or plunger assembly

some sites use "plunger rod" but unless you have a strong preference I think the "rod" is redundant.

see here
http://www.marchesini.com/it/news-events/starletter/dettagli...

The english translation they use here for astinatura is plunger application. Not sure that is good English. Perhaps plunger assembly or plunger insertion would sound more authentic - see this link

equipping a complete syringe line, from filling to ... - ONdrugDelivery
www.ondrugdelivery.com/.../Prefilled Syringes 2009/...
File Format: PDF/Adobe Acrobat - Quick View
machine or to the plunger insertion and label- ling machine (series AEC by Corima). The insertion of the plunger on the syringe, likewise for the syringe pick-up ...

Bosch Showcases New Rod Insertion and Labeling Machine for ...
news.thomasnet.com/.../Bosch-Showcases-New-Rod-Ins... - United States
1 May 2012 – The syringes are delivered to the rod insertion and labeling machine from the filling line and a plunger is inserted. This is performed by gently ...

Syringe Assembly & Labeling Equipment - MG America
www.mgamerica.com/.../syringe-assembly-labeling-equipment/
Hasta syringe insertion machine, syringe assembly equipment, machinery for ... This syringe assembly machine is designed specifically to insert plunger rods ... a P&A labeler with vision systems and control devices for trouble-free production.