A printer includes: a printhead; a platen positioned below the printhead for supporting print media conveyed along a media feed direction through a print zone, the platen defining a particle-collection slot upstream of the print zone relative to the media feed direction; and a vacuum chamber in fluid communication with the particle-collection slot. An upper surface of the platen has a plurality of raised ribs extending along the platen in the media feed direction and a dam wall extending across the platen transverse to the ribs. The dam wall is positioned at a downstream side of the particle-collection slot and the ribs extend towards the dam wall from an upstream side of the particle-collection slot.