METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF ONE OR MORE LAYERS OF A MULTI-LAYER FILM OBTAINED BY BLOW EXTRUSION PROCESS

Abstract

A method is described for measuring, in a multi-layer film obtained by blow extrusion process and formed by one or more layers of a first material and a second material, the total thickness of the first material and/or the second material. The method includes the steps of: a) acquiring by means of an X-ray sensor a first measurement signal 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 materials 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 signals, the total thickness (L.sub.1 of the first material and/or the total thickness (L.sub.2) of the second material.

Claims

1. A method for measuring, during production by blow extrusion process of a multi-layer film formed by 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 electromagnetic radiation sensor a first measurement signal (S.sub.x) representative of the total thickness of the film; (b) acquiring by means of a capacitive sensor a second measuring signal (S.sub.cap) which is the sum of the signals given by the first and the second material of the film, wherein the signal given by each material of the film 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.x, 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 electromagnetic radiation sensor is an X-ray retro-reflection sensor.

3. The method according to claim 1, wherein said steps a) and b) of acquiring a first measurement signal (S.sub.x) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S.sub.cap) by means of a capacitive sensor, are performed with said sensors arranged facing an outer surface of the bubble (B).

4. The method according to claim 3, wherein said steps a) and b) of acquiring a first measurement signal (S.sub.x) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S.sub.cap) by means of a capacitive sensor, respectively, are performed with said sensors arranged above each other and at a certain distance from each other, in a same plane oriented tangentially with respect to an outer surface of the bubble (B).

5. The method according to claim 4, further comprising the step of measuring the distance of said plane from the outer surface of the bubble (B) by means of an ultrasonic sensor.

6. The method according to claim 2, wherein said steps a) and b) of acquiring a first measurement signal (S.sub.x) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S.sub.cap) by means of a capacitive sensor, respectively, are performed by moving said sensors along a circular path around the bubble (B).

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: $S_X=k_X.Math.\left(L_1+L_2\right)S_{\mathrm{cap}}=k_1.Math.L_1+k_2.Math.L_2,$ wherein the parameter k.sub.x is continuously derived from data supplied by dosing means of the film production plant, which measure the quantities of the first material and the second material fed into the plant, wherein the parameter k.sub.1 is determined, during the start-up phase of the film production plant, based on the value of said second measurement signal (S.sub.cap) when the film (F) is formed by the first material only, and wherein the parameter k.sub.2 is determined during the film production cycle, based on the average value of said second measurement 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 provided by said dosing means, from the following equation: $k_2=\left(\stackrel{\_}{S_{\mathrm{cap}}}-k_1.Math.\stackrel{\_}{L_1}\right)/\stackrel{\_}{L_2}.$

8. An apparatus for measuring, in a multi-layer film obtained by blow extrusion process and formed of 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, said apparatus comprising: an electromagnetic radiation sensor arranged to provide a first measurement signal (S.sub.x) representative of the total thickness of the film; a capacitive sensor arranged to provide a second measurement signal (S.sub.cap) which is the sum of the signals given by the first and second materials of the film, wherein the signal given by each material of the film is a function of the thickness (L.sub.1, L.sub.2) of said material; and processing means for calculating, from said first and second measurement signals (S.sub.x, 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.

9. The apparatus according to claim 8, wherein said electromagnetic radiation sensor is an X-ray retro-reflection sensor.

10. A blown film extrusion plant for producing a multi-layer film, comprising an apparatus according to claim 8.

11. The plant according to claim 10, wherein said apparatus further comprises a measuring head on which said electromagnetic radiation sensor and said capacitive sensor are mounted so as to lie in a same plane oriented tangentially with respect to the outer surface of the bubble (B), guide means arranged around said bubble (B), coaxially to said bubble (B), and on which said measuring head is movably mounted along a circular path, and driving means for controlling the movement of said measuring head on said guide means.

12. The plant according to claim 11, wherein said apparatus further comprises an ultrasonic sensor arranged to measure the distance between the plane on which said electromagnetic radiation sensor and said capacitive sensor lie and the outer surface of the bubble (B).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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:

[0026] FIG. 1 is a perspective view 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

[0027] FIG. 2 is a perspective view showing on an enlarged scale the measuring head of the apparatus of FIG. 1.

DETAILED DESCRIPTION

[0028] Referring first to FIG. 1, a measuring apparatus configured to measure the thickness of one or more layers of a multi-layer film, in particular a multi-layer film comprising 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 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 multi-layer films having a different number of layers and/or a different composition.

[0029] More specifically, the measuring apparatus 10 is intended to be used in a plant for the production of multi-layer films by blow extrusion process, in particular in a plant in which the haul-off means are fixed, i.e. do not rotate.

[0030] The measuring apparatus 10 comprises a thrust bearing 12 (or, more generally, other guide means) arranged around the bubble B (the axis of which is denoted by z), coaxially thereto, and a measuring head 14 which is mounted on the thrust bearing 12 so as to be movable in a horizontal plane (i.e. in a plane perpendicular to the axis z of the bubble B) along a circular path around the bubble B. Driving means (not shown, but anyway of a per-se-known type) are associated with the measuring head 14 for controlling the movement of the measuring head 14 along the aforementioned circular path on the thrust bearing 12.

[0031] With reference also to FIG. 2, the measuring head 14 is provided with an electromagnetic radiation sensor 16, in particular an X-ray retro-reflection sensor, and a capacitive sensor 18, both arranged facing the outer surface of the bubble B, at a certain distance therefrom. The two sensors 16 and 18 are positioned one above the other, at a certain distance from each other, in a same plane oriented tangentially with respect to the outer surface of the bubble B.

[0032] Preferably, the measurements with the two sensors 16 and 18 will be performed at different times, in particular with a delay from each other corresponding to the time required for the same point on the bubble B to travel the distance separating the two sensors. Thus, depending on the linear speed with which the bubble B moves upwards, the measurement of the electromagnetic radiation sensor 16 will be performed with a certain delay with respect to the measurement performed by the capacitive sensor 18 so that the two sensors measure the thickness of the bubble B at exactly the same point each time.

[0033] Preferably, the measuring head 14 is moved at discrete intervals around the bubble, taking the measurement with the two sensors at a given angular position, then moving by a certain angle and taking a new measurement with the two sensors, and so on. Preferably, the measuring head 14 further comprises a third sensor 20, in particular an ultrasonic sensor, arranged to measure the distance between the measuring head 14, and thus the plane in which the two sensors 16 and 18 lie, and the outer surface of the bubble B.

[0034] 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 (per se known) by solving the system of equations (1) and (2) above based on the values of the signals S.sub.x and S.sub.cap supplied to such processing means by the electromagnetic radiation 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 therefore L.sub.2=0), while the latter will advantageously be determined during operation by 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 are preferably provided by the dosing devices, for example gravimetric dosing devices, which measure the quantities of the first material and the second material fed into the plant. Alternatively, the average values of L.sub.1 and L.sub.2 can be set manually by the operator during calibration, as equal to the nominal values of L.sub.1 and L.sub.2.

[0035] 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 consists of PE and 5 m consists of EVOH, and with a structure comprising a first 12.5 m layer of PE, a 5 m layer of EVOH and a second 12.5 m layer of PE.

[0036] Once started-up, the plant will begin to produce a 25 m film of PE, whereby the electromagnetic radiation sensor will provide a measurement signal S.sub.x, as a result of the calibration with the production recorded by the dosing devices, equal to:

S.sub.x=L.sub.1=25 m.

[0037] 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 (L.sub.2 being equal to 0):

[00003]$S_{\mathrm{cap}}=k_1.Math.L_2=40m.$

[0038] By entering the value L.sub.1=25 m measured with the electromagnetic radiation sensor, the value of the first calibration coefficient is obtained:

K.sub.1=S.sub.cap/L.sub.1=40 m/25 m=1,6.

[0039] When EVOH is introduced into the line, and thus the film contains both the layers of thickness L.sub.1 and the layers of thickness L.sub.2, the electromagnetic radiation sensor will provide a measurement signal

[00004]$S_X=L_1+L_2=30m,$

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

[00005]$S_{\mathrm{cap}}=1,6.Math.L_1+k_2.Math.L_2=50m.$

[0040] 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 for example provided by the dosing devices of the plant or entered manually by the operator based on the nominal values or based on the values measured in laboratory.

[0041] From the previous relationship, the following one is obtained:

[00006]$k_2=\left(50m-1,6.Math.25m\right)/5m=2.$

[0042] From this time onwards, the measuring apparatus will therefore be able to measure the thickness L.sub.2 at any time.

[0043] If for some reason the plant were then 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 electromagnetic radiation sensor and the capacitive sensor would provide the following signals:

[00007]$S_X=26m+6m=32m{S}_{\mathrm{cap}}=1,6.Math.26m+2.Math.6m=53,6m.$

[0044] Based on these values of the signals S.sub.x and S.sub.cap provided by the electromagnetic radiation sensor and the capacitive sensor, respectively, as well as 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 thus obtains 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):

[00008]$L_2=(\left(S_{\mathrm{cap}}-k_1.Math.S_X\right)/\left(k_2-k_1\right)=\left(53,6m-1,6.Math.32m\right)/\left(2-1,6\right)=6m{L}_1=S_X-L_2=32m-6m=26m.$

[0045] 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.