Platinum/Addition Cure Slicone

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Platinum/Addition Cure Slicone

Joined: April 18th, 2005, 5:47 pm

July 14th, 2012, 10:58 am #1

Hi all,

I just bought some of this stuff http://www.tomps.com/shop/rapimould-rtv ... p-215.html
Never used this stuff before, as I have only ever tin cure stuff previously.
So am wondering what is the safest way to make a mold box?, I was going to use some paper cups, and either lego blocks or a plasticard mold box.
Are these going to be OK?
Also, whilst I am asking, I have 2 things that I am needing copies of, some resin boots and a resin torso and a whitemetal torso, are these OK to use as masters with this stuff?
As I am really unsure of what will inhibit the curing, I know onion and garlic will, but that's it.

Thanks for any info/advice.

Jon
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Joined: January 29th, 2006, 7:01 pm

July 14th, 2012, 4:02 pm #2

Here's one:

http://www.silicones-inc.com/pgeninfo.pdf

And another:

http://www.smooth-on.com/tb/files/MOLD_ ... _30_TB.pdf


Cure Inhibition - Addition cured silicone rubber may be inhibited by certain contaminants in or on the pattern to be molded
resulting in tackiness at the pattern interface or a total lack of cure throughout the mold. Latex, sulfur clays, certain wood surfaces,
newly cast polyester, epoxy or urethane rubber my cause inhibition. If compatibility between the rubber and the surface is a
concern, a small-scale test is recommended. Apply a small amount of rubber onto a non-critical area of the pattern. Inhibition
has occurred if the rubber is gummy or uncured after the recommended cure time has passed. To prevent inhibition, one or more
coatings of a clear acrylic lacquer applied to the model surface is usually effective. Allow any sealer to thoroughly dry before
applying rubber.
Even with a sealer, Mold Star® silicones will not cure against surfaces containing sulfur. If you are not sure if your clay
contains sulfur, do a small compatibility test before using for an important project.

KL



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Joined: December 27th, 2010, 1:13 am

July 15th, 2012, 7:01 am #3

Hi all,

I just bought some of this stuff http://www.tomps.com/shop/rapimould-rtv ... p-215.html
Never used this stuff before, as I have only ever tin cure stuff previously.
So am wondering what is the safest way to make a mold box?, I was going to use some paper cups, and either lego blocks or a plasticard mold box.
Are these going to be OK?
Also, whilst I am asking, I have 2 things that I am needing copies of, some resin boots and a resin torso and a whitemetal torso, are these OK to use as masters with this stuff?
As I am really unsure of what will inhibit the curing, I know onion and garlic will, but that's it.

Thanks for any info/advice.

Jon
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.


Cheers


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!!
Last edited by ModelStarter on July 15th, 2012, 7:09 am, edited 1 time in total.
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Joined: January 4th, 2004, 2:26 am

July 15th, 2012, 5:50 pm #4

Hi all,

I just bought some of this stuff http://www.tomps.com/shop/rapimould-rtv ... p-215.html
Never used this stuff before, as I have only ever tin cure stuff previously.
So am wondering what is the safest way to make a mold box?, I was going to use some paper cups, and either lego blocks or a plasticard mold box.
Are these going to be OK?
Also, whilst I am asking, I have 2 things that I am needing copies of, some resin boots and a resin torso and a whitemetal torso, are these OK to use as masters with this stuff?
As I am really unsure of what will inhibit the curing, I know onion and garlic will, but that's it.

Thanks for any info/advice.

Jon
Hi Jon,
I've used platinum cure for one-sided molds; plastic masters and lego mold boxes no problem. I was always under the impression that they were a bit more resistant to mold expansion then the tin cure silicon. I've used TAP Plastics version:http://www.tapplastics.com/product/mold ... licone/494 and more recently Smooth-On's version Mold Star:http://www.smooth-on.com/Silicone-Rubbe ... index.html . I use sulphur-free modelling clay for two-part molds if needed. Also I always wrap the lego mold box with masking tape to prevent seepage of the rubber. A small amount will always find it's way between the bricks (especially with vacuum or pressure) but at least the tape keeps it from flowing out.

Jim
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Joined: December 27th, 2010, 1:13 am

July 16th, 2012, 2:06 am #5

I second the comments on use of a sulphur-free clay for two part moulds etc over normal "plasticine" type materials, which can react with the curing RTVs.

These are formulated specifically for use with RTVs etc.

Local product here in Sydney from Barnes (no connection, but a satisfied customer) is Kleen Klay. It comes in three grades of hardness. I use the softest almost exclusively - it remains easy to work even in cold weather and seems to last forever without going rancid or drying out.


Cheers
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Joined: April 18th, 2005, 5:47 pm

July 16th, 2012, 4:11 pm #6

Hi all,

I just bought some of this stuff http://www.tomps.com/shop/rapimould-rtv ... p-215.html
Never used this stuff before, as I have only ever tin cure stuff previously.
So am wondering what is the safest way to make a mold box?, I was going to use some paper cups, and either lego blocks or a plasticard mold box.
Are these going to be OK?
Also, whilst I am asking, I have 2 things that I am needing copies of, some resin boots and a resin torso and a whitemetal torso, are these OK to use as masters with this stuff?
As I am really unsure of what will inhibit the curing, I know onion and garlic will, but that's it.

Thanks for any info/advice.

Jon
I am going to do a small test of the clay/plasticine I have, it's by Newplast and apparently is sulphur free..apparently they use this stuff to make Wallace and Gromit, so I'd rather use this stuff then anything else
The other alternatives are quite expensive, Chavant and I have found Kleen Klay that's a bit cheaper.If this stuff I have dont work, I'll get some of the Kleen Klay.I think, as I ain't doing a 2 part mold so dont need anything to elaborate, I'll just use a dam/box outta the clay.It should be easy enough for a simple block mold.

All this hassle to get some decent boots for figures....why can't someone just produce a set of 1/35th scale boots?, for various nations/periods...grrrrrrr.

But thanks a lot for the advice/info guys, put my mind at ease and set me on the right track.

Jon
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