WO1990008025A1

WO1990008025A1 - Blown-film extrusion die

Info

Publication number: WO1990008025A1
Application number: PCT/GB1990/000073
Authority: WO
Inventors: Ricardo Pablo Rodriguez
Original assignee: Polysystem International Inc.; Asquith, Anthony
Priority date: 1989-01-19
Filing date: 1990-01-18
Publication date: 1990-07-26
Also published as: GB2227446A; GB9001201D0; AU4826490A; GB8901204D0

Abstract

The die is of the spiral feed-groove type, in which the annulus of liquid resin emerging from the groove chamber has bands of unequal temperature. The die is provided with a flow-dividing and re-combining facility, which takes the form of two series of open channels, one cut into the outer wall, and one cut into the inner wall, just below the die nozzle. One series of channels is inclined or offset to the right, the other to the left. The effect is that liquid from the hot band may be recombined with a matched quantity of liquid from the cool band. This leads to a marked improvement in temperature equalization band to band. As a result, the gauge thickness of the as-manufactured plastic film is more consistent.

Description

BLOWN-FILM EXTRUSION DIE

This invention relates to the manufacture of plastic film. Conventionally, in the manufacture of film, liquid plastic is extruded from an annular nozzle as a continuous tube; the tube is inflated to form a bubble and, upon cooling and soldifying, the tube is flattened and the plastic film is collected on rolls. An example of the kind of extrusion die with which the invention is concerned is described in PCT WO 87/06879 (BENTIVOGLIO).

In such a die, molten liquid-plastic material is fed under pressure into a central supply duct of the die, and the liquid flows through the die, and out through the annular extrusion nozzle. It has been a matter of intense

development within the industry to provide the facility within the die whereby the liquid spreads out evenly from the single duct into the annular stream, and whereby the liquid is forced out of the annular extrusion nozzle

absolutely evenly all around the circumference of the nozzle.

One of the conventional measures taken to ensure this even spread around the nozzle is to provide, in the die, a series of spiral feed-grooves, as shown in BENTIVOGLIO. The liquid progressively leaks out of these feed-grooves into an annular chamber. The usual design is that each feed-groove extends approximately once around the full annular circumference.

The said helical feed-grooves ensure a good degree of evenness and equality in the properties of the liquid at all points around the circumference of the nozzle. The present invention is concerned with enhancing still further the equality of properties; the manufactured plastic film is sold on the basis of its minimum properties (of mechanical strength etc), not the average properties, so that the more even the properties, the more economical the film.

PCT WO 88/01226 (GATES) shows an example of the kinds of measures that are taken to promote greater evenness of temperature around the circumference, and discloses an arrangement for feeding the liquid into the spiral

feed-grooves wherein most of the temperature gradients within the liquid, at the point where the liquid enters the feed-grooves, tend to be evened out.

Such measures are extremely effective in reducing the variations in physical properties in the manufactured plastic film. The extra equality of properties that is the aim of the present invention is intended not so much as a replacement for the previous measurures, but as an addition to, and as a refinement that goes beyond, the previous measures.

In fact it is possible, by proper design of a spiral-feed die according to the conventional principles, to provide a die in which the through-flow rate of the liquid plastic (measured in grams per second) at any point on the extrusion nozzle is the same, within about 1%, as the flow rate at any other point around the nozzle. Nevertheless, even when such a die is set up as perfectly as that, it turns out that there are still variations in the gauge thickness around the circumference of the manufactured tube of film. The gauge thickness may be seen to vary cyclically in peaks and troughs around the circumference; and it may be easily observed that there are as many thickness peaks (and as many troughs) as there are spiral feed grooves.

It is recognized in the present invention that although the volumetric flow rate of liquid may be completely even at all points around the circumference of the nozzle, such

uniformity in the flow rate unfortunately does not ensure that all the flowing liquid is at the same temperature. It may be observed, in fact, that the temperature of the liquid that reaches the nozzle from the very ends or tips of the feed-grooves may be several degrees cooler than the liquid that reaches the nozzle from a point intermediate between the ends of the feed-grooves.

The viscosities of the types of plastic materials from which blown film is usually made are very critically dependent on temperature. Even a tiny change in temperature can make a significant alteration to the viscosity. Thus, when the liquid emerges from the nozzle, and is inflated to form a bubble, the hotter portions, between the feed-grooves, are more "runny", and become stretched to a significantly greater extent than the intercalated cooler portions.

A primary aim of the invention is to provide a means for reducing the effect of these temperature peaks and troughs in the liquid plastic between adjacent feed-grooves. It may be noted that the variation in thickness of a film due to these temperature variations is especially troublesome when the material is very temperature-sensitive, and when the film is thin, even though the die may have been set up perfectly.

DESCRIPTION OF MAIN FEATURES OF THE INVENTION

The conventional extrusion die as described above, includes a gathering chamber, which is the enclosed annular space between the spiral feed grooves and the extrusion nozzle. The liquid plastic material, after emerging, in an annulus, from the feed-grooves, is gathered in the gathering chamber prior to passing out through the nozzle.

In the invention, a means is provided, in the gathering chamber, for dividing the liquid flow emanating from each point around the annulus, and for recombining the flow. In a preferred form of the invention, one portion of the divided flow is diverted to the left, the other portion to the right, the arrangement of the means, being such that the effect of this measure is to double the number of the said temperature peaks and troughs.

As a consequence of the inherent characteristics of a die with spiral feed-grooves (in the invention as in the prior art designs), the annulus of liquid that emerges therefrom is characterized by hot bands and cool bands: the cool bands correspond to the tips of the feed-grooves, and the hot bands correspond to intermediate points between the tips. Thus, the spacing of the hot bands ( ie. the peaks)

corresponds, on a one-to-one basis, to the spacing of the feed-grooves. This splitting up of the annulus into heat bands cannot be avoided, even if the die were set up

perfectly, with perfectly even flow rate around the

circumference.

Each hot band of high temperature liquid travelling up the bubble is flanked by corresponding cool bands. The dividing means of the invention has the effect (in the preferred form of the invention) that the circumferential distance apart of the cooler bands (and thus the width of the hot band) is only half the corresponding distance in the prior art designs. Since the hot bands are narrower, the amount of stretching (and consequent thinning) of the hot bands due to inflation of the bubble is reduced. Thus, the amount of thinning of the bubble liquid that takes place in the hot bands is more nearly equal to the amount of thinning that takes place in the cool bands. The result is that the gauge thickness of the manufactured plastic film becomes less variable.

It may be noted that this advantage -- of making the degree of thinning in the hot and cool bands of the bubble liquid more nearly equal -- still occurs even though the temperatures of the hot and cool bands in the bubble remain as different as the temperatures of the hot and cool bands in the annulus of liquid when emerging from the feed-grooves. The advantage arises because the hot and cool bands are narrower and closer together, not because the temperatures in the bands have become equalized.

On the other hand, it may be noted that the temperatures of the hot and cool bands in the bubble do in fact become more nearly. equalized as compared with the prior art gathering chambers.

This tendency towards equalization of temperature takes place firstly because the bands are physically closer together, and secondly because of a mixing facility that is preferably incorporated into the dividing and recombining means of the invention. The means preferably comprises two sets or series of open channels, which are arranged to overlap in a criss-cross fashion. Some of the liquid flowing in one of the channels may leak into, and mix with, the liquid flowing in another of the channels. Thus, some of the liquid from the hot bands is caused to mix with some of the liquid from .the cool bands.

However, it should be noted that preferably the amount of mixing should be kept comparatively small, ie only a little of the liquid should be permitted to leave its allotted channel. This is because it is the fact that the liquid is conveyed by means of the channels to different points around the circumference of the annulus which accounts for the doubling of the number of bands.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

By way of further explanation of the invention, an exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Fig 1 is a cross-sectional side-elevation of an extrusion die which embodies the invention;

Fig 2 is a cross-sectioned plan view taken on the line 2-2 of Fig 1;

Fig 3 is a developed view taken on the plane indicated by the lines 3-3 in Figs 1 and 2; Fig 4 is a diagrammatic view of a portion of Fig 3;

Fig 5 is a view corresponding to Fig 4 of another die;

Fig 6 is a view corresponding to Fig 4 of yet another die;

Fig 7 is a view corresponding to Fig 1 of a further die;

Fig 8 is a view corresponding to Fig 3 of the die of Fig 7.

The dies shown in the accompanying drawings and described below are examples of dies which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by features of specific embodiments.

The die 20 shown in the drawings receives hot, molten, plastic material in the form of liquid resin, under pressure from an injector (not shown). The liquid resin travels up through a central supply duct 21 of the die.

A number of conduits 23 fan out from the central supply duct 21, and these conduits convey the liquid to a number of spiral feed-grooves 25. The spiral feed-grooves 25 are cut into the face 27 of a cylindrical mandrel 29, which

constitutes the inner wall of an annular feed-groove chamber 30. The outer wall of the chamber 30 is constituted by a cylindrical bore 32 in an outer component 34 of the die 20. Located above the feed-groove chamber 30 is a gathering chamber 36, in which the liquid resin emerging from the feed-groove chamber is gathered prior to passing through the extrusion nozzle 38 of the die.

As described thus far, the die is conventional. The

invention lies in the manner in which the liquid resin is treated as it passes through the gathering chamber 36, which will now be described.

The gathering chamber 36 includes relatively wide-open upper and lower liquid-reception sub-chambers 39,40. Between these two sub-chambers, the inner and outer walls 41,43 of the gathering chamber are provided with inner and outer protruding lands 45,47. Open channels are cut into the lands 45,47, and these channels are passageways which comprise the dividing and recombining means of the

invention.

The lands 45,47 are close enough together, radially, that little of the total flow of liquid passing up through the gathering chamber 36 can pass directly through the gap 49 between them. Thus, virtually all the flow passes through the channels. The channels are however open to each other, and so it is possible for liquid flowing in the channels cut in the inner land to spill or leak into, and to mix with, the liquid flowing in the channels cut in the outer land, and vice versa.

The channels 50 cut into the inner land 45 comprise a first series 52 of channels. The channels 54 cut into the outer land 47 comprise a second series 56 of channels.

Each channel has an entry mouth T, and an exit mouth X.

The channels are disposed in a helical or spiral fashion, in that the exit mouth X of each channel is displaced a

distance D, measured in the circumferential sense, with respect to the entry mouth T: the channels 50 of the first or inner series 52 are displaced to the right, and the channels 54 of the second series 56 are displaced to the left, as shown in Fig 3.

(It does not matter which of the two series goes to the left and which to the right: the important aspect is that the two series take the opposite directions.)

In Fig 4, the criss-cross lines shown therein represent the paths taken by the liquid flowing along the various

channels.

It may be seen that the dispositions of the channels relative to the feed-grooves 25 are such that there are twelve times as many channels as feed-grooves, six in each of the series 52,56. Every sixth channel of the first (inner) series may be designated as a first sub-series 58; it will be appreciated that there are as many channels in the first sub-series 58 as spiral feed-grooves 25, and that each channel in the first sub-series lies in the same circumferential position relative to a respective associated one of the spiral feed-grooves. A corresponding second sub-series 60 may be designated in relation to the second

(outer) series 56 of channels, the entry mouths T of the channels in the second sub-series 60 lying at the same circumferential locations as the entry mouths of the

channels of the first sub-series 58.

The displacement D, ie the circumferential displacement between the entry and exit mouths of the channels, is the same for both sub-series, and is such that the exit mouths of the channels of the first sub-series lie half way between the exit mouths of the channels of the second sub-series.

As shown in Fig 4, the displacement D is equal to 3/4 of the distance G between adjacent feed-grooves.

As shown in Fig 5, the displacement D is equal to 1/4 of the distance G between adjacent feed-grooves.

As shown in Fig 6, the displacement D is equal to the distance G between adjacent feed-grooves.

The "D = 3/4 G" arrangement of Fig 4 is preferred for the following reason.

As mentioned, the liquid that emerges from the feed-groove chamber 30 is characterized by having hot bands and cool bands, the difference between the temperatures of the two kinds of bands being several degrees in some cases. The cool bands 61 are aligned with the tips 63 of adjacent feed-grooves 25, the hot bands 65 being aligned with the midpoints between the tips. The letters H and C represent the relative temperatures of the hot and cool bands, and of the areas between them.

The rising annulus of liquid from the groove chamber, with its temperature bands, reaches the mouths of the channels. The mouths of the channels 50 of the inner series 52 and the mouths of the channels 54 of the outer series 54 are presented equally to the approaching liquid. Each band -- hot, cool, and all the bands in between -- is split into an inner component, which travels up the inner channels 50, and an outer component, which travels up the outer channels 54. The inner and outer components of the cool bands 61 are designated 67 and 69 respectively, and the inner and outer components of the hot bands 65 are designated 70 and 72 respectively. The bands and their divided components are repeated every feed-groove, as shown.

In both the Fig 5 and the Fig 4 arrangements, the inner components 70 of the hot bands 65 emerge from the exit mouths of the appropriate channels. It will be noted that these hot, inner, components are in circumferential register with the cool, outer, components from the associated cool bands 61. In fact, it will be noted that the arrangements of both Fig 4 and Fig 5 provide recombined flows in which the hot and cool streams are mixed in equal proportions.

This equality would not, in practice, be as perfect as indicated on the diagrams: one reason being that the hot band is not exactly halfway between the adjoining cool bands but is skewed somewhat to one side. However, the

improvement in temperature equality that may be attributed to the arrangements as shown is considerable, and more than worth the expense of providing the channels.

The Fig 4 version is preferred because in Fig 5 there is less opportunity for the inner and outer streams to touch each other across the open channels, and therefore less opportunity for mixing and temperature transfer.

On the other hand, in some circumstances the Fig 5 version may be preferred.

In the invention, it is contemplated that the open channels, as shown in the drawings, may be replaced by closed pipes, aligned in a similar criss-cross fashion. Thus, the lines in Fig 4 may be taken to represent the axes of such pipes, as opposed to the axes of open channels. With closed pipes, there is no mixing at all that could be regarded as

equivalent to the channel-to-channel leakage, as described, but it will be seen that even without the channel-to-channel spillover, all the recombined streams still have equal components from the hot and cool bands.

Against the use of enclosed pipes, however, is the fact that some kinds of molten plastic material are over-sensitive to being broken into separate streams, in that they acquire a "seam" upon recombining which can be difficult to get rid of.

In the non-preferred arrangement shown in Fig 6, the

distance D is 4/4 of the distance G. It will be observed that in Fig 6 the separation between the hot and cool bands has been preserved just as in the annulus of liquid emerging from the feed-groove chamber 30. However, that is not to say that the arrangement of Fig 6 incompletely outside the invention, because some degree of temperature equalization will occur due to the leakage and spillover effects between the open channels, as described.

A general rule may be formulated regarding the preferred relationship between D and G: in the general formula

D = G N/4

the temperature-equalization is maximum if N is an odd whole number, and minimum if N is an even whole number. If N is not a whole number the temperature-equalization will lie correspondingly somewhere between maximum and minimum.

In practice, there is little point in going to the odd numbers higher than 3, as the possible slight further gain in equalization would not generally be worth the extra difficulty of manufacture. It might however sometimes be worthwhile incorporating a further, or duplicate, set of inner and outer channels, in line above the set as

described, as a second stage of equalization.

In fact, it is contemplated that in some cases it would be possible to replace the set of spiral feed-grooves 25 with a separate stage of the criss-cross passageways, and the scope of the invention should be construed accordingly.

As described, the preferred arrangement is that all the channels in the outer land are inclined to the left, and all the channels in the inner land are inclined to the right (or vice versa).

It might be considered that it would be acceptable to replace this arrangement with one in which both the

left-inclined and the right-inclined channels are cut into the same land. The effect of this arrangement would be to leave a series of diamond-shaped islands of metal between the channels.

Such an arrangement would not work, according to the invention. Although the liquid would be deflected slightly by the diamond shaped islands, basically the liquid would continue to flow straight upwards; as far as the temperature equalization between the hot bands and the cool bands is concerned it would be largely as if no channels had been cut at all.

The arrangement of open channels as described is in one respect similar to the diamond-island arrangement, in that liquid may flow freely from channel to channel. However, the arrangement of the invention is superior to the diamond arrangement because in the diamond arrangement the streams cannot flow independently, but must intermingle and mix completely at every point of intersection. Thus, in the invention, the divided flows can be kept apart until they can be recombined in the most favourable and advantageous manner. In the invention, it can be arranged that the hot-content of every inner element of the flow is (at least theoretically) exactly matched by the cool-content of the outer element with which it combines.

As shown in Figs 1-3, the channels are large enough, and open enough, that very little flow-resistance is caused by the presence of the channels; such resistance, if present, would cause a back pressure, and this back pressure would have to be overcome by a corresponding increase in the pressure at which the liquid resin is injected into the die. Often, such an increase cannot be tolerated, because many types of resin are adversely affected by increases in injection pressure.

In some cases, injection pressure is not so critical and then some flow-resistance in the channels can be allowed. In Figs 7 and 8 the lower extremities 80 of the channels 82 start from nothing and become progressively deeper; the upper extremities 84 of the channels 82 are so formed as to become progressively shallower, in an equivalent manner. It may be noted that the spiral feed-grooves 25 similarly become progressively shallower at the tips 63. The depth of the channels is controlled of course by controlling the depth of the tool which cuts the channel.

When the channels 82 are formed with run-ins and run-outs, as at 80,84, not only is the flow-resistance increased, but the flow is not separated and divided so cleanly as was the case when the channels were open at the ends, as in Figs 1-3. However, the degree of mixing together of adjacent streams may be expected to be greater when the channels are in the Figs 7-8 configuration than when in the Figs 1-3 configuration.

It would be possible to reduce the back pressure due to flow-resistance in the Figs 7-8 configuration, by increasing the annular spacing 86 between the inner and outer

components of the die. However, it should be noted that it would be outside the invention to increase the annular spacing so much that the main flow of liquid resin was not, in substance, conditioned by the channels.

It will be appreciated from the above descriptions that, in the invention, it is inevitable that some small proportion of the flowing liquid will be able to pass through the channels region without being displaced circumferentially barely at all: the invention is concerned with conditioning the main body of the flowing liquid, rather than with what happens to insignificant elements of the flow, and the scope of the invention should be construed accordingly.

Preferably, in the invention, substantially all the flowing liquid resin enters the channels, and the channels are so configured that the point at which any given element of the liquid emerges from the channels is substantially displaced circumferentially from the point at which that element entered the channels

Claims

CLAIM 1. Extrusion die for blown plastic film, wherein: the die includes an annular extrusion nozzle; the die includes an annular feed chamber, through which passes plastic material in liquid form; the feed chamber includes regularly circumferentially spaced sources of temperature inequality, whereby the liquid emerges from the feed chamber having regularly circumferentially spaced bands of alternating relatively hot and cool temperatures; the die includes an annular gathering-chamber, in which plastic material in liquid-resin form is gathered prior to being extruded through the nozzle; the gathering-chamber is so located in the die that the total flow of liquid resin passing from the feed chamber enters the gathering-chamber, and then passes from the gathering chamber to the extrusion nozzle; the gathering chamber is provided with many passageways; the passageways are so disposed within the gathering chamber that at least a major proportion of the total flow of liquid passing through the gathering chamber passes through the passageways; the passageways have respective entry mouths, through which the flowing liquid enters the passageways, and exit mouths, through which the flowing liquid leaves the passageways; and the passageways are angled circumferentially with respect to the gathering chamber, in that the exit mouths are displaced circumferentially with respect to the entry mouths.

CLAIM 2. Die of claim 1, wherein: the passageways in the gathering chamber are arranged in first and second series of passageways; the passageways in the first series are angled

circumferentially to the left, and the passageways in the second series are angled circumferentially to the right, with respect to the gathering chamber.

CLAIM 3. Die of claim 2, wherein: the passageways of one of the first or second series are in the form of open channels cut into the outer annular wall of the gathering chamber, and the passageways of the other of the series are in the form of open channels cut into the inner annular wall of the gathering chamber; the open channels which comprise the passageways of the first series are in direct facing oppositon, across the gathering chamber, to the open channels which comprise the passageways of the second series.

CLAIM 4. Die of claim 3, wherein: the dimensions of the gathering chamber are such that a minor proportion of the total flow of liquid passing through the open channels leaks from the channels of one series into the channels of the other series.

CLAIM 5. Die of claim 2, wherein: the entry mouths of the passageways of the first series are disposed evenly around the gathering chamber, and the entry mouths of the passageways of the second series are disposed evenly around the gathering chamber; and the entry mouths are so arranged that the flows of liquid resin entering the two series are approximately equal.

CLAIM 6. Die of claim 5, wherein: the distance D by which the exit mouths of the passageways of the first series are displaced with respect to their entry mouths to the left, is equal to the distance by which the exit mouths of the passageways of the second series are displaced with respect to their entry mouths to the right.

CLAIM 7. Die of claim 6, wherein: the said displacement distance D is related to a dimension G, which is the circumferential distance between

circumferentially-adjacent ones of the said sources of heat inequality in the feed chamber, and to the number F of the said sources in such a manner that; a first sub-series of the passage-ways of the first series comprises those F passageways of the first series which lie spaced apart from each other a distance G; a second sub-series of the passageways of the second series comprises those F passageways of the second series which lie spaced apart from each other a distance G; the entry mouths of the passageways of the first sub-series lie at substantially the same circumferential location as the entry mouths of the passageways of the second sub- series ; and the exit mouths of the passageways of the first

sub-series lie substantially half-way between the exit mouths of the passageways of the second sub-series.

CLAIM 8. Die of claim 6, wherein: the said displacement distance D is related to a dimension G, which is the circumferential distance between

circumferentially-adjacent ones of the said sources in the feed chamber, by the formula: D = G * N/4, where N is an odd number.

CLAIM 9. Die of claim 8, wherein N

CLAIM 10. Die of claim 8, wherein N = 1.

CLAIM 11. Die of claim 1, wherein: the feed chamber comprises a feed-groove chamber; the sources of temperature inequality comprise respective liquid-conveying feed-grooves, which are formed in a wall of the feed-groove chamber; and the feed-grooves extend in a helical or spiral sense with respect to the feed-groove chamber.

CLAIM 12. Die of claim 3, wherein: the said open channels are so arranged, in relation to the flow of the liquid emerging from the channels, as to become progressively shallower.

CLAIM 13. Die of claim 3, wherein: the said open channels are so arranged, in relation to the flow of liquid entering into and then emerging from the channels, as to become progressively deeper and then progressively shallower.