Unique Acrylic Dome

[Steve_01]

I have a very unique clock that needs an acrylic dome. The old dome was blown glass and broke many years ago. I decided that instead of a new glass dome (which are extremely expensive) I would go with an acrylic dome. I have since spent the past 6 months trying to find someone who will do it and have had no luck. Every single company that I have called has told me that this is too hard to do. Anyway, I've come to the conclusion that I'll probably have to do it myself which is fine, as long as I know exactly what I'm doing...and forming acrylic is a little foreign to me.

I've posted a couple pictures of the dome that I designed in AutoCAD. The first is semi-transparent (to show the inside of the base) and the second is solid. The base of the dome is 13.5" wide and 8" front to back. It stands 24" tall but it starts to curve inward at 18" (for the actual dome part).

I'm starting to think that maybe a silicon rubber mold in which to pour liquid acrylic, might work best because of the size of the dome. The vacuum forming method just seems a little too hard for the size....but I don't know for sure since I have no experience. Any help would be greatly appreciated!

Steve

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[scratch_6057]

Steve,

You MIGHT want to check with the folks who frequent the
Casting Group at YAHOO. If your interested http://groups.yahoo.com/group/casting/
I don’t know what your chances are of working your project in acrylic but many clear
resins are available and with careful vacuuming of the mixed resin (to remove entrapped air)
and pressurizing the casting clear and bubble free castings have been made.
You might find someone on the YAHOO list willing to give you a quote on doing the project for you.

You might also want to contact Alumilite Corp. Kalamazoo, MI 269 488-4000
http://www.alumilite.com/ They sell casting materials and could advise you,
they also do some contract work.

There are LOTS of resins available Alumilite just happens to be what I found the easiest for
the work I do (I have never cast any clear projects) and they have people who are willing
to talk to you instead of just SELL you something.

[Steve_01]

Thanks for the help!

I tried to click the yahoo link and it wouldn't work but I'll try to look it up under yahoo's website. I'll also call up alumilite to see if they might be able to help me out.

I was wondering though if you could just pour the resin into a mold and try to vibrate the bubbles to the surface? That way I could probably do it myself. I'll reasearch it first though to make sure. Any other suggestions would be greatly appreciated. Thanks

[Geof]

I was wondering though if you could just pour the resin into a mold and try to vibrate the bubbles to the surface?

This is a valid procedure provided your resin is not too viscous and cures slowly but your biggest problem I think will be the mold. You will need a mold for the outside shape and a plug for the inside because you want to cast a thin walled shell. The plug will need an optically smooth finish because polishing the inside would be very difficult. Also the plug will need to be multipart so that it can be collapsed inside the cast shape and be removed. Similarly the mold will need to be optically finished and it will need to be in two parts so you can separate them to remove the part. If you can accept a fairly noticeable taper to the part you might get away with a one piece mold and plug but you will still need the optical finish.

[Steve_01]

This is a valid procedure provided your resin is not too viscous and cures slowly but your biggest problem I think will be the mold. You will need a mold for the outside shape and a plug for the inside because you want to cast a thin walled shell. The plug will need an optically smooth finish because polishing the inside would be very difficult. Also the plug will need to be multipart so that it can be collapsed inside the cast shape and be removed. Similarly the mold will need to be optically finished and it will need to be in two parts so you can separate them to remove the part. If you can accept a fairly noticeable taper to the part you might get away with a one piece mold and plug but you will still need the optical finish.

I was thinking about creating a plaster model for both molds. I was gonna spend a lot of time on them, sanding them, to make sure they were as smooth as possible. Would this provide an optical finish? I was then gonna create a negative (or a mold) from each plaster model from silicon rubber (or some other similar material). From there I was thinking about trying to leave each mold as one piece, suspending the plug mold into the outer mold while the resin cures. Once cured I was thinking about trying to blow compressed air inbetween the mold and the resin to try and release it. Basically I was thinking about creating a flat straw type thing that could slide down the inside to then introduce air as far down as possible. It's just a thought, but I really don't want to deal with a seam that you would probably get with a 2 piece mold. Worst case, I would end up having to cut a slice around the silicon rubber to release it, thus making it a 2 piece mold and then if I had to remold (because of air bubbles or whatever) I would just put the 2 pieces back together as close as possible and then deal with the seam. As for the plug mold, I was also thinking about building a hollow wooden box that fits inside the silicon rubber (it would be put in place when I mold the silicon). I could then disassemble the box and fold the silicon walls in to the middle to help release it....or I could use compressed air again, if it works?

This is what I was thinking, but I've never molded resin or worked with plastics before. I work with wood and metal all the time but no plastic, so any thoughts, concerns or criticisms would be appreciated.

Thanks,

[tobyaxis]

I'm Not a Mold Specialist of any kind or a Master Machinist for that matter, but I would think that making a plaster mold may create a lot of problems for you. The first issue is that plaster to my knowlege is like sand paper when it dries. Second, if you sand it down you will loose your shape to a degree because plaster isn't that wear resistant. Not to mention that if you have to make a two piece mold the realignment would be a real pain without dowel pins. I'm thinking aluminum or steel for a mold material.

You may try calling a place that does Glass Blowing. Corning, New York has a place that blows glass. I have no idea what the name of the company is or if they are still around. It may even be a museium by now.

I hope you find what you need. It looks like a beautiful clock.

[Geof]

Steve01; The procedure you describe will be tedious but has a good chance of working. The comment by tobyaxis about plaster not be smooth is correct and this is where you will have the greatest challenge. You might be able to coat the plaster with something like gelcoat. I think that is the name and I may not have the details correct but I think it is used in fiberglassing to put a high gloss finish on the mold which then transfers to the part. Sealing the porosity in the plaster before doing the final coating may be difficult and you will have to do some experimenting. The silicone mold rubber; two part RTV silicone I guess you mean, will replicate the surface on your plaster mold very accurately but whether it will truly replicate an optically smooth surface I don't know. I have used it but only with opaque resins and the resin surface come off glossy that I know. However, I am pretty sure that you are going to need a vacuum system for the RTV. You need to pull a vacuum on this after you have mixed it but before you pour it to thoroughly degass it. If you don't there will be bubbles adjacent to the the plaster mold surface. Again you will have to experiment; you need a good vacuum down to about 25" Hg and the silicone will foam up to several times its volume as the gas comes out. Make sure your containers are way oversized otherwise you get a horrible mess (experience talking). Your compressed air idea for releasing the the cast resin from the silicone sounds workable. For the silicone plug I would try making this as a 1/2" layer of silicone over a smaller replica of the shape in plaster with the same good finish (a modification of your wooden box idea that should be reusable). A plaster plug within a silicone plug sort of thing. After making this two part plug separate them and then reassemble them with a thin layer of silicone lubricant. This way after casting the resin you can withdraw the inner piece and then peel the silicone plug out with your air idea.

As a final comment because I have run on far too long I suggest you try to make some contacts in the preservation department in a big museum. What you are trying to do is similar to what museums do to restore things.

[replicapro]

I cast clear resins all the time. You can not get a optical clear casting unless the casting is cured under 60 psi of pressure in a pressure chamber. There will be air bubbles trapped inside other wise.

If you machine your master in metal and buff it to a high polish, you can then make a silicon mold with a product called dragon skin. www.smooth-on.com
Then use a painter pot http://www.harborfreight.com/cpi/cta...emnumber=93119
to cure the mold in under 60 to 80 psi.
Then make a core plug for the inside to make the casting hollow. You do not want to have mill the casting hollow, the chances of gettting a glass smooth finish on the inside are 1 in 1000.
Then use crystal clear 202 resin from www.smooth-on.com and cast it under 60-80 psi.
Take a look at this casting I did using that method
http://www.replicaprops.com/images/lananeu.jpg


Vac forming is easier and cheaper in materials, but the equipment is expensive and hard to find.

[sulfide]

Casting plastic in this size will likely produce distracting optical defects.

Probably the cheapest good solution is to vacuume mold two shells, front & back - then solvent-weld them together. With a little care and work the seam should be nearly unnoticable.

If this clock is unique, I wouldl consider going to a stained glass worker and have them assemble a dome out of flat and curved glass elements. Many glass workers can create curved pannels up to 12x12 by slump heating in a kiln. Sort of like vacuume casting except it uses heat and gravity.

Best of luck!

[scratch_6057]

Well I don't know what it is but I've had the problem before,
when I put in links (esp. Yahoo) they get badly MUNDGED

Yahoo Casting GROUP should be

groups (dot) yahoo (dot) com / group / casting /

Note that's GROUPS not GROUP and the additional (dot) between
GROUPS and YAHOO

[Steve_01]

Whoa!!! I thought I was confused before....Now my head is spinning.

Ok, I now realize that plaster isn't going to work. There's just way too much work invested in it for it to come out good. Geof, I might still consider your gelcoat idea if all else fails, but that's gonna be a pain. After reading all the responses, I think I'm gonna have to go with a metal master, or set of masters. But now we're talking a lot of money. I can't cut aluminum or steel due to the fact that I haven't built my CNC machine yet and even if I did, it would have to be rediculously accurate to cut this master. I'm also not even going to consider glass because the dome needs to be removed about once a week to wind the clock and this is how the old dome broke...it chipped on the lip of the clock base, then cracked, then the crack spread up the side and eventually broke completely. The only way that I'd do it is if I could get it done for cheaper than resin, but that's doubtful. I'm still checking though...

Ok, so this is where I need some help. Please correct me if I am wrong.

1) I get a master carved for each the inside and the outside? Or is it one for the outside and not the inside because that would be almost impossible to get a smooth finish (is that what you were saying replicapro?). Or could it be possible to machine the inner in two pieces that fit together precisely, then pour the silicon material in it to make a plug mold??? I'm guessing that the best results would come from a one piece master for the outside mold and a 2 piece master for the inner that could come apart after the silicon plug was poured into it.

2) I'm shooting for about 1/4" thickness unless that is too thin? The base of the clock has a lip on it that is about 3/8" for the dome to fit on, so I could go to just under 3/8" at most, because I want to leave some clearance for the dome to fit over the inner lip of the base on the clock. By the way the base of the clock is determining the size of this dome since it has to fit exactly on the base.....which is probably another reason why machining a master is the right choice (assuming the price isn't too high).

3) I need to find a pressure pot that will fit a 24" tall dome......and actually it has to fit more than 24" because of the height of the mold as well. The width is 13.5" plus the width of the mold. I'm worried about this as far as cost? Does anyone have any suggestions here?

I want to thank everyone for responding and being a help to me. If this ends up working, you can be sure that I'll post pics as well as full details of the whole process to maybe help somebody else out down the road. Thank you all so much!!!

Steve