1. Reaction Injection Molding
Tooling
Andrew Hollcraft

2. Introduction
• Injection of monomers into the
mold cavity, followed by
polymerization1
• Advantages
• Low pressure and temperatures
• Reduced tooling and machining costs
• Prototyping
• Large and thick parts
• Cons
• High costs for large volume
production
• Target designs
• Large parts with low production runs
• Prototypes

3. Resin Introduction
• After mixer (sprue)
• Some materials require additional shear
mixing
• Elastomers
• Proprietary designs based on the
materials2
• Gates3
• Direct fill
• Used for radially symmetric parts
• Can cause blemishing and air entrapment
• Fan gate
• Take up a lot of platen space
• Cold slug well analogous feature
• Dam gate
• difficult to machine but are more compact
• Path of least resistance
• Slow down the resin to reduce air
entrapment
• Largest contributor to scrap

4. Tool Requirements• Exothermic reactions
• Venting more critical than injection molding
• Heat removal to prevent degradation4
• Cooling lines
• Mold temperature: ±2°C
• Controls reaction rate
• Controls mechanical properties
• 50-80°C
• Water
• Low viscosity during injection
• RIM molds must seal tighter around cores, pins, and the
parting line4
• Parting line
• Economical due to reduced required pressures
• Machine a relief around the parting line to
increase local molding clamp pressure
• Gasket

5. Tool Design• Foaming Process
• Typically the mold is rotated
into an elevated position for
molding3
• Rib location: prevent air
entrapment
• Vent location: highest
location on the part
• Parting line
• Gusset angle
• Ejection
• Can have large ejection
forces required due to
increased mold surface wet
out
• Air ejection
• Large parts typical
• Large diameter ejector pins
• Parts are not fully cured
at time of ejection, so
they have a low modulus

6. Mold Materials• Low pressures and temperatures required
• Much less expensive mold materials
• Epoxies5
• Surface must be free of residual catalyst
and unreacted functional groups
• Poor temperature control
• Aluminum reinforced
• Fiber reinforced backing (200-300 parts)
• Geometrical stability for large parts
• Poor surface finish
• Nickel shell (can be supported by
materials other than epoxy)
• Silicones (25-50 parts)3
• Lead time: days
• Stereolithography masters
• Low viscosity during filling
• Reactants wet out the surface more that
injection molding
• Release difficulties
• High surface finish required

7. Takeaway
• Low upfront costs
• Inexpensive tooling
• Short lead time
• Epoxies and silicones
• Scalable mold materials
• Transition from prototyping to production

8. References
1. "RIM Mold Design." RIM Manufacturing. Web.
<http://www.reactioninjectionmolding.com/rim-mold-design/>.
2. "Engineering Polyurethanes – RIM Part and Mold Design Guide." Bayer
Material Science, 1 Nov. 2008. Web. . <
http://www.reactioninjectionmolding.com/wp-
content/uploads/2013/09/RIM-PartMoldDesignGuide.pdf >.
3. Macosko, C. W. (1989). Fundamentals of Reaction Injection Molding.
Munich: Hanser Publishers.
4. Sweeney, F. Melvin. "Mold Design." Reaction Injection Molding
Machinery and Processes. New York: M. Dekker, 1987. 253-278. Print.
5. "Reaction Injection Molding Design Guide Download." World-Class
Supplier of Reaction Injection Molding. PREMOLD CORP, 1995. Web. 6
May 2015. <http://www.premoldcorp.com/design-guide-download/>.