vapor polishing

[Anchor]

Anybody know anything about vapor polishing? Mainly the chemical used to do it.

 

[Evan]

Methylene chloride

 

[surplusjohn]

i do if you are talking about plastic. Methlene Cloride was used, works great on ABS and Lexan. THis material is pretty much banned I believe. A vapor degreaser is the ideal method. the solvent is evaporated and allowed to condense on the part and flow back to the tank, a refridgerated blanket of air keeps the vapors in. There are some alternate tricks that I learned depending on what you are doing

 

[Anchor]

Thanks Evan, I just found that out doing a more elaborate search.

I have a job machining some polycarbonate. I just heard of this method a few minutes before I posted this. Is it practical to vapor polish these parts that I have to make? Expenisve? A real nice finish is not called for, but I do like to sell nice looking parts.

 

[Evan]

Methylene chloride is pretty nasty stuff. I haven't looked it up but if you do you will probably find you don't really want to vapor polish your parts after all.

 

[surplusjohn]

MC is used because it is non-flamable and since it has to be boiled that is a good thing. You could probably boil a small amount in a container in a larger container [out doors] and simply pass the part thru. Another trick is to spray it with clear krylon, the MEK and lacqure will melt the surface. this works really well. dipping may work on small parts [really short dip]

 

[Ries]

Depends on the exact plastic, but a trick I have seen used in plastics fab shops is to set up a small oxy-fuel torch with oxygen and hydrogen, and then, using a very small tip, flame the cut or milled edges of the plastic. This makes it clear and shiny, even though it does not take out all the machining lines.
I used to have my shop next to a couple of guys who did signs, point of display stuff, business card holders, and the like, and that is what they did.
Cheap, non-toxic, and it worked well on plexiglas and lexan.
Not sure about polycarbonates, though, but it might be worth talking to a plastics fabricator and see if it would work, and maybe you could even farm it out.

 

[Evan]

Flame polishing works great on acrylics but not on polycarbonate. It just tries to burn.

As for using oxy/hydrogen please tell me when you do that so I can leave the area. Washington is too close. Tank hydrogen has a flame speed of double that of acetylene in free air and mixed with oxygen it is over 8000 fps.

 

[Mud]

Evan - I used Hydrogen for fuel gas with oxygen when I was metallizing aluminum, and still have the equipment. Is it really that dangerous to use? I was only advised that a leak would collect in the rafters and blow my roof off.

 

[Evan]

We used to use it when I was working for a scientific glassblowing company years ago. It's the only flame hot enough to melt quartz. When the master glass blower needed to do quartz we would retire outside while he worked. It can blow back with enough force to destroy the torch. It detonates instead of burning. Not only is the flame speed much higher but an unignited hydrogen/oxy mixture can explode on contact with metals that act as catalysts including platinum and I believe nickel and silver.

It burns with an almost invisible flame and emits high levels of extreme ultraviolet. It can spontaneously ignite unexpectedly due to extreme sensitivity to very small static discharges. The flowing gas itself can produce static discharges that ignite it because it is an excellent insulator. It has a very wide range between the LEL and UEL, 4% to 75% in air and nearly any mixture in oxygen.

 

[Mud]

Are you sure that's Hydrogen he was using? The flame on my equipment was blue in color, plenty visible, and it was used when spraying aluminum specificly because it was cooler than oxy/acetylene.

And most of the plastic fabricators I've dealt with use hydrogen to flame polish acrylic specificly because it is a cool flame, and with very basic looking equipment. Some have a little rig that reduces water to hydrogen and oxygen at low pressure which is enough to do the job well.

Hmm - obviously something new here for me to learn about.

 

[BrianH]

I'll throw in a few more vapor polishing tips....

Clean it with alcohol...get all the fingerprints and oils off...get the fuzzys off...make sure the parts are totally dry.

1/2 to 1 inch of MC in a pot...leave the top on until you are about to dip...maximize the vapors.

I use bent paperclips through a hole or something....dip in the vapor and count to 3...more time and the polycarb starts to melt. Hang it to cool...it comes out tacky. If you need to get down into a slot or pocket, face pocket up....the vapor doesn't rise, it drops.

 

[Evan]

Are you sure that's Hydrogen he was using? The flame on my equipment was blue in color, plenty visible, and it was used when spraying aluminum specificly because it was cooler than oxy/acetylene.

You weren't using hydrogen. Hydrogen has an almost invisible flame. Just look at the main engines on the shuttle when it launches.

Hydrogen/oxygen burns at almost 6000 degrees F, far hotter than oxy/A.

[edit]

On thinking about it may be possible you were using hydrogen but with an extremely lean mixture. I'm not sure what that would do to the flame color but it would reduce the heat content of the flame although not the actual burning temperature. That's possible because of the extreme insensitivity of the fuel to proper stoichiometric mixture.

 

[metlmunchr]

I have to agree with Mudflap's take on hydrogen. Hydrogen and oxygen is used for gas welding aluminum specifically because the flame is cooler and therefore more controllable.

From a chemistry perspective, hydrogen should produce the coolest flame among common gases because it has the lowest bond energy between atoms. Acetylene tends to be at the top of the temperature scale because it has a triple bond between the carbon atoms IIRC. Somewhere between these limits would be methane (natural gas) with multiple single bonds to the single carbon atom, and propane, which has a double bond between the carbons if memory (from chemistry class 35 yrs ago) serves. One thing I do remember specifically from chemistry is the prof telling us that the flame temperature of any stochiometrically correct mix of a combustible gas and oxygen will be directly related to the bond energy between the atoms in the gas. More bond energy = higher flame temperature.

 

[Ries]

Here is a quick chart off a torch suppliers website (Smiths Little Torch)

Gas Flame Temp. w/ Oxygen (°F)
Acetylene 6000
MAPP® 5300
Hydrogen 4900
Propane 4800
Natural Gas 4600

And here is what they say about
Hydrogen / Oxygen: Produces concentrated flame with low (BTU) heat output. Very clean burning. Excellent for brazing or casting platinum. Flame is difficult to see. Much lighter than air, will disperse with air movement if leakage occurs. Extremely flammable and uses very little oxygen for amount of heat energy produced. Unlike other fuels, comes in high pressure cylinders which may be obtained from most welding gas suppliers.

 

[metlmunchr]

Take a look at this site for some real gas info

http://www.nottaughtinschools.com/Yull-Brown/Free-Energy-Demo.html

This guy makes PT Barnum look like a rank amateur

 

[Evan]

Flame temperatures

Last updated April 6, 2005.

Flame temperature is not as simple as it sounds. It depends on many factors, such as:

* Whether the (gaseous) fuel flow is laminar (smooth) or turbulent
* Whether the fuel and oxidixer are premixed, or simply diffuse together while burning
* Whether the flame is "adiabatic", meaning that it does not lose heat. While impossible in practice, the theoretical adiabatic temperature is often quoted.
* Where in a (laminar) flame the temperature is measured
* For flames in air, the temperature, humidity, pressure, present gas mixture, etc. of the air
* The mixture of fuel vs. oxidixer in use. For example, an acetylene/oxygene flame may be neutral (equal mix: complete combustion, white rounded cone); carburising (excess acetylene: large smoky yellow flame); or oxidising (excess oxygen: short, sharp inner cone, deeper purple hissing flame).

Sites discussing practical welding and similar tasks tend to give much lower numbers than science-oriented sites, probably because of rapid heat loss in typical use. A number of sites warn that measurements done with thermocouples can be wildly inaccurate. More definitive information is available in the North American Combustion Handbook.

As a general practical guideline, consider one site's list of these heats for their jewelers' (gas/oxygen) torch:

* 2200°C = 3992°F, for Propane/Oxygen
* 2927°C = 5300°F, for MAPP Gas/Oxygen
* 2700°C = 4892°F, for Acetylene/Oxygen
* 3200°C = 5792°F, for Hydrogen/Oxygen

http://www.derose.net/steve/resources/engtables/flametemp.html

 

[ooyeniyi]

I've got a vapor polished acrylic piece, with significant improvements to the clarity. However, I would like to get rid off the misty/glossy look that appears after vapor polishing. Using Novus polisher 3,2,1 after vapor polishing removes most of the glossy look, but I worry that process re-introduces scratches to the acrylic piece. Do you have a recommendation for how to effectively wash off the glossy finish without introducing micro-scratches?

This image is of the piece before vapor polishing.


This image is of the piece after vapor polishing.


This image is of the piece after vapor polishing and buffing with Novus plastic polish 3,2 and 1.