RE: Tacky Exactomer parts, take 2

From: Mark Newton (Motorola  Inc.)
To: RP-ML
Date: Friday, July 21, 1995
Subject: RE: Tacky Exactomer parts, take 2
Hi Jeff, 
The most humidity sensitive SLA resin system is the Ciba Geigy's epoxy-acrylate 
("QuickCast") followed closely by Allied Signal's vinyl ether (Exactomer).  Ciba 
and 3D systems are developing a new epoxy system that should be more moisture 
resistant.  The older acrylate systems are not particularly humidity sensitive, 
but they suffer from inferior physical and dimensional properties.  3D systems 
recommends that the R.H. should stay below 50% for the epoxy system an we have 
found that this is critical for the vinyl ether system as well.  The ambient 
R.H. at 3D System's facility in Valencia, CA is almost always less than 40%.  If 
the lab and build chamber is not humidity controlled, however, Exactomer parts 
can become irreversibly tacky.  The cleaning solvents must also be dry.  We've 
had the best luck cleaning with dry isopropanol (IPA) in a dry (<40% R.H.) lab.  
Others have found chlorinated hydrocarbon solvents effective, however we do not 
use these because of regulatory restrictions. The trick is to clean parts as 
quickly as possible with a solvent that evaporates easily and will not distort 
or soften the part during the cleaning operation.  Several quick rinsings, 
brushings, or hand wipes with CLEAN solvent are better than a single cleaning 
step for removing uncured resin from the surface.  It is usually uncured resin 
containing adsorbed water that produces a tacky surface.  Postcuring for 20 to 
40 minutes in a humidity controlled UV oven (PCA) often helps stiffen the parts 
and in the case of the vinyl ether resins, will also produce a hard, tack free, 
workable finish that is even resistant to subsequent water exposure.  For more 
information I suggest that you contact either Rob Connelly at Becton Dickenson 
(919) 990 2263, or Frank Garcia (305) 723 5729 and Derek Smith (305) 723 4790 at 
Motorola.  They have lots of practical experience with both the vinyl ether and 
epoxy systems.

Regards,
Mark Newton
Motorola, Inc.
305 723 4416