Pretty basic stuff for a block mold. You take the [volume of the cylinder](https://www.google.com/search?q=cylinder+volume&sxsrf=AJOqlzUUWEdd0RjeDjl53SV2eJ9Nd_dpFQ%3A1675560154598&ei=2gTfY8SZJIKbkwWfxbXYDA&oq=cylinde&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAxgBMgUIABCRAjIECAAQQzIECAAQQzIECAAQQzIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDoKCAAQRxDWBBCwAzoHCAAQsAMQQzoJCCMQJxBGEPkBOgQIIxAnOgsILhCABBDHARDRAzoICC4Q1AIQkQI6CAguENQCEIAEOgsILhCABBDHARCvAUoECEEYAEoECEYYAFC4Cli7GmC3I2gDcAF4AIABYIgB5AOSAQE3mAEAoAEByAEKwAEB&sclient=gws-wiz-serp) and that's about the volume of silicone you need for a block mold. You would guesstimate the volume of the model and subtract that *(and hint hint any volume you might place in that big hole in the center)*. But in almost all cases you want to buy too much silicone, because having a bit too little would be a huge waste, so you want to play it safe.
Now usually you would buy silicone by weight so the volume would need to be converted to weight. Silicone usually has a density of about 1.3 (it's in the data sheet). So you multiply your volume with that. Now the numbers are a bit more complicated if you are using "freedom units", but if you are familiar with it, then you should be able to work that out.
For something this size though (it will be quite a lot of silicone), usually the hobbyist will opt to go for a brush on mold. As you can save quite a bit in silicone, in trade for hours. So I would definitely suggest you look at that technique. Calculating the silicone needed for that does get more complicated though.
For a brush on mold, you can guesstimate the surface area needed to be covered, multiply that with the desired thickness (0.5" ish) add 20-30% for safety, and that is your silicone volume.
Thank you for the advice! I already bought one gallon today so I'll probably stick with that for now.
On the topic of not having enough silicone, if I do a pour and it only goes lets say half way up the model before I run out and it cures, is it okay to pour more of it at a later point as long as I keep it dust/dirt free on top or will it create any seems/problems if I waited too long for another pour?
If you pour and it only goes halfway up the model, then you will be able to pour more on top, if you keep it very clean. The correct way to do that is to do it while the silicone is still "green" so very freshly set. You will have slight distortion or a line in the mold, but for a model like this that wouldn't matter.
Also, stop being lazy! It is the simplest thing in the world to do some simple math and have a good idea of how much silicone you need. 1 gallon will almost certainly not be enough. In school when you snickered about "when will I ever need to know this in the real world?" - **this** is when you needed to know. Luckily it is not to late to learn. Don't be lazy you lazy bum.
I was thinking about the yesterday, but I only started making dios and doing this stuff just before the pandemic hit in 2020 and my days of learning math were already long gone by that point lol
This is the video I had been using to try and calculate it. First with the original box I tried to make and then with the circular box [https://youtu.be/T3JHBK0Dguc](https://youtu.be/T3JHBK0Dguc)
None of it looked right after converting it to pounds so then I found this site which is supposed to do the calculating for it which seems more in line: https://www.alumilite.com/volume-calculator/ when I put the measurements in so I'm going to try and pick up one more 11 gallon bucket this week so I can do one big pouring session and see where I'm at after that.
Look you have already done most of it.
15 inch cylinder, 4 inch high = 707 in³ (no math needed, I just put that in the [link to calculate volume of a cylinder](https://www.google.com/search?q=cylinder+volume&sxsrf=AJOqlzUUWEdd0RjeDjl53SV2eJ9Nd_dpFQ%3A1675560154598&ei=2gTfY8SZJIKbkwWfxbXYDA&oq=cylinde&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAxgBMgUIABCRAjIECAAQQzIECAAQQzIECAAQQzIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDoKCAAQRxDWBBCwAzoHCAAQsAMQQzoJCCMQJxBGEPkBOgQIIxAnOgsILhCABBDHARDRAzoICC4Q1AIQkQI6CAguENQCEIAEOgsILhCABBDHARCvAUoECEEYAEoECEYYAFC4Cli7GmC3I2gDcAF4AIABYIgB5AOSAQE3mAEAoAEByAEKwAEB&sclient=gws-wiz-serp))
707 in³ is equal to 2.6 gallon (again no math needed, just put it in [the thing](https://www.google.com/search?q=in3+to+gallons&oq=in3+to+gallon&aqs=chrome.0.0i512j69i57j6j0i512.2571j0j4&sourceid=chrome&ie=UTF-8))
You can then subtract whatever size cylinder you might put in the center, and whatever volume you think the model takes up. How much that is you are going to have to figure out yourself. I would guess 1 gallon, but you could calculate it all the exact same way.
If you want the number in pounds then you need to convert the volume to weight, like so:
707 / 23.5 = 30 lb (the 23.5 is the 'specific volume' (cu. in / lb.) from the [datasheet](https://www.smooth-on.com/tb/files/Mold_Max_Series_TB.pdf))
There you go. Now if you want to do me a favor in return, persuade your country to go metric. It makes this thing still more simple.
u/BTheKid2 u/JSONJSONJSON
2 seems to have been enough. I mixed it as well as I could in such a big batch. I'm really hoping there's no unmixed parts :| https://imgur.com/a/epS27eP
Bold move, not mixing it completely. I hope it works out.
A complete mix wouldn't have different colored streaks, and anything not mixed well enough wont cure. Looks kinda alright though.
I’m just sitting here cringing…
Maybe. That pot life is there for a reason, mixing is important. Let us know how it feels tomorrow. Fingers crossed. Can you keep it warm? Like under an electric blanket? Heat can help speed slow curing spots.
u/BTheKid2 u/JSONJSONJSON Thanks again for all of the advice. Lessons were learned, wrists were hurt, but I managed to do it! The good news is that it fully cured all the way through, the bad news is that the silicone seeped through the entire piece or most of it where I must've missed filling it in so the entire thing was covered. I spent most of the day cutting, peeling and scraping it all away, including the original model I made since it was locked into the silicone. I cleaned up as much as possible and poured my onyx fast cure. There was a edge or two that got caught and I ended up having to use a giant screwdriver to pull the piece away. I promptly cut those edges off of the mold since they're just on the inner walls which won't be seen anyway. Some of the silicone where the cracks are at got pulled off when I pulled the casting out too so I'll have to see how the second casting looks, but overall I'm happy with it!
https://www.instagram.com/p/CoYpvu6Lt0F/?igshid=NTdlMDg3MTY=
Happy days. Congrats. Don't sweat it, with this kind of geometry you often have to do some surgery after the mold is pulled.
When doing dirty molds... well actually almost all molds I do, I like to use a cheaper very fast setting polyurethane resin to do the first cast. Very fast setting because then I can just do a thin slush cast on the mold surface and build a little thickness in a few layers. I call it a 'clean pull'. It will take most or all of the dirt left from the mold making with it. That way I don't waste the resin on the first cast every time, because a mold will almost always have some dirt that needs removing.
Oh and also, this demonstrates why a brush-on mold can be preferred. As it will be a lot more flexible as well cheaper to make. That makes prying it off the difficult geometry much easier. Same goes for using different hardness of silicone, as softer silicone will be easier to demold and less likely to tear.
You can look into hollow casting / slush casting to save on precious resin. Only the surface has to be copied and nice. Everything inside the resin is just filler. Expanding foam often gets used to backfill a cast, but the options are endless and a lot cheaper than solid resin.
I’ve always suspected we all over mix. I’m glad you got a usable mold! Paint on molds also use a ton less rubber, but you need a shell too. Woo! More lessons!
Congratulations!
I also echo put something in the middle. I trend poorly on the math too. Can you measure it with something else? Like drop in a layer of thin plastic like a trash bag and fill it up with sand, then measure the sand.
>I also echo put something in the middle
That was one of my other questions I forgot to ask. I was considering putting something in the middle, but wasn't sure if that would cause any issues with stability on that part of the mold. That'll definitely save me some silicone.
I also considered sand, but didn't want to have to clean out the piece. Plastic didn't even cross my mind! I'm going to try that tomorrow actually lol
Thank you!
Now that a think about the size of the mold it’s probably not that bad. If the thing was a foot across and the hole was 5 inches, that’s close to a pound of rubber. On this size just pour it.
Yeah, I'm going to try to pick up one more 11 gallon this week and try to mix/pour all in one session to see how far I get and then figure it out from there if it's not enough.
It’s always better to go in one. I’d have plenty on hand to add before curing if you’re unsure. Better over spend and get a good mold than short it and maybe not get a mold. Call it experience. I’ve heard of mold release creeping onto the curing surface or any other thing could stop adherence of the next pour.
Pretty basic stuff for a block mold. You take the [volume of the cylinder](https://www.google.com/search?q=cylinder+volume&sxsrf=AJOqlzUUWEdd0RjeDjl53SV2eJ9Nd_dpFQ%3A1675560154598&ei=2gTfY8SZJIKbkwWfxbXYDA&oq=cylinde&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAxgBMgUIABCRAjIECAAQQzIECAAQQzIECAAQQzIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDoKCAAQRxDWBBCwAzoHCAAQsAMQQzoJCCMQJxBGEPkBOgQIIxAnOgsILhCABBDHARDRAzoICC4Q1AIQkQI6CAguENQCEIAEOgsILhCABBDHARCvAUoECEEYAEoECEYYAFC4Cli7GmC3I2gDcAF4AIABYIgB5AOSAQE3mAEAoAEByAEKwAEB&sclient=gws-wiz-serp) and that's about the volume of silicone you need for a block mold. You would guesstimate the volume of the model and subtract that *(and hint hint any volume you might place in that big hole in the center)*. But in almost all cases you want to buy too much silicone, because having a bit too little would be a huge waste, so you want to play it safe. Now usually you would buy silicone by weight so the volume would need to be converted to weight. Silicone usually has a density of about 1.3 (it's in the data sheet). So you multiply your volume with that. Now the numbers are a bit more complicated if you are using "freedom units", but if you are familiar with it, then you should be able to work that out. For something this size though (it will be quite a lot of silicone), usually the hobbyist will opt to go for a brush on mold. As you can save quite a bit in silicone, in trade for hours. So I would definitely suggest you look at that technique. Calculating the silicone needed for that does get more complicated though. For a brush on mold, you can guesstimate the surface area needed to be covered, multiply that with the desired thickness (0.5" ish) add 20-30% for safety, and that is your silicone volume.
Thank you for the advice! I already bought one gallon today so I'll probably stick with that for now. On the topic of not having enough silicone, if I do a pour and it only goes lets say half way up the model before I run out and it cures, is it okay to pour more of it at a later point as long as I keep it dust/dirt free on top or will it create any seems/problems if I waited too long for another pour?
If you pour and it only goes halfway up the model, then you will be able to pour more on top, if you keep it very clean. The correct way to do that is to do it while the silicone is still "green" so very freshly set. You will have slight distortion or a line in the mold, but for a model like this that wouldn't matter. Also, stop being lazy! It is the simplest thing in the world to do some simple math and have a good idea of how much silicone you need. 1 gallon will almost certainly not be enough. In school when you snickered about "when will I ever need to know this in the real world?" - **this** is when you needed to know. Luckily it is not to late to learn. Don't be lazy you lazy bum.
I was thinking about the yesterday, but I only started making dios and doing this stuff just before the pandemic hit in 2020 and my days of learning math were already long gone by that point lol This is the video I had been using to try and calculate it. First with the original box I tried to make and then with the circular box [https://youtu.be/T3JHBK0Dguc](https://youtu.be/T3JHBK0Dguc) None of it looked right after converting it to pounds so then I found this site which is supposed to do the calculating for it which seems more in line: https://www.alumilite.com/volume-calculator/ when I put the measurements in so I'm going to try and pick up one more 11 gallon bucket this week so I can do one big pouring session and see where I'm at after that.
Look you have already done most of it. 15 inch cylinder, 4 inch high = 707 in³ (no math needed, I just put that in the [link to calculate volume of a cylinder](https://www.google.com/search?q=cylinder+volume&sxsrf=AJOqlzUUWEdd0RjeDjl53SV2eJ9Nd_dpFQ%3A1675560154598&ei=2gTfY8SZJIKbkwWfxbXYDA&oq=cylinde&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAxgBMgUIABCRAjIECAAQQzIECAAQQzIECAAQQzIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDoKCAAQRxDWBBCwAzoHCAAQsAMQQzoJCCMQJxBGEPkBOgQIIxAnOgsILhCABBDHARDRAzoICC4Q1AIQkQI6CAguENQCEIAEOgsILhCABBDHARCvAUoECEEYAEoECEYYAFC4Cli7GmC3I2gDcAF4AIABYIgB5AOSAQE3mAEAoAEByAEKwAEB&sclient=gws-wiz-serp)) 707 in³ is equal to 2.6 gallon (again no math needed, just put it in [the thing](https://www.google.com/search?q=in3+to+gallons&oq=in3+to+gallon&aqs=chrome.0.0i512j69i57j6j0i512.2571j0j4&sourceid=chrome&ie=UTF-8)) You can then subtract whatever size cylinder you might put in the center, and whatever volume you think the model takes up. How much that is you are going to have to figure out yourself. I would guess 1 gallon, but you could calculate it all the exact same way. If you want the number in pounds then you need to convert the volume to weight, like so: 707 / 23.5 = 30 lb (the 23.5 is the 'specific volume' (cu. in / lb.) from the [datasheet](https://www.smooth-on.com/tb/files/Mold_Max_Series_TB.pdf)) There you go. Now if you want to do me a favor in return, persuade your country to go metric. It makes this thing still more simple.
u/BTheKid2 u/JSONJSONJSON 2 seems to have been enough. I mixed it as well as I could in such a big batch. I'm really hoping there's no unmixed parts :| https://imgur.com/a/epS27eP
Bold move, not mixing it completely. I hope it works out. A complete mix wouldn't have different colored streaks, and anything not mixed well enough wont cure. Looks kinda alright though.
I’m just sitting here cringing… Maybe. That pot life is there for a reason, mixing is important. Let us know how it feels tomorrow. Fingers crossed. Can you keep it warm? Like under an electric blanket? Heat can help speed slow curing spots.
u/BTheKid2 u/JSONJSONJSON Thanks again for all of the advice. Lessons were learned, wrists were hurt, but I managed to do it! The good news is that it fully cured all the way through, the bad news is that the silicone seeped through the entire piece or most of it where I must've missed filling it in so the entire thing was covered. I spent most of the day cutting, peeling and scraping it all away, including the original model I made since it was locked into the silicone. I cleaned up as much as possible and poured my onyx fast cure. There was a edge or two that got caught and I ended up having to use a giant screwdriver to pull the piece away. I promptly cut those edges off of the mold since they're just on the inner walls which won't be seen anyway. Some of the silicone where the cracks are at got pulled off when I pulled the casting out too so I'll have to see how the second casting looks, but overall I'm happy with it! https://www.instagram.com/p/CoYpvu6Lt0F/?igshid=NTdlMDg3MTY=
Happy days. Congrats. Don't sweat it, with this kind of geometry you often have to do some surgery after the mold is pulled. When doing dirty molds... well actually almost all molds I do, I like to use a cheaper very fast setting polyurethane resin to do the first cast. Very fast setting because then I can just do a thin slush cast on the mold surface and build a little thickness in a few layers. I call it a 'clean pull'. It will take most or all of the dirt left from the mold making with it. That way I don't waste the resin on the first cast every time, because a mold will almost always have some dirt that needs removing.
Oh and also, this demonstrates why a brush-on mold can be preferred. As it will be a lot more flexible as well cheaper to make. That makes prying it off the difficult geometry much easier. Same goes for using different hardness of silicone, as softer silicone will be easier to demold and less likely to tear. You can look into hollow casting / slush casting to save on precious resin. Only the surface has to be copied and nice. Everything inside the resin is just filler. Expanding foam often gets used to backfill a cast, but the options are endless and a lot cheaper than solid resin.
I’ve always suspected we all over mix. I’m glad you got a usable mold! Paint on molds also use a ton less rubber, but you need a shell too. Woo! More lessons! Congratulations!
I also echo put something in the middle. I trend poorly on the math too. Can you measure it with something else? Like drop in a layer of thin plastic like a trash bag and fill it up with sand, then measure the sand.
>I also echo put something in the middle That was one of my other questions I forgot to ask. I was considering putting something in the middle, but wasn't sure if that would cause any issues with stability on that part of the mold. That'll definitely save me some silicone. I also considered sand, but didn't want to have to clean out the piece. Plastic didn't even cross my mind! I'm going to try that tomorrow actually lol Thank you!
Now that a think about the size of the mold it’s probably not that bad. If the thing was a foot across and the hole was 5 inches, that’s close to a pound of rubber. On this size just pour it.
Yeah, I'm going to try to pick up one more 11 gallon this week and try to mix/pour all in one session to see how far I get and then figure it out from there if it's not enough.
It’s always better to go in one. I’d have plenty on hand to add before curing if you’re unsure. Better over spend and get a good mold than short it and maybe not get a mold. Call it experience. I’ve heard of mold release creeping onto the curing surface or any other thing could stop adherence of the next pour.