Abstract: A screen unit for removing solid particles from a viscous mass, for example molten plastic. The screen unit includes a housing provided with a supply channel and a discharge channel for the viscous mass, a screen element in the housing between the supply channel and discharge channel, a backflush channel for discharging during backflushing solid particles screened by the screen element and connected to an outflow channel by a shutoff valve, and a pressure element which can be operated to flush back the viscous mass through the screen element at a pressure higher than the pressure in the supply channel. The shutter element is placed along the screen element, and the shutter element is caused to move with respect to the screen element such that during the screening phase the screen element is gradually more exposed for screening and during the backflushing phase the screen element is gradually less exposed to the backflushing action.