Abstract:
An apparatus for testing the wall thickness of a moving tube of plastics or other electrically non-conducting material being produced by an extruder, comprises an electrically conductive measurement body arranged in the moving tube, a ferromagnetic holding member which is arranged in the tube within effective range of a holding magnet located outside the tube and which has a tie connection to the measurement body to locate the measurement body axially within the effective range of a measuring sensor located outside the tube. The holding member and the measurement body may be introduced into the tube through a channel in the extruder head, shot into the tube from a forward position, or inserted through an aperture cut in the tube wall.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for testing the wall thickness of a tube of electrically non-conducting material produced by an extruder. 
     The apparatus according to the invention comprises an electrically conductive measurement body held within the moving tube and a measuring sensor situated outside the tube within effective range of the measurement body, and arrangements of this general type are described in, for example, Swiss Pat. No. 563,567. In most cases, the measurement body is located within the tube by attachment through some kind of rod to the mandrel of the extrusion nozzle, and thereby held in a predetermined axial position. This construction has the disadvantage that the measurement body must be fixed to the nozzle of the extruder before the latter is brought into operation, which complicates the operation because the measurement body is then immediately in the path of movement of the leading end of the tube issuing from the nozzle. 
     It is also known to introduce the measurement body freely into the tube to be measured and, by means of a magnet, to hold the measurement body in the desired axial position and against the wall of the tube. However, in this case the measurement body consists of a single rigid component which introduces the risk that, under the magnetic attraction which influences the measurement body from the side where the measuring sensor is situated, the measurement body will, on encountering irregularities or curvatures in the tube, bear tightly against the tube wall in the region of the holding magnet but have a certain amount of clearance from the tube wall in the region of the measuring sensor. 
     It is an object of the present invention to allow the measurement body to be held within the tube so that it will always be reliably seated against the inside of the tube, and to allow the introduction of the measurement body into the tube simply and without disturbing the production process. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an apparatus for testing the wall thickness of a moving tube of electrically non-conducting material produced by an extruder, said apparatus comprising a measurement body arranged in the moving tube, a ferromagnetic holding member which is arranged in the tube within effective range of a holding magnet located outside the tube and which has a tie connection to the measurement body to locate the measurement body axially within the effective range of a measuring sensor located outside the tube. 
     In the apparatus according to the present invention, the forces which determine the axial positioning of the measurement body in the tube are separated from the measurement body itself and cannot in any case act upon the latter, so that a constant close fit of the measurement body against the inside of the tube is ensured. The connection between the holding member and the measurement body may be effected by means of a flexible member such as a wire or cord. In such a case the measurement body can be held against the tube wall either by gravitational force or else, in a known manner, under the action of a second holding magnet. The division of the arrangement into a measurement body and a holding member, which can be flexibly connected together, means that these are comparatively small individual parts which can be correspondingly more easily handled. In particular, they can be introduced in a simple manner either from the rear side of the extruder in the direction of the delivery of the tube, or else, and usually preferably, they can be introduced from a forward position entering through the leading end of the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic elevation showing tube producing plant incorporating apparatus according to the invention; 
     FIGS. 2 and 3 are enlarged detail views of parts of the plant shown in FIG. 1; 
     FIGS. 4 to 6 are diagrammatic views illustrating one method of introducing a measurement body and holding member into the tube; 
     FIGS. 7 and 8 are diagrammatic views illustrating another method of introducing the measurement body and holding member into the tube; and, 
     FIGS. 9 and 10 show, diagrammatically, a device for the automatic introduction of the measurement body and the holding member. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, FIG. 1 shows an extruder 1, a cooling device 2, a switching device 3 and a reeling station 4 in a plant for producing synthetic plastics tube 5. The switching device 3 cuts off the tube automatically when a reel is full, and feeds the leading end of the tube onto an empty adjacent reel so as to ensure continuous production. 
     Between the cooling device 2 and the switching device 3, there is situated the testing apparatus according to the invention, which is also shown diagrammatically and which includes a measuring device 6 standing upon the floor and having a support 7 for a measuring head 8. At a predetermined distance before the measuring head, measured in the direction of travel of the tube being produced, there is provided a holding magnet 9a, the pole shoes of which are diagrammatically represented as rectangles. FIG. 3 shows the construction of the measuring head 8 which comprises a permanent magnet 9 of the U or pot type, and an electromagnetic measuring sensor system 10, of a known type, located within said magnet. In the operating condition, the permanent magnet 9 holds a measurement body 11 of ferromagnetic conducting material in the region of the measuring system 10 in close contact with the inner wall of the tube 5; in this arrangement, a particularly snug seating of the measurement body is achieved in the region of the measuring system 10 by virtue of the magnetic attraction operating at both sides of the measuring system. As shown in FIGS. 1 and 2, the extruder 1 is provided with an extruder head 12 to which the plastics material is fed through a side conduit 13 in such a manner that an open-ended channel 14 is formed in the extruder head 12 and leads into the centre of the extruder nozzle. As indicated in FIG. 2, the measurement body 11 can be introduced through the channel 14 into the tube 5. 
     FIG. 3 shows the operating condition, in which the measurement body 11 is situated in the region of the measuring head 8. The measurement body 11 has a flexible connection, by means of a cord 16, to a holding member 17 of ferromagnetic material. The holding member 17 is secured in its axial position by the holding magnet 9a which is preferred to be an electromagnet fed with alternating current so that the consequent vibration will minimise friction of contact between the holding member 17 and the inner face of the tube. The electromagnet 9a is controlled by means of a switch 18. 
     As already indicated, the measurement body 11 and the holding member 17 are introduced through the channel 14 of the extruder head 12 into the tube at a suitable instant during operation or while setting the plant in operation, and the measurement body and the holding member are then carried along by the tube 5 moving in the direction from the right to the left as depicted in FIG. 1. When the body 11 and the member 17 reach the positions shown in FIGS. 1 and 3, the magnet 18 should be energised to retain the holding member in the position shown in FIG. 3. The correct positioning of the parts 11 and 17 can be ensured by means of appropriate measuring sensors. It is however also possible, by means of the measuring system 10, to detect a change in the measuring conditions caused by the first effective entrance of the measurement body 11, and at that instant to switch the electromagnet 9a into circuit. At that time, the holding member 17 will in fact be situated within the effective range of influence of the magnet 9a, and will then be drawn into and retained in the desired position. As a result, the measurement body 11 will be attracted against the inner wall of the tube by the permanent magnet 9 situated in the region of the measuring system 10, the measurement body thus allowing a correct measurement of the wall thickness in a manner known per se. 
     In many cases, the extruder is not provided, as it is in FIG. 2, with a channel 14 in the head 12, in which case the measurement body and the holding member cannot be introduced from a forward position through the leading end of the tube 5. Such a possibility is illustrated in FIGS. 4 to 6 where an impelling device 20, shown diagrammatically, is provided to project the holding member 17 and the measurement body 11 from a forward position into the tube 5 by releasing a compressed spring. The launching velocity of both the parts must be so chosen that the holding member 17 will reach at least the area of influence of the holding magnet 9a, and the measurement body 11 comes within the area of influence of the magnet 9 and the measuring system 10. As shown in FIG. 4, the measuring head 8, together with the magnet 9, is lifted away from the tube when shooting in the parts 11 and 17, so that the magnet 9 exerts practically no retarding influence upon the parts 11 and 17 entering the tube. Between the measuring head 8 and the holding magnet 9a there is positioned a detecting sensor 21, for example an inductive sensor, which releases a pulse through a suitable measuring circuit whenever one of the parts 11 or 17 passes its position. 
     As FIG. 4 illustrates, the launching velocity of the measurement body 11 and the holding member 17 is so selected that both of these members shoot past the sensor 21. As a result, two pulses are released. The parts 11 and 17 then stop and rest on the tube so that they are carried along as indicated in FIG. 5, whilst the magnet 9a still remains switched out of circuit. The measurement body 11 soon reaches the area of influence of the sensor 21, which releases a further pulse. At the end of this pulse, when the body 11 has reached the position shown in FIG. 6 which is in the effective region of the measuring head 8, the switch 18 is closed and thereby the electromagnet 9a is energised. Thus, the holding member 17 is held in the desired position, and the measuring head 8 can then be moved towards the tube, whereby the magnet 9 of the measuring head attracts the measurement body 11 in the described manner against the inner wall of the tube. Instead of using an electromagnet for this holding purpose it is also possible to employ a permanent magnet which would be moved towards the tube at the appropriate time. 
     FIGS. 7 and 8 show, diagrammatically, a further possible means of introducing the holding member 17 and the measurement body 11 into the tube 5 from a forward position. In this case, an accessory device includes a horseshoe magnet 22 which retains the holding member 17 in the tube. It is therefore only necessary to introduce the holding member 17 into the tube from a forward position, and to retain the member in a suitable position by the external application of the magnet 22. The measurement body 11 is then also introduced into the tube, and the magnet 22 is moved along the tube 5 from a forward position until the holding member 17 comes into the range of the electromagnet 9a. The electromagnet is then switched into circuit, and the measuring head 8 is brought close to the tube as shown in FIG. 8, whereupon the horseshoe magnet 22 can be withdrawn. The holding member 17 is now secured in the area of the magnet 9a, and ensures that the measurement body 11 is secured in the desired position in the effective region of the measuring system 10 and the measuring head 8. 
     Previously, it has been assumed that the measuring head 8 must be provided with a magnet 9 in order to attract the measurement body against the tube wall. This may be necessary if the wall thickness of the tube is to be measured all the way round, in which case the measuring head 8 is rotated about the tube to carry the measurement body with it. It may, however, be sufficient if the wall thickness is determined along one axial section of the tube, in which case a measuring head 8, without a magnet 9, can be applied to the tube from below, whilst the measurement body 11 lies upon the tube wall under gravitational force. 
     The flexible connection between the holding member 17 and the measurement body 11 may be effected in an alternative manner. In particular, if these two parts are to be introduced into the tube from a forward position by means of an impelling device or an auxiliary magnet, it may be of advantage to provide between them a somewhat rigid, but still movable connection. Such a connection may comprise, for example, a helical spring or a leaf spring, or an elastic tie, and may facilitate the introduction of these parts as described with reference to FIGS. 7 and 8. 
     The auxiliary device including the magnet 22 according to FIGS. 7 and 8 may be provided with guide means to facilitate its manipulation. It is possible, for example, to provide a fork engaging around the tube to prevent the magnet from deflecting sideways when being pressed against the tube. 
     A fully automatic auxiliary device for introducing the measurement body and the holding member into the tube is shown diagrammatically in FIGS. 9 and 10. This device is shown in two working positions, and comprises a frame 30 having a guide ring 31 through which the tube 5 travels to the right as shown in the drawings, in the direction of the arrow. At the one end face of the ring 31, there is mounted a knife 32 which can be driven downwardly by a piston and cylinder device 33. On a support 34, which can be lifted upwardly by means of a piston and cylinder device 35, there are secured two permanent magnets 36 and 37 upon which are secured brushes 38 and 39. The measurement body 11 and the holding member 17 can be placed on these brushes where they are held by the fields of the magnets 36 and 37. Above the brush 39, in front of and behind the tube as seen in plan view, there are arranged holding electromagnets 9a which can be energised in the described manner. Upon a support 40, which is arranged to swing downwardly to the right, there is mounted a support roller 41 for the tube 5. The frame structure 30 carrying the parts mounted upon it is movable in the direction of travel of the tube 5. 
     The mode of operation of the auxiliary device is as follows: 
     At the start of an operation, the newly-formed tube 5 travels out of the extruder 1 and the cooling device, and passes through the rings 31 and over the support roller 41 as shown in FIG. 9. The first operation leading to the introduction of the measurement body 11 and the holding member 17, is to sever the tube in front of the ring 31 by rapid downward operation of the knife 32, whilst the device together with the tube proceeds towards the right. The knife is immediately retracted and, at the same time, the support 34 is raised by means of the piston and cylinder 35; as shown in FIG. 10, this causes the support roller 41 to raise the severed forward end of the tube which is removed from the normal path of movement of the tube. At the same time, the brushes 38 and 39 are raised to such an extent that the measurement body 11 and the holding member 17 are positioned in front of the new leading end of the tube, which then passes through the ring 31 as soon as the frame 30 no longer moves together with the tube to the right. The leading end of the travelling tube now passes over the brushes 38 and 39, whereby the holding member 17 and the measurement body 11 successively enter the tube, because they are prevented by the magnets 36 and 37 from moving along with the tube. The leading end of the moving tube then impinges against the support roller 41 and swings the latter downwardly so that the tube can continue to travel. The support 34 is now lowered to return to the rest position shown in FIG. 9, whereby the retaining influence of the magnets 36 and 37 upon the parts 17 and 11 is removed and these parts then travel along with the tube 5 to the right. At a suitable instant, the holding magnets 9a are switched into circuit in order to retain the holding member 17 in position within the tube. The rest position of the auxiliary device is so selected that now the measurement body 11 is situated at the correct position within range of the measuring device, which is not shown in FIGS. 9 and 10. 
     The auxiliary device illustrated in FIGS. 9 and 10 permits an automatic introduction of the measurement body and holding member into the tube, virtually irrespective of the condition and shape of the leading end of the tube, which often is not so open and clean as to allow the measurement body and the holding member to be introduced through that end. Due to the support afforded to the tube in the guide ring 31, the cross-section of the tube remains substantially unchanged when cutting, so that the measurement body and the holding member can be received therein without particular difficulty. 
     If an extruder according to FIG. 2 is provided, there exists the possibility of providing only a measurement body 11 and of anchoring this to the extruder by means of a cord. Such an anchoring cord F is indicated in FIG. 1. In this case, the holding member (17) and the holding magnet (9a) can be omitted. By suitable choice of the length of the cord, the position of the measurement body in the tube can be determined sufficiently accurately, especially if the measuring point is not too far removed from the extruder. 
     In certain cases a measurement body (11) could be provided, which, by means of a holding magnet associated with it, is attracted to make intimate contact with the inside of the tube and is ensured against axial displacement out of the range of the measurement device. 
     A further possible method for introducing the measurement body and/or the holding member, which is particularly applicable in the case of large and relatively slowly travelling tubes, consists in cutting along the travelling tube from above with a knife or a milling tool, and allowing the measurement body and/or the holding member to fall into the slot which is made behind the knife or milling tool. The holding magnet, or the holding magnets, for the measurement body and/or the holding member may be directly situated at the place or places, at which said members descend, or else these parts may be carried along by the tube to be then be held in the region of the holding magnets in a manner already described with reference to FIGS. 1 to 3. In this case also, it is possible to provide an automatic device for introducing the measurement body and/or the holding member, said device having, for example, a milling tool automatically adjustable from above with reference to the tube, behind which milling tool there is provided a mechanical or magnetic holding device for the measurement body and/or the holding member, which allows these parts to fall as soon as a sufficiently long slot is formed in the upper wall of the tube. 
     When, for measuring the wall thickness at various peripheral points of the tube, the holding magnet for the measurement body is moved around the tube, it may also be necessary, particularly when measuring large tubes, to rotate the holding magnet for the holding member likewise around the tube, in order that the measurement body and the holding member shall always be situated in approximate axial alignment. 
     Between the measurement body and the holding member there may be provided a common guide tube, for example surrounding the wire connection between them. By arranging that this common guide tube is of smaller external diameter than the measurement body and the holding member, the desired relative positioning upon introducing the parts into the tube can be ensured. 
     As a propellant for shooting the measurement body 11 and the holding member 17 into the tube from a forward position it is also possible to employ a compressed gas, for example nitrous oxide.