GB2047887A

GB2047887A - Extrudate Thickness Measurement by X-ray Absorption

Info

Publication number: GB2047887A
Application number: GB8012356A
Authority: GB
Original assignee: Polymer-Physik GmbH and Co KG; Polymer Physik GmbH and Co KG
Current assignee: Polymer-Physik GmbH and Co KG; Polymer Physik GmbH and Co KG
Priority date: 1979-04-17
Filing date: 1980-04-15
Publication date: 1980-12-03
Also published as: JPS55141608A; DE2915472A1

Abstract

Continuous measurement of the thickness of extruded synthetic resin profile sections 4 is made by determining the degree of absorption of X-rays upon penetration through the extruded synthetic resin profile sections, wherein, for a weight per unit area in the range of from 4,000 to 14,000 g/m<2>, there are used X-rays with energy of from 20 to 40 keV. A device for carrying out this method comprises a screened radiation source 1 and a counter 6, the radiation source emitting X-rays having a photon energy of from 20 to 40 keV, said X-rays being generated by a radionuclide, e.g. Am 241, acting as primary emitter, by means of a target from an element having an atomic number of from 40 to 60, e.g. Rb, Ag, Mo, Ba. Other target materials are Cu and Tb. <IMAGE>

Description

SPECIFICATION Method and Device for Thickness Measurement of Extruded Synthetic Resin Profile Sections The present invention is concerned with a method and a device for the continuous thickness measurement of extruded synthetic resin profile sections by determining the degree of absorption of gamma rays upon penetration through the extruded synthetic resin profile sections.

It is known that gamma and beta rays can be used for measuring the weight-thickness of materials. The emitter and the receiver (counter) are each mounted on opposite sides of the material to be measured. The material thereby partially weakens the radiation in dependence upon the weight per unit area, F, which represents the product of the thickness and the density of the material to be measured. If the density is assumed to be constant, the indication is inversely proportional to the thickness. This method has hitherto been used only for the measurement of the thickness of objects with a large surface area, such as paper webs, webs of synthetic resin sheeting, metal sheeting and the like, and for flowthrough measurements in pipes.In these cases, for the measurements of weight per unit area of up to 10,000 g/m2, beta-ray emitters with an energy of up to 2.27 MeV (Sr 90) have been used. This type of application requires considerable screening from secondary X-ray emission. In the case of thickness measurements above 40,000 g/m2, gamma ray emitters, such as Am 241, Cs 137 or Co 60, are used as sources of radiation and these also require extensive screening and safety precautions.

Therefore, it is an object of the present invention to provide a method and a device which, on the one hand, permit extensive screening and safety precautions to be dispensed with and, on the other hand, make it possible to measure accurately and continuously the thickness of synthetic resin profiles within a range of thickness of from 4,000 to 14,000 g/m2, without using beta rays. Continuously increasing prices for raw materials in organic chemistry and the demand for uniform quality necessitate the precise maintenance of the thickness of the material during the production of extruded synthetic resin materials.

Thus, according to the present invention, there is provided a method for the continuous measurement of the thickness of extruded synthetic resin profile sections by determining the degree of absorption of gamma rays upon penetration through the extruded synthetic resin profile sections, wherein, for a weight-thickness range of from 4,000 to 1 4,000 g/m2, there are used X-rays with a photon energy of from 20 to 40 keV as gamma rays.

The X-rays emitted by the radiation source are preferably focussed so that they impinge vertically on the profile section. Those protons which come into contact with surfaces which are not at right angles to the photon beam are preferably masked.

It is preferable to support the profile in the region of the photon source and of a counter (receiver) in a device which is secured to the photon source and the counter in such a manner that, when the profile section is moving vertically with respect to the extrusion apparatus, the whole measuring equipment moves with it, measuring errors thus being avoided.

Furthermore, the photons transmitted through the profile section are preferably picked up in an energy-dispersive manner.

The present invention also provides a device for carrying out the above-described method, comprising a screened radiation source and a counter, the radiation source emitting X-rays having a photon energy of from 20 to 40 keV, said X-rays being generated by a radionuclide, acting as primary emitter, by means of a target from an element having an atomic number of from 40 to 60.

The primary source can be, for example, 10 mCi Americium 241, which emits 59.5 keV photons and is preferably enclosed in a ceramic jacket. Different targets can be swung into the path of the primary photons and, in turn, emit characteristic X-rays, the photon energy of which is lower than the primary energy.

These primary photons have energies of between 8.04 keV for copper targets and 44.23 keV for terbium targets, 13.37 keV for rubidium targets, 17.44 keV for molybdenum targets, 22.10 keV for silver targets and 32.06 keV for barium targets.

Thus the photon energy can be adjusted to an optimum degree to the particular measuring problem, i.e. photon energies are used which produce as high a measuring signal as possible with the smallest possible change in thickness of the material.

It has proved to be particularly advantageous when the X-rays emerge focussed from the source and are thus also suitable for measuring the thickness of relatively complex profile sections. For this purpose, it is necessary to locate a position on the profile sections which is representative of the whole profile section and which has surfaces which intersect a preferably approximately 10 mm. thick photon beam vertically.

Other surfaces contacted by the photon beam and which are not at right-angles thereto are preferably masked in order to exclude inaccuracies which these would introduce into the measuring process.

In the device according to the present invention different targets can be spaced out side by side on a rotating disc disposed below the primary source. Furthermore, the device can be provided with a diaphragm for limiting the solid angle of the photons emitted and also with another diaphragm which limits the entry of the photon beam into the counter.

The counter can be an energy-proportional counter and especially a sodium iodide counter capable of separating the information-carrying photons from the background.

The radiation source and the counter can be secured to a support device through which the profile section to be measured passes during the measurement.

The device can also be adapted to convert pulses arriving in the counter in a convertor into a direct current which is proportional to the incident radiation and which drives an indicator, a recorder and/or a process control system.

Figure 1 of the accompanying drawings serves to illustrate the fact that the correct selection of photon energy is decisive for a meaningful resolution of a change in the thickness of a material. The diagram shown in Figure 1 uses, as an example, polyvinyl chloride with a thickness of 10 mm.

Along the ordinate is plotted the difference in the rate of counting with a 1% change in layer thickness per 1,000 primary pulses.

Along the abscissa, the energy of the photons emitted from the source is plotted directly. The curve shows that an optimum value for the primary energy lies between 32 and 43 keV.

Under the given measuring conditions, i.e. a measuring time of 3 minutes and 1% resolution in layer thickness, the problem can only be solved optimally in the hatched area.

Figure 2 of the accompanying drawings illustrates an embodiment of the device according to the present invention. This device comprises a photon source (1), a diaphragm (2) for limiting the solid angle of the photons emitted, limit stops (3) for lateral guiding of a synthetic resin-profile section (4), a diaphragm (5) limiting the entry of the photon beam into a counter (6) and a frame (7) for maintaining the exact distance between the source and the counter.

The following Example is given for the purpose of illustrating the present invention:- Example For continuous material thickness measurement at an extruder for window profile sections, the last calibrating nozzle is followed by a measuring arrangement comprising a variable X-ray source having an Am 241 primary source, constructed as an enclosed ceramic source, and a barium target as a secondary X-ray emission source.

The secondary X-rays, which have an energy of 32.06 keV and are focussed to a solid angle of approximately 0.5 sterad, penetrate a window profile section which is held in two lateral jaws acting as mechanical supports, at a position at which the profile surfaces run vertically to the direction of the photon beam. Before the photon rays enter a sodium iodide counter, the rays which have spread out to a diameter of more than 10 mm. are absorbed by a diaphragm so that they do not enter the counter and distort the result of the measurement.

The number of photons reaching the counter is reduced with respect to the number of photons emitted by the source of radiation by the number of photons which have been absorbed in the window profile section due to the thickness of the material. The thicker is the object, the less photons penetrate the profile section. The photons still reaching the counter are converted into a current signal, which is amplified and fed to a rate meter. There is a relationship between the counting rate and thickness of material which can be used directly for controlling, for example, the extruding speed at the extruder in the case of deviations from the desired thickness of material.

Technical data for such a thickness measuring device: thickness of material: 12,000 g/m2 thickness of profile section (PVC): 10 mm.

primary source: Americium 241 source intensity: 10 mCi barium target energy of secondary photons 32.06 keV flow of secondary photons: 46,500 photons/sec sr extrusion rate: 0.8 m/min.

Thickness resolution with a counting period of 10 sec. ,um; referred to a profile thickness of 10 mm., this corresponds to a resolution of 1%.

Claims

1. A method for the continuous measurement of the thickness of extruded synthetic resin profile sections by determining the degree of absorption of gamma rays upon penetration through the extruded synthetic resin profile sections, wherein, for a weight-thickness range of from 4,000 to 14,000 g/m2, there are used X-rays with a photon energy of from 20 to 40 keV as gamma rays.

2. A method according to claim 1, wherein the X-rays emitted by the radiation source are focussed and impinge vertically on the profile section.

3. A method according to claim 1 or 2, wherein the photons which come into contact with surfaces which are not at right angles to the photon beam are masked.

4. A method according to any of the preceding claims, wherein the profile is guided in the region of the photon source and of a counter (receiver) in a de ice which is secured to the photon source and the counter in such a manner that, when the profile section is moving vertically with respect to the extrusion apparatus, the whole measuring equipment moves with it, measuring errors thus being avoided.

5. A method according to any of the preceding claims, wherein the photons transmitted through the profile section are picked up in an energy-dispersive manner.

6. A method according to claim 1 for the continuous measurement of the thickness of extruded synthetic resin profile sections, substantially as hereinbefore described and exemplified.

7. A device for carrying out the method according to claim 1, comprising a screened radiation source and a counter, the radiation source emitting X-rays having a photon energy of from 20 to 40 keV, said X-rays being generated by a radionuclide, acting as primary emitter, by means of a target from an element having an atomic number of from 40 to 60.

8. A device according to claim 7, wherein the primary source is 10 mCi Americium 241 enclosed in a ceramic jacket.

9. A device according to claim 7 or 8, wherein different targets are spaced out side by side on a rotating disc disposed below the primary source.

10. A device according to any of claims 7 to 9, wherein the target is a barium, silver, molybdenum, rubidium, terbium or copper target.

11. A device according to any of claims 7 to 10, wherein it is provided with a diaphragm for limiting the solid angle of the photons emitted.

12. A device according to any of claims 7 to 11, wherein it is provided with a diaphragm which limits the entry of the photon beam into the counter.

13. A device according to any of claims 7 to 12, wherein the counter is an energy-proportional counter capable of separating the information-carrying photons from the background.

14. A device according to claim 13, wherein the energy-proportional counter is a sodium iodide counter.

1 5. A device according to any of claims 7 to 14, wherein the radiation source and the counter are secured to a support device through which the profile section passes during the measurement.

1 6. A device according to any of claims 7 to 15, wherein it is adapted to convert pulses arriving in the counter in a converter into a direct current which is proportional to the incident radiation and which drives an indicator, a recorder and/or a process control system.

1 7. A device for carrying out the method according to claim 1, substantially as hereinbefore described and exemplified and with reference to Fig. 2 of the accompanying drawings.