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RIM
(Reaction
Injection Molding)
Large
Complex Shapes Structural and
Aesthetic Pleasing Curved
Surfaces
What is RIM?
Polyurethane reaction injection molding (RIM) technology
was developed in the late 1960s. Since
then, the technology has evolved dramatically as more and more product
designers and manufacturers have learned to tap the unique capabilities and
benefits of RIM for an ever-growing range of products. The universal physical
characteristics of polyurethane RIM parts are high strength and low weight.
Like thermoplastic injection molding, RIM is a plastics-forming process that
uses molds to form parts. However, the similarity ends there. It is helpful to view RIM not as a specific
resin with narrowly defined properties, but as a process capable of achieving
a broad range of properties. As its name implies, the polyurethane RIM
process uses polyurethanes to produce molded parts. The polyurethanes begin
as two liquid components, compared with the pellet form of most
thermoplastics. These liquid components - an isocyanate and a polyol - are
developed in two-part formulations, which are often called polyurethane RIM
systems.
Depending on how the polyurethane RIM system is formulated, the parts molded
with it can be a foam or a solid, and they can vary from flexible to
extremely rigid. Thus, polyurethane RIM processing can produce virtually
anything from a very flexible foam-core part to a rigid solid part. Part
density can vary widely, too, with specific gravities ranging from 0.2 to
1.6.
Molds can be resin, spray metal, cast
aluminum or machined aluminum.
  
Inserts Rim Overmold –
Metal Casting Multi-Densities
Strengths
of RIM:
- RIM is the ideal process for
achieving a superior finish on high-quality, limited-run (1-2000 or more
per year)
- Rapid Tooling Options (Molds can be resin, spray metal, cast aluminum or machined
aluminum)
- Very Low tooling cost with
ease of modification for design changes
- Structural integrity - the
universal physical characteristics of polyurethane RIM parts are high
strength and low weight.
- Large parts in low volume –
most cost effective process
- Curved surfaces - most cost
effective process.
- Allows design of clean and attractive
and structural shells/frames
- Capable of a wide range of
wall thicknesses, allowing for deep reinforcing ribs, molded-in
components and assembly hardware
- A variety of castings, sheet metal and threaded
inserts can be molded in place.
- Allows extensive design
freedom
- Encapsulation of carbon fiber
or fiberglass mats gives unprecedented strength
- RIM material meets industry
standards for combustibility and impact resistance required on most
consumer products (UL rating 94V-O, 94-5VA).
- Alternative foams are available which offer a
range of insulating properties.
- Mold in names, logos, instructions
- UL traceability.
RIM Relative
to other processes:
Tooling
for RIM
Depending on
several variables, including quantity, part material, speed of delivery required
and others, RIM molds can be made from
Silicone – for a few pieces, Epoxy for up to 100, Spray Metal for a couple of
hundred, Machined Aluminum and Steel for high volumes.
   
   
A RIM part is
created
When should
RIM be considered?
- For prototype through medium volume
applications (1 - 2,000 pieces).
- Design freedom required to produce complex
shapes.
- Parts larger than two square feet
- Rounded corners and curved surfaces
- Parts heavier than 2.5 pounds
- Parts that must be laminated together
- High degree of dimensional stability required
- Extreme wall thickness variations desired
- Parts where the surface must meet very
demanding
requirements
of appearance or texture
- Surface repeatability
- Parts that require stiffness and the ability to
resist impact
- Parts requiring that inserts (such as heating
or cooling coils)
be
molded into the product
- Parts requiring overmolding of metal, glass,
electrical components,
or
even electronic circuit boards
- Parts that would benefit from insulation from
the environmental effects
of
thermal, vibration, shock, and EMI
     
Curvaceous
Encapsulated
Multi-Densities Weather
Sealed Very Large
How RIM Works
The principle
of the polyurethane RIM process is a chemical reaction between the two liquid
components, isocyanate and alcohol (Polyol), which are held in separate,
temperature-controlled feed tanks equipped with agitators. From these tanks,
the isocyanate and polyol feed through supply lines to metering units that
precisely meter both components, at high pressure, to a mixhead device. When
injection of the liquids into the mold begins, the valves in the mixhead
open. The liquid reactants enter a chamber in the mixhead at pressures
between 1,500 and 3,000 psi, and they are intensively mixed by high-velocity
impingement. From the mix chamber, the liquid then flows into the mold at
approximately atmospheric pressure. Inside the mold, the liquid undergoes an exothermic
chemical reaction, which forms the polyurethane polymer in the mold. Cross-linking occurs between isocyanate
groups (-NCO) and the polyol’s hydroxyl end groups (-OH).
Thermoplastic
PU (TPU) have some cross-linking, but purely by physical means. These bonds
can be broken reversibly by raising the materials temperature, as in molding
or extrusion. The ratio between the
two gives a range of properties between flexible foam (some cross-linking) to
a rigid urethane (high degree of cross-linking). In PUR foams, density can range from 1 lb/
cu ft to 70 lb/ cu ft. Foams are
produced by chemical blowing agents. Catalysts
are used to initiate the reaction
Shot and cycle times vary, depending on the part size and the polyurethane
system used. An average mold for an elastomeric part may be filled in one
second or less and be ready for demolding in 30-60 seconds. Special extended
gel-time polyurethane RIM systems allow the processor enough time to fill
very large molds using equipment originally designed for smaller molds.
What types
of plastic raw materials are used in
RIM?
Some of the materials used
in RIM molding: rigid urethanes, urethane elastomers, urethane foam, DCPDs,
SRIM. Selection of the correct material for a job is an exacting science.
Comparison Chart RIM vs. Injection
Mold Properties
|
Material
|
Units
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R I M
|
Polystyrene
|
ABS
|
Modified
PPO
|
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Specific gravity
|
|
0.85
|
0.85
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0.89
|
0.85
|
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Thickness
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(in.)
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0.25
|
0.25
|
0.25
|
0.25
|
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Flexural modulus
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(psi)
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220,000
|
275,000
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240,000
|
261,000
|
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Flexural strength
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(psi)
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9,500
|
5,700
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6,000
|
6,800
|
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Tensile strength
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(psi)
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5,800
|
2,300
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3,000
|
3,400
|
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Elongation
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(%)
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10
|
20
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- -
|
16
|
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Gardner impact + +
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(in-lb)
|
74
|
28
|
52
|
55
|
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Falling ball
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(ft-lb)
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- -
|
- -
|
- -
|
18
|
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Dart impact
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(ft-lb)
|
- -
|
- -
|
10
|
- -
|
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Heat distortion temperature
|
|
|
|
|
|
|
@ 66 psi
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(F)
|
212
|
183
|
177
|
205
|
|
@ 264 psi
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(F)
|
180
|
168
|
162
|
180
|
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The above properties are
intended as a guide only and may vary depending on thickness and shape.
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RIM (Reaction Injection Molding) Applications:
Electronic,
Medical, Appliance
• Keyboards • Bezels • Housings • Bases
• Speaker
enclosures • Visually critical applications
• Custom structural enclosures, Housings and Frameworks
• Insulation overmolds – Thermal, Vibration, Shock, EMI, RFI,
etc.
• Insert fasteners, stampings, castings, mounting
arms, metal for weight
• Doors • Encapsulation
requirements (metal parts & joinery hardware)
Automotive
• Dashboards • Interior panels • Under-body shields
Truck
components
• Bumper-beams • Front-ends • Grills
 
Very Complex Shapes with
Metal Inserts Multiple
Densities for Insulation
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PORTABLE CLEAN ROOM DEVICE BREAKS THE DESIGN MOLD
USING POLYURETHANE REACTION INJECTION MOLDING
This mobile air filtration
system brings fresh, attractive product design to clean room equipment,
thanks to the design freedom of polyurethane reaction injection molding
(RIM). The nine housing and structural components of the device are molded
with a structural foam polyurethane RIM system. Other than a steel plate in
the base of the 160-pound device, the molded polyurethane is the only
structural material in the device.
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Contact me for a
quick response with the options for meeting your objectives for the design,
speed and performance of your custom-engineered mechanical components
Ron
Humphrey 425.450.0099
rhumphrey@humphrey-associates.com
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