METHOD AND MEASURING ARRANGEMENT FOR DETERMINING A RHEOLOGICAL PROPERTY OF A FLUID

Abstract

In order to determine a rheological property of a fluid, the fluid is conveyed with a constant volume flow rate through a nozzle and the fluid strand thereby generated is deposited on a substrate. A relative movement takes place between the nozzle and the substrate at a forward feed velocity value. A contour of the liquid strand between the nozzle and the substrate is optically measured, and an extensional viscosity as a rheological property is deduced from knowledge of the volume flow rate, the forward feed velocity value and the contour of the fluid strand.

Claims

1. A method for determining a rheological property of a fluid, which comprises the steps of: conveying the fluid with a constant volume flow rate through a nozzle; depositing a fluid strand thereby generated on a substrate and a relative movement takes place between the nozzle and the substrate at a forward feed velocity value; optically measuring a contour of the liquid strand between the nozzle and the substrate; and deducing a strain rate-dependent extensional viscosity as the rheological property derived from the constant volume flow rate, the forward feed velocity value and the contour of the liquid strand.

2. The method according to claim 1, which further comprises thermally regulating the nozzle to a target temperature value.

3. The method according to claim 1, which further comprises selecting the forward feed velocity value for the relative movement between the nozzle and the substrate to be constant.

4. The method according to claim 1, which further comprises detecting a trajectory and a diameter of the fluid strand between the nozzle and the substrate as parameters of the contour.

5. The method according to claim 1, which further comprises carrying out the optically measuring by means of at least one camera, which is disposed stationary relative to the nozzle.

6. The method according to claim 1, which further comprises carrying out the optically measuring by means of at least one camera, which is moved synchronously with the nozzle.

7. The method according to claim 1, which further comprises optically detecting a spreading of the fluid strand deposited on the substrate, and deducing a wettability and/or a shear viscosity of the fluid with an aid of an temporal profile of the spreading.

8. The method according to claim 1, which further comprises carrying out the depositing of the fluid strand inside a thermally regulated housing.

9. The method according to claim 1, which further comprises carrying out the depositing of the fluid strand inside a housing flooded with protective gas or placed at a reduced pressure.

10. The method according to claim 1, which further comprises applying an electrical voltage for generating an electric field between the nozzle and the substrate.

11. The method according to claim 2, which further comprises thermally regulating a fluid reservoir upstream of the nozzle to a further target temperature value being a same as the target temperature value of the nozzle.

12. A measuring configuration for determining a rheological property of a fluid, comprising: a nozzle; a conveyor conveying the fluid through said nozzle during normal operation; a substrate, on which a fluid strand generated by said nozzle is deposited during the normal operation; a forward feed device for inducing a relative movement between said nozzle and said substrate at a forward feed velocity value; an optical detector for measuring a contour of the fluid strand between said nozzle and said substrate; and a controller being adapted to deduce a strain rate-dependent extensional viscosity as the rheological property from knowledge of a volume flow rate, the forward feed velocity value and the contour of the liquid strand.

13. The measuring configuration according to claim 12, wherein said optical detector is formed by at least one camera.

14. The measuring configuration according to claim 12, further comprising a further optical detector being adapted and disposed to detect spreading of the fluid strand deposited on the substrate.

15. The measuring configuration according to claim 14, wherein said optical detector is disposed stationary relative to said nozzle and said substrate.

16. The measuring configuration according to claim 14, wherein said further optical detector is a further camera.

17. The measuring configuration according to claim 14, wherein said optical detector and said further optical detector are disposed stationary relative to said nozzle and said substrate.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0082] FIG. 1 is a diagrammatic, side view of a measuring arrangement for determining a rheological property of a fluid'

[0083] FIG. 2 is a flow chart of a method for determining the rheological property, which is carried out by means of the measuring arrangement;

[0084] FIG. 3 is an illustration showing a detail representation III according to FIG. 1 of a nozzle of the measuring arrangement and a fluid strand extruded by means of the nozzle; and

[0085] FIG. 4 is a perspective partial detail of an extension of the measuring arrangement according to another exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0086] Parts and quantities which correspond to one another are always provided with the same references in all the figures.

[0087] Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown schematically a measuring arrangement, referred to below in brief as the “measuring instrument 1”, which is adapted and provided to be used for determining a rheological property of a fluid. The measuring instrument 1 contains an instrument table 2, which carries a substrate (not represented in detail). The measuring instrument 1 furthermore contains a gantry-like frame 4, on which a fluid reservoir 6 and a nozzle 8 connected to the fluid reservoir 6 are mounted displaceably at least in a forward feed direction 10. In order to displace the fluid reservoir 6 and the nozzle 8, the measuring instrument 1 also has a forward feed device, here in the form of a drive 12, which has an electric motor, a downstream transmission as well as a belt drive for force transmission to the module containing the fluid reservoir 6 and the nozzle 8. The measuring instrument 1 furthermore contains a conveyor device 14, which is adapted to convey the fluid from the fluid reservoir 6 through the nozzle 8 and therefore to extrude a fluid strand 16. The substrate is used as a depositing surface for the fluid strand 16.

[0088] The conveyor device 14 is for example—in particular depending on the fluid to be studied—a pump, a piston which displaces the fluid from the fluid reservoir 6, or the like. For the case in which a plastic melt is used as the fluid, the pump is for example configured as a melt pump.

[0089] The fluid reservoir 6 and (in a manner not represented) also the nozzle 8 contain a thermal regulation device which is used to thermally regulate the fluid to a target temperature value. Optionally a common thermal regulation device is provided, by means of which the fluid reservoir 6 and the nozzle 8 are thermally regulated to the same target temperature value during normal operation. In one alternative exemplary embodiment, the fluid reservoir 6 and the nozzle 8 respectively contain mutually separate thermal regulation devices—for example electrical heating elements or fluid heating elements, in particular heating sleeves, integrated heating tubes, or the like.

[0090] The measuring instrument 1 furthermore contains a housing 18, which accommodates the instrument table 2, the frame 4 and, in the exemplary embodiment represented, also the nozzle 8, the fluid reservoir 6 and the conveyor device 14. The measuring instrument 1 furthermore contains a thermal regulation system (not represented), which is used to thermally regulate the housing interior 20 accommodated by the housing 18 to a target temperature value.

[0091] The measuring instrument 1 furthermore contains a controller 22, which is adapted to control the thermal regulation devices of the fluid reservoir 6, of the nozzle 8, the thermal regulation system for the housing 18, the drive 12 and the conveyor device 14. For example, the controller 22 is part of an (optionally industrial) PC.

[0092] In order to be able to determine the rheological property, the measuring instrument 1 furthermore contains an optical detection unit in the form of a (digital) camera 24. In the exemplary embodiment represented, the camera 24 is arranged stationary. The camera 24 is aligned with its viewing direction perpendicular to a nozzle axis and to the forward feed direction 10. The camera 24 is furthermore aligned in such a way that its optical detection region 26 includes at least the nozzle tip of the nozzle 8 and a part of the substrate lying below the nozzle 8. The extruded fluid strand 16 can therefore be detected by the camera 24 between the nozzle 8 and the substrate.

[0093] The controller 22 is adapted to carry out a method described in more detail below, inter alia with the aid of FIG. 2, in order to determine the rheological property. The method is explained here by way of example for a plastic, specifically a plastic melt, as the fluid.

[0094] In a first method step S1, the controller 22 controls the thermal regulation device of the plastic-filled fluid reservoir 6 and of the nozzle 8 to a target temperature value, which lies above the melting temperature value of the plastic. The controller 22 likewise controls the thermal regulation system for the housing 18, or the housing interior 20, to a further target temperature value, which in the present variant is set equal to the target temperature value of the nozzle 8 (in the case of a polyamide, for example, at about 240 degrees Celsius).

[0095] In a second method step S2, the controller 22 controls the conveyor device 14 so that it presses the molten plastic out of the fluid reservoir 6 through the nozzle 8 with a constant volume flow rate, and thus extrudes the fluid strand 16. The controller 22 also controls the drive 12 in such a way that the fluid reservoir 6 is displaced with the nozzle 8 with a constant and predeterminable forward feed velocity value relative to the instrument table 2, and therefore the substrate. The fluid strand 16 is therefore deflected laterally after contact with the substrate and—because of a correspondingly high selection of the forward feed velocity value—drawn away from the nozzle 8. A trajectory y(x), i.e. curve, of the fluid strand 16 is therefore set up, which extends curved or bent in the forward feed direction 10 between the nozzle 8 and the substrate. The fluid strand 16 furthermore tapers with an increasing distance from the nozzle 8. The tapering is in this case dependent on the forward feed velocity, which therefore represents a kind of withdrawal velocity of the fluid strand 16 from the nozzle 8, and the viscosity, in particular the extensional viscosity. The shape of the trajectory y(x) is also dependent on these two quantities, since a relatively inviscid material (in this case i.e. a low-viscosity melt) will flow away rapidly from the nozzle 8 and therefore be bent with a relatively tight radius in the direction of the horizontal only just before the substrate.

[0096] In a third method step S3, the controller 22 therefore optically detects the profile, i.e. the trajectory y(x), of the fluid strand 16 by means of the camera 24. The trajectory y(x) is optionally imaged in a Cartesian coordinate system and its profile is “fitted” by a polynomial. From this, the controller 22 derives a “contour coordinate s” by means of which all points of a “neutral fiber” (and therefore of the trajectory y(x)) of the fluid strand 16 can be described along its longitudinal extent. The controller 22 furthermore determines a diameter d(s) of the fluid strand 16 along its longitudinal extent, i.e. as a function of the contour coordinates (see for example d(s1) and d(s2) in FIG. 3). As an alternative to the above-described use of the polynomial, the controller uses a monotonically increasing (preferably when regarding the trajectory y(x) from the substrate in the direction of the nozzle) function to approximate the trajectory y(x). Optionally, trigonometric functions are also used in order to describe the trajectory y(x).

[0097] The controller 22 uses knowledge of the diameter D of the nozzle 8 in order to measure the diameter d(s) of the fluid strand. The (nozzle) diameter D therefore serves as a scale for the controller 22.

[0098] The controller 22 furthermore determines the profile of the angle a(s) between the trajectory y(x), specifically the tangent of the trajectory y(x), and the horizontal along the contour coordinate s.

[0099] Subsequently, in a fourth method step S4, the controller 22 determines the extensional viscosity of the plastic melt, i.e. its actual value, from the profile of the diameter d(s), the volume flow rate, the (known) density of the fluid, the acceleration due to gravity, the angle a(s), the shape of the trajectory y(x) and the surface tension, with the aid of the above-described Formulas (1) to (6).

[0100] In one optional exemplary embodiment, which refines the exemplary embodiment described above, the measuring instrument 1 contains an additional optical detection unit, specifically a further camera 30 (see FIG. 4). This camera 30 is arranged with the viewing direction perpendicular to the instrument table 2 and therefore to the substrate. The detection region of the camera 30 is selected in such a way that the fluid strand 16 can be detected when it is deposited on the substrate. By means of the camera 30, the controller 22 detects the strand diameter a(t) of the deposited fluid strand 16 over a period of time t—schematically represented in FIG. 4 with the aid of the fluid strand 16 at two instants t1 and t2. With the aid of the aforementioned Formulas (7) to (10), the spread of the fluid strand 16 and therefore of the plastic melt on the substrate may be deduced from the temporal profile of the strand diameter a(t). This in turn gives information about the wetting behavior of the plastic melt and about its shear viscosity.

[0101] In one optional exemplary embodiment, the measuring instrument 1 is an independent instrument for studying the extensional viscosity, and optionally the shear viscosity and the wetting behavior.

[0102] In one alternative exemplary embodiment, the measuring instrument 1 is a 3D printer which is expanded with the camera 24, and optionally the camera 30, and the control instrument of which is expanded by installing a method as described above as code-containing analysis software for the controller 22.

[0103] The subject-matter of the invention is not restricted to the exemplary embodiments described above. Rather, further embodiments of the invention may be derived by the person skilled in the art from the description above. In particular, the individual features described with the aid of the various exemplary embodiments and their configuration variants may be combined with one another in a different way.

[0104] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: [0105] 1 measuring instrument [0106] 2 instrument table [0107] 4 frame [0108] 6 fluid reservoir [0109] 8 nozzle [0110] 10 forward feed direction [0111] 12 drive [0112] 14 conveyor device [0113] 16 fluid strand [0114] 18 housing [0115] 20 housing interior [0116] 22 controller [0117] 24 digital camera [0118] 26 detection region [0119] 30 camera [0120] S1-S4 method steps [0121] d(s) diameter [0122] D diameter [0123] a(t) strand diameter of the deposited fluid strand [0124] s contour coordinate [0125] t period of time [0126] y(x) trajectory [0127] a(s) angle