RIM — Reaction Injection Molding

Reactive injection molding of polyurethane in a closed mold.

Identification & TL;DR

Family: Discontinuous process for rigid and semi-rigid polyurethane Principle: Impingement mixing at 100–250 bar Cavity pressure: <4 bar Typical cycle: 30 s to 3 min per part Ideal volume: Medium to high; parts of 0.1 to 20 kg Dominant sectors: Automotive, construction, agriculture, industrial

Discontinuous process in which reactive liquid components — polyol and isocyanate — are mixed by high-pressure impingement and injected into a closed mold, polymerizing in seconds to form the final part. Cornerstone process of the structural polyurethane industry.

Principle of operation

The RIM cycle has five distinct moments:

Component preparation. Polyol and isocyanate start from day tanks with controlled temperature and viscosity. Thermal stability of the components is one of the most sensitive process variables — see section 2.

Metering. High-pressure metering units (lance-type pistons in specialized machines, or gear pumps in simpler units) deliver each component to the mixing head, which is mounted directly on the mold. Between shots, components recirculate through the mixing head and return to the tanks.

Impingement mixing. At the shot, polyol and isocyanate streams meet in a small chamber, colliding at opposite angles at 100–250 bar. The impact energy dissipates as turbulence and heat — and it is this turbulence that generates the mix. Unlike low-pressure mixing heads, there is no mechanical impeller. Modern self-cleaning mixing heads (L-type or MQ) incorporate a piston that sweeps the chamber between shots, eliminating the need for purge solvent.

Mold filling. The reactive mass exits the mixing head at low pressure (typically <4 bar inside the cavity) and fills the mold by laminar flow. The low initial viscosity of the system (<2,000 cP) makes this possible: the mold is filled by the pressure generated by the flow itself, without external mechanical force. This is the fundamental reason why RIM molds are much lighter than thermoplastic molds.

Reaction and demolding. Polymerization begins immediately. The exotherm is significant — around 50 kcal/kg — and needs to be managed by the mold's thermal control. In 1–10 s the mixture gels; in 30 s to 3 min the part can be demolded, depending on the chemical system and thickness.

Operating window

The eight essential parameters that define a stable RIM window:

Parameter	Typical range	Consequence of deviation
Stream pressure	100–250 bar	Below, compromises mixing; above, accelerates wear
Total throughput	50–2,000 g/s	Outside window alters filling pattern
Mix ratio	System-specific (±1%)	Drift causes incomplete cure or wrong properties
Polyol temperature	35–45 °C typical	High accelerates reaction; low increases viscosity
Isocyanate temperature	30–40 °C typical	Narrower window than polyol
Mold temperature	60–90 °C typical	Affects cure and surface finish
Cream time	1–5 s	Defines flow window before gelation
Demold time	30 s–3 min	Warped part if removed prematurely

Chemical systems