What is Reaction Injection Molding (RIM Molding)?

Benefits of Reaction Injection Molding (RIM Molding)

Reaction Injection Molding (RIM Molding) vs. Thermoplastics

Reaction Injection Molding (RIM Molding) vs. Structural Foam

Reaction Injection Molding (RIM Molding)vs. Fiberglass

Reaction Injection Molding (RIM Molding) vs. Thermoforming

Reaction Injection Molding (RIM Molding) vs. Metal

Reaction Injection Molding (RIM Molding) Encapsulation Alternative

Reaction Injection Molding (RIM Molding) Materials

Reaction Injection Molding (RIM Molding) Design Guidelines

Designing a part using the RIM process involves selecting the proper material system and developing the part geometry for Reaction Injection Molding. The information below highlights some of the important considerations involved in the design of the part's geometry for Reaction Injection Molding.

Part Stiffness

To improve stiffness while minimizing wall thickness, consider using a higher modulus system, improving part geometry or adding reinforcements and encapsulations.

Wall Thickness

Parts made from polyurethane systems can be designed with varying wall thickness. If you double wall thickness in a flat part, the part's stiffness will increase by a factor of eight.

Wall thickness for solid materials is typically 1/4", although parts with walls as thick as 1 1/2" have been molded successfully.

Another method to stiffen a side wall in the direction of draw is to curve it at the base, or redesign the flat section so that it has steps, angles, or corrugations.

Wall thickness for parts made of Baydur structural foam can range from 1/4" to 1/2".

Because a part's thickest cross section determines molding time, excessively thick cross sections may cause uneconomical and long molding cycles. Unusually thick cross sections can also cause dimensional difficulties. Because the material in thick cross sections takes longer to cool, parts may shrink more and can possibly warp. Whenever possible, core thick sections to avoid this effect. Consider using ribs or other local reinforcements to increase part stiffness as an alternative.

Rib Design and Configuration

Taller, thinner ribs are more effective than shorter, wider ones. Ribs should run continuously from side-to-side or corner-to-corner whenever possible. Taller ribs with draft may lead to a wide base, resulting in problems in processing, cycle time, and product appearance.

Ribs and other protrusions that are thicker than the nominal wall can cause "readthrough" - sink marks or visual blemishes on the opposite show surface.

If you need the support of a thick rib, design it as a series of thinner ribs with equivalent height and cross-sectional area. The space between these thinner ribs should be no less than the nominal wall thickness.

Side walls may need to be stiffened in the direction of draw and/or perpendicular to the direction of draw. For stiffening in the direction of draw, use simple ribbing. When perpendicular ribbing is necessary, such as in walls, you may have to use sliding cores, which may add significantly to mold and finishing costs. Radius the inner corners of all ribs, bosses, and walls at least 1/16" to reduce stress concentrations and help avoid air entrapment.

Additional design considerations for ribs include:

  • Ribbing increases stiffness only in the ribbing direction of the rib.
  • If a rib is notched, the lower section of the rib will determine strength, unless the notch is bridged with a metal stiffener.

If your part needs torsional stiffness as well as longitudinal stiffness in both directions of the plane surface, use diagonal ribbing.

Draft

Every surface parallel to the direction of draw needs a draft angle to facilitate demolding. The recommended draft angle increases with part height.

Generally, draft is more important on the core side (usually the top half of the mold) than it is on the cavity side, because parts generally shrink onto the core during cooling. Other rules of thumb for draft angles include:

  • A minimum of 1/2 degree is usually adequate for parts with low side walls or ribs, typically those up to 1" deep.
  • Add at least 1/4 degree of draft for every additional inch of draw, such that a 5 inch draw would require a minimum of 1-1/2 degree draft.
Bosses

Attach bosses and other projections on the inside of parts to the side walls with connectors that allow air to escape during molding. Avoid isolated bosses, also known as "blind bosses." If you cannot attach a boss to a side wall because of interference or distance from the wall, design gussets or vent the boss with a core. All bosses should have at least a 1/16 inch radii at their bases.

If you are using a boss to accommodate an inset, such as a screw or press fit, make the hole as deep as possible, preferably leaving only one nominal wall thickness to prevent sink marks.

Other design guidelines include:

  • If you cannot avoid an isolated boss, add gussets that extend from the base to the top on the side in the direction of flow to facilitate air removal and mold filling.
  • Attach bosses to side walls with a connector of nominal wall thickness for foamed materials, 3/4 nominal wall thickness for solid materials.
  • Design bosses away from corners unless the boss can be connected to the wall directly or indirectly. This will help prevent localized heat build up and possible warpage.
  • Consider molding a hollow boss to maintain nominal wall thickness.
  • Core bosses instead of drilling when using thread-cutting screws and thread-cutting inserts in parts made of structural foam to increase pullout strength.
  • Consider designing an elongated boss and having the excess ground off as a post-molding operation, only as a last resort.
Holes, Grooves, and Slots

Holes can be post-drilled, molded in direction of draw, or formed by a retractable pin actuated by a hydraulic cylinder. A hole in a side wall with enough draft can also be formed by having the mold core and cavity meet at the hole. With this design, holes can be positioned anywhere on the wall.

Orient grooves and slots in the flow direction to minimize air entrapment or knit lines. Make sure that grooves are rounded or chamfered rather than sharp to help flow, vent air, and reduce stress concentrations. Grooves should not reduce the wall thickness to the extent that foam flow is impeded. As a rule of thumb, do not recess grooves more than 3/16" for foamed materials and 3/32" for solid materials.

Consider positioning slots in a side wall, curled around the base plate, to allow for molding without slides. Another option is to design slots with stepped cutouts, positioned in a sloping section thicker. If using this last option, do not make mating sections too sharp, as this could damage the mold.

Design slots and grooves with a minimum 1 1/2 degree draft to help with demolding.

Inserts

Polyurethanes have low molding temperatures and pressures, making them ideal for encapsulating reinforcing inserts. The insert should not impede material flow. If using a hollow insert, the ends must be sealed. Thermoplastic end caps have been successfully used to seal inserts. To promote good adhesion with the polyurethane, clean and roughen the inserts and, if necessary, treat them with an adhesion promoter.

The type of Reaction Injection Molding system used determines the recommended minimum distance between insert and the mold wall. For solid materials, this minimum distance is 1/8"; for foamed systems, 1/4".

Encapsulated inserts are used for any number of reasons. For example, they:

  • Increase stiffness
  • Reduce wall thickness
  • Absorb high stresses
  • Control thermal expansion
Metal Stiffening Inserts

Molding metal inserts into RIM polyurethane material will increase stiffness significantly. Inserts of all types - including flat plates, extrusions, tubes, and bars - have been easily and successfully encapsulated. Fully encapsulating inserts eliminates metal corrosion, while reducing thick cross sections, controlling deflection and thermal elongation, and absorbing high stresses.

Wood Stiffening Inserts

Wood inserts - generally less expensive and lighter than metal inserts - can also be used as stiffening inserts in polyurethane parts. When a finished part is subjected to repeated loads, wood inserts may separate from molded polyurethane if the wood's moisture content exceeds 6%. If the wood insert cannot be dried to meet this limit, it must be sealed with lacquer before molding.

Threaded Inserts

Threaded inserts are particularly useful when components must be attached to RIM-molded parts. Use appropriately sized, press-fit inserts with respect to boss-hole diameter. Use threaded inserts if your part is going to be frequently assembled and disassembled.

When using Baydur structural foam, molded-in inserts may offer greater pullout strength, because skin forms over the entire insert surface. Generally, we prefer press-fit inserts, even though these inserts may not be as strong as molded-in ones. Placing inserts on pins inside the mold can increase cycle time significantly.

The insert design, hole diameter, part density, and screw size determine the pullout force and stripping torque of threaded inserts.

Undercuts

If possible, avoid undercuts when designing parts made of rigid RIM polyurethane. They add to cost and may create demolding problems. Modify the part geometry or mold orientation, or divide your part into two separate molds to avoid undercuts.

Snap Fits, Wire Guides and Hinges

A simple, economical, and rapid joining method, snap-fit joints offer a wide range of design possibilities. All snap fits have a protruding part on one component - a hook, stud, or bead - which deflects briefly during joining and catches in a recess in the mating component, thus relieving the deflection force.

Used extensively in business-machine and appliance housings, wire guides offer simple design solutions to keep cables in position. Generally molded into the part, wire guides can be designed as a restraint that is molded without undercuts.

When designing hinges, consider the end use: Will it be a permanent connection? Will it be used often? Will it have to disengage after a certain opening angle? All of these factors will affect design. For permanent, frequently used joints, consider metal hinges, which can be molded-in or post-molded assembled. While they add to costs, they may be optimum in long-term applications. For permanent, infrequently used hinges, consider the living hinge. Typically, they are made of the same material as the part, but can be made of different material. Bayflex elastomeric materials have excellent flexural strength. Molded strips of Bayflex elastomers can be cut and placed into a mold to form a living hinge for a more rigid part. However, if such a hinge breaks, it will be virtually impossible to repair.

Another-hinging method is to mold a part that looks and operates like a metal hinge, with alternating sections on opposite part halves. These partial hinges offer a designer a method of forming hinges without undercuts. While they have reduced load-carrying capability, partial hinges offer lower tooling costs and use hinge pins, as full metal hinges do. A rod pushed through the assembly completes the hinge. This design will disengage when the joint angle reaches 180 degrees. If you do not want the hinge to disengage, consider designing full holes at the ends and a retractable core pin.

Fillers

Using materials that have glass and other inert fillers will affect your part's shrinkage, coefficient of linear thermal expansion (CLTE), stiffness, and impact strength. A filled Bayflex elastomeric polyurethane material can have a CLTE closer to steel. Generally, fillers include fiberglass flakes, short glass fibers, or other mineral fillers. Usually, fillers need to have a sizing treatment to promote adhesion.

As filler content increases, stiffness increases. Short fibers usually orient in the direction of flow, causing greater stiffness and lower CLTE parallel to the fiber orientation. Adding 15% glass filler to a Bayflex elastomer can almost double its flexural modulus. Test your part to ensure that it performs acceptably with suggested filler content.

Warpage in Part Design

Warpage has many causes, including uneven mold and part cooling, incorrect positioning of inserts, unfavorable part geometry, and forces caused by incorrect stacking before a part has fully cured. As a designer, you should be aware of the potential for part warpage early in the design process.

Plastics have significantly higher coefficient of linear thermal expansion (CLTE) than metals, a major consideration if you are designing a part with structural metal inserts. Warpage is more noticeable in flat parts than in those with more complex geometries.

Creep Considerations

All materials show a certain amount of irreversible deformation under long-term load, known as creep. Polymer-chain movement under stress causes creep in polyurethane materials. Creep is usually measured in tension or flexure, with measurements taken at several different temperatures and at different loads.

Fatigue Considerations

Repeated loading causes fatigue, a progressive, permanent change in a part subjected to cycling stresses and strains. While at first no noticeable damage may appear, over time and with continued stresses, parts may begin to fail. Typically fatigue testing consists of repeatedly putting a sample under tension. Generated results show a material's ability to endure these repeated loads.

 Reaction Injection RIM Molding Design Guide

 

 

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