GB2243930A

GB2243930A - Heating control means

Info

Publication number: GB2243930A
Application number: GB9105952A
Authority: GB
Inventors: Gerald Buttery
Original assignee: CEETAK DEVELOPMENTS Ltd; CEETAK HOLDINGS Ltd
Current assignee: CEETAK DEVELOPMENTS Ltd; CEETAK HOLDINGS Ltd
Priority date: 1990-03-22
Filing date: 1991-03-21
Publication date: 1991-11-13
Also published as: GB9006427D0; GB9105952D0

Abstract

Controlling the temperature of an electrical resistance heating element for plastics packaging film comprises the steps of supplying electrical current to the element in a series of square wave D.C. pulses, monitoring the temperature of the element or of a member to be heated thereby and reducing, in step-wise fashion, the frequency and/or the duration of the current pulses successively from an initial value which is employed when the temperature of the element or of the member to be heated thereby is low to a second lower pulse frequency and/or duration as the temperature approaches a required level and preferably to a third even lower pulse frequency and/or duration when the temperature reaches the final required level. The heating element has an input lead secured to its mid-position and a pair of output leads connected to respective ends. This said to reduce stress due to differential expansion and contraction. <IMAGE>

Description

TITLE: HEATING CONTROL MEANS DESCRIPTION The invention relates to heating means and more particularly to means for controlling heaters for use in apparatus for heat sealing plastics packaging film.

It is known to seal or join flexible plastics packaging material by welding using heat supplied for example by hot air or by heated metal dies. It is also known when heat sealing plastics film to use heated dies faced with thermally conductive silicone rubber which are able to conform more closely to the shape of the article to be heat sealed than is the case with rigid dies.

Although rubber faced dies are therefore desirable they are not able to withstand high temperatures without deterioration and thus their heating must be closely controlled. Although, unlike metal dies, rubber faced dies can be made to have the desirable characteristic of low thermal inertia, the known method of heating such rubber faced dies nevertheless tends to allow premature failure of the bond between the rubber and the electrical heating element thus creating undesirable temperature gradients along the die so that the efficiency of the heat seal is impaired.

It is an object of the invention to provide a method of controlling the temperature of a heating element, which can for example be embedded in a flexible or resilient heat sealing die for use in welding flexible plastics packaging material.

According to the invention a method of controlling the temperature of an electrical resistance heating element comprises the steps of supplying electrical current to the element in a series of square wave D.C.

pulses, monitoring the temperature of the element or of a member to be heated thereby and reducing, in step-wise fashion, the frequency or duration of the current pulses as the temperature rises.

If desired the pulse frequency can be reduced from an initial value to a final lower value when the temperature of the element or of the member to be heated thereby reaches a given level. Preferably however the arrangement is such that the pulse frequency is reduced successively from an initial value which is employed when the temperature of the element or of the member to be heated thereby is low to a second lower pulse frequency as the temperature approaches a desired level and to a final even lower pulse frequency when the temperature reaches the desired level, whereby the initial heating rate is rapid when the temperature is low and is slowed as the temperature approaches the desired level to avoid overshooting the desired level and whereby the final pulse frequency is applied when the temperature is at the desired level to assist in maintaining the temperature at the desired level.

The duration of the pulses may be of the order of 10 milliseconds and the pulse frequency may be in the range of 3 to 12 per second depending on the temperature.

Where the heating element is of significant length it is preferred that the element is divided into a series of sections each of which can be pulsed successively with current e.g. in cascade. By this method a heating element and particularly a heating member consisting of an electrical resistance heating element embedded in a flexible sheath of silicone rubber or the like can be heated to a desired temperature and maintained at that temperature with less risk of causing local hot spots which might result in the deterioration of the bond between the rubber and the element.

The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which: Figure 1 is a block circuit diagram of a temperature control device; Figure 2 is a circuit diagram of a pulse control gate, and Figure 3 is a sectional view of a silicone rubber faced heated die for a heat sealing machine.

There now follows a description of a method of controlling the temperature of an electrical resistance heating element used to heat thermal conductive flexible materials such as glass fibre, or silicone rubber, and such, for example, as P.T.F.E. coated glass fibre tape, particularly when such materials are used to form flexible heated dies for the welding of flexible films of plastics material, e.g. polyethylene film, as used in the packaging industry. The method therefore must be capable of generating heat between a range of about 1000C and 2200C, of maintaining an even temperature throughout the length of the die and of ensuring that any desired temperature is maintained within close tolerances of, say, + 20C.

The die may consist of an electrical resistance element chemically bonded between two layers of a flexible material such as thermally conductive silicone rubber, the arrangement being such that current can be passed through the element at such a rate that it will heat the layers of material. It is however important that the heat generated within the heating element does not reach a temperature which would be detrimental to the chemical bonding between the heating element and the flexible material. In the case of silicone rubber this critical temperature is around 27O0C. If the temperature is exceeded there will be a breakdown of the chemical bond between the resistance element and the silicone rubber material causing cold spots to occur at the point of the breakdown due to the lack of heat transfer from the element to the flexible material.The heating circuit described below takes this problem into account by reducing the current as the temperature rises towards the critical temperature of this bonding.

Figure 1 of the drawings shows a circuit in block diagram form which meets the welding conditions required for flexible packaging material which requires that the circuit should pass current through the heating element to enable it to heat up the die quickly, to lose heat quickly on command and to hold a temperature which has been pre-selected by the operator.

The circuit can be described as a power frequency switching unit where the power (e.g. 24 v D.C.) is drawn from a transformer through the heating element and through a switching element, the frequency of the switching element being controlled by the circuit. It can be seen that there are three main frequency controls, one being designated as - 100 C, the second as + 1000C, and the third one 1100C.

The operation of the circuit is as follows. First the required temperature of the heated dies is selected by the operator who adjusts a readout on a control panel.

This sends to a temperature control gate a signal proportional to the required temperature. A temperature transducer is attached ,to the heating element and feeds back into the temperature control gate the temperature existing at the heating element. The temperature control gate then selects which frequency it will allow through to a power switch. If the temperature is below 1000C, high frequency switching can take place without the danger of overheating the resistance element and creating breakdown of the bonding between the resistance element and the material being heated. The temperature control gate would thus select this high frequency pulse generator until the temperature rises to 1000C. At this point the temperature control gate would switch off the high frequency unit and switch to the + 1000C pulse generator.This frequency is lower and therefore will only allow the temperature of the heating element to rise at a reduced rate so that surface temperature of the element does not exceed 270 C. On reaching the temperature which the operator has set, the unit then switches to the third pulse generator unit which is calibrated to pass a frequency that will only allow the heating element to rise to 1100C.

To give an example of this, assume that the operator has selected a temperature of 1500C and the heating system has been set in operation. The circuit would therefore behave in the following way. First the high frequency unit would be switched on to raise the heating element to 1000C rapidly. At this point the intermediate frequency would be switched on and the high frequency switched off to allow the temperature to continue rising to 1500C at which point the circuit would switch to the lowest frequency thus allowing the temperature to start falling slightly from 1500C but still maintaining current at low frequency going through the element to maintain the temperature required.As soon as the temperature drops below the 1500 preselected temperature the unit would automatically switch back to the intermediate frequency of + 1000C raising the temperature again.

Therefore by continually switching between the + 100C frequency and the 1100 frequency, an overall frequency has been sent to the switching unit which will hold the heating element at the desired temperature.

The reason for the lowest frequency being set to operate at 1100C is in view of the fact that one of the most common failures in packaging machinery, much to the embarrassment to the user, is failure of temperature control. In a system in which this feedback temperature control is an on and off system, i.e. either there is current flowing through the heaters or no current flowing through the heaters, a situation can develop where if there is a break in the feedback line the temperature drops at the weld point below a working level and the result is that products are processed without any welding taking place. Using the circuit of the present invention it can be seen that if the failure of feedback occurs the switch off from the selected temperature is not to no current at all but to a current which would generate 1100C at the weld point.This would mean that in spite of the failure in the system, alarms would be raised in the normal way, the products would still be welded at a temperature which in a majority of cases would be sufficient to create a seal, albeit of reduced strength.

The block diagram circuit configuration of Figure 1 comprises six sections as follows: 1. Temperature selection 2. Temperature transducer 3. Pulse generators 4. Pulse control gate 5. Pulse shapers 6. Power switching of thermal current Section 1 transfers voltage into digital readout.

Its purpose is to enable the operator to select the operating temperature and to indicate, after the selection has been made, the temperature which has been achieved by the device. The following facilities are available to the operator: (1) a selection switch having a temperature selection mode and a mode which reads the temperature at the heating element, and (2) an adjustment control by which the readout temperature can be varied once the selection switch has been switched to the selection mode.

The readout represents millivolts and each millivolt is equivalent to one degree Centigrade i.e. a readout of 120 would mean that the device reading 120 millivolts on input. The input to the readout unit is either from the temperature transducer or records a voltage which has been sent to the pulse control gate.

Section 2 is a temperature transducer which has been bonded to the silicone rubber or flexible material surrounding the heating element. The transducer comprises an element the resistance of which changes according to the temperature applied to the face of the transducer. This is fed into an operational amplifier which translates that resistance change into a one millivolt-per-degree Centigrade change in temperature.

Therefore the voltage generated by the operational amplifier is directly proportional to the resistance change in the transducer per degree Centigrade. The output of this transducer is passed to the pulse gate control unit.

Section 3 of the circuit consists of three pulse generators in the form of voltage control oscillators, the frequencies of which are preset to meet the design requirements of the circuit. The first frequency generator marked on the logic diagram - 1000 has a high frequency output, of the order of 12 pulses per second.

This is to enable quick heating up of the heating element when the operational conditions are such that the unit is being heated up from start temperature e.g. ambient temperature. The second voltage control unit has a frequency which is adjusted so that as the heater approaches the critical temperature of 2700C, no over shooting will take place. This is marked on the diagram as + 1000. The pulse range of this generator may be of the order of 8 to 9 pulses per second. The third frequency unit marked 1200 acts to top up the heater when the selected temperature has been reached. The pulse range of the third generator may be of the order of 3 to 4 pulses per second.

These three pulse generators pass the correct amount of thermal current through the heating element according to its temperature i.e. the faster the frequency the more current that will be passed. Preferably the thermal current is pulsed in 10 millisecond square wave pulses, but the pulses can be in the range 5 to 25 milliseconds in duration.

Section 4 is a pulse control gate which selects, according to the temperature sensed by the temperature transducer, which of the pulse generators should be applied to the pulse shaper and so control the pulse switching through the heating element. The unit consists of two operational amplifiers and a gating system which is shown in more detail in Figure 2. The first operational amplifier is the +100 -100 amplifier. The transducer feedback is fed to one leg of this amplifier whilst the other leg has been preset to a voltage which will operate the unit when coincidence of the two voltages have been achieved. The second amplifier is the top-up amplifier controlling the 1100 frequency output.

Here the target leg is controlled from the temperature selection. As the temperature selection voltage is changed according to the readout so this voltage will vary and reset position at which this operational amplifier will function. The outputs of these amplifiers pass through a gating system giving the output requirement for the pulse shaper to accept.

Section 5 shows a pulse shaper which consists of a conventional circuit for reading the leading edge of the impulse pulse transmitted by the pulse control gate. On seeing this leading edge it transmits this into a square wave pulse the width of which can be governed to approximately 10 milliseconds in length. Within the design configuration facilities are available for increasing or decreasing the length of this pulse in the range 5 to 25 milliseconds. Preferably this would be a factory setting within the circuit to enable versatility of the circuit design. The pulses that are generated are pulses which are cascaded one after the other. The logic circuit shows two pulses being generated, one going immediately to power switch 1, the second one going to power switch 2. This part of the circuit therefore operates in the following manner: on receiving a pulse from the pulse control gate a 10 millisecond square wave pulse will be generated going directly to power switch 1.

As soon as this pulse has been completed a second pulse would be generated which will be transmitted to power switch 2. The circuit configuration allows a number of these pulses to follow one another in a cascade, whereby a series of heating elements can be heated successively, e.g. a series of such elements making up a long die, or a series of such heating elements making up two adjacent pairs of dies.

Section 6. This part of the circuit switches the thermal current through the heating element. The positive connection to a 24 volt D.C. power supply is permanently maintained in the centre of the heating element with the two ends being connected, one to the power switch 1, and the other to power switch 2. As these power switches are switched on and off alternately to zero volts, thermal current in 10 millisecond pulses will be switched alternately through the left-hand side and then through the right-hand side of the heating element thus maintaining an even current flow through the heating element itself without creating hot spots at each end of the heating element. The switches themselves consist of conventional power transistors with the base connected directly to the pulse shaper, the drain and source being connected directly to the power supply and heating element.

Figure 2 is a block diagram of the pulse control gate. The logic diagram comprises integrated circuits (IC's) referenced IC 1 to IC 12 respectively. These ICs appear to be individual but in practice some of them will be contained within one IC block. There are five inputs and one output to the logic control diagram. The five inputs are: 1. Input of + 1000 frequency 2. Input to - 1000 frequency 3. 1200C frequency 4. The target temperature set by the user of the equipment 5. Input of the temperature transducer IC 1 and IC 2 are operational amplifiers, IC 3 and IC 4 are Darlington arrays, and IC 5 to IC 12 are standard AND gates. The circuit calls for the output to give out one of three frequencies. The highest frequency will be for fast heating up which is the frequency of 1000C pulse generator.A lower frequency is generated by the + 1000 pulse generator and is used for the approach to the target temperature and to ensure that no overshooting of temperature takes place. The third frequency is the topping up frequency which comes into operation after the target temperature has been reached to ensure that the temperature remains constant within + 20 of the target temperature. The main purpose of keeping this frequency running once the target has been reached is to ensure there is no rapid reduction in the temperature thus maintaining close tolerances to the target that has been set.

The way the circuit functions is as follows: if the temperature is below 1 O OOC, say at switch on, both operational amplifiers and therefore Darlington array outputs will be high therefore through the AND gate system the only AND gate capable of passing its frequency would be the minus one. On reaching 1000C the output from the operational amplifier IC 1 will go low therefore this AND gate frequency will be switched on but the AND gate will switch on the frequency for + 1000 frequency.

This will be maintained until the target temperature is reached when both operational amplifiers will be low and Darlington arrays low. Under this configuration the AND gate system will only permit the 1200 frequency AND gate to be switched on. Therefore the three output AND gates IC 5, IC 6 and IC 10 which are passing the frequencies to IC 12 are controlled by the configurations of the Darlington arrays IC 3 and IC 4 which in turn are controlled by the operational amplifiers which are detecting the temperature at the transducer and the target temperature that has been set. The position of the outputs of these amplifiers are as follows: if the temperature is below 1000C both amplifiers will be on, above 1000 but below the target temperature only IC 2 will be on when the target temperature is reached both amplifiers will be switched off.

Although the circuit described above controls temperature by adjusting the frequency of heating pulses, it will be appreciated by those skilled in the art that it would be possible alternatively or additionally to vary the duration of the pulses in order to control the temperature.

Referring to Figure 3 of the drawings there is shown a flexible faced die for a heat sealing machine for packaging film. Usually such dies operate in pairs which can be moved into and out of contact with one another, but a single die is shown in the drawing in the interests of simplicity. The die consists of a rigid backing member 10 by means of which the die can be fixed in heat sealing apparatus by screws or bolts not shown. Usually the backing member 10 will be made from a material such as glass fibre reinforced plastics. The rear face 11 of the block 10 will in use be fixed against the heat sealing machine and the front face 12 of the block 10 carries a layer 13 of a thermally conductive silicone rubber. Such rubbers are known as such and are for example filled with materials such as aluminium oxide or zinc oxide in order to increase their thermal conductivity.A second layer 14 of thermally conductive silicone rubber is applied to the layer 13 and sandwiched between the two layers 13 and 14 is an elongate electrical resistance heating element 15 of a kind known Der se. The layers 13 and 14 are bonded one to the other, e.g. by means of a suitable adhesive, to trap the heating element in surface to surface contact with the rubber layers.

The device thus far described in Figure 3 is known Der se in the art but there is now proposed an improvement in the known device consisting of an input lead 16 secured to the mid-position of the heating element 15 and a pair of output leads 17 connected to the respective ends of the heating element. By this design it is possible to apply current alternately to one half and to the other half of the resistance element by means of the circuit described above to reduce stresses on the element and particularly on the electrical connections to the element due to differential expansion and contraction.

Claims

1. A method of controlling the temperature of an electrical resistance heating element comprising the steps of supplying electrical current to the element in a series of sqaure wave D.C. pulses, monitoring the temperature of the element or of a member to be heated thereby, in step-wsie fashion, and reducing the frequency and/or the duration of the current pulses as the temperature rises.

2. A method according to claim 1, wherein the pulse frequency and/or duration is reduced from an initial value to a final lower value when the temperature of the element or of the member to be heated thereby reaches a given level.

3. A method according to claim 1, wherein the pulse frequency and/or duration is reduced successively from an initial value which is employed when the temperature of the element or of the member to be heated thereby is low to a second lower pulse frequency and/or duration as the temperature approaches a required level and to a final even lower pulse frequency and/or duration when the temperature reaches the required level.

4. A method according to claim 3, comprising selecting the second and/or the final pulse frequency andior duration dependent on the required temperature.

5. A method according to any preceding claim, wherein the duration of the pulses is in the region of 5 to 25 milliseconds.

6. A method according to any preceding claim, wherein the duration of the pulses is of the order of 10 milliseconds.

7. A method according to any preceding claim, wherein the pulse frequency is in the range of 3 to 12 per second.

8. A method according to any preceding claim, comprising successively pulsing different sections of the element with current.

9. A method of controlling the temperature of a plastics packaing film sealing member substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

10. Means for controlling the temperature of a member heated by an electrical resistance heating element comprising means for supplying electrical square wave D.C. pulses, means for monitoring the temperature of the member and/or the element, and means responsive to the sensed temperature for reducing the frequency and/or duration of the current pulses in step-wise fashion as the temperature of the element and/or the member rises.

11. Temperature control means according to claim 10, comprising means for reducing the pulse frequency and/or duration from an initial value to a final lower value when the temperature of the element or of the member reaches a given level.

12. Temperature control means according to claim 10 or claim 11, comprising means for reducing the pulse frequency and/or duration successively from an initial value which is applied when the temperature of the element and/or the member is low, to a second lower pulse frequency and/or duration as the temperature approaches a required level, and to a final still lower pulse frequency and/or duration when the temperature reaches the desired level.

13. Temperature control means according to claim 12, comprising means for selecting a required temperature for the heating element or the member heated thereby and means for selecting the second and/or the final frequency and/or duration dependent on the required temperature.

14. Temperature control means according to any one of claims 10 to 13, wherein the heating element has at least two sections and comprising switch means connecting the pulsed current supply successively to the different sections of the element.

15. Temperature control means according to any one of claims 10 to 14, wherein the pulse supply means supply square wave pulses.

16. The combination of plastics packaging film sealing means and temperature control means therefor substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.