Water cooling on plastic ?

[Hannu Rinne]

I saw somewhere an instruction to install a plastic sheet on the water surface to avoid twisting (water cooling method) - but has anyone ever tried to engrave a sheet which is totally (slightly) immersed under the water ? Would it work with a low power laser ? - just one of my unsolved questions...(before I'll do any unconsidered tests :D)

Regards, Hannu

[Joe Pelonio]

If I understand the question, I have had problems with engraving plastic when the copy is very busy, so much that the heat causes the sheet to warp while running and then get out of focus. I tried misting it with water but it dries before getting to the end. Placing the sheet onto a layer of damp paper towels helped, but I had better luck using an aerosol window cleaner that contains no solvent or ammonia and comes out foamy. I spray it onto the surface before engraving and it lasts to help cool the plastic.When done any that's left gets wiped off and cleans off any fingerprints.

I would be worried that if under water, there would be splash off the beam that could get onto the lens and cause it to crack so if you experiment be careful.

[Martin Boekers]

Just tossing this out, haven't tried it.

How about placing it on a cooled sheet of aluminum,
or a laptop cooler?

Chances are the movement of the lens may cause ripples
in the water.

Marty

[Hannu Rinne]

Joe & Marty, thanks for your replys

And sorry about my English ! In that instruction, there was a pool filled with water and the sheet was on the top of the water - size of the sheet was something like 15"x20" and the method was to engrave the most of the area of the sheet. There isn't any problem if you are placing this engraved sign by yourself but if you're just engraving it and give this twisted sign to your customer - then there might be some problems :D (they might be sceptical about my skill )

Regards, Hannu

[Dee Gallo]

How about placing it on one of those blue freezer things? They are extra cold and rigid ones are nice and flat. Or, freezing the plastic before putting in the laser?

:) dee

[Bruce Volden]

I would be worried that if under water, there would be splash off the beam that could get onto the lens and cause it to crack so if you experiment be careful.[/QUOTE]

Joe, I mean no disrespect. I was and have always been told that there was no way this could happen (by the laser mfgrs). I asked this same type of question pertinent to mirrors, shiny metals, etc. and was told "It can't happen.". I recall a thread awhile back (Dan?) which posted pic's of refracted beams and resulting damages. I have engraved many things over the past 15+ years and have yet to have any lens crack. I even challenged the laser mfgr. to the point where he said he would personnaly repair any damages resultive of refractions! ???? In my mind a refracting beam would certainly be possible to cause damages, I just have yet to have had any.

Anyway, I hope somebody here can set me straight.

Bruce

[Dan Hintz]

Hannu,

Water heavily absorbs the IR wavelength used for CO2 laser processing, so you wouldn't want to submerge anything in water. Also, plastic is a very poor conductor of heat, so floating it on water will do practically zero for keeping the plastic itself cool.




Bruce,

I was in on the discussion about reflected beams. I was (still am?) a non-believer in the ability to reflect enough of the beam back in a focused manner to cause the damage claimed by some (such as scribed lines on the surface of the lid's glass). If memory serves, the test showed something like a darkened line on a piece of paper placed on the lid... at the time (as it is now, unfortunately), I did not have the free time to ru my own experiments.

Reflection is a real problem for high-power systems (kW level) because the beam melts the metal, creating a highly-reflective pool of molten metal (think like a mercury mirror) that has the ability to reflect the exact "image" of the beam back towards the focusing optics (and therefore back into the lasing chamber). This is the reason they sell one-way, non-reflective optics for the higher-powered systems.

At low power (such as our systems), such a reflection counts strictly upon the reflectivity of the metal in its solid state. To etch a line on the glass lid 8-12" away from the focal point means the metal was bowl-shaped oh so perfectly for the entire length of the line etched into the glass... highly unlikely (nay, impossible), but I wasn't going to get into the battle until I've had time to set up my own experiment, so I acquiesced. I'm sure someone will see this thread and rip me a new one... so be it.

[Dee Gallo]

So what's the solution? Start the job and stop it partway through to let things cool off, then resume the job?

[Dan Hintz]

Dee,

That's one solution, though a slow one. My suggestion would be used a smaller grid on the vector table for support and get a higher powered vacuum on it to keep the sheet as flat as possible throughout the run.

[Doug Griffith]

Since it's a heat differential issue, I'd work with the laser instead of against it. I'd try heating the acrylic before placing it in the laser. Finding the optimum temperature without damaging/warping the sheet prior to lasering is the tricky part. I'm guessing somewhere under 180F which is the annealing temperature. It may need to be supported on a carrier unaffected by the heat. Might be worth an experiment.

[Joe Pelonio]

Hannu,


Bruce,

I was in on the discussion about reflected beams. I was (still am?) a non-believer in the ability to reflect enough of the beam back in a focused manner to cause the damage claimed by some (such as scribed lines on the surface of the lid's glass).

I'm sure someone will see this thread and rip me a new one... so be it.
Bruce,

I'm not going to object to your theory, or waste time arguing this.

But. . .I do have large etch marks on the inside of my glass lid that I saw happen when cutting mirror acrylic face up, and since then I always apply transfer tape or cut it upside down in reverse.

Do try it some time, but be careful. I was running 100% power 6 speed.

[Rodne Gold]

Air assist will cool the plastics down

[Hannu Rinne]

Thanks to you all for your answers,

So, the water is out of the question. As Dee said some refrigerant pad might still be worth of test, also a powerfull vacuum table (coming) as Dan mentioned. Heating the plastic sheet would be very interesting to test (thanks Doug) using for example some thermostat adjusted "heating plate" (I don't know the correct word in English, sorry). Added a photo - heater with ceramic surface, temperature range 30-95C (86-203F) - price something like $ 200,- work area about 600x400mm (at least ideal to keep your meal warm). Also avoiding the mistakes what I've done so far would help; Engraving too deep with too much power, cutting with too slow speed, using too high DPI/PPI settings, using thinner sheets instead of thicker when doing some larger signs, forgetting to use air assist (which is very good cooler as Rodne said) - and finally just forgetting to use my minor brain(s) :D !

Best regards, Hannu
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[Scott Shepherd]

Rowmark's technical support site mentions the water too. It says to put water on the table and place the plastic on the water. So just a heavy spray of water on the table, then lay the plastic on that, engrave it.

Haven't tried it, but it did come directly from Rowmark's site.

[Dan Hintz]

Can't say I've ever tried to vector with any power directly on the table top, but I would think the amount of reflecti0on back into the material would create a nasty kerf.

Same scenario with the hot plate underneath...

[Martin Boekers]

I do it from time to time, mostly on military nametags that I do on the spot.

I do a handful of those a day so I don't want to wear any hinges out. (or my back):D

kerf isn't bad, just needs to go over the back with a bit of windex an a magic sponge.

Marty

[Rodne Gold]

I vector directly on a sheet of anodised aluminium for all stuff under 5mm (acrylics and plastics) , no flashback and no melt issues (if you use a backing paper) , in fact the ally , over time gets nice and sticky and almosts acts like one of those sticky mats.

[Scott Shepherd]

You can go ahead and wipe "engraving while sitting in water" off the list of things that will help. Have a repeat job in house right now, one I advised the customer against repeatedly, but they knew the risks and wanted it anyway. It's a 4" x 9" "sign" and it's got all the material burned away except for the text and a border. So it's basically just a massive area to engrave and it bowes like a banana. I told them about it, I laughed when they picked it up, saying "Here is your order of bananas" and they keep ordering them.

So this time I got a fair amount of water on my table, sat the material in it, ran the first pass through, noticed some bowing, ran a light finish pass across it, bowed like a banana. No change in doing it dry.

That myth is busted for me.

[Robert Walters]

Since what's really needed is fast displacement of heat...

Why not use a radiator or heater core type element as a cutting table and run chilled water/glycol through it using a small aquarium type pump and a bucket of ice water?

Only issue I can think of is condensation.


http://www.evanstempcon.com/serviceimages/RV218093.jpg

[Doug Griffith]

For one thing, you are not going to get enough surface contact area to be effective. You'd be better off with a very large Peltier chip mounted to an anodized aluminum plate. And, if you went that route, you'd also need a very large heatsink on the opposite side and some ducting to your vac system. I'd go as I mentioned before. Reverse the voltage and use the Peltier to heat the plastic. Might not be hot enough though and it would be easy to burn out the peltier if heating.

[Doug Griffith]

Rowmark's technical support site mentions the water too. It says to put water on the table and place the plastic on the water. So just a heavy spray of water on the table, then lay the plastic on that, engrave it.

Haven't tried it, but it did come directly from Rowmark's site.

I don't think the water is actually cooling anything. It is acting as thermal grease as in a heat sink on a processor.

[Rodne Gold]

A chiller under the plastic isnt going to work as the heat put into the material is at the point of lasering and it cant wick away fast enough , you could use superchilled air blasting on the piece as your air assist , this would probably help , however the fact is that the laser works by generating so much heat at the focal point that the substrate vaporises - trying to stop that heat will affect the way the laser engravers.
For large area engraving , the best strategy is not to use solid fills , rather use a shaded fill as this is engraved as a 1/2 tone and discrete dots which are spaced far away enough to stop the extreme heat affected zones and still close enough to give a solid effect.

[Scott Shepherd]

the best strategy is not to use solid fills , rather use a shaded fill as this is engraved as a 1/2 tone and discrete dots which are spaced far away enough to stop the extreme heat affected zones and still close enough to give a solid effect.

Exactly what I have been doing on these pieces. I think double sided tape solves the warping while running problem far better than the water trick :)

[Dan Hintz]

Steve,

Next time you do one, try making the background a black/white checkerboard. Try different sized squares... 1/2", 1/4", etc. I'm curious if that would spread out the HAZ enough to at least cut down on the amount of warpage...

Run it with one background, then run it again with the inverse. It will take twice as long, but it may help do those "undoable" signs... assuming it works, of course.

[Richard Rumancik]

I don't know where the water idea came from, but Rowmark is suggesting it in a specific way for a specific purpose. Rowmark suggests the option of the spray mist on the table with the cap layer facing the table. This applies to vector cutting only.

I think the idea is to have the water in contact with the face so it cools the plastic (presumably reducing raised edges when cutting). Excess laser energy probably just goes into the water, at least in theory. It probably still requires an optimization of the laser power setting so you are not more than 5 or 10% above the minimum for cut-through.

I haven't tried this, or needed to. I cut Rowmark Flexicolor from the back with paper mask on the front resting on honeycomb, and it works fine with mimimal raised edges. But it will still warp when heavily rastered (or vector marked) from the front. I use tabs of PSA on my fixture to hold down the edges. I wish I could do Flexicolor from one side (combo vector/raster) but I had big problems with raised edges.

Rowmark does not suggest that you can/should use water to cool the sheet when rastering the front. I agree with others that it probably won't have much effect. One thing that can be done is to try not to raster heavily in one spot. This might require some use of layers to control which parts get plotted in sequence.

Why don't the laser drivers have an "interlace" option for rastering? eg if you are rastering text at 300 dpi it could do a first pass with every second line, then come back and fill in. This would reduce the heat buildup on the material as it would have some time to cool before the second pass. It would not require much more laser time to do 2 passes at 150 compared to 1 pass at 300.