BACKGROUND TO THE INVENTION: It is well known to form synthetic thermoplastic polymers into lengths of material by passing the polymer whilst in the soft or molten state through an orifice in a die.

Such a process is known as extrusion moulding and can be applied to a wide range of materials which are in a fluid or semi-fluid state, for example materials which undergo a physical, chemical or crystallographic change from a fluid or malleable state to a solid state. Thus, the extrusion process can be applied to thermosetting materials which undergo a chemical reaction to form a solid cured product; to materials which dry either by the evaporation of water therefrom or by the absorption of water into a different crystal or morphological form; or to molten or thermoplastic materials which solidify on cooling. The extrusion process is of especial application in the extrusion of thermoplastic polymers such as polyalkylene resins, notably polyethylenes, polypropylenes and alloys or blends thereof. For convenience, the invention will be described hereinafter in terms of the extrusion of a polymer and the term extrudate will be used herein to denote in general all materials which can exist in a viscous, malleable, fluid or semi-fluid form which can be extruded under pressure through the orifice of a die and

the term malleable will be used herein to denote the physical state of such a material as it is extruded.

Typically, the material to be extruded is fed as a particulate solid to a cylindrical tube known as the barrel of the extrusion apparatus and is fed along the barrel by a rotating screw drive or auger, a reciprocating ram or other positive transport means. If necessary, the barrel can be heated or cooled to maintain the material within the barrel at the optimum temperature for flow of material through the barrel. The material is forced through an orifice at the end of the barrel, typically an interchangeable die made from a tool steel or other wear resistant material and having an orifice whose cross sectional geometry corresponds to that of the shape which is to be produced. Typically, the cross section of the bore which forms the flow path of the extrudate downstream of the barrel progressively changes until it has the shape of the die orifice. Such change preferably occurs smoothly and the terminal portion of bore may have a cross section which is substantially uniform and corresponds to that of the orifice located at the downstream end of the bore. For convenience, the term die will be used hereinafter to denote that portion of the extrusion equipment through which the extrudate flows downstream of the barrel; the term bore of the die will be used to denote the passage within the die through which the extrudate flows; and the term die orifice will be used to denote the orifice through which the extrudate flows from the downstream end of the bore. The die can be formed as a unitary member with the bore and orifice being machined or otherwise formed in a single metal component.

Alternatively, the die can be formed in sections so that only the terminal section or sections need be replaced to enable the shape of the die orifice and hence of the product to be changed.

For convenience, the invention will be described hereinafter in terms of equipment using a screw auger to transport extrudate along a generally cylindrical barrel and through a terminal single component die. Such forms of extrusion equipment are commercially available for extruding a wide range of materials.

As the extrudate is transported along the extruder barrel and through the die bore and orifice by the positive driving action of the screw, shear forces are generated within the mass of the extrudate. These are caused not only by the working of the extrudate by the screw, but also by the frictional forces between the wall of the barrel or the die and the extrudate as it travels towards the die orifice. This friction causes a generally parabolic velocity profile to develop across the bore of the barrel and the die. The profile varies from a virtually static layer of the extrudate adjacent the wall of the barrel or die to a faster moving central core of extrudate. Where the viscosity of the extrudate is low, for example less than about 500, for example about 5 to 100, milliPascal second, these shear forces can be accommodated without significantly affecting the extrusion process. However, as the shear forces increase, their effects become more marked and can lead to localised overheating and degradation of the extrudate. The shear forces also require the use increasingly greater extrusion J. U/UJ/UU

pressures to force the extrudate along the barrel and through the die orifice. The pressure required may limit the through put of extrudate which can be achieved in practice.

Furthermore, as the extrudate exits through the die orifice into the ambient pressure environment beyond the die, it undergoes swelling so that the extruded product no longer conforms to the dimensions and shape of the die orifice. As a result, the designer of the die must compensate for this swelling to achieve a product having the desired shape and dimensions.

Furthermore, surface ripples or other imperfections are formed on the surface of the extrudate as it exits the die orifice.

In order to reduce these effects, operators can raise the temperature of the extrudate within the barrel so as to reduce its viscosity. However, when a molten or near molten material is extruded through a die orifice, it takes a finite time to solidify and can deform or lose its shape during the period before solidification occurs. It is therefore desirable that the distance between the die orifice and point at which the extrudate solidifies, known as the frost line, be as short as possible. It is often necessary to provide a cooling tunnel or other means to accelerate cooling of the extrudate and especial equipment for handling the partially solidified extrudate to preserve its required shape. All this has to be compensated for when the die is designed and the extrusion temperature and other conditions are selected.

As indicated above, these problems become more accentuated as the viscosity of the extrudate increases. In extreme cases, the extrudate cannot be extruded successfully at satisfactory extrusion rates and commercially excessively low rates have to be tolerated in order to achieve a satisfactory product.

In order to reduce the friction between the extrudate and the wall of the barrel or die, it has been proposed to coat the walls of the barrel and die with a low friction material, for example a polyfluorohydrocarbon such as PTFE, or to provide a sleeve of such a material within the barrel or die. However, many extrudates are processed at high temperatures and are often abrasive. As a result, the low friction coating or sleeve becomes warped or is readily abraded and has only a limited operating life.

Such coatings or sleeves are expensive and require frequent replacement if they are to maintain a smooth finish to the bore of the barrel or die and offer any significant processing benefit. As result, such coatings or sleeves have not been widely used in practice.

It has also been proposed to incorporate a liquid lubricant into the extrudate. Whilst this may reduce the friction between the extrudate and the wall of the barrel or die, it also reduces the friction between the extrudate and the flights of the screw used to drive the extrudate along the barrel, reducing the efficiency of transport of the extrudate along the barrel. Furthermore, the lubricant often leaves a surface residue upon the extruded product, which must be removed. In some cases, the

lubricant can cause surface blemishes on the extruded product. In order to reduce the slip between the extrudate and the screw, it has been proposed to inject the lubricant into the die. However, this accentuates the problems of production of a product with surface blemishes and residues of the lubricant. The use of a liquid lubricant injected into the extrudate is not claimed to achieve any benefit other than reducing the pressure required to force the extrudate through the die orifice.

In order to reduce the frictional forces between the extrudate and the die walls, it has been proposed, for example in Russian Patent No 1717398A1 and Japanese Patent Publication No 06-270233, to introduce a gas between the material being extruded and the wall of the die by injecting gas through a micro-porous wall of the die. The presence of such a gas is claimed to reduce the friction between the extrudate and the wall of the die, enabling extrusion to be achieved with a lower extrusion pressure.

In these proposals, the material to be extruded has been a very viscous material which is virtually solid in the die.

We believe that the material has been sufficiently stiff for it to bridge the gap between adjacent gas bubbles on the die wall and that the discrete gas bubbles therefore lift the material as an intermittently supported solid away from the wall of the die. However, we have found that where the material to be extruded has a lower viscosity, for example less than about 20,000 Pascal seconds, the extruded product suffers from excessive and erratic surface blemishes. Such a proposal cannot

therefore be used to process the commonly used polyalkylene polymers.

We believe that malleable extrudates having such lower viscosities, notably a molten polymer, readily adhere to any exposed surfaces of the die wall and that gas bubbles are absorbed into the extrudate. In the case of the use of a porous die wall, the gas forms a series of discrete gas bubbles on the surface of the wall and we believe that the extrudate adheres to the remaining exposed surfaces of the die wall. As a result, erratic and varying blemishes are formed on the surface of the extrudate where it adheres to the die wall.

Surprisingly, we have found that if the gas is introduced as a stream which extends substantially continuously both circumferentially and axially over the surface of the wall of the bore of the die, preferably at a single circumferential location in the wall of the bore of the, this gas stream does not form discrete separate bubbles of gas as would have been expected from the results from the use of porous walled barrels as in the Russian and Japanese proposals. The gas stream can be caused to form a surprisingly stable film between the extrudate and the wall of the die. This film acts as a lubricant to reduce the formation of the stationary layer of extrudate against the wall of the barrel or die, and thus assists formation of a full slip boundary condition between the die and the extrudate. This assists relative movement between the material and the die, permitting the use of a lower extrusion pressure to achieve the same flow rate, or to achieve a higher flow rate at the same extrusion pressure.

Surprisingly, the gas film also assists cooling of the

extrudate within the die so that the extruded product solidifies rapidly after it exits the die orifice, that is the distance between the die orifice and the frost line is reduced, often sufficiently to avoid the need to use a cooling tunnel or other means for accelerating the solidification of the extrudate. We have found that the presence of the gas film also reduces the stresses within the extrudate and aids retention of shape within the extruded product. The presence of the gas film thus results in a product which has an improved profile quality and the invention can achieve greater precision moulding than extrusion processes which do not use gas injection.

Surprisingly, we have also found that the presence of the gas film reduces the extent by which the extruded material swells after it exits the die orifice and that the extruded product retains its shape better after exiting the die orifice than where extrusion is carried out in the absence of the gas film.

The presence of the gas film thus simplifies or obviates the compensations which the die designer and the process operator would normally have to make to ensure that the extruded product had the required form as well as producing a product with an improved surface finish.

SUMMARY OF THE INVENTION: Accordingly, the present invention provides a method for extruding a material through the orifice of a die, which method comprises forming and maintaining a substantially continuous film of gas as an interface between the

material being extruded and at least part of the axial length of the wall of the bore of the die; and extruding the material through the die orifice.

Preferably, the material being extruded is maintained above, at or near its melting point and is a thermoplastic material, notably a polymer. Preferably, the gas film is from 5 to 100 micrometres thick and flows axially along the die wall as an interface between the extrudate and the wall of the die to exit with the extrudate through the die orifice. Preferably, the gas film is formed by introducing gas into the bore of the die through one or more gas injection ports or apertures extending substantially circumferentially around the internal wall of the bore of the die and at substantially the same axial location along the axis of the die so that the gas film forms an interface between substantially the whole of the wall of the bore of the die and the extrudate downstream of the gas injection aperture (s).

The invention also provides an extrusion die which comprises means for forming a film of gas upon the internal wall of the bore of die, which means extends substantially continuously circumferentially within the bore of the die; and also an extrusion machine provided with such a die. Preferably, the means comprises a gas injection port or aperture, for example a circumferential slot, by which the gas is introduced as a continuous film between the extrudate and the wall of the bore of the die.

Preferably, the gas film is formed downstream of a circumferentially extending gas inlet port; and the flow path of the bore of the die at or immediately downstream

of the gas injection port is greater than that immediately upstream of the port. Such an increase in the bore flow path assists formation of the gas film and the flow of the gas stream as a coherent layer between the wall of the bore of the die and the extrudate. It is particularly preferred to form the gas injection port as a circumferential slit, notably one which is directed radially with respect to the flow of extrudate through the bore, in the wall of the bore which has a width of from 5 to 250 micrometres; and that the radial dimensions of the bore of the die at or immediately downstream of the gas injection port be from 5 to 250 micrometres greater than those immediately upstream of the port. As is known in the extrusion art, the bore within the die may have a complex configuration and may converge or diverge towards the die orifice. The increase in the flow path dimensions at or adjacent the gas injection port may thus be localised and be subsumed into other variations in the shape or the flow path further downstream towards the die orifice.

The gas injection port preferably extends substantially continuously over the circumference of the wall of the bore of the die. However, the port may be interrupted where the gas issuing from adjacent portions of the port can bridge the interruption so as to form a continuous film of gas immediately downstream of the port. Thus, the port can extend along both long sides of the rectangular cross section portion of a die having a slit orifice, but need not extend along the short sides where these are less than about 0.1 cms or less than about 2.5% of the total periphery or circumference of the gas port.

The invention can be applied to dies having a wide variety of cross sectional shapes to the die orifice and the internal bore of the die upstream of the orifice. For example the die orifice can have a polygonal, squared, rectangular, triangular, cruciform, star shaped, oval, elliptical or true circular cross sections and the bore of the die can taper or otherwise change along its length from a circular cross section at the upstream end adjacent the barrel to the approximate or actual shape and dimensions of the die orifice. The bore of the die may include a central spigot or torpedo which partially occludes the bore so as to form tubular or other hollow products. Such a torpedo can also be provided with a gas injection port terminally or part way along its length so as to reduce friction between the extrudate and the surface of the torpedo and to reduce the formation of blemishes within the bore of such hollow products. Since the presence of the gas film in the bore of the die reduces swelling and distortion of the extruded product, the invention enables complex shapes to be extruded with greater accuracy that where no gas is used and thus enables precision moulding of complex profiles to be achieved.

For convenience, the invention will be described hereinafter in terms of a die having a substantially circular cross section bore and die orifice, and having a continuous circumferential gas injection port.

The gas port can be directed radially or axially and is preferably located at the annular shoulder in the internal

face of the wall of the bore of the die where the change in radial dimension of the bore occurs.

As stated above, the gas injection port extends substantially over the whole circumference of the wall of the bore of the die. If desired, the injection port can follow a spiral path, so that the gas film is formed progressively over the surface of the wall or the bore.

However, it is preferred that the gas injection port be formed upon a true circular circumference of the wall of the bore so that the gas film is formed simultaneously around substantially the whole of the circumference of the bore and at the same axial position within the bore of the die. However, if desired, the gas may be injected through a plurality of axially separated circumferential ports.

However, as stated above, it is desirable to ensure that such injection ports form a substantially continuous gas film between the wall of the bore of the die and the extrudate. Therefore, it is preferred that any axial separation or circumferential interruptions between such ports or portions thereof should not be so great that the gas stream from the ports cannot span such separation of interruptions to form the continuous gas film. Typically, such separations of interruptions should not individually exceed about 1 mm, preferably less than 0.5 mms, notably less than 0.25 mms or total more than about 5%, preferably 2.5% or less, notably less than 1.5%, of the total circumference or periphery of the port.

If desired, the port (s) can have flared orifice (s) to aid spread of the gas over the surface of the wall of the bore of the die.

The gas injection port can be located at any suitable point axially within the bore of the die upstream of the die orifice. However, it is preferred that the gas port be located approximately half way along the axial length of the bore of the die so that the gas film can provide its benefits over a substantial proportion of the flow of the extrudate through the die. The optimum axial location of the gas port can readily be determined by simple trial and error tests, but we have found that the optimum position for the port will usually be within from 15 to 75, preferably about 40 to 60%, of the length of the bore upstream of the die orifice. Where the bore has a terminal portion immediately upstream of the die orifice which is a substantially parallel walled portion terminating in the die orifice, it preferred to locate the gas injection port within the length of this terminal portion.

If desired, the barrel of the extrusion machine can also be provided with one or more such gas injection ports to form a lubricant gas film within the barrel upstream of the die. However, we have found that the provision of the gas film solely within the bore of the die enables satisfactory results to be achieved in most cases. For convenience, the invention will be described hereinafter in terms of the formation of a gas film within the bore of the die using gas injected via a circumferential port

located at a single axial location within the length of the bore of the die.

The gas can be selected from a wide range of gases which do not deleteriously affect the material being extruded.

Suitable gases for present use thus include the inert gases, notably nitrogen, air, carbon dioxide or mixtures thereof. The suitability of a gas for present use can readily be determined by simple trial and error tests.

The gas is fed to the gas port from any suitable source, for example a conventional gas storage cylinder, via a suitable flow control mechanism. We have found that it is necessary to regulate the flow of gas so as to avoid excessive flow of gas, which is wasteful and can lead to localised surface blemishes. On the other hand, insufficient gas flow can lead to break down of the continuous gas layer between the extrudate and the wall of the bore of the die. The optimum gas flow rate will vary from material to material, with the extrusion pressure and the extrusion through put required and can readily be determined by simple trial and error results. Variation of the gas pressure and flow rate within the acceptable limits established by such tests can also be used as described below to optimise the cooling of the extrudate and other benefits of the presence of the gas film.

It is also necessary to feed the gas to the port at a pressure which is not sufficient to cause localised lifting of the gas stream away from the surface of the bore wall. This is to be contrasted with the prior proposals using a porous die wall where the gas was

required to lift the material being extruded away from the wall of the bore as a coherent mass. Significant over pressures could be accepted in those cases because significant collapse of the material between gas bubbles would not occur due to the semi-rigid nature of the material being extruded. This is to be contrasted with the materials for present use, which are malleable or viscous materials. We prefer to apply the gas to the injection port at a pressure which is less than about 105, notably about 5%, above the hydrostatic pressure in the extrudate in the die bore at that point. Typically this will result in the feeding of the gas at a pressure of less than 2 bar, for example less than 1 bar, notably about 0.5 bar, above the hydrostatic pressure in the extrudate at that point of the die. The optimum pressure can readily be determined by simple trial and errors tests.

As indicated above, the gas film flows with the material being extruded and acts in part to remove heat from the material. To assist optimum heat removal, as evidenced by rapid solidification of the extruded product as or immediately after it exits the die orifice, it may be desirable to increase the rate of supply of gas to the gas inlet port above the minimum required to form and maintain the stable film of gas between the material being extruded and the bore wall. Typically such excess will be in the range of from 2 to 10 times the minimum, and the maximum flow rate for the gas stream is determined by economic considerations and the onset of the formation of surface defects or blemishes in the extruded product. The optimum gas rate to achieve adequate cooling of the extruded

product can readily be determined by simple trial and error tests.

As stated above, the gas film is preferably at least 5 micrometres thick, so as to supply an adequate slip film between the material being extruded and the wall of the bore of the die. Such a thickness of the gas film is achieved by a combination of the gas flow rate and gas injection pressure as well as the design of the bore of the die. As will be appreciated, the thickness of the gas film may vary downstream of the gas injection port due to variations in the geometry of the bore and absorption of some of the gas film by the extrudate and the thickness quoted above is for the gas film immediately downstream of the gas injection port. Typically, the gas film thickness is the minimum required to achieve adequate through put of the extrudate and the other benefits set out above by maintaining the gas film interface between the extrudate and the wall of the bore substantially completely from the gas injection port to the die orifice. However, it may be necessary to form a thicker film of gas than the minimum required in order to achieve one or more of the benefits of the invention to a satisfactory extent.

The thickness of the gas film will also depend upon the dimensions of the gas injection port. In general, the larger the port and/or the larger the increase in radial dimension of the bore of the die immediately downstream of the port as described below, the larger the thickness of gas film achieved. The optimum combination of port size, gas flow rate and gas pressure can be varied to achieve the desired benefits using simple trial and error tests.

The thickness of the gas film is also dependent upon the increase in the flow path within the bore of the die, usually an increase in the diameter of the die bore and/or a reduction in the diameter of any torpedo, at or downstream of the gas injection port. Typically, the gas injection port is formed as a circumferential port at the axial lip of an annular shoulder in the wall of the die or the torpedo and directs the gas axially and/or radially into the bore of the die so that the change in bore dimensions occurs immediately upstream or downstream of the port. If desired, the orifice of the gas injection port can be provided with a shaped or belled lip to guide the gas stream along the downstream wall of the bore of the die or the torpedo. Typically, the increase in the flow path of the bore is achieved as a step change in the diameter of the bore or torpedo. However, the increase may be achieved by a suitable taper. Typically, the increase in the flow path corresponds approximately to the desired thickness of the gas film and will be from 5 to 250 micrometres. However, this need not be the case, notably where a substantial excess flow of gas is required, for example to achieve the desired extent of cooling of the extrudate. Preferably, the radial increase in the dimensions of the flow path is from 50 to 125 micrometres and corresponds to the width of the slot of the gas injection port.

The change in dimension of the bore of the die quoted herein are those immediately adjacent to the gas injection port since the geometry of the bore of the die may be complex and the flow path may taper or otherwise converge

or diverge downstream of the gas injection port to the shape and dimensions of the die orifice. However, it is preferred that the change in dimensions of the bore and/or the torpedo extend downstream of the gas injection port substantially uniformly to the die orifice so as to maintain a gas film of substantially uniform radial thickness between the extrudate and the wall of the bore.

The gas is typically fed to the gas injection port at ambient temperature. However, since the gas will flow through at least part of the wall of the die, it will in general pick up heat from the die wall and be injected into the bore of the die at an elevated temperature.

Surprisingly, we have found the despite such heating of the gas feed, adequate cooling of the extrudate can be achieved to reduce swelling and shape change of the extruded product to an acceptable level and that cooling of the gas is not usually necessary. However, if desired, the gas feed can be cooled using any suitable technique.

Apart from the provision of the gas injection port and the optional increase in the radial dimensions of the flow path of the die bore immediately adjacent the gas injection port, the die design, construction and operation may of conventional form. However, as stated above, the invention enables the same through put of extrudate to be achieved at a lower pressure or a higher through put to be achieved at the same extrusion pressure; reduces the formation of surface blemishes; and may reduce the need to carry out cooling and other treatment of the extruded material downstream of the die orifice to reduce swell and shape change. These benefits simplify the operation of

the extrusion process using conventional extrusion equipment and operating techniques. However, we have found that it will usually be desired to commence injection of the gas stream before material to be extruded is fed to the die bore so as to minimise the risk of the extrudate adhering to the wall of the die before the gas film can be established.

The invention has been described above in terms of the formation of a gas film over substantially all of the wall of the die bore downstream of the gas injection port.

However, where it is acceptable to produce a product which has surface blemishes on some exposed faces thereof and/or it is acceptable to achieve only part of the potential benefits offered by the invention, it may be appropriate to form the gas film on only part of the circumference of the die, for example one face of a slit to produce a film of extruded material which has one smooth face and the other face with blemishes.

As indicated above, the invention can be applied to a wide range of materials to be extruded and its use is not limited to the extrusion of polyalkylene or other polymers. Thus, the invention can be applied to the extrusion of pasta, chocolate or other foodstuffs where the benefits of reduced extrusion pressure and the lack of surface contamination of the extruded product are of value.

The invention can be applied to existing extrusion equipment by modification of the existing die or by the use of a replacement die to provide the gas injection

port. However, such modification will usually not require other modification to the equipment and can readily be achieved at low cost to existing or new equipment.

DESCRIPTION OF THE DRAWINGS: To aid understanding of the invention, a preferred embodiment thereof will now be described by way of illustration with respect to the accompanying drawing which is a diagrammatic axial cross section through a die of the invention attached to a conventional screw type extrusion apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT: The extrusion equipment comprises an extrusion barrel 1 within which is rotatably journalled a screw auger 2 driven by a motor and gear box 3. Solid particles of that extrusion grade high density polyethylene sold under the Trade Mark Rigidex are fed to the upstream end of the barrel 1 from a hopper 4 and are transported along the barrel towards a die 5 at the distal end of the barrel.

The barrel 1 is maintained at a temperature of from 160°C at the feed end of the screw to 180°C at the die by means of external electrical heating coils (not shown). The die 5 has an internal bore 6 which tapers from a circular cross section portion 6a to a parallel walled rectangular cross section portion 6b of dimensions corresponding to the slot die orifice 7 through which a ribbon of polyethylene product is to be extruded. Such equipment is of conventional design, construction and operation.

The die is provided with an internal annular shoulder 10 at which the diameter of bore 6 increases by 200 micrometres. At the downstream edge of the shoulder, the radial wall of bore 6 is provided with an annular circumferential slot 11 which extends axially downstream of the shoulder 10 for 100 micrometres. A series of radial bores 12 extend from the slot 11 to a circumferential manifold 13 connected to a supply 14 of nitrogen gas via flow and pressure regulators 15 and 16.

The shoulder 10 and slot 11 are located approximately midway within the length of the substantially parallel walled portion 6b of the bore closely upstream of the die orifice as shown solidly in Figure 1. However, the shoulder and slot could be located within the upstream tapering portion 6a of the die bore as shown dotted in Figure 1.

The flow of nitrogen gas to slot 11 is initiated at a flow rate of about 0.21 g/sec at a pressure of about 5% above the hydrostatic pressure in the extrudate at that point in the bore of the die, and the screw 2 is rotated at 1 rpm to feed the polyethylene to the die orifice 7. With this flow of gas a smooth surfaced ribbon of polyethylene was extruded through the die. Little or no swelling of the extruded product was detected and the frost line at which the product solidified was within 3.5 cms of the die orifice.

By way of comparison, when no gas was injected through slot 11, the product showed a swell ratio of 1.38 and the frost line was 24 cms from the die orifice.

The rate of gas flow was reduced to 0.52 g/sec and the speed of rotation of the screw 2 reduced to 0.1 rpm. The extruded product demonstrated virtually no swelling and solidified within 1 cm of the die orifice when gas was injected as before through slot 11.

In further tests, the gas flow rate was varied between 0.5 and 3 g/sec at screw speeds of from 0.1 to 1 rpm and gave consistently good surface finished product with little or no discernible swelling (swell ratios between 1.03 and 1.13) and which solidified within 3.5 cms of the die orifice. By way of further comparison, when no gas was injected at slot 11 under similar operating conditions, the extruded products had poor surface finish, demonstrated swell ratios of from 1.33 to 1.38 and solidified at from 4 to 24 cms from the die orifice.

Inspection of the extrudates through clear viewing panels in the die wall using birefringence techniques demonstrated that the stresses within the extrudates were substantially completely relaxed when gas was injected through slot 11, whereas high stresses were observed in all samples where no gas was injected.

In a further comparison, the wall of the die was replaced with a porous ceramic frit and gas was injected through the pores in the wall to form a plurality of localised gas sources on the surface of the wall of the die bore. The extruded product demonstrated poor surface finish and required a higher extrusion pressure than where the gas was injected though slot 11 to form and maintain a coherent gas film over the wall of the die bore.

Further tests were carried out using the circular cross section bore and orifice die shown in Figure 2 in place of the tapering die 5 of Figure 1. The die had bore and gas inlet dimensions in mms as follows: Axial length of bore 55, diameter of die orifice 9.6, diameter of inlet to die 9.5.

Gas injection port located at downstream edge of 0.5 deep radial step increase in bore diameter formed 7.5 from exit of die orifice, the gas injection port had an axial width of 0.1 and extended for the full circumference of the bore.

Rigidex (Trade Mark) extrusion grade high density polyethylene (melt flow index 0.2g/lOmin and a density of 954 kg/m3) was fed using a variable speed gear pump to the barrel of a commercial extruder having a screw diameter of 18 mm, and L/D ratio of 20 and a compression ratio of 2: 1.

The barrel was heated using band heaters to give a temperature profile of 160,160,175 and 180°C. The extrudate was fed from the outlet of the extruder barrel into a melt flow cell which was fitted with the die orifice described above. The extrusion pressure was measured using a transducer installed in the flow cell.

The melt flow rate through the extruder was controlled by varying the gear pump rotation speed from 0.1 to 1.2 rpm.

Nitrogen gas was fed to the gas injection port in the die at a pressure of about 0.5 bar over the pressure in the die as measured in the flow cell and flowed out of the die

orifice with the extrudate and formed a gas film interface between the extrudate and the wall of the bore in the die.

The extruder was operated over a range of pump speeds and with and without gas injection. Using a single light source at 546nm, birefringence observations were carried out through glass windows in the wall of the die to determine the stress patterns within the extrudate. The properties of the extruded product were observed and the distance of the frost line from the die orifice and the swelling of the extruded product were measured. The results of these tests are set out in the accompanying charts in which Chart 1 demonstrates that there was a significant drop in extrusion pressure when gas was injected into the bore of the die. Chart 1 also demonstrates numerical model calculations for when gas was and was not injected, which closely follow the experimental results. Chart 1 also shows the extrusion pressures achieved when the die was omitted from the melt cell and demonstrates that the gas film in the bore of the die is achieving virtually full slip boundary conditions within bore.

Chart 2 demonstrates the cooling effect that the gas has on the extrudate and how the frost line, as determined by the point at which the extrudate loses its transparency, is closer to the die orifice with gas injection than when no gas injection is used.

Chart 3 demonstrates that die swell of the extrudate as measured in the extruded product is significantly reduced by the injection of gas into the bore of the die. The die

swell is quoted in Chart 3 as the ratio of the diameter of the extruded product to the diameter of die orifice and a value of 10 on the chart corresponds to no swelling.

The visual examination of the birefringence tests also demonstrated that the stresses within the extrudate had virtually completed relaxed when the extrudate passed through the die orifice. By way of comparison, stresses were still present when no gas was injected into the bore.