The RIM Process: (Elements of the RIM process)

The RIM Process: (Elements of the RIM process):

The major equipment components are:

1.       Storage tanks

2.       The chemical system

3.       Metering units

4.       Mixing head

5.       Other miscellaneous components

6.       Mould support

7.       The mould

8.       The press.

1.       Storage tanks:

It should be capable of being maintained at constant temperature of 27-38^o C and must be fitted for recirculation of fluids.

2.       The chemical system:

There is a wide range of chemical system which produces urethane polymers having a wide range of physical properties. The chemical system should react to form a polymer having the physical properties required for the part being moulded, and its viscosity must be within limits imposed by the metering system and the impingement mixer.

3.       Metering units:

The RIM machine should include at least two high pressure metering pumps, capable of delivering the liquid material to the mixing head at pressure of 1500-3000 psi. The pumps must also be capable of completely filling the mould within 2-3 seconds.

4.       Mixing head:

It is a small mixing chamber (1-6 cc). Here two or more liquids at pressure up to 3000 psi impinges and mixes thoroughly and then transferred to the mould cavity. The mixing heads are usually self cleaning, between injections, by means of recirculating action of the pumps. Hence no solvent flush is required to keep the mixing head clean.

5.       Other miscellaneous components:

The RIM machine not only includes metering unit and mixing head but also includes a set of temperature controlled conditioning tanks, the piping, hoses, filters and controls for operation.

6.       Mould support: OR The mould handling system:

It must position the mould to facilitate the expansion process.

7.       The mould:

The moulds can be made of Aluminum, Kirksite (Aluminum + Zinc) or even epoxy because the mould pressures are relatively low (50-70 psi). For high production, the moulds are usually made of polished of Nickel plated steel. The clamping pressures are also low due to the low mould pressures. In RIM, the mould orientation is very important. The mould must be positioned to allow flow entry with minimum turbulence and rapid gas venting. Depending on the compound the mould temperature range from ambient to a maximum of approximately 66^oC.

8.       The press:

The process is ideal for automation because it is designed for rapid production. Most presses are fully automatic in opening and closing of mould, feeding the resin, demoulding etc.

RIM Terminologies

Explanation of terms:

RIM: Reaction Injection Moulding:

It is a variation of the basic injection moulding process involving high pressure impingement (impact) mixing of two or more reactive thermoplastic or thermoset liquid component (either cold or warm) in a mixing head. After mixing, the liquid stream is injected into a closed, warm mould, at low pressure, where the reaction continues and the mixture solidifies. The finished parts can be cellular or solid elastomer, with a wide range of hardness and modulus.

LIM: Liquid Injection Moulding:

It is another term used for the same process – RIM

HPIM: High Pressure Impingement Mixing:

It is used especially with urethanes, which is a most important one in the overall moulding process.