my experience with molds is limited, but why not just get the real stuff and use real plaster or dental plaster?

Just wondering

I've had a lot of success (for a complete novice) with casting my scratchbuilt homeworld ships in resin. I stocked up on silicone and PE resin from here:

http://www.tomps.com/

I found a good couple of tutorials online, and then went ahead and had a go. Its a real learning experience, but very worthwhile when you pull a copy of something you've just made. The most important things to remember are to think about your mold design, and not to forget the mold release!

I can post some pictures if anyone is interested?

I only found out recently that the Forum had stopped sending mail to the Yahoo list, I thought the Forums had died! Didn't see your post, because I was using the list to keep up with anything interesting on the forum.

What do you mean by 'real stuff' - the mold making stuff that costs a hundred dollars a gallon?

Real plaster is pretty weak. Dental plaster isn't a common item, and costs as much or more than fiberglass resin. Fiberglass resin is in about every Walmart I've ever walked into, most Auto parts stores, and quite a few other places.

I just did some fiberglass castings from my crappy old 'kitty litter' mold, just to evaluate Fiberglass resins with fillers. It's likely not worth it. Makes the resin too thick to pour well, and it's thick as it is (well, my 20+ year old quart of resin was, anyway). I did come away with some idea of how tough the stuff was, and I'd be hard-pressed to say that any kind of plaster would be worth it for minis. YMMV, however....

Sure, post a link....

I checked their resin price, and it's 9.26 Pounds for a 1 kilogram tin that appears to be about a quart. For us here in the USA, the shipping would be more expensive than just taking a short drive (for most of us) to Walmart to grab a quart of 12$ auto-repair resin. That's what I used on my test figures recently, though it was 20+ years old - I'd had it that long!

..worked fine. Didn't use a mold release, and the mold was the 100% silicone caulk I mentioned earlier.

One thing I've noted on all the casting and molding lists I'm on is that everyone has their preferences, and the vendors always think their stuff is the best. Got nothing against making a buck, but if my el-cheapo stuff works for me, then I'm sticking with that... Unless someone has a cheaper or better way.

If I were to gear up to make 'em for a living or a little extra cash, having an easier to use product might be worth it.

Well, I gave the process a test.  I used it on some model kit sprue I had because they had some really fine pressure pin lines and I wanted to see if it would pick them up.  I tried the method above (straight silicone for the first layer) but I didn't like the process.  It was a major pain to work with.

So I tried the tinned method.  Much better.  But too slow to cure.  The post on that Myheap site said that using glycerin or paint with the thinner would make it lumpy and that it was counter productive.  But I tried it anyway.  I thinned down some silicone caulk with some camp fuel (naphtha white gas) and added glycerin and paint.  It didn't turn lumpy at all.  I painted it on (and it went on easily when thinned) and set the sprue chunk aside.  It cured fast and normally.  It didn't lump up or shrink or anything.  Naphtha as a thinner and glycerin and paint as a catalyst work together just fine.

My friend has some plaster blocks in which he carved a stone pattern that we wants to test it on, so I'm going to do that Saturday.  Plaster is pretty porous, so I'm going to have to do some reading on how to properly seal it.

Oh, and the silicone replicated the detail just fine.  It even picked up some pin indents that I couldn't see with my naked eye.  I saw them when I looked through a magnifying lense and thought something was wrong with the mould and then grabbed the sprue and saw them on the sprue itself.

I've always not bothered doing much more than the occasional conversion because I thought replicating what I made wouldn't be economical.  But now... now there's scratch building to be done.

3 parts silicone, 1 part naphtha based thinner.  It was enough to make it paintable with easy but no more.  One guy quoted on that myheap site says you could go 50/50 and it should still work.  My thinking is to not thin it any more than necessary though.

Did you also add a catalyst?  I also just painted the mixture onto some glass to see how thin I could get it and the 3:1 silicone/naphtha with paint as a catalyst didn't deform, even at the edges where it was thin enough to be translucent.  I forgot about the mould release though and couldn't get it all off the glass (which is made from sand/silica, so silicone sticks to it).  I'm going to get some cheap corrugated plastic "for sale" signs at the dollar store for future mould box use.

We ended up postponing the trial mould making after it came to light that some of his pieces will require a two part mould.  So we're going to need some non-sulfur clay for that process.  It's going to be a challenge, combining brush on mould making with the techniques for making a pourable mould.  I think we'll give the PVA a go.  Vegetable oil spray seems to be working fine as a mould release agent off of the glass and sprue I've tested the silicone on so far.

Amazing!!

I'm shocked it worked so well on a piece of lava rock.  I would have thought the silicone would have torn and the mould rendered useless.  That's amazing.

My thinking is that even if I get 5-10 casts out of a mould, for the cost and ease of getting the materials, that's worth it.

I've done a bit of casting (hirst arts moulds) with dental stone and it's quite strong.  I had this crazy micro resin bubble reinforced stuff with like a 20,000 psi break strength.  Bought it at a local dental supply place.  Check your phone book and give them a call.  Tell them you want to get some high psi (10,000+) dental stone.  Expect to pay about a dollar per pound.

I could spike the 20,000 psi stuff I had on concrete and I'd only get corners chipping off the blocks on a single throw.  Took a couple throws to make it shatter.  Thin pieces though, broke far more easily, that was an inch x half inch x half inch block.

It's not light though.

Clint Sales sells Merlin's Magic for hobby use.

http://www.clintsales.com/dental.htm

A tutorial sort of thing:

http://www.hirstarts.com/casting/dental.html

it's a plaster-type powder about $1 per pound all-in all including shipping

for me, I have about 40 pounds left, so I'm going to use it whether it's cost-effective or not.

I'm slightly surprised the mold worked too.  But work, it did.

Did you affix the wooden hexigon to another surface and make a skin that goes wide around it?  Or does the mould end right at the hexagon?

Here's an example of what I'm talking about, step 9 and 10:
http://www.myheap.com/book/chapter-07/example-01/example-01.php

Likely, I'll work up a few models first. That's going to take a while. I've worked with cements and plasters before....

Did you do the Hirst Arts thing? Most of the links that folks been putting up are familiar, I've been there before.

THAT is the information that I needed. A "foot wide cube" is like 7.48 gallons IIRC....that's a BUTTLOAD of resin, and answers my cost question. Unless the diestones are totally unsuitable for the model in question, I'm thinking that they are much cheaper for the given volume, given what you and Jim have said.

I've also read up on casting pot metals and aluminum, but that's likely going to be overkill.

As long as it's not more than 4X the water volume, you're still ahead.

Pretty much - unless the thing is really delicate, diestone comes out ahead in my thinking.

Everyone keeps telling me that.

I dump it in a trash can...might be able to recycle some chunks. Yeah, I'm THAT cheap...

I'd just cast the whole thing in plaster. Time is money...or, you could slush cast the exterior in a diestone, and fill with plaster. Can you slush cast with a diestone?

Yeah, the fumes and ease on the molds is a biggie....

another reason not to dispose of this stuff down the pipes is that it's illegal.  Not that big brother is gonna come knockin at your door for it...but if they do...ouch.