[drcrash]

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]

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]

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]

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.

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]

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]

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.