Hulk-mania

two things too consider when picking master materials;

1. sulphur: will stop rtv from v'ing. A lot of modelling clays have sulphur in them, so need to be avoided.

2. Absorbant materials: will, not surprisingly, absorb rtv, so need to be sealed, which means coating with varnish/lacquer/paint/pva or some such.

I think that's probably about it.

As for minimun thickness, that's just as thin as you can go. Shouldn't be a problem, you can cast wafer thin thin layers of resin in rtv molds without much hassle.

I have recently learned that one of the world's leading exponents in molding and casting is currently in Wellington, so I'm going to to try and plug him for tips, maybe even get him on the board.

Naturally I agree with everthing Phil says . I've created and molded resin details that weren't thicker than say 2mm.

Also I'd definitely stick with one sided molded. Two sided molding is a hassle and almost impossible to get working nicely without serious effort. Terrain is typically is excellent for one sided molding as well.

If you buy your products from TopMark they also sell a modelling clay called 'Klean Klay' that is SPECIFICALLY made for casting work. So it's 100% sulphur free. It's also cheap ($10 for a 500gm block) and completely re-useable - I've used the same ball for 3 years now .

Also feel free to bug me for any tips if you like...I'd add:

- For detail work you DEFINITELY want to brush a thin coat of RTV on the master before pouring the rest.

- Make your 'enclosures' out of foam board and seal the with Klean Klay beading and masking tape.

Yes I've have numerous people recommend lego . Alas I don't own any unless you count 'Mega Blocks' for 2 year olds.

Foam card is pretty painless because you can cut one side and fold the corners without breaking the card. That way you get an enclosure cut to size with a single join you have to seal. Plus you have to seal it to the base (I use CD jewel case covers mostly) but surely you have to do that with Lego too?

The other advantage with foam card that you probably don't have with Lego is you can make irregular polygonal bounding boxes rather than just square or rectangular ones. It doesn't sound like much but when it costs $40 for 550gms of RTV I want to 'cut as many corners' (literally) as possible!

What RTV product are you using Dustan? Your Dad's? Also what are you considering casting in? Resin or plaster?

Plaster is cheap and friendlier to your molds. Plus a good quality plaster will catch just as much detail as resin.

Resin is more expensive and hard on RTV molds without talc dusting and/or fancy mold release (although I've used Pam's cooking spray in the past).

You can probably tell from my blog that I'm biased towards Ultracal 30 (or even better Hydrostone).

You might want to consider keeping the size of your masters down if possible too...better to mold a series of modular pieces that you can mix and match rather than one giant slab...but I guess it depends what you're after...I like making smaller molds...again because of the co$t of RTV.

Another thing to keep in mind is NEVER THROW AWAY SET RTV. Once it's set you can 'dice' it up into little cubes and use it as filler for new molds. Using this method stretches your RTV even further! It's imperative you keep the set stuff clean though...I always seal mine in freezer baggies.

Plaster for these definately. You'll be looking at a massive resin bill otherwise.

The world's best plaster isn't necessarily a necessary necessity. Try the bulk cheap stuff from bunnings and places. I have had luck in the past mixing bog standard plasters with things like like gedeos resin plaster. Also, I've never had to replace a single mold I use with plaster, ulike the resin ones which may have a lifespan of a couple of weeks.

I like lego best for my mold boxes, but the biggest problem with it is actually finding the stuff. What you need are the old regular building blocks; most of the modern stuff is no good. I was lucky to find a big box for $5 in a second hand shop, and that should last me fo the rest of my life.

The downside is like you said the lack of flexibility. It's surprising just how few different sized rectangles you can make, particularly as you get smaller.

On the plus though, your boxes are quick to make, easy to recycle and very sturdy. This is the big one for me, as it lets me drop the molds from a great height to get the air out. And you get loads of really cool shaped bits of rubber from where it seeps into the bricks.

You do still have to seal the bottom of the box, I use plasticene, or glue if I need a stronger box still.

If you are trying to mold something irregular in a rectangular box, just use your old molds as filler as stu describes. You can get away without shredding the stuff too, depending on what you're doing. Say you want to mold something circular in a square box, just stick big triangular pieces in each corner. Once it's all setup, you can cut the corners off again to reuse so long as you leave enough ruber round the mold proper.

Like stu says never throw it away, but not just the old molds, any drips or spills can be peeled up and reused when set, as can the residue from your tools and containers. Don't try to clean the stuff off anyhting untill it has set then peel it away and reuse.

I'ts the most reusable stuff ever, I've never thrown any out. Just recently I had a catering size milo tin of shredded rubber, and used the whole lot up in fresh molds. So in effect that means every cent I had ever spent on molds was, at that point, actually still in use as molds. Pretty freaky. Reminds of the terry pratchett's wizards who always put the last bit of their roly back in their baccy pouches where it get's mixed back in. So there's some primordial shred of tobacco that's lasted forever.

For your floor squares it might be worth trying something else since you need quite so many. You'll just be pig sick by the time you finish, if you only made the one mold a conservative estimate would be about 100 hours to do them. I was wondering if you could find some sort of embossed plastic or card or tiles or something? Even big sheets that you could cut with a guillotine.

And I agree with Phil, dicing isn't really req'd. I just usually re-used diced RTV because most of the mold work I'm doing is pretty small stuff and I like to keep the mold walls thin (saving RTV again!). IMHO you don't need a mold wall to be thicker than 5mm or so with Ultrasil RTV. It has a low 'shore' rating which means it's pretty flexible once set...the trade off meaning it can tear reasonably easily so be careful.

Phil, interesting comment about dropping molds to release air bubbles! Brushing the RTV over the master almost 100% removes air bubbles in my experience...which is why I started doing it. I use a hog's bristle brush. Have you tried brushing? Do you use any kind of mold release on the masters? I've tried this in the past as well (Pam's cooking spray, or vaseline) but found it's not req'd at all.

In fact the mold release can actually add some undesired texture if you're not careful. I don't even bother sealing my masters these days. I just create them from non-porous products like plasticard and various epoxy putties. I would probably seal a master containing balsa wood though since that stuff is really porous.

Also what RTV products are you using? Topmark's stuff as well? They actually have a whole slew of products for molding and casting but I've only ever used the three I mention above and have grown to know their properties. It would be nice to find a slightly stiffer RTV though...but I suspect you just pay more for a higher quality of rubber.

Mastering molding massively expands your modelling possibilities Dustan so go for it man! Nothing like mastering your own pieces and then creating wicked terrain from them .

You own a compressor! Lovely...I assume you've got airbrushes too then. Lucky.

1:50 white two part rubber! Aieee...the horror. I think I'll stick to Ultrasil! 1:50 would require some incredibly large molds - or some incredibly fine measuring...neither of which I am currently capable of!

Oops my prices were a bit off too...550gms of Ultrasil is actually $48.20 + GST. My brain obviously rounds down.

Just email TopMark for their latest prices Dustan. They have some other interesting products too.

Wacker M4470 Red/Brown Tin based silicone looks interesting. It's over twice the Shore A (60 vs 25) rating of Ultrasil for roughly the same price. ($101 for 1.04kg of Wacker vs $90 for 1.1kg of Ultrasil). Might be worth investigating for larger 'product' molds molds. TopMark only sell the 1.04kg kit which is why I haven't used it yet.

They have an interesting range of quite similar resins too...

I use Easycast (2 min working time Shore D 73) but have considered Procast (5 min working time Shore D 65) in the past. FYI Shore D 65 is roughly the same hardness as skateboard polyurethane wheels.

Man I can't wait to see what your mad modelling brain comes up with...I'm so sad I model vicariously .

Interesting. Easycast is good for fine details...the only problem I have is the shorter working time before it starts to set.

However I've noticed the resin seems sensitive to temperature and you get a longer working time in the winter...in fact I from memory one of the staff at TopMark recommended keeping the two parts in the fridge to extend the working time.

The problem there is that the resin is also very sensitive to moisture which ages it. So you want to make sure you always keep the bottles WELL sealed.