Hi Gareth,
Stardraw is able to define a minimum pen width for solid lines; this is defined in tenths of a millimeter by the document attribute "_DefaultPenWidth". Lines that use a style other than solid cannot support pen widths other than 0 - this is a Windows thing and beyond our control - so they will have a width that is interpreted by your printer as "as thin as I can print". This is obviously a high-res printer, which is why the lines are faint.
As far as we know there is no setting either in Windows or the printer that could beef up the minimum printed line width. You will be able to work around this, however, if you simply switch from using dashed or dotted lines to solid lines.