A 0.5 % difference in masterbatch can be the line between a part that is on colour and a rejected pallet. And if the masterbatch is expensive, every extra tenth is paid for every day. This guide explains how to calculate the percentage, how to convert it into grams per shot and how to set a masterbatch doser so that it delivers what the setpoint says.
The percentage formula
The masterbatch supplier gives a recommended dosage, usually between 1 and 5 % for colour, according to the datasheet. The first thing is to know what basis it refers to, because there are two ways of stating it:
- Percentage of the total (the most common): % = masterbatch / (masterbatch + natural material) × 100.
- Let-down ratio : 1:50 means 1 part of masterbatch per 50 parts of natural material, i.e. 1 / 51 ≈ 1.96 % of the total.
Mixing up the two bases is a small error on paper and an expensive one in production. If in doubt, ask the supplier.
From percentage to grams per shot: example
Injection moulding works in shots, not kilos. The calculation starts from the full shot weight, including the runner if it is not a hot runner.
Example data:
- Shot weight (part + runner): 250 g.
- Masterbatch dosage: 2 % of the total.
- Cycle: 30 s.
- Screw recovery time: 8 s.
| Quantity | Calculation | Result |
|---|---|---|
| Masterbatch per shot | 250 g × 0.02 | 5 g |
| Natural material per shot | 250 g − 5 g | 245 g |
| Shots per hour | 3600 s / 30 s | 120 |
| Masterbatch consumption | 5 g × 120 | 0.6 kg/h |
| Flow rate during recovery | 5 g / 8 s | 0.63 g/s |
The last figure is the one that matters for setting the unit. The doser works in sync with screw recovery, not during the whole cycle. If you calculate it over 30 s instead of 8 s, you will dose almost four times less than needed.
Volumetric or gravimetric doser
| Criterion | Volumetric | Gravimetric |
|---|---|---|
| What it controls | Volume: screw turns or speed | Actual weight, with a load cell |
| Calibration | Manual, every time the material changes | Self-correcting during production |
| Sensitivity to bulk density | High | Low |
| Investment | Minor | Higher |
| When it fits | A stable colour, moderately priced masterbatch | Expensive additives, tight colour tolerances, traceability |
A well-calibrated volumetric doser gives very repeatable results with the same masterbatch. The problem appears when the pellet size or the batch changes and nobody recalibrates.
How to calibrate a volumetric doser
- Fill the doser hopper to its usual working level.
- Put the unit in test mode at the starting speed and collect the material for a fixed time, for example 60 s.
- Weigh the sample on a precision scale and calculate the actual flow rate in g/s.
- Adjust the speed proportionally: if you need 0.63 g/s and get 0.50 g/s, increase the speed by 26 %.
- Repeat the test until the difference is within your tolerance and record the value for that masterbatch.
Recalibrate whenever the masterbatch, the batch, the recovery time or the shot weight changes.
To check the real dosage in production, record the masterbatch consumption over a shift and divide it by the total material processed in that shift. If the result drifts from the setpoint, the problem is usually the calibration or the recovery time, not the masterbatch.
Single or double dosing
A single doser adds one component to the natural material: normally a colour masterbatch. A double doser adds two independent components, each with its own setpoint. It is useful when the part needs colour plus an additive (antistatic, UV, slip) or when you want to adjust each one separately without preparing premixes.
If you work with regrind, remember that it also counts as part of the blend and that its proportion affects the final colour. You can see the options in our range of industrial dispensers.
Typical mistakes
- Calculating on the part weight and forgetting the runner.
- Using the full cycle instead of the recovery time for the flow rate.
- Not recalibrating after changing the masterbatch batch or supplier.
- Doser hopper almost empty: in a volumetric unit, the level affects how the screw fills.
- Wet masterbatch: some carriers, such as PA or PET, also need drying. See our guide to plastic drying.
- Poor mixing at the feed throat: colour streaks usually point to poor mixing or low back pressure, not always to a dosing error.
The loader also plays a part: a vacuum loader that leaves the machine hopper empty intermittently causes ratio variations.
Frequently asked questions
What percentage of masterbatch is used?
The supplier states it on the datasheet. For colour it is usually between 1 and 5 %, although it depends on the pigment, the part thickness and the target shade.
How do I convert a percentage into grams per shot?
Multiply the full shot weight, including the runner, by the percentage. With 250 g and 2 %, that is 5 g per shot.
How often should a volumetric doser be calibrated?
Every time the masterbatch, the batch, the shot weight or the recovery time changes, and periodically as a check.
When is a gravimetric doser worth it?
When the additive is expensive, the colour tolerance is tight or you need to record actual consumption per batch.
Want to fine-tune your dosing?
Tell us the shot weight, the cycle and the dosage your supplier recommends, and we will help you choose the equipment. Try the equipment selector or talk to a technician.