METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF ONE OR MORE LAYERS OF A MULTI-LAYER FILM

Abstract

A method is described for measuring, in a multi-layer film having one or more layers of a first and a second material, the total thickness of the first material and/or the second material. The method includes: a) acquiring, by means of an optical or an ionizing radiation sensor, a first measurement signal (S.sub.ott) representative of the total thickness of the film; b) acquiring, by means of a capacitive sensor, a second measurement signal which is the sum of the signals given by the first and second material of the film. The signal given by each material of the film is a function of the thickness of the material; and c) calculating, from the first and second signal, the total thickness of the first material and/or the second material.

Claims

1. A method for measuring, in a multi-layer film (F) having one or more layers of a first material and one or more layers of a second material, the total thickness (L.sub.1) of the first material and/or the total thickness (L.sub.2) of the second material, comprising: a) acquiring, by means of an optical sensor or an ionizing radiation sensor, a first measurement signal (S.sub.ott) representing in an absolute manner the total thickness of the film (F); b) acquiring, by means of a capacitive sensor, a second measurement signal (S.sub.cap) which is the sum of the signals given by the first and second material of the film (F), wherein the signal given by each material of the film (F) is a function of the thickness (L.sub.1, L.sub.2) of that material; and c) calculating, from said first and second measurement signals (S.sub.ott, S.sub.cap), the total thickness (L.sub.1) of the first material and/or the total thickness (L.sub.2) of the second material.

2. The method according to claim 1, wherein said steps a) and b) of acquiring a first measurement signal (S.sub.ott) and acquiring a second measurement signal (S.sub.cap) are performed while the film (F) is being moved over a cylinder which deviates its path.

3. The method according to claim 2, wherein said step a) is performed by hitting the film (F) with a focused optical beam (OB) emitted by an emitting head of the optical sensor placed on one side of the cylinder and detecting the shadow projected by the film (F) by means of a receiving head of the optical sensor placed on the opposite side of the cylinder relative to the emitting head.

4. The method according to claim 2, wherein said step b) is performed with the capacitive sensor arranged with its measurement axis (z) lying in a plane passing through the axis (x) of the cylinder.

5. The method according to claim 2, further comprising the step of measuring the distance of the capacitive sensor from the cylinder by means of an inductive sensor.

6. The method according to claim 1, wherein said step a) is performed by hitting the film (F) with an ionizing radiation beam (RB) emitted by an emitting head of the ionizing radiation sensor placed on one side of the film (F) and detecting the ionizing radiation beam (RB) emitted by the emitting head by means of a receiving head of the ionizing radiation sensor placed on the opposite side of the film (F).

7. The method according to claim 1, wherein said step c) of calculating the total thickness (L.sub.1) of the first material and/or the total thickness (L.sub.2) of the second material is based on solving the following system of equations: $S_{\mathrm{ott}}=L_1+L_2$ $S_{\mathrm{cap}}=k_1.Math.L_1+k_2.Math.L_2,$ wherein the parameter k.sub.1 is determined, during the start-up of the film production plant, based on the value of said second signal (S.sub.cap) when the film (F) is formed by the first material only, and wherein the second parameter k.sub.2 is determined during the production cycle of the film (F), based on the average value of said second signal (S.sub.cap) and the average values of the total thickness (L.sub.1) of the first material and the total thickness (L.sub.2) of the second material, from the following equation: $k_2=\left(\stackrel{\_}{S_{\mathrm{cap}}}-k_1.Math.\stackrel{\_}{L_1}\right)/\stackrel{\_}{L_2}.$

8. The method according to claim 7, wherein said average values of the total thickness (L.sub.1) of the first material and the total thickness (L.sub.2) of the second material are provided by dosing means of the film production plant.

9. An apparatus for measuring, in a multi-layer film (F) having one or more layers of a first material and one or more layers of a second material, the total thickness (L.sub.1) of the first material and/or the total thickness (L.sub.2) of the second material, comprising: an optical sensor or an ionizing radiation sensor for providing a first measurement signal (S.sub.ott) representing in an absolute manner the total thickness of the film (F); a capacitive sensor for providing a second measurement signal (S.sub.cap) which is the sum of the signals given by the first and second material of the film (F), wherein the signal given by each material of the film (F) is a function of the thickness (L.sub.1, L.sub.2) of that material; and processing means for calculating, from said first and second measurement signals (S.sub.ott, S.sub.cap), the total thickness (L.sub.1) of the first material and/or the total thickness (L.sub.2) of the second material.

10. The apparatus according to claim 9, further comprising a cylinder on which the film (F) is caused to move to deviate its path, wherein the optical sensor and the capacitive sensor are arranged close to the cylinder to acquire said first measurement signal and said second measurement signal, respectively, on a section of film (F) in contact with the outer surface of the cylinder.

11. The apparatus according to claim 9, further comprising an inductive sensor associated with the capacitive sensor for measuring the distance between said capacitive sensor and the cylinder.

12. The apparatus according to claim 9, wherein the optical sensor comprises an emitting head placed on one side of the cylinder and a receiving head placed on the opposite side of the cylinder with respect to the emitting head, wherein the emitting head is configured to hit the film (F) with a focused optical beam (OB), and wherein the receiving head is configured to analyse the shadow generated by the film (F) that is being hit by the optical beam (OB) emitted by the emitting head.

13. The apparatus according to claim 9, wherein the ionizing radiation sensor comprises an emitting head placed on one side of the film (F) and a receiving head placed on the opposite side of the film (F), wherein the emitting head is configured to hit the film (F) with an ionizing radiation beam (RB) and wherein the receiving head is configured to analyse the ionizing radiation beam (RB) that has passed through the film (F).

14. The apparatus according to claim 13, wherein the capacitive sensor is mounted on the emitting head of the ionizing radiation sensor and has a through-hole aligned with the ionizing radiation beam (RB) emitted by said emitting head to allow the ionizing radiation beam (RB) to pass through the capacitive sensor.

15. A plant for producing a multi-layer film (F), comprising a measuring apparatus according to claim 9.

16. The method according to claim 5, wherein the inductive sensor is integrated in the capacitive sensor.

17. The apparatus according to claim 11, wherein the inductive sensor is integrated in the capacitive sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further features and advantages of the present invention will become clearer from the following description, given purely by way of non-limiting example with reference to the accompanying drawings, in which:

[0027] FIGS. 1 and 2 are a perspective view and a front view, respectively, of an apparatus for measuring the thickness of one or more layers of a multi-layer film according to an embodiment of the present invention; and

[0028] FIG. 3 is a schematic view of an apparatus for measuring the thickness of one or more layers of a multi-layer film according to a further embodiment of the present invention.

DETAILED DESCRIPTION

[0029] Referring first to FIGS. 1 and 2, a measuring apparatus configured to measure the thickness of one or more layers of a multi-layer film F, in particular a multi-layer film formed by one or more layers of a first material and one or more layers of a second material, is generally indicated 10. In the following description reference will be made to the case in which the film F comprises a pair of outer layers of PE as the first material and an inner layer of EVOH as the second material, but it is clear that the invention is also applicable to the measurement of the thickness of layers of a multi-layer films having a different number of layers and/or a different composition.

[0030] The measuring apparatus 10 can be installed at any point downstream of the extrusion head, in the case of a plant for the production of a multi-layer film by cast extrusion method, or downstream of the haul-off means, in the case of a plant for the production of a multi-layer film by blow extrusion process.

[0031] The measuring apparatus 10 comprises a cylinder 12 of metallic material, electrically grounded. The cylinder 12 is supported for rotation about its own axis (indicated by x), which is preferably oriented horizontally. The film F, which in case of use of the apparatus in a blow extrusion plant will be the flattened tubular film coming out of the haul-off means, is caused to move over the cylinder 12 (FIG. 2), so that the path of the film is deviated, that is, so that the film branch (indicated by arrow F.sub.OUT) leaving the cylinder 12 does not extend along the same direction as the incoming film branch (indicated by arrow F.sub.IN), but forms a certain angle with the latter, in particular an angle between 90 and 150, preferably an angle between 90 and 120.

[0032] The measuring apparatus 10 further comprises a capacitive sensor 14 arranged with its measurement axis (indicated z) in a plane, in particular a vertical plane, passing through the axis x of the cylinder 12, at a certain distance from the side surface of the cylinder 12. This distance depends on the specific sensor being used, but will typically be of the order of several millimetres.

[0033] The measuring apparatus 10 further comprises an optical sensor adapted to measure in a non-interferometric way the total thickness of the film F. In the example proposed herein, the optical sensor is a shadow projection optical sensor and is configured to detect the shadow projected by the film F as the latter is caused to move over the cylinder 12 and at the same time is hit by a focused optical beam. Thus, in this case, the optical sensor comprises an emitting head 16, which is placed on one side of the cylinder 12 (to the right of the cylinder, with respect to the point of view of a person looking at FIGS. 1 and 2) and is configured to emit an optical beam B, and a receiving head 18, which is placed on the opposite side of the cylinder 12 relative to the emitting head 16 (therefore, in the present case, on the left side of the cylinder, with respect to the point of view of a person looking at FIGS. 1 and 2) and is configured to analyse the shadow generated by the film F hit by the optical beam B in order to determine the total thickness of the film. The direction of the optical beam B lies in a plane perpendicular to the axis x of the cylinder 12 and is perpendicular to the axis z of the capacitive sensor 14.

[0034] Both the capacitive sensor 14 and the optical sensor 16, 18 therefore acquire their respective measurement signals on a section of film F in contact with the outer surface of the cylinder 12.

[0035] Advantageously, an inductive sensor (not shown, but in any case of a per-se-known type) is associated to the capacitive sensor 14, preferably integrated in the same capacitive sensor, which inductive sensor is arranged to measure the distance between the capacitive sensor 14 and the cylinder 12, so as to simultaneously provide a zero offset to the optical sensor and, via the inductive signal provided by the inductive sensor, remove the contribution of the air to the capacitive signal provided by the capacitive sensor 14.

[0036] As explained above, given a multi-layer film comprising one or more layers of a first material (for example, a neutral material such as PE) of total thickness L.sub.1 and one or more layers of a second material (for example, a barrier material such as EVOH) of total thickness L.sub.2, the values of the thicknesses L.sub.1 and L.sub.2 will be calculated by appropriate processing means (known per se) by solving the system of the above equations (1) and (2) based on the values of the signals S.sub.ott and S.sub.cap supplied to those processing means by the optical sensor and the capacitive sensor, respectively. With regard to the parameters k.sub.1 and k.sub.2 appearing in equation (2), the former will advantageously be determined, during the first start-up phase of the film production plant, based on the signal S.sub.cap provided by the capacitive sensor when the film is formed by the first material only (and thus L.sub.2=0), while the latter will advantageously be determined during operation from equation (3) above, based on the average value of the signal S.sub.cap and the average values of L.sub.1 and L.sub.2. The average values of L.sub.1 and L.sub.2 can, for example, be provided by gravimetric dosing devices, which measure the quantities of the first material and of the second material fed into the plant. Alternatively, the average values of L.sub.1 and L.sub.2 can be provided by the operator during calibration.

[0037] As an example, the measurement method is illustrated here in the case of a multi-layer film with a total thickness of 30 m, of which 25 m are made of PE and 5 m are made of EVOH, and with a structure comprising a first layer of 12.5 m of PE, a layer of 5 m of EVOH and a second layer of 12.5 m of PE. Once started-up, the plant will begin to produce a 25 m film of PE, for which the optical sensor will provide a signal:

[00003]$S_{\mathrm{ott}}=L_1=25m.$

[0038] During this phase, the capacitive sensor will be measuring a non-calibrated (and therefore non-important) value, for example 40 m. Equation (2) above will then become (since L.sub.2=0):

[00004]$S_{\mathrm{cap}}=k_1.Math.L_1=40m.$

[0039] By entering the value L.sub.1=25 m measured with the optical sensor, the value of the first calibration coefficient is obtained:

[00005]$k_1=S_{\mathrm{cap}}/L_1=40m/25m=1.6.$

[0040] When EVOH is introduced into the plant, and thus the film contains both the layers of thickness L.sub.1 and the layers of thickness L.sub.2, the optical sensor will provide a measurement signal

[00006]$S_{\mathrm{ott}}=L_1+L_2=30m.$

while the capacitive sensor will still provide a non-calibrated measurement signal, for example 50 m. Equation (2) above will then become:

[00007]$S_{\mathrm{cap}}=1.6.Math.L_1+k_2.Math.L_2=50m.$

[0041] At this point, a second calibration is performed to determine the coefficient k.sub.2, using the average values of L.sub.1 and L.sub.2, i.e. L.sub.1=25 m and L.sub.2=5 m, which are provided, for example, by the dosing devices of the plant or are entered manually by the operator based on the nominal values or based on the values measured in laboratory.

[0042] From the above relationship, the following is obtained

[00008]$k_2=\left(50m-1.6.Math.25m\right)/5m=2.$

[0043] From this time onwards, the measuring apparatus will therefore be able to measure the thickness L.sub.2 at any time.

[0044] If for some reason the plant were to produce a film with a varied structure, for example with a first layer of PE of 12 m thickness, with an intermediate layer of EVOH of 6 m thickness and with a second layer of PE of 14 m thickness, the optical sensor and the capacitive sensor would provide the following signals:

[00009]$S_{\mathrm{ott}}=26m+6m=32m$$S_{\mathrm{cap}}=1.6.Math.26m+2.Math.6m=53.6m.$

[0045] Based on these values of the signals S.sub.ott and S.sub.cap provided by the optical sensor and the capacitive sensor, respectively, as well as on the values of the parameters k.sub.1 and k.sub.2 determined as described above, the measuring apparatus calculates the thicknesses L.sub.1 and L.sub.2 by solving the system of equations (1) and (2) and therefore obtaining the following results (which correspond exactly to the sum of the thicknesses of the two layers of PE and the thickness of the intermediate layer of EVOH):

[00010]$L_2=\left(S_{\mathrm{cap}}-k_1.Math.S_{\mathrm{ott}}\right)/\left(k_2-k_1\right)=\left(53.6m-1.6.Math.32m\right)/\left(2-1.6\right)=6m$$L_1=S_{\mathrm{ott}}-L_2=32m-6m=26m.$

[0046] With reference now to FIG. 3, in which parts and elements identical or corresponding to those of FIGS. 1 and 2 have been assigned the same reference numerals, according to another embodiment of the invention there is provided, instead of the optical sensor, an ionizing radiation sensor arranged to perform the same function as the optical sensor, i.e. measuring the total thickness of the film independently of the composition of the latter, i.e. without any effect by the chemical composition of the film on the measurement provided by this sensor.

[0047] The ionizing radiation sensor is, for example, made as a transmission sensor, in which case it comprises an emitting head 20 provided with a source 22 for emitting a ionizing radiation beam RB towards the film F and a receiving head 24 for detecting the signal of the ionizing radiation beam RB passing through the film F, calculating its absorption and thus obtaining the total thickness of the film F. Preferably, as in the illustrated example, the capacitive sensor 14 (the function of which is identical to that described above with reference to the embodiment of FIGS. 1 and 2) is mounted on the emitting head 20 of the ionizing radiation sensor and has a through-hole 26 for allowing the passage of the ionizing radiation beam RB. The capacitive sensor 14 may be formed by two armatures, one on the face of the emitting head 20 facing the film F and the other on the face of the receiving head 24 facing the film F. Alternatively, the capacitive sensor 14 may be integrated into a single body comprising both armatures, so that the electric field lines start from and return to the capacitive sensor itself, passing through the film F.

[0048] The ionizing radiation sensor may, however, be made as a reflection sensor instead of a transmission sensor.

[0049] What has already been described above with reference to the embodiment of FIGS. 1 and 2 applies in all other respects.

[0050] The present invention has been described so far with reference to a preferred example thereof. It is to be understood that other embodiments and modes of carrying out the invention may be envisaged, which are based on the same inventive core as defined by the appended claims.