First, the two parts of the polymer are
mixed together. The mixture is then
injected into the mold under high
pressure using an impinging mixer. The
mixture is allowed to sit in the mold
long enough for it to expand and
If reinforcing agents are added to the
mixture then the process is known as
reinforced reaction injection molding (RRIM).
Common reinforcing agents include glass
fibers and mica. This process is usually
used to produce rigid foam automotive
A subset of RIM is structural reaction
injection molding (SRIM), which uses
fiber meshes for the reinforcing agent.
The fiber mesh is first arranged in the
mold and then the polymer mixture is
injection molded over it.
The most common RIM processable material
is polyurethane (known generally as PU-RIM),
but others include polyureas,
polyphenols, polyepoxides, and nylon 6.
For polyurethane one component of the
mixture is polyisocyanate and the other
component is a blend of polyol,
surfactant, catalyst, and blowing
Advantages and disadvantages
Reaction injection molding can produce
strong, flexible, lightweight parts
which can easily be painted.[citation
needed] It also has the advantage of
quick cycle times compared to typical
vacuum cast materials.
The bi-component mixture injected into
the mold has a much lower viscosity than
molten thermoplastic polymers, therefore
large, light-weight, and thin-walled
items can be successfully RIM processed.
This thinner mixture also requires less
clamping forces, which leads to smaller
equipment and ultimately lower capital
expenditures. Another advantages of RIM
processed foam is that a high-density
skin is formed with a low-density
The disadvantages are slow cycle times,
compared to injection molding, and
expensive raw materials.[citation
Machined steel or aluminum; cast
aluminum; silicone rubber;epoxy resin.
The machines can be large or small
depending on the size of part required.