A two-component adhesive is only as good as the package it arrives in. The cartridge is not a bottle: it is a volumetric metering device. Its two chambers, its pistons and its outlet together decide how much resin and how much hardener reach the static mixer on every stroke of the gun. When a bond cures soft, leaks in transit or blocks a nozzle, the cause is often the packaging rather than the chemistry.

This guide covers the four decisions that shape a cartridge order - mix ratio, capacity, material and the mechanical details most buyers never see - and closes with a specification routine and the questions worth asking before a sample is approved.

Key takeaways

• The mix ratio is volumetric and fixed by chamber geometry; the gun only supplies force.
• Size the cartridge from bead cross-section and run length, then add a purge allowance.
• PP covers most 1:1 and 2:1 work; PBT is the usual choice for slim high-ratio chambers; PA suits aggressive solvents.
• Wall thickness, piston design, outlet interface and cap quality decide whether a cartridge survives pressure and transport.

1. Why the ratio comes before everything else

Every 2K cartridge is built around a fixed volumetric ratio: 1:1, 2:1, 3:1, 4:1 or 10:1. The ratio is not set by the gun or by the operator. It is set by the cross-sectional area of each chamber and by the fact that both pistons are advanced by the same plunger or push-rod pair. If chamber A has twice the area of chamber B, the cartridge delivers twice the volume, on every stroke, without adjustment.

That is also why the established industrial systems are organised by ratio and geometry rather than by brand name. The Sulzer Mixpac side-by-side C- and J-Systems cover 235 mL to 1500 mL in 1:1, 2:1, 3:1, 4:1 and 10:1, while its coaxial Q-System is built around 10:1 in 150, 310 and 380 mL. The choice between side-by-side and coaxial is really a choice about how to package a small B fraction: a 10:1 side-by-side cartridge needs a very slim hardener chamber, whereas a coaxial design places the hardener in an inner tube and keeps the outer body round and pressure-resistant.

One warning that costs real money: confirm whether your adhesive's ratio is specified by volume or by weight. Resin and hardener rarely share the same density, so a formulation quoted 1:1 by weight is not 1:1 by volume. Filling a volumetric cartridge to a weight-based ratio shifts the mix before the product has even left the plant.

Table 1. Standard mix ratios and how they are usually packaged

2. Match capacity to the job, not to the price list

Capacity is usually chosen by habit, or by unit price. A more reliable method is to calculate the bead. For a triangular bead the cross-section is about 0.433 times the square of the width, so a 6 mm bead carries roughly 15.6 mm2 of adhesive - close to 15.6 mL per metre of run.

A 400 mL cartridge, allowing a few millilitres of purge, therefore gives about 24 to 25 m of 6 mm bead. The same arithmetic shows that a 50 mL cartridge is a repair tool at roughly 3 m, while a 600 mL cartridge is a production tool at roughly 38 m.

Capacity carries two hidden costs as well. The first is dead volume: at every cartridge change the mixer and outlet hold adhesive that will never be used, and that penalty is proportionally larger on small packs. The second is shelf life after opening. A capped cartridge can be stored and reused, but each opening exposes the material to air and moisture, so a two-week job does not always need the largest pack on the list.

Table 2. Approximate bead length per cartridge (6 mm triangular bead, before purge)

3. Material: PP, PBT and PA are not interchangeable

Three thermoplastics dominate 2K cartridge barrels, and the difference appears under pressure rather than on a datasheet.

• PP (polypropylene). The default for 1:1 and 2:1 cartridges. Good resistance to most epoxy, PU and silicone systems, easy to mould, lowest cost. Its weakness is stiffness: thin PP walls deflect under load.
• PBT (polybutylene terephthalate). Higher stiffness and much better dimensional stability, with low moisture uptake. On a 10:1 cartridge the hardener chamber is small and thin, so keeping that section round under pressure is exactly what protects the ratio, which is why high-ratio cartridges are commonly moulded in PBT.
• PA (polyamide, nylon). The best choice against aggressive solvents and for moisture-sensitive fills, because it forms a tighter barrier. It is also hygroscopic, so PA cartridges must be stored dry and sealed.
• PE and LDPE. Not a barrel material. These are the standard grades for pistons, where a softer, self-lubricating polymer seals better against the bore.

Table 3. Barrel material selection

4. The parts you cannot see in a product photo

Two cartridges can look identical in a catalogue and behave completely differently in the field. Four features account for most of that gap.

• Wall thickness and ribbing. Thin, unribbed cartridges deflect as the pistons advance; when chamber A deflects more than chamber B, the delivered ratio drifts rich in resin and the adhesive cures soft or tacky. Reinforced walls, typically 1.2 to 1.5 mm with ribs between the chambers, hold their section at working pressure.
• Piston design. A wiper-lip piston scrapes the bore clean, which reduces dripping and prevents a film of cured material behind the piston. Vented pistons are the other half of the story: they let air escape during filling instead of trapping it under the piston.
• Outlet interface. Bell-mouth with a retaining nut, coarse thread with an O-ring, or bayonet and interlock: each family has a different sealing surface and its own pressure ceiling. The interface must match the series of the mixer already on your shelf, not merely the ratio.
• Caps. A good tip cap has a knurled grip and a vent so it can be fitted without pressurising the cartridge; an end cap needs a precision fit and a positive seal. Cheap caps are the most common reason a cartridge leaks in a container rather than on a job site.

5. A six-step specification routine

1. Read the TDS and write down the ratio, and whether it is expressed by volume or by weight.

2. Screen the chemistry against the barrel material: solvents, acidity, amine content and filler loading all matter.

3. Calculate total volume from bead cross-section times run length, then add 5 to 10 percent for purge.

4. Set the pressure class from the dispensing method; manual guns are gentle, pneumatic guns and meter-mix lines are not.

5. Lock the outlet interface to the mixer family already in stock, or plan to change both together.

6. Check the filling and logistics side: nozzle or plug configuration, case pack, pallet pattern and container utilisation.

6. Before you approve a sample, ask for these

• A dimensional report covering bore diameter, wall thickness and chamber depth, not only overall height.
• A ratio verification, ideally by dispensing A and B separately into tared cups and comparing the mass ratio with the density ratio.
• Leak test data. A 48-hour water-fill test on every mould, as JAPI runs in-house, is a practical benchmark.
• A drop test and a transport trial, because the most expensive failure happens between the filling line and the customer's shelf.
• Batch traceability and a retained sample, so a complaint can be traced back to a mould and a material lot.

JAPI moulds PP, PBT and PA cartridges from 30 mL to 1500 mL in 1:1, 2:1, 3:1, 4:1 and 10:1, and supplies the matching static mixers as one system, which is the simplest way to avoid an interface mismatch. For a closer look at how the gun side of the system works, see our article on how adhesive dispensing guns work.

Specifying a new 2K pack? Send us the adhesive TDS, the target fill volume and the gun you intend to use. JAPI will match cartridge, piston and mixer as one set, and send filled samples for a line trial.