Quick answer: The let-down ratio for flame retardant masterbatch is calculated from the active flame retardant your compound needs, not from a rule of thumb. If your formulation requires 20% active flame retardant and the masterbatch carries 50% active content, you dose 40% masterbatch — a 1:1.5 let-down. Typical flame retardant masterbatch addition runs 10–40% of total throughput, far higher than the 2–5% people expect from colour masterbatch, because flame retardancy needs bulk loading while pigment needs only traces.
This is the single most common technical question we get, and it usually arrives in a form we cannot answer: “what percentage of your masterbatch should I add?” There is no universal number, and any supplier who gives you one without asking what is in the masterbatch is guessing. But the calculation behind it is genuinely simple — three numbers and one division — and once you can do it yourself, you can compare quotes from different suppliers on equal terms instead of comparing prices on incomparable products.
The Only Formula You Need
Q: How do I calculate flame retardant masterbatch let-down ratio? Divide the active flame retardant level your compound needs by the active content of the masterbatch.
Masterbatch addition (%) = Required active FR in final compound (%) ÷ Active FR content in masterbatch (%) × 100
Worked example. Your PP part needs UL94 V-0 at 1.6 mm, and the qualified formulation calls for 22% of an intumescent flame retardant system in the finished part. You are quoted a masterbatch at 55% active content on a PP carrier:
22 ÷ 55 × 100 = 40% masterbatch addition — expressed as a let-down ratio, 1:1.5 (one part masterbatch to 1.5 parts virgin resin).
Note what that means physically: 40% of everything going into your hopper is masterbatch, and 22 of every 100 kg leaving as finished part is flame retardant. This is why flame retardant masterbatch behaves nothing like colour masterbatch on a production line — you are not tinting a polymer, you are co-feeding a second material stream.
Reading Quotes Correctly: Active Content Is the Only Comparable Number
Two suppliers quote “flame retardant masterbatch for PP.” One is $3.20/kg, the other $4.40/kg. Which is cheaper?
| Supplier A | Supplier B | |
|---|---|---|
| Masterbatch price | $3.20/kg | $4.40/kg |
| Active FR content | 35% | 55% |
| Addition needed for 22% active | 63% | 40% |
| Masterbatch cost per kg of compound | $2.02 | $1.76 |
| Virgin resin displaced | 63% (more carrier in your part) | 40% |
The “expensive” masterbatch is the cheaper one per finished kilogram — and it also leaves you far more control over the resin in your part, because 60% of the polymer is your own specified grade rather than someone else’s carrier. Always ask for active content in writing. A supplier who will not state it is selling you an unknown dilution.
Typical Addition Ranges by System
| System / target | Active FR in compound | Typical masterbatch addition |
|---|---|---|
| Brominated + Sb₂O₃ in PP/PE, V-2 | 8–14% | 15–25% |
| Brominated + Sb₂O₃ in PP/PE, V-0 | 15–22% | 25–40% |
| Intumescent (APP) halogen-free PP, V-0 | 20–28% | 35–50% |
| Mineral (ATH/MDH) LSZH compound | 55–65% | Usually supplied as full compound, not masterbatch |
| Colour masterbatch (for comparison) | — | 1–4% |
The bottom two rows explain a lot of confusion. Mineral systems need such high loading that masterbatch stops making sense — at 60% ATH there is barely any carrier left to make pellets from, which is why LSZH material is sold as a finished compound instead. And the colour masterbatch row is the mental model most people arrive with; forget it here, the numbers are an order of magnitude apart.
Five Ways the Calculation Goes Wrong in Practice
- Confusing masterbatch percentage with active percentage. “We’re running 30% flame retardant” can mean 30% masterbatch (perhaps 16% active) or 30% active (perhaps 55% masterbatch). We have seen a whole production run fail UL94 because two engineers used the same sentence to mean different things. Write both numbers on the work order.
- Ignoring what the carrier does to your part. At 40% masterbatch addition, nearly half your polymer comes from the masterbatch carrier. If that carrier is a general-purpose PP and your part needs a high-flow impact copolymer, you have quietly changed the resin. Carrier grade matters at these loadings, and it is a legitimate thing to specify.
- Forgetting the regrind. Regrind already contains flame retardant. Feeding 25% regrind alongside a full-rate masterbatch dose overshoots the target, costs money and can hurt mechanicals. Account for the FR already in the loop.
- Volumetric feeders on materials with different densities. A flame retardant masterbatch is often significantly denser than virgin resin. Volumetric dosing set by percentage of volume will deliver the wrong mass fraction. Gravimetric dosing is strongly preferred at these addition rates.
- Assuming rated thickness transfers. A loading validated for V-0 at 1.6 mm will not necessarily hold at 0.8 mm. Thinner walls need more, not less. Every thickness is its own qualification.
When Masterbatch Stops Being the Right Format
Honest answer to a question we get asked less often than we should: above roughly 40–50% addition, the masterbatch route starts losing its advantages. You are handling two bulk streams instead of one, dosing accuracy becomes critical, and you are paying a compounding step for material that is mostly flame retardant anyway. At that point a fully formulated compound — where we do the compounding and you run a single-material line — is usually cheaper per finished part and far more consistent.
Below that threshold, masterbatch wins on the things that matter to a moulder: no powder handling, no dust, no dispersion equipment needed, easy grade switching, and the ability to run the same base resin for flame retardant and non-flame retardant parts. The trade-offs between the two formats are laid out in our comparison of flame retardant additives vs masterbatch, and the product range itself on the flame retardant masterbatch page, with resin-specific pages for PP and PE.
If you want the calculation done against your actual part rather than a worked example, send us the resin grade, wall thickness at the test point, target rating and whether halogen is permitted — we will come back with the active level, the recommended masterbatch and the exact let-down, plus burn data from our lab before you commit a trial.
FAQ
How much flame retardant masterbatch do I add to get UL94 V-0?
For a typical brominated system in polyolefins at 1.6 mm, roughly 25–40% masterbatch depending on active content; for halogen-free intumescent systems, 35–50%. These are starting ranges, not specifications — the exact figure comes from the active content of the specific masterbatch and the thickness of your part.
What is a let-down ratio?
The proportion of masterbatch to virgin resin, written as a ratio. A 1:4 let-down means one part masterbatch to four parts resin, or 20% addition. Flame retardant masterbatch typically runs 1:1.5 to 1:4, versus 1:25 to 1:50 for colour.
Can I use more masterbatch than recommended to be safe?
Overdosing costs money, can reduce impact strength and flow, and in some systems actually worsens performance — excess antimony without matching halogen, for example, does nothing useful. It also risks blooming and plate-out on tooling. Dose to the qualified level, and if the part is failing, diagnose the cause rather than adding margin.
Does the masterbatch carrier resin need to match my base resin?
At the loadings used for flame retardancy, yes, this matters. A PP carrier in PP is straightforward; mismatched carriers can cause phase separation, poor dispersion and property loss. Where a customer’s resin is unusual, we compound the masterbatch on a matched carrier — it is a standard part of a custom package rather than an exotic request.
—— Rectivas Materials 团队 老陈