GB835950A - Melt spinning process - Google Patents

Abstract

<PICT:0835950/IV (a)/1> In melt-spinning, the pressure drop between the points of entry to, and exit from, the spinning pack is kept substantially constant for a prolonged period of time by passing the melt through a pressure zone in series with the spinning pack, the pressure drop across the melt in said zone being controlled so that the sum of the pressure drops across the pressure zone and the spinning pack remains substantially constant. Suitable apparatus comprises a pump for pumping melt to the spinning pack via an orifice in series with the pack, and a pressure means controlling the size of the orifice by virtue of the pressure exerted thereon by the melt coming from the pump, so that as pressure builds up in the pack the size of the orifice is increased. As shown (Fig. 1), a pack body 1 contains an open-ended cylinder 2 partly immersed in the filter 3 and supported on a porous plate 4 above distribution plate 5. A piston 6 in the cylinder 2 is supported on a spring 7 and a port 8 communicates between the upper portion of cylinder 2 and the space 9 above the filter. The spring 7 acts through piston 6 against the flow of melt from the pump (not shown) with a thrust substantially equal to that developed by the pump. When the pump is not working piston 6 closes port 8, but when the pump is in operation the flow of melt forces down piston 6, opening port 8 so that melt flows through the pack. The thrust P developed by spring 7 is balanced by the sum of the pressure drops p1 across the entry point of the inlet to the port 8 and the pressure drop p2 across filter 3. At the start of spinning p2 is comparatively small and p1 large. As p2 increases with time, due to blocking of the filter 3, the thrust P tends to increase and spring 7 compresses further, thus further opening port 8 and so reducing the pressure drop p1 so that the relationship P=p1+p2 still holds. For this to occur the spring must be of such length that, when port 8 is fully open, the total thrust developed by the spring is still substantially P. Fig. 2 shows a modified apparatus which compensates for the increase in thrust of the spring 7 as said spring is compressed. The piston 10 is provided with a shoulder 11, and ports 12, 13 provide access between the upper narrow portion 14 of the cylinder and the lower wider portion 15 and space 9 above filter 3. As p2 rises, the thrust on the piston shoulder increases and the spring compresses, thus further opening port 12. Port dimensions and spring characteristics are so chosen that the reduction in pressure drop at the port 12 (p1) will be equal to the increase in p2. Thus the thrust on the piston remains substantially constant and a shorter spring may be used. Fig. 3 shows an equivalent construction to Fig. 2, in which the piston and port are replaced by a needle valve 16 and the shoulder 11 of Fig. 2 is replaced by the sloping flanks of the needle. The apparatus is particularly suitable for the melt-spinning of filaments and tows of polyethylene terephthalate.