I've been meaning to post on this for some time, but was somehow hoping to have photos to illustrate the point. Doesn't make all that much sense, I admit, since once I discovered the risk, I wasn't about to repeat the mistake just for a photo's sake.
OK, so what am I talking about? If you put a pen or pen part into an ultrasonic cleaner so that only part of it is immersed (typically, just the nib or nib and section), the ultrasonic waves can travel through the portion that isn't immersed and create a hot spot where the waves converge. That spot can end up hot enough to blister celluloid, as I discovered when cleaning a later-production Sheaffer plunger-filler's Triumph nib with the internal filling unit still attached.
This doesn't happen if the part is fully immersed, since the cleaning solution disperses the heat and probably also dampens the natural resonance of the part. There also seems to be more risk if the barrel is sitting at a slant, as opposed to held vertically -- but in any case, either immerse the assembly to be cleaned completely, or go slow and use multiple short cleaning cycles instead of a single longer one. Depending on the power output of your cleaner, you might want to go with cycles of 15-30 seconds rather than 90-180.
The vulnerability is greatest with thin-walled barrels. Pelikans and other similar celluloid-barreled piston-fillers are at risk, along with plunger-fillers as noted above. I have not experienced problems with barrels of hard rubber or acrylic to date, nor with the thicker celluloid barrels of Vacumatics.
ADDENDUM: Here is a photo of a blistered Sheaffer plunger-filler barrel. It was almost fully immersed while in the ultrasonic, but "almost" clearly wasn't enough, as the hot spot formed right above the waterline. The cycle time was 180 seconds.
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