Dosing
Volumetric dosing is a method that measures the additive not by its weight but by its volume — by the speed of the dosing screw, the rotation of a disc or the speed of a belt. It is simple and cheap to install; but when the bulk density changes (a new batch, regrind, a different granule size) the real ratio drifts and it does not measure this. That is why it is sufficient for low-ratio, stable material and inadequate for regrind and multi-component mixtures.
A volumetric device is accurate under the conditions in which it was calibrated. The problem is that conditions change quietly: the masterbatch supplier changes the granule size, the hopper fill level changes the feed pressure to the screw, and the temperature difference between morning and midday affects the flow. When the ratio drifts, it gives no signal.
Belt-type volumetric dosers give a smoother flow than the screw type; there is no pulsation effect and they are suitable for very low ratios. Even so, because there is no weighing, they do not replace a gravimetric system.
When there is a single additive, a fixed supplier, no regrind and a wide color tolerance. If the recipe or material changes often, if records are required or if regrind is used, you should move to a gravimetric system.