Reaction Injection Molding Tooling
-
Upload
andrew-hollcraft -
Category
Documents
-
view
361 -
download
3
Transcript of Reaction Injection Molding Tooling
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/>.