How to calculate let-down ratio for black masterbatch
Two quotes can differ by 30% per kilo and still cost you the same — or one can be cheaper per kilo and cost you more. The number that settles it is the let-down ratio. Here is how to work it out, and where most buyers get it wrong.
1. What let-down ratio actually means
Let-down ratio is simply how much natural polymer you dilute one part of masterbatch into. A 2% let-down means 2 kg of black masterbatch mixed into 98 kg of natural resin to make 100 kg of black product. A 5% let-down means 5 kg into 95 kg.
The ratio is set by how much pigment the masterbatch carries and how dark the finished part has to be. A masterbatch loaded at 40% carbon black can run a much lower let-down than one loaded at 20% — and that is exactly why the cheaper bag is often not the cheaper choice.
2. The formula
Three numbers decide everything:
- Carbon loading — the % of carbon black inside the masterbatch (we supply roughly 9% to 60%).
- Target loading in the finished part — the % of carbon black the part itself needs. For most general-purpose black, that is around 1.0–2.5%; for UV-protected outdoor products such as pipe and geomembrane it is higher.
- Let-down ratio — what you get when you divide one by the other.
Let-down % = (target carbon % in the part) ÷ (carbon % in the masterbatch) × 100
Example A: your part needs 2.0% carbon black and the masterbatch carries 40%.
2.0 ÷ 40 × 100 = 5% let-down — 5 kg masterbatch per 95 kg natural resin.
Example B: same part, but the masterbatch carries only 20%.
2.0 ÷ 20 × 100 = 10% let-down — 10 kg masterbatch per 90 kg natural resin.
3. Why the cheaper bag per kilo can cost more
Take those two examples and put a price on them. A 20%-loaded grade might sell for noticeably less per kilo than a 40% grade — but you have to buy twice as much of it.
| Grade A — 20% carbon | Grade B — 40% carbon | |
|---|---|---|
| Let-down needed | 10% | 5% |
| Masterbatch per 1,000 kg of product | 100 kg | 50 kg |
| Natural resin per 1,000 kg | 900 kg | 950 kg |
Even before you price the resin, Grade B halves your additive volume. In practice the higher-loaded grade usually wins on total cost — but not always, because very high loadings are harder to disperse, and that has a cost of its own.
4. Three mistakes that quietly inflate real cost
- Comparing price per kg instead of cost per finished part. This is the big one. Always convert two quotes to "cost to make 1,000 kg of my product" before comparing.
- Pushing let-down too low to save money. Below a certain level the masterbatch cannot distribute evenly through the melt, and you get streaks, shade variation or weak spots. The saving disappears in scrap.
- Ignoring the carrier. A grade designed for PE will not disperse the same way in PP or PET. Running the wrong carrier costs you more in defects than you saved on price.
5. How we set let-down at the plant
When you send us a specification, we ask for the polymer, the application, how dark the part has to be, and — for outdoor products — the service life you need. From that we work back to a carbon loading and a let-down that disperses cleanly on your equipment, rather than the highest number we can technically load.
Every sample ships with let-down guidance so your operators can run it straight away. If a loading will not disperse properly in your polymer, we say so instead of shipping it.
Related pages
- Black masterbatch pricing — what drives cost per kg and how to get a firm factory quote
- Film grades — thin-gauge film where let-down matters most
- Garbage bag grades — high let-down economy grades
- All black masterbatch — full grade range and selection guide