PDA

View Full Version : air flow through unsealed MDF under vacuum



drcrash
09-06-2007, 11:17 AM
Does anybody know how much air flows through unsealed MDF under vacuum?

My understanding is that it's reasonably high; MDF is pretty porous stuff.

Anybody have an actual data point? (Say, a tank of a certain size filling up at a certain rate just from flow through the bottom of an MDF platen, or vacuum-clamping surface, of a certain size?)

I'm also wondering what's the easiest, cheapest way to seal MDF pretty well. My understanding is that a lot of paints and varnishes don't do a good job---as they dry, they become microscopically porous where the solvent evaporates away. Even a bunch of coats won't help as much as you think. (I've used polyester & epoxy resins, but that's a bit of a pain in the ass, and a little more costly than I'd like.)

mxtras
09-06-2007, 01:11 PM
I have over 10 years experience working with an MDF vacuum table but I have no data points. I do know it flows better when it is skim cut. I know this because I have a commercial router with a gauge in the chamber and when the 1" waste board is not cut, the gauge will read a higher vacuum pressure than when both sides of the board are skimmed. Higher pressure in the chamber when the table is wide open indicates a restricted surface. In theory, a guage below an open table with no restrictions will read next to nothing for vacuum pressure. To me, this is the most desirable condition. Effective distribution is also important.

The best way I have found to seal unused surfaces is with clear plastic packing tape. It's cheap and very effective.

Scott

martinw
09-06-2007, 02:50 PM
I'm also wondering what's the easiest, cheapest way to seal MDF pretty well. My understanding is that a lot of paints and varnishes don't do a good job---as they dry, they become microscopically porous where the solvent evaporates away. Even a bunch of coats won't help as much as you think. (I've used polyester & epoxy resins, but that's a bit of a pain in the ass, and a little more costly than I'd like.)

Dear drcrash,

A really easy way to seal MDF is to mix three parts of water to one part of PVA glue. Just paint on a few coats. You can then paint on some oil based paint if you want (you may not need to, but I did). I used this PVA and oil based paint approach on a "vacuum bag" set up which was about 5 ft by 10 feet on plan. The bottom of the "bag" was a sheet of 18mm MDF. A PVC cover-sheet went on top of the MDF and was sealed to the MDF by rubber gaskets. Without the PVA and paint, I just couldn't get any vacuum at all. With it, it worked a treat.

Best wishes

Martin

kayaker43
09-06-2007, 09:09 PM
I've been meaning to set up some tests, but never find the time. I figure if I stall long enough someone else may do it :rolleyes:

You can see the denser surface layers on a fresh cut edge, it looks like maybe 1/16 deep? The center looks less dense. It varies between brands too, some are darker, harder and heavier.

There are higher densities, sometimes called harboard or tempered hardboard but I've only seen them in thinner sheets. One brand name is masonite. This density may not flow much air at all, need to do tests? McMaster Carr sells the same thing only much harder, its hard like a rock and nearly black, I've used it for making router templates.

There's bound to be some common goop that will seal ordinary MDF, or even particle board for that matter,.. mobile home roof coating or something?

coonhunter
09-07-2007, 03:10 PM
Duct Tape.

drcrash
09-11-2007, 09:49 AM
Thanks all for the suggestions and useful info.

I've got a half-made new platen, and when I get around to finishing it, I may get a chance to do an experiment.

I'm thinking of testing it unsealed, then sealing the bottom (back side) with a couple of coats of sandable primer to see if that has much effect, and if not, some diluted glue. (I'm sure duct tape would work, but it eventually creeps and gets gummy, and maybe peels.)

I'm wondering if the sandable primer might be good enough, if it's a reasonably high-solids product. (As a sandable primer should be.)

kayaker43
09-12-2007, 10:59 PM
I'm pretty sure primers dry porous? As a kid I would start some bodywork on my car and just prime the bare metal till I could finish it. It usually shows rust within a week without a top coat. High solids but I think they're not as fine as regular paint pigment?

Try gloss latex house paint? I peeled a patch off a waxed metal plate and it was like a thick rubbery plastic sheet.

drcrash
09-12-2007, 11:44 PM
I'm pretty sure primers dry porous? As a kid I would start some bodywork on my car and just prime the bare metal till I could finish it. It usually shows rust within a week without a top coat.


Yeah, I got to thinking about that. (That's why my posting about paint that hot plastic won't stick to had a preference for smooth, maybe gloss.)


High solids but I think they're not as fine as regular paint pigment?

Yes... I suspect they dry porous so that whatever you put on them can insinuate itself into the pores and make a good bond.

I don't think it's the pigment that's different, though... and now that I think about it, I don't know if there's not much difference between gloss and matte paint in terms of actual smoothness.

As I understand it the difference between gloss and matte paint is the size of the pigment particles. The ones in gloss paint are so tiny---comparable to a wavelength of light---that light reflects off of them as though they were a solid sheet. The ones in matte paint are really microscopically tiny, too, but not quite that tiny, and light waves bounce off them in random directions. I suspect the difference in the scale of bumpiness doesn't matter for vacuum sealing, because the binder smooths over them at that scale and it's just light that "cares." (The light goes through the transparent binder and is or isn't scattered.)

I could be wrong, though; the bumps might affect the surface and be big enough to make channels for seepage.

Geof
09-12-2007, 11:57 PM
I think practically all paints dry porous. The explanation for this that I got from my Dad who was a painting contractor was that paint has to be porous so that water vapor can pass through. If it is not porous, or not porous enough, that is when you get separation of the paint film and bubbling.

automizer
09-13-2007, 12:15 AM
I have run large table routers 5' by 10' with MDF spoil board , and in a pinch for a complex shape a simple shop garbage bag works really well. They worst part of this design is as soon as your tool breaks threw your product you have now made a leak, Unless your blower motor is huge to compensate for these leaks it will become a nightmare.

drcrash
09-13-2007, 12:48 AM
I think practically all paints dry porous. The explanation for this that I got from my Dad who was a painting contractor was that paint has to be porous so that water vapor can pass through. If it is not porous, or not porous enough, that is when you get separation of the paint film and bubbling.

That sounds familiar, now that you mention it. I read up on paints a few years ago, and I probably knew that.

I wonder if auto paints are the same way? I could imagine not, on the assumption that metal or fiberglass doesn't have any significant moisture content if you paint it when it's dry, and you really don't want moisture getting in if it's steel... but I could imagine it the other way, too... if a little moisture is in there, you want it to get out rather than rusting the metal under the paint.

I've been using an epoxy coating that's mostly sold for sealing furniture. I can imagine that even that is designed to be porous, with just enough solvents that evaporate out after it's set to leave microscopic air channels for wood to breathe. Any sealer designed for wood seems likely to be the same way, unless its designed for continuous immersion in water.

(And even that's not a gimme. It might be designed so that gangs of hand-holding water molecules can't get in, but individual water vapor molecules can get out, and loose gas molecules can get in or out, vaguely like Gore-Tex.)

This is ridiculous. I could always laminate a sheet of plastic or foil to the bottom of a platen, but there's got to be something you can just spray on that makes a reasonably air-tight coating.

My guess is that just primer will slow the air inflitration, by clogging up the big pores in MDF, and that adding a coat of regular will slow it more, by clogging up the medium-sized pores in primer. Then there'll be the tiny pores in housepaint left.

I can imagine that'd cut air inflitration by 99 percent, with each step blocking 90 percent of the air. Or maybe just by 50 percent, with each step not doing much---just replacing big pores with smaller pores, but more of them.

:confused:

drcrash
09-15-2007, 02:48 PM
This is ridiculous. I could always laminate a sheet of plastic or foil to the bottom of a platen, but there's got to be something you can just spray on that makes a reasonably air-tight coating.


Hmmm... now I'm wondering if something like Armor All would do the trick. It's supposed to seal & protect plastics from oxidation (as well as UV). Paint is basically porous plastic that needs sealing to keep air out.

So prime, paint, and coat with Armor All?

martinw
09-15-2007, 08:59 PM
OK, I'm getting boring...

One part PVA glue, and three parts water.

Why not try it?

Add a coat of gloss paint if you want.

BTW, it works

Best wishes,

Martin

drcrash
09-15-2007, 10:26 PM
OK, I'm getting boring...
One part PVA glue, and three parts water.


Sorry, I didn't mean to make it sound like that wasn't a fine solution. (In fact I've passed it along on another forum.)

One of the things that's going on is that I'm also looking for things you can use on the top of the platen, where sometimes they'll have hot plastic against them (and sometimes you'll use tape to cover), so I'm exploring different possibilities. My understanbding is that PVA glue melts at a pretty low temperature, so it might not be a good choice for that.

(On the other hand, Armor All may have the same problem.)

Geof
09-15-2007, 10:27 PM
OK, I'm getting boring......One part PVA glue, and three parts water......

No you are not getting boring: I think it is a Ballpoint/Biro, Scotch tape/Sellotape kind of thing; in other words different terminology. I don't think it is known as PVA glue over here, it tends to go by Tradenames; one is PROBOND which is available from Home Depot. I think another is Elmers or White glue.

lgalla
09-15-2007, 10:45 PM
OK I am confused.:confused: Martin can attest to that.But I agree with Martin.:cheers: PVA or Elymers,or Pro Bond or LePage or bondfast will work.
If polyester or epoxy works,"stick with it"
Arborite or formica will work very well.
I am not sure what you want to seal.A vacuum table is usually a grid with a spoil board of MDF.We seal the edges with iron on PVC edging.
Milling the spoil board increases the vacuum as MDF is pressed and this results in high density skins and porous core.
If you don't want any vacuum to escape a surface,use cheap Melomine.
Larry

kayaker43
09-15-2007, 11:23 PM
I'm pretty sure Armor All is just diluted silicone lubricant with UV additives? It doesn't leave much of anything behind to seal with, but will make sure nothing ever sticks afterward. Probably a good mold release. Cheapo silicone spray does the same job to make rubber and plastic look shiny and new. Belt dressings and tire treatments are other variations of mostly silicone with other stuff. I hate marketing hype!

I learned that all the different GOOP brand adhesives they make are identical but some have UV additives. The prices are all different depending on the intended markets, Houshold, Marine, Automotive, etc..

drcrash
09-16-2007, 12:15 AM
OK I am confused.:confused: Martin can attest to that.But I agree with Martin.:cheers: PVA or Elymers,or Pro Bond or LePage or bondfast will work.
If polyester or epoxy works,"stick with it"
Arborite or formica will work very well.


I'm familiar with PVA glues... just more going on than I made clear. I should probably have made it clearer that I'm trying to solve several different problems, ideally with one solution.



I am not sure what you want to seal.


Some vacuum forming platens. For the bottoms, PVA seems fine. For the tops, heat seems like a problem for PVA. If something works for both, that's even better.



A vacuum table is usually a grid with a spoil board of MDF.


I'm talking about some (somewhat novel) vacuum forming platens.

I've been under the impression that when people were talking about "vacuum tables" and "spoil boards" they were talking about vacuum clamping / work hold-down stuff for CNC machines... but maybe I'm completely wrong about that and I've been missing the fact that people are talking about the same thing I'm trying to do.



We seal the edges with iron on PVC edging.


Could you elaborate on that? Are you talking about a vacuum forming platen edge that you mold hot plastic around to make a seal, or just sealing a board on top of another board?

Either way, what's the iron-on PVC edging you're talking about? I'm not familiar with that... or is it regular countertop or shelf laminate edging stuff?

For my application (or one of my applications) iron-on stuff seems iffy... when it gets hot, it may delaminate.



If you don't want any vacuum to escape a surface,use cheap Melomine.


Can melamine take the heat of hot plastic formed on it?

Geof
09-16-2007, 12:35 AM
.......Can melamine take the heat of hot plastic formed on it?

I can (maybe) answer one question. Yes, provided they are using the name melamine correctly. Melamine is a urea formaldehyde based thermosetting plastic that is rigid and quite heat resistant, it does not soften, melt or depolymerize but at high enough temperatures will char. However, be careful, I have run across some MDF with a coating that was described as melamine but it was flexible and did soften with heat. I think sometimes terminology is use loosely.

automizer
09-16-2007, 01:44 AM
One thing you might want to try is a kitchen counter top and cut threw it in some places to allow air to pass I have placed pots of boiling water on my counter tops with no problems and its a cheep top from homedepot. They sell the stuff in sheets too and with some contact cement it sticks to MDF really well. Try the kitchen stuff its meant to take heavy heat

drcrash
09-16-2007, 07:45 AM
One thing you might want to try is a kitchen counter top and cut threw it in some places to allow air to pass I have placed pots of boiling water on my counter tops with no problems and its a cheep top from homedepot. They sell the stuff in sheets too and with some contact cement it sticks to MDF really well. Try the kitchen stuff its meant to take heavy heat

Thanks for the suggestions.

Doug (kayaker43) has already suggested the countertop stuff to me privately, but ideally I'm looking for something dirt cheap. (MDF plus PVA is good in that respect.)

I will look into the sheets that you contact-cement in place yourself. That may be very useful for a different part of the setup I'm working out, if it's cheap enough.

drcrash
09-16-2007, 10:12 AM
If polyester or epoxy works,"stick with it"

Arborite or formica will work very well.

The polyester and epoxy work okay for the bottom of the platen, but your PVA solution is probably less trouble.

I started with epoxy and polyester so that I could get two different effects---an airtight seal (most important across the bottom) and a very smooth, heat-resistant, durable top surface.

Ideally I'd like an airtight seal across the outer part of the top, too, for reasons that only make sense for my odd designs. Maybe I should explain that. (Sorry if this is long-winded.)

---

I have an interchangeable gasket system for different-sized plastic sheets; each gasket is on a "tape-down sheet" that covers the platen holes outward of the gasket. You tape it down to the platen to mask it down to the size you want, covering the extra holes. The platen holes don't go all the way to the edges, so you don't need a full-sized tape-down sheet to stop down the platen to the size you want---as long as it covers the central region with the holes, you're good, if the top surface around the edges is close to airtight. Here's a simple low-end version of that, using a platen with one big hole in the middle: http://www.instructables.com/id/E8RW98YF3C4XLCQ/ and a video of it in operation: http://www.youtube.com/watch?v=hGBRiYhxRTM

There are several subtleties to get this to work well for high vacuum and many-hole platens, as opposed to low vacuum (vacuum cleaner) and a one-hole platen. The gasket thing itself doesn't make a particularly good seal, but you get a better seal once the plastic sucks down to the platen around the mold, or just inside the gasket. (The gasket only has to work well enough that the plastic sucks down to the platen.)

If there are holes exposed just outside of that (potential) seal, or if the platen isn't smooth, you may never get a good seal. A vacuum cleaner may be able to cope with the leakage, but a cheap vacuum pump just can't do the CFMs to keep up. (I'm aiming for really really cheap, so that's a big issue.)

One solution to this is to make the platen glassy smooth, and ensure that there's a boundary of unholed platen that the the plastic can suck down to and seal (reasonably) well.

A different solution is to make the tape-down sheet extend about an inch inward of the gasket, and let the plastic seal to that instead. (The tape-down sheet is sealed to the platen with tape, so that works, and the platen surface doesn't have to be smooth or well-sealed. It should tolerate having tape applied and removed, though, at least on the outer parts.)

That requires that the tape-down sheet be made of something thin and smooth but heat-resistant, so that the hot plastic can seal to it instead of the platen. (Maybe melamine sheet sold to be laminated to particle board? Is there something cheaper that would work?)

If I can't keep this cheap enough and easy enough to make, it may make more sense to just make different-sized platens and interchange them, at least when you need maximum performance.

That's partly what I do (for myself) now... I have two platens for the sizes I use most frequently, plus an oversized platen for doing the tape-down sheet trick when I want a different size/shape and don't need the absolute best possible seal. (That is, for thin plastic that forms well before the vacuum in my tank runs out due to leakage.)

---

Unfortunately, the polyester laminating resin is a bit too thick to easily make a nice thin coating before it starts to kick and make a mess. (I could likely overcome that with some care, but I want something dead easy to recommend to other people.) It worked fine for platen bottoms, but was too uneven for me to want to try it on tops. (Smooth, but ripply.)

The epoxy I used is a very thin, slow-setting product mostly for furniture (Enviro-Tex brand Pour On (TM) finish) which worked better for the tops.

Unfortunately, I had a problem with small bubbles forming, because the MDF is so porous that the liquidy epoxy soaks in and displaces air, which comes up as bubbles. At first it looks great, but after a few minutes bubbles start to come up... and keep coming up until it sets. You can get rid of most of them by heating it so that the epoxy is less viscous and most of the bubbles spontaneously pop (I just left it in the sun), and you can pop most of the rest by smoothing it with a gloved finger before it sets up much... but there are still some flaws and it's too much trouble.

I think that's going to happen with any liquid coating that doesn't mostly dry fast like a solvent-borne paint or primer, so the MDF needs to be sealed first. (The instructions of the Pour On stuff pretty much say that---"open grained" woods need to be sealed first, to avoid bubbling. I guess MDF would count as "open grained.")

One problem there is that if I seal the MDF with something that can't take a fair bit of heat, that may defeat the purpose of coating it with something that can---if the sealer or primer or whatever degrades, it may start to delaminate, fracture, etc. Bleah.

Unless I can find easy and not-too expensive sprays that can both seal the MDF and take heat, this is all too much hassle and I should go with countertop stuff or (real) melamine laminate.

On the other hand, if I do find something like that---maybe appliance epoxy or high-temp oven paint?---it may be worth it. I might not need anything else.

Geof
09-16-2007, 10:54 AM
For what its worth I think you should bite the bullet and go with the melamine; this what countertops are made of anyway. It is also called Formica.

Are there any cabinet making/architectural millwork places near you? Often these outfits chuck out quite large odd shaped pieces that are too small for their products. You could easily 'tile' your surface using smaller pieces.

drcrash
09-16-2007, 11:08 AM
I'm pretty sure Armor All is just diluted silicone lubricant with UV additives? It doesn't leave much of anything behind to seal with, but will make sure nothing ever sticks afterward. Probably a good mold release. Cheapo silicone spray does the same job to make rubber and plastic look shiny and new.

Interesting. I've used silicone spray to soften rubber seals, but didn't know that Armor All was mostly the same thing.

Even so, it might work to seal the pores in paint reasonably well. Even a very thin film of oil can slow gas diffusion a lot. (A barely noticeable sheen of oil on a pond can suffocate fish.)

The non-stick aspect has pros and cons I haven't worked out. If it doesn't work well enough as a sealer, that would make it hard to fix by putting on a coat of something else.

I'm wondering about nonstick platen surfaces. I could imagine that's a very good thing, but I don't understand the issues in getting hot plastic to seal-but-not-stick to a platen edge or top. :confused:



I hate marketing hype!


Speaking of which, there's another Armor All product, Armor All Body Shield, with the following hype:

"Special polymer nano-technology bonds with glass, paint and trim to give your vehicle the best shine and water repellency."

I'm guessing that's only "nanotechnology" in the same sense that pores in paint and polymers in Elmer's glue are "nanotechnology." (Polymer molecules are pretty danged small, after all.)

Even so, that *might* be just the sort of thing I'm looking for---just a spray-on plastic that's not porous, so that it seals porous plastics like paint, even easier to apply than PVA (which admittedly isn't hard) and maybe more heat-tolerant.


I learned that all the different GOOP brand adhesives they make are identical but some have UV additives. The prices are all different depending on the intended markets, Houshold, Marine, Automotive, etc..

That might explain why I've seen people talking about coating things with GOOP, without bothering to specify which one---maybe they assume their audience knows they're all basically the same. Thanks for cluing me in.

drcrash
09-16-2007, 12:30 PM
For what its worth I think you should bite the bullet and go with the melamine; this what countertops are made of anyway. It is also called Formica.

I expect you're right. Melamine on top with PVA on the bottom seems like the best safe option so far.



Are there any cabinet making/architectural millwork places near you? Often these outfits chuck out quite large odd shaped pieces that are too small for their products. You could easily 'tile' your surface using smaller pieces.That would work for me, but I'm trying to come up with free plans I can put up on my site, which random hobbyists and DIY-ers will actually follow, with no special tools or skills. (A drill and a portable jigsaw, and *maybe* a chisel, but no table saw, router, or welding gear.)

Anything that requires any scrounging whatsoever decreases the appeal for most people---they'd rather just run over to Home Depot and buy *something* and build *something*, even if it costs a bit more and the result isn't as good. (As long as it doesn't cost "too much" more, and everybody has a different threshold.)

Any "new" technique likewise decreases the chances anybody else will actually do it. (Spraying paint from a can is "better" than having to mix and spread epoxy, for example. Most people are comfortable with spray paint and say "I can do that!")

My intended audience is mostly like the people over at Instructables, or maybe at RC Groups. Most people at Instructables are wannabe-"makers" who don't actually have any cool tools or skills yet, and are looking for something very cool that's not very challenging or expensive. Most of the people at RC Groups are model-makers who don't really care that much about vacuum forming unless the vacuum former costs less than the next model that they use it for. (And there's an easy upgrade path to the version that can handle the model after that.) The bottom line is that most of my potential "market" is lazy and/or cheap, or both. (For good reasons or bad ones.)

I enjoy scrounging, but it's increasingly clear to me that most people don't.
If I put in too many design alternatives and possible scrounging sources for the parts, they complain that it's "too complicated." They generally want a parts list with easy sources---preferably Home Depot, *maybe* online---but don't want to run around town or even make a single phone call to a likely source of free stuff.

I've got piles of good scrounged (s)crap to build stuff out of myself---I'm all set---but I don't use most of it because if I do, nobody else can replicate what I build without paying a lot more than I did. I can even afford to buy some premium materials new now and then, but I generally don't, because most other people won't. They'll just make a box with pegboard on top that will collapse if they ever add a vacuum pump. (And maybe break their molds even if they don't.)

Those are some pretty challenging constraints. It's the standard "good, fast, cheap: pick two" thing I'm trying to beat. :tired:

I do realize that for people who are already serious about vacuum forming and willing to put real money or effort into it, this is probably a pretty tedious discussion of a solved problem, and I really appreciate all the feedback.

martinw
09-16-2007, 01:21 PM
My understanbding is that PVA glue melts at a pretty low temperature, so it might not be a good choice for that.

.)

Dear drcrash,

You could well be right. My guess is that PVA won't hold up well at high temperatures, but it is just a guess. Another disadvantage of it is that the film it forms is not particularly tough , unlike epoxy or whatever.

Best wishes

Martin

automizer
09-17-2007, 12:40 AM
well if you are looking at scroungers you can make a few suggestions such as cabinet shops, sign shops places like that use formica and do throw away scraps some very large ones like Geof said. Or you can head to Ikea and they have a DIY section (they call it "As IS") with pre lam table tops that can be in the 5 to 10 buck range.

coonhunter
09-17-2007, 10:26 PM
I may be a little confussed with what you doing but here go's When forming are you loosing all your vac ? Your vac should stay on thru out the forming process for the most part and then shut off and then air eject on.
The molds your trying to form, do you put in a vac chamber? If you do, use your mdf or plywood as the back with the vac hookup in the center, and just run tape right around the edges where the mold and back comes together.That should help with bleed off. Now if you using several molds with the same platen and your loosing vac try milling a .375dp x .500 wide chamber around the platen about 3" from edge of mold. The plastic should draw down in this and this should seal.












,vacuum drill

drcrash
09-19-2007, 10:58 AM
I may be a little confussed with what you doing but here go's When forming are you loosing all your vac ? Your vac should stay on thru out the forming process for the most part and then shut off and then air eject on.

I'm using very inexpensive homebuilt gear. The pumps are very cheap and small, and there's no air eject. The "high vacuum" (from some sort of cheap vacuum pump & tank) fades quickly, losing about half the vacuum in maybe 5 to 15 seconds, and when it falls really low, a check valve opens and a vacuum cleaner holds a steady 5 inches of mercury or so. (I'm trying to figure out the cost/benefit of having the vacuum fade more slowly.)

The usual scheme is drape forming over a male mold on a plain platen. Sometimes it's a platen that the plastic seals directly to the edge of, and sometimes there's a gasket.

The gasket can be either one that seals directly to the hot plastic, or one that that the bottom clamping frame sits on. (In that case, the seal isn't usually very good at that point, but gets better when the plastic sucks down to the platen surface between the gasket and the mold.)


Now if you using several molds with the same platen and your loosing vac try milling a .375dp x .500 wide chamber around the platen about 3" from edge of mold. The plastic should draw down in this and this should seal.Interesting. One of the things I do for small stuff on a large platen is to put a gasket on a tape-down sheet that stops down a platen to the size you want. (It covers any extra platen holes, outside the footprint of the plastic sheet, but has a big hole inside the gasket to expose the platen holes you need to pull the plastic down.)

I've considered putting a *raised* edge on the sheet, about an inch inside the gasket so that the plastic can suck down to that and make a better seal. (The sheet would be cut away inside that to expose the platen holes.) A groove would be more difficult in that context.

This is for prototyping and maybe *very* low volume production stuff, so I'd like to avoid having to make new platens (with grooves in the right places) for particular molds.

Unfortunately, I'm not clear on what makes for the best seal---a hard edge vs. a rubbery one, a raised ridge vs. a groove, etc.

I would think that a rubbery one would make the best seal. Once the hot plastic forms around something, it cools enough to become a solid, and hard things against hard things generally don't seal as well as hard things against soft things... or am I wrong in this context.

Getting a great seal would be a big advantage for the kind of setup I'm talking about, with a small pump and a small tank. (The better the seal, the longer the vacuum will last and the slower it will weaken.)

My impression is that in production environments, people almost always form the plastic around a hard platen edge or upper edge of a female mold, but I'm not sure why.

I would guess that doesn't make the optimal seal, but it's just not worth hassling with soft stuff that will wear out and need to be maintained (dressed) and periodically replaced. Rather than go for the best seal, you just go for a faster pump and a somewhat bigger tank to compensate. (?)

For the low-volume stuff I'm talking about, soft seals make somewhat more sense, if they're easy enough to make, especially if they're easy enought to interchange for different very-low-volume projects.

Unfortunately, I have zero data points on how well various types of seal can be expected to perform. (Like CFM of leakage foot of platen perimeter.)
















,vacuum drill[/quote]

lgalla
09-21-2007, 10:22 PM
Drcrash,melomine is 2 sides and really cheap,which only requires sealing or the edges.Heat should not be a problem,as from the video only the plastic gets heated and should cool down fast.
Amor All,Sillycone?NO,NO!
Nothing in your work place will ever stick again.You will not be able to post finish your parts.It is called silicone contamination.Sanding with 40 grit sandpaper will not remove silicone as it is round micro slippery balls which just roll around.
larry

drcrash
09-22-2007, 08:07 AM
Amor All,Sillycone?NO,NO!
Nothing in your work place will ever stick again.You will not be able to post finish your parts.It is called silicone contamination.Sanding with 40 grit sandpaper will not remove silicone as it is round micro slippery balls which just roll around.


Thanks very much for this tip. I was guessing that I could just try it, and sand off the paint w/Armor All on it if I didn't like it, and that would be okay. I guess not.

spektr
01-29-2008, 08:59 AM
A 2 step process. Shellac. This seals the MDF preventing airflow. It also can bridge those little gaps where we didn't glue stuff together too well. Step 2. Seperate the MDF from hot plastic using any silicone seal caulking product. I use GE Silicone II. Hot plastic releases from it EXTREMELY well.

Scott.