How does CD-RW work w/o melting the plastic?

[NeoPTLD]

You can read in literatures that CD-RWs work by phase change of special alloy by heating them to hundreds of degrees C. You can use rewrite CD-RWs many times. How does it heat something attached to a plastic subtrate without causing damage to it?

 

[Matthias99]

They use a very focused laser to do the heating, and pulse it very precisely; they have to, otherwise you'd erase the whole disk. Even if the spot directly under the laser is at 200 degrees Celcius, a spot 1mm away might be at ambient, as might the underside of the disk. I'm sure if you somehow defeated the safety mechanisms and sat the disk still with the write laser turned on, you'd melt a hole right through it eventually.

article

 

[friarfraust]

pretty interesting. I wonder if anyone ever tried making some sort of laser gun with a cdrw burner.

 

[Gord]

Originally posted by: friarfraust
pretty interesting. I wonder if anyone ever tried making some sort of laser gun with a cdrw burner.

All shall tremble before the mighty Plextor, destroyer of worlds!

 

[cirthix]

lol, very low powerd laser, but focused on such a small area it can change that tiny bit because of the extreme heat, BUT it is such a small area that it requires a TINY amount of energy to heat, so no L4Z0R guns from cdrws lol

 

[numb]

You can find old grocery checkout scanners on ebay or in the junkyards... those can be made into lower power lasers.

 

[Howard]

EDIT: Oops

Doesn't seem like enough heat. The metallic surface is very thin.

 

[Mday]

The plastic used has a relatively high melting point. The laser used is a class 1 laser, iirc, and is low power by definition. And, the plastic is clear, so it would absorb relatively little of the energy of the laser used to burn the write layer.

 

[Howard]

Originally posted by: Mday
The plastic used has a relatively high melting point. The laser used is a class 1 laser, iirc, and is low power by definition. And, the plastic is clear, so it would absorb relatively little of the energy of the laser used to burn the write layer.

But the heat conducted to the plastic by the heated data layer? That's what the OP was asking about, I think.

 

[Mark R]

The phase change layer is not directly adherent to the plastic substrate - there is a ceramic dielectric between the recording layer and the substrate. I would expect that this provides sufficient thermal insulation to prevent the maximum temperature of the substrate from being exceeded.

Also, do be careful if playing with optical drive lasers - yes, the old CD reader lasers were pretty harmless (0.1 mW) but the new 16x DVD-R drives use high power lasers (0.2-0.3 W). You can certainly get skin burns from these, not to mention severe eye injuries.

 

[Gioron]

Originally posted by: Mday
The plastic used has a relatively high melting point. The laser used is a class 1 laser, iirc, and is low power by definition. And, the plastic is clear, so it would absorb relatively little of the energy of the laser used to burn the write layer.

I see this a lot, and its not true. You can't assume that a class 1 laser is harmless, because there are two different types of class 1 lasers. The first type are the ones you're thinking of, dinky little ones that can only hurt you if you look into the beam for a minute or so. The second kind are "enclosed" lasers, where the laser can be any power, but it won't be exposed in normal operation. Since the laser is only on with the drive is closed, the laser can't get at your eyes or skin and is safe enough that you don't need to deal with laser safety procedures. However, if you take it out of the burner it will revert back to its previous rating (which might be anything up to class 4, the kind that can cause skin damage) and can cause serious injury if you manage to turn it on and point it the wrong way.

I work with a couple 10W class 4 lasers at work, in what looks kinda like the typical Hollywood image of a laser setup (with one exception: unfortunately, the laser is in the IR range so it is completely invisible without a proper viewer). Despite extensive safety precautions, we've had a couple incidents where a beam went somewhere unexpected and caused damage, and I could show you a monitor with a rather interesting melted plastic burn mark just below the screen. This isn't your normal laser pointer. However, if I took the very same laser we use (which is about the size of a computer case), stuck it in a steel box and then welded it shut, I could call it a class 1 laser and it would be considered safe. That doesn't mean you can cut a hole in the box, wave the laser beam around, and not loose an eye.

 

[Fencer128]

Originally posted by: Gioron

Originally posted by: Mday
The plastic used has a relatively high melting point. The laser used is a class 1 laser, iirc, and is low power by definition. And, the plastic is clear, so it would absorb relatively little of the energy of the laser used to burn the write layer.

I see this a lot, and its not true. You can't assume that a class 1 laser is harmless, because there are two different types of class 1 lasers. The first type are the ones you're thinking of, dinky little ones that can only hurt you if you look into the beam for a minute or so. The second kind are "enclosed" lasers, where the laser can be any power, but it won't be exposed in normal operation. Since the laser is only on with the drive is closed, the laser can't get at your eyes or skin and is safe enough that you don't need to deal with laser safety procedures. However, if you take it out of the burner it will revert back to its previous rating (which might be anything up to class 4, the kind that can cause skin damage) and can cause serious injury if you manage to turn it on and point it the wrong way.

I work with a couple 10W class 4 lasers at work, in what looks kinda like the typical Hollywood image of a laser setup (with one exception: unfortunately, the laser is in the IR range so it is completely invisible without a proper viewer). Despite extensive safety precautions, we've had a couple incidents where a beam went somewhere unexpected and caused damage, and I could show you a monitor with a rather interesting melted plastic burn mark just below the screen. This isn't your normal laser pointer. However, if I took the very same laser we use (which is about the size of a computer case), stuck it in a steel box and then welded it shut, I could call it a class 1 laser and it would be considered safe. That doesn't mean you can cut a hole in the box, wave the laser beam around, and not loose an eye.

Very true. Not that I've checked, but I'd assume the substrate on which the information is etched is more absorbing of the frequency of the laser radiation used than that of the transparent protective polymer layer.

Cheers,

Andy

 

[PsharkJF]

Basically, there's a chemical mixture in a CDRW, that, when the burner does:
Temperature A: Writes a non-reflective point.
Temperature B: Erases CD
Temperature C: Writes a reflective point.

 

[klaviernista]

Originally posted by: Mark R
The phase change layer is not directly adherent to the plastic substrate - there is a ceramic dielectric between the recording layer and the substrate. I would expect that this provides sufficient thermal insulation to prevent the maximum temperature of the substrate from being exceeded.

I concur, typically layer just below the polymer surface of the disc is diamond like carbon or anodized aluminum, both of which can be highly insulative to heat.

Also note: this is assuming a far-field magneto-optical media (one where the laser first passes through the substrate before it hits the recording layer. IN near field media (where the laser hits the record layer before the substrate) things work a bit differently, but insualtive/protective layers are employed to a similar effect.