Mixing head with aftermixer

Family: Mixing head — see hub Principle: Primary impingement + secondary back-pressure Operating pressure: 100–250 bar at the streams; additional drop of 10–50 bar in the aftermixer Compatible processes: RRIM, high-density systems, pigmented tri-component systems, low high-pressure throughputs, high-viscosity systems

The mixing head with aftermixer is a specialized variant of the high-pressure family. It couples a secondary chamber after the impingement mixing chamber — the aftermixer — that generates additional back-pressure and forces new turbulence into the flow before delivery to the mold. It is the correct choice when impingement alone does not produce sufficiently homogeneous mixing, typically in systems with mineral fillers, high density, or elevated viscosity.

This page assumes familiarity with the high-pressure mixing head (impingement). Common elements — metering, recirculation, self-cleaning, general suppliers — are described on that page and are not repeated here.

Function in the process

The position of the mixing head with aftermixer in the production flow is identical to that of the standard high-pressure mixing head: it is mounted on the mold, receives pressurized streams from the metering units, delivers homogenized mixture to the cavity. The difference is internal — the mixture goes through two stages instead of one.

Operating principle

The mixing head with aftermixer keeps all elements of a standard high-pressure mixing head (body, cleaning piston, primary chamber, hydraulic actuator) and adds a secondary chamber between the primary chamber and the outlet.

The cycle works as follows:

Recirculation state. Same as the standard mixing head — piston down, streams bypass the piston and return to the tanks. The aftermixer remains full of material from the previous operation or with cleaning fluid.

Shot state. Piston retracted, streams collide in the primary chamber (impingement at 100–250 bar). The resulting mixture does not exit directly to the mold — it is forced to pass through the aftermixer, where it encounters the central pin (or other geometry, depending on variant).

The aftermixer fulfills three functions simultaneously:

Generates back-pressure. By restricting the outlet flow, it forces the pressure in the primary chamber up above what it would be if the mixture exited freely. This increases impingement intensity — incoming streams hit against higher pressure, expanding turbulence and improving initial mixing.

Adds secondary turbulence. The central pin (or equivalent geometry) forces the flow to bypass obstacles, generating local eddies that continue homogenizing the mixture after impingement. For systems that impingement alone does not fully mix — high viscosity, fillers — this second stage makes the difference.

Increases residence time. A mixture that passes through primary chamber + aftermixer has total residence time of 5–15 ms, versus <1 ms in pure impingement. This is too short to initiate cure, but enough for more robust homogenization in slow systems.

The downside of this arrangement: total pressure drop through the system is higher, requiring slightly more powerful metering units; cycle time is marginally longer; and aftermixer cleaning is a new problem — some models incorporate self-cleaning piston extension into the aftermixer, others do not.