Masterbatch Dosers

A masterbatch doser is a unit mounted on the feed throat of an injection molding machine or an extruder that adds color or additive masterbatch to the base resin at a set ratio. Masterbatch is the smallest share of the blend, but it is the most expensive component per kilogram; the doser therefore has two jobs: to hold the ratio on every shot and to keep that share no larger than necessary.

Three common problems in masterbatch dosing

1. Shade variation. The ratio drifts unnoticed; the part comes out lighter on one shift and darker on the next. In units that measure by volume, the cause is usually a change in the pellet size of the masterbatch lot or in the fill level of the hopper. 2. Overconsumption. For fear of missing the shade, the recipe ratio is rounded up on the shop floor. The difference is a cost item nobody sees but that is paid every month. 3. Segregation. Material premixed in the bag or at a remote station separates by pellet size, even while it is being conveyed; the blend that reaches the screw is no longer the blend in the recipe. A doser that adds the masterbatch at the feed throat, just before it enters the screw, eliminates this problem by design.

Volumetric or gravimetric?

The short rule is this: for a job with a single color, a fixed masterbatch supplier and no need for consumption records, a volumetric unit is sufficient and economical. Jobs with more than one color or additive, regrind in the blend, or a customer who requires lot-based records call for a gravimetric doser that measures by weight. A comparison table of the two methods and the capacities of all series are on our plastic raw material dosing systems page.

The right size: masterbatch throughput, not machine throughput

When selecting a doser, two throughputs are considered together: the machine's hourly material consumption and the masterbatch share within it. On a machine processing 40 kg per hour, masterbatch added at 2% amounts to only 0.8 kg per hour. The lower the ratio, the more it matters how evenly the doser delivers small quantities; for additives below 0.5%, micro-dosing units or a belt-type gravimetric doser are preferred. You can find the masterbatch throughput for your own machine with the masterbatch consumption calculator.

Points to watch during installation

• Height: above the feed throat come, in order, the doser, the base material hopper and the hopper loader. The total height must be checked from the start against the ceiling, the crane runway and maintenance access.
• Masterbatch feeding: small hoppers can be filled by hand; if consumption is high, the masterbatch hopper is also connected to the central material conveying line.
• Calibration: on a volumetric unit, a one-minute gram test is run whenever the masterbatch lot changes, and the dosing screw speed is adjusted accordingly. On a gravimetric unit, the zero and weight check of the load cell is written into the maintenance plan.
• Color change: a hopper that can be emptied quickly and dosing parts that can be removed without tools directly shorten the changeover time. Details are in our article on cleaning time during color changes.

Payback of the doser

A machine processing 30 kg per hour and running 5,000 hours a year consumes 150,000 kg of material a year. If the recipe needs 2.6% but 3% is added on the shop floor, the difference of 0.4 percentage points amounts to 600 kg of masterbatch a year. You can see this difference with your own ratios and masterbatch price in the masterbatch savings calculator.