Mould Making... an attempt at

[Connectamabob]

There's also the question of whether you're planning on pressure casing or not.

If you are going to pressure cast your parts, then you definitely want to vac the molds. It'll get the most bubbles possible out compared to other methods, and that's important for pressure casting as bubbles will collapse when the mold is pressurized, causing distortion and/or hills/bumps in the castings.

If you're not going to be pressure casting, then other methods mentioned like vibration or lay-up coat brushing can do you fine. For non-pressure casting, you're mostly just trying to make sure there aren't any bubbles on the inside surface of the mold, but bubbles in the "solid" areas of the rubber aren't really an issue.

Building a vac chamber is, as others have noted, pretty simple to do on the cheap. The chamber itself only gets expensive if it's intended to do double-duty as both a vac and pressure vessel, as a pressure chamber needs to be a lot stronger than a vac chamber.

However, since there isn't really a need for a vac chamber unless you're also pressure casting IMO, there isn't really much reason to ever build a cheap vac chamber like the ones described. If you're doing pressure casting, then it's more economical to make one double-duty chamber instead of two specialty ones. You need both anyway, so you can't escape the higher total cost by building a cheap vac chamber, and they both have to accommodate the same molds, so there's no reason for one to be more custom-shaped than the other. You only need separate chambers if you're running a pro shop where you might need to do both jobs for different projects/molds at the same time.

[Mr. Peabody]

I don't bother with silicone, preferring polyurethane instead.

It mixes to a near water-like consistency and pours almost clear, so you can see exactly where & if there are any bubbles that may be of concern.

I work part time in a museum and one of the chaps there put me on to this route. He was very much of the opinion that silicone was overkill for most jobs and barring the need for high heat or extreme detail reproduction, the poly will do a superb job at a fraction of the cost.

He was right. Poly will reproduce fine textures, like that of balsa-wood, with no trouble at all.

 

[Papa Spanky]

I would avoid using legos. They make for a untidy mold (it leaks out) and you have to clean the legos of tiny bits of silicone before you use them again. I now use cheap foamcore for my molds and dispose of it when im done.

[Connectamabob]

Lego dams can be brushed with liquid mask to prevent that kind of seepage. It seals the seams without penetrating, so afterward peels off as a sheet, nice and clean.

The main trade off with urethane mold rubber vs silicon is that you absolutely need to use a mold release with urethane. Urethane will permanently adhere to many materials, including of course other urethanes, so you have to be very fastidious with your your mold releases in both molding and casting. For this reason IMO silicons are still more desirable in many common applications, despite it being true that urethanes are both vastly cheaper, and can perform quite well if use correctly.

IMO a beginner is better off learning using silicone, as it will be much more forgiving. And a GK manufacturer is better off using silicon, as it will allow them to eliminate labor/fuss and potentially catastrophic slip-ups from their workflow. I've seen what happens when a small operator trying to maximize turnover tries to lower their costs by using urethane molds. Silicone wears out faster, but is relatively forgiving along the way. One small misstep can ruin a urethane mold on the second pull.

But for people who aren't in a hurry, and already have a good hand on what they're doing, it can be a great savings.

There are clear silicone mold rubbers, and not all urethane rubbers are clear, so that's not a point either way.

[Dewbakuk]

Just run some cellotape around the inside of the Lego box. Prevents any leakage and easily replaced, certainly cheaper than foam board

[oldskoolrebel]

Thank's for the advice all;

Obviously I'd started the project before reading the advice! I've learned a lot in my wee attempt. Please bear in mind that it was Hirst Arts blocks that I was moulding, not miniatures.

Masters: PVA glue is not strong enough when removing masters from the mould! In several instances the master broke during the process. If I was to do this in future I'd probably use superglue or the likes.

Undercut: So I've got a pretty large undercut on the masters, in the form of the floor section. This caused issues in removing them from the mould. In future I'd probably cast the walls and floor sections separately.



Fronging: I've ended up with quite a lot of additional frongs. Should I remove this? What is the best way of doing so? I think in future a bit of time spent with filler on the blocks would help to remove this problem.




Sorry about the poor quality photos


Cheers
Andy

[6milPhil]

That's fine for a first effort. Well done.  

You can superglue your master into the box, so long as it's of a material which won't be damaged should you apply acetone to get the superglue off. I use PVA glue on all my masters, but they're rock solid so rarely get damaged when removing them from the mould.

You have to watch out for undercuts, you can smooth them out with some filler or vent them. As you say for this one moulding the components is probably the best way to go.

Snip the frongs off with scissors, any left may come out with subsequent casts. These will have added to any damage the masters suffered. You're best filling all masters, sure it's timely but you save a lot of mould tidying.

Photo's are fine... looking forward to seeing the cast.

[bishop odo]

Great first attempt on mold making, and good luck.  There where a lot of good suggestion already, but here are a few more, with Tin Based RVT and especially Platinum cure, you have to watch the "Sulphur" it can cause cure inhibition and Sulphur is in most everything. White PVA glue, clay, unless it says Sulphur free, its not. Unless you want to invest in a vacuum chamber and duel stage vacuum pump, pressure pot, and compressor and a good scale, you have to pay attention, every product is different, to the RVT and Resin data sheets, that list Viscosity, shore hardness, and mix ratio of the mold making product you want to use.  Smooth on, a company that makes RVT and Resin has some great .PDF and Videos, educate yourself.

[snitcythedog]

Sorry, I missed this before.  Very nice first attempt.  You can cut those tendrils out with a pair of small scissors.  Just leave about a 1/16 of an inch so the crack is still visible on the cast part. 
A couple of suggestions.  Firstly even if you do not spend out for a vacuum setup you can still get bubble free casts pretty easily.  All you need to do is paint the silicone onto your masters using cheap disposable brushes.  When you do this really force the silicone into the details and get a good clean coat.  I use these. 

After you paint the silicone on all surfaces then you do your pour.  Bubbles that end up embedded in the silicone walls do not matter since they are not touching detail.  Secondly with your pour.  When you pour try to create a long thin stream.  The thinner the better since the bubbles that are formed during your mix  will travel down the stream and pop on the way down.  Also try to pour into one corner and let the silicone move into the rest of the mold. 
Concerning the preparation of your masters.  All major gaps should be filled.  Silicone will always find its way into the smallest of cracks.  If you take the time to fill the gaps you will not have that problem at all.  Another helpful suggestion is to seal the masters.  I use the Future method.  Here is a link to find the product in most countries.  http://www.swannysmodels.com/TheCompleteFuture.html  It is a bit of overkill but has worked well for my masters.  For the mold box that is up to you.  I have used both legos and foamcore in the past and they both work.  It is really up to you.  The only difference is how tidy the exterior of the mold looks and since that is not the part of the mold we are worried about it really does not matter. 
Hope that helps and just to let you know again great first effort on the mold.  It could have been much worse.  My first effort had huge bubbles sat right on the detail. 
Snitchy sends.

[Mr. Peabody]

+1 for Future / Klear to seal or protect masters.

This has allowed me to use all sorts of materials that would otherwise have failed or even ruined a mould. And I got my masters back ready to be used again as needed.

[snitcythedog]

Masters: PVA glue is not strong enough when removing masters from the mould! In several instances the master broke during the process. If I was to do this in future I'd probably use superglue or the likes.

Sorry Andy, missed this in the first go around.  PVA will be both too week and too flexible.  Superglue might be too messy and expensive.  If you check your local DIY like B&Q look for Gorilla Glue wood glue.

It is similar to PVA and other wood glues in that it is water soluble and cleans up easily when wet.  Where it is different is that it dries rock hard.  I now use this product exclusively when building terrain and when making masters to mold from.  Hope that helps and good casting.
Snitchy sends.

[bishop odo]

In the US Gorilla wood glue, foams as it dries, making it a little hard to use unless you monitor it and wipe as it dries. Is that the case elsewhere?

[Mr. Peabody]

Pretty sure the 'foamy' Gorilla Glue is their original formula. They have a lot of products... 

There is both "original GG" - the water activated, expanding monster - and "GG white", which is also water activated, but does not expand.

[snitcythedog]

In the US Gorilla wood glue, foams as it dries, making it a little hard to use unless you monitor it and wipe as it dries. Is that the case elsewhere?

I know what you are talking about but this stuff is pretty much the same formula as Elmer's wood glue.  No foaming.  Since I immigrated over here I have tried out about twenty different types of glues.  This is the only one I really like since I know how it will behave both wet and dry.   PVA and wood PVA are way too much like Elmer's white glue for my tastes.  They are too runny and they are flexible as hell when set and that does not work well for terrain in my view.  Enough thread hijacking by talking about the finer points of adhesives.   
Snitchy sends