Enter what is in the bag and what you need in the compound. The calculator gives you the let-down ratio, the dosing rate for your line, and what the masterbatch actually adds per tonne of finished material.
The arithmetic is let-down % = target carbon black % ÷ masterbatch carbon black % × 100. Everything else follows from that.
The let-down ratio is the only number that converts a masterbatch price into a real cost. Two suppliers can quote $2.00 and $2.20 per kilogram, and the $2.20 bag can still produce the cheaper compound — if it carries more carbon black and disperses it properly.
Worked example. A 40% carbon black masterbatch at $2.00/kg, targeting 2.5% in the compound, runs at a 6.25% let-down and adds about $125 per tonne of compound. A 50% grade at $2.20/kg runs at 5.0% let-down and adds about $110 per tonne. The more expensive bag is cheaper per tonne of compound, by roughly 12%.
That comparison only holds if the higher-loaded grade actually disperses at that loading. Undispersed carbon black does not colour the way the bulk number suggests, so operators raise the let-down to compensate — and the advantage evaporates. This is the point where a formulation problem turns into a screen-pressure problem on the line.
| Masterbatch carbon black | Target in compound | Let-down | As a ratio |
|---|---|---|---|
| 20% | 2.0% | 10.0% | 10 : 1 |
| 30% | 2.0% | 6.7% | 15 : 1 |
| 40% | 2.0% | 5.0% | 20 : 1 |
| 40% | 2.5% | 6.25% | 16 : 1 |
| 50% | 2.5% | 5.0% | 20 : 1 |
| 60% | 2.5% | 4.2% | 24 : 1 |
Note that the ratio and the percentage run in opposite directions: a higher let-down percentage means a lower ratio figure. Buyers who mix the two conventions up end up quoting the wrong dosing rate, which is a common source of "your masterbatch is off-spec" disputes.
The let-down ratio is how much masterbatch you dose into the base polymer to reach a target carbon black content in the finished part or film. If the masterbatch contains 40% carbon black and you need 2.5% in the compound, the let-down is 6.25% by weight — about 16 kg of polymer for every 1 kg of masterbatch.
No. A higher-loaded masterbatch usually carries a higher price per kilogram, so the advantage depends on whether the carbon black is genuinely dispersed at that loading. If dispersion is poor, the practical let-down has to be raised to hit colour and UV targets, and the cost advantage disappears — it shows up instead as screen changes and rejected film.
Divide 100 by the let-down percentage. A 6.25% let-down is the same as 16:1 (16 parts polymer to 1 part masterbatch). A 5% let-down is 20:1, and a 4% let-down is 25:1.
Three common causes: the feeder is set by habit rather than by weight, the masterbatch has picked up moisture and is dosing unevenly, or fines in the pellet are feeding inconsistently. Weigh what actually leaves the feeder for one hour before assuming the masterbatch is at fault.
Yes. The loadings differ substantially — white masterbatch is often dosed at 2–5% while a 50% black concentrate may run at 4–6% — but the arithmetic is identical: target loading divided by masterbatch content, multiplied by 100.
Send us your polymer, your target carbon black content and your line output. We will come back with a grade and a let-down recommendation — and a free 1–5 kg sample compounded on your own carrier resin if you want to verify the number on your own line before committing.