GB2225272A

GB2225272A - Extrusion die for blown plastic film

Info

Publication number: GB2225272A
Application number: GB8824511A
Authority: GB
Inventors: Peter Clifford Gates
Original assignee: Polysystem Machinery Manufacturing Inc
Current assignee: Polysystem Machinery Manufacturing Inc
Priority date: 1988-10-19
Filing date: 1988-10-19
Publication date: 1990-05-30
Also published as: GB8824511D0

Abstract

In an extrusion die, liquid polymer is conveyed from a central supply duct, through conduits and feed channels, to the conventional set helical grooves. Each groove is fed by a pair of main feed channels, and one or more subsidiary feed channels, the latter being positioned between the main feed-channels. The feed channels are fed from the conduits in such a manner that the hot liquid from the middle of the main supply duct appears along the walls of the groove, and the cooler liquid from the walls of the central supply duct is broken up and dispersed in the middle of the stream flowing up the groove. The effect is to create more uniform temperatures in the polymer as the polymer enters the helical grooves than has been possible hitherto. The invention leads to a more homogeneous film, which allows the production rate to be stepped up. One benefit is that hard -to-extrude materials such as high-density polypropylene can be accommodated. <IMAGE>

Description

#X'PRU##(?N nrp: wap flT#Wfl PTAS"'TC WTT,M This invention relates to the manufacture of plastic film.

BACKGROUND TO THE INVENTION International patent publication number WO 88/01226 shows a plastic film extrusion die of the kind with which the present invention is concerned.

In such a die, hot liquid resin is fed up through a central supply duct (numeral 48 in '226). The resin travels outward from the duct via conduits (47 in '226), which fan out in a conical formation from the top end of the supply duct. The conduits open into feed-channels (45,46 in '226), through which the liquid resin passes to start-channels (43 in '226) and thence to the helical grooves of the die.

The present invention is concerned with the manner in which these various passages are arranged, from the point of view of ensuring that the flowing streams of hot liquid resin are as smooth and even as possible, especially as regards the temperature and viscosity of the liquid resin.

As a general rule of hydrodynamics, any liquid flowing along any passage tends to have a variable velocity profile -- in that the liquid at the centre of the passage travels faster than the liquid near the walls of the passage. In the liquid resin in the central supply duct of an extrusion die, however, this difference in velocities is especially marked, for the following reason.

Over the range of temperature experienced in the die, the viscosity of the resin is very sensitive to changes in the temperature of the resin. A drop in temperature of only a few degrees results in a considerable increase in the viscosity of the resin. Therefore, in the flow of resin supplied to the die, flowing along the central supply duct, the hotter, more runny, resin tends to congregate in the middle of the duct where it can flow quickly, whereas the colder, more viscous, resin tends to migrate towards the walls of the duct. The colder resin, because it moves more sluggishly, tends to congregate in such pockets as may exist in the passages: the colder the resin, the more it tends to congregate in the pockets, and along the walls. It will be appreciated that this effect does not correct itself, but tends to snowball.

The central supply duct therefore unfortunately acts as a separator, in that the central supply duct effectively acts to detect any slight differences in the temperature of the flowing resin and acts to exaggerate the effect of those differences. The central supply duct acts to force the runny hot liquid into the centre of the duct, and to force the sluggish cold liquid to the walls of the duct. The greater the difference in temperature, viscosity, and velocity between the hot and cold sectors of a flowing stream, the greater the tendency for the hot and cold sectors to become even further separated.

This tendency of the duct to act as a temperature-separator would be present also in the other passages within the die, unless precautions are taken against it.

Also, this tendency of the duct and passages to act as temperature separators is present notwithstanding the fact that the die is heated. Even though the walls of the passages may be heated to a temperature that is nominally hotter than the temperature of the resin, nevertheless, the colder portions of the resin stream, because of their reduced viscosity, tend to congregate at places where they can move more sluggishly, ie along the walls of the passages, and in any pockets as may exist. However, the degree of separation that occurs will of course be mitigated to some extent by heating the die, and by thus keeping the walls of the passages hot.

Thus, it Is important that special care should be taken by the designer of the die to make sure the streams of flowing liquid resin remain at an even temperature. If the resin streams are allowed to become, or to remain, separated into hot and cold sectors, there will be a marked variability in the residence time of the resin in the die. Such variability can lead to non-homogeneities in the properties of the resulting plastic film.

Referring again to '226, we now consider the disposition of the liquid resin as it enters and flows along the conduits.

The liquid that enters the bottom sector of the conduit 30 is liquid that comes from the walls of the central supply duct, whereas the liquid that enters the top sector of the conduit is liquid that comes from the centre of the central supply duct. In each of the conduits, therefore, the liquid resin in the top sector of the conduit is hot, whereas the liquid resin in the bottom sector of the conduit is cold.

As the liquid streams emerge from the conduits and enter the feed channels, the hot streams from the top sectors of the conduits lie at the top sectors of the feed channels.

In '226, each start channel is associated with a pair of the feed channels. The arrangement is that the hot streams from the top sectors of the feed channels, when they enter the starter channel, lie adjacent to the walls of the starter channel, whereas the cold streams from the bottom sectors of the feed channels are trapped between the two hot streams, and the two cold streams are therefore kept away from the walls of the starter channel.

Thus, in '226, the homogeneity of the film is enhanced because within the starter channel the cold portions of the stream of liquid resin lie not along the walls and in pockets, but lie in the centre of the stream, where they are swept along with the surrounding hotter, more runny, liquid.

The present invention is aimed at providing a means for enhancing still further the process by which the separated hot and cold flows are recombined, for the purpose of improving the homogeneity of the plastic film.

GENERAL DESCRIPTION OF THE INVENTION In the invention, each start-channel (ie, in effect, each helical-groove) is fed with the liquid resin from at least three feed-channels. Two of the said at least three feedchannels are termed the main feed-channels in respect of that start-channel, and the other feed-channel(s) are termed the subsidiary feed-channels in respect of that start-channel.

The two main feed-channels may be termed the left and right main feed-channels, and are so positioned in relation to the subsidiary feed-channel(s) that the subsidiary feedchannel(s) lie between the left main feed-channel and the right main feed-channel. The relationship of the channels may be further explained thus: each main feed-channel has a respective top wall and a bottom wall; the start-channel has left and right side walls -- the left side wall of the start-channel merges directly into the top wall of the left main feed-channel, and the right side wall of the startchannel merges directly into the top wall of the right main feed-channel. The subsidiary feed-channels lie between, and merge directly with, the respective bottom walls of the left and right main feed-channels.

Each feed-channel receives liquid resin from the conduit associated with that feed-channel. In the invention (as in the arrangement shown in '226) the liquid which flows along the top wall of the associated feed-channel is the hot, runny, liquid from the top sector of the conduit, whilst the liquid which flows along the bottom wall of the feedchannels is the colder, sluggish liquid from the bottom sector of the conduit.

In '226, there were no subsidiary feed-channels, but only the pair of main feed-channels, feeding each start-channel.

In '226, the cold streams from the bottom walls of the two feed-channels therefore merged directly together upon entry into the start-channel. The temperature profile across the start-channel in '226 therefore may be regarded as: HOT-COLD-COLD-HOT.

In the invention, the two cold streams from the main feed channels are separated by the streams from the subsidiary feed channels, so that the temperature profile in the start-channel, with the invent on, may be regarded as: HOT-COLD-SUBSIDIARY-COLD-HOT.

Hence, in the invention, the two cold streams from the main feed-channels are broken up by the streams from the subsidiary feed-channels of the invention. The result is that the agglomeration of cold sluggish material is dispersed, and the temperature distribution across the start-channel is still more even and constant than was achieved in the '226 arrangement.

A number of different arrangements of the subsidiary streams are possible, within the scope of the invention, as will be described.

In the invention, the fast-flowing hot liquid resin entering the start-channel tends to be very well mixed with the slower, colder liquid resin. This has two effects:- first, that the residence time of both the hot and the cold liquid resin tends to become more uniform; and second, that more heat transfer can take place between the streams because the streams are in intimate contact with each other, which tends to even out the differences in temperature. The invention provides a thorough mixing together of the hot and cold streams, which enhances the homogeneity of the resulting plastic film. In turn, this allows the throughput of liquid, and thus the production rate of the plastic film, to be increased.

The improved mixing together of the hot and cold streams in the start-channels that results from the invention also permits such notoriously difficult materials as high-density polyethylene to be extruded at economical production rates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An example of the manner in which the invention may be incorporated into a practical die will now be described, with reference to the accompanying drawings, in which: Fig 1 is a pictorial view of the mandrel of an extrusion die; Figs 2 and 3 are cross-sections of the mandrel of Fig 1, and its associated components, on lines 2-2 and 3-3 of Fig 4; Fig 4 is a diagrammatic representation of a part of the mandrel of Fig 1, showing some of the channels and conduits; Fig 5 is a close-up of an area of Fig 4; Fig 6 is a cross-section, corresponding to Fig 3, of a further mandrel; Fig 7 is a diagram, corresponding to Fig 4, of the mandrel of Fig 6; Fig 8 is a diagram, corresponding to Fig 4, of another mandrel; Fig 9 is a diagram, corresponding to Fig 4, of yet another mandrel.

The mandrel 30 of Figs 1-5 is provided with many helical grooves 40 which spiral around the outer cylindrical surface 50 of the mandrel. Liquid resin oozes from these helical grooves 40 into the annular chamber 52 formed between the surface 50 and the inner surface 54 of the outer die member 56.

The top of the annular chamber 52 narrows to form an annular nozzle 58, through which the liquid resin is extruded. The plastic resin emerges from the annular nozzle 58 in the form of a tube, and the tube is inflated by means of air blown into the the hollow interior 59 of the mandrel 30.

Each one of the helical grooves 40 is provided with a respective start-channel 41. The liquid-resin enters the start-channel 41 through an arrangement of feed-channels and conduits which will be described in detail.

Each start-channel 41 is supplied from four feed-channels: two main feed-channels, which are the left and right upper channels 45,46 and two subsidiary feed-channels, which are the left and right lower feed-channels 47,48. The two upper or main feed-channels 45,46 are supplied from main conduits 68,69 and the two subsidiary or lower feed-channels 47,48 are supplied from subsidiary coinduits 70,71.

The conduits 68-71 are supplied with liquid resin from the central supply duct 80. The line 81 (Fig 3) is a graph or profile of the temperature distribution of the liquid resin flowing upwards in the central supply duct 80. The line 81 may also be regarded as a profile of the velocity of the liquid resin, measured across the cross-section of the central supply duct 80. The liquid resin in the centre of the central supply duct 80 is considerably hotter, and moves considerably faster, than the liquid resin nearer to the walls of the central supply duct.

In this specification, the terms HOT and COLD are applied to the different regions of the liquid resin. However, the transition between the two temperatures is gradual, and there is no clear demarcation between the two.

Typically, the HOT liquid resin may be at 260 deg C, and the COLD liquid resin may be at 230 deg C. As may be seen in Fig 3, the liquid resin that enters the top sector of the conduit 68 is the hot liquid resin from the centre of the central supply duct 80, whereas the liquid resin that enters the bottom sector of the conduit 68 is the cold liquid resin from the walls of the central supply duct.

It should be noted again that this disposition of the hot and cold regions of the liquid resin in the central supply duct is not caused by the walls of the central supply duct being cold. The reason is that the colder regions of the flowing liquid resin, being more viscous, and therefore apt to move more slowly, tend naturally to migrate to the walls of the central supply duct, whereas the hotter regions of the flowing liquid resin tend to stay with the fast-moving liquid resin in the centre of the central supply duct. The plastic materials which tend to be very difficult to extrude into film, such as high-density polyethylene, are those materials in which the viscosity is unusually sensitive to temperature changes, ie in which a small change in the temperature gives rise to a large change in the viscosity.

The more the viscosity of a material is sensitive to temperature, the more a moving stream of the material tends to separate itself, within the duct, into a central, fast-moving, hot stream, surrounded by a slower-moving, colder stream.

The resin stream flowing in each of the conduits 68-71 has a respective hot sector located at the top of the conduit, and a respective cold sector located at the bottom of the conduit. Upon reaching the exit-mouths 78,79 of the conduits, the liquid resin stream divides: half the stream from the main conduit 69 goes into the associated right main feed-channel 46, and the other half of the stream from the conduit 69 goes into the associated left main feed-channel 49, on the other side of the conduit 69. (The "left" and "right" designations are with respect to the starter-channel 41, in Figs 4 and 5.) Each half-stream, as it enters and travels along the associated feed-channel, retains a fast-moving hot sector at the top and a slow-moving cold sector at the bottom.

The feed-channels are arranged to run toqether, so as to cause the liquid resin streams to mix within the start-channel. An upper-left spit 86, an upper-right spit 87, and a lower spit 88 define the points of convergence of the feed-channels 45,46,47,48, as shown.

The liquid that enters the conduits comes directly from the central supply duct 80. As it enters the conduits 68-71, the liquid resin should be as uniform as possible as to its temperature and speed. It is therefore preferred, in the invention, that the upper conduits 68,69 be fed from the same location along the longitudinal axis 89 of the central supply duct 80 as the lower conduits 70,71. It may be seen from Fig 3 that the entry mouths 90,91 of both the upper (main) 68 and the lower (subsidiary) 70 conduits are all at the same level or location around the central supply duct.

Thus, the exit-mouths 78 of the upper conduits 68 are at different locations from the exit-mouths 79 of the lower conduits 70 -- with respect to the longitudinal axis 89 of the central supply duct 80 -- whereas the entry-mouths 90,91 of both the upper 68 and the lower 70 conduits are all at the same location with respect to that axis. Therefore, it follows that the upper conduits 68,69 must be set, in the die, at a different angle or orientation to the lower conduits 70,71. This difference in the orientations of the upper conduits from the lower conduits, so that both may take their resin from the same longitudinal location in the central supply duct 80, may be seen in Figs 3 and 4.

It is important that there should be no opportunity for the liquid resin to collect in slow-moving pockets. This aspect should be borne in mind in the design of the top of the central supply duct 80, where the conduits join into, and merge with, the central supply duct. Of course, the layout of the various passageways is dictated by the practical limitations of drilling and boring and other machining operations, but, bearing those limitations in mind, the conduits should connect with the central supply duct in such a manner as to leave no pockets.

Fig 5 Is a close-up of the spit area, and shows how the streams of liquid resin merge together. The resulting temperature profile can be seen to be: HOT-COLD-HOT-COLD-COLD-HOT-COLD-HOT It will be noted that, as compared with the design in shown in '226, the COLD portion in the middle of the starter-channel stream is broken up, which leads to a greater degree of homogeneity in the resulting plastic film.

The two subsidiary feed-channels 47,48 comprise a pair, in which the cold sectors of the streams in those two feed-channels 47,48 are brought together at the point of convergence 88, and the two hot sectors from that pair surround the combined cold sectors from that pair, in the form of a sandwich. The main feed-channels 45,46 associated with that same start-channel 41 then, in turn, sandwich between them the combined flow from the subsidiary pair 47,48 of feed-channels.

It is recognised in the invention that the above temperature profile serves to alleviate the tendency of the channels and conduits to act as separators. In the invention, the liquid resin in the start-channels is much more even as to its temperature than has been the case in the previously known arrangements.

In addition to the excellent mixing together of the hot and cold streams which occurs in the invention, it will be noted that the liquid that is in contact with the walls of the start-channel 41 is the fast-moving hot liquid.

The invention provides that there is very little variation in the temperature of the liquid resin flowing through the helical grooves 40 and the extrusion nozzle 58. Sirrtllarly, there is very little variation in the velocity with which the liquid resin enters the helical grooves. Consequently, there is very little variation in the residence-time me or the liquid-resin within the die.

In the invention, the average residence-time tends also to be low, since there is little opportunity for any portions of the liquid-resin to remain in the die for long periods.

Any slow-moving liquid that might tend to accumulate at or near the spits S6-88 for example would be swept along by the fast-moving liquid on the walls of the channel.

It is important that the channels, and the junctions and confluences between the channels, should streamline the flowing liquid-resin. There should be no shapes or formations at the confluences which might promote the formation of pockets of slow-moving resin.

In the embodiment illustrated, the four feed-channels associated with a particular start-channel are disposed symmetrically about the start-channel. While symmetry is preferred, symmetry is not essential: a non-symmetrical arrangement of the feed-channels could still produce the excellent mixing of the liquid resin that is a characteristic of the invention.

A low residence time can be important when the die is used to produce batches of film in different materials or in different colours, because a low residence time means that only a little time and material need be lost waiting for all traces of an earlier liquid resin material to pass out of the die.

Although the Fig 1-5 arrangement provides an excellent mixing together of the hot and cold streams, it is not essential in the invention that the entry mouths 90,91 be at the same level with respect to the central supply duct 80.

In the embodiment shown in Figs 6 and 7, for example, the entry mouths 790 of the main conduits 768,769 are at different levels along the height of the central supply duct than the entry mouths 791 of the subsidiary conduits 770,771.

One possible problem that might come to be associated with the Fig 3-5 arrangement, at least in some sizes of die, is that so many holes converge on the end of the central supply duct that the manner in which liquid enters and flows into the conduits may be somewhat confused, and the desired smoothness of transition of the liquid streams might thereby be affected.

The Figs 6,7 arrangement should provide for a more predictable entry flow-pattern into the conduits 768,769,770,771.

As shown in Fig 7, the feed-channels are arranged as they were in Fig 4. The resulting temperature profile in the start channel may be seen as: HOT-WARM-COOL-COLD-COLD-COOL-WARM-HOT Again, it may be seen that the cold sector flowing up the middle of the start-channel has been broken up. One benefit of placing the entry mouths 791 of the subsidiary conduits further down the central supply duct is that the hot liquid from the middle of the duct tends to spread out more: therefore the liquid in the WARM and COOL sectors tends to be rather closer in temperature to the HOT than to the COLD liquid, with the result that heat readily flows from the WARM to the COOL and thence to the COLD sectors.

Another embodiment of the invention Is shown in Fig 8.

Here, there is only one subsidiary feed-channel 847. In accordance with the invention, this one subsidiary feed channel is sandwiched between the two main feed-channels 845,846. Again, the entry mouths of the conduits may be at the same or at differenct levels along the height of the main supply duct.

It may be noted that in the Fig 8 version, the HOT and COLD sectors emerging from the subsidiary conduit 847 enter the start channel with the COLD sector radially, outside the HOT sector, as distinct from the HOT and COLD sectors being side by side, as in the previous embodiments. This means that even the liquid flowing along the back wall or base of the start-channel is HOT, the COLD liquid being located adjacent to the inside surface 54 of the outer die member. The surface 54 being regular, the COLD liquid tends to be carried along with the fast moving HOT stream. In Fig 8, the temperature profile may therefore be seen as: --HOT HOT-COLD- -COLD-HOT -COLD It may be seen from the Fig 9 embodiment that the subsidiary feed-channel 947 need not be of substantial length to achieve the excellent mixing, and smooth flow, of the invention. It will be noted that in the Fig 9 embodiment that the main feed-channels 945,946 are formed as a continuous groove encircling the mandrel. This means that the feed-channels may be cut into the mandrel simply by turning the mandrel on a lathe, which is a much less expensive process than end-milling the channels, as is required in the other embodiments.

Claims

CLAIM 1. Extrusion die, havIng an annular chamber leading

to an annular nozzle, wherein: the die includes a cylindrical mandrel, the outer cylindrical surface of which comprises an inner wall of the annular chamber, and is provided with a series of helical grooves; the die includes a central supply duct, through which molten or liquid resin enters the die under pressure, the said duct being located inside the mandrel, and substantially in line with the cylindrical axis of the mandrel; the die includes a system of passageways for conducting the pressurised liquid from the central supply duct to the helical grooves; the system includes starter channels, main feeder channels, subsidiary feeder channels, main conduits, and subsidiary conduits, so arranged that the liquid flows from the central supply duct into the conduits, thence into the feeder channels, thence into the starter channels, thence into the helical grooves, the chamber, and out of the nozzle; tha arrangement of the system is such that the streams of liquid from the main conduits flow into and through the main feeder channels, and the streams of liquid from the subsidiary conduits flow into and through the subsidiary feeder channels; the said channels each comprises a respective groove formed into the outer cylindrical surface of the mandrel, the groove having a respective base and side walls; the conduits comprise holes through the mandrel, which fan out from the central supply duct to the outer cylindrical surface of the mandrel, whereby entry mouths of the conduits are at the duct, and exit mouths of the conduits are at the surface; the arrangement at the entry mouths of the main conduits is such that liquid which enters the top sector of the entry mouth, and which flows along and adjacent to the top wall of the main conduit, is hot liquid from the most central portion of the central supply duct, and liquid which enters the bottom sector of the entry mouth, and which flows along and adjacent to the bottom wall of the main conduit, is colder liquid from a less central portion of the central supply duct; the arrangement at the exit mouths of the main conduits is such that the stream of hot liquid flowing adjacent to the top wall of the main conduit flows adjacent to the top wall of the associated main feeder channel, and the stream of cold liquid flowing adjacent to the bottom wall of the main conduit flows adjacent to the bottom wall of the said associated main feeder channel; each starter channel is in liquid-stream-conveying communication, at a junction, with a respective pair of the main feeder channels, termed the left and right main feeder channels, associated with that starter channel; the arrangement at the junction between the said left and right main feeder channels and the associated starter channel is such that the stream of hot liquid flowing adjacent to the top wall of the left main feeder channel, upon entry into the starter channel, flows adjacent to the left wall of the associated starter channel, and the stream of hot liquid flowing adjacent to the top wall of the right main feeder channel, upon entry into the starter channel, flows adjacent to the right wall of the associated starter channel; the arrangement at the junction is such that the respective streams of cold liquid flowing adjacent to the bottom walls of the two main feeder channels flow into the associated starter channel, and are spaced away from the walls thereof, and lie between the left and right streams of hot liquid flowing adjacent to the left and right walls of the associated starter channel; the subsidiary feeder channels are associated with the starter channels, at least one subsidiary feeder channel to each associated starter channel; the said at least one or each subsidiary feeder channel is positioned between the said two main feed channels making up the pair of main feed channels associated with that starter channel; the arrangement at the junction is such that the whole stream of liquid from the said at least one or each subsidiary feeder channel flows into the associated starter channel, and is spaced from the walls thereof; the arrangement at the junction is such that, in the starter channel, the said whole stream from the said at least one or each subsidiary channel is located between the said hot streams from the top walls of the associated left and right main feeder channels.
CLAIM 2. Die of claim 1, wherein the die includes only one subsidiary feed-channel per associated starter channel.
CLAIM 3. Die of claim 1, wherein the die includes two subsidiary feed-channels per associated starter channel.
CLAIM 4. Die of claim 3, wherein: the arrangement at the entry mouths of the subsidiary conduits is such that liquid which enters the top sector of the entry mouth, and which flows along and adjacent to the top wall of the subsidiary conduit, is liquid from a first portion of the central supply duct, and liquid which enters the bottom sector of the entry mouth, and which flows along and adjacent to the bottom wall of the subsidiary conduit, is cooler liquid from a second portion of the central supply duct; the arrangement at the exit mouths of the subsidiary conduits is such that the stream of liquid flowing adjacent to the top wall of the subsidiary conduit flows adjacent to the top wall of the associated subsidiary feeder channel, and the stream of cooler liquid flowing adjacent to the bottom wall of the subsidiary conduit flows adjacent to the bottom wall of the said associated subsidiary feeder channel; and the entry mouths of the subsidiary conduits are so arranged in relation to the central supply duct that the liquid in the first portion is hotter than the liquid in the second portion.
CLAIM 5. Die of claim 4, wherein: the arrangement at the entry mouths of the subsidiary conduits is such that liquid which enters the top sector of the entry mouth, and which flows along and adjacent to the top wall of the subsidiary conduit, is hot liquid from the most central portion of the central supply duct, and liquid which enters the bottom sector of the entry mouth, and which flows along and adjacent to the bottom wall of the subsidiary conduit, is colder liquid from a less central portion of the central supply duct; and the arrangement at the exit mouths of the subsidiary conduits is such that the stream of hot liquid flowing adjacent to the top wall of the subsidiary conduit flows adjacent to the top wall of the associated subsidiary feeder channel, and the stream of cold liquid flowing adjacent to the bottom wall of the subsidiary conduit flows adjacent to the bottom wall of the said associated subsidiary feeder channel.
CLAIM 6. Die of claim 4, wherein: the arrangement at the entry mouths of the subsidiary conduits is such that the liquid which enters the top sector of the entry mouth, and which flows along and adjacent to the top wall of the subsidiary conduit, is liquid from a less central portion of the main supply duct.