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Hanaldo
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Looking good mate! Is there any real reason to be concerned about the hardness of the silicone? You only need to get one pull out of them, so they don't need to be super hard wearing. And the extra flexibility will just make demoulding your casted moulds even easier.
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TURK
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Anyway guys ......... as it turns out, my silicone moulds made of CS25 came out absolutely superb !  >> Now I've got two perfectly formed and blemish free silicone moulds for the next stage ......... resin casting >>  If I had one thing to add about the CS25, I think it's a little too soft ( flexible ) for my liking. But then, I was taking a gamble with the OOMOO 30 as well, as that's just a Shore A of 30. It's not detailed intricate moulding I'm working with, so something a little firmer was what I originally was looking for. |
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TURK
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What is it with this site, that you can't use a quote from a member and that it adds your response to the entire quote !? No amount of editing on my part can fix that ........ just wasted 20 minutes trying to fix that !  TURK
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TURK
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Yup, precisely Hanaldo ........ that's the method I've been using, easy when you're using a 1:1 ratio like OOMOO 30 though >>  OK, here's some figures for you ......... as I got it wrong but can't find where I could of got it so wrong ! To help explain my situation better, here's a picture of the mould set up box and the plug positioned inside it. So that' the volume I'm trying to fill with silicone >>  And here are the figures I used for working out the volume of the box, minus the volume of the plug in cubic centimetres ; 12cm X 32cm X 20cm = 7680 cm³ ( interior dimensions of mould box )Then the dimensions of the model or Plug in this case 10cm X 29cm X 16cm = 4640 cm³ Deduct the size of the plug from the mould box : 7680 - 4640 = 3040 cm³ Multiply that number by the specific Gravity of the material you're using, in this case the density of the mixed CS25 silicon will be 1.14 ( g/cc ) = 3465.6 So that means I'll be needing 3465.6 grams of silicon in total. As a general rule of thumb I think it's a good idea to have a little extra left over ( the little bits of material left on the stirring sticks and in the bottom of the bucket ), so the addition of the catalyst to that figure will be the 'extra' amount of silicone just in case. So, 3465.6 grams of silicone would require 173.28 grams of catalyst as it's 5% Giving me a total of 3638.88 to fill that volume mentioned above ....... the thing is, as I was pouring the silicone, I knew I wasn't going to make it !  Luckily CS25 has a pot life of 60 minutes so I had ample time to weigh, mix and degas another 1000 grams of silicone plus another 50 grams of catalyst.
Where did I go wrong ? it must be something to do with the way I worked out the volume, but I can't see it !
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TURK
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Group: Forum Members
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That's OK Warren, at least now I'll know for next time ........ as I constructed my mould set up boxes before I bought the degassing system. It's also good to know that the chamber can be used on it's side like that without any safety issues. As you're here, maybe you can answer me a question ......... since having setup my degassing system and playing around with it, by which I mean running a few tests. I noticed that my vacuum gauge never settles on '0'. I pierced the bung as directed, thinking that it takes a while for the gauge to 'acclimatise' itself, but still no joy. I even removed the rubber bung and replaced it, still no joy, there's also a VERY large bubble in the gauge, is this normal ? I've used the setup about a dozen times now, but the gauge is so far out ....... begins at about 5, and goes way past the -30 Hg ! to the point where it can't go any further. TURK
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Warren (Staff)
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Turk, if its only 5mm, I would shave down the chipboard ends of the box or use something thinner. It only needs to be strong enough to withstand the weight of the silicone once you pour it into the box. A Simple solution would be some 1mm steel or alloy as its thin and plenty strong enough and if you do it to both ends you should easily gain more than the 5mm you need.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Hanaldo
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Glad it helps mate! One of the things I quite like about the casting process is you can work out your material requirements quite accurately by filling the cavity with dry rice and then measuring the volume. Becomes somewhat trickier when you need to convert that volume to a weight for the sake of your ratios, but it is at least a starting point.
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TURK
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As I'm at the beginning stage, learning the ropes as it were, I think that 'exact' measurements is what I should be learning. Personally I don't like 'guestimating' anyway ! It's very interesting the way that you came about the same result Hanaldo, I was a little confused at first on how you come to get '1.05' !?  but I see now that it's the decimal equivalent of 100:5 ........ I certainly wouldn't of thought of that. But interestingly enough, I have used that % formula before, when I catalysed my Pattern-Coat Primer @ 2%
I think it's a good idea ( particularly for the beginner ) to familiarise themselves with at least two different methods for their calculations, whether by volume or weight, but also to have an understanding of the differences of grams and ratio's as percentages. I will eventually settle on one 'formula' that works for me ( pretty much as you have done, your true and tested method ), but in the meantime I'll use two different methods as I think it's a good way of confirming my math !
It's always a good idea to know exactly how much material you need, given the dimensions of your mould set up box, but it's also a good idea I think to include a little extra ....... just in case ! But that little extra amount of material is always estimated isn't it. So knowing the exact amounts, you then know the quantity of the 'extra' you have left over, or should have left over. But it could also be a disaster in the making whereas, if you're pouring two moulds as I am ........ you could end up with insufficient material for your last mould, even 100 grams could ruin your last mould !
Many thanks guys for taking the time to explain things in such detail, it's all becoming crystal clear to me know  |
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Hanaldo
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Just on the topic of calculating catalyst ratios, thought I would add my formula to the mix in case it helps. As Richie said, there's a thousand ways to work it out and as long as you get the correct answer then none of them are wrong, but this is the way I find the easiest. As you are looking for a percentage, you can simply divide by the ratio given. So if you need an exact quantity, pets use your example of 2000 grams of silicone, then simply divide by 1.05. This gives you the amount of silicone that you need to pour, which in this case is 1905g (1904.76 if you don't like to round up). You can then taken that number from your required amount to work out how much catalyst you need; 2000g-1905g = 95g. In the case of the CS25, 5% isn't a whole lot of wastage so you can just say ok well I need 2000g so I'll just pour 2000g and then add 5% catalyst and end up with 2100g as you did with your calculation. However I find this way easier because you can use it for whatever mix ratio you are given. So say for the VacCast epoxy resin system that you want to use to form your actual moulds, this mix ratio is 100:6 (not a whole lot different to 5% I know, but for the sake of sticking with products you are using!). So if you need 1500g to pour your moulds, you simply do: 1500 divided by 1.06 = 1415g of part A. And then 1500-1415 = 85g of part B. You can also manipulate the formula to solve any errors you make pouring the stuff. So say you need 1500g of the VacCast in total and you already worked out that means you need 1415g of part A, but then you accidentally pour a bit much and end up with 1484g of part A. Now you could try to tip the excess back into the container and end up back at your original amount, or you could reverse the equation and solve for how much hardener you need for your new amount: 1484 multiplied by 1.06 = 1573. 1520-1484 = 89g of Part B.
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TURK
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Group: Forum Members
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Why have my pictures been reduced to the size of a postage stamp !? They were 900 x 600 when I uploaded them. TURK
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