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Unread 09-16-2013, 03:06 AM   #1
Tomecko
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Looking for best clear resin

Hi! I run an Etsy shop for cosplay resin gems, and have been using Castin' Craft Clear Epoxy Resin for most of my items. However, I noticed that it cures with a faint yellow tint, and was wondering if any other resin casters out there had an opinion on what the clearest 'clear' resin is? I would like to make some truly crystal-clear pieces soon.
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Unread 09-16-2013, 03:30 PM   #2
verdatum
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As far as the best product to use, I'd say it's Polytek's Poly-optic 1411. As long as you don't leave it in direct sunlight for a few decades, it won't tinge yellow or brown the way epoxy and polyester resins do.2 pints is $45, and you'll want to use it within a year, if not sooner.

Even this product on its own isn't perfect. If you cast with this, you stand to get cloudy or bubble-ridden gems.

To get resin trulycrystal clear, you need a pressure pot and an air compressor. You also gotta plug up some of the fittings with caps from the hardware store. After pouring the resin, you put the mold in the pot and screw the lid down tight. You then pump compressed air into the pot until it reaches 80PSI and leave the thing to cure. The pressure will make all the microscopic air bubbles dissolve into the resin and go invisible.
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Unread 11-12-2013, 01:39 AM   #3
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Old thread revive! Finally looking into pressure casting. Does one leave the resin to cure while still inside the pressure tank at 80 PSI, or can you take it out after the bubbles have been eradicated?
For instance, I tried casting with some Alumilite brand resin, and although it mixed up beautiful and bubble-free, it actually formed big hideous bubbles while curing. I imagine that would have to be kept at-pressure (or I did something very wrong).
But epoxy resin, my usual resin, doesn't re-form bubbles while curing, so would it be safe to take it out of the tank and crush the bubbles out of something else?
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Unread 11-12-2013, 11:12 AM   #4
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Yes, you leave the mold in the pot under pressure until it has fully cured. The bubbles don't exactly leave the resin when you use a pressure pot. The pressure compresses some existing bubbles to the point that they dissolve into the resin and prevents already dissolved gas from turning into bubbles during the cure. It's all the same rules as how carbon dioxide is dissolved into soda. When the soda bottle is under pressure, you don't see many bubbles, but when the pressure is released, cavitation points on the bottle cause the CO2 to come out of solution and form bubbles. (Since the soda is a nonviscous liquid, the bubbles rise to the top and escape.) Likewise, resin might look clear when you pour it, but as it begins to cure, the air that is naturally dissolved into it during mixing falls out of solution, and forms bubbles that get locked in place by the hardening resin before the have any chance to rise to the surface and pop.

On the other hand, there is a process called vacuum degassification. In this process, the gasses in the solution expand to the point that they rise to the surface and escape. So once all the gasses have been pulled out, you can take the solution out of the vacuum chamber without the worry of new gasses entering. However, vacuum degassification is mostly used to remove gasses from thicker, slower curing substances; particularly RTV mold rubbers after they've been mixed, but before they've been poured. It isn't a very effective process for getting clear resins.

Also, on review, 80PSI might be a little high. 60PSI is plenty, and 45 PSI is often perfectly fine. The important thing is that you've got a working safety valve on the pot (I think it comes with one but can't recall). The safety valve has a spring that opens up if the pressure gets to high, allowing excess air to escape at a safe rate.
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Unread 11-12-2013, 12:47 PM   #5
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You should only pressure cast molds that have been degassed, otherwise you will get warping issues and/or damage your mold.

EDIT
Smooth-On just posted up a video showing degassing resin a few weeks ago:

http://www.youtube.com/watch?v=l7WRGfnJQaM

Last edited by msleeper : 11-12-2013 at 07:29 PM.
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Unread 11-12-2013, 11:36 PM   #6
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Ah, didn't think about the molds getting pressurized. Ouch. I don't do a degassing on my molds, just the usual tap & shake to let the bubbles rise. Vacuum chambers can get expensive. :P And you definitely can't degas resin while it's in the mold, because of the expansion.

Would a pressure pot help get rid of bubbles trapped against the corner of the mold, too? I'm working on a piece with tons of undercuts (Sailor Saturn's crystal brooch) that would be all too easy to get air bubbles trapped in during the pouring process, even if I put in a couple extra bubble-escape holes.

Thanks for all the advice, guys!
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Unread 11-13-2013, 03:29 AM   #7
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I hadn't thought about that issue. There are a few tricks that can be done.

First, if you don't want to vacuum degas your molds, you can make it using the brush-on method. Brushing thixotropic mold rubber onto your prototype does a heck of a lot for forcing bubbles in a mold to pop. You can render regular silicones into thixotropic ones using special thickeners (such as tin-thix and plat-thix in the Polytek line of products), or you can use universal thickeners such as Cabosil (which is a touch nasty to work with since it's a lightweight powder that flies all over the place).

There are specially formulated mold rubbers that have less of a need to be vacuum degassed. these products tend to be a bit less viscous, and tend to have longer cure times, sometimes as much as 12 hours, giving bubbles plenty of time to rise to the top.

There are tricks where you use....uh...back massagers, like the Hitatchi Magic Wand to vibrate your mold after pouring to encourage the rubber to settle and the air bubbles to rise and escape.

There's also the trick where you put your mold rubber in a wax paper cup and rest it on the edge of a table and put the mold box on the floor. Then punch a hole in the cup such that by the time the rubber touches the mold box, it is the thickness of a hair. When the rubber is thinned out like this, most of the bubbles escape naturally.

tiny bubbles aren't a super big deal in a pressure pot. They collapse, the mold stretches a touch to compensate, worst case, you get a couple deformities that you might need to sand/buff out. Molds would probably only break if you are dealing with large bubbles within a brittle rigid mold.

If you'd like to get into vacuum degassing, and you've already got a pressure pot setup, you aren't too far off from pulling a vacuum. the pot is sturdy enough to serve as a vacuum chamber. To convert mine, I just bought a hunk of acrylic, tapped a 1/4'' NPT hole in the center to run the hose, and wrapped the acrylic/pot joint with vacuum-seal tape (imagine Silly Putty as a big long coil). And depending on the stats on your compressor, you can potentially pull a vacuum by combining the compressor with a device called a "venturi", which is cheap, since it has no moving parts. It doesn't pull as strong of a vacuum as a proper pump, and if your compressor reservoir is small, you might need to get another pressure take, also, it's a touch noisy, but I'm lead to believe it can be used for degassification.

Personally, I just bought my own vacuum pump. It took me months of watching to find a good pump at a reasonable price, and even when you get a good deal, it ain't cheap.

As far as airbubbles in the nooks and crannies of molds, pressure casting it helps a little, but it can't work miracles. That is better dealt with by carving or molding vent-channels into those undercuts so the gas has someplace to escape. You gotta cut them off and sand/polish them away after casting, but with a bit of practice, it becomes pretty quick work. Another trick is to pour the resin into the mold, slosh it and squish around a bit, pour the resin back into the mixing container, and then pour it into the mold a second time. This allows the resin to creep into the corners, and helps to form a skin over the mold, breaking surface tension, so air can more easily flow up and escape through the sprue or primary vent.
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Unread 11-13-2013, 06:29 PM   #8
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Quote:
Originally Posted by Tomecko View Post
Would a pressure pot help get rid of bubbles trapped against the corner of the mold, too? I'm working on a piece with tons of undercuts (Sailor Saturn's crystal brooch) that would be all too easy to get air bubbles trapped in during the pouring process, even if I put in a couple extra bubble-escape holes.
While it isn't exactly what you're talking about, if you are getting bubbles along the surface then you may want to lightly dust the inside face of your mold with talc prior to casting. As verdatum was saying, this helps to break surface tension (especially in smaller areas). You might also want to cut your pour spout in such a way that you can replace it and use it as a plug, that way you can twist and roll and flip the mold to try and get air to float up to the top easier. But again as verdatum said, you probably just need more vents.

If you have a hobby shop that carries molding and casting supplies, see if they have rental vacuum pots. I have one near me that rents them for $25/day and it's totally worth it to rent for 2 days to make properly degassed molds.

Quote:
Originally Posted by verdatum View Post
There are tricks where you use....uh...back massagers, like the Hitatchi Magic Wand
What are you talking about? That is totally their only intended use.
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Unread 11-14-2013, 03:22 AM   #9
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I never even considered 'plug' pour spouts! I've always just made sure my models had at least one flat plane to pour into later. Y'all are geniuses.

Quote:
Originally Posted by verdatum View Post
There are tricks where you use....uh...back massagers, like the Hitatchi Magic Wand to vibrate your mold after pouring to encourage the rubber to settle and the air bubbles to rise and escape.
That seems like just one more very good reason to buy one! (Though I've heard turning a dremel tool to a low setting will do the same thing.)
(For the molds)
(Not for your... back)

I've heard of dusting with talc, but not why I should do it. I can think of a couple of my current molds that could benefit from that. Does it affect surface shine? Most of my items are high-gloss crystals/gems (my shop is here, btw). Not that I can't hit it up with some gloss spraypaint, but it does distort the facets a bit.
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Unread 11-17-2013, 12:43 PM   #10
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Quote:
Originally Posted by Tomecko View Post
I never even considered 'plug' pour spouts! I've always just made sure my models had at least one flat plane to pour into later. Y'all are geniuses.

I've heard of dusting with talc, but not why I should do it. I can think of a couple of my current molds that could benefit from that. Does it affect surface shine? Most of my items are high-gloss crystals/gems (my shop is here, btw). Not that I can't hit it up with some gloss spraypaint, but it does distort the facets a bit.
If you are doing a basic dump box mold, and not any sort of 2 part mold, then you probably don't need an actual pour spout. I could be wrong though. Could you show us a photo of the typical molds you're using?

As far as affecting the surface, that I honestly don't know. My intuition says that yes it will affect the surface, but to what degree and how solveable that is, will be something you'll just have to test with. Do you own a rock tumbler? I bet if the casts do come out with a cloudy surface, you could put them in the tumbler to polish them up and remove the surface talcum. Just make sure you are putting as light of a coat as possible on the surface of the molds.
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Unread 11-17-2013, 02:35 PM   #11
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I just tried pouring into my first talc-dusted mold last night, so I'll see how it affects the surface when I demold it tonight. I kinda think it'll be dull-looking, too. But so far, it does seem to have helped the bubbles clinging to the side of the mold. I've only really had a problem with clingy bubbles in this current batch of resin, which as far as I can tell, is 'newer.' Part B tends to get a yellow color the longer it sits on the shelf, and this one's nearly clear - maybe there is an upside to older resin batches?

As for pour spouts, most of my items have a flatback, so there's a large pouring surface. I've only (successfully) made one two-part mold so far, and it's far from perfect.

Pictures (hello to the tireless mods who approve them):

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Unread 11-18-2013, 02:38 PM   #12
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The only other thing I can suggest is, like was said before, vacuum degas your molds and pressure cast your clear resin. That's really your best bet at this point I think. Instructables has guides on how to build both out of a $100 paint pressure tank.
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