46.1 REACTION INJECTION MOLDING (RIM)
· Basic Principle - A mold is brought together, and a multipart thermosetting polymer is mixed and injected. After the part sets, the mold is opened, and the part is removed. Post curing may be used.
· Features:
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- Very little energy is required, most energy is provided by the chemical reaction.
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- Parts up to 100 lb produced.
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- Surfaces don't have flow lines found in traditional injection moulding.
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- RIM materials tend to be low weight, corrosion resistant, and variable properties can be obtained by additives and ratio adjustment.
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- RIM machines use lower pressures, and are therefore less expensive.
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- Typical parts are; bus bumpers, large electronics cases, shoes, etc.
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- Fillers can be used to increase strength.
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- The basic process involves mixing the raw components.
· Metering and mixing are critical
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- special metering pumps are required for the components to maintain material properties.
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- mixing must thoroughly mix the components at high speed, and inject them without becoming clogged when the mixture sets.
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· While the previous mix head is for a runnerless system, it is also common to have an extra component that is between the mix head and the part (the runner).
· An aftermix may also be used to increase mixing. A typical design will split the stream and cause it to impinge at 180°, then continue on to the mold.
· A comparison of the materials shows the advantages over a similar injection molded material.
