WO 98/21568, for instance, describes apparatus for inspecting and mark- ing defects in sheet material, such as a strip of polythene. The entry side of equip- ment of this type is provided with at least one roll onto which a plastic compound is extruded and stretched to a strip, tape or film by rotation of the roll. Alternatively double rolls may be placed at the entry side, between which the strip, tape or film passes. Additional rolls may also be provided, mainly to ensure uniform thickness across the whole width of the strip, film or tape.

The temperature of the roll at the entry side of the equipment must be controlled, normally cooled, in order to control the temperature of the strip, tape or film, and thus its thickness and tension. The temperature of the chill roll should normally be in the range of 50-60°C, sometimes approaching 100°C or even slightly above this temperature. By way of example, it may be mentioned that polypropylene has an extrusion temperature of typically 300°C and cooling is re- quired to reduce the temperature to below 100°C. In many cases it is important that the temperature of the material to be inspected is lowered below the"frost line", at which the plastic resembles frosted glass, so that the plastic material once again becomes transparent. These rolls have previously been filled with wa- ter and comprise a cylinder of steel rotating about central shafts in its ends, at least one shaft being hollow for the supply of coolant. Special precautions must therefore be taken to ensure that the roll is water-tight and to avoid leakage, mak- ing the design of the chill roll relatively complicated and expensive. The roll is also locally heated by the tape or strip which often covers only a part of the roll width and, since steel is a poor conductor of heat, this results in uneven temperature distribution in the roll.

The object of the present invention is to provide a greatly simplified chill roll apparatus which also enables quicker and more exact temperature control.

This object is achieved with a chill roll apparatus of the type described in the introduction and having the characterizing features defined in claim 1.

A general impression in technical circles has been that air cooling is not satisfactory for the present application. However, the present invention has sur-

prisingly shown that an air-cooled embodiment is an extremely advantageous so- lution. Practical experiments have even shown that air-cooling can sometimes lower the temperature of the roll unnecessarily much. The design of the chill roll is simplified by means of the chill roll apparatus in accordance with the invention, the problem of water leakage is totally eliminated and the possibility of rapid and ex- act temperature control is improved.

In accordance with advantageous embodiments of the chill roll apparatus according to the invention, the roll is designed to permit air to be blown through it in the direction of its axis. For this purpose the roll may be provided with a plurality of holes running through it in axial direction and at least one fan may be arranged to blow air into the holes at the end wall of the roll. A simple and robust chill roll apparatus is thus obtained, by means of which effective cooling is achieved.

In accordance with other advantageous embodiments of the chill roll ap- paratus according to the invention, at least one heating element is arranged to heat the roll. This heating element may advantageously comprise an IR heater ar- ranged to heat one end wall of the roll. During operation the chill roll is normally heated by the extruded tape or film which usually has a temperature in the range of 100-200°C. However, under certain circumstances this heating can be too slow and special heating elements may be necessary. The use of such a heating ele- ment is mainly useful for quickly increasing the roll temperature from"cold"chill roll to the desired operating temperature. Since it is possible to both cool and heat the roll in an arrangement in accordance with the invention, rapid and efficient control of its temperature can thus be achieved.

In accordance with another advantageous embodiment of the chill roll ap- paratus according to the invention, the roll is made of copper. Since this material is an extremely good thermal conductor, uniform temperature distribution is en- sured throughout the roll and the above-mentioned problem of uneven tempera- ture distribution encountered with the steel rolls used previously is eliminated.

In accordance with other advantageous embodiments of the chill roll ap- paratus according to the invention, at least one temperature sensor is arranged to measure the temperature of the roll. The temperature sensor preferably compris- es an IR temperature sensor which is arranged to measure the temperature at the outer edge of the roll without coming into contact therewith. Means are also pro- vided to enable the temperature of the roll to be read. This allows the actual tem-

perature of the roll to be continuously monitored by the operator as well. In pre- viously known liquid-cooled chill rolls of the type in question the temperature of the coolant is measured in a storage container that in fact does not give much in- formation as to the actual temperature of the roll.

In accordance with other advantageous embodiments of the chill roll ap- paratus according to the invention a temperature control unit is arranged to control the heating element and/or fan depending on the actual value of the roll tempera- ture measured by the temperature sensor and a predetermined set point for the temperature supplied to the control unit. The fan may be constructed for pulse operation and the temperature control unit may be arranged to control the cooling by varying the length of the pulses during which the fan is in operation. The tem- perature control unit may be arranged to control the speed of the fan. The appara- tus may also comprise several cooling fans and/or several heating elements con- trolled individually by the temperature control unit. This enables automatic, rapid and exact temperature control of the chill roll. Equivalent control of a water-cooled roll is considerably slower and more complicated.

The invention also relates to equipment for continuous inspection of sheet plastic material, comprising a detecting unit for detecting defects in the material, wherein a chill roll apparatus in accordance with the invention is arranged at the entrance of the equipment, to receive the material supplied to the equipment for inspection. In accordance with an advantageous embodiment of such equipment the chill roll is arranged to be driven by a geared-down servo-motor via belt driv- ing. As mentioned above, plastic compound is extruded or cast on a chill roll to form tape, strip or film. In known equipment of this type the chill rolls are driven by means of chain drive from a motor. Variations in the angular velocity of the rolls, as a result of the stepwise engagement of the gears in the chain often leads to an unsharp, blurred picture in the subsequent inspection equipment, which disturbs the inspection. The above design of the invention in accordance with the inven- tion, with belt driving from a greatly geared-down servo-motor which runs extreme- ly smoothly, no variations in the angular velocity of the chill roll occur and uniform and correct speed and tensioning in the material are always ensured for the fol- lowing inspection.

In order to explain the invention in more detail selected embodiments of the chill roll apparatus in accordance with the invention will be described more

fully with reference to the accompanying drawings, in which Figure 1 illustrates the principle structure of one example of the inspection equipment for sheet material in which the chill roll apparatus in ac- cordance with the invention is intended to be used, Figure 2 shows an end view, Figure 3 a longitudinal view of the chill roll for one embodiment of the appara- tus in accordance with the invention, and Figs. 4 and 5show examples of electric circuits for controlling the temperature of the chill roll.

Figure 1 illustrates the principle structure of one example of inspection equipment for sheet plastic material of the type for which the chill roll apparatus according to the invention is intended. Chill roll apparatus in accordance with the invention, schematically indicated at 1, is arranged at the entry side of the inspec- tion equipment, onto which roll an almost fluid plastic compound, e. g. polythene compound, shall be extruded for stretching to a tape or film by rotation of the roll.

2 indicates a test chamber through which the sheet tape or film shall pass for detection of any defects in the material. The actual inspection equipment in- side the test chamber suitably comprises a light source for illuminating the mate- rial and a detector for detecting defects in the sheet material from spread light transmitted through the material and/or reflected by the material. The detector may advantageously comprise a linear camera.

The inspection equipment also comprises a thickness gauge 5 for the sheet material, and a gauge 3 for measuring the tension in the material. The vol- ume and/or mass of the material being inspected can be determined from the val- ues presented by the thickness gauge. The values from the thickness gauge 5 can also be used to control the speed of rotation of the chill roll, and thus the thickness of the tape or film. It is important for correct tension to be maintained in the tape or film so that it will pass through the inspection equipment in the same, reproducible way all the time. If the tension in the tape or film is too low there is a risk of it passing outside the focus of the optical parts of the inspection equipment.

If the tension is too high, there is a risk of the tape or film starting to vibrate. The chill roll is preferably driven via belt drive from a greatly geared-down servo-motor which runs extremely evenly and continuously gives correct, uniform material speed and correct, uniform tension in the material in the subsequently inspection

equipment. A length gauge 4 is provided to continuously measure the length of the sheet material being inspected in order, for instance, to calculate the running speed of the material, synchronise the detector (linear camera) to the sheet mate- rial, localise detected defects for marking with a label, for instance, or cutting away defective parts. A storage unit 6 is also shown, onto which inspected mate- rial is wound. Figure 1 also shows a PC 7 which acts as operator interface, and a printer 8 for printing out inspection reports.

Figure 2 shows one end of an embodiment of a chill roll 10 shown by way of example in the apparatus in accordance with the invention. The chill roll 10 consists of a solid metal cylinder, suitably of copper by virtue of its good thermal conductivity, through which a plurality of axially running holes 12 are drilled.

The chill roll 10 is fitted on an attachment plate 14 forming a part of the machine stand of the inspection equipment, see fig. 3. This figure also illustrates support of the roll 10 comprising bearing support 16 and belt wheel 18. A washer is shown at 36 for screwing the roll 10 at its axis.

Two cooling fans 20 are also attached to the plate 14 in front of holes 22 in the plate 14, see fig. 2, for blowing cooling air towards the end surface of the chill roll 10 and through its channels 12.

Heating elements in the form of flat IR heaters 24 are also secured in front of openings 26 provided in the attachment plate 14 and fitting the IR heaters 24, see fig. 2, for contactless heating of the end wall of the chill roll 10. Both the cooling fans 20 and the heating elements 24 in the embodiment shown are screwed firmly to the attachment plate 14. Corresponding screw holes 28 and 30, respectively, are shown in fig. 2.

A temperature sensor 32, suitably an IR temperature sensor, is further- more arranged in an aperture 34 through the attachment plate 14 in order to measure the temperature of the chill roll 10 at its outer edge without coming into contact therewith.

One advantage with an air-cooled roll is that the tape or film can be caused to move at a very uniform speed through the inspection equipment, which is most important for ensuring the requisite good correlation between tape or film position and camera. It is therefore advantageous to integrate the chill roll in the actual inspection equipment and make sure that the tape is always suitably ten- sioned, as discussed above.

Figure 4 shows an electric circuit diagram for automatic control of the temperature of the chill roll. A temperature control unit 38 is connected via fuses 40 to the mains supply 42. A set value, e. g. 70°, is supplied from a computer, for instance, via the cable 01019 to the control unit 38. U17 represents a temperature sensor, e. g. an IR temperature sensor as described above, from which the actual value of the roll temperature is supplied to the control unit 38. The actual value measures can be transmitted via the cable 10126 to the computer (PC), enabling the operator to read the temperature value. The temperature read may also be entered into reports concerning inspections performed.

A semiconductor breaker or relay d4 is connected to the control unit 38 at 1 C which, when activated, influences a breaker 44 so that the heating element U19, e. g. an IR heater as described above, is connected to the network 42 via fuses 46.

A second semiconductor breaker or relay d2 is connected to the control unit 38 at 1A. Upon activation of the breaker or relay d2 the fans U27, U28 and U29, shown in fig. 5, are connected. In the embodiment shown the fans are de- signed for pulsing operation, the cooling being controlled by variation, from the control unit, of the pulse lengths during which the fans are in operation. The cool- ing is thus controlled by the fans operating for longer or shorter periods.

Many alternative control methods are naturally possibly. The fans could thus be individually controlled, and the number of fans connected may be varied as well as their speed, etc. Naturally several individually controlled heating ele- ments may also be used to achieve effective temperature control. The embodi- ment shown in fig. s 2 and 3 comprises two cooling fans and two heating ele- ments, and the embodiment in fig. 5 shows three cooling fans.