GB2354965A - Extrusion sizing/calibration dies with gas pressure control - Google Patents

Abstract

In an extrusion sizing on calibration die 5 and method of extrusion gas is supplied in a controlled manner to produce a layer 9 between the extrudate 10 and the die surfaces to facilitate the extrusion process. The gas pressure is monitored and controlled at both the entrance 6 to and exit 8 from the die to produce a back pressure to maintain a gas pressure in the die space between the die surface and the extrudate. Gas may flow upstream or downstream through the die, and may be recovered for recycling. In one embodiment (figure 7.1 - not shown) the gas is infected at a substantially central portion so that it flows both upstream and downstream. The gas provides a lubricating effect reducing adhesion of the extrudate to the die and the force opposing its passage through the die.

Description

2354965 IMPROVEMENTS RELATING TO EXTRUSION SIZING AND CALIBRATION DIES The

present invention relates to an extrusion sizing and calibration die and to a method of extrusion using such a die.

Extrusion of fluid, semi-fluid, and even solid materials through shape-defining dies to form continuous lengths of consistent crosssection shapes or profiles is well known and practised in many different forms to manufacture a very wide range of products using well established techniques, methods and apparatus.

The production costs of extruded products is influenced by many factors, but specifically the rates or speeds of material extruded from the dies is directly proportional to the cost per unit length. Hence it is desirable to increase the speed of extrudate movement, thereby reducing the unit cost of production.

There is a continuing need to maintain or improve the quality of extruded products. This may include improving the surface finish, dimensional accuracy with reduced tolerances, and maintaining physical properties of the extruded material. In many products a good visual appearance is important. In some cases post-extrusion finishing such as printing requires a smooth surface free of intermittent blemishes or continuous lines.

-2 This invention aims to both reduce costs of production and to improve product quality. Whilst the basic principles of the invention may be applied to virtually any extruded product or material, initial applications will be for thermoplastic materials, and therefore this patent is based on descriptions, methods and apparatus principally applied to plastic materials.

They may equally well apply to other materials such as aluminium, zinc alloys, coppers, ferrous materials, glass, ceramics, rubber, and to the food and chemical industries.

There is often a tendency for the material to swell after emerging from the forming dies, and/or to change shape and deviate from the required shape or dimensions referred to as 'die swell'. It then becomes necessary to 'recalibrate' or 'resize' the extrudate into the final shape and dimensions required of the product. It is common practice to pass the extrudate through one or more sizing dies positioned downstream of the first forming die. It consequently becomes necessary to pull or haul off the extrudate in order to pull it through the sizing dies which normally reduce the section size by compressing and, in some cases, stretching or elongating the material, when in a semi-molten to solid state.

There is consequent frictional resistance between the extrudate and the sizing die surfaces. The haul off force required may be substantially increased as a result of the reactive force resisting the flow of extrudate through the dies.

In some cases the changing of shapes can amount to cold-forming the material when in a near rigid or semi-flexible form, particularly when the extrudate and the sizing dies are in a water or other cooling bath to hasten the rate of cooling and solidification of the material.

It is therefore well known that the secondary process of sizing or calibrating extrudates requires haul off under a continuous tensile force which therefore has a power and energy usage which can be a significant cost of production.

In some cases there is difficulty in gripping the extruclate by haul off means, which may be based on a series of opposing pulley wheels or caterpillar belts. These may create indentation or other surface marks in the extrudate, or even distort the extruded form.

It is therefore apparent that there could be substantial practical processing and economic benefits if the frictional force opposing the passage of the extrudate through the sizing and calibration dies can be reduced.

There have been attempts to reduce the opposing frictional force in the first line extruder forming dies. These have been based on reducing the co-efficient of friction between the extrudate material and the dies by:

-4 premixing a slip additive or lubricant to the material so that the surface of the extrudate is more slippery. This can have a detrimental effect on the processing of the material in the extruder when the slip factor adversely affects the interaction between the material and the extrusion screw, or reduces physical properties of the finished product.

attempts have also been made to introduce a liquid lubricant at the surface of the extrudate in or upstream of the dies. A further version of this has been to inject gas through porous material, forming the whole of or part of the extrusion dies. This has proved to be difficult to control, and there is a tendency for the porous material to vary in porosity, thereby varying the flow of gas, and/or to clog up with plastic at its surface, thereby varying and restricting the passage of gas. It therefore appears not to have been developed into well established practical processing.

Attempts have been made to reduce the friction of interaction between the extrudate and the dies by coating the die surface to produce a surface with reduced co-efficient of friction. For example coating with PTFE is sometimes used for this purpose, but this has a limited life before recoating is necessary.

If the haul off force required to pull the material through the die is too S -5 great, unacceptable elongation of the material can result, thereby again adversely affecting the quality of the extruded product, and in some cases further changing the overall cross-sectional dimensions or area of the product.

Product applications which are considered likely to benefit from the use of the invention are not confined to, but may include, solid rod sections of different outside configurations, large and small dimension tubular and pipe-like products, profiles including building and furniture products, automotive trim - internal and external, and other vehicular applications, foamed or cellular cored sections where there is a greater tendency to expand post-extrusion, sections used in structural applications, mechanical handling and material storage systems.

The objective of this invention is primarily to reduce the reactive friction between the extrudate and the downstream sizing and calibration dies in order that:

the force required to pull or haul off the material through the dies is reduced.

the surface finish of the extrudate is improved as a result of avoiding surface pick-up marks caused by local catching or temporary adhesion of the material to the die surface at small or localised areas.

-6 the effect of die swell is reduced, i.e. the expansion of material section as it emerges from the downstream end of the dies is reduced.

the tendency for elongation or stretching of the extrudate as a result of excessive force required to pull the material through the sizing dies is reduced.

the energy/power consumption required to maintain a significant haul off force in the extrudate is reduced.

economic advantages can include increased production rates resulting from increased speed of passage of the extrudate through the dies as a result of additional cooling effect of the gas on the surface finish, more efficient sizing or calibration of the extrudate, and enabling passage through shorter lengths of sizing dies because it is possible to increase the angle of compression resulting from the reduced frictional reaction force According to one aspect of the invention, there is provided an extrusion calibration or sizing member comprising means for supplying gas to the member and means for controlling the pressure of the gas whereby to produce in operation a layer of gas between the surface of the member and extrudate passing therethrough.

According to another aspect of the invention there is provided a method of extrusion including thesteps of passing an extrudate through a calibration or sizing member, supplying gas to the die and controlling the pressure of the gas in order to produce in operation a layer of gas between the surface of the member and extrudate passing therethrough.

This invention is therefore based on the creation of one or more layers or films of gas between the extrudate and the sizing or calibration die surfaces in a controlled manner, so that the physical contact force between the extrudate and the die surface is reduced and the frictional reaction force between the moving extrudate and the die is reduced. The gas may form a static film In relation to the dies but Is more likely to flow across the surface of the dies and the extrudate. The gas may flow downstream in the some direction as the extrudate, or may flow upstream against the flow of extrudate, depending on the direction of lowest pressure between the extrudate and the dies. The gas will flow from a high pressure to a low pressure area.

In every case it is necessary to control the pressure of the gas feed at the entrance to the die space, and in many cases it is preferable to provide means to control the pressure at its exit from the dies. In the first case the gas may flow to atmosphere from the die exits. Alternatively the gas may be directed to an outlet gas circuit so that a back pressure can be created to restrain the exit flow of gas and, if required, to create a back pressure in the gas within the dies. Controlling the outflow of gas also enables the loss of gas to be reduced and to ensure the gas layer or film is maintained up to or near to the extrudate exit from the dies. If the gas is directed to an outlet circuit it will be possible to recover the gas for recycling.

It is important to introduce the gas at a uniform flow pressure around the periphery of the extrudate, and therefore preferably through a continuous gap between two sections of the die. The gas must be prevented from leaking to atmosphere in order that the gas pressure can be controlled at normally between 10 and 200 Bar, depending on the material the die configuration, and the force required to compress or move the material to enable the gas to be forced between the extrudate and the die surfaces.

The gas is therefore normally introduced from a peripheral gas channel surrounding the die space which is in turn surrounded by a mechanical seal between two die sections. The gas channel is usually in communication with the die space through a small, sometimes adjustable, gap between the die sections (normally between 0.5 and 0.02mm). This space should be small enough to resist the ingress of molten plastic but large enough to allow the unrestricted flow of gas.

It has been found preferable to assist the introduction of the gas and its uniform distribution to the extrudate surface by including a temporary reduction of pressure between the extrudate and the die surface or where the extrudate temporarily leaves contact with the die surfaces. In order to achieve this the downstream die section is relieved i.e. the die section aperture is increased in area for a short length, normally between 1 and 5mm and with a relieved thickness of between 0.5 and 3mm. Other shapes of die surface at the point of gas injection or gas return may be advantageous, including semi-circular grooves, rectangular sections, or, in some cases, no change in the die surface is necessary.

Frequently the gas is allowed to exit from the die6 and flow to atmosphere, but it is sometimes desirable to include a second relieved section downstream of the gas flow, to provide a means of collecting the gas so that the gas outlet can be controlled, and in some cases so that a back pressure can be applied, when restricting the outflow of the gas. Also the gas may be captured and recovered for recycling or may be ducted safely to the atmosphere outside a production area.

In some cases a low pressure flexible seal can be used at an upstream and/or downstream position relative to the dies so that the space between the die entry and/or its exit can be used to collect and encapsulate the gas, which has leaked from the die, for recovery or for ducting to atmosphere.

_10 The preferred gas is nitrogen due to it being chemically inert and therefore does not oxidise or react with the extrudate, it does not support combustion, and it is non-toxic and therefore safe unless in excessive quantities to the exclusion of oxygen in a local atmosphere.

As previously stated it is necessary to accurately control the pressure of the feed of the gas to the die. Adjustable pressure relieving valves are preferably provided so that the gas pressure is relieved down from a high pressure gas source. Advantageously, the pressure is monitored by the use of pressure gauges and/or pressure transducer means. The latter will also enable continuous control of pressure through a variable pressure regulating valve, which in turn may be controlled mechanically, electrically or through a separate compressed air feed from an ele ctro -converter used to convert electronic signals from a pressure transducer via a computer and/or PLC (programmed logic control), which in turn are fed to a pressure reducing valve as indicated. The use of computer or microprocessor continuous fast reacting closed loop control means is now established as normal practice in the plastics industry for numerous applications, but for this invention is used to control the pressure, and in some cases the volumetric flow of gas between the die surfaces and the extrudate.

In some cases it is found desirable to include pressure transducer rneans for monitoring the pressures inside the sizing or calibration dies so that the pressure exerted by the gas and/or the extrudate can be monitored and regulated. In some cases it may be advantageous to use this method of measuring pressure as a basis for controlling the gas feed pressure and exit back pressures when the transducer is positioned at a selected position or at a number of positions along the length of the dies and at the surface of the dies.

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 illustrates a conventional extruder apparatus, Figure 2 illustrates an extruder apparatus with a downstream calibrating/sizing die according to the invention, Figure 3 illustrates calibration/sizing die of figure 2 in greater detail, Figure 4 shows sectional view along the line 4-4 in Figure 3 Figure 5 shows sectional view along the line 5-5 in Figure 3 Figure 6 illustrates a modified calibration/sizing die with gas entry and gas exit.

Figure 7.1 shows enlarged view of section (A) in Figure 6 and 8, Figure 7.2 shows an alternative gas flow in the die of figure 6.

Figure 8 shows alternative gas flow in the die of figure 6, Figure 9 illustrates a gas control circuit diagram, Figure 10 shows an advanced control circuit diagram of means for controlling gas at entry and exit of the dies described in figures 2 to 9, Figure 11 illustrates the use of an internal sizing core, Figure 12 illustrates an internal calibration/sizing core in combination with a calibration /sizing die, and Figure 13 illustrates a part in detail of the combination of figure 12.

Referring to figure 1, the conventional extrusion apparatus diagrammatically illustrated in figure 1 comprises an extruder 1, an extruder forming die 2, a calibration or sizing die 3 downstream of die 2 and a haul off means 4 to pull the extrude through the sizing/calibration die. The characteristic die swell in the extrudate downstream of the extruder forming die and prior to passing through the calibration or sizing die is illustrated at 2a. Normally the process will include a means of cooling the extrudate after emerging from the forming dies and whilst passing through the calibration/sizing dies. This may be based on air or water cooling systerns.

Referring to figure 2, the conventional calibration or sizing die 2 of figure 1 is replaced by a calibration or sizing die 5 of the invention. This die comprises means of reducing the friction or resistance force to the extrudate as it passes through the calibration or sizing dies, thereby reducing the haul off or pull through force required from the haul off means.

1 4 The means for reducing friction comprises a gas inlet 5a and a gas outlet 5b for supplying gas to and exhausting gas from the die 5.

The frictional force between the extrudate and the surface of the ca lib ration /sizing die 5, is reduced by injecting gas into the body of the calibration dies in such a manner so that the gas forms a film or layer of gas 9, as shown in Figure 3, 4 and 5 between an extrudate 10 and the inner surface(s) of the calibration die 5, thereby reducing the contact force between the extrudate 10 and the contact areas of the die surfaces. This achieves a lubricating effect, enabling the extrudate 10 to pass through the calibration die 5 more easily, with less resisting force and with less tendency for the extrudate to adhere to the die surfaces. Such local adhesion of the extrudate to the surfaces may cause surface pick up or other marks on the finished product. The gas enters the die near the extrudate entry point 6 and exhausts therefrom to atmosphere at exit 8.

The gas may be injected through the die at one or more positions, as shown in more detail in Figures 6,7 and 8, within the die 5 surrounding the extrudate 10, and preferably forms a continuous gas 3 completely surrounding the extrudate so that equalised gas pressure is in connection with and fed to the extrudate total surface as it passes through the gas entry position.

The gas may be injected at one or more positions throughout the length of the calibration die with the objective of extending and maintaining the film of gas between the extrudate and the die surface and over as long an area as possible. The gas will tend to flow from a high pressure position to a lower pressure position. It will therefore normally be beneficial to inject the gas at an upstream position 6.1 as shown in Figure 3, so that the gas will tend to flow downstream in the same direction as the extrudate. In some cases it may be advantageous for the gas to be injected at a more central position 6.2 as shown in Figure 7.2 so that the gas will flow both up and downstream. In other cases it may be preferred to inject the gas downstream at 13 so that it flows upstream contra to the extrudate flow as shown at 15 in Figure 8, the gas exiting at 14.

The gas may be injected at a position or from the calibration dies where the pressure at the surface of the extrudate is temporarily relieved by enlarging over a short distance the die internal cross-sectional area. This may be achieved by including a peripheral groove or indent 12 in the die surface surrounding the extrudate. Preferably the enlarged section or groove may be formed in one or both positions of two adjoining surfaces of two die members, as illustrated in Figures 3, 4, 5, 6, 7.1 and 8.

The groove or relieved section 12 communicates with a gas supply channel 8, also within the die or between two die members as shown in Figures 6, 7.1 and 8. In some cases it is beneficial to form a continuous gas channel surrounding the die space, and for this to be in connection with the gas entry by a thin connecting channel 3 through which gas may pass without restriction but into which molten plastic will not flow. Typically the thickness of the gas gaps is between 0.5 and 0.02mm.

Referring particularly -to Figure 7.1, in order to prevent the gas from leaking from the surrounding gas channel 8 to atmosphere, it is preferable to completely surround the gas channel with a mechanical '0' ring seal 7, which is also positioned between the adjoining die members..

In one format the invention assumes the outflow of gas to atmosphere at the downflow position where the extrudate exits from the calibration die see 8 in Figure 3.

It may be advantageous to control the exit gas pressure in order to create back pressure to assist and maintain the film of gas over as long and extensive an area of the extrudate as possible. This will therefore require a means of collecting and channelling the gas from the dies to a pressure and/or flow control means. The collection of gas may be more effective if a relieved section 12.1 or peripheral groove similar to the means of injecting the gas is used as illustrated in figure 6.

Further it may be beneficial to duct the exhausted gas from the die to atmosphere at a safe position, e.g. outside the production factory area, or to channel it to a means of recovery for filtering, cleaning and recycling -the gas. In order to more effectively collect and channel the gas from the above it may be advantageous to encapsulate the gas at its exit from the die by adding flexible seals 41 surrounding the extrudate after or at the position of exit from the die as shown in figure 6.

The gas used in the application of the invention may be any gaseous fluid, and in some cases it may be advantageous to use a more viscous fluid in liquid form. The liquid would be required to resist the normal high temperature of extrusion processes, and therefore have a high evaporation temperature point.

Further it is normally preferable to use nitrogen gas because it is chemically inert and will not oxidise the extrudate when at an elevated temperature, will not support combustion and it is non-toxic and therefore safe. However in some cases air may be safe and effective, particularly in lower pressure and temperature applications.

The gas has a beneficial cooling effect on the surface of the extrudate, and in some cases it may be further enhanced by. pre-chilling or cooling the gas.

As previously stated it is fundamental to the invention that the pressure of the gas at entry to the die 5 is effectively controlled. Gas will not enter or create a space between the extrudate and the surface of the dies unless the gas pressure is above the pressure exerted by the extrudate -17 on the die surface. The gas pressure at entry is therefore controlled and capable of being adjusted. Referring to Figure 9, in its basic form the control is by the means of manually adjusted pressure relieving valve 16 and flow regulator 17 so that the gas from a high pressure source 21 is controlled down to the required pressure. The pressure upstream and downstream of the control valve is monitored by pressure gauges 18.1 and 18.2.

Similarly the pressures of the gas at exit 19 from the die may be controlled manually by manually adjusted pressure relieving valve 20, thereby creating a back pressure to more effectively control and maintain the gas pressure in the die space between the die surface and the extrudate.

In more advanced and demanding applications of the invention it is necessary to exercise more precise and variable control of the gas pressure supply at the entry and exit from the dies. In order to achieve this, the pressures may be variably controlled and regulated through closed loop monitored systems including microprocessor and PLC program logic control controls.

Referring to Figure 10, which shows a control circuit diagrams the high pressure gas outlet to the die 27 is controlled and regulated by a ratio regulating valve 26 supplied with high pressure gas from a high pressure receiver 37 and high pressure source 38. The pressure of the gas outlet is monitored by pressure transducer 28 which is directly connected to micro computer PLC control means 25. The control means is connected to an IP converter 29 which converts an electronic signal into a compressed air variable feed, which in turn is fed through a solenoid valve 39 and then via a conduit 30 to the regulating valve 26. Such regulating valve regulates from a low pressure compressed air feed to the high pressure gas feed in a typical ratio of 65:1.

A similar means of controlling the gas recovery outlet from the die 40 through a ratio regulating valve 35 and then to a recovery means 36 may be provided. Alternatively the gas may be exhausted to atmosphere 37 through the vent solenoid valve 32 or via dump valve 33. It is possible to select the means of recovering or exhausting the gas through vent solenoid valves 32, 33 and the flow regulating valve 34.

It is possible to recover the gas at a low pressure for further compression so that it may be recycled if required.

The same principle of using a creation of a film or layer of gas between the extrudate and sizing and calibration cores is applicable in cases where an internal sizing or calibration core is used and where it is desirable to reduce the frictional resistance to flow when passing over a core to achieve a precise internal shape and/or dimensions, as illustrated in figures 11 and 12. This method and apparatus may be used with or without the previously described external applications of sizing or calibration dies and/or without the gas film or layer between the extrudate sizing or calibration dies.

Referring to figure 11, an internal sizing core is shown expanding the extruded section to required internal dimensions. The internal core assembly is retained upstream at the extruder die. Gas is supplied through a channel 40 to a peripheral ring 41 through a gap 42 to a relieved section 43, enabling gas to be injected at a lower pressure position and to flow into and form a film or layer 44 between the extrudate, 45 and the sizing core 46. Gas exists downstream at a position 47 where the sizing core ceases contact with the extrudate at position 48.

Referring to figures 12 and 13 the application of gas assistance to an internal sizing core in combination with an external sizing/calibration die is shown. The gas is channelled to the internal core through a connecting channel 50 connected to a peripheral distribution channel 51 through a gas gap 52 connected to a relieved section 53 to enable the gas to form a film or layer 54 between the extrudate 55 and the sizing core 56. There is an external sizing/calibration die 57 which may or may not be used with gas assistance. The supply and exhaust of the gas may be controlled as previously described.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For example, the above described production of a gas layer may also be applied to an upstream first in line extruder die or core.

Claims (26)

-21CLAIMS

1 An extrusion calibration or sizing member comprising means for supplying gas to the member and means for controlling the pressure of the gas whereby to produce in operation a layer of gas between the surface of the member and extrudate passing therethrough.

2. An extrusion calibration or sizing member according to claim 1, in which the means for controlling the pressure of the gas is disposed to control the pressure of the gas at the entrance to the member.

3. An extrusion calibration or sizing member according to claim 1 or 2, in which the means for controlling the pressure of the gas is disposed to control the pressure of the gas at the entrance to and the exit from the member.

4. An extrusion calibration or sizing member according to claim 2, in which the means for controlling the pressure comprises means for exhausting gas to atmosphere at the exit from the member.

5. An extrusion calibration or sizing member according to claim 3, in which the means for controlling the pressure at the exit from the member comprises means for directing gas to a gas outlet circuit operative to create a back pressure to restrain the exit flow of gas.

6. An extrusion calibration or sizing member according to claim 5, in which means are provided to recover the gas from the gas outlet circuit.

7. An extrusion calibration or sizing member according to any preceding claim, in which the means for supplying gas to the member comprises means for introducing gas at a uniform flow pressure around the periphery of the extrudate.

8. An extrusion calibration or sizing member as claimed in claim 7, in which the means for introducing gas comprises a continuous gap disposed between two sections of the member.

9. An extrusion calibration or sizing member as claimed in claim 8, in which the means for introducing gas comprises a peripheral gas channel surrounded by a mechanical seal disposed between the two sections of the member.

10. An extrusion calibration or sizing member as claimed in claim 9, in which the gas channel communicates with the interior of the member through the gap between two sections of the member.

11. An extrusion calibration or sizing member as claimed in claim 8, 9 or 10, in which the gap is adjustable.

12. An extrusion calibration or sizing member as claimed in any preceding claim, in which the member is relieved at the downstream side to provide an area where, in operation extrudate temporarily leaves contact with the member surface.

13. An extrusion calibration or sizing member as claimed in any preceding claim, in which a low pressure seal is provided so that leaked gas can be encapsulated for recovery or ducting to atmosphere.

14. An extrusion calibration or sizing member as claimed in any preceding claim, in which a pressure relieving valve is provided operative to relieve gas pressure from a high to a low value.

15. An extrusion calibration or sizing member as claimed in any preceding claim, in which means for monitoring pressure are provided.

16. An extrusion calibration or sizing member substantially as hereinbefore described with reference to figures 2 to 5 or to these figures with the modification of any of figures 6 to 13 of the accompanying drawings.

17. A method of extrusion including the steps of passing an extrudate through a calibration or sizing member, supplying gas to the member and controlling the pressure of the gas in order to produce in operation a layer of gas between the surface of the member and extrudate passing therethrough.

18. A method of extrusion as claimed in claim 17, in which the pressure of the gas is controlled at the entrance to the member.

19. A method of extrusion as claimed in claim 18 in which the pressure of the gas is controlled at the exit from the member.

20. A method of extrusion as claimed in claim 17, 18 or 19, in which the gas flows upstream in the member.

21. A method of extrusion as claimed in claim 17, 18 or 19, in which the gas flows downstream in the member.

22. A method of extrusion as claimed in any of claims 17 to 21 in which a back gas pressure is produced at the exit from the member.

23. A method of extrusion as claimed in any of claims 17 to 22, in which the gas flowing through the member is recycled.

24. A method of extrusion as claimed in any of claims 17 to 23 in which gas pressure is temporarily reduced at the gas inlet.

25. A method of extrusion as claimed in any of claims 17 to 24, in which the gas pressure is monitored.

26. A method of extrusion substantially as hereinbefore described, with reference to figures 2 to 5 or to these figures with the modification of any of figures 6 to 13 of the accompanying drawings.