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MPC Polyurethane Knowledge Center

5 Design Tips for RIM

It's 3:33 am, and you found yourself frantically awakening from a product design nightmare. Let me guess - you're concerned about transitioning your prototypes into a first production run? In this post we focus on 5 tips you can use when designing for Reaction Injection Molding (RIM). If you haven't already, check out our previous post What is Reaction Injection Molding, it will provide good context. In summary, unlike injection and cast molding, RIM lets you mold intricate, large geometries with unique surface finishes, while taking advantage of superior engineered thermoset polyurethanes. If you are considering RIM to manufacture your product, here are 5 tips to make the process better and avoid those nasty night-sweats.

RIM polyurethane partsAvoid right angles and undercuts

  • If possible, put a radius on everything! Right angles are air traps, and will result in bubbles and voids being left on the molded part. By including a fillet radii in your design, you can easily allow the air in the mold to escape
  • Sharp angles, cavities and/or recessed areas in a design will often create undercuts. These undercuts will make the part difficult to remove from the mold, which in turn may damage the part.

Strengthen your part walls

  • Long unsupported spans, excessive wall thickness, and sharp internal corners often lead to dipped, warped parts. To avoid this design flaw, simply use ribs, fillets and/or a radii for a stress free part! 
  • For extra durability, design thin exterior walls into a shell and add vertical ribs to the interior. 

Consider manufacturability in your rib design

  • Try avoiding ribs running perpendicular to the direction of the flow of the material as it is introduced into the mold. Moreover, bidirectional ribs often entrap air, resulting in voids and unfinished parts. 
  • To avoid sink marks, design a step in your design where the rib meets the mating wall. If aesthetics are not a major concern, ribs can be thicker than the nominal wall thickness of your design. 
Molded wood finish Reaction Injection Molded part demonstrating ribs

Determine your encapsulated insert  

  • Encapsulated inserts are ideal to reduce wall thickness, increase stiffness, and absorb high tension. Common inserts typically include metal, wood, glass and threaded inserts. Depending on your performance requirements, these inserts should not obstruct the flow of the material as it is introduced into the mold. 
  •   For quicker lead times, consider reasonable size inserts that will be easier for the operator to handle during production. 

Know your surface finish

  • RIM offers a wide range of aesthetics, ranging from molded to painted surface finishes. Molded finishes can often accommodate a variety of textures whereas painted finishes can offer additional physical properties to your part. 

By incorporating these 5 tips above you can get your RIM design manufacturing ready! If you are still having a product design nightmare, contact us here and let our thermoset experts bring your RIM design to life.  

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