Quick answer: Once you have decided to buy masterbatch rather than powder, four things decide which one — carrier resin, active content, particle size behind the pellet, and the dosing accuracy your line can hold. Get the carrier wrong and you quietly change the base resin you already qualified. Get active content wrong and you compare quotes that are not comparable.
There is plenty written about whether to use masterbatch. Much less about how to pick one after that decision is made. This is the sequence we would work through with you, in the order that actually eliminates options.
Step 1: The Carrier Decides More Than People Expect
At a 30% let-down ratio, roughly a third of what you add to your part is somebody else’s polymer. It does not disappear at the hopper — it stays in the finished component, and it changes properties you may have already signed off.
The default rule is to match the carrier to your base resin: PP carrier into PP, PE into PE. Where that gets interesting is the blends and the engineering resins, and where buyers get caught is the word “universal”.
A universal EVA carrier will run in most things, which is exactly why it is offered. What it costs you is a small compromise in almost every property — stiffness, heat resistance, sometimes clarity. If your part has tight mechanical targets, that compromise was made for you by someone who never saw your specification. The full matching logic, including melt flow index and when a custom carrier is worth paying for, is in carrier resin selection.
Step 2: Active Content, Because Price Per Kilo Lies
This is the single most common way a masterbatch purchase goes wrong, and it is pure arithmetic.
A masterbatch at 40% active against one at 60% active is not 33% cheaper. You will dose half as much again to reach the same loading in the part, so the cheaper-looking quote can easily cost more per finished kilo. Ask every supplier for active content in the same enquiry and normalise the quotes yourself before ranking them.
Two follow-up questions worth asking once you have that number:
- What is the batch-to-batch tolerance on active content? If a supplier cannot answer in numbers, nobody is measuring it. Drifting active content is why a compound passes UL94 in spring and fails in autumn.
- Is the flame retardant a single chemistry or a package? A blended package at 70% active behaves differently from a single-chemistry concentrate at 70%, even at the same dosage.
The dosage arithmetic itself, plus the five ways it goes wrong, is worked through in loading ratio and let-down calculation.
Step 3: Match It to Your Process, Not Just Your Resin
The same chemistry, in the same carrier, at the same active content, behaves differently on different equipment. Process is the constraint people check last and should check third.
| Your Process | What to Specify | The Usual Failure |
|---|---|---|
| Injection moulding | The most forgiving case. Standard grades work; focus on dosing accuracy | Metering error at high let-down ratios — a 2% dosing swing at 25% loading is a real rating risk |
| Profile / pipe extrusion | Melt strength matters. Carrier MFI should not undercut your base resin | Sagging and dimensional drift when the carrier is too fluid |
| Blown film | The hard case. Particle size has to be well below your film gauge | Gels, pinholes and bubble instability. A grade that runs beautifully in moulding can be unusable here |
| Cast sheet / thermoforming | Surface finish and clarity constraints drive the choice | Visible specks from incomplete dispersion at low residence time |
Blown film deserves the warning it gets. Film thickness and filler particle size compete directly, and no amount of screw adjustment fixes a grade whose particles are too coarse for your gauge. Process-by-process detail is in masterbatch by process.
Step 4: Decide How You Will Verify It
Specify this before the first order arrives, not after a problem. Two checks cover most of what goes wrong:
Ash content on incoming lots. The cheapest test that catches loading drift — if the mineral content of the pellet has moved, the ash tells you before the burn test does.
A burn result at your own wall thickness. Not at 1.6 mm, not on the supplier’s plaque. Your thinnest section is where a rating fails, and a datasheet rating measured at a thicker wall tells you very little about it.
If results start moving between lots, work through symptoms rather than swapping supplier immediately — shear, moisture and metering account for most of it, and dispersion troubleshooting maps symptom to cause.
What We Would Actually Advise
If your part has tight mechanical or optical targets, pay for a carrier matched to your resin rather than accepting a universal grade. It is a small premium against a property compromise you did not choose and may not notice until qualification.
If you are comparing three quotes, normalise them on active content first and only then look at price. In our experience the ranking changes often enough that skipping this step is the most expensive five minutes you can save.
If your loading is heading above roughly 40–50%, masterbatch is reaching its practical limit. Dosing that much on a moulding machine is unreliable, and finished compound usually costs less per acceptable part once scrap is counted. We would rather point you there than sell you a format you will fight with.
And if you own twin-screw capacity, you probably should not be buying masterbatch at all — powder additives give a lower cost per kilo of active, and the specification side of that is covered in compounding flame retardants.
FAQ
What is a flame retardant additive masterbatch?
A pellet concentrate in which flame retardant additives are pre-dispersed into a carrier polymer, so you can dose it on a moulding or extrusion line without handling powder. You buy it to remove dispersion and dust handling from your process; the trade is a higher cost per kilo of active than buying powder and compounding yourself.
How much masterbatch do I add?
It depends on the active content and your target loading in the finished part, not on a fixed percentage. Divide your target additive loading by the masterbatch active content — a 20% target with a 50% active masterbatch means 40% addition. This is why active content, not price per kilo, is the number to compare across quotes.
Can I use one masterbatch across several base resins?
Sometimes, using a universal carrier such as EVA, and it costs you something in each of them. Where your parts have loose property requirements it is a practical simplification; where any of them has tight mechanical, thermal or optical targets, a matched carrier is worth the extra grade.
Why does my masterbatch disperse well in moulding but badly in film?
Residence time and particle size. Injection moulding gives more mixing history and a thicker section to hide in; film gives less of both, and any particle approaching your gauge shows up as a gel or pinhole. It is usually a grade problem rather than a process problem — ask for a film-specific grade.
Do you supply custom carrier masterbatch?
Yes, where the standard carriers do not fit your resin or property targets. Tell us the base resin grade, process, target rating and wall thickness, and we will propose the carrier and active content together rather than quoting a catalogue number.
What should I ask for with the first order?
Active content on the TDS, batch tolerance on that figure, the COA for the specific lot, and a burn result at your own wall thickness. Ask for trial quantity and production minimum in the same enquiry so you can plan qualification properly.
Final Thoughts
Choosing masterbatch is mostly about refusing to compare the wrong numbers. Carrier first, because it ends up in your part; active content second, because it decides both dosage and real price; process third, because it quietly rules grades out. Send us your base resin grade, process, target rating and wall thickness, and we will propose a specific carrier and active content rather than a product list — see our masterbatch range, or the resin-specific pages for PP, PE and PVC.
—— Rectivas Materials 团队 技术工程师老陈