What the duplex unit on your office MFP actually does
Quick definition
The duplex unit is the internal mechanism that flips a printed sheet and runs it back through the print engine to print the second side. Without a duplex unit the device prints only one side at a time and the operator must manually reload paper to print the second side. Modern office MFPs include duplex units as standard equipment.
How the duplex unit works
After the first side prints, the diverter at the fuser exit routes the sheet into the duplex loop rather than to the output bin. The duplex loop flips the page using a series of curved guides and rollers, then feeds the sheet back to the registration assembly with the unprinted side facing the print engine. The page receives its second-side image and exits to the output bin as a finished duplex sheet.
Why duplex is slower than simplex
Duplex throughput is typically 60-75% of simplex throughput because the duplex loop holds each page briefly while the prior page completes its first side. The engine cannot process pages continuously; it interleaves between first-side and second-side prints. The trade-off is acceptable for paper savings produced by routine duplex printing.
Paper limitations on the duplex path
Duplex units handle standard paper weights well (60-160 gsm typically) but cannot route heavier stocks through the loop's bends. Envelopes, labels, transparencies, and heavy cardstock cannot duplex. The device's paper type setting controls whether duplex is available — selecting heavy stock automatically disables duplex.
Why duplex defaults matter
Offices configuring duplex as the default print mode reduce paper consumption by 35-45% across typical workloads. The setting lives in the print driver defaults or the MFP's web admin. Setting duplex as default is among the highest-impact sustainability and cost optimisations available on office MFPs.