High-pressure mixing head (impingement)

Family: Mixing head — see hub Principle: Impingement mixing of pressurized streams Operating pressure: 100–250 bar at the inlet streams Compatible processes: RIM, RRIM, SRIM, LFI, technical applications in rigid polyurethane

The high-pressure mixing head is the industry standard for RIM and its derivatives. It mixes polyol and isocyanate by direct collision of pressurized streams — without moving parts in the path of the reactive flow — and delivers the homogenized mass to the mold in fractions of a second. The vast majority of modern models are self-cleaning, eliminating the need for solvent purge between shots.

Function in the process

In the production sequence, the high-pressure mixing head is mounted directly on the mold. It receives the pressurized streams from the high-pressure metering units and delivers the reactive mixture into the cavity in fractions of a second — typically a shot lasts 0.5 to 5 seconds, depending on shot weight and part.

Between shots, material does not stop circulating. The streams continue flowing through the metering units and the mixing head, but are diverted internally back to the day tanks. This continuous recirculation regime keeps pressure, temperature and hydraulic conditioning stable — the next shot finds the system already prepared. See recirculation details on the RIM process page.

Operating principle

The mixing head consists of four functional elements: the body, which contains internal channels and connection ports; the cleaning piston, a vertical hydraulically actuated plunger; the mixing chamber, a small volume (typically 1–10 cm³) where mixing actually occurs; and the hydraulic actuator, responsible for moving the piston.

The cycle works in two alternating states, controlled by the piston position:

Recirculation state (between shots). The piston is down, occupying the mixing chamber. In this position, the polyol and isocyanate streams entering through the side ports meet carefully machined grooves in the piston itself, which connect the inlets to the return channels. Material simply bypasses the piston and returns to the tanks — without ever contacting the opposite stream.

Shot state. The actuator retracts the piston upward, releasing the mixing chamber. With return channels interrupted and the chamber unobstructed, the pressurized streams now flow through the inlet ports directly into the chamber.

This is where the magic of impingement happens. Streams arrive at 100–250 bar at velocities of 100 to 200 m/s through 1–3 mm diameter conduits. They collide at 180° (collinearly opposed, in most designs) at the chamber center. The impact's kinetic energy dissipates as intensely chaotic turbulence — the Reynolds number in the impact zone exceeds 50,000, ensuring fully turbulent regime. Under this condition, complete molecular mixing is achieved in less than 1 millisecond.

Exiting the chamber, the already-homogenized mixture falls by gravity and the flow's own momentum to the mold inlet. The pressure drop is dramatic: from 100–250 bar at arrival to less than 4 bar inside the cavity. This is a fundamental characteristic — the RIM mold is filled by low-pressure laminar flow, not by forced injection, which is why RIM molds are much lighter than equivalent thermoplastic molds.

At the end of the shot, the piston comes back down, mechanically sweeping any mixture residue from the chamber and restoring the recirculation path. This movement is why the mixing head is called self-cleaning: cleaning is mechanical, not chemical, eliminating solvent.