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Hello;
2 weeks ago Guy Allen Brady posted a question regarding long-term, in-use
stability of epoxy stereolithographic resins. Guy's original message and a
response fron the DuPont Somos Materials Group follows.
Guy Allen Brady wrote the following:
> I have a question regarding the perishabilty of stereolithography
> resins, particularly SL 5170 epoxy.
>
> As resin is used and replenished with new resin as needed, at what point
> do users "purge" the old resin and start with 100% new resin? Does it
> ever need to be "refreshed"? If so, what criteria is used to determine
> when this needs to be done? Is it too much crap at the bottom of the
> vat, viscosity measurements, the "feeling" of the operator, part
> quality-strength, rate of resin usage, laser hours, cash on hand to buy
> resin....etc.
>
> Is the "old" resin filtered and resued at some point??
>
> Or, are there cases where new resin is added continuously without any
> purging process???
>
> I would appreciate any comments on this issue. It would be best to get
> both 3D's Resin R&D folks (or equivalent) as well as anecdotal evidence
> from typical users.
>
> Thanks, all.
> --
> 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 48105
> work: 313/936-0177 fax: 313/747-4807 email:
> galbrady@umich.edu
> http://www-personal.engin.umich.edu/~galbrady
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Response form the DuPont Somos Solid Imaging Materials Group:
Somos(r) Technical Bulletin #2
Version 1.1
Stability of Epoxy Stereolithography Resins
---------------------------------------
The following briefly explains some of the issues related to the long-
term and in-use stability of epoxy based stereolithographic
photopolymers.
Epoxy resins, such as DuPont Somos(r) 6100/6110 (and probably
Cibatool(r) SL-5170/5180/5190), are likely to have a
shorter in-use life than acrylate resins. The stability of acrylates
is largely maintained by oxygen which inhibits the polymerization and
is constantly replenished by the atmosphere. Epoxy resins can be
stabilized by various mechanisms, but the stabilizer is generally
consumed without the benefit of an automatic replenishment as in the
case of atmospheric oxygen for acrylate resins.
The polymerization will continue within the epoxy resin unless it is
stopped by a stabilizing inhibitor. The primary indicator of epoxy
resin instability is a viscosity change, which typically increases
exponentially with time. Addition of new resin will lower the
viscosity of unstable vat resin and may provide additional
stabilizer. However, the new stabilizer may be quickly consumed
allowing the viscosity to continue to increase. At some time, when
the vat resin viscosity begins to affect build performance, the resin
in the vat should be completely replaced and the vat should be
cleaned.
Our testing of Somos epoxy resins shows that exposure to light,
which does not result in hardening, is a primary cause of epoxy resin
instability. Under such conditions, where the exposed epoxy remains
soluble in the surrounding resin, the epoxy polymerization reaction
continues. If the epoxy hardens due to the exposure and becomes
essentially insoluble in the surrounding resin, such as when part
regions are correctly imaged, the polymerization is substantially
contained within the hardening matrix and will not lead to vat resin
instability. Examples of light exposure that lead to instability of
epoxy resins are:
+ Exposure from the low intensity portion of the laser beam's
Gaussian profile (outside the high intensity portion which
hardens the resin during part fabrication); this occurs most
often when drawing supports, borders, and QuickCast(tm) style or
other high surface-area-to-volume type parts; + Exposure given
during scanner jumps; + Exposures due to scattered laser light
within the build chamber; + Exposure due to window sunlight and
fluorescent room light.
A second major cause of reduced in-use life of epoxy resins is due to
the diffusion of resin into and out of hardened part regions. In such
cases, the unexposed resin may "extract" the reactive components from
the hardened region and initiate slow polymerization within the main
volume of the vat resin. Some examples of conditions that might lead to
epoxy instability by such an extraction process are as follows:
+ Draining of QuickCast(tm)-like parts, and extensive draining of
standard parts, over the vat region; + Broken part sections and
supports that sink to the bottom of the vat; + Overexposure of
parts, high vat temperatures, and/or very low humidity conditions.
Practices that minimize the conditions that lead to epoxy instability
due to stray light or due to "extraction" will provide a longer in-use
life for stereolithographic epoxy resins. Some practices a user, wishing
to extend the in-use life of epoxy resins, can implement are:
+ Reduce ambient UV Light: - Install "gold" colored, or UV
shielded, fluorescent lamps in the work area; - Install tubular
light traps around each beam sensor and at the beam rest position;
- Use the maximum jump speed recommended by the equipment
vendor; - Use the minimum necessary overcures on borders and
down-facing regions.
+
Reduce vat resin contamination:
- Avoid re-using resin drained from QuickCast(tm)-like parts;
- Minimize draining parts over the vat after fabrication;
- Remove scrap pieces of parts and supports from the vat;
- Maintain the build chamber environment within the resin
manufacturer's specifications.
For Further information contact:
The DuPont Company
Somos(r) Materials Group
Two Penn's Way, Suite 401 Tel: 302-328-5435
New Castle, DE 19720 Fax: 302-328-5693
--------------------------------------------
Cibatool(r) is a registered trademark of Ciba
Somos(r) is a registered trademark of E. I. du Pont de Nemours & Co.
QuickCast(tm) is a trademark of 3D Systems, Inc.
================================================================
Bronson R. Hokuf USMail: DuPont Company
DuPont Company 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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