|
Post by agoralyx on Feb 16, 2019 21:41:07 GMT -5
Hello!
I have a kind of weird project, and a number of questions about how best to go about tackling it. I've never done any mold making or casting before.
I'll be making a clear sword prop out of plexiglass. I'd like to make a mold of it so that I can reproduce the shape with relative ease, while still keeping it clear. The sword is roughly 61" long, average 1.5" wide, but 8" wide at the widest part, and .5" thick at it's thickest.
I'm assuming doing a two part mold would be the best way to mold it? What would be the best kind of silicone to use? Are there any kind of interactions between plexiglass and silicone I need to account for? I don't have access to a degassing chamber, could I use one of the kitchen vacuum sealers?
Also, what kind of resin (or other material) would be the best to use? I want it to be as clear as possible, either out of the mold, or able to be polished (I understand that epoxy can't be buffed like other types of resin, while otherwise being easier to use?) I want it to be hard, but not brittle to the point of being fragile.
I don't need to mess with resin in the immediate future, but I want to make a mold relatively quickly, because the plexiglass original is needed for a prop.
Any insights are appreciated!
|
|
|
Post by notoes on Feb 17, 2019 7:51:55 GMT -5
That's a big project, be aware it's going to be a lot of work and expense. That said, you've come to the right place for advice.
First you will need to build a mold box to hold the silicone while it cures. Anything from wood to stiff cardboard can be used. For that size, I'd suggest wood just cause it's easy for me to get ahold of in large sizes. Be sure to seal the seams so you don't have silicone leaking out. Any tube caulk would work or even a large piece of plastic sheeting if you can find one that reaches up all the sides without any seams. Be sure to do it in two parts with "registration keys" - I've used bolts to make these. And be sure the first half is completely cured first before doing the second. Also be sure to put a "release agent" on the first half before pouring the second or you'll have to cut them apart and you might mess up the mold doing that. Plus be sure to add "vents" for any trapped air to get out for when you pour the copy.
Yes a two part mold would be the best for you. A liquid silicone would give you the best detailing as they pick up the tiniest details. I'd go with one of the HS (High Strength) series, either 2 or 1 would be good for that size and stiffness needed for it. You should have no interactions between it and plexiglass. I don't have a degassing chamber either and I have no problem without it. No definitely do not use a kitchen vacuum sealer - they won't work for this as you can't do the pressure fluctuations needed to mimic a degassing chamber.
Any of the clear resins would be good, just be sure you have "vents" already in your mold for air to escape or you'll get bubbles. Also follow the directions as exactly as possible - mix slow and steady for the entire mix time and pour it slow and thin into the mold. Take your time and you will have less chance of having bubbles in the casting.
I hope this helps and feel free to ask if you need more help.
|
|
|
Post by agoralyx on Feb 17, 2019 16:44:57 GMT -5
The goal is to have something that is reproducible and of a quality that is high enough to be worth selling, so I'm counting the cost as an upfront investment. I want to do it as correctly and finely as possible, given the utilities I have access to. If I've done my math correctly, I'll be needing 250.12 cubic inches of silicone, if I keep an inch border on all sides. If I'm reading the calculation page correctly I multiply that by 21g to get 5,252.52g divide by 453g/lb to get 11.6 lbs of silicone? Do I need to do anything to seal the wood other than at the seams so that moisture doesn't mess with curing? Or is that only an issue for the platinum cure? I've read that vibrations can also help get rid of air bubbles, would putting the mold on the dryer during curing be sufficient? Unnecessary?
|
|
|
Post by agoralyx on Feb 17, 2019 16:55:13 GMT -5
For reference, this is what I'm working on, and what I am planning on the mold box being shaped like. Any feedback is welcome and appreciated.
|
|
|
Post by notoes on Feb 18, 2019 20:37:19 GMT -5
I'm not sure about the calculation, I don't normally have to do much math as most of my molds are small but if that's what the calculation page said then it should be right. But I'd go a bit over that to be sure I had enough silicone. And that 1" around the sides sounds good though I would go closer to 2" around since it's going to be a bit big. You also might want to keep the wood box for when you pour the copy. At that size, it never hurts to have extra support plus if you have to store it for a while, a dedicated mold box would help keep it in the proper shape and you could be sure that nothing heavy is put on top of it that might warp the shape.
Yes platinum cures are a little "picky" but the tin cures like the HS series don't have the same problem. I've poured HS over a clay casting with no problem and some clays would give the Plat series a problem - case in point, don't use the Flex resin with Plat 25 or you will have a big mess (learned that the hard way) that will never cure. But as long as you use one of the tin cures, you shouldn't need to seal the wood except to prevent leaks at the seams. I've used wood to make vent holes and never had a problem with curing around them.
Yes a dryer would work for something of that size but it's only needed for the open time and once that is up then there's no need for extra vibration. If you pour from a high height (6-12 inches is good) and pour slowly in a thin stream (think the size of a paper clip wire diameter) then you shouldn't have any problems with bubbles even without any vibration.
|
|
|
Post by agoralyx on Feb 19, 2019 14:58:36 GMT -5
Would storing the mold with the negative space filled help it retain its shape too? Or would that not make much of a difference as long as it's protected externally?
Thank you so much for all of your insight, I really appreciate it.
|
|
|
Post by notoes on Feb 21, 2019 8:28:12 GMT -5
Actually if you can do that, it would probably help too. Just be sure the filler isn't heavy in case it tries to deform the bottom of the mold inside.
You're welcome!
|
|