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How to.... (1) make an RTV open mould

Captain Sci-Fi

Hi Guys,

Here is something I have been meaning to do for ages, provide a clear step by step guide to some of the most important model making techniques that new guys might find useful.

I have to make a mould to cast a part for my new UFO model so thought you might like to follow along as this technique can be used to replicate kit parts, small detail parts and larger items used in scratch building. With flexible RTV (Room Temperature Vulcanising) Silicones in their great ranges on the market it is possible to have a go for even the most novice builder and get a useful result.

Silicones fall into two basic types, those that need a chemical additive to cure (become solid) and those that don't (like bath sealer) both are useful materials but the two part material is the most useful to us. It's cured state will be self supporting in shape, hold the detail we want down to fingerprint level and be completely ready to use in a day or less.

One of the compounds I use frequently is Flexil - J24 available from Jacobson Chemical Ltd in the UK, there are other companies and I will provide a glossary later. This thick sticky treacle like fluid is like white paint, drips in long strands is self levelling and to a greater extent self degassing. The cure time from mix to set can be controlled with different grades of catalyst usefully dyed to aid in thorough mixing.

I prefer the fast set Blue which gives a useful pot life (how long you have between adding the catalyst and it being unable to pour from the mixing pot) and the point at which the mould is ready for use (the cure time). Usually this means any further changes in the RTV are minuscule and don't effect any outcome to the finished mould.

Usually no release agents are required and as long as you can provide a leak free containment, even temperature, clean equipment and non porous master you will be happy with the result. So first we need a master part, it needs to be clean, held stable (in this case stuck down to a glass table) and somewhat held level. The RTV will find it's own level so if you plan to pour a liquid plastic into this mould to cast parts it will be so much easier if the mould is pre levelled. The top becomes the bottom actually the base of the mould so it needs to be as level as possible.

So where to start?

I usually prepare the moulding surface first, in this case a length of glass shelving bought as a bathroom accessory from B&Q (a hardware store) it is 4 mm thick and like most float glass, flat enough for my needs. From the same store I bought double sided carpet tape, this has the strongest contact adhesive I have ever come across. Initially I used this directly on the glass but it left a lot of clean up work after use as I had to scrape with a wood chisel to remove it, then clean with alcohol to remove the sticky residue. Now I use brown parcel tape as a first layer with the carpet tape applied over that. The vinyl tape makes an easy release for the heavily bonded carpet tape.I laid this out in a square large enough to enclose the round shape I will be moulding plus a margin to allow for enough thickness to be self supporting. I finished by applying the double sided tape edge to edge in lines over it.

Master preparation
As you can see I have prepared the master part by raising it of the glass with a 2 mm acrylic spacer (shim) tacked glued to the underside so the RTV can flow underneath a little way to capture the edge of the part. This was placed in the middle of the sticky area and pressed down firmly, the carpet tape is pressure sensitive so a good firm press is all round evenly is all thats required.

The Enclosure
Next a ring of Styrene sheet wide enough to make a tall enclosure at least 1.5 times the height of the part was cut and cemented with a large overlap. Be sure this joint is stuck from top to bottom and that the bottom edge lines up perfectly. I clipped the ends together with a clothes peg and stood the ring on it's edge to be sure they were even then used liquid cement to sel the deal. I have several of these rings in different sizes, they are reusable. the material thickness is quite important I would use less than 0.5mm and not more than 1 mm thickness, less will be tricky to stick down, more won't be flexible enough for a curve much less than 100mm (4"). Any smaller than that and I use a tube section, cut down paint can tops, old mixing beakers and vending cups are all useful. I know it's hard to believe but even on this mould at some 200mm (8") across and 40mm (1 1/2" ) high is plenty to hold the rubber water tight on it's edge.

My earliest attempts at RTV work would have me guessing how much rubber to use at this point, these days I put cling film over the master and fill the enclosure with dry uncooked rice to give a rough volume, it's rough but it works....

So we know how much material we need to use, we have removed the rice in it's cling film bag and poured it into a measuring cup to check the volume, make a not of this as you will need it later.

Gather together all that will be needed, I use the following.

Icing knife (stainless steel) bought from the hardware store used to smooth ice cakes - I use it to mix the silicone base and catalyst, the long straight blade can scrape clean the sides of the tallest mixing cups.

Measuring beakers bought on ebay and used for mixing resins, they are cheap and meant to be disposable but I clean and reuse mine.

Small scales - I use a digital version found on eBay as postal scales these are 5kg capacity have a tare (reset a measured weight to zero) and mode (both metric and imperial scale) function. ALWAYS measure, NEVER guess.

Sections of paper towel - kitchen roll is perfect, I cut large sheets down into quarters and use them for wiping clean the icing knife.

I keep a dirty measuring cup handy to use as a desk bin for Kitchen paper and other debris.

Here is the sequence I use:
Note the volume used from the rice, this is the volume of rubber needed plus 10% for catalyst. Now we have volumes of material but the RTV measure is by weight, so place the empty cup on the scales and tare function to remove the weight of the cup (reset to zero) then fill the cup up to the volume that was measured from the rice. For this example I will use a figure of 200gms which gives us the volume measured. 10% of 200gms is 20gms so this is the weight of catalyst needed to make the RTV fully cure.

The blue catalyst I use is heavily dyed to make mixing easier basically you stir (not forgetting to scrape the sides and bottom of the cup) until everything is a pale sky blue colour. I take a disposable brush (ebay see acid brushes) and brush the first coat all over and around the master, this reduces the risk of trapping air next to the the master. I like to pour the remainder down one side with the mould slightly elevated on the opposite side to allow the heavy liquid to push the air ahead of it as it gradually fills the cavity and runs around the master part. Use a cocktail stick or skewer to burst any bigger bubbles you see and encourage the liquid rubber where needed. Remove any wedges and let the mould sit flat to see where it settles and check for leaks gently repress anywhere that shows any signs of escaping RTV, there shouldn't be any.

Except.... this:
I noticed this tiny leak after the mould had cured but notice the styrene is not on the double sided tape at this location.

Tomorrow we check out the results...

Captain Sci-Fi

How to.... (2) make an RTV open mould and jacket

After removal of the styrene ring here is what we have, not a bubble anywhere. The fine line detail painting guide is present and it's ready to put into service.

The meniscus around the edge has to be removed so the mould will lay flat on it's back on the table as this mould was intended be filled to the brim.

Small confession time....
I have improved my technique leaps and bounds since I originally made this mould. I am used to the characteristics of the materials and have altered the method to thin the RTV skin to about 5 mm with a jacket to support the shape.

Here is the epoxy impregnated into the glass strand mat and allowed to cure

Removal of the styrene containment ring shows the flash of fibres, watch your hands at this stage as it is easy and painful to stab yourself on them. I cut off the edge and rub a rough glass paper over the surface to knock down any wayward fibres. The blue line indicated the overlap mark reproduced from the styrene ring, I use this as a quick alignment indicator.

Edge flash removed and quick sanding of the outside, the bottom of this jacket is flat but I could easily use filler to stick a wide acrylic tube section levelling the mould top and leave to bond in position if needed.

Here is the West System catalyst I use, it's the quick set type. Any auto repair and servicing store will stock similar products, boat yards are also a good source of these materials. You could use the traditional polyester resin of course but it stinks the place up, is bad for the environment and my health generally. This is something I am paying more attention to lately.

I usually notch both the epoxy jacket and RTV mould to make alignment simple. The RTV liner will sit perfectly supported this way and it stops frustration and removes doubt later, keep it simple..!

So how did I ensure an even thickness for the RTV? I used a small paintbrush married with a fast cure catalyst and simply kept pushing the liquid silicone up the sides for the 15 mins it takes to gel (just the point where the liquid stops flowing). It won't be cured but will stay put, brush marks and other lumps and bumps can be evened out further with a dab of isoprop alchol or methalyted spirit on your finger. This step is to prevent undercuts on the outside surface of your mould so the epoxy jacket will locate and relax back into it's original position and be totally supported.

This method despite using expensive epoxy saved 2/3rds the price in silicone alone so is well worth the extra effort. The first mould weighs in at a hefty 1024 grams, the jacket method RTV mould weighs in at 303 gms and I could have made the diameter smaller with more forethought saving more money.

Next I will demonstrate how to produce a simple plug to use with this mould to make a part hollow and later in this series I will make a closed mould suitable for hollow casting larger parts.

So next time we use Urethane resin know as Fast Cast.


I did a tutorial and then found the captain had already covered!

A different approach for a very different type of part.
I used a transparent RTV, the part in question is the head of a figure for Warhammer (not mine); at some point some more common figures will have the same done to them :yes:

See attached .zip / .pdf

ps. strange, could not upload a 400Kb .pdf but allowed to upload 4gb .zip


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