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jzawacki
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Joined: 12 Jan 2007
Posts: 70
Location: Illinois
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 Posted: Wed Jan 24, 2007 3:45 pm Post subject: Single Hole Design? |
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Ok.. so after reading all this info on single hole vs. many hole, I want to make sure I better understand exactly what that would look like.. Obviously the many hole is easy to describe as it is has a pegboard look basically on top of a single hole.
So, is this what a single hole design might look like?
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tubachris85x
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Joined: 15 Jan 2007
Posts: 351
Location: Ft. Lauderdale, Florida
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| Posted: Wed Jan 24, 2007 3:47 pm Post subject: |
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Hmm.....that a unique design, I just figured you put a holw in the middle and thats it.
-tubachris
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jegner
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Joined: 30 May 2003
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Location: Texas, USA
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| Posted: Wed Jan 24, 2007 4:50 pm Post subject: |
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| That is one way to do it. Another would be to router a grid, say 1 inch apart. That way there would not be any area that could not get air pulled under. |
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jzawacki
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Joined: 12 Jan 2007
Posts: 70
Location: Illinois
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| Posted: Wed Jan 24, 2007 5:39 pm Post subject: |
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Ahh.. makes sense.. so like this.
But what.. 1" apart, and how deep/wide should the grooves be, 1/8" you think?
Edit: Now that I look at it more, it's beginning to "look" more like a many hole design. I can see where the benefit would be because you really have more paths for the air to travel when you compare it to a many hole design and your molds covering some of the holes. No matter where your mold is, it doesn't block any of the paths.. I like it! |
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jegner
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Joined: 30 May 2003
Posts: 2144
Location: Texas, USA
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| Posted: Wed Jan 24, 2007 5:58 pm Post subject: |
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Thats the design I had in my mind. Yes, just 1/8th inch deep, saw or router blade width.
Might even be better than the TJ platen! |
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drcrash
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Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Wed Jan 24, 2007 8:20 pm Post subject: |
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| tubachris85x wrote: | | Hmm.....that a unique design, I just figured you put a holw in the middle and thats it. |
That's generally what people actually do, and it works fine as long as your mold is up on spacers at least 1/8" thick. Then there's plenty of room under the mold for the air to get through and out the hole.
The radial grooves would be convenient, if you have enough of them, because then you could put things down flat without blocking the pathways to the hole. The grooves would act as little pipes radiating out from the hole and sucking air from around the mold.
To do that, though, you need a fair number of grooves, or fairly large grooves, right around the hole, or you'll have a bottleneck.
Suppose you've determined that a 3/4" pipe is sufficient for your platen size and the kinds of stuff you'll be doing. (That's usually a good ballpark figure for a 24" square platen.)
A 3/4" round pipe has a cross-sectional area of about 1/2". To avoid a bottleneck, the grooves radiating out from the hole should have at least that much cross sectional area, too. So for example, if they're only 1/8" wide and 1/8" deep, that's 1/64 of a square inch per little tube. So you need 32 of them to have the same capacity as your pipe, and not be a bottleneck.
If you make them 1/4" deep by 1/8" wide, you only need 16 of them, and if you make them 1/4" x 1/4", you only need 8.
I'd actually double that, just to make sure that the radial grooves are not near being the limiting factor and the air pressure is very equalized.
The way I'd do it, the edge of the hole should be mostly the ends of grooves that terminate at the hole, with a little wood or MDF or whatever left to provide support whatever goes over that. (A mold, or a vacuum distribution layer like a screen or something.)
In addition to the radial grooves, you want something that allows air to flow in other directions, so that it can find its way around to the other side of the hole. If you don't have that, then you need more and/or bigger radial grooves, because sometimes you may have to suck air mostly from one side and not the other. (E.g., if you have a tall mold on one side, and short ones on the other.) Having paths that hop across grooves lets air bypass a bottleneck on one side of the hole, find its way around, and get to the hole through other radial grooves, maybe even on the opposite side of the hole.
I wouldn't build all that into the platen, though. I'd only put the radial grooves in the platen, and I'd only make them extend about half way from the hole to the edges.
Then if you put some kind of porous-layer spacer on top, like windowscreen only bigger, air can flow through the porous space between the plastic and the platen and get to a groove.
If that porous layer is 1/8" thick, and 50 percent porous, it's equivalent to a 1/16" gap, and will provide a lot of air channel capacity. It's like a thin but very wide pipe that meets up with the radial grooves... an 8-inch wide swath of 1/16" gap has a cross section of of 1/2 square inch, like our pipe.
That's enough everywhere on the platen except right around the hole, where the air has to narrow down through less than 8 inches as it converges to the hole---and that's where the radial grooves take over to keep there from being a bottleneck there.
Doug Walsh had this almost right when he wrote his book, and recommended a one-hole platen with windowscreen on top to sit the molds on. He underestimated how thick the porous layer that provides would need to be, especially right around the hole, to avoid having a bottleneck.
He realized that eventually, and edited his book a little bit. Right at the end he throws in something saying that you need to put your molds on spacers, because the windowscreen isn't enough.
If the windowscreen it was a little thicker---say three layers of windowscreen, oriented differently so that they can't just settle into each other---and you had radial grooves right around the hole, it would be enough. You wouldn't need spacers.
Windowscreen is cool---better than a routed mesh of grooves---because it effectively provides a zillion tiny air channels right around the edge of the mold. (All the little gaps that you get because the windowscreen is not flat.) So even if you do rout a mesh into the platen, you likely want a layer of windowscreen too. I'd say just use the radial grooves and some thick kind of screen, and that's all you need.
One reason for that is that it's more flexible. If the platen itself only has the radial grooves, and they only go out far enough to prevent a bottleneck around the hole, you can tape stuff down to the platen and make a seal, as long as it's bigger than the radially-grooved area. Just take the windowscreen-like stuff off, and you have a flat platen except in the middle.
Then you can tape down a sheet with a smaller gasket on it, and put another, smaller windowscreenlike porous-layer thing in the middle of that area.
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Wed Jan 24, 2007 8:40 pm Post subject: |
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Here's my high-tech graphic of what I'm suggesting:
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| \ ----- / |
| / one \ |
| -------| big |------- |
| \ hole / |
| / ----- \ |
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+--------------------------------------------------------------+
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Not to scale, with regard to hole size. And there'd be about 2x as many radial grooves, and windowscreenish stuff over most of it.
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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jzawacki
Novice
Joined: 12 Jan 2007
Posts: 70
Location: Illinois
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| Posted: Wed Jan 24, 2007 11:32 pm Post subject: |
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Ok.. I guess the plain flat makes just as much sense with standoffs just a little smaller than your mold would be good as well.. Anyway, based off your details I drew these up real quick:
With screen:
and with screen and baffle (or whatever it would be called if you wanted to use a smaller piece of plastic:
So basically.. this thread was a waste.. Sorry.. But, I better understand how it all works if that matters..  I'm actually thinking I will just use a plain blank board with a single hole in it and just use spacers (are pennies a good idea?) and attach them to the bottom of my molds. That way I don't have to worry about forgetting to put them down. As my projects will mostly be rectangles anyway, I could probably get away with just the 4 corners? Oh, and I don't plan on having solid molds.. would that be a problem? |
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jegner
Site Admin
Joined: 30 May 2003
Posts: 2144
Location: Texas, USA
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| Posted: Thu Jan 25, 2007 7:14 am Post subject: |
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I don't really see this thread as a waste, some good ideas came up here, and I think the grid option is a good one, easy to cut, and effective, but yes, pennies or washers will do fine to raise your molds. I guess it all depends on how much time you want to spend making a platen.
The TJ version is rugged, and effective but a bit more complex to make. |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Thu Jan 25, 2007 8:36 am Post subject: |
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BTW, if you want to plop molds right down on a TJ platen without worrying about spacers, having a piece of aluminum windowscreen over most of the middle of is probably a good idea. That way, the holes under the mold can't get completely blocked, and there are tiny air channels all around the edge of the mold, rather than just at particular points near the edge where there are holes.
I'd think one layer of regular window screen would be fine, given all the little holes. You just need to avoid having the mold sit absolutely flat over them. For the basic pulldown, you'll get plenty of air flow from the holes not under the mold, and after that, when those are covered by pulled-down plastic, you don't need a lot of air flow. The porous layer provided by the screen will be enough to keep the holes under the mold working, to get detail.
Paul
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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drcrash
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Joined: 04 Sep 2006
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Location: Austin, Texas
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| Posted: Thu Jan 25, 2007 9:42 am Post subject: |
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| jzawacki wrote: |
So basically.. this thread was a waste..
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I don't think so.
| Quote: | | Sorry.. But, I better understand how it all works if that matters.. |
Right. It even made me think of a new design, which is easy to make for people without a router---just a drill and a saw. Cool. Most people don't have routers.
I'm actually thinking I will just use a plain blank board with a single hole in it and just use spacers (are pennies a good idea?) and attach them to the bottom of my molds.
I'd use nickels, or something else thicker than pennies.
One good option is hunks of paint stir sticks, which they give away free at the hardware store. They're about 1/8" thick, which is about right.
If you clamp several of them together, they're easy to cut up with a saw. I have a variety of lengths lying around for mold supports, in a plastic cup near my platen. You could glue them to the bottom of molds.
| Quote: | | Oh, and I don't plan on having solid molds.. would that be a problem? |
Yes, if your molds aren't strong. Even a good vacuum cleaner pulls about 2 pounds per square inch. That's 100 lbs of force over (say) a 5 x 10 inch rectangular part. And if you have high vac, it'll be something like 500 lbs. Over a torso, it's over a ton.
Also, don't scrimp on the supports. If you put a solid plaster face cast on 4 nickels, for example, with a several-inch span between the nickels, you'll likely break it. And make sure your mold sits reasonably flat on the supports, flat on the table, so that it's not just supported mainly by a couple of points and stressed in between.
I usually fill my hollow molds in with something reasonably strong, like water putty reinforced with drywall tape. (1/4' yellow 2" wide fiberglass mesh, that comes in a roll at the hardware store.)
When you do that, though, you should think about venting your mold. If your mold has concavities, it likely needs a few vent holes. (E.g., for a face, around the nostrils and they inside corners of the eyes.) Otherwise the plastic may seal over those concavities, and the air has no way out.
Drilling tiny holes through a solid mold is a pain in the ass. (Plaster eats drill bits.) So it's better to drill the tiny vent holes while it's thin, and preserve them by putting straws or something from there when you pour in the filler. That way you don't end up trying to drill little holes from front side with piano wire, and bigger holes from the back, and trying to get them to meet in the middle...
(By the way, water putty expands a tiny bit when it sets. It can crack an already-set thin-shell mold if you put it in all at once. Oops.)
Even better is to cast the vent holes right in. If you're casting your buck, and you can guess where you need them, you can insert little greased wires into the not-quite-set plaster or water putty or bondo or whatever. Then after it sets, you can just slide them out and, voila, you've got tiny vents without any trouble. (It helps to make sure you don't bend the wires while filling in the mold.)
Paul
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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badger
Novice
Joined: 03 Sep 2006
Posts: 63
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| Posted: Thu Jan 25, 2007 11:53 am Post subject: |
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I think one thing I got out of this discussion (not a waste at all), was the concept of "what and how many objects to you pull at a time?
If you usually do one big mold in the middle, single hole and spacers (pennies are cheap. heh heh) to give it some space under is probably just fine.
If you have a larger mold, and generally do several small molds, lots of holes, or grooves of some kind would be a good idea.
This backs up my experience so far with my single hole design. No grooves or screen on mine. When I pull the pilot helmet or rebel trooper helmet, no problem. but when I did some accessories for the chin cups, and comm box (five of each) I had to put some 1x2 scrap down to raise it all up for air flow.
I might have to get me some screen now to help out with that. What type is generally used? Hrm. or maybe I should just suck it up and build a new former, one I can use in the garage. Oh man, I don't need more projects.
badger |
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ANH trooper
Master
Joined: 20 Oct 2005
Posts: 305
Location: UK
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| Posted: Thu Feb 01, 2007 11:30 am Post subject: |
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I use a sinlge hole platen and get very good results. If you use very small spacers under your mould, when you suck the hot plastic down it will try sucking the plastic under your mould resulting in a much sharper and tighter pull. Spacers any bigger than 2mm and the plastic will pull under and blow a hole it in. I use this method only because I don't have a high vacuum set up, and would more than likely use the peg board platen then. Using a peg board with a shop vac dramatically loses power from my experience and pulls are not too sharp. If I want to pull multiple parts, I swap my 24"x24" platen for a 12"x24" and use a pegboard and butt up parts when I can or space them out evenly. I get better results using the smaller platen and not trying to pull too many parts at once. I use .100 HIPS and my formings are as sharp as any high vac that I have seen posted.
Hope that helped
-Paul. |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Thu Feb 01, 2007 12:08 pm Post subject: |
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| ANH trooper wrote: |
Using a peg board with a shop vac dramatically loses power from my experience and pulls are not too sharp. |
I'm trying to figure out how it could make any difference, if your mold is on spacers and there's no bottleneck.
One thing I noticed when I had crummy gaskets was that I got better pulls for the same mold with a larger piece of plastic.
I guessed then---and I still think I believe it---that I was losing air around the edge, and that with a larger piece of plastic, I got a better seal when the plastic sucked down flat to the board around the edges. With the smaller frame and gasket, the plastic didn't pull all the way down to the board in a couple of places. (It pulled down past the buck to the riser level, but not quite to the board.)
(I've made better gaskets since then, but I haven't done controlled experments to validate my guesswork about this.)
In that kind of situation, where the edge seal is not very good, you can lose power by sucking in air around the edges. Having many holes, including holes very near the edges, may make it worse, because the plastic may not suck down flat there and make a better seal---it'll just suck air from the outside through leaks in the gasket seal instead.
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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ANH trooper
Master
Joined: 20 Oct 2005
Posts: 305
Location: UK
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| Posted: Sat Feb 03, 2007 5:08 pm Post subject: |
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| The way I see it is that the peg board design spreads out the suction and loses power. One big hole in the middle gives you the full power of your vac and I get awesome tight pulls where the plastic is trying to pull under the moulds with no need for big spacers which I see many using on here and still not getting very tight pulls. I have tried the pegboard design and haven't had any good pulls really with the pulls being very weak but as soon as I use the single hole, much better. The only draw back is forming multiple moulds can be tricky but not impossible. The hole must be covered with a mould or the plastic will blow out instantly and ruin the pull. |
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