Reaction Injection Molding Tooling

Andrew Hollcraft

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

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

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

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

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

Takeaway• Low upfront costs• Inexpensive tooling

• Short lead time• Epoxies and silicones

• Scalable mold materials• Transition from prototyping to production

References1. "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/>.