air flow through unsealed MDF under vacuum

[drcrash]

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.

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]

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]

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]

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]

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]

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]

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]

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.