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.
By Zoe Lin, Export Manager, E-LUCK (Guangdong Yijuyuan Plastic Industry Technology Co., Ltd.)
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.
Questions buyers ask about the let-down ratio
The calculation is simple; the mistakes are not. These are the ones that show up in almost every enquiry we receive.
What does a let-down ratio of 5:1 actually mean?
It means five parts of your finished part contain one part of masterbatch and four parts of carrier resin, by weight. A 5:1 ratio is therefore 20% masterbatch in the final part. Buyers who say “5% loading” and buyers who say “5:1 let-down” are describing two different things, and the confusion is one of the most common causes of a batch that arrives too light or too heavy.
Should I compare prices per kilogram of masterbatch or per finished tonne?
Per finished tonne, always. Masterbatch price per kilogram rewards a high-loading grade, because you are also paying freight on carrier resin you did not need. The honest comparison is masterbatch price multiplied by the let-down ratio, then divided by the fraction of the finished part that the colour actually has to cover. On a 13% grade at 5:1 and a 47% grade at 1.4:1, the two can reach the same blackness at very different delivered cost.
What is the most common let-down mistake?
Weighing by volume on the line instead of by weight in the hopper. Carbon black masterbatch is a dense, high-loading pellet; a volume-measured scoop can be 10-15% off the intended ratio without anyone noticing. If you are dosing with a gravimetric feeder, the ratio is whatever the feeder is told — check that the setpoint matches the ratio your supplier wrote on the TDS, not the ratio on an old purchase order.
Does the let-down ratio change the dispersion quality?
No, not directly. Dispersion is decided in the compounding step, before your part is ever made. What the let-down ratio changes is how much pigment you deliver per tonne of product, and how hard the carrier has to work to wet it out. A high loading at a low ratio is gentler on the line; a low loading at a high ratio means more carrier to melt through, and more chances for a streak if the compound is marginal to begin with.
Can I change the let-down ratio on an existing grade?
Within reason, yes — grades are supplied to a loading range, not a single number. What you must not do is change it without telling your supplier, because the CoA you receive is written for the loading you ordered. A grade specified at 40% carbon black and used at 12% will not behave like a grade specified at 12% and used at 12%: you will get a lighter appearance, a higher tolerance for error, and a lot number whose certificate does not describe what is in your silo.
Continue reading
- Black masterbatch defects: gels, streaks and specks — the line-side problems that show up when let-down is wrong.
- How to read a black masterbatch CoA — verify the carbon content your let-down math depends on.
- Black masterbatch vs compound vs liquid color — cost-per-part comparison across three formats.
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
- 508C — sewer & drainage pipe grade — a high let-down economy grade where cost-per-part math pays off fastest
- CM50 — 47% carbon film grade — the low let-down, high loading side of the same equation