Brother HL-1800 series - Manual (Page 41)

CHAPTER 1 ABOUT THIS PRINTER
1-31
☛ ☛ ☛ ☛ 3.
For Windows User
Open the Properties dialog box in the printer driver. Select Duplex Printing
mode in the Advanced tab, make sure that ‘Use Duplex Unit’ has been
selected, and then click the OK button. The printer will print on both sides of
the paper automatically.
<PCL driver>
<PS driver>