LSR Mixing Ratio Control and Why 1:1 Systems Scale

When an LSR part comes off the press soft, tacky at the surface, or weaker than the last batch, the material is usually blamed first and the tool second. In practice the cause is often upstream of both. LSR mixing ratio control determines whether the silicone cures to the crosslink density it was formulated for, and a small, gradual drift in that ratio produces exactly the ambiguous defects that are hardest to attribute.

The reason it is hard to spot is that off-ratio parts are not obviously defective. They demould, they hold their shape, and they pass a visual check. The deficit shows up later as tear strength, compression set or an unexpected extractables result.

This article covers why the ratio governs cure, where drift comes from, what the static mixer contributes, and why a fixed ratio changes the validation picture at scale-up.

Why the A to B ratio governs cure

Liquid silicone rubber is supplied as two components and cures by a platinum-catalysed addition reaction. Part A carries the platinum catalyst, present at only a few parts per million and not consumed by the reaction — it enables the cure rather than being used up in it. Part B carries the crosslinker, which reacts with vinyl groups on the base polymer present in both parts. It is this crosslinker-to-vinyl stoichiometry that the formulator sets when specifying the material, and it is what the delivered A:B ratio ultimately controls.

Deliver the components off-ratio and the crosslink network is affected. A deficient crosslinker (Part B) leaves the network genuinely under-built — fewer crosslinks form regardless of cure time. A deficient catalyst (Part A) behaves differently: it slows the reaction rather than starving it, so the part can still come out under-cured within a fixed press cycle even though, given enough time and heat, it would eventually cure out. Either way, modulus and tear strength fall, compression set worsens, and unreacted material remains in the part — which matters for anything with a biocompatibility or extractables position to maintain.

The important point for a process engineer is that, across most of the practical range, this is a continuous relationship rather than a threshold — performance degrades progressively, which is why ratio problems tend to be diagnosed only once a trend appears in test data. At the extremes this changes: severe catalyst deficiency can fail to gel within the mold cycle at all, and a large crosslinker excess can generate hydrogen gas as a by-product, producing porosity rather than simply weaker parts.

Where ratio drift comes from

Drift is rarely a single fault. It usually accumulates from several small effects that share the characteristic of being invisible at the dispense point.

• Viscosity imbalance. If the A and B sides sit at different temperatures, their viscosities differ and the pressure needed to move each one differs with them.
• Hose expansion. Long flexible hoses carrying individually metered components expand under pressure and contract when it drops, so what is metered at the pump is not what arrives at the mixer instant by instant.
• Air entrainment. Air drawn in during a pail or cartridge change is compressible, so a volumetric system delivers less material than it thinks on that side until the air clears.
• Element wear. Metering components wear at different rates on the two sides, producing a slow, monotonic drift over months.
• Feed level effects. As a container empties, follower plate behaviour and head pressure change, which can bias one side.

What these share is that they degrade gradually. A system that was correct at installation can be meaningfully off a year later with nothing having visibly broken.

Reading the symptom back to the cause

What you observeLikely ratio-side causeFirst checkTacky or uncured surfaceOff-ratio, catalyst side deficientVerify delivered ratio by mass at the mixer outletParts soft but fully formedLow crosslink density from mild driftCompare against a retained reference shotDefects only after a container changeAir entrained during changeoverPurge routine and follower plate seatingGradual decline over monthsMetering element wearScheduled ratio verification historyVariation between first and last shot of a runTemperature or feed level effectsMaterial conditioning and pail level at faultStreaked or marbled appearanceIncomplete mixing, not ratioStatic mixer element count and condition

The last row is the one most often misread. A marbled part is a mixing failure, not a metering failure, and changing pump settings in response makes the process worse rather than better.

What the static mixer contributes

Correct ratio at the pump is necessary but not sufficient. The two streams still have to be brought to homogeneity, and that is the static mixer's job: a tube of fixed helical elements, no moving parts, each element dividing and recombining the flow.

Two practical consequences follow. Element count matters, because too few elements leave the shot inhomogeneous even at a perfect ratio. And mixer position matters, because every metre of hose between the mixer and the tool is a length in which mixed, catalysed material is sitting and slowly reacting. Placing the mixer close to the dispense point limits both the residence time and the hose-expansion effect described above.

The production LSR metering and mixing system from MonoBiologics is built around this arrangement: Part A and Part B are drawn from separate 5-gallon pails, metered at a locked 1:1 ratio, and dispensed through a static mixer at up to 600 cc/min, across a viscosity range of 50,000 to 2,000,000 cps at 0 to 5,000 psi output pressure.

Why a fixed ratio changes the validation picture

An adjustable-ratio system is more flexible, and that flexibility has a cost that is easy to underestimate. Ratio becomes a process parameter: it must be set, verified, recorded and controlled at every scale, and any discrepancy between the bench setting and the production setting becomes a variable in every subsequent investigation.

A fixed 1:1 system removes the parameter. There is no setting to transfer, no drift between machines to reconcile, and no ratio to revalidate when a formulation moves from development to production. The benchtop meter/mix system uses the same fixed 1:1 principle as the production unit, differing in feed and throughput — cartridge-fed at up to 50 cc/stroke and 0 to 3,000 psi rather than pail-fed at 600 cc/min — so process parameters developed at bench scale carry forward directly.

That is the practical argument for fixed-ratio equipment in regulated work: not that it meters more accurately in principle, but that it eliminates a class of scale-up discrepancy rather than managing it. The rest of the LSR overmolding machine range is arranged on the same basis, and the scale-up decision between the two systems comes down to throughput and feed rather than to ratio.

A verification routine worth adopting

1. Verify delivered ratio by mass at the mixer outlet on a fixed schedule, not only when a problem appears.
2. Record the result each time so that slow drift is visible as a trend rather than as a surprise.
3. Condition both components to the same temperature before a run, and record it.
4. Purge deliberately after every container change and discard the purge volume.
5. Track static mixer service life by shot count, and replace on that basis rather than on appearance.
6. Retain a reference shot from a known-good run for physical comparison.
7. Keep the mixer as close to the dispense point as the tooling allows.

Frequently asked questions

Why does the A to B ratio matter so much in LSR?

Liquid silicone rubber is a two-part platinum-cured system. Part A carries the platinum catalyst, present only in trace amounts and not consumed by the reaction; Part B carries the crosslinker, which reacts with vinyl groups on the base polymer in a defined proportion set by the formulator. If the delivered ratio drifts, the crosslink density is wrong. The part may still look correct while being under-cured, with lower tear strength and a changed extractables profile.

What causes LSR ratio drift during a production run?

Common causes are unequal viscosity between the A and B sides as drum temperature changes, expansion of long flexible hoses under pressure, air entrained during a pail change, and wear in the metering elements. Most of these produce a slow drift rather than a sudden fault, which is why the resulting cure problems are often blamed on the material or the tool instead.

What does the static mixer actually do?

A static mixer is a tube containing fixed helical elements and no moving parts. As the two metered streams pass through, each element divides and recombines the flow until the two components are homogeneous. Correct ratio at the pump is not sufficient on its own: if mixing is incomplete, parts of the shot are effectively off-ratio even though the overall proportion is right.

Why does a fixed 1:1 ratio simplify scale-up?

An adjustable ratio is a process parameter that has to be set, verified and controlled at every scale, and any difference between bench and production settings becomes a variable in the investigation when a part fails. A fixed 1:1 system removes the parameter entirely, so a formulation qualified on a benchtop system transfers to a production system without the ratio itself needing to be revalidated.

Ratio problems are under-diagnosed because they do not announce themselves. Parts come out whole, the press behaves normally, and the trend only becomes obvious in test data weeks later.

Verifying the delivered ratio on a schedule, and recording it, converts that slow drift into something visible. If you are specifying a cell and want to talk through how the ratio side would be verified in your setup, send us your formulation details.