air flow through unsealed MDF under vacuum

[martinw]

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]

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]

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]

OK I am confused. Martin can attest to that.But I agree with Martin. 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]

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]

OK I am confused. Martin can attest to that.But I agree with Martin. 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]

.......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]

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]

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:

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.