Dosing
Dosing, in plastics processing, is adding color masterbatch, additives or regrind to the base resin at a defined ratio. The doser is usually mounted on the feed throat of the injection molding machine, directly below the hopper. There are two basic methods: volumetric dosing measures material by volume (screw speed, belt speed), gravimetric dosing by weight. The accuracy of the ratio determines both color and property consistency and the consumption of masterbatch, the most expensive component.
The word dosing is also used in water treatment and the food industry, where a liquid chemical is usually pumped into a line. In plastics the dosed material is a solid in granule or flake form, and the real difficulty is that the bulk density of the granules changes from lot to lot, even within the same bag.
Three questions decide the method: how many components are there, is regrind used, and must the ratio be recorded? With a single additive and stable material, a volumetric doser is sufficient and economical. When regrind, a multi-component recipe or traceability is involved, a gravimetric system is the choice; weighing is not affected by density changes.
The position of the doser also affects the result. A doser mounted on the machine feeds the blend directly to the screw, and the risk of segregation is low. A blend prepared at a central station can segregate by granule size in the conveying line; with components of different sizes this shows up as color fluctuation.
No. Dosing means feeding the components in a measured ratio; mixing means making the fed components homogeneous. Batch-type gravimetric systems do both; with a volumetric doser the blending mostly takes place in the screw itself.
Masterbatch is usually added at 1–4%. Start with the ratio recommended by the supplier and verify it by color measurement; at low ratios the precision limit of the doser becomes decisive.
Throughput (kg/h), the number of components and the use of regrind decide. For gravimetric systems the series is selected with the capacity table on the category page; volumetric units are sufficient for simple single-additive applications.