Centring Adhesive in Three Chemistries: How to Choose

Centring adhesive is the last structural bond a loudspeaker driver receives, and Aosibo Adhesives lists it in three versions — one-part, epoxy and halogen-free. Buyers new to the category read that as a quality ladder and ask for the best one. It is not a ladder. Those three options describe how the adhesive cures and what it is allowed to contain, and choosing between them is a decision about your oven, your line takt and your customer's restricted-substance list, not about which bond is stronger.

Getting it wrong is expensive in a quiet way. Centring adhesive locks in the concentricity of the voice coil, so a bad choice does not produce a driver that fails on the test bench. It produces a driver that measures acceptably, ships, and comes back as a rub complaint after the customer has taken delivery of six months of production.

What the centring bond is actually doing

The sequence is familiar to anyone who has watched a driver line. The voice coil former goes into the magnetic gap over a shim or gauge that holds it concentric — a few tenths of a millimetre of clearance on each side, held mechanically by something that will be pulled out again. The spider and cone are then fixed at their necks. Adhesive is applied, the assembly is cured, the gauge is removed, and whatever alignment existed at the moment of cure becomes permanent.

That last sentence is the whole specification problem. The centring adhesive is not primarily a strength component. It is a dimensional component. Three consequences follow, and the first is the one most buyers miss.

Cure shrinkage matters more than peak shear strength. An adhesive that shrinks measurably as it cures pulls on the joint while the gauge is still in place, or worse, after the gauge is out and the assembly is still hot. A high-strength grade with high shrinkage will lock a coil slightly off-centre with great authority. When you ask a supplier for data on a centring grade, ask for linear cure shrinkage and post-cure dimensional movement before you ask for a shear number.

The joint sits at a stiffness transition. On one side is a rigid former; on the other is a spider that is designed to flex several millimetres, millions of times. A glassy, brittle fillet at that boundary concentrates the flexural stress into a narrow band and cracks there. The same reasoning drives grade selection at the damper and spider joint, and for the same reason: the adhesive has to hand the load off gradually rather than create a hard edge.

Cure heat reaches parts that cannot take it. By the time the centring adhesive goes on, the assembly already contains a surround, a cone and often a lead-wire bond, each with its own thermal limit. A 150 °C cure will not care that the foam surround underneath it is rated to 90 °C. The cure schedule of the last adhesive applied is constrained by the least tolerant part already in the assembly.

The three chemistries, and what each one costs you on the line

One-part

One component, no mixing, no pot life, no ratio error, no mixed-waste at the end of a shift. The dispensing equipment is simpler and the process variable count drops sharply, which is why high-volume driver lines like one-part systems. The trade is that the cure has to be driven by something — usually heat — so the adhesive consumes oven capacity and oven time you may not have spare. One-part heat-cure systems also tend to want cool or cold storage, and their shelf life is shorter than a two-part kit's, so the logistics question becomes real: what temperature does it ship at, what is its shelf life from date of manufacture, and how much of that life is left when the pallet lands.

Ask for: cure temperature and dwell, minimum and maximum oven window, storage temperature, shelf life from date of manufacture, and open time at line temperature before the parts have to be closed.

Epoxy

Epoxy chemistry buys crosslink density, and crosslink density buys heat resistance, solvent resistance and gap filling. If the driver is destined for continuous high power or an automotive environment, this is usually the direction the specification moves. The cost is process discipline. A two-part epoxy has a mix ratio that has to be right, a pot life that starts the moment the parts meet, and a metering and mixing step that has to be either equipment-controlled or supplied as a pre-measured kit.

Two traps live here. First, mix ratios are quoted by weight or by volume and the two numbers are different — a ratio quoted without units is a scrap incident waiting for a night shift. Second, pot life is temperature-dependent; as a rough rule, reaction rate roughly doubles for every 10 °C rise, so a pot life measured in a 23 °C laboratory shortens substantially in a Dongguan or Jakarta assembly hall in July. Ask for pot life at your actual shop temperature, not at standard conditions.

Ask for: mix ratio by weight and by volume, pot life at 25 °C and at 35 °C, fixture time, full-cure schedule, whether static-mix nozzles or pre-measured kits are supplied, and exotherm behaviour in the bead size you dispense.

Halogen-free

This is the version most often misunderstood. Halogen-free is a compositional restriction, not a performance class. It says something about what the formulation may not contain — chlorinated and brominated compounds, usually because a brand customer's restricted-substance list or a fire-safety specification for the end equipment demands it. It says nothing at all about strength, cure speed or temperature capability.

More importantly, "halogen-free" is a numeric claim, not a description. The definition that electronics buyers usually mean comes from IEC 61249-2-21, commonly cited as chlorine at or below 900 ppm, bromine at or below 900 ppm, and total halogens at or below 1,500 ppm. Other customer specifications name different thresholds, and some restrict fluorine and iodine as well. So the request is specific: ask for the test report, the issuing laboratory, the analytical method, the measured values for each element, and the threshold those values were compared against. A supplier statement that a grade is halogen-free, without those five items, is not something you can pass on to your own customer.

There is a cost to asking for it when you do not need it. Restricting the formulation space can change flexibility, cure behaviour and thermal performance, and it narrows the number of grades available to you. Specify halogen-free when a customer specification names a threshold. Do not specify it as a general precaution.

Ask for: the test report and threshold, plus everything you would have asked about the underlying chemistry anyway — because halogen-free is an attribute layered on top of a one-part or epoxy base, not a third base chemistry.

Side by side

Aosibo publishes no technical data sheet for any of the three, and describes viscosity, cure schedule, pot life and substrate data as shared at inquiry. The table below is therefore a question list as much as a comparison — the "what to confirm" column is what turns it into something you can put in a contract.

Choose against the constraint you cannot move

Work down this list in order. The first hard constraint you hit decides more than the rest combined.

1. Does a customer specification name a halogen threshold? If yes, that sets the boundary before anything else. Get the exact clause and the exact numbers from your customer, not a paraphrase, then select the base chemistry inside that boundary.
2. What cure energy do you actually have? Not the oven you have requisitioned — the oven capacity free at the point in the line where this bond happens. If the answer is "none", ambient-cure options move to the front and a one-part heat-cure grade drops out regardless of its other merits.
3. What is the least heat-tolerant part already in the assembly? Foam surrounds, certain adhesives already cured nearby, and plastic formers all set ceilings. The centring cure schedule has to fit under the lowest of them — and if the magnet-circuit bond shares the same oven pass, its schedule is part of the same constraint.
4. What is your takt time and how many fixtures do you own? Fixture time multiplied by station count is your throughput. An adhesive with a 20-minute fixture time and a line moving every 40 seconds needs 30 fixtures in parallel, which is a capital question disguised as a chemistry question.
5. What temperature does the finished driver reach in continuous use? This is where an epoxy usually earns its process cost, and it is the same criterion that governs bonds around the voice coil itself.
6. What is your rework policy? Some chemistries permit a joint to be broken and re-bonded; some do not. Decide before you find out.

When you send the inquiry, send the answers rather than the questions. Aosibo's own guidance is to send both substrates and the cure window you have available — which is another way of saying that a request for "your best centring adhesive" cannot be answered, and a request naming two substrates, a maximum cure temperature, a maximum dwell and a halogen threshold can be. Suppliers price and specify against the information they are given, which is a large part of why quotations for the same nominal spec differ so much.

Common questions

Is the epoxy version stronger than the one-part version?

Usually it has higher crosslink density and better heat and chemical resistance, but "stronger" is the wrong frame for a centring bond. The bond is holding an alignment, not a load. A grade that shrinks less and stays slightly compliant at the spider transition often produces better drivers than a stiffer, stronger one. Compare shrinkage, modulus at temperature and behaviour at the stiffness transition before you compare shear figures.

Can I use the halogen-free grade for everything, to simplify purchasing?

You can, but check what it costs before you commit. Restricting the formulation may change flexibility or thermal capability, and it usually narrows your options if you later need a second source. If only one of your customers demands a halogen threshold, running one restricted grade and one unrestricted grade is often cheaper than restricting the whole line — as long as the two are clearly separated on the shop floor.

How do I stop a chemistry change from becoming a hidden defect?

Ask for a written formulation-change notification commitment before the first order, and ask what batch documentation travels with each shipment. A centring adhesive that changes shrinkage behaviour between lots will not show up on any incoming inspection you are likely to run; it shows up as a shift in rub-and-buzz yield three weeks later. Retain a sample from every lot, cured on your own schedule, so you have something to compare against when the yield moves.

What to ask the supplier next

Send this as a single message and judge the reply by how much of it comes back with numbers rather than reassurance:

1. For each of the three centring versions: cure temperature, dwell, fixture time and full-cure schedule.
2. Whether the epoxy version is one-part or two-part, and if two-part, the mix ratio by weight and by volume.
3. Pot life at 25 °C and at 35 °C for any two-part grade.
4. Linear cure shrinkage and post-cure dimensional movement for each version.
5. Substrate compatibility against your actual former, spider and cone materials — sent as material names, not categories, in the same disciplined form as any other specification sheet you send a Chinese factory.
6. For the halogen-free version: the test report, the laboratory, the method, the measured chlorine, bromine and total-halogen values, and the threshold they were compared against.
7. Storage temperature, shelf life from date of manufacture, and a minimum remaining shelf life on arrival as a contract term.
8. The unit behind the stated 1,000-unit minimum order — bottle, cartridge, syringe or kilogram — plus pack size and price basis, so competing quotations can actually be compared.
9. A written commitment to notify any formulation change before shipment.

Sampling is stated at 7 and production at 15, given as bare numbers on the company's application and read as days. A week is enough to bond coupons on your real substrates, run the cure schedule you actually have, check whether the fillet cracks under flex, and measure alignment drift before and after cure. It is not enough for thermal ageing, which belongs to the qualification rather than the sample review.

Aosibo Adhesives is a reasonable test case for this list. Its stated range covers centring adhesive in exactly these three versions alongside the rest of a driver's bond schedule, and its own note on the halogen-free grade — that halogen-free is a numeric claim rather than a description, and that you should ask for the report and the threshold — is the right instinct applied to its own product. The Aosibo Adhesives profile sets out what the company states about its range and terms and which product data was not available at review, so you know before you write which of the nine items above still has to be asked for.