Reaction Injection Molding

Principle

Reaction injection is a process of molding of articles by reaction or curing of reactants inside the mold.

Two reactive components are metered together and mixed inline so that they being to react in either polymerization or cross linking reaction. It is one of the important molding process for making foamed or non foamed parts.

Materials Used:

Mostly RIM system uses urethane formulations. RIM systems utilizing epoxies, polyesters, polyamides, and nylon 6 are currently under development. System utilizing reinforced nylon are approaching.commercialization for use in the automobile industry.

Urethane systems have traditionally been used for RIM because they readily meet processing and performance requirements. The RIM process requires liquid intermediates which can be catalyzed to provide rapid polymerization at low temperatures without producing gaseous by-products. Most urethane systems are comprised of two liquid intermediate feeds. When blowing agents are incorporated in the isocyanate feed, foam products are formed.

Additives:

Antioxidants, Blowing agents, fillers, colorants, 1 lubricants,urethane catalysts, UV stabilizers, Reinforcing agent.

Process Description:

Two liquid reactants-polyisocyanate components and resin mixture-are held in separate temperature controlled feed tanks equipped with agitators. From these tanks, the polyol and isocyanate are fed through supply lines to metering units that precisely meter the reactants, as high pressure, to the mixing head. When injection beings and valves in the mixhead open, the liquid reactants enter a chamber in the mixing head at pressure between 105 and 210  kg/cm²
where they are intensively mixed by high-velocity impingement. From the mix chamber, the liquid flows into the mold at approximately atmospheric pressure and undergoes an exothermic chemical reaction, forming the polyurethane system used. An average mold for an elastomeric part may be filled in one second or less and be ready for demolding in 30-60 seconds. Special extended geltime polyurethane RIM systems allow the processor to fill very large molds using equipment originally designed for molds with smaller volumes.

The schematic diagram of RIM

Advantages:

• Low tool cost.
• Complete design freedom
• Higher strength to weight ratio.
• Improve or eliminate secondary operations.
• No sink marks.
• Lower weight.
• wide range of physical properties 
• In addition to high strength and low weight, polyurethane Reaction Injection Molding (RIM) parts exhibit heat resistance, thermal insulation, dimensional stability, and a high level of dynamic properties.
• They also offer resistance to inorganic and organic acids as well as many other potentially damaging materials and chemicals including a large number of solvents.
• Resistance to weathering and aging is another plus, though extended exposure to the sun's ultraviolet rays typically results in a color shift at the surface.
• Low processing temperatures (35^oC to 65^oC) and low injection pressure (2-7 kg/cm²) make the Reaction Injection Molding (RIM) process more economical than other molding methods for large parts.