Speed of Stereolithography Resins

From: Bronson R. Hokuf (hokufbr@a1.esvax.umc.dupont.com)
Date: Tue Jan 28 1997 - 19:08:01 EET

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Andy recently wrote approximately the following:

    ...an Ec of 5 makes a resin the fastest on the planet...

Looking at Ec alone to determine the speed of a resin is a mistake. One
must consider both the Ec and Dp. The exposure required to expose a
layer is given by the formula:

    Energy = Ec * e ^ (Thickness / Dp) or

    E = Ec * e ^ (T / Dp),

    Energy (E) is the energy required to produce a layer of Thickness (T)
            E = mJ /cm^2
    Thickness (T) is the thickness of the desired layer (mils or mm)
    Ec = mJ / cm^2 from the working curve
    Dp = Slope of the Working curve (mils or mm)

For example:
    To produce a layer 10 mils (0.25 mm) thick,
    using a resin with Ec = 8 mJ/cm^2 and Dp = 5.6 mils (~0.14mm),
    one would calculate:

    E = 8 * e ^ (10/5.6) = 47.7 mJ/cm^2

DuPont, 3D and Ciba have used the terminology E-10 to refer to such a
number, where E-10 is to be interpreted as "the energy required to cure a
layer 10 mils thick".

Other layer thicknesses can be reported, for example, E-5 would refer to
a 5 mil layer.

One must still be careful when using such an approach. Real parts in
real equipment have many different exposures used to create the layers.
A more careful analysis of the actual typical or dominate exposures is
required to make the most meaningful comparison. At the simplest level
one should compare the hatching exposures since hatching tends to
predominate in typical part geometries. If one images a free floating
layer of a part and measures the actual thickness produced you may find
that for a 6 mil (0.15 mm) layer, the total thickness, with overcure, is
around 10 mils (0.25 mm). Consequently the E-10 comparison is not a bad
one to make when comparing materials being used for 6 mil layers.

In the example cited earlier, an Ec of 5 was mentioned, Dp was not
mentioned. Consider the following E-10 values computed for 3 different
Dp values:

Dp = 4, Ec = 5, E-10 = 5 * e^(10/4) = 61 mJ/cm^2

Dp = 5, Ec = 5, E-10 = 5 * e^(10/5) = 37 mJ/cm^2

Dp = 6, Ec = 5, E-10 = 5 * e^(10/6) = 26 mJ/cm^2

The third material is twice as fast as the first by this measure, even
though they all have the same Ec of 5.

Historically we were able to compare the earliest resins by only looking
at Ec, since they all had approximately the same Dp. We can no longer do
this. We must take both Ec & Dp into account.

Thanks, Bronson

PS. The opinions are my own, don't blame anybody else!
Bronson R. Hokuf USMail: DuPont Company
DuPont Company (NCCC/PWay) 2 Penn's Way, Suite 401
Advanced Material Systems (P&IP) New Castle, DE 19720
Somos(tm) Solid Imaging Materials Tel: 302-328-5635
E-Mail: hokufbr@esvax.dnet.dupont.com Fax: 302-328-5693

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