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Vacuum Forming, Movie Prop, Sci-fi and GIjOE Forum
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Plasticman
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Joined: 08 Jul 2006
Posts: 109
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 Posted: Mon Jun 04, 2007 7:29 pm Post subject: I think I have too much vacuum!! |
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| I have noticed on molds that are negative.....like say a square that is 3 inches tall by 5 inches wide and has a 1 inch recess in it, that the vacuum will actually seal the sqaure and pull no vacuum in the center. I am hesitant to drill relief holes as I dont want to have them show in my pull, but I dont know what else to do. Has anyone else come across this? How do I deal with TOO MUCH vacuum??? I thought about putting scotchbright or some other pouress type material under the square to keep at least that part from sealing too quickly. Hmmm I dont know what to do. Any ideas? |
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remarque
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Joined: 06 Oct 2006
Posts: 22
Location: The Carolinas
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| Posted: Mon Jun 04, 2007 9:18 pm Post subject: |
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Thats the same problem I had with molds, it would seal around the rim of the mold and wouldnt mold the inside. Once I did cut a couple of holes in my mold I found out my mold needed alot more reinforcements. At 20hg my vacuum crushed two 7 inch high 3 layer fiberglass molds down to about an inch. I didnt think it would be that strong, it almost pulled screws out of one of my wood reinforced molds  |
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drcrash
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Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Mon Jun 04, 2007 9:59 pm Post subject: Re: I think I have too much vacuum!! |
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| Plasticman wrote: | | I have noticed on molds that are negative.....like say a square that is 3 inches tall by 5 inches wide and has a 1 inch recess in it, that the vacuum will actually seal the sqaure and pull no vacuum in the center. I am hesitant to drill relief holes as I dont want to have them show in my pull, but I dont know what else to do. Has anyone else come across this? How do I deal with TOO MUCH vacuum??? I thought about putting scotchbright or some other pouress type material under the square to keep at least that part from sealing too quickly. Hmmm I dont know what to do. Any ideas? |
The safest easy thing to do might be to make an alginate mold and cast a new water putty buck from that, and cast the vent holes in the copy, rather than drilling them. (Using greased skewers or something, that you can pull out later to leave vent holes.)
As far as I know, there's no good substitute for vent holes in serious concavities. If you have porous or textured stuff around the concavities, air MAY seep out of them fast enough... especially if you poke them with a soft-gloved finger during forming, to help squish the air out of the concavity... but it's pretty iffy.
How thick is the plastic you're forming? (The thinner it is, the smaller the vent holes need to be to keep them from showing up in the pull.)
What is the shape of the concavity? Is there a flat place you could put something like felt, over little vent holes, so that the vent holes wouldn't show?
Sometimes you can use reasonably large vent holes, and put something like fine screen over them, and something like felt over that to keep the screen pattern from coming through. The trick is getting all that flush and where you want it.
Details of the concavity shape may make it easy to hide the holes one way or another. Got pictures of your molds?
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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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Plasticman
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Joined: 08 Jul 2006
Posts: 109
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| Posted: Mon Jun 04, 2007 11:30 pm Post subject: |
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| no, no pics. But Its basicly a stormtrooper back plate. The plastic seals around the edges and wont pull in the center. If I drill holes in the mold, I think you will see them. At this point, however, thats my only option. I have tried massaging the plastic as you say, but with bad results. [/i] |
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jegner
Site Admin
Joined: 30 May 2003
Posts: 2144
Location: Texas, USA
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| Posted: Tue Jun 05, 2007 6:32 am Post subject: |
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| For that trooper back plate, just drill 4 holes in the very corners on the inside of the lip, and in the center of the sprocket. 1/8th inch is big enough. On the bottom of the mold on the side that touches the platen, be sure you increase the hole to 1 inch, or larger so that it can get air. |
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Plasticman
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Joined: 08 Jul 2006
Posts: 109
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| Posted: Tue Jun 05, 2007 8:00 am Post subject: |
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| I guess thats what I will have to try. |
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