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Re: Platinum/Addition Cure Slicone

July 15 2012 at 3:01 AM
Peter Hartup  (Login ModelStarter)
Missing-Lynx members
from IP address

Response to Platinum/Addition Cure Slicone

Hi Jon,

My only experience with platinum catalysed rtv is with a product from Barnes here in Oz called Pinkysil, so my remarks are limited to that product, but I think they will apply.

As with all things you are dealing with a set of compromises.

Pinky is a two part 50/50 mix which is nice in that it makes for easy measurement when compared to the usual mix-by-weight ratios you normally get.

The stuff is very thin compared with the usual tin catalysed RTVs, which means it pours easily and if you pour indirectly (ie not directly onto the master) then you are less prone to trapping air on the surface of the master.

It also cures very quickly, so if you want to mould something quickly and start popping out casts ASAP, you can probably achieve this in an afternoon. I'd think it would be good in this regard if you want to give demos etc, particularly of multi-part moulds.

The cured moulds themselves are extremely flexible.

Now for the relative downsides. The price you pay for that flexibility (and softness) is limited mould life. If you've got a very detailed master, it'll pick up the details as well as anything else, but it will deteriorate a lot sooner than the harder RTVs. Sorry I can't be more specific but I only use it for "limited run" stuff myself.

Also, I'm not sure about library / shelf life of the finished moulds, but I suspect they might not keep as well as long. I must pull out one of the older moulds I've done and check this out!

As for the dams / mould boxes, I use Lego bricks and although Pinky is thinner that the tin-based RTVs, leakage isn't that bad thru the blocks. I guess this is partially due to the fast set and cure time - it doesn't really have enough time to leak a lot thru the wall! Also, I use those miniature quick-release clamps, one per side, make sure that the base I'm using on the box is nice and tight up against the bottom of the box / dam. For the bases I've got a collection of 2-3mm sheet styrene cut to the most common dimensions I use (measured in Lego brick units!).

I normally put the RTVs in a vacuum chamber I built from a pressure cooker after pouring to make sure all trapped air bubbles are eliminated. For tin-based RTVs you have plenty of time to get the mould into the chamber and get the pressure down before it starts to set. With Pinky you get a lot less time so I used a smaller (transparent) vessel. The amount of bubbles pulled out of the Pinky under vacuum was quite alarming and left a frothy top to the mould (like an Aero chocolate!). The cavity around the master was OK and not distorted, but I still haven't decided whether all the bubbles were air trapped, or more likely, dissolved in the Pinky, or whether the vacuum was pulling some sort of solvent out of the RTV.

Either way, I haven't used a vacuum with Pinky since. Careful pouring and stirring seems to have done the trick.

Overall, I quite like the stuff for quick jobs and maybe pushing the envelope a bit in terms of undercuts which may damage castings during extraction with a harder less flexible RTV, but which the flexibility of the Pinky can accommodate easily. But I still use the tin-based RTVs for the vast majority of moulding, especially larger moulds where the stiffer tin-based RTVs are less prone to flex under gravity if unsupported.

Also, I'm not sure about using Pinky for two or more part moulds where you need to press the parts together to make a seal when pouring the resin. I'm not confident that the Pinky wouldn't distort the shape being cast. So I'll stick to the harder RTVs (ie tin-based) for those.

Hope this helps.


PS: I've only used Pinky for masters built from sheet styrene and / or resin without probs, but I'm pretty sure it'd be OK for metal, though I'd test it on something I don't love first!!

This message has been edited by ModelStarter from IP address on Jul 15, 2012 3:09 AM

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