One barrel of the rheometer was fitted with a capillary die of L/D ratio 16 and the other bore was fitted with an orifice die. The bore diameter of both capillary dies was 1 mm, with die entry angle of
180◦, whilst the lengths were 16 and less than 0.25 mm for long and
orifice dies, respectively. Capillary dies were fitted into the bottom of the barrels and pressure transducers located directly above the dies. The capillary dies were made from tungsten carbide–cobalt alloy in order to maintain tight geometrical integrity.
The polymer was dried in an oven at 50 ◦C for 24 h before rhe-
ological investigation. The flow behaviour of each grade of HPC
was characterised at 140, 145 and 150 ◦C. The rheometer was set
at test temperature and allowed to stabilize. Temperatures were controlled within ±0.5 ◦C of the set values and monitored by plat-
inum resistance thermometers fitted in the three (top, middle and bottom) zones of the barrel. The polymer was fed into both bores of the barrel and manually compressed before the test was started. The polymer was subjected to pre-compression pressure of 0.5 MPa, and a total pre-heating time of 240 s. The instrument was run in
8-stage discrete speed programme. The piston speed was automat- ically converted to shear rate by software and is summarised in Table 1.