Quick answer: If your LSZH compound passes the burn test but arrives with poor elongation, gels or an inconsistent melt index between lots, the formulation is probably fine and the process is not. At 60 % mineral loading the LSZH cable compound production process has almost no tolerance left — feed order, melt temperature and screw configuration decide the mechanical result more than the recipe does.
Buyers ask us for a formulation. Compounders who have actually run this material ask a different question: how is it fed, and at what temperature. That second question is the one that separates a compound you can extrude at line speed from one that looks right on paper.
Why this material is hard to make
An LSZH cable compound is not a polymer with additives in it. At 55–65 % aluminium trihydroxide or magnesium hydroxide flame retardant by weight, it is a mineral held together by just enough polymer to extrude. Every processing decision follows from that one fact.
Two constraints run against each other the whole way through. You need enough shear to wet out and disperse the filler, because undispersed mineral is both a mechanical defect and a flame-performance defect. But the filler decomposes endothermically — that is the entire flame retardant mechanism — and ATH starts releasing its water around 200 °C. Shear generates heat. So you are trying to disperse hard while staying cool, using a material whose whole purpose is to react to heat.
That is why heavily filled halogen-free work sits on co-rotating twin-screws rather than single screws, and why the practical advice is controlled shear rather than brute force: the screw has to disperse without cooking the filler.
The feed order, and why it is not negotiable
The single most common mistake we see from compounders new to this material is feeding everything at the main throat.
It does not work, for a mechanical reason before a thermal one. Sixty percent of a bulk powder at the throat starves the feed section: the mineral has low bulk density, it entrains air, and the screw cannot convey it consistently. Output drops, surging starts, and the operator compensates with speed, which puts heat exactly where it should not be.
The working sequence is:
| Zone | What goes in | Why here | What goes wrong if not |
|---|---|---|---|
| Main throat | Polymer, coupling agent, part of the antioxidant | Needs to melt before it can wet anything | Filler at the throat starves feed and surges output |
| First mixing section | — | Establish a stable melt seal before the filler arrives | Powder blows back up the side feeder |
| Side feeder 1 | Most of the ATH or MDH | Into an existing melt, so it is wetted not conveyed | Dry mineral pockets survive to the die as gels |
| Distributive section | — | Spread, do not grind — shear here becomes heat | Melt climbs past 200 °C and ATH starts releasing water |
| Side feeder 2 (optional) | Balance of filler, synergist, carbon black | Splitting the load lowers peak torque and heat | Single-shot feeding spikes specific energy |
| Vacuum vent | — (removes) | Pulls surface moisture and volatiles before the die | Porosity, voids, and a jacket that fails hot-set |
Practical machine note: heavily filled HFFR work generally wants 28–32:1 L/D with real mixing capability, not a short compounding screw. And the barrels wear — mineral at this loading is abrasive enough that bimetallic construction is the baseline and tungsten carbide is normal for heavy loading. If a quotation for LSZH compound looks unusually cheap, wear parts are one of the places the cost went.
The coupling agent step, which decides your elongation
Filler surface treatment is where a compound is won or lost mechanically, and it happens in one of two places.
Pre-coated filler. You buy ATH or MDH already treated — stearate for dispersion and melt flow, silane where the surface hydroxyls need capping. Simpler, more consistent, more expensive per kilo.
In-situ treatment. The coupling agent goes in at the throat and reacts on the filler surface during compounding. Cheaper on paper. It also means your dispersion result now depends on residence time, moisture and mixing quality on that particular day.
The mechanism is worth understanding rather than taking on faith: silane coupling agents work by displacing or modifying the moisture layer on the particle surface, which reduces clumping and lets the polymer wet the filler. That is why moisture control matters so much here. Mineral fillers carry surface water; if it is still there when the coupling agent should be reacting, you get neither good coupling nor good dispersion, and the loss shows up as elongation at break, not as a burn failure.
This is the step that separates a compound holding 140–155 % elongation from one that arrives at 90 % and cannot survive drum handling.
Where crosslinked grades change the picture
For grades that need heat resistance beyond thermoplastic HFFR, silane crosslinking is the usual route: a vinyl silane such as VTMS or VTES is grafted onto the polymer during compounding in the presence of an organic peroxide, and the crosslinks form later when the extruded cable is exposed to moisture with a catalyst present.
The practical consequence is that a silane-crosslinkable LSZH compound has a shelf life and a moisture-sensitive storage requirement that a plain thermoplastic grade does not. We have had material land in a humid port warehouse for six weeks and start reacting in the bag. If your grade is crosslinkable, ask about storage before you ask about price.
A lot that passed everything and still got rejected
An Indian cable maker took a 20 MT trial of a black HFFR sheath grade. Burn results were clean — IEC 60332-1 comfortable, smoke well inside the IEC 61034 60 % light transmittance requirement. They rejected the lot anyway.
The problem was surface: intermittent rough patches on the extruded jacket, maybe one metre in forty. Not enough to fail a test, enough to fail their customer’s visual inspection.
The first two theories were both wrong. Screen pack was suspected, changed, no difference. Then die temperature, adjusted, no difference. What actually turned up was on the compounding side: the filler was being fed through a single side feeder at full load, and the torque trace showed a repeating spike. Splitting the feed across two ports flattened it and the surface defects went with it.
Six weeks, and the fix cost nothing except admitting that the recipe had never been the issue. That job is why I now ask about feed configuration before I look at a formulation sheet — and why I am sceptical when a compound problem gets diagnosed as a chemistry problem in the first phone call.
What you can check without visiting the plant
Most buyers cannot audit a compounding line, and honestly most audits that do happen look at paperwork rather than screw configuration. Three things you can ask for that actually tell you something:
Melt index or melt volume rate across lots, not just within one. Lot-to-lot MFI drift on a heavily filled compound usually means thermal history is varying, which means the process is not under control.
Elongation at break on the compound, not the cable. If the compound leaves at 150 % and the cable tests at 120 %, that is your extrusion. If the compound itself is at 100 %, it was made that way.
Ash content or filler content per lot. Cheap to run, and it catches the substitution that a burn test will not: a compound quietly re-balanced toward more filler and less polymer still passes the flame test and loses mechanicals.
What we would actually advise
If you compound in-house: fix the feed configuration before touching the recipe. Split the filler across two side feeders, keep the melt under the ATH decomposition onset, and use pre-coated filler while you are still stabilising the process — move to in-situ treatment later if the volume justifies it. The general processability rules across all flame retardant families are in compounding flame retardants.
If you extrude cable but do not compound: do not start now. At 60 % loading the capital and the learning curve are real, and a finished flame retardant compound is almost always the cheaper answer for a cable maker. This is one of the few cases where we will actively talk you out of the additive route.
If the cable goes outdoors: the filler coating decision is no longer only about dispersion — it also decides weathering life, which is set out in UV resistant LSZH compound.
If you are qualifying a new supplier: ask for three consecutive lot COAs rather than one. A single good lot proves a good day. Three consecutive lots start to prove a process.
FAQ
What is the LSZH cable compound production process, in short?
Polymer and coupling agent at the main throat, a melt seal established, then 55–65 % ATH or MDH fed downstream through one or two side feeders into the existing melt, distributive mixing at controlled shear, vacuum devolatilisation, then pelletising. The order matters more than the ingredient list.
Why can’t the filler go in at the main feed throat?
Bulk density and air entrainment. Mineral powder at that proportion starves the feed section and causes surging, which the operator usually corrects with screw speed, which adds heat where you least want it. Side feeding puts the filler into a melt that can wet it immediately.
What melt temperature limit applies?
ATH begins releasing water around 200 °C, so the melt has to stay below that with margin. MDH decomposes around 330 °C and gives a much wider window, which is the main technical reason to pay more for it. Choosing between them is covered on our halogen-free flame retardant page.
Should I buy pre-coated filler or treat it in-situ?
Pre-coated while you are stabilising, in-situ once your process is repeatable and the volume justifies the saving. In-situ treatment makes your dispersion depend on residence time and moisture control on the day, which is a poor trade when you are still learning the material.
Can I make LSZH compound on a single screw?
Not well at these loadings. Heavily filled halogen-free work belongs on a co-rotating twin-screw with side feeding and vacuum venting. A single screw cannot feed the filler downstream or devolatilise properly.
Why does my compound pass the burn test but fail mechanically?
Because flame performance is driven by how much filler is present and mechanical performance by how well it is bonded and dispersed. You can have the first without the second. Check elongation and filler content per lot, not just the flame result.
Do you supply the compound or only the additives?
Both. Flame retardant additives as powder packages for compounders and finished compound for cable makers who do not compound. We are a sourcing and technical partner rather than a factory owner, so the compound is produced at the partner factory whose line suits the grade, with UL94 and LOI screening we arrange and cable-level reports through an accredited laboratory. Cable standards and the wider system view sit on the flame retardant for wire and cable page.
Final thoughts
The recipe for an LSZH compound is not a secret and has not been for twenty years. What separates lots that run from lots that do not is process discipline at a filler loading that leaves no margin: where the mineral enters, how hot it gets, whether the coupling agent had dry surfaces to react with. When a compound underperforms, look at the process trace before the formulation sheet — in our experience that is where the answer is more often than not. Send the standard you have to meet, your line speed and your elongation minimum, and we will tell you which route fits, including when the honest answer is to buy finished compound instead of learning to make it. The filler grade you feed matters as much as how you feed it; surface area and purity are covered in flame retardant filler selection.
—— Rectivas Materials 团队 技术工程师老陈