Patent GB-1360310-A (GB)
Title: Producing plastics products
IPC: B29C48/74
Filed: 1972-02-25
Abstract: 1360310 Mixing screws BARMAG BARMER MASCHINENFABRIK AG 25 Feb 1972 [25 Feb 1971] 8855/72 Heading B1C [Also in Division B5] A thermoplastics extrusion, having foamed and non-foamed portions of the same thermoplastics, is extruded from a die 3 to which the thermoplastics of each portion is delivered as separate molten streams from a single extrusion screw source, with a liquid or gaseous expanding agent being added only to the stream or streams defining the foamed portion. The extrusion may be of solid section, with a foamed or non-foamed core respectively covered by a non-foamed or foamed layer, or may be tubular with a foamed or non-foamed layer respectively intermediate exterior non- foamed or foamed layers. The extrusion passes through trimming means 4. The extrusion screw source comprises an upstream screw region 2a, Fig. 1, from the screw channel of which one of the streams is by-passed to the die by a channel 10, and from which another of the streams passes to the die along the screw channel, via a region 5 of decreased channel volume, intermediate larger channel volumes of the upstream region 2a and a downstream region 2b, and which region 5 functions to prevent mixing between the foamable and non-foamable streams. In this embodiment, the expanding agent is added as a liquid at barrel inlet 7 in the region 2b, and the non-foamable stream is by-passed through a channel 10 in the barrel wall, with the streams being supplied to the die 3 through flow control valves 11, 12. The non-foamable material may be by-passed from the region 2a by a pipe (10), Figs. 1b and 2 (not shown), extending to the die (3) externally of the barrel and which may be provided with heating means; the pipe (10) may be provided with mixing means (not shown) comprising axially adjoining and relatively offset fixed baffle elements, or a similar mixer may be provided between the die and screw; the screw tip may be provided with radially extending mixing projections (14); and the pipe (10) may be provided with two separate branches into the die 3 providing non- foamed exterior tubular layers to a foamed inner layer, with each layer being extruded from one of concentric die nozzles. In the Fig. 3 embodiment (not shown) the non-foamable stream is similarly by-passed by a pipe (10) externally of the barrel, and the liquid expanding agent is admitted through a screw core channel (9) to a blind bore at the end of the screw, and containing baffle elements (18a &c.) which effect mixing with the thermoplastic melt, which is admitted and discharged to the bore from the screw region (2b), via radial screw bores (20) located upstream and downstream of an enlarged screw pressure bead (19) provided downstream of the screw region (5). In the Fig. 4a embodiment, the non-foamable stream passes along the screw -region 2b, and the foamable stream to the die 3 via radial screw bores 22 and an axial screw bore 21 to which the expanding agent is delivered through a screw passage 9, a stationary mixing device 24 comprising radial cylindrical pegs 23 being located in the bore 21. In the similar Fig. 4b embodiment (not shown), the device (24) is replaced by a device similar to that of the Fig. 3 embodiment.