A typical single-screw vertical mixer is shown in figure 2. Note the location of the mixer discharge. A considerable amount of feed will remain in the mixer after the last feed leaves the discharge opening. With proportioner mixers, volumetric measures are used to meter the required proportions of each ingredient. An advantage of this system is continuous operation over extended periods.
Calibration can be a problem.
When this is not done, the feed produced can differ significantly from the formula being used. Calibrations should also be made whenever changes of ingredients occur, including new shipments. Accurate metering of drug premixes is difficult with a volumetric system, and may result in over- or undermedicating of feeds. Medicated feed carryover in the mixing chamber at the back of these units can be significant. Withdraw feeds mixed after medicated feeds may have significant drug residues.