Dosing
Gravimetric dosing is a dosing method that blends each component by weighing it according to its weight, not its volume. A load cell measures each batch by its real weight; even if the granule size, bulk density and regrind ratio change, the recipe ratio stays constant. Compared with the volumetric method it reduces masterbatch consumption by 10–30% and keeps a record of every batch.
A volumetric doser relies on the screw speed or the time it stays open; when the material density changes the ratio drifts, and the operator overdoses the additive to stay on the safe side. A gravimetric system weighs and verifies; the extra masterbatch is thus eliminated.
There are two basic designs: in the batch type the components are weighed in sequence and discharged into a mixing chamber; in the continuous type each component flows from its own loss-in-weight line. In injection molding the batch type is common; the batch weight is the parameter that limits precision with low-ratio additives.
It depends on masterbatch consumption. The calculator gives the annual saving from the current ratio, target ratio and masterbatch price; with high-ratio color dosing the payback can come within a few months.
In the order of ±0.1% on modern batch-type systems; but this depends on the batch weight and on the share of the smallest component within that batch. Weighing a 0.5% additive in a 1 kg batch does not give the same precision as weighing it in a 100 g batch.