Making a high pressure vacuum chamber

[No_Shorty]

Hey, so I'm not sure if anyone here has the relevant experience or knowledge to answer this but fingers crossed!:

I am attempting to find a high pressure vacuum chamber for OCA film or maybe just LOCA glue. I am guessing the latter needs a lower vacuum as it is liquid, but just a guess.

I am looking to use this 'stretched bar' LCD in a project:



The manufacturer 'WiseCoCo' will of course sell this item with the LCD already bonded to the glass touch panel. The problem with this is that they seem to be doing this by hand/without a 'jig' - and the alignment is VERY poor. They are the only supplier of this panel and I am wanting to use them for a project which I may sell a few of that I would have the body CNCd out of aluminium. As such I need good alignment.

I could try to see if WiseCoCo would use a jig I manufacture myself... but given the numbers I would be ordering (less than 10) - I think it might be a struggle to overcome the distance/language barrier. It's still an option but I have leant toward aligning and bonding the screens myself.

I have done this once with LOCA, but its suuuper messy squeezing all the bubbles out and my first application seemed to cure the glue in a weird way, leaving visible smudges.

I would love to be able to try out OCA film, which is the standard way of bonding these screens now. The problem being you need an 'autoclave' - or vacuum chamber with a recommended pressure of 6 bar. This seems to be rather a lot higher than any of the 'pressure cooker' DIY builds you might find on the web.

I was trying to find a 'plug and play' solution like this: https://www.sznasan.com/shop/lcd-repair-machine/na-b3max/

But unfortunately they are really designed for phones - so the capacity of their 'cavity' is too small. I double checked with NASAN and they confirmed it was only 325mm.

I can't find anything remotely affordable (or a sensible size) that is capable.

So this has all lead me to wondering if I can make something myself. I am aware that 6 bar is a lot of pressure and the realistic answer to this might just be no. But the idea I had was as follows:

To use 3 pieces of Acrylic, 30mm in depth. The centre piece I would cut a hole big enough to fit my screen. I would then seal these to one another - not sure what sealant, maybe just silicone? I would then mount valves to the top piece, near an edge.

I am however unsure if the acrylic could take this kind of pressure, or if I would be able to effectively attach valves that would not leak at these pressures, or if the sealant between the acrylic would leak at this pressure.

If 6 bar is going to be unachievable for a DIY solution, what kind of pressures do you think I might be able to achieve?

[jonovid]

high pressure vacuum chamber    the contradiction   

[Kleinstein]

Pressureized air can be quite dangerous. Acrylic is relatively brittle and not a suitable material at this size.
Even just vaccum has some danger from an implosion, accerating the parts in and than having parts bounce back.
If ever possible avoid brittle material, so that failure would be more gradual leakage and not an explosion.
The forces are quite high even at moderate pressure and the construction with flat sides and corners would have local stress concentrations that add to the difficulties. The suitable material would be more like relatively thick steel, especially around seals / openings.
So this does not look like a sensible DIY project.

[jpanhalt]

I agree with prior comments on the dangers of pressure.  If in doubt, look at how ruggedly a medical steam sterilizer/autoclave is constructed.  Typical conditions are 121°C and 15# above ambient.  Six bar is a lot more.

[wraper]

LOCA doesn't need vacuum chamber. The problem is how to avoid glue getting inside of LCD on the edges and screwing up light diffuser.

[james_s]

I'm confused, do you need a vacuum chamber or a pressure chamber? You mention 6 Bar, that makes no sense in terms of vacuum, atmospheric pressure is only 1 Bar to begin with and you can't go down below zero.

[No_Shorty]

As it turns out I have got how these autoclave machines work very wrong.

I had assumed they were working via vacuum, to remove any bubbles from the adhesive. This is what made sense to me having seen a vacuum remove bubbles.

However the replies here made me look into it a little more depth, and these 'autoclave' machines do in fact crease a pressure vessel, not a vacuum. The pressure vessel actually forces the bubbles to be 'absorbed' the the adhesive. I don't understand how that works, but I guess thats not important. That these machines have heaters makes more sense with this in mind, as the glue may need to be warm in order to do this.

LOCA doesn't need vacuum chamber. The problem is how to avoid glue getting inside of LCD on the edges and screwing up light diffuser.

Technically, no - but it is a royal pain removing bubbles from a large LCD, I would hope a vacuum would do a better job. The LCD and it's backlight/diffuser come as one unit, and the edge is wrapped in a foil. So I think this isn't so much of an issue for what I am using it for.

So - given I was off to a completely incorrect start thinking I needed to achieve '6 bar' in a vacuum, I not wonder two things:

Is there any way to achieve 6 bar in a DIY pressure vessel?

Is it worth trying to achieve a much lower vacuum in either a acrylic or steel (thanks for that suggestion) chamber to extract bubbles from LOCA?

[james_s]

The most common application for an autoclave machine is sterilizing medical devices such as surgical instruments. They are just a fancy pressure cooker, using superheated steam in a sealed pressure vessel to sterilize the tools.

There are small labs such as universities using "Instant Pot" pressure cookers as low cost autoclave machines, perhaps that would work? Conventional pressure cookers that you put on a stove sometimes turn up in thrift stores.

[HighVoltage]

There are plenty of spezilized companies that make special glass for pressure chambers to be safe.

I have built a pressure chamber once for up to 200 bar and it required a 32mm thick glass at about 120mm diameter. It was tested for up to 400 bar, so I was on the safe side.

DO NOT play with acrylic and pressure chambers!!!

[TimFox]

A specific danger:  laboratory supply companies sell acrylic (or similar) plastic shields that are designed to keep spills, boil-overs, or splatters from wet-chemistry setups away from personnel.
E.g.:  https://www.coleparmer.com/c/safety-shields?searchterm=Safety%20Shields
When hit by an exploding pressure vessel, they can sent sharp shards as shrapnel in your direction (pardon my alliteration).
I first heard of this when "critical-point drying" became popular to prepare specimens for electron microscopes, where the "bombs" required very high pressures.

[No_Shorty]

There are small labs such as universities using "Instant Pot" pressure cookers as low cost autoclave machines, perhaps that would work? Conventional pressure cookers that you put on a stove sometimes turn up in thrift stores.

Yea I had started to look in to this as a pressure cooker is a common thing to use in a 'DIY' vacuum chamber for degassing epoxy and the like. They are typically rated for only 15psi however, so believing I needed something that could do 100psi I abandoned the idea (and then subsequently learned the 100psi was for a pressure chamber, not a vacuum chamber). However, 15psi may be plenty to suck the bubbles out from under a screen using the LOCA glue, as it is fully liquid.

The most common application for an autoclave machine is sterilizing medical devices such as surgical instruments. They are just a fancy pressure cooker, using superheated steam in a sealed pressure vessel to sterilize the tools.

Yes, this was confusing when I first started looking up 'autoclaves' - as the ones for electronics obviously do not use steam. They are more commonly referred to as "bubble remover machine" when referring to mobile phone repair or assembly. Although "autoclave" is sometimes used. They do not use steam to create the pressure but a pump.

There are plenty of spezilized companies that make special glass for pressure chambers to be safe.

I have built a pressure chamber once for up to 200 bar and it required a 32mm thick glass at about 120mm diameter. It was tested for up to 400 bar, so I was on the safe side.

DO NOT play with acrylic and pressure chambers!!!

It was only as acrylic is used for a lot of DIY chamber 'guides' you find online that I had been considering it (and of course the ability to see whats going on inside) - do you think there is no safe level of vacuum in which acrylic can be used?

I would have thought glass - if you get it wrong - would be even more dangerous, and I would guess specialised glass that is specced for such things is gonna be pretty expensive to 'experiment' with?

At this stage I have pretty much abandoned the idea of a high vacuum chamber. As it seems unachievable and the tools for removing bubbles use pressure chambers not vacuum chambers in any case.

I a considering the kind of vacuums that seem to be common for 'pressure cooker' builds - so around 15psi (in lieu of knowing how to trqnslate that from pressure to vacuum unit, if that is erroneous) - this is why I was still considering acrylic, as it is common in these builds and would allow me to make something the right size. As opposed to having to buy a heouge pressure cooker.

[dmills]

Acrylic is horrible for this, it just loves to propagate cracks if you even look at it funny, and that is not a good thing.

Were I trying this I would be taking a leaf from the model engineering communities boiler making practises, and obviously doing some fairly hard core hydrostatic tests before even thinking of pressurising with air.


 

[Ian.M]

+1
Acrylic plastics are notorious for poor impact resistance, micro-cracking, and sudden catastrophic failure under tensile or bending stress.

Polycarbonates are a lot better (tougher) as long as you respect their chemical compatibility limitations.  Using the wrong sealant or plasticizer migration from the wrong choice of gasket material or exposure to solvent vapours can trigger catastrophic failure by removing its toughness, making the Polycarbonate plastic extremely brittle with little or no visible signs of the incipient failure.  With this known issue, I would personally not be comfortable with a Polycarbonate viewport design unless it was protected against chemical contamination, and was constrained by a strong metal mesh and supporting grid on the outside to contain any catastrophic failure under pressure.

If you *NEED* visibility inside your vacuum/pressure chamber and cant afford a commercially available, new pressure rated viewport, consider a video camera inside the chamber.

[No_Shorty]

Ok, noted. Given the acrylic would need to host a metal valve as well I can see that being a source of fractures. As I said I had only really leant toward it as it was a common material in DIY projects, but I know those are often done without any knowledge of the materials.

The visibility was more for inspection of bubbles, not really achievable with a camera I don't think. I would have also allowed me to pull the vacuum as well as cure the LOCA whilst under vacuum. to ensure no bubbles creep back in - but this was a 'nice to have' idea, not an essential.

I could afford to consider something up to about £1.5k if it was the 'right' solution (like one of the bubble remover machines linked above) but I certainly can't be experimenting with how well a vacuum chamber works for LOCA for that kinda money. I probably can afford to drop a couple of hundred on a pressure cooker style vacuum/degassing chamber from Amazon, so may go down that route for testing.

If I was to DIY a solution then I would probably go for 3 pieces of thick steel or alu, with a hole cut out of the center piece - so same as my original idea but no visibility.

[Ian.M]

Vacuum bagging would be worth a try.  Its pretty easy to DIY, easy to inspect the lamination process, and doesn't require a pressure vessel.  However it only gives a maximum of 1 bar pressure, and in practice significantly less, because its generally desirable to reduce the vacuum to reduce the size of residual bubbles after pulling a higher vacuum to de-gas the adhesive.   You *MIGHT* be able to reach higher pressures without a pressure vessel using a press, and an elastomer pressure pad on top of the vacuum bag, but that would only be practical for thermal or chemical cure adhesives as it would block access for the light source for UV curing.

[james_s]

It was only as acrylic is used for a lot of DIY chamber 'guides' you find online that I had been considering it (and of course the ability to see whats going on inside) - do you think there is no safe level of vacuum in which acrylic can be used?

You keep going back and forth between pressure and vacuum. Vacuum is really just the absence of pressure, 1 atmosphere is about 15 psi absolute, anything under that is a vacuum, a perfect vacuum is 0 psi absolute, but vacuums are typically measured in inches of mercury for partial vacuums, and microns of mercury for deep vacuums. The difference in mechanical stress on a vessel between the sort of vacuum you can get from a cheap single stage rotary pump or even a hand pump, and a deep vacuum as you'd find inside a CRT is actually pretty small.

[jpanhalt]

Yes, please clarify whether you mean vacuum or pressure.  In a recent post, you mention using it to degas epoxy.  That is usually done with vacuum.  You can also try sparging with helium, which has a low solubility, but vacuum is far simpler and cheaper.   
  ( https://en.wikipedia.org/wiki/Sparging_(chemistry) )

[No_Shorty]

I am at this stage not sure why there is confusion between how I am referencing vacuum and pressure. I did look back at my posts but I am unsure why they are still causing confusion.

For the debubble machines - like the one linked in my OP - I now understand these work at high pressures - not vacuums. I am still interested in these machines, but I do not believe I can achieve what they do via a DIY route.

I am now concentrating on the idea of trying to remove bubbles from LOCA (the liquid/glue, not the OCA film) with a vacuum chamber.

The only point I have referred to pressure again was in describing the "pressure cooker style vacuum chambers" - by this I mean this kind of thing, I am aware it is not a pressure vessel, just that it looks like a "Pressure cooker": https://www.amazon.co.uk/BACOENG-Stainless-Stabilizing-Degassing-Silicones/dp/B07GR5FTQJ/ref=sr_1_5

I hope that makes sense.

I am broadly a little confused by the use of the word 'pressure' when describing vacuum - the word "pressure" comes up frequently when looking into vacuums, and if I google what units vacuums should be measured in a link like this: https://sens4.com/vacuumunits.html - talks repeatedly about pressure and measurement in mbar. I expect I am missing something simple here, so I have been trying to avoid the use of the word pressure when referencing a vacuum.

edit: Ian.m - thanks for that suggestion, I have a sous vide machine so the thought had crossed my mind (not to use the soud vide machine itself, but bagging as a form of creating a vacuum). It guess it would also provide some pressure (right word this time?!  ) to the screen as the bag constricts around it.

[james_s]

I am broadly a little confused by the use of the word 'pressure' when describing vacuum - the word "pressure" comes up frequently when looking into vacuums, and if I google what units vacuums should be measured in a link like this: https://sens4.com/vacuumunits.html - talks repeatedly about pressure and measurement in mbar. I expect I am missing something simple here, so I have been trying to avoid the use of the word pressure when referencing a vacuum.

Because pressure and vacuum are like heat and cold. Cold is not really a thing itself, it is the absence of heat. Vacuum likewise is the absence of pressure. There are two ways to discuss pressure, one is absolute pressure, which is relative to a perfect vacuum. The other is gauge pressure (psig) which is relative to atmospheric pressure. Atmospheric pressure is around 15 psi absolute, and it is by definition 0 psig. A vacuum is negative psig, but positive absolute, you cannot go below 0 psi absolute just like you cannot get colder than absolute zero, because absolute zero in both cases is the complete absence of heat or pressure.

In the real world an absolutely perfect vacuum is impossible to achieve, there will always be some pressure, which is created by some number of remaining gas molecules flying around and bouncing off the walls of the vessel. A vacuum is by definition lower than atmospheric pressure, so there will also always be pressure on the outside of the vessel caused by air molecules exerting force on it. With a vessel at atmospheric pressure you have an equal amount of pressure inside and out so the net pressure is zero.

[TimFox]

With vacuum and high pressure, there are two measurements:
"Absolute pressure" is the actual pressure inside the chamber.  It must be positive. 
"Gauge pressure" is the difference in pressure between the chamber and the local atmosphere.  It can be positive or negative, but the most negative value equals the absolute pressure of the ambient air.
You may see a gauge calibrated in "PSIG" for positive gauge pressure (pounds per square inch gauge, pronounced pigs' eye) and "Inch Hg" (inches of mercury) for negative gauge pressure (vacuum).
Vacuum gauges for "high vacuum" are usually for absolute pressure, with an interesting collection of units:
"mm Hg" = "Torr", where 760 mm is standard air pressure, based on traditional units for mercury barometers
"microns" = micrometer Hg, defined similarly
"mbar", roughly equal to "mm Hg", since 1 bar = standard air pressure = 760 mm Hg.
In physics labs, I found the ultra-high-vacuum gauges calibrated in mm Hg to be amusing, since we could obtain a very good vacuum of 10^-10 Torr, and a mercury atom (300 pm diameter) is much larger than 10^-10 mm = 0.1 pm.

[No_Shorty]

Thanks, I think it is making sense to me (more or less) now.

[james_s]

For additional reading, this is more that you ever wanted to know about vacuums.

https://www.repairfaq.org/sam/lasercva.htm

[No_Shorty]

Haha, that sure a lot 

[wraper]

LOCA doesn't need vacuum chamber. The problem is how to avoid glue getting inside of LCD on the edges and screwing up light diffuser.

Technically, no - but it is a royal pain removing bubbles from a large LCD, I would hope a vacuum would do a better job.

You don't need to get rid of bubbles as long as glue was applied in proper pattern and touch is slowly placed under slight angle so no air is entrapped.  I personally glued touch to 11" LCD. Making a jig will make the job much easier.

[jpanhalt]

Added confusion: getting rid of bubbles v. degassing

1) Vacuum is used for degassing usually. Sparging can also be used to reduce gas content or change it from say air to argon/helium.
2) One can easily use vacuum to remove bubbles.  Take a fiberglass layup.  You may get bubbles.  If you put it into a flexible bag and apply vacuum such that atmospheric pressure is on the wet layup, then one removes those bubbles.  That might be analogous to using vacuum to add transformer varnish without bubbles.
3) Of course since the resin (e.g., epoxy) is likely saturated with air at atmospheric pressure and tends to degass once mixed, if you add enough pressure to a wet layup one could reduce those bubbles to very small size or force them to redissolve in the matrix.  Once the epoxy sets, they will appear to be gone.  My concern is whether with time, they may reform.

The problem with vacuum is that one is limited to a pressure of about 12.7 psi depending where you are.  With pressure, you can exceed that pressure pretty much at will.  I only have experience with #'s 1 & 2.