Re: What is "critical exposure"?

From: Guy Allen Brady (galbrady@engin.umich.edu)
Date: Wed Jan 21 1998 - 19:31:14 EET


On Wed, 21 Jan 1998, Werdinius Christian wrote:

> Im going do to some experiments on stereolitografi, trying to get an
> extremly small cured point, with a very focused laser. Ive got some Ciba
> Tool SL5190 to do the experiment with. According to the label on the bottle
> its supposed to work with a Nd-YVO4 laser. Is that the standard laser in
> the SLA-machines?

Not all. Some are He-Cd, some are Ar+. These are continuous wave lasers,
the Nd-YVO4 is a diode-pumped pulsed laser. In other words, always on
versus pulsed.

> Ive heard that the wavelength of the laser is 354 nm, is that correct?
> Im going to work with a pulsed dye-laser, 50Hz with a variable wavelength
> of 210-750 nm - do you think there will be any problems working with that
> laser, if I tune it to the right wavelength?

He-Cd is 325 nm, Ar+ is 351 (I think) and Nd-YO4 is 354 nm. As for using a
tunable laser, I think you have to worry about the power you get at the
wavelength you choose.

> The "critical exposure" for the resin is 14,9 mJ/cm2 and the "penetration
> depth" 4,6 mils (is mils=millimeter?). Whats the meaning of these two
> numbers?

Critical exposure is the critical dose needed to gel the polymer. In
practice, however, you'll need more than this to get a good spot cure of
any measurable thickness.

Penetration Depth is 4.6 mils = 0.0046 inches = 117 microns = 0.117 mm.
Dp is the depth to which the irradiance is reduced to 1/e of its original
value. (Jacobs, 1992)

The meaning of the numbers in practial terms gives the SLA machine the
ability to calculate how fast to draw a vector on the resin to cure it to
a prescribed depth according to:

Cd = Dp*ln(E/Ec)

where Cd is cure depth, Dp is penetration depth, Ec is critical exposure
and E is average exposure dose. Given your resin parameters (Dp and Ec)
and your Cd, you can calculate the exposure needed, and from that, the
speed to draw your line (or time to expose a spot) if you know the power
of the laser.

> Ive heard that you get a better finish with a longer exposuretime at a
> lower power than the opposite, is that true?
> In which ranges can you then vary the time and power parameters?

Yes, but I don't know anything about the effect of trading time for power
on the surface finish.

> I want to make my focused point on a flat plate , with a good adhesion
> towards the polymer. Which material do you suggest?

Material for what? The flat plate? Remember that you may get reflection
from the plate back into the resin if you only have a thin layer - this
could well turn your nice small spot into a big blob.

Hope this helps.

          G. Allen Brady -- Graduate Research Assistant
Materials Science and Engineering - The University of Michigan
2219 H.H. Dow Bldg. 2300 Hayward Street Ann Arbor, MI 48109-2136
work: 313/936-0177 fax: 313/747-4807 email: galbrady@umich.edu
           http://www-personal.engin.umich.edu/~galbrady



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