Pipe Extrusion with Rotating Die Systems
April 16th, 1986
Paper to the 6th Plastics and Rubber Institute Conference on Pipes, York, UK, 25th-27th March, 1985.
S F Bush with W G Harland and S Bilgin
Abstract
In an earlier paper[1] the principle of extruding short (around ½ mm) glass fibre reinforced poly-propylene in pipe form, using a rotating mandrel and stationary 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 (around 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 figures 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 strength-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, HDPE, 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.
| Material | Mandrel | Outer Die | Hoop Strength MN/m2 | Hoop Modulus GN/m2 | Results |
|---|---|---|---|---|---|
| PP | Static | Static | 35 | 1.2 | Ref 1 |
| PP | Rotating | Static | 36 | 1.3 | Ref 1 |
| PP + SGF | Static | Static | 36 | 2.0 | Ref 1 |
| PP + CA | Rotating | Static | 46 | 6.3 | Ref 1 |
| PP + 20% SGF | Static | Static | 35 | 2.0 | Latest |
| PP + 20% SGF | Static | Static | 35 | 2.0 | Latest |
| PP + CA | Static | Rotating | 55 | 6.8 | Latest |
| PP + 20% NF | Static | Static | 41 | 2.3 | Latest |
| PP + 20% NF | Static | Static | 41 | 2.1 | Latest |
| PP + CA | Static | Rotating | 74 | >8.5 | Latest |
Key: SGF – short glass fibre; NF – new fibre; CA – coupling agent.
Ref 1: S F Bush, W G Harland, S Bilgin, “Extrusion of Fibre-reinforced Thermoplastic Pipe”, 233, SERC Polymer Engineering 2nd Biennial Review Meeting, Loughborough, 13th-15th April 1983.