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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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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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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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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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TURK
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Group: Forum Members
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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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Visits: 662
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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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Group: Forum Members
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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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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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Warren (Staff)
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Hi Turk,
Glad the moulds came out ok. With densities and volume, you will find in most cases, the densities are listed as a range of figures rather than one set figure to account for batch variation. Also remember that the relationship between volume and density is effected by temperature which could account for you being slightly under in quantity.
CS25 is quite a soft silicone and for most people this is ideal, although often for far more complicated shapes than you are doing where the flexibility is great to get the parts out. Our AS40 Addition cure is a fair bit firmer at Shore A 40, but is also a bit more expensive. Realistically for a simple casting I think you will be fine.
What altitude are you at? sometimes that can effect the gauges a little. If you're not especially high, then it sounds like your gauge has had a heavy knock to be off centre like that. If that is the case, contact me or any of our staff and we will happily replace the defective gauge.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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TURK
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Group: Forum Members
Posts: 101,
Visits: 662
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Hi guys, Thanks for those kind words guys ........ yeah, really happy with the way it turned out, resulting in two immaculate silicone moulds
After only having one attempt at mixing and pouring RTV silicone prior to these ( the outdated OOMOO 30 fiasco ), preparing and mixing just over 4Kg of silicone can be a stressful experience for a novice such as myself. What calms the situation is the knowledge that the CS25 has a pot life of around 60 minutes, so there's no need to panic as you would probably would do with other silicones with shorter pot lifes. Things did almost go pear-shaped immediately though ....... the scales I bought from EC, really aren't up to the job I don't think. Not EC's fault as they didn't design them, who in his right mind would design a product like that, that are an 'Auto Shut-OFF' system !? I was midway through pouring the silicone into my weighing bucket when the scales turned themselves OFF ! then when I tried to re-weigh again, to the point where the scales shut down, I got different readings !  everytime I re-weighed, I got different weights !
I have bought myself some decent postal scales made by Steinberg accurate to 0.5 of a gram, but I won't be receiving them from Germany till the end of February ! and they're mains powered 
I happened to mention that ideally I would of liked a silicone with a higher Shore A rating, purely because of the situation that can be observed in the picture below. The cured silicone mould on the left was the one I had just completed ........ the completed silicone mould on the right however, is the one I had made the previous day. Already you can see that the mould' wall is moving away from the mould set up box under it's own weight. I think because the silicone is quite flexible, thus soft, it's 'falling' inward. A firmer silicone would undoubtedly be able to support itself. It's not a major disaster, and I think the only remedy would be to tape the side of the silicone to the mould set up box wall to give more support >>
Thanks Warren for the info ...... I've come to the conclusion that the gauge could be faulty also. I understood that by piercing the bung would in effect 'set' the gauge to the atmospheric pressure that I was working at ( on average 450m above sea level where I am ). But even after a couple of days it still hadn't set itself to zero, so then I removed the rubber bung for an entire day ........ nothing really happened so I replaced the bung. Now that I've used the system about a dozen times, it's still doing the same. After about 3 minutes of the vacuum degassing process, the needle goes way past the -30 Hg mark ! As for the air bubble in the gauge itself, it's the entire top third of the gauge ......... I'm by no means an expert with these things, but I suspect that' there's something wrong here
As with anything in life, particularly work related, we all develop our own styles of doing things, I'm still felling my way around this business, learning the processes and the different materials. I don't leave things to chance I research everything ...... Am I dissatisfied with CS25 ? ...... no, not all, I'm very happy with it and believe I will continue using it on this project. I did look at the AS40 Addition Cure silicone originally, and will undoubtedly be trying it out in the future, as I now have the correct equipment. But for the time being I'm very happy using CS25 on my headlight project.
Next up ............ casting 'Vac-Cast Epoxy Resin' |
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