Patent GB-1137993-A (GB)
Title: Improvements in or relating to flat film dies
IPC: B29C48/08
Filed: 1967-03-01
Abstract: 1,137,993. Flat film extrusion die. ALLIED CHEMICAL CORP. 1 March, 1967 [7 March, 1966], No. 9764/67. Heading B5B. A flat film extrusion die comprises a feed passage, at least one extrudate distribution passage communicating with the feed passage and extending therefrom, a passageway connecting the distribution passage with a pressure equalizing channel which leads to a slot-shaped extrusion orifice, the passageway varying in width from a minimum width adjacent the feed passage to a maximum width adjacent the end of the extrudate distribution passage so as to result in a close approximation of equal pressure distribution in the extrude along the length of the extrusion orifice. As shown in Fig. 1, molten filament forming material is supplied to the interior of the die through a central feed passage 42 and flows through extrudate distribution passage 16 by means of which the melt is distributed along the length of the die. From the passage 16 the melt flows through the passageway 41 and the pressure equalizing channel 54 to the elongated extrusion slot 13. The passageway 41 is defined by an inner wall 14<SP>1</SP> of a flat flexible insert 14 and by an upper land 41<SP>1</SP> of a die cavity insert 15. The pressure equalizing channel 54 is formed in the die cavity insert 15 and extends the full length of the extrusion slot. The bottom edge of the lower land 37 of the die cavity insert forms one lip of the slot-shaped extrusion orifice 13, the other lip being formed by the lower edge of the insert 14. The insert 14 is positioned in a supporting block 11 and a screw 24 bears against the insert at a point 26. Such screws are positioned at regular intervals along the extrusion slot 13 and by means of these screws the width of the slot 13 can be finely adjusted in order to compensate for any irregularities in the pressure of the melt which may exist over the length of the extrusion slot. The die cavity insert 15 is positioned by means of screws 17 in a supporting block 12. The inner end of the central feed passage 42 is secured to flat flexible insert 14 by means of threads 42<SP>1</SP> while its outer end is positioned by means of threads 55<SP>1 </SP>in a collar 55. Keys 51 and 52 ensure alignment, respectively, of the supporting blocks 11 and 12, and inserts 14 and 15. Clamping bolts 18 in threaded holes 20 are used to maintain supporting blocks 11 and 12 in clamped engagement. As shown in Fig. 3, which is a fragmentary elevational view on a reduced scale of a portion of the die of Fig. 1, taken on a line 2-2 of Fig. 1, reference numeral 116 indicates a level extrudate distribution passage through which the melt is distributed from central feed passage 142, reference numeral 120 indicates the holes into which clamping bolts shown at 18 in Fig. 1 are threaded, and reference numeral 141<SP>1</SP> indicates an upper land surface corresponding to upper land 41<SP>1</SP> in Fig. 1, the upper land 141<SP>1</SP> being of uniform width L over the entire extent of the extrusion slot 113. The melt flows into the pressure equalizing channel 154 after flowing over the upper land 141<SP>1</SP>, from thence flows over a lower land 137 and emerges from the extrusion slot 113. As shown in Fig. 5, which is a fragmentary cross-sectional view taken along line 4-4 of Fig. 1, reference numerals 111 and 112 indicate supporting blocks corresponding to blocks 11 and 12 in Fig. 1, and reference numerals 114 and 115 indicate respectively the flat flexible insert 14 and the die cavity insert 15 of Fig. 1. Reference numeral 160 indicates a tapering width passageway between melt distribution passage 116 and pressure equalizing channel 154 which is the critical feature of the invention. Passageway 160 varies in width from a narrow point 162 adjacent central feed passage 142 to a wide point at its end 161, the increase in width being in direct proportion to the distance from central feed passage 142. Since the ends of the flat flexible insert 14 and the die cavity insert 15 are not restrained and the inserts are therefore free to expand and contract with temperature changes, the width of the extrusion slot is not distorted due to differential contraction or expansion and remains uniform in width, both the inserts being constructed of the same material. The die is a modification of that described by the Applicants in Specification 1,020,379.