Reaction Injection Molding Processing

Andrew Hollcraft

Introduction• Reaction Injection Molding (RIM): Injection of

thermoset monomers into a mold, followed by polymerization.

• Historically driven by the automotive industry in the early 1970’s.[8]

• Polyurethane• Physical properties• Foamed – reduced part mass gives better fuel economy• Polymerization at room temperature• Wealth of information at the time

• Bumpers and crash structures • Result of 1960 and 1970’s Federal Motor Vehicle Safety

Standards legislation

• Niche markets• Large parts with low production runs

• Medical equipment• Petroleum and energy industries• Recreation

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Process1. Storage

• Constant recirculation• Cooling

2. Supply• Carefully measured to maintain

desired stoichiometry

3. Mixing Head• High pressure to achieve

homogeneity • Self Cleaning• ~1200 psi

4. After Mixer• Ensures complete mixing

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[3]http://www.premoldcorp.com/process_chart.html

Process4. After-mixer

• Ensures complete mixing5. Filling

• Pressure drops to ~ 100 psi6. Cure

• Exothermic reaction near room temperatures

7. Demolding• Knockout pins • Air assisted ejection

8. Post Curing

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Technologies• Homogeneous parts

• Rigid or elastomer thermosets• Alternative Resins

• Nylon, polyurea, dolydicyclopentadiene, polyester, epoxy, etc [1]

• Additives• Bridge molecules, chain extenders,

surfactants, flame retardants, blowing agents, pigment, fillers, and reinforcement

• Adhesives• Depend on the thermoset and

formulation• High bond strength due to chemical

bonding

• Foams• Addition of a blowing agent in the

mixing head

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Technologies• Reinforced reaction injection molding

(RRIM) [4]

• Addition of reinforcements such as short chopped fibers, mica, or regrind

• Structural reaction injection molding (SRIM)• Addition of fiber packages and or inserts into

the mold cavity before injection

• Back Molding• One sided RIM, RRIM, or SRIM• Can add foam to substrate in one process

• Reduces labor costs

[4]

Long Fiber Injection• Long chop strand, monomers, and

additives are applied to the mold cavity using a robotic arm [9]

• Reduction in machine wear• Glass loading comparable to sheet

molding compound• Fiber length and resin formulation

can be controlled layer by layer or by position

• Beneficial for higher volume production in structural application • Aerospace

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Painting• Pigment addition to

polyurethanes is limited and the low injection pressures can reduce surface finish [2]

• Solutions• In mold painting• Texture: Hides imperfections

and customer wear• Post painting: Highest

cosmetic finish, high gloss• Hydrographic films• Thermoplastic films

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Disadvantages• Higher cost in large volume

production• Cosmetic parts must be painted• Reduced part detail• Shrinkage

Advantages Lower pressures required Lower temperatures required Reduced tooling cost Large amounts of

reinforcement possible Longer fiber reinforcement

possible Thicker parts, up to one inch Lower cycle time for large parts

Summary

References1. Macosko, C. W. (1989). Fundamentals of Reaction Injection Molding. Munich: Hanser Publishers.2. "Reaction Injection Molding (RIM) Process." RIM Manufacturing, n.d. Web. .

<http://www.reactioninjectionmolding.com/about-rim-manufacturing/>.3. Becker, Walter E.. Reaction injection molding. New York: Van Nostrand Reinhold, 1979. Print.4. "Engineering Polyurethanes – RIM Part and Mold Design Guide." Bayer Material Science, 1 Nov. 2008. Web. .

<http://ebookbrowsee.net/rim-partmolddesignguide-pdf-d595404426>.5. Werner, E. "Subproject B6." . Technische Universitat Chemnitz, n.d. Web. .

<http://www.pt-piesa.tu-chemnitz.de/P_2/en/b6.php?druck>.6. "Long Fiber Glass or Natural Fiber Injection Technology ." . Krauss-Maffei, n.d. Web. .

<http://www.freethailand.com/indexsite.php?username=ultimagroups&cat=3018&act=mc>.7. "In-Mold Painting." . Wausaukee Composites , n.d. Web. . <http://www.wauscomp.com/what-we-do/manufacturing-

processes/>.8. E.T. Lloyd and M.C. Cornell. “Polyurethane RIM: A Competitive Plastics Molding Process”. American Chemical Society –

Division of Polymeric Materials Science and Engineering: Proceedings of 186th American Chemical Society Symposium, Washington, D.C., August 28-September 1983. Detroit. Library of Congress, 1984. 15-24. Print.

9. Mason, Karen. "Long-Fiber Injection Advances Polyurethane Composites." Composites World, 1 Dec. 2006. Web. . <http://www.compositesworld.com/articles/long-fiber-injection-advances-polyurethane-composites>.