The invention relates to heating means and more particularly, but not exclusively, to means for controlling electric heaters for use in apparatus and 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 often nowadays faced with other materials. It is usually important that the temperature of the dies is closely controlled since too low a temperature will result in poor or no sealing of the plastics film, whereas too high a temperature can damage even destroy, the plastic film again resulting in poor or no sealing. Historically, heated metal dies have comprised quite large metal blocks having a high thermal inertia in an attempt to provide temperature stability. This however means that it is impossible quickly to change or even correct the die temperature relative to a required target temperature.

At least for heat sealing plastics film it is known to use heated dies faced with thermally conductive silicone rubber able to conform more closely to shape for what is to be heat-sealed than is the case for bare rigid metal dies. Although rubber-faced dies, or other faced dies, can be desirable, they are often adversely susceptible to temperatures higher than target, particularly to protracted such over-heating, so their heating should be closely controlled. It is advantageous for such dies, particularly rubber-faced dies, to have low thermal inertia, otherwise there can be early and unpredictable failure of the bond between the facing, such as rubber, and the electrical heating element or other carrier, which can create undesirable temperature gradients along the die with consequent inefficiency of resulting heat seals.

It is an object of the invention to provide means of controlling the temperature of a low thermal inertia heat

sealing die, whether or not faced and regardless of any facing material used.

According to the invention heating control means for controlling the temperature of a member to be heated by an electrical resistance heating element comprises means for generating a control signal proportional to a target temperature, means for generating a control signal proportional to the actual temperature of the member, means comparing the two control signals to gauge the temperature difference, and means responsive to the control signal comparison for switching a power supply to the heating element, the switching being such that the power supply goes On' and 'off at a variable frequency and that the 'on' phases are of variable duration, both dependent on the temperature difference. In this way, more power can be supplied to the heater element when it is relatively cold both by relative high switching frequency and by relatively long 'on' phases.

Conveniently, for heating up heat sealing dies and until they are within a first and largest prescribed temperature difference below the target temperature, both of said frequency and said duration can be at their permitted maxima; though there coul- , ©*? course, be an initial stage of continuously applied electrical power, if desired for last and largest prescribed temperature difference above the target temperature both of said frequency and duration can be at their permitted minima. Advantageously, variation of said frequency is used in preferred embodiments of this invention, for relatively fine control, compared with use made of variation of said duration.

In some such preferred embodiments, application of electrical power at maxima for both of said frequency and said duration, is followed first by beginning to reduce only said duration, then with reduction of said frequency beginning only after achieving a second and lower prescribed temperature difference below the target

temperature. At and above the target temperature, variable control can be by way of further reductions of both of said frequency and said durations, say with minimum (short of complete switch off as could be further provided) for said durations at a higher further prescribed temperature difference above the target temperature than applies to minimum for said frequency.

Relative to temperature increasing through the target temperature, preferred reductions of both of said frequency and said duration for control purposes are progressive, i.e. so that less electrical power is applied at and above the target temperature than below it; and duration reductions can start before and continue after frequency reductions. The reductions in either or both of said frequency and said duration could be continuous and follow any desired characteristic, but are particularly readily implemented in a step-wise manner relative to the above and other suitable or desired prescribed temperature differences, some of which can coincide for both of frequency and duration reductions (or increases considered against falling temperature of the dies).

Thus, both of frequency and duration reductions may take place at or close to achieving the target temperature on heating up, and/or at the prescribed temperature difference for frequency reductions to start on heating up, and/or at a prescribed temperature difference representing minimum frequency; and there can be intermediate further prescribed temperature differences at which at least frequency reduction occurs.

Other preferred embodiments of this invention can readily and advantageously operate with fewer reductions of said durations, even only one such reduction, usually at or preferably before actual temperature reaches said target temperature; depending, of course, on whether or not reductions of said frequency begin before or at the one duration reduction.

It is practical and highly advantageous for the same

control provision to be operative in respect of more than one heater element. Examples include situations where cut or melted severance or tear-off weakening is required between two good seals, or beyond one good seal, normally requiring two or three heater elements; or long heating dies where end and intermediate dies or die parts may have different heat loss or heat take-off characteristics, particularly if used for difference widths of articles to be sealed; or angled heating dies, typically in an L- or V- or U-configuration, but not limited thereto. Embodiments of the invention thus allow such operation to control plural heat sealing dies or die parts, particularly where control involves use of electrical pulses.

Whilst it is feasible to assign consecutive periods of said frequency individually to different ones of plural die or die part heaters to be controlled in whatever sequence may be desired, it is preferred to deal with powering each of the die heaters concerned in desired sequence within each period of said frequency. It is convenient for each of a stream of pulses at said frequency to trigger pulse length determining means effectively setting said durations, and further to have such duration setting pulses each triggered within the sequence from the trailing edge of the immediately preceding duration setting pulse in the sequence.

Control of plural heat-sealing dies or die parts can be relative to individual control temperature sensing means associated therewith. This approach is perhaps preferable where characteristics of different dies or die parts and their heating elements are not sufficiently similar for a single temperature sensor to suffice, or their temperature requirements differ and they are individually available for association with temperature sensors. Alternatively, pulse lengths corresponding to application of electrical power, or even a proportioning of applied or available power, could be set for different dies or die parts, whether because their said characteristics are different in known

relative ways or because one or more of the dies or die parts are to be heated differentially. This approach is perhaps preferable where it is impractical to have individual temperature sensors, say where a desiredly hotter die part is in a sandwich structure between desiredly less hot die parts.

Specific implementation for this invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:

Figure 1 is an overall block diagram for heat sealing die provision including three side-by-side heaters; Figure 2 is a diagram for plural heat sealing dies or die parts each with a heater;

Figure 3 is a circuit diagram for one stage of means for setting voltage by reference to temperature difference; and

Figure 4 is another circuit diagram for changing control pulse duration directly according to temperature difference.

Generally, it is desired accurately to control the conversion of electrical energy into heat energy in the heating elements of a heat-sealing die, particularly a heat-sealing die having low thermal inertia, usually where the mass of material of the or each heating die itself is low. A conventional heating system operating simply to switch-on and switch-off electrical power under the control of a thermostat would result in unacceptably inaccurate heating of the die, particularly for its surface, relative to a target temperature. It is proposed herein to modulate the electrical current flowing through the heating element to enable more accurate temperature control and stability to be maintained for the heating die concerned. The circuitry which will now be described meets these requirements and can be considered as a power-modulating circuit.

Operation is based on measuring the difference between voltage signals arranged to represent a selected or target

temperature and actual sensed temperature of a heat sealing die, and controlling switching signals for solid-state switching circuitry for supplying either AC or DC current to at least one heating element.

The block diagram of Figure 1 is first described for main components involved in controlling one heating element 10B of a heat sealing die 12 that actually has three heating elements as will be further described later. An electrical power source 14 is shown supplying the heater element 10B through a solid-state electronic switch 16B. The heat-sealing die is shown with an actual temperature sensor 18.

Solid state electronic power switches are well-known operative for determining 'on' states by durations of control pulse signals, see line 20B for switch 16B. These control pulse signals are controlled as to both frequency and duration, see idealised pulses 22B that can vary in a controlled way as to length or duration of each and as to intervals between them, i.e repetition rate or frequency.

Durations of the control pulse signals 22B are shown determined by voltage controlled pulse length/width setting circuit 24B, and initiations and repetition rate by voltage controlled oscillator 26. The circuit 24B and the voltage controlled oscillator 26 can and preferably do have base states, i.e. with no voltage signals on their control lines 28 and 30, for pre-set maximum control pulse duration and preset maximum frequency or repetition rate.

Voltage control signals for the pulse duration setting circuit 24B and the voltage controlled oscillator 26 are shown coming from circuits 28 and 30, respectively. The circuits 28 and 30 can be of similar types, conveniently resistance ladder networks that give voltage output changes in steps and according to sequential energisation of plural inputs. For each of the circuits 28 and 30, five inputs are shown, see 32A-E and 34A-E, respectively. For pulse- length/width and frequency control (at circuit 24B and oscillator 26) proportional to control voltages following

7 decreasing temperature difference to the target temperature then increasing beyond, resistance ladder networks (as circuits 28 and 30) can be so arranged that more of their inputs (32A-E, 34A-E) are energised, see further below regarding Figure 3.

Energisation of inputs 32A-E and 34A-E of the voltage setting circuits 28 and 30 for the circuit 24B and oscillator 26, respectively, is shown in accordance with outputs of comparator circuity 36 and 38, respectively. These comparator circuits 36 and 38 receive both of signals representing a selected or target temperature at branches from line 40 from temperature selector 42, and at branches from line 44 for actual heat-sealing die temperature from the temperature sensor 18. For energisation of resistive ladder networks as voltage setting circuits 28 an 30, the comparator circuits 36 and 38 operate on, as mentioned above, a basis of energising lines 32A-E and lines 34A-E sequentially according to prescribed temperature differences between its inputs, which prescribed voltages extend from the actual temperature being below through to being above the target temperature. For one suitable scheme for particularly close die temperature control prescribed temperature differences and responses are as follows:-

1. First prescribed temperature difference, say 10°C below target temperature, for energisation of output 32A, thus first reduction of the control pulse duration by circuit 24B during heat-up.

2. Second prescribed temperature difference, say 2°C below the target temperature, for energisation of outputs 32B and 34A, thus second reduction of the control pulse duration by circuit 24B, and first reduction of the frequency or repetition rate of oscillator 26.

3. Third prescribed temperature difference, say 1°C below the target temperature, for energisation of output 34B thus second reduction of the frequency or repetition rate at oscillator 26.

4. Fourth prescribed temperature difference, say zero, i.e. when the actual temperature first matches the target temperature during heating up, for energisation of outputs 32C and 34C, thus third reduction of the control pulse duration by circuit 24B, and third reduction of the frequency or repetition rate at oscillator 26.

5. Fifth prescribed temperature difference, say 1°C above the target temperature, for energisation of output 34D thus fourth reduction of the frequency or repetition rate at oscillator 26.

6. Sixth prescribed temperature difference, say 2°C above the target temperature, for energisation of outputs 32D and 34E, thus fourth reduction of the control pulse duration by circiut 24B and fifth reduction of the frequency or repetition rate at oscillator 26.

7. Seventh prescribed temperature difference, say 5°C or 10°C above the target temperature, for energisation of output 32E, thus fifth reduction of the control pulse duration by circuit 24B.

Such a control regime gives good accuracy of actual temperature relative to target temperature, even for dies with very low thermal inertias, but should not be taken as ruling out alternatives, whether for more duration and/or frequency reductions, or for less as will be described for Figure 4, and including as to temperature differences, if any, at which both of the comparators 36 and 38 change their output states.

For a heating element made from an alloy of 80% Nickel and 20% Chrome, with an effective electrical resistance of 0.5 ohms, and a DC supply of 30 Volts normal, effective available control pulse durations as above can be 30, 20, 10, 8 and 5 milliseconds and available frequencies can have periods as above corresponding to repetition rates of 30, 17, 15, 10 and 2 per minute.

The comparators 36 and 38 can, as indicated, usefully be based on operational amplifiers, one for each output,

say each arranged to change state for detected equality of input signals, then with the target temperature signal off line 40 subject to successive offsets. Suitable such offsets can be minus 100, minus 20, zero, plus 20 and plus 50 or 100 millivolts for the comparator 36; and minus 20, minus 10, zero, plus 10 and plus 20 millivolts, where the temperature sensor 18 and the temperature setting means 42 operate or have their outputs adjusted on the basis of 10 millivolts per degree Centigrade (Celsius).

It will be appreciated that illustrated components as so far described can service a heat sealing die having a single heating element, or a heat sealing die or dies having plural heating elements all to be energised together. However, particular practical advantage is seen arising from separate individual driving of plural heating elements sequentially within each of single periods of the voltage controlled oscillator 26, see dashed pulses 22C, 22A following first solid pulse 22B in Figure 1.

That is readily achieved where all heating elements are to be similarly driven as indicated by solid lines 20A,B,C to solid-state switches 16A,B,C for heating elements 10A,B,C in Figure 1, and with control according to a single die surface temperature sensor (18), but with control signals for the lines 20A,B,C coming from successive stages (TB, TC, TA) of the pulse generator 24B, each preferably and conveniently simply triggered from trailing edge of the control pulse from the preceding active stage, as is readily achieved, say using Schmitt Trigger circuitry 24S between those stages. Alternatively, the voltage controlled oscillator 26 could have a successive phasing or stage operation or structure, see dashed extensions therefrom in Figure 1 with dashed connection to pulse generating stages that could then be independent of each other. Repeated use of a single pulse generator 24 is feasible with switching of its output connection to the lines 20A,B,C and/or its input connection to outputs of the voltage controlled oscillator 26. More

individual control is possible using more temperature sensors, up to one per heating element concerned, then with switching of actual temperature signal lines at input of the duration setting circuit 36, but one chosen to set the frequency or repetition rate via the circuit 38, and clearly permitting maintaining any desired or required temperature differences for the die or dies concerned.

Another way to achieve differential die/die part temperatures is and to adjust the lengths of control signal pulses on the lines 20A,B,C after they have left the circuitry 24, whether in accordance with different actual temperature sensors, or by pulse shortening, even pulse stretching, circuitry to utilise a single temperature sensor 18 and operate according to offsets representing known relative parameters or simply adjusted to be effective. Figure 1 shows such offsetting by way of pulse shortening circuitry, see dashed at 46A,C in lines 20A,C as is appropriate for a three heater elements and die part system (10A,B,C;12) as used for heat sealing and severing/ weakening within overall sealing or between two distinct seals, where the central die part (10B) will ordinarily require a higher temperature and it is convenient to set the flanking die part (10A,C) relative thereto. Ramp-action blocking amplifiers are suitable and may well be best provided as indicated adjustable units 46A,C. Alternatively, adjustment could be within the extended stages of the circuitry 24.

At least provisions such as control pulse length adjustors can also be particularly useful in coping with a dies or die-parts system where there is differential heat loss/take-off, and in order to seek to maintain a steady desired temperature, as sensed at 18 (or some other characteristic if more appropriate) .

Figure 2 shows a die system 52 with four heating elements 50A,B,C,D as may be applicable simply to a long heat sealing die, or to a die structure with more than one direction of required sealing, say an asymmetric L— or a U-

or C- or V- configuration, and each with its own solid- state pulse length controlled switch 56A,B,C,D from common power supply 54. This arrangement is readily controlled by circuitry as described for Figure 1, indeed that Figure shows dashed extension of its control pulse generator circuitry 24 and/or voltage controlled oscillator 26 for four-way use.

Figure 3 shows part of one suitable resistive ladder type control for the voltage controlled oscillator 26, for which its control voltage needs to decrease for lower frequencies. Specifically, as shown for the output 34A, the related operational amplifier 60A has inputs 61A,62A for voltage signals representing actual and appropriately offset target temperatures, respectively, and output 34A going high when the actual temperature (61A) exceeds the target temperature (62A) as appropriately offset. Then, transistor 65A will be switched into conduction and adjustable resistance of potentiometer 66A will be shorted out and replaced by lower resistor 67A in the emitter/collector circuit of the transistor 65A going to earth rail 68. It will be appreciated that there will be other similar circuit provisions at the indicated break 69, one for each of the operational amplifiers and outputs 34B-E, and that the voltage seen at 64 for controlling the frequency of the voltage controlled oscillator 26 will depend on how many of the transistors are sequentially switched on and their variable potentiometers shorted out. Adjustability of potentiometers 66A-E allows individual set-up of changes of frequency for particular controlled systems.

It will be appreciated that a multi-stage cascaded ladder-type circuit like Figure 3 can be used for control pulse length/duration setting as at 28, which might then operate on an up or down ramping basis to convert the voltage concerned to time, specifically duration of control pulse.

However, there are many instances where up to five or

more frequency changes is adequate relative to controlling sealing dies with low thermal inertia even where deployed with a lesser number of changes of control pulse duration, even as low as two. Figure 4 shows a particularly simple way of handling just two available control pulse lengths or durations. A single operational amplifier 70 has two inputs 71 and 72 for signals with voltage levels representing actual and target temperatures, actually conveniently some suitable offset from the target temperature, which will usually be below. The operational amplifier 70 has its output 74 taken to bases of transistors 75 and 76, the latter via inverter 77. The transistors 75 and 76 are connected with their emitter/collector circuits going in common to ground at 78 from different capacitors 79 and 80 alternatively cooperating with resistor 81 to set time constant for a suitable pulse duration setting circuit 24. It will be appreciated that the operational amplifier 70 will operate at some prescribed temperature difference to switch from capacitor 80 to capacitor 79 for a single reduction in control pulse length/duration. A change from 12 to 10 millisecond duration is found to be enough, with the switching at as low as 5°C below target, or at about 2°C, or as otherwise desired relative to any useful overlap with frequency changing, say as above described.

It would, of course, be feasible to control frequency and duration according to single sequential output signals for each change required, and for temperature comparator circuitry to operate accordingly, further feasibly for a single temperature comparator provision to provide outputs selectively gated for frequency and/or duration control purposes.