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Armour Downunder
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Joined: 26 Oct 2005
Posts: 27
Location: Australia

PostPosted: Sat Mar 25, 2006 8:35 pm    Post subject: Resin Casting Reply with quote

I’ve just started into the wonderful world of RTV Silicone mould making and resin casting. I made a singlepoint scope and a DH-17 blaster tip then made the 2 half silicone moulds, which looked to be good enough to try casting. I found that the finished resin casts tended to have a slight bow in them, which makes them completely useless. One thing I noticed is that 1 half of each of the moulds seems to have less hardener in it than the other and this is the direction the bow occurs. Would this be the cause and would I need to re-make the weaker half with more hardener in it.
Is there someone out there with the experience to help out with this problem?
Thanks in advance.

Mitch
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jegner
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Joined: 30 May 2003
Posts: 2144
Location: Texas, USA

PostPosted: Sun Mar 26, 2006 11:44 am    Post subject: Reply with quote

Can you post some pictures? I reinforce my molds with styrene or thin boards, then, it's like the cover of a hardback book, and I just rubber band the two parts molds together.

Last edited by jegner on Mon Mar 27, 2006 8:35 am; edited 1 time in total
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Armour Downunder
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Joined: 26 Oct 2005
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Location: Australia

PostPosted: Sun Mar 26, 2006 6:21 pm    Post subject: Reply with quote

I watched your video on resin casting before I did anything. I use plywood to help make the moulds rigid.
My guess is that one side of the mould has less hardener in it because it is more flexible than the other
and also a lighter shade of blue (the colour of the catalyst).
This is the side the casts tend to bow into. I figured maybe someone has had a similar problem when starting out with RTV silicone.
I used a 200ml jug for measuring the silicone and a syringe for the catalyst (its 10 to 1 ratio) and at some stage I’m sure I thought the jug was only 100ml and would have used less hardener.
Maybe I should just re-make the side that is more flexible and see how that goes.
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TK161
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Joined: 09 Mar 2006
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PostPosted: Mon Mar 27, 2006 7:53 am    Post subject: Reply with quote

Is it possible to make a one piece silicon mold with a 2 piece rigid mother mold to hold the the shape of the silicon. You can than serparate the mother mold and then carefully cut the piece out. You need to make sure that you leave a hole for filling the mold with resin when you're done. That way you're whole silicon mold is made from the same mix.
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Armour Downunder
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Joined: 26 Oct 2005
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Location: Australia

PostPosted: Mon Mar 27, 2006 6:37 pm    Post subject: Reply with quote

OK, that sounds like a plausible possibility.
With simple items such as the scope I could just make a box around the scope while it is standing upright
and pour the silicone over it. The only thing missing would be the male / female lugs that are created
when making a 2 part mould that align the 2 halves.
I will give this method a go when I get some more silicone.
Thanks

Mitch
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Armour Downunder
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Joined: 26 Oct 2005
Posts: 27
Location: Australia

PostPosted: Tue Apr 04, 2006 1:48 am    Post subject: Reply with quote

I have found a way to solve the problem of bowed pieces.
It may not be orthodox but it works. I de-mould the casts while the resin
is still warm and a bit pliable, this way I can straighten the bowed piece by hand
and it has worked pretty well so far. RTV Silicone over here is rather expensive at AU$50 (US$36)
for a 32oz (10 to 1 ratio) kit so I had to find an alternative way to get the results required.
One day I will make new moulds and try the one piece as suggested.

Mitch
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Armour Downunder
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Joined: 26 Oct 2005
Posts: 27
Location: Australia

PostPosted: Mon May 08, 2006 6:39 am    Post subject: Reply with quote

Well, some issues with the resin casting have been resolved but now I’m faced with another couple. I had a go at using pigment in the resin which made the resin really thick and I thought I wouldn’t get the amount of detail due to the thicker resin not flowing into all the under cuts etc. but my first 2 pours were near perfect. The problem I’ve now come across is when I don’t use pigment I end up with air bubble problems. The silicone mould has "escape vents" at the crucial points and these seem to work but the top of the cast where the resin is poured into the mould still has bubble problems. Another thing is that I don’t seem to be getting the same details that I did with the thicker resin. Its been suggested that thinning the resin with acetone might help get the resin to the details but I’m not sure what quantity to use or if it will even work. And I still have the bubble problem.
Id sure appreciate any help with these problems if anyone has worked out the secrets to the perfect resin cast.
Thanks

Mitch
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jegner
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Location: Texas, USA

PostPosted: Mon May 08, 2006 12:32 pm    Post subject: Reply with quote

Some resins are designed for pigment, some not. Usually you mix the dye with the 'B' part, then mix with the curative.

As for thick vs thin and the amount of bubbles, well that has alot to do with the way the two parts are mixed. A careful slow 'figure 8' is the preferred method. I like the fast curing 3 min pot life resin, with a near water-like viscosity.

Bubble can occur is the pour is slow, and if your molds don't have a high place for the bubbles to rise to. Also, tapping the molds helps shake the bubbles to the surface.

Jim
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Armour Downunder
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Joined: 26 Oct 2005
Posts: 27
Location: Australia

PostPosted: Tue May 16, 2006 6:26 pm    Post subject: Reply with quote

I also use the 3 minute water like resin. The only thing I haven’t been doing is pouring the resin quickly, I just assumed slow was the way to go. I tried the suggested method with my last attempt and the resin did not cure. It was mixed properly with the curative but for some reason just wouldn’t cure, not even after leaving it for 24 hours. It just got a thin cured shell and the rest remained liquid. I did notice that the curative was an opaque colour instead of the usual almost clear colour that the first couple of batches were. I asked the supplier about this and he said I was the only one to have problems with the particular batch Confused I will get some more resin sent down next week and try again. But, I was wondering if temperature plays a major part in the curing process. It was pretty cold out in the shed when I did the last pour.
I'll let you know how the next lot goes.


Cheers
Mitch
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jegner
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PostPosted: Thu May 18, 2006 8:50 am    Post subject: Reply with quote

I'm not sure, but it sounds like the resin either did not get mixed well, or was old, or there was too much of one of the two parts. I take two throw-away plastic cups, stick a ruler in and measure up the side of the cup and mark a line. I do this for both cups, marking one 'A' and the other 'B'. I then place the cups on a level surface, and fill to the line on the side of the cup. If one is more than the other by 10% the resin may not cure. If the color of the resin has changed to an tan or orange color, then chances are, air got in and oxodized the resin, making it bad. Be sure to keep the lids on the resin at all times, and the lids are tight. Resin usually have a 6 month shelf life once opened. I rarely get that long out of it, though. By about 4 months, the curative has started to turn orange, and the crusties start to settle in the bottom of the curative. It's best to sit down and make one big pour once you get your molds ready, and use up all the resin you have.


Jim
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AlterEgos
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Joined: 30 May 2006
Posts: 115
Location: Centreville, VA

PostPosted: Wed May 31, 2006 9:31 am    Post subject: Reply with quote

See, this is the very reason I always try to use 1:1 ratio mix resins and silicones. By using them, it's practically guaranteed to eliminate any chance of flaws.

Smooth-On makes the best 1:1 products on the market... unfortunately, this doesn't help anyone who lives outside of the U.S. :/
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Armour Downunder
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Joined: 26 Oct 2005
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Location: Australia

PostPosted: Wed May 31, 2006 4:10 pm    Post subject: Reply with quote

The resin I use is 1:1 but it is by weight which makes it that little bit more fiddly having to use kitchen scales. The silicone is a 10:1 ratio and again this is by weight. If it was all just 1:1 ratio by volume it would be just peachy.
Its all good now though because Ive found another supplier and this guy used to do exactly what Im doing so he has been instructing me in some of the finer points of silicone moulding & resin casting. He has also been involved with vacuuforming so will be helpful when the time comes for me to make a start on that project.

Chers
Mitch
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AlterEgos
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Location: Centreville, VA

PostPosted: Wed May 31, 2006 11:07 pm    Post subject: Reply with quote

That sounds great, Mitch! Glad you found a better product and someone local to offer instruction.

Wish I could have been more help, but I'm just not familiar with all of the products currently on the market. I just stick with what works best for me when I find it and then forget about all the other products I've used in the past.
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