GB2271856A - Improvements in and relating to rheology. - Google Patents

Abstract

A rheometer (7) is connected to a conveyor (1) of material for a production process. The rheometer (17) comprises a flow passageway (11), which is in communication with flowable material to be tested, a rheometer pump (10) adapted to generate pressure in the material in the flow passageway (11), and first and second dies (23) defining first and second die paths (23) for the material. The first and second dies (23) can each be coupled operatively to the flow passage by automatic die changing means (14, 16) to communicate the flow passageway (11) with the first or the second dies (23). The control means (16) receives a signal representing flow rate from the pump (10) and a signal representing pressure in passageway (11) from a transducer (17). <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO RHEOLOGY This invention relates to improvements in and relating to rheology, in particular to improvements in apparatus and methods for measuring the viscosity of materials.

It has long been very important to know how a material will flow and/or process. Whole industries, such as plastics moulding and pharmaceuticals, depend upon the predicability of the flow of materials.

On-line viscosity measuring instruments, rheometers, are known and are incorporated into production lines.

These monitor the flow characteristics of a material as it is being extruded, or as it flows. The basic concept of such rheometers is to take a bleed of the material flowing and to have it formed through a die of known geometry at a known flow rate, or pressure. Using the change in pressure over the die, the geometry of the die, and the flow rate through the die, the viscosity of the material can be calculated, usually automatically by the machine (shear viscosity is shear stress, derived from the pressure drop, divided by shear strain rate, derived from volume flow rate).

The viscosity of a material can alter with flow rate, and with temperature. In order to measure the changes in viscosity of a material over a large range, or of different materials having different viscosities, it is necessary to use dies of different geometries each die giving an acceptable "window" of measuring capability. To change a die it is simply necessary to take the rheometer off-line from the main extrusion equipment, stop the flow of material in the rheometer, detach the old die and re-attach a new die.

A further delay ig use by tha hee t euEfi to the desired temperature, and stabilise at that temperature. If, however, a new material is to be tested the bleed supply tube to the rheometer needs to be emptied of old material and cleaned.

According to a first aspect we provide a rheometer having automatic die changing means.

Preferably the rheometer is on-line.

This enables the die through which the material being tested is flowing to be changed whilst the rheometer is still on-line - whilst the extruding or flow-generating of a main material dispenser is still working. Furthermore, since the die can be changed automatically we can replace a die which is, for example, blocked or partially blocked, with a fresh die in a matter of seconds, rather than minutes as was required to change a die manually and without the need for personnel to be present.

The die may be changed for a similar or identical die, or it may be changed for a die having different characteristics. Changing the die for one with different characteristics enables us to test the material for different properties, or for a greater range of the same property, preferably in the same operational run of the main extruding or like flow-generating apparatus from which the flow to the rheometer is taken.

Preferably the die changing means is controlled by automatic control means, such as a microprocessor, computer or the like.

The automatic die changing means preferably has a plurality of interchangeable dies of different characteristics. The rheometer can change one die from being in an operative condition to an inoperative condition, and the previously inoperative die to being operative, thereby varying the flow characteristics of the die being used.

Preferably the automatic die changing means comprises a cartridge or block having a plurality of dies and a corresponding plurality of operative positions with respect to a flow passageway of the rheometer with which the operative die is in communication. Cartridge moving means are preferably provided to move the cartridge between its operative positions. The cartridge moving means is preferably under the control of the automatic control means. The cartridge of dies preferably has dies arranged in a line, or around an arc of a circle.

The rheometer preferably has seal means to seal the die that is in use to the flow passageway. The seal means preferably includes clamping means adapted to urge the die against an abutment face provided on the rheometer. The clamping is preferably under the control of the automatic control means.

The rheometer may have sensing means to detect automatically when a die is blocked, the control means then preferably changing the die automatically.

There may be dies of the same length, but different diameters. The shear strain rate of a material is dependent upon the cube of the diameter, thus a trebling of the diameter gives a 27 times increase in shear strain rate range which can be measured for a given flow rate of material.

There may be dies of the same diameter, but different lengths. By measuring the performance of different length dies both end corrections for the main viscosity measurement may be applied and the elongational viscosity of a material (as opposed to the normal shear viscosity) can be determined.

Furthermore, we can measure the die slip automatically. Although in classical fluid mechanics fluids have zero velocity at the wall of a bore through which they flow, this is not always true: they can have a wall velocity - the die slip. The flow rate Q of a material through a cylindrical pipe can be represented by 3 2 0 = f (fd y) + fd V w We can measure Q directly, and if we test with a variety of dies with the varying diameter, but the same diameter to length ratio we can estimate Vw According to a second aspect of the invention we provide a rheometer comprising a flow passageway communicable with flowable material to be tested, pressure generating means adapted to generate pressure in said material in said flow passageway, a first die defining a first die path for said material, a second die defining a second die path for said material, and automatic die changing means, the arrangement being such that the first die is capable of being operatively coupled to the flow passageway so as to communicate the first die path with the flow passageway, and as an alternative the second die is capable of being operatively coupled to the flow passageway instead of the first die, so as to communicate the second die path with the flow passageway, the automatic die changing means performing the change of die automatically.

Preferably the first and second dies each have operative and inoperative positions or configurations with respect to the flow passageway and the automatic die changing means moves them between the operative and inoperative positions. There may be more than two dies provided for some tests to obtain required information.

All of the operative and sequenced dies are preferably held at the same temperature to each other and to the temperature of the flow chamber of the rheometer. Heating and control means may be provided to achieve this.

According to a third aspect the invention comprises a production line comprising flowable material delivering, extruding, moulding or like apparatus operating on flowable material, and an on-line rheometer in accordance with the first or second aspects of the invention.

According to a fourth aspect of the invention we provide a method of improving the efficiency of an on-line rheometer comprising providing the rheometer with an automatic die changer.

A further problem with rheometers is that for the equations upon which calculations of viscosity are based to be appropriate they rely upon the flowable material being at an equilibrium temperature.

An on-line rheometer takes flowable material from a main flow conduit (usually hot material) via a bleed line. The bleed line is usually heated to prevent the flowable material from cooling, and perhaps setting.

The temperature of the flowable material needs to be at equilibrium for sensible readings.

According to a fifth aspect of the invention we provide an on-line rheometer having a bleed line adapted to take a supply of flowable material from a main on-line flow process flow conduit, in which the bleed line has at least a portion of non-round internal cross-section.

The bleed line may have a substantially uniform internal cross-section, or it may not.

Preferably a substantial portion of the length of the bleed line has an internal cross-section which is not round. Preferably all, or substantially all, of the length of the bleed line has a non-round internal cross-section. The cross-section of the bleed line is preferably generally flat, preferably a flat oblong, having two sides substantially longer than its other pair of sides.

Preferably the geometry of the internal cross-section of the bleed line is such that the surface area of the internal bleed line is large in comparison with the internal volume of the bleed line, consistent with minimising the restriction of flow through the bleed line.

We have appreciated that despite the fact that in the art round cross-section bleed lines have always been used, they are in fact one of the worst cross-sections that could have been chosen. A round cross-section passageway has the smallest surface area compared to its volume, and the temperature of its contents are therefore proportionately least effected by a heater provided in the walls of the conduit defining the bleed line - they take a long time for temperature fluctuations to even out, to reach an equilibrium temperature. A bleed line with a flat, thin, internal cross-section has a much greater temperature-influencing wall surface area in comparison with the volume of material flowing in it and the temperature of the material can be controlled faster.

It reaches equilibrium faster.

It is very surprising that we are the first to appreciate this.

According to a sixth aspect of the invention we provide a method of reducing the time for a bleed line of an on-line rheometer to achieve temperature equilibrium comprising providing the bleed line with a non-round internal cross-section.

There is a further problem with on-line rheometers associated with equilibrium conditions. The pressure in the flowable material must also be at equilibrium before readings can be used properly. The pressure in a flow passageway of a rheometer approaches equilibrium asymptotically. Conventionally the user simply waits before starting measurements until he is sure that a long enough time has passed to ensure that the pressure must, surely, have reached equilibrium (within experimental error). He may have a guideline chart setting out a few examples of suitable times for a variety of materials and initial pressure changes to which he can refer in order to assess where his case falls on the look up table, and he then adds a bit more on for luck. This is all very well, but it can waste a lot of time if, in fact, the user has been too cautious.If the user has in fact not been cautious enough this can cause significant error in the measurements.

According to a seventh aspect of the invention we provide a rheometer which monitors the pressure of a flowable material awaiting testing and which detects or calculates when equilibrium has been reached.

Preferably the achievement of equilibrium pressure within an allowable margin of error is determined by monitoring the pressure/time characteristics of the material as it approaches the equilibrium pressure from below, and by monitoring the pressure/time characteristics of the material as it approaches the equilibrium pressure from above.

Since both the rising and falling pressure curves will be approaching the same equilibrium pressure it is possible to tell when the curves are close enough together to be within the required experimental error of equilibrium. This avoids the problem of not knowing whether the pressure curve being monitored is nearly at equilibrium, or is just rising slowly, but is away from equilibrium.

According to an eighth aspect of the invention we provide a way of deciding when an equilibrium condition has been reached, within an allowable margin, by comparing the asymptotic approach to equilibrium from above and below equilibrium in order to assess when they are close enough together to identify an eventual equilibrium value, within an allowable margin of error.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings of which: Figure 1 shows schematically an on-line rheometer having an automatic die changer; Figure 2 shows another schematic version of an extruder system fitted with a rheometer similar to that of Figure 1; Figure 3 shows a view on arrow A of Figure 2; Figures 4 to 6 show a cross-section on line BB, CC, and DD respectively of Figure 2; Figure 7 illustrates the cross-section of the bleed line of the rheometer of Figure 2, and schematically shows the relative position of a temperature sensor; Figures 8 to 11 show details of a die block used with the arrangement of Figure 1; Figure 12 shows schematically a clamping arrangement for the automatic die changer of Figure 1; and Figure 13 shows a graph of pressure against time for flowable material in the rheometer of Figure 1.

A conveyor 1 of material for a production process, for example a material supply to plastics injection moulding tool, or material supply to a filling station, as shown in Figure 1 comprises a heater 2 surrounding a barrel 3 which defines a bore for the transport of flowable material 4. A screw auger 5 conveys the material along the barrel 3 towards an outlet for the material 6. An on-line rheometer 7 is attached to the conveyor 1. Heaters 8a are provided to heat various parts of the apparatus. The heaters ensure that the material is at a uniform temperature, and does not cool or begin to solidify.

The rheometer 7 comprises a connector barrel 8 defining a bleed line 9 which communicates the interior of the barrel 3 with a rheometer pump 10. The rheometer pump 10 is adapted to pump flowable material 4 from the bleed line 9 to a flow passageway 11 defined in pressurisable chamber means 12. A moveable die block 13 is provided at one end of the passageway 11. Die changing means 14 is provided to move the die block with respect to the chamber means 12. A purge pump 15 is provided and is in communication with the bleed line 9. Control means 16 controls the operation of the purge pump, the rheometer pump 10, the heaters 8a, and the die changing means 14. The heaters 8a are controlled independently in response to respective temperature sensors. Pressure signals indicative of the pressure in the flow passageway 11 are fed to the control means 16 by a pressure transducer 17 provided mounted in the chamber means.

The chamber means has a fluid entrance aperture 18, a cleaning aperture 19 which is closed by a screw threaded bolt 20, and a sealing face 21 which is adapted to sealingly engage corresponding sealing faces 22 provided on the die block 13. The sealing faces 22 each surround a respective die 23 each of which defines a die passageway extending through the thickness of the block 13. Clamping means 24 is provided beneath the die block 13 and is controlled by the control means 16.

In use the rheometer 7 is set up to bleed a supply of flowable material from the barrel 3 and pass it through a die 23, with the rheometer pump 10 running at a constant speed so that an equilibrium condition is achieved in the flow passageway 11, and through the die 23. Once a flowable material 4 has passed through the die 23 it either falls to waste, or is collected once it reaches substantially zero pressure and is returned to the main conveyor 1. Pressure transducer 17 indicates to the control means, which is preferably controlled by a microprocessor, the pressure in the flow passageway 11. The drive to the rheometer pump 10 indicates the flow rate to the control means 16.From an indication of the pressure and the flow rate at an equilibrium condition, with a knowledge of the diameter of the die 23 and its length, the control means 16 can evaluate the shear viscosity of the flowable material.

Should it be desired to change the die being used, and there are a number of reasons why this may be desirable, the control means 16 stops the rheometer pump 10 (and may sometimes reverse its action a little), the clamping means 24 unclamps the die block 13 from the sealing face 21 of the chamber means 12 so as to allow the die block 13 to move relative to the chamber means 12, and the controller controls the die changing means 14 to move the die block automatically to bring the next die selected for use into registration with the flow passageway. The clamping means 24 then re-clamps the die block 13 in its new position, with a new die 23 in operative relationship with the passageway 11, and the clamping pressure seals the faces 21 and 22, preventing the flowable material 4 from escaping from between them.

Further pressure and flow rate measurements can then be made using the new die. In the case where the new die is of the same diameter as the old die, for example if the die were changed because the old die were blocked, then usable measurements can be obtained quite quickly after the change.

The die changing operation can be completed in a matter of a few seconds.

Details of the die block 13 and die changing means 14 can be seen from Figures 8 to 12. The die block in this example has six dies 23a to 23f, each surrounded by a copper die seal ring 25 which defines the sealing faces 22 and is clamped against corresponding sealing face 21 of the chamber means 12 (this may also have a seal insert of suitable material). The block 13 has an array of location holes 26 in its rear face into which a corresponding moving plug (not shown) connected to a movable arm 27 is introducable. The movable arm 27 is capable of moving transversely relative to the passageway 11, and is itself movable in a direction perpendicular to its direction of movement by another movable arm 28. Each of the movable arms is controlled by the control means 16.Thus the movable arm 27 can move longitudinally relative to the die block 13 to a position adjacent location hole 26 and then move its moving connection plug into engagement with a location hole so as to gain a purchase to index the die block 13 along one position. We prefer to move the die block by one position at a time since this enables us to have a more accurate, and simpler, die changing means. Since the die block is indexed by one position at a time we need many location holes 26, the moving plug indexing the die along by one step at a time. The dies may be removably held in the die block so that the user can arrange them in a suitable order for use.

The clamping means 24 comprises a screw member 29 mounted by means of a corresponding screw thread in a support 30. The screw 29 has a lever 31 extending radially away from it by means of which it is adapted to be screwed upwards and downwards by a part of a turn. A moving linkage 32 is connected to the lever 31 and is operated by a pneumatic mechanism 33 which is under control of the control means 16. Movement of the screw 29 by a part of a turn is enough to free the die block 13 for sliding movement relative to the support 30 and the passageway 11. The use of a screw member 29, a lever 31, and a pneumatic mechanism 33 enables a high clamping force to be applied by the clamping means 24. For example, we envisage applying sufficient force to resist test pressures of 10,000 to 20,000 psi.

The automatic die changing facility enables the rheometer 7 to change its die during a run of the rheometer - during analysis of a material.

Whilst the description shown moves the die linearly, it will be appreciated that we may choose to move it angularly, for example by rotational movement about an axis, or by a combination of translational and rotational movement. It willXalso be appreciated that the dies themselves may be provided as die inserts in a carrier block (the dies themselves may need to be made of expensive hard material, such as tungsten carbide whereas the carrier block may be made of cheap material).

Since we can change the die in use to a selected one of many dies, automatically, the rheometer can be used to do sophisticated analyses quickly, and automatically. We can use dies of different lengths, but the same diameter, dies of different diameters but the same length, or dies of different diameters and of different lengths, all under the control of the control means which can use the results from a plurality of tests on the same material to calculate values not previously readily obtainable. For example we can estimate the die slip of a material, and/or its elongational viscosity (as well as its shear viscosity).

When the material to be tested is to be changed the connector barrel 8 is disconnected from the conveyor 1 and connected to a source of cleaning material, and the purge pump 15 used to clean the bleed line 9. The rheometer pump 10 can be used to clean the flow passageway 11, or it can be assisted by the purge pump 15. The entire die block 13 can be changed (the old die block being cleaned for re-use). The bolt 20 can be removed and the flow passageway 11 cleaned through the cleaning aperture 19.

Figure 2 shows another rheometer which is in many ways similar to that shown in Figure 1, and similar components have been given similar reference numerals.

Figure 2 illustrates in more detail features of the connector barrel and bleed line. The connector barrel 8' in this example has three heaters 40 each of which is controlled by a microprocessor, and each of which has a temperature sensor 50 which feeds temperature signals to the microprocessor. (The heaters on the connector barrel 8 of Figure 1 are not shown or described).

Figure 3 illustrates the provision of a clamping flange 51 at the end of the connector barrel 8 to which the chamber means 12 is clamped.

Figure 4 shows a detail of the construction of the barrel 8'. The barrel 8' is made of two longitudinal components 52 and 53 held together by screws 54 and dowel pins 55.

Figure 5 illustrates the relationship between a temperature sensor 50 and the bleed line 9', as does Figure 7. The bleed line 9' has a flat rectangular cross-section. This enables the temperature of the flowable material in the bleed line 9' to reach equilibrium quicker. The bleed line 9' has a large surface area defined by the connector barrel 8'. The connector barrel 8' is heated by the heaters 40 and is made of a material with a high heat capacity and adequate thermal conductivity (for example steel). Thus the heaters can control the temperature of the barrel 8' to be uniform, and that uniform temperature can be quickly transmitted to the flowable material 4 by the appropriate choice of cross-section for the bleed line 9'.

The temperature sensor 50, which is usually a thermocouple or platinum resistence thermometer, does not extend radially towards the flowable material, but instead extends in an offset manner, for example tangentially. We have surprisingly found that this gives better temperature sensing since it avoids local fluctuations in temperature. This may constitute another invention which may be applicable to other temperature sensors.

Figure 6 shows a cross-section through the connection between the conveyor 1' and the barrel 3', and again shows the temperature sensor extending tangentially with respect to the passageway through which the flowable material flows (passageway 56).

Before meaningful readings can be taken the rheometer has to be in pressure equilibrium (some materials are compressible). When the rheometer pump is turned on the system has to be allowed to flow for a while with the pump at constant speed for equilibrium to be achieved. Deciding how long is long enough for any particular material and flow rate can be difficult.

Some materials have a gradual approach to equilibrium, so that longer is needed to get there (within an allowable margin), other materials have a step approach.

For example when the rheometer pump is set to do a test at a particular flow rate the control means looks at the pressure to decide when equilibrium has been reached. For a series of tests on the same material, at the same flow rate, but using dies of different geometries, it can be a great advantage to keep the equilibrium wait as short as possible. In order to identify the equilibrium pressure the rheometer firstly steps the speed of the pump up to the desired, constant, rate and takes measurements of the pressure (with digital filtering of the results to eliminate noise) so as to give data to evaluate the equilibrium pressure (usually using the rate of change of pressure at different times), and then increases the speed of the pump above the desired, constant rate and re-sets it down to the desired rate, thereby approaching pressure equilibrium from above. Similar measurements are taken and the system uses the rising pressure data and falling pressure data to identify the equilibrium pressure for a given material at a given flow rate (at a given temperature). Figure 13 illustrates this principle.

In subsequent test runs on the different dies the system then knows what the equilibrium pressure is and can tell when the pressure in the flowable material has approached it closely enough following a stoppage (for example to change the die). If several readings are to be taken this can save a lot of time.

Claims (27)

1. A rheometer having automatic die changing means.

2. A rheometer comprising a flow passageway communicable with flowable material to be tested, pressure generating means adapted to generate pressure in said material in said flow passageway, a first die defining a first die path for said material, a second die defining a second die path for said material, and automatic die changing means, the arrangement being such that the first die is capable of being operatively coupled to the flow passageway so as to communicate the first die path with the flow passageway, and as an alternative the second die is capable of being operatively coupled to the flow passageway instead of the first die, so as to communicate the second die path with the flow passageway, the automatic die changing means performing the change of die automatically.

3. A rheometer according to claim 1 or claim 2 in which the rheometer is on-line.

4. A rheometer according to any preceding claim in which the automatic die changing means has a plurality of interchangeable dies of different characteristics.

5. A rheometer according to any preceding claim in which first and second dies are provided each having operative and inoperative positions or configurations with respect to a flow passageway.

6. A rheometer according to any preceding claim in which the automatic die changing means comprises a cartridge or block having a plurality of dies and a corresponding plurality of operative positions with respect to a flow passageway of the rheometer with which the operative die is in communication.

7. A rheometer according to claim 6 in which cartridge moving means are provided to move the cartridge between its operative positions.

8. A rheometer according to claim 6 or claim 7 in which the cartridge of dies has dies arranged in a line, or around an arc of a circle.

9. A rheometer according to any preceding claim in which the die changing means is controlled by automatic control means, such as a microprocessor, computer or the like.

10. A rheometer according to claim 7, claim 8 or claim 9 as it depends from claim 7 or claim 8 in which the cartridge moving means is under the control of the automatic control means.

11. A rheometer according to any preceding claim in which the rheometer has sensing means to detect automatically when a die is blocked.

12. A rheometer according to any one of claims 9 to 11 in which the control means changes the die automatically.

13. A rheometer according to any preceding claim which has dies of a same length, but of different diameters.

14. A rheometer according to any one of claims 1 to 12 which has dies of a same diameter, but of different lengths.

15. A rheometer according to any one of the preceding claims which has operative and sequenced dies which are held at the same temperature to each other and to the temperature of the flow chamber of the rheometer.

16. A rheometer substantially as described herein with reference to Figures 1 to 6 and 8 to 12 of the accompanying drawings.

17. A production line comprising flowable material delivering, extruding, moulding or like apparatus operating on flowable material, and an on-line rheometer in accordance with any one of claims 3 to 16.

18. A production line substantially as described herein.

19. A method of improving the efficiency of an on-line rheometer comprising providing the rheometer with an automatic die changer.

20. A method of improving the efficiency of an on-line rheometer substantially as described herein.

21. An on-line rheometer having a bleed line adapted to take a supply of flowable material from a main on-line flow process flow conduit, in which the bleed line has at least a portion of a non-round internal cross-section.

22. An on-line rheometer according to claim 21 in which a substantial portion of the length of the bleed line has an internal cross-section which is not round.

23. An on-line rheometer according to claim 21 or claim 22 in which all, or substantially all, of the length of the bleed line has a non-round internal cross-section.

24. An on-line rheometer according to any one of claims 21 to 23 in which the cross-section of the bleed line is generally flat, having two sides substantially longer than its other pair of sides.

25. An on-line rheometer having a bleed line substantially as described herein with reference to Figure 7 of the accompanying drawings.

26. A method of reducing the time for a bleed line of an on-line rheometer to achieve temperature equilibrium comprising providing the bleed line with a non-round internal cross-section.

27. A method of reducing the line for a bleed line of an on-line rheometer to reach temperature substantially as described herein with reference to Figure 13 of the accompanying drawings.