Pipe Extrusion with Rotating Die Systems

In an earlier paper the principle of extruding short (c. 5 mm) glass fibre reinforced polypropylene
in pipe form, using a rotating mandrel and stationery outer die, was described, together
with the results shown in the first section of the Table below. Generally, desirable increases in
both hoop strength and hoop modulus were obtained at relatively low !c. 10 rpm) rates of mandrel
rotation. This improvement was ascribed to two factors !a) elimination of the weld line (this
particularly affects hoop strength) and (b) partial alignment of the fibres in the hoop
(circumferential) direction (this particularly increases hoop modulus, and reduces longitudinal
crack propagation).

The present paper reports work which extends the earlier results in three directions.
These are (i) use of a wider range of fibre lengths, (ii) use of other polyolefine matrices including
rubber modified types, (iii) development of the die system design. The object of (i) and (ii) has
been primarily to improve further the strenqth-modulus-toughness combination while (iii) is aimed
at enabling (i) and (ii) to be carried out and also at improving the surface quality. A third
objective under (iii) has been the design of a rotating die system which is easy to install on
standard machines. Basically the results indicate that a commercially applicable process for making
tubes with an attractive combination of properties has been developed.

The polymer systems investigated include PP, HOPE, mixtures of the two, and PP containing a
proportion of EPDM, together with varying amounts and lengths of glass fibre. The process systems
investigated include inner and outer die rotation together with an optimised cooling profile. The
effects of different process configurations on pipe surface finish as well as on the primary
mechanical properties have been assessed. Fibre orientations have been determined as a function
of process parameters and compared with model predictions. Other measurements made comprise fibre
length distribution in the pipe, burst strengths and dart impact resistance, as functions of the
combined polymer-process system. The results obtained for two of the many matrix-fibre combinations
investigated are shown in the Table with earlier values for comparison. While not as yet quantified
the results obtained under dart impact with the latest fibre-matrix-process system are especially
noteworthy: the tendency for cracks to propagate and collapse the pipe specimen is greatly if not
entirely inhibited.