A sieve unit for melted plastics comprises two parallel ducts, each contg. a replaceable sieve, a main cut-off valve downstream of the sieve, a discharge opening with its own cut-off valve at or upstream of the sieve, and a further valve upstream of the sieve, this being also pref. a main cut-off valve. The ducts near the main cut-off valves are pref. shaped to avoid dead zones, more specif. with curved portions contg. the valve seating. When replacing one sieve, the associated downstream valve is fully closed, the further valve is throttled; the sudden increase in available volume in the duct resulting from the introduction of a new, empty sieve, e.g., in 1/10th sec., does not cause feed to be abstracted from the other, operational duct, since the throttle acts as a barrier; therefore the total throughput continues uninterrupted. Also, the discharge permits the branch to be flushed through with a charge of the matl.
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