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Old 23-02-2012, 12:18 AM   #21
Neo-Uk
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I've finished the floodlight now and was pleasantly surprised to see the first casting (I need four) come out looking like this with all the bars intact. I gave it a blast of primer.



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Old 23-02-2012, 07:57 AM   #22
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Coming along very nicely! You have to make a model sea monster to go with it...
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Old 23-02-2012, 07:41 PM   #23
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That's some nice casting!
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Old 23-02-2012, 10:32 PM   #24
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Thanks Dave, but that's the part of the build I'm hating the most.
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Old 05-03-2012, 12:08 AM   #25
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Can anyone tell me why I'm getting resin casts coming out like this ?

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Old 05-03-2012, 04:41 PM   #26
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It looks to me as though the moulding rubber had a load of air-bubbles which formed on the surface of the master/pattern (check the mould cavity). I've had the odd one when making RTV moulds before, though never so many. These voids in the mould become protuberances on anything you cast in the mould. They can usually be picked off with a thumbnail and the marks buffed away with fine sand paper.

How did you pour the rubber onto the master?

Last edited by McTodd; 05-03-2012 at 04:49 PM.
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Old 05-03-2012, 11:20 PM   #27
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It wasnt rubber it was silicone (left over from some make up effects I did for a film).
I poured it nice and slow and from a bit of height and started from the corner of the mold wall. As I said to Dave I hate this bit of scratch building, I'd rather pay someone to do bits like these for me.
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Old 06-03-2012, 01:13 AM   #28
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I know you meant RTV silicone rubber, I referred to it as 'rubber' for brevity, but so that people don't get confused in future from now on I'll always refer to it as 'RTV silicone rubber'. What brand and type did you use? RTV silicone moulding rubbers and RTV silicone rubbers for makeup effects don't necessarily have the same characteristics. If you weren't using an RTV silicone rubber specifically intended for mouldmaking that may be where your trouble lies. Alec Tiranti stock some excellent RTV silicone moulding rubbers:

Alec Tiranti RTV Silicone Moulding Rubber

I've used T20 RTV silicone moulding rubber for years without any real problems - as I say, I've had the odd small bubbles but nothing like your piece appears to have suffered from. T20 RTV silicone moulding rubber is pretty stretchy as well so you can make castings with quite severe undercuts and not risk tearing it when de-moulding.

Last edited by McTodd; 06-03-2012 at 01:25 AM.
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Old 06-03-2012, 08:42 PM   #29
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Try dusting the inside of the mould with baby powder.
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Old 07-03-2012, 11:35 AM   #30
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That wouldn't help in this case, because the mould itself is defective, i.e. it has spherical holes or pits along the surface of the moulding cavity. Talcum powder does help in cutting down on air bubbles during the casting process, i.e. when pouring liquid resin into a mould during casting, but if he'd had problems with air bubbles in the mould during casting his cast item would have had pits or holes in the surface, the exact opposite of what it's suffering from here.

Looking at that casting again, there is evidence of so many bubbles on the surface of the master while you were pouring your silicone rubber during the mouldmaking process that I doubt they were air bubbles at all but that rather somehow the silicone rubber you used reacted with the surface of the master causing a gas to generate and form the bubbles.

You say you didn't actually use RTV silicone moulding rubber but a silicone rubber meant for prosthetics. I suspect that is the root of the problem.
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Old 07-03-2012, 12:19 PM   #31
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Hi Neo,

To avoid this problem on your next mould paint a layer of rubber onto the surface of your master first, then take a cocktail stick and agitate the rubber next to and around the surface of the part. You will shift all the air bubbles that way. This is a best attempt scenario if you don't have degassing or pressure casting equipment.

Make sure that the parts to be cast are totally clean. It looks to me like you have had a problem with the bubbles sticking to the surface due to surface tension from contaminants like oil or grease, Lanolin being one, it's the naturally occurring oil in skin. The cocktail stick and paint brush trick will agitate the air away from the surface usually.

The same trick works with Urethane casting the final parts, poke around to make sure the resin has flowed into every detail then tease any air bubbles to the surface edge and pop or lift them out.

Hope this helps.
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Old 07-03-2012, 11:20 PM   #32
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Thanks for the advice peps I'll be sure to take heed.
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Old 14-03-2012, 08:38 AM   #33
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I need help from somebody who knows about rtv molding rubber.
As you can see by this picture I got big wad of rtv on my diving bell on top of the main viewport which took me ages to get right.
Problem being it's been like that now for three days and I'm starting to panic.
I dont think its going to go off so the question is how do I get it off without damaging whats underneath (a sculpt of the window in plasteline ).

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Old 14-03-2012, 07:30 PM   #34
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Are you saying that the RTV silicone moulding rubber has been in that position for three days and it's not cured? If so, then given your master underneath is clay (plastiline I take it is rather like firm plasticine or chavant?) and given that the RTV silicone moulding rubber won't cure at all if it hasn't by now, there's not much you can do. Turn the whole thing upside down and pour out as much as you can - perhaps set up some kind of jig to hold the entire thing in situ for several hours (overnight even) so as much of the RTV silicone moulding rubber pours off as possible.

But you will need to remove every trace of uncured RTV silicone moulding rubber from your master before having a go at making a new mould because there's clearly something wrong with it and if you try to pour new RTV silicone moulding rubber without having got rid of every trace of the defective stuff, you risk it bollocksing up again, because it may inhibit the new RTV silicone moulding rubber's curing. If your master was made of rigid materials like styrene, wood etc., it wouldn't be a problem, you could use solvents, brushes and cloths to really scrub away all traces of the defective RTV silicone moulding rubber, but with clay I think you may have to scrap it and start again. But try cleaning it first, you never know.

You do seem to be having a lot of problems that you really shouldn't be having, so could you post exactly what you are using? Without knowing that, it's impossible to advise further. If necessary, provide a link to the exact RTV silicone moulding rubber you bought, and if you know how long you've had it that would help - the stuff doesn't last forever, especially if you've opened it, used a bit then closed it for a long period before using it again.

I get the impression from earlier posts that you aren't using proper RTV silicone moulding rubber anyway, but stuff designed for making prosthetics from; that may be part of the problem. And if what you're using is old, it may simply have gone off.
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Old 14-03-2012, 07:53 PM   #35
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Quote:
Originally Posted by Neo-Uk View Post
I need help from somebody who knows about rtv molding rubber.
As you can see by this picture I got big wad of rtv on my diving bell on top of the main viewport which took me ages to get right.
Problem being it's been like that now for three days and I'm starting to panic.
I dont think its going to go off so the question is how do I get it off without damaging whats underneath (a sculpt of the window in plasteline ).

I have had this problem and held my nerve and waited, and waited, then waited a bit more and it eventually went off! From memory it was still squidgy after a fortnight!!
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Old 14-03-2012, 09:20 PM   #36
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Quote:
Originally Posted by Steve Zodiac View Post
I have had this problem and held my nerve and waited, and waited, then waited a bit more and it eventually went off! From memory it was still squidgy after a fortnight!!
A fortnight?!?!? Jaysus, never heard of RTV silicone rubber taking that long. I've used rather old stuff from a can that's been opened, closed, left a while and used again and it's taken two days to cure and I considered that to be pushing it.

So there you go, Neo-UK, give it another eleven days minimum and see how it turns out.

But please post details of what you're using anyway because it doesn't sound at all right.
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Old 18-03-2012, 01:56 AM   #37
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So I got fed up of waiting and decided to get this useless lump off so I can move on with the build. After I go most of it off this is what is left to clean up.



It must be the stickiest substance in the world. What a $h!t job I've got in front of me..I feel like crying
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Old 18-03-2012, 08:10 AM   #38
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Again, exactly what are you using? There are experienced people here trying to advise but without knowing exact details of what you've got nobody can help.
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Old 18-03-2012, 04:53 PM   #39
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I had a look on the tub and all I can see on there is Moldsil ,but you're right McTodd it was opened, closed, left a while and used again but the stuff a bit expensive and I didnt want to throw what I had away.
Funny thing was a did a small test with this stuff first just to make sure and it cured but just took a day longer than normal.
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Old 18-03-2012, 06:29 PM   #40
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Thanks for the extra info - I've had a google and found this (it's an Aussie site, ignore that):

http://rubbermoulds.com.au/moldsil-kit-w-catalyst/

I assume it's the same stuff in which case yes, it is designed for mould-making, so that's good.

I know what you mean about wanting to use old stuff that you still have in stock because it is damn expensive - like you, I've used RTV rubber from tins that have been opened, closed and ignored for months (even a year or so) and whilst it's taken longer to cure, only an extra day or so. You've had such terrible problems with your RTV rubber that I seriously wonder if you've been really unlucky and it was a dodgy batch because it shouldn't be that screwed up.

All I can say is I feel for you, what a pain in the arse! I don't know where you're based, other than somewhere in the UK, but if you can get to Alec Tiranti's shop (one in London, one in Reading I think) then you can't go far wrong with the stuff they sell:

Alec Tiranti's RTV Moulding Rubbers

I've had a fair few tins of their stuff over the years and never had any problems.

DISCLAIMER: I am not related to anyone who works for or with Alec Tiranti, nor do I know anyone etc. I have no connection other than having bought a fair bit of various materials and tools over the years from them and I've always been happy.
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