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page 268

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.

 

recirculation lines

component A

component B

 

mixing head

 

(pull back to

 

mix and mold)

 

precise heaters

 

large volume

 

capacity pumps

M

M

This way to mold

page 269

Component A

Component B

Features:

-Very little energy is required, most energy is provided by the chemical reaction.

-Parts up to 100 lb produced.

-Surfaces don’t have flow lines found in traditional injection moulding.

-RIM materials tend to be low weight, corrosion resistant, and variable properties can be obtained by additives and ratio adjustment.

-RIM machines use lower pressures, and are therefore less expensive.

-Typical parts are; bus bumpers, large electronics cases, shoes, etc.

-Fillers can be used to increase strength.

-The basic process involves mixing the raw components.

page 270

Synchronized metering units

Isocyanate or

Prepolymer mixing chamber

Pour

Polyol Mix

Mold

Preparation

Cure

Demold

Optional

Components

After process operations

Metering and mixing are critical

-special metering pumps are required for the components to maintain material properties.

Fluid In

Normally closed inlet valve

Fluid Out

piston

Normally Closed Valve

In this common form of piston pump, the piston rod is drawn back creating suction that holds the valve closed, and pulls fluid into the chamber. When the cylinder is full of fluid

the piston motion is reversed, creating a pressure, and forcing the inlet vale closed, and the outlet valve open, and the

fluid is pumped out. The fluid volume can be controlled by using the cylinder size, and piston strokes

- mixing must thoroughly mix the components at high speed, and inject them without becoming clogged when the mixture sets.

 

page 271

 

component

component

component

component

A

B

A

B

pump/tank

pump/tank

pump/tank

pump/tank

Mixing head is advanced, and the two components are mixed, and they pour into the mold

The two component have been mixed, or the mold is empty, and the mixing head is retracted, allowing the components to recirculate

• 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).

page 272

component is Hydraulic pumped in here fluid actuates

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.