Post-Mixing Processes

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12 Post-Mixing Processes
12.1 Introduction
The results of mixing obviously have a decisive influence on the down-stream processes.
Some of the critical points of the influence will be discussed in this chapter. Extrusion
and injection moulding are selected as examples, although other processes such as
calendering are equally important. Extrusion and injection moulding are automated; as
a result they have a strict requirement for the consistency of the compounds. The operation
of extrusion will be discussed with respect to feeding problems, the compound uniformity
and flow mechanisms. An example of the extrusion problem of silica-filled compounds
is also discussed. For the injection moulding a method of dealing with the non-uniformity
of the compound is described.
12.2 Extrusion
A brief description of the extrusion of a rubber compound is given with emphasis on the
uniformity of the operation. One source of non-uniformity is that originating from the
mixing and the other is caused by the extrusion itself. Only the former is related to
mixing, but both cases are described here, because they may not be clearly separable in
the actual extrusion operation. The purpose of extrusion is to fabricate a continuous
object with a precisely defined cross-sectional geometry, i.e., a profile. It follows that the
material must be uniform in every part over the cross-section as well as in the machine
direction. The material, being viscoelastic, carries deformational memory. Therefore, the
uniformity of the memory as well as that of the composition must be concerned with the
recovery or else the memory will distort the profile.
12.2.1 Causes of non-uniformity in feeding
The compound fed into extruder may be a ribbon, pellets or premixed powdered rubber.
The ribbon is usually cut from sheet taken up from the sheeting mill-roll. The width of
the ribbon is uniform but the thickness may be irregular. This is one of the potential