Low-Pressure Reaction Injection Molding (RIM) is a process in which two groups of extremely low-viscosity reactive resins (commonly polyurethane PU, polyurea PUA or nylon RIM) are metered and mixed in a mixing head, and then injected into a mold or glass fiber reinforced epoxy mold at a low pressure of only 0.3-1 MPa, and the exothermic polymerization of the material itself is used to quickly solidify and form in the mold.

Its main features include:
⚡ This process can produce large covers or bumper shells with uneven thickness (2-20 mm) and a maximum size of more than 2 meters in one molding within 2-5 minutes;
⚡ Due to the low cavity pressure, the mold structure is simplified and the life span can reach thousands of pieces, and the cost is only one-third of that of traditional injection molds.
⚡ The resin has low viscosity and the filling is much smoother than thermoplastic injection, and can be formulated into Class-A paint-grade appearance or 30% glass fiber reinforced high-impact formula.

Overall advantages – low pressure and low wear, low mold cost, large size and high degree of freedom in mixing thick and thin, short cycle and surface quality that can directly reach mass production level (50 – 3000 pieces) – make low pressure infusion become the best manufacturing solution for medium-volume automotive exteriors, industrial equipment panels, medical bed panels and other large structural/appearance parts.

	Soft Tooling (vacuum casting)	Reaction Injection Molding (Low pressure injection)	Rapid Tooling / Injection Molding (aluminum mold or soft steel mold + mass production injection)
Mold material	RTV Silicone (Soft Mold)	Silicone, FRP/GRP, Aluminum Powder Epoxy Resin	Aluminum alloy/mild steel P20, S50C, NAK80, etc.
Mold cost (relative)	$	$$	Aluminum Mould $$$ / Steel Mould $$$$$
Mold delivery time (general situation)	3 – 7 working days	5 – 15 working days	15 – 35 working days
Single mode life	15 – 25 pieces	50 (Silicone) – 3,000 pcs (Aluminum Powder Epoxy Resin)	Aluminum moulds 10,000 – 20,000 pieces Steel moulds 100,000 – 500,000 pieces
Cavity pressure/ Clamping force	Vacuum → 0.05 MPa / Almost no clamping	0.3 – 1 MPa / 5 – 30 ton	70 – 200 MPa / 75 – 800 tons
Common materials for finished products	* PU (ABS‑like, PC‑like, transparent PMMA‑like) * High heat resistant PU (120 °C) * Elastomer PU / Silicone (Shore A 30‑90) * Conductive, flame retardant, filling grade special resin	* Solid or micro-foamed PU / PUA * Glass fiber reinforced PU (RRIM / SRIM) * DCPD, Nylon-RIM (Caprolactam) * Reactive EP, Polyester	* ABS, PC, PC/ABS, PP, PE, POM, PBT * PA6 / PA66 (including GF, CF) * PMMA, ASA, TPU/TPE and other thermoplastics
Cycle time/piece	30 – 60 min	2 – 5 min	20 – 60 s
Recommended ordering batch	8 items↑	50 items↑	5,000 pieces↑ (MOQ 5,000 pcs)
Common size range	≤ 0.5 m	≤ 2 m Large enclosures possible	≤ 1 m; potentially limited by the clamping force of the injection molding machine
Possible wall thickness	1 – 6 mm; uniform thickness is the most stable	2 – 20 mm, thick and thin can be mixed	0.8 – 4 mm; thickness difference requires flow channel balance
Dimensional accuracy	±0.20 mm /100 mm	±0.15 mm /100 mm	±0.05 mm /100 mm
Surface quality	0.1 mm texture can be copied; spray painting is required	Available in Class-A paint or leather texture	The mold surface can directly achieve mass production texture or high gloss
Typical advantages	Low mold opening fee, fast delivery, high flexibility	Low mold cost, large size, mixed thickness, impact resistance	Lowest unit cost, best precision, most materials and colors
Common Applications	Appearance/functional prototypes, samples, small batch internal components	Automobile bumpers, medical/industrial large panels, structural foam parts	Mass production of consumer goods, precision gears, fasteners, and large-volume structural parts