Re: What is "critical exposure"?

From: Marshall Burns (
Date: Wed Jan 21 1998 - 20:26:53 EET

Werdinius Christian wrote:

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

Dear Christian,

    Here is an except from "Automated Fabrication" (page 259..60) on these two

     "Light incident on the surface of a resin is absorbed as it passes into
and through the liquid. The PENETRATION DEPTH, Dp, is defined as the depth at
which the intensity of the light has been reduced to about 1/3 (actually to
(1/e)=0.37) of its value at the surface. Do not confuse this quantity with the
gel depth, defined below. Dp is a fundamental property of a resin; gel depth
depends also on the intensity and duration of the incident light.

    "As light impinges upon a particular region of resin over time, if the
total exposure (incident energy per unit area) is less than a certain value,
then the photochemical reactions do not proceed far enough for the resin to gel
or acquire stiffness. In other words, that region of resin does not turn solid.
If, on the other hand, the region is exposed to more than this amount, then it
does turn solid. This amount of exposure required for gelation is the CRITICAL
EXPOSURE, Ec, and the stiffness acquired by the resin depends on how much the
exposure exceeds this value. The existence of this critical value of exposure
is one of the fundamental assumptions of the Evans-Jacobs theory.

    "Dp and Ec are of great importance to the fabricator operator because,
together with the intensity and duration of incident light, they determine the
GEL DEPTH. This is the maximum depth of resin that acquires any stiffness under
a particular exposure. (This is often called the “cure depth,” but this term
may be misleading because curing reactions actually take place far beyond this
depth.) The operator uses the gel depth, in conjunction with parameters
controlling the incident light, to determine important aspects of the build
process, such as layer thickness, the width of the gel region (if the light
source is a laser), and gelation efficiency. Published values of Dp and Ec for
many commercial fabricator resins are given in Table 7-4(b)."

    I hope this helps.

Marshall Burns

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