Inconsistent Masterbatch Dosing: Causes and Fixes

Inconsistent masterbatch dosing shows up as color or additive-content variation between parts or between production batches, even when the doser setting hasn't changed. The most common causes are a change in material bulk density (regrind, humidity, particle size), a doser type that isn't matched to how variable the material is, or variation in the masterbatch itself from batch to batch.

TL;DR - check these first:

• Check whether bulk density of the base material has changed - new regrind ratio, humidity, different pellet batch
• Confirm whether the doser is volumetric or gravimetric, and whether that matches how variable the material is
• Compare the current masterbatch lot against a previous one for pigment concentration
• Run a timed output/weight-check on the doser itself, independent of the part
• Rule out a partially blocked or worn feed screw causing inconsistent flow

Is bulk-density variation the cause?

A volumetric doser meters a fixed volume per cycle; if the bulk density of what fills that volume changes, the actual weight of material - and masterbatch percentage - dosed per shot changes too, even though nothing on the doser was adjusted.

How to confirm: Compare current bulk density of the material blend (including any regrind) against the value the doser was originally set up for; a rising regrind ratio, humidity pickup or a new pellet batch are the usual triggers.

Fix: Recalibrate the volumetric doser whenever the material blend changes, or move to a gravimetric dosing unit, which meters by weight and holds the set percentage regardless of bulk density.

Is the doser type mismatched to the material?

A volumetric unit is sufficient with a stable, consistent material stream and looser tolerance on additive content. Once regrind of varying density, humidity variation, or a specified additive tolerance is involved, weight-based dosing is what actually holds the target consistently.

How to confirm: Check how much bulk density realistically varies in your material stream day to day; if it varies meaningfully and the part has a specified additive content, a volumetric doser is the wrong tool regardless of how well it's calibrated.

Fix: Where the application needs it, use a gravimetric dosing unit instead of a volumetric one - it doses by weight, so it compensates automatically for bulk-density changes that would throw off a volumetric setting.

Is masterbatch batch-to-batch variation the cause?

The doser can be metering exactly the same weight percentage shot to shot, but if the masterbatch itself carries a different pigment concentration from one batch to the next, the parts still come out different.

How to confirm: Mold a sample using the same doser setting but switching only the masterbatch lot; if color shifts with the lot change and not with the doser setting, the masterbatch batch is the variable.

Fix: Request a certificate of pigment concentration per masterbatch lot from the supplier, and re-check color match whenever a new lot arrives, independent of any doser adjustment.

Cause overview

Perotec Systems supplies Plastic Systems volumetric and gravimetric dosing units, sized to your material stream and additive tolerance. See also Sink Marks on Injection Molded Parts: Causes and Fixes for related material-preparation diagnostics. Contact us with your current doser type and material mix - we help determine whether the fix is calibration, doser type, or the masterbatch supply itself.