM
mmmmm....BEER
Hi Odysis,

We've used the hairspray method a lot (on glass and carbon) on a number of moulded models we've made and haven't had any problems with it yet. As Flyonline points out, you only need a small amount to do the job. I've also used it on kevlar with no problems.

Steve
Sep 15, 2010, 04:21 AM
Life begins at transition
Thanks, I've recently started 77ing 3/4oz to the heavier stuff to make it easier to handle, especially bias. Maybe I should look into the hairspray. After all, there's some in the bathroom
Sep 15, 2010, 06:45 AM
Detail Freak
target's Avatar
What brand of hairspray are you guys using?

R,
Target
Sep 15, 2010, 07:54 AM
mmmmm....BEER
Target,

I've just used the cheapest that I could find. The generic supermarket brand. One could steal the wife/partners good stuff from the bathroom............but thats fraught with danger!

Steve


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