Reaction injection molding
Encyclopedia
Reaction injection molding (RIM) is similar to injection molding
except thermosetting polymer
s are used, which requires a curing
reaction to occur within the mold.
Common items made via RIM include automotive bumper
s, air spoilers, and fender
s.
.
If reinforcing agents are added to the mixture then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass
fiber
s and mica
. This process is usually used to produce rigid foam automotive panels.
A subset of RRIM 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 polyurea
s, polyisocyanurate
s, polyester
s, polyphenol
s, 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 agent
.
s. Another advantages of RIM processed foam is that a high-density skin is formed with a low-density core.
The disadvantages are slow cycle times, compared to injection molding, and expensive raw materials.
Injection molding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...
except thermosetting polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s are used, which requires a curing
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...
reaction to occur within the mold.
Common items made via RIM include automotive bumper
Bumper
An automobile's bumper is the front-most or rear-most part, ostensibly designed to allow the car to sustain an impact without damage to the vehicle's safety systems...
s, air spoilers, and fender
Fender
Fender Musical Instruments Corporation, commonly referred to as simply Fender, of Scottsdale, Arizona is a manufacturer of stringed instruments and amplifiers, such as solid-body electric guitars, including the Stratocaster and the Telecaster...
s.
Process
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 cureCure
A cure is a completely effective treatment for a disease.The Cure is an English rock band.Cure, or similar, may also refer to:-Film and television:* The Cure , a short film starring Charlie Chaplin...
.
If reinforcing agents are added to the mixture then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s and mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
. This process is usually used to produce rigid foam automotive panels.
A subset of RRIM 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
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
(known generally as PU-RIM), but others include polyurea
Polyurea
Polyurea is a type of elastomer that is derived from the reaction product of an isocyanate component and a synthetic resin blend component through step-growth polymerization. The isocyanate can be aromatic or aliphatic in nature. It can be monomer, polymer, or any variant reaction of isocyanates,...
s, polyisocyanurate
Polyisocyanurate
Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is essentially an improvement on polyurethane . The proportion of methylene diphenyl diisocyanate is higher than for PUR and instead of a polyether polyol, a polyester derived polyol is used in the reaction...
s, polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
s, polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...
s, polyepoxides, and nylon 6
Nylon 6
Nylon 6 or polycaprolactam is a polymer developed by Paul Schlack at IG Farben to reproduce the properties of nylon 6,6 without violating the patent on its production. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization. This makes...
. For polyurethane one component of the mixture is polyisocyanate and the other component is a blend of polyol
Polyol
A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
, surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
, catalyst, and blowing agent
Blowing agent
A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers, plastics, and metals. They are typically applied when the blown material is in a liquid stage...
.
Advantages and disadvantages
Reaction injection molding can produce strong, flexible, lightweight parts which can easily be painted. 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 expenditureCapital expenditure
Capital expenditures are expenditures creating future benefits. A capital expenditure is incurred when a business spends money either to buy fixed assets or to add to the value of an existing fixed asset with a useful life extending beyond the taxable year...
s. Another advantages of RIM processed foam is that a high-density skin is formed with a low-density core.
The disadvantages are slow cycle times, compared to injection molding, and expensive raw materials.