RIM Molding Design Considerations

When considering RIM molding technology for your next low volume plastic enclosure, housing or assembly, there are many things to consider including, part size, part function, value added assembly, design for assembly techniques, molding in features, part cost, post mold assembly costs, tooling investment, part quality, repeatability and the ability for the tools and parts to retain quality, function and dimensional stability throughout the entire lifetime of the series production. Our design considerations will assist you to do that.

Functional Considerations

Environment the part will operate in
Necessity for high impact resistance or stiffness
Parts loading conditions, attaching requirements, structural conditions
Chemical Resistance needed
Is Insert molding needed
Do you need a foamed system to cut down on part weight?

Economic Considerations

When making cost considerations, remember it’s the net shape of part that is important, not the weight. So make cost per volume calculation instead of cost per pound material decisions. Remember a 1” wall section in a foamed polyurethane will be much less than other materials systems including metals.

Look at the part geometry for load bearing applications.
Optimize wall thickness and rib ratios to reduce cost and weight.
Consider if the parts require paint.

General Part Design

Part Stiffness

  • Balance wall thickness and flexural modulus
  • Balance wall thickness and material density in foamed systems
  • Improve stiffness by adding glass (where possible)
  • Improve stiffness and reduce wall sections by using higher modulus systems, improving part geometry or adding reinforcements and encapsulations

Wall Thickness

  • Thicker Walls have higher stiffness
  • Double the wall thickness of a flat and the stiffness will increase by a factor of eight
  • Wall thickness can range from 1/8” up to 2” depending on the material system used and geometry
  • Thicker wall section require more cure time
  • Watch abnormally thick walls as they can cause dimensional stability issues due to longer cooling.

Rib Design and Configuration

  • Taller thinner ribs are more effective than shorter, wider ribs.
  • Run ribs continuously from side to side.
  • Design ribs in the direction of draw to minimize mold cost


  • Every surface parallel to the direction of draw needs draft (1/2 a degree min) to facilitate demolding.
  • As the part height increases over 1”, draft should increase.
  • Add ¼” degree draft for each additional inch of draw over 1”.


  • Attach bosses to the side walls of the part to allow air to escape during the molding process
  • Avoid isolated bosses
  • When design bosses that are not attached to a side wall, use gussets or vent the boss with a core
  • All bosses need radii at their bases

Holes, Grooves and Slots

  • Holes can be drilled in the part post mold, molded in the direction of draw or formed by a retractable pin using hydraulics in the tool.
  • Holes in side wall are also possible having the core and cavity meet at the hole.
  • Orient grooves and slots in the flow direction to minimize air entrapments.
  • Make sure grooves and slots are rounded or chamfered rather than sharp to help flow and vent air.
  • Consider positioning slots in a side wall curled around the base plane to allow for molding without slides.


  • Minimum distance for the insert to the part wall should be 1/8 “ for a solid PUR or ¼” for a foamed system.
  • Metal, plastic, glass and electronics can be encapsulated in PUR materials
  • Threaded Inserts can be used if the parts are to be frequently disassembled by the end-user.
  • Threaded inserts can be either post mold or molded in the part
  • The insert design, hole diameter, part density and screw size determine the pullout force and stripping torque of threaded inserts.


  • If possible, avoid undercuts.
  • Undercuts can be molded but may be costly and only possible in localized areas