Tutorial: Mold Making II

Continuing from our previous post about mold making…

Estimating the Mold Volume and Mixing RTV

4. Assemble your materials.

The RTV product I use is called ‘Ultrasil’ manufactured by Barnes, an Australian company and available from TopMark in New Zealand. I’ve also grabbed a small previously set mold, a cheap hog bristle brush, a permanent marker, a clean pot to mix RTV in, a mixing tool (cleverly disguised as an HB pencil) and a large 1L bottle of beach sand. I’ll describe the use of each below.

Ultrasil is a 1:10 RTV (room temperature vulcanising) product consisting of a large pot of white rubber and a small bottle of blue hardener. I’d seriously recommend purchasing RTV products with coloured parts like this, because mixing a 1:10 product by weight when both components are white is an absolute nightmare. Particularly when one of them is a thick, sticky, viscous rubber. Having a coloured hardener also makes it very easy to see your RTV is fully and uniformly mixed prior to pouring which is obviously important.

However the real beauty to a coloured hardener is you can mix all of your molds ‘by eye’ using colour matching to determine when you’ve got roughly enough hardener into the mix to guarantee a set mold. This saves you measuring anything and is fast and reliable. I’ve mixed roughly 30 molds this way and only botched one early pour.

The only down side to this method is you tend to over use hardener which could lead to you running out. Although that’s never happened to me. I suspect Barnes gives you excess anyway in anticipation of most molders being lazy devils and mixing by colour. Varying the amount of hardner added will change the working time and final strength of the set RTV, but most of these products should have an ample working time anyway. Definitely check the MDS (material data sheet) for your products before mixing them though.

5. Estimating mold volume using sand.

RTV is an expensive product here in New Zealand so I want to waste as little as possible. This means I need a way to get a good measurement of the volume of RTV I’ll need to fill a given pour box. The easiest way to estimate the volume is simply to pour some other ‘liquid’ into the assembled box.

Water springs to mind and I know at least one professional caster that does use water for mold volume estimation, but to me water is far too messy and makes your masters and pour box wet! So I use good old washed and dried beach sand (nabbed from Whangamata several summers ago).

Take your sand and fill the pour box with it, completely covering the masters and then some. Gently shuffle the box around until the sand is level then take a tool/finger and poke around in it until you hit a master. Do this to make sure your masters are buried sufficiently deeply – like the external mold walls you should be aiming for 4-5mm of sand covering the highest point of the tallest master in the box.

6. Marking the mixing pot.

Once you’re happy the sand sufficiently fills the pour box, gently pour it out into some large container (I left that out of the above photos) being careful not to lose too much onto your work area. Pour the larger container into your mixing pot and mark where the sand fills the pot to. This is how much of the basic rubber component you’ll need to fill your pour box.

Tip the sand out of your mixing pot and brush any extra grains out. You’ve also got to clean any extra sand out of the pour box. I use a combination of gentle shaking, blowing and brushing off the masters with the hog’s bristle brush. It’ll only take a couple of minutes to clean up and you’ll have a dry box ready for filling with RTV.

7. Mixing RTV by colour matching.

Finally we’re ready to mix! Carefully pour the white rubber component into your mixing pot until it’s been filled to the mark. This can be a messy process and I find it handy to have a scrap of foam card nearby to scrape the edge of the rubber pot clean before putting the lid back on tightly.

I believe RTV products like these can be degraded by exposure to air moisture so don’t leave the lid off your rubber pot for longer than required. The shelf life of an opened pot may be reduced too, so it’s often a good idea to plan and pour a series of molds together in order to use your RTV up in a reasonable space of time. Discuss these details with the manufacturer/reseller when you purchase your RTV.

Add hardener a fair amount at a time and thoroughly mix it in with your mixing tool. Be particularly careful that you mix in the white rubber on the sides and bottom of the pot. I find it handy to use transparent or semi-transparent mixing pots for this reason. I have a cat and a three year old in the house so have an endless supply of plastic pet food containers and jelly pots that get seconded to the garage once empty and cleaned.

Compare your mix to any set mold you have handy. Mix in more hardener until you have a rough colour match to the set mold. Always keeping in mind that a mold that’s seen a lot of either resin or plaster can get a little lighter in colour.

Of course if this is the first mold you’re mixing then the above paragraph is a little useless. In this instance I’d recommend either following whatever instructions came with your RTV or use a roughly regular shaped mixing pot (no curves or sloped walls) and dividing the space between the bottom of the pot and the ‘fill’ mark into 10 roughly similar proportions. Fill nine of those with white rubber and one of them with coloured hardener then mix thoroughly. If anything you want to be a little too generous with the hardener for your first mold, just to make absolutely sure the RTV will set. I’d also suggest your first mold pour be reasonably modest in size. For future molds colour match to your first one as appropriate.

Pouring the Mold

8. Brushing on the first RTV coat.

Now you have a pour box and your mixed RTV ready to go you may be tempted to just slop it in and leave it to set. Beware! For this will almost certainly lead to trapping air bubbles in your masters and related annoying blobs appearing in any casts you make. ‘Act in haste, repent in leisure’ as they say, and remember that air bubbles are the garage caster’s nemesis.

Without fail, I always brush the first coat of RTV over the masters by hand using the stiff hog bristle brush you have handy. Brushing the first coat of RTV on like this will reduce the chances of trapping an annoying air bubble somewhere on the surface of a master. It won’t guarantee you have no air bubbles in your mold but it will significantly reduce the chance of it happening.

So take a deep breath, load up your brush with mixed RTV and start painting it over your masters. You’ll need a good stiff brush (hence hog’s bristle) because RTV is a very thick liquid. You should have ample time to carefully paint each and every master in your pour box before the RTV is anywhere near the end of its working period. But check your RTV MDS to be sure!

Once you’re done, put the brush aside in a cleaning fluid. RTV cleans up in painter’s turpentine and make sure you come back and thoroughly clean this brush once your mold is fully poured. It’s a little ironic while working up this tutorial I was so busy taking photos I completely forgot about the brush I’d used until the next morning. A brush loaded with set RTV is a throw away. This is also why I use cheap hog bristle brushes for all my RTV and plaster casting work, because you can throw them away! I buy a pack of 10 various brushes from a local ‘$2 shop’ regularly.

9. Filling the mold with RTV.

Finally, take your remaining RTV and slowly pour it into the box to fill your mold. I generally try to pour in at a corner or edge and let the thick RTV ‘roll’ across the masters to cover them completely. I believe this should introduce less bubbles than say sluicing RTV all over the mold.

Another tip with pouring is make sure the rim of your mixing pot is clean. You’ve just mixed the rubber and hardener in there and there may be some unmixed components around the rim of the pot which you do not want to pour into your mold.

While pouring you may notice there’s a little colour variation in the RTV as it goes in. Usually this is obvious because you can see the ‘ribbon’ of RTV you’ve just poured laying on to of the RTV already in the box. Don’t worry about this too much unless it’s very clear you have unmixed rubber at the bottom of the pot. If so take a moment to regret not completely mixing the components before pouring. Then stop pouring and try gently stirring up the RTV in the mold with a brush or tool to mix it in with the rest. You can also try to recover the partially mixed rubber left in the pot by adding a little more hardener.

It’s also worth noting that RTV is ‘self adhering’ which means you can later pour additional RTV into a box partially filled with set RTV – providing the set RTV is reasonably clean and free from dust etc. I wouldn’t recommend it but you can use this to recover botched pours where something has gone wrong with your mixed RTV halfway through.

10. Leaving the mold to set.

And you’re done! Stand back and admire your handiwork for a few minutes. Pay particular attention to the edges and seams of your pour box because if you notice RTV escaping anywhere you’ll have to deal with it now. The truely paranoid might light to keep a blob of Klean Klay handy just in case they need to shore up any gaps or holes. This hasn’t happened to me yet but you never know…

Make sure the mold is placed somewhere level and out of the way to set. Level is very important because eventually you want to turn this mold over and pour resin or plaster into it which means the bottom has got to be flat and level. Out of the way because there’s normally a long setting time involved. Again check your RTV’s MDS for details. Then wait at least twice as long as that before you go back to it.

Seriously, I usually pour in the evenings and wait 24 hours for a product with an 8 hour recommended setting time. Do not be tempted to mess with your mold until you’re sure it’s set. RTV is a deceptive product – the surface can appear set while the interior (next to your masters) is still goey. Imagine your surprise at discovering that while trying to peel the mold off! Yes I speak from bitter experience.

There are three techniques I use to avoid this issue:

  1. The first I’ve already mentioned – be patient!
  2. The second is to keep the mixing pot next to your setting mold as in the photo. The pot will likely contain some dregs of RTV from your pour. You can examine these dregs to see how set the poured RTV is likely to be.
  3. The third I like to call the ‘spring test’. Come back to your mold later, when you suspect it may be set. If the surface looks set and the dregs in the mixing pot are fully set take a blunt tool (eg. paintbrush handle) and give the mold surface a good robust prod and hold the tool there. Then suddenly pull it away. If the RTV quickly springs back to a flat surface it’s likely the interior is well set too. If the RTV exhibits any reluctance to return to a flat surface, with the indent you made slowly fading away then for goodness sake walk away because the interior is not fully set.

I realise this last test is quite subjective but believe me it works. Cast a handful of molds and you’ll likely see at least one that does this when you expect it should be completely set. Be patient and give it another day of curing time. If you’ve mixed the RTV well enough that at least the surface has set then eventually the interior should set too.

If you finally give up and peel the mold away from the masters only to discover some or all of the interior has failed to set then it’s likely that your rubber has been contaminated or has aged (consider throwing the batch away) or you constructed your masters from some material that retards the RTV setting process. This is unfortunate. As I mentioned in the previous post sulphur containing plasticene can have this effect, but that’s the only mastering material I know to avoid. There may be others – I use a lot of DIY products, epoxy putties including green stuff, resin pieces and plasticard for my masters without difficulty.

In the next post where we reveal the final mold!

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