EP0044837A1 - A process and a device for hot working - Google Patents

Abstract

A hot working device, preferably for forming or heat welding plastic materials, comprises electric heating means and a control device for supplying electric heating energy to said heating means. This control device comprises a) power reduction means (3), b) means (9) for adjusting a maximum heating energy, said adjustment means (9) being connected to a control unit (4) which controls said energy limiting means, c) means (6) for measuring the heating temperature, said means being connected to the control unit (4), the signal coming from these temperature measuring means (6 ) acting through the control unit (4) on the power limiting means (3) to limit the heating energy when it exceeds a particular predetermined temperature, and this continuously or in several stages, preferably at least three stages, preferably of the same magnitude, and d) pulse control means (8) for keeping the heating power supply connected for a period of adjustable time for at least a period d continuous impulse. A method of hot working, preferably of forming or heat welding of plastic materials, using electric heating means for said hot working, especially heating resistance means, or the supply of heating power electric to said heating means is controlled for at least one continuous pulse period.

Description

A PROCESS AND A DEVICE FOR HOT WORKING

This invention is related to a process and a device for heating and especially for hot working of materials, such as plastics, elastomers and similar, i.e. polymers of organic or inorganic type, especially for welding or hot forming such materials, e.g. hot sealing, vacuum forming, blowing, extruding, injection moulding, pressure moulding, etc.

According to one embodiment the invention is related to a process and a device for welding and hot forming, e.g. hot sealing plastics and similar materials in which the supply of the required amount of heat is controlled in a particular way. Preferably the required heat is supplied to the material with heated tools for welding or hot forming.

According to the invention heat is supplied with an electrical heating means preferably resistance heating means. The heating power supplied by the heating means is adjustable, preferably continuously or in a plurality of steps, such as at least 3, 4, 5, 7 or 10 steps, which pre¬ ferably constitute equally large parts of the maximum power, and can be restricted to an adjustable maximum value and controlled continuously or stepwise, as mentioned above, to a minimum value which may be 0 or an adjustable set minimum power value. The adjustment of the power can be achieved with a thyristor, e.g. triac, which is connected into the electric heating circuit and restricts the electric current supplied, e.g. to a resistance heating means.

The heating power output can also be controlled with regard to a measured temperature, preferably the temperature of the heating means which supplies heat to the material, such as a welding tool or a forming tool, or the temperature of the material being subjected to working. Said control can be set to shut of the supply of electric heating power entirely or to restrict the power to a lowest value of not above 50, 30, 20 or 10% of the set maximum power, when an adjustable set maximum temperature is reached. The supply of electric heating energy is adjusted preferably so that the electric heating effect is reduced continuously or stepwise, as mentioned above, within a temperature range from a lower measured temp -^~erature tmm. to the set maximum temperature t_ . This can be achieved with a temperature measuring means, preferably a thermistor, which measures the temperature, e.g. the temperature of a heat releasing electric heating means, such as a welding tool or a forming tool, said temperature measuring means being connected to supply a signal corresponding to the temperature to a control means which influences the heating power supplied to said heating means. This can be achieved by connecting said temperature measuring means so that it controls said thyristor, preferably through a control unit, e.g. comprisin an amplifier. Preferably the temperature measuring means is connected to control the dissipated electric heating power with control means which are adjustable so that the electric heating power delivered as a function of the temperature, also denoted f(t), can be adjusted and set in any desired matter, e.g. to be nearly linear from a lower control temperature t . to a maximum temperature ^{fro a hi}?^her P°^{wer p}tmin ^{to a lower po er P}tmax ^as mentioned above. For said purpose one may use the change of the resistance of a thermistor as a function of the temperature. A suitable temperature range, Δ_t, between the control minimum temperature and maximum temperature can e.g. be up to 100°C, preferably up to 75°C or even lower, e.g. up to 50, 40, 30, 2θ, 10 or 5°C.

A suitable temperature range, -Δ_tr, for the power control, may also be up to 75, 60, 50, 40, 30, 20, 10 or 5% of the set maximum control temperature t expressed in °K or preferably C and especially as a %-part of the temperature range between the ambient temperature at the hot working operation in question and t . The reduction of the power with temperature increase in said range may be set or adjustable to a delivered power at t of down to 80, 60, 50, 40, 30, 20, 10, 5 or 0% of the highest adjustable maximum power or of the maximum power set for the operation.

The steepness of the power control, i.e. the reduction of power with increasing temperature, **Vdt, p meaning power and t temperature, is thus preferably adjustable. Suitable values may vary e.g. from an upper value of 40, 30, 20, 15, 10, 5 or 3% per °C to a lower value of 20, 15, 10, 5, 3, 1, 0.5 or 0.1% per °C and can preferably be linear.

The temperature measuring means should preferably measure the temperature of the heating means or optionally the temperature of the heated material as accurately as possible. The temperature measuring means, e.g. thermistor, may therefore be arranged with low heat transmission resistance in relation to the heating means and as close to .the heating means as possible, e.g. at a distance of at most 1 mm, preferably at most 0.5 mm and especially at most 0.2 mm.

In some cases it is preferable not to cut off the supply of electrical heating power at the upper temperature limit tma_x but only ^' to reduce said heating p ^cower to a lower value, _m-_jn, said lower heating power being adjusted so that it does not give a temperature increase above the desired maximum value but rather a temperature reduction at said maximum value. The temperature control can of course also be performed with the aid of a plurality of temperature measuring means, in which case temperature control should preferably be performed so that the maximum value is not exceeded by any of said temperature measuring means. Heating can also be performed with a number of electrical heating means which are controlled by said temperature measuring means.

According to the invention the heating power can also be supplied as heating pulses, i.e. preferably as electrical current pulses with a pulse duration (pulse time) which is longer than the period time of the line frequency i.e. usually 50 periods per second, so that each current pulse comprises a plurality of line frequency periods. Preferably the duration of the heating pulses as well as the duration of the interval (pause) between said pulses are arbitrarily adjustable within certain ranges as well as the number of heating pulses in a sequence, which are automati¬ cally delivered in a working operation. In many cases, e.g. when welding or sealing plastics a pulse time of up to 100, preferably up to 30, especially up to 10 seconds and an

« interval time of up to 100, preferably up to 30 and especi¬ ally up to 10 seconds can be used. The lower limit of the pulse time which can be pre-set may vary, e.g. down to 0.1, especially down to 0.05 or down to 1 second, and said values may also be used for the lower limit of the interval time which can be preset.

The number of pulses may e.g. be pre-set to between 1 and 10 but also a larger pre-set number of pulses may be desirable, e.g. up to 20 or up to 50 pulses.

It is also possible to vary or adjust the duration of the pulse time and/or the interval time in one and the same pulse sequence, i.e. in one and the same heating operation, preferably gradually from a lower pulse and/or interval time at the beginning of the operation to a shorter time at the end of the operation, i.e. the end of the pre-set puls sequence at the operation in question. Said gradual varia¬ tion of the duration of the pulse and/or interval time may also be combined with temperature surveying and may be controlled by the temperature so that the pulse and/or interval time is gradually reduced when the temperature approximates a pre-set upper temperature value, which may be the maximum temperature value mentioned above. The pulse and/or interval time may e.g. be reduced to 75%, 50%, 25% or 10% of the starting value at the beginning of the opera¬ tion, i.e. the beginning of the pulse sequence, when the upper temperature limit, e,g, the maximum temperature is reached, preferably with a gradual reduction up to said temperature, said gradual reduction may be linear or adjustable in any other way. The reduction of the puls and/or inteval time may also be made time dependent and preferably varying from maximum value at the beginning of the operation to a minimum value at the end of the opera- tion or pulse sequence, e.g. the minimum values mentioned above, expressed as % of the starting values. It is possible to combine a gradual or stepwise reduction of the delivered heating power and a gradual or stepwise reduction of the duration of the pulses and/or intervals in one and the same operation.

In the foliosing an embodiment of the invention is explained with reference to the enclosed drawing, in which Figure 1 is a block diagram, Figure 2 is a circuit diagram of the blocks according to Figure 1,

Figure 3 is a time diagram of the tension in various parts of the circuit diagram according to Figure 2, Figure 4 is a power temperature diagram of a theoretical welding sequence,

Figure 5 discloses the tool arrangement used for heat seal¬ ing foils, Figure 6 shows the shape of the heating means for heat sealing of semi-circular shape,

Figure 7 shows schematically the arrangement of the means according to Figure 6 in a welding tongs, Figure 8 shows the arrangement of two tubes which are overlap welded and

Figure 9 shows the arrangement of the welding means for welding the tubes according to Figure 8.

With reference to Figure 1 there is disclosed a block diagram of an embodiment of the invention which is suited e.g. for welding plastics. Figure 1 shows a load 1 which may consist of one or more welding tools with electrical resistance heating means. For reducing the personal hazards said load is energized with a low voltage 48 Volts from a transformer 2. Said transformer is fed from a triac 3 to which are supplied trigger impulses from a control unit 4. To said control unit 4 there is connected a temperature sensor 6, preferably consisting of a thermistor, which preferably may be arranged close to the resistance heating means, and pre-selection means for a maximum tem¬ perature 5 as well as pre-selection means for a maximum effect 9 which may e.g. consist of a potentiometer for manual adjustment graduated e.g. in a scale of 1-10. Furthermore, there is connected to said control unit 4 a pulse control unit 8 with which the pulsed supply of the heating effect to the load mentioned above can be achieved. Connected to said pulse control unit are means 7 for selecti the duration of each pulse and means 11 for selecting the duration of an intervening pause as well as means 10 for selecting the number of pulses supplied during an operation or pulse sequence. Having selected desired values of the various parameters with said means the operation is started, e.g. for welding or hot forming of plastics articles, by actuating the start switch 12. Referring to the circuit scheme on Figure 2 the following is a detailed explanation of the way in which one embodi¬ ment of the invention works.

When grounding the input START connected to the OR-gate

101 a trigger pulse is obtained in a first time circuit 102. Said circuit emits a pulse, the duration of which is selected by setting a potentiometer 105. A binary counter 103 is hereby counted up one step. The output of the timing circuit 102 is connected on one hand through a trigger net C-, R, to the input o'f the second timing circuit 104 and on the other hand to both inputs of an AND-gate 106. The output from the AND-gate 106 is fed to a relay 107. The relay switch 108 of said relay 107 is arranged to switch on the trigger pulses from the control circuit IC1. The other timing circuit 104 is triggered with the rear edge of the pulse delivered by said first timing circuit 102. When the output from said second timing circuit 104 goes low after a period of time which is determined by the setting of the potentio- meter 116 , a starting impulse is supplied to said first timing circuit at the gate 101 and a repetitive sequence is obtained.

The number of pulses which are to be delivered at the welding is set with a thumb wheel switch 109 which supplies BCD-code corresponding to a set digit on the four output circuits 11, 12, 13, 14. The logic output signal from the counter 103 is compared with the code from the pre-setting device 109, and at coincidence a logic signal is obtained at the output of the logic comparator 110. This turns over the flip-flop consisting of the AND-gates 114 and 113 and through the relay circuit C-R^C^' a reset pulse is obtained at the counter 103. Simultaneously the timing circuits 102 and 104 are deactivated by a signal from said flip-flop 114 and 113. At the same time a light emitting diod "READY" is switched on. During the pulse time of said first timing circuit 102 the light emitting diode "HEAT" is switched on .

The maximum welding power is set with the potentiometer

Ry - in the voltage divider consisting of the resistances R and R, , . The control voltage to the control circuit

IC1 is tapped at the point U *Ά and fed through the diode

D, to the input 5 of TCI. In the instant integrated circuit the control circuit is a DC voltage with the control range between 1.4 V to 5.6 V. The maximum power is obtained at the lowest voltage. The power is controlled by phase control according to the graph on Figure 3 in which Vv__* denotes to control voltage and Tn is the value of said control voltage at a set maximum power and T is the value of the control voltage when reaching a set maximum temperature.

The voltage at the anode of the diode D. is at the onset of a welding operation higher than the voltage at the anode of diode D_. Thus, D is conducting and D_ is non-conduct- ing. When the temperature of the welding tools exceeds a pre-determined temperature which is set with the potentio¬ meter R„- the voltage at the anode of the diode D„ rises due to the decrease of the resistance of the NTC-resistance or the thermistor, and the diode D_ becomes conducting and D, non-conducting. The voltage divider R and are disconnected and the triac is now controlled by the variations of the NTC-resistance. With heated-upwelding tools the welding sequence is thus controlled by the thermistor until the welding is continued and the tempera- ture of the welding tools again decreases to a lower value at which the diode D again becomes conducting and D_ non-conducting and the voltage dividers with the starting voltage U according to Figure 1 are again switched on. Instead of one thermistor two or more thermistors can of course be connected in parallel or in series. Figure 3 shows various curve shapes in the control device according

- to the invention described above. Figure 3a shows the voltage at the primary side of the welding transformer as a function of time in a particular welding operation. T denotes the heating time and T denotes the cool-off time. The ignition angle of the triac is denoted of and varies between 0 and Jf radianes.

Figures 3b-3e show signal voltages as function of time from various blocks of the integrated circuit IC1. The instant embodiment uses phase control of the thermistor and curve shapes 3b-3e are drafted with Figure 3a as a reference. The integrated circuit IC1 consists basically of five function blocks, i.e. a D.C. voltage source, zero level crossing detector, difference amplifier, ramp function generator and trigger pulse amplifier.

Figure 3b proves the signal at one of the inputs to the difference amplifier, and said graph also discloses the level T of the control voltage V , which gives the set maximum power and the control voltage level T ,which gives control of the power corresponding to a particular set maximum temperature of the welding tools.

Figure 3c shows the output signal from the difference amplifier,

Figure 3d shows a signal in said ramp function generator and Figure 3e shows the output signal from the trigger pulse amplifier.

Figure 4 shows the reduction of power as a function of the temperature in a theoretical welding operation. The set value of the maximum welding power is half the maximum available welding power corresponding to an ignition angle of ] ^*/. The pre-selected maximum temperature is 100 C. ^~

When using said device for e.g. welding plastics, such as

O.YPI hot sealing plastic foils between two tools, if is pre¬ ferable to start by pre-selecting a pulse time which by experience is selected at a suitable, preferable low value. Furthermore, the maximum temperature is pre-selected to a value which is suitable for the material, preferably at a low value within the range which by experience is suitable. Furthermore, the maximum power is pre-selected at a value which can be expected to give the desired temperature within the pulse time, preferably at the end of the pulse tim After arranging the material, e.g. the plastic foils, in the device, the device is put into operation. If the puls- time is shorter than the pre-selected time the maximum temperature is reached during said pulse which may be indicated by a separate signal means, e.g. a signal lamp. After the test welding operation the weld is inspected, and the pre-selected values may thereafter gradually be increas¬ ed or decreased until the best welding result is obtained. If an unsatisfactory welding result is obtained with one pulse it is possible to test the use of two or more power pulses , preferably with the same total switch-on time as the originally tested single pulse. The use of said pulses is especially suited for welding thicker materials or vulnerable materials which give a longer heat diffusion path or require lower temperature gradients.

As an example reference can be made to welding or forming plastics, the maximum power of the device being 3 kW. The tools are provided with 4 resistance heating means with the power 500 W each which at full effect reach the temperature 450 C. A suitable working temperature is about 200 C and the expected forming time 4 seconds. Said time is set and the temperature and the maximum power are set at or below the expected suitable values, whereafter a test operation is performed. If the result is not satisfac- torily the values of power and temperature are increased stepwise until the desired result is obtained. The device may optionally be provided with means with which a series of test operations with stepwise changed parameter values is automatically performed in accordance with a pre-determined schedule, and thereafter the best result in said test series is determined and the device is set for said values.

In the following examples are given of the use of the process and the device according to the invention for welding plastics. The heating means consisted of iron- -constantan with the resistance 0,73 ohm/m at room tempe¬ rature and 0,775 ohm/ at 200°C.

Example 1. In this experiment the control device disclosed above was used in combination with a couple of straight welding tools (welding clamps) of steel of the shape disclos-ed on Figure 5. Said figure shows a couple of straight welding clamps 1 with the longitudinal extension 400 mm. At the inner surface of each welding clamp there is a resistance means consisting of a resistance strip 2 the size which is 4 x 0,25 x 420 mm. Said resistance strips are connected in series and connec¬ ted to the output of a transformer which is controlled with the device according to the invention. Furthermore, a thermistor 3 is arranged in the upper welding clamp closely to said resistance strip.

The welding material consisted of soft polyvinyl chloride with the thickness 0,10 mm and the melting point about 140 C. Two layers of said material are joined by welding. In said welding operation the following settings of the controls on the control device are used:

Power 3.9 (index) Temperature 4 (index) Time 7 (index) (= about 7 seconds)

Number of pulses 1 By measuring amperage and voltage at the resistance strrps connected in series with voltage meter and ampere meter which show the effective values the following results were obtained:

_Starting voltage ^{7 volt}

Amperage ,_nι . ,_{7 A} at switch-on about 1⁷ A after about 1 second 16 after about 4 seconds 8 ^A. at the end of the pulse about 2 ^A

For the first 2-3 seconds of the pulse time the effect was calculated to about 100 watts.

The clamping pressure used for welding amounted to in average about 0.1 k_P/cm longitudinal extension of the weld, calculated as the load divided with the contact surface of the heating strip. A very good weld was obtained.

Example 2. in this experiment a weld of semicircular shape was produ^¬ ced with a pair of welding tongs. The pair of welding tongs is shown on Figures 6 and 7. Figure 6 shows the shape of the heating means 2 and the arrangement of the thermistor 3. Figure 7 shows the shape of the welding tongs with a fⁱxed part 4 with a semicircular resistance means 2 and a movable part 5 which is likewise provided with a semicircular resistance heating means and which can be moved against the fixed part as is indicated on the Figure. The resistance means consisted of a welding strip with the cross sectxon 0 4 x 0.25 mm and the extension about 82 mm. Said means were connected in series and as in the experiment according to example 1 were connected to the output of a transformer which was controlled _by a device according to the invention. The thermistor 3 was arranged close to one of the 5 resistance strips. The welded material consisted of a rigid, clear polyvinyl chloride foil with the melting point about 145°C and the thickness 0,35 mm. Two such foils were welded together.

In the welding operation the following pre-selected index values were used in the device according to the invention: Power about 3.8

Temperature 5

Time 5 seconds Numbe'r of pulses 1

The welding time amounted to 2 seconds and voltage and amperage, measured as stated in example 1, where measured to in average 5.5 volts and 24.5 amperes. The amperage decreased from the maximum of 25-28A at switch-on to a stable value of 20,5 A after about 1 second.

In said experiments a very good welding result was obtained.

When using a pre-selected power index value of below 3 an entirely satisfactory weld could not be obtained even when using 5 pulses with a pulse time of 3 seconds and the same pause time.

Also when using a temperature index value below 4 a good weld could not be obtained.

In all said experiments an average pressure force of about

0,1 - 0.15 kp/cm weld extension was used.

Example 3.

In this experiment plastic tubes were welded as disclosed sketch-like on Figures 8 and 9. Figure 8 shows an inner tube

6 of polyethylene coated aluminum foil, to the end of which is welded an outer tube 7 of polypropylene. The thickness of the polyethylene layer was about 10 microns and the thickness of the polypropylene layer was about 0,35 mm. The melting temperature of the polypropylene tube was about 130°C.

Figure 9 shows the shape of the resistance means 2 which consisted of welding strip with the cross-section size 6 x 0,15 m and the extension of 130 mm and comprised a semicircular part with the extension 110 mm and two radially directed parts with the extension 10 mm for electric connection of the heating strips. The heating strips were connected in series and connected to the output side of a transformer which was controlled by a device according to the invention as disclosed in connection with the preceding examples.

A thermistor 3 was arranged close to the lower semi¬ circular resistance heating strip 2.

The following pre-selected values were used in the welding operation with the device according to the invention:

Power 3,5

Temperature 3,5

Time 4,5 (about 4 seconds)

Number of pulses 1

Voltage and and amperage were measured in the same way as stated in preceding examples. The voltage at switch-on amounted to 4,8 volts. The amperage at switch-on amounted to 26-27 amperes and after 1 second to about 24 amperes, and thereafter the amperage gradually decreased to a final value of 8-10 amperes at the end of the pulse time. The real pulse time was about 4 seconds.

A perfect weld was obtained. The welding could be repeated with intervals of about 10 seconds with the same qood result

GVFI When performing welding operations, e.g. hot sealing, gluing and similar operations according to the invention it is also possible to control the pressure force with which the heating means are applied against the material being treated. The pressure force can be controlled during the operation depending upon time and/or temperature and/or electrical output power, and said control can be performed through the same device which is used for controlling the heating power or with a separate control device. The pressure used in the welding operation can e.g. be controll¬ ed from a high original pressure force which gives good heat transfer, to a lower pressure force at higher tempera¬ ture in the material, which prevents that the tool intrudes too deep into the heat softened material. For welding (hot sealing) e.g. polyolefines, polyvinyl chloride and other materials an average pressure force of below 1 kp/cm, e.g. below 0.5 or 0,25 kp/cm and preferably above 0,01 e.g. above 0.05 or 0.1 kp/cm of the extension of the weld in many cases has been found suitable.

As examples of materials which can be treated with the process and device according to the invention reference can be made to thermoplastics in general, such as polyolefines, e.g. polyethylene (low density and high density), poly- propylene and polybutylene, polymethylpentene, and copolymers of these and also with other copolymerisable monomers, e.g. ethyl acrylat and vinyl acetate, especially with ethylene, styrene plastics, polystyrene, styrene-acrylonitrile plastics, acrylonitrile-butadiene-styrene plastics, methacrylate- -butadiene-styrene plastics, methacrylate-styrene plastics, thermoplastic styrene-butadiene rubber, vinyl plastics, e.g. vinylchloride plastics, polyvinylchloride, vinylacetate plastics, polyvinylacetate, vinylacetal plastics, polyvinyl- acetal, vinyl alcohol plastics, polyvinyl alcohol, amide plastics, polyamides, nylon, cellulose plastics, cellulose acetate, cellulose acetate- butyrate, cellulose acetate propionate, ethylcellulose, acryl plastics, polyacrylate, polymethyl acrylate, formaldehyde plastics, polyformaldehyde, acetal plastics, carbonate plastics, polycarbonate, fluoro-chloro-olefin plastics, fluoroethylene plastics, polyfluoroethylene, polytetrafluoroethylene, polyfluoro- ethylene propylene, polychlorotrifluoroethylene, polyviny- lidene fluoride, polyvinyl fluoride, chloroether plastics, linear estei* plastics, linear polyesters, polyethylene- terephthalate, polyethyleneglycolterephthalate, phenyleneoxide plastics, polyphenyleneoxide, aromatic sulphone plastics, polysulphones, phenoxy plastics, and furthermore also curable plastics, such as pheno plastics a ino plastics, unsaturated ester plastics, alkyd plastics, allyl plastics, epoxy plastics, casein plastics, silicon plastics, furan plastics, furfuryl plastics, urethane plastic polyurethane.

Said plastics may be used in the shape of e.g. foils, sheets, foam,sponge, fibres, beads, grains, etc.

To said plastics other materials can be bonded or combined, e.g. metal, ceramics, glass, textiles, woven and non-woven, such as cotton, wool, viscose, etc.

The following comprises a description of an embodiment of a tool which is especially suited for use according to the invention and comprises a substrate body, preferably in the shape of a straight ruler or a prismatic body, e.g. an elongated body with essentially rectangular cross-section shape, and an electric heating means arranged in the longi¬ tudinal direction of the body with the body as a supporting substrate, preferably on one of the narrow longitudinal side surfaces of the body. According to the invention said heating means is maintained under traction tension with a

GiVP tensioning means, e.g. helical springs or similar spring means, at one or both ends of said heating means. One such device is disclosed schematically on Figure 10 in a side view and on Figure 11 in a section along A-A. On said elongated substrate body 101 there is arranged a resistance element 102 in the shape of a metal strip extending along a central part intended to act upon a treated material, between two edges 103 of said substrate body acting as deflection points for said strip so that said strip by the influence of a tension at the end part can be maintain¬ ed bent in an angle against said central part at said edges 103. The figure shows also a heat insulating and/or electrically insulating spacer 105 between the resistance strip 102 and the substrate body 101. The end parts 104 of said strip are fixed to a tensioning means 106 extending through an opening 107 in a flange-like part or protrusion 108 at the end part of the body 101. Said tensioning means 106 has the shape of a bolt with a threaded end part which extends outwardly from said flange 108, said outwardly extending part being surrounded by a helical spring 109 urging against a nut 110 on said threaded end of said means 106 which with a further nut 111 holds a cable clip 112 at the end of an electric supply cable 113. A corresponding tensioning means can be arranged at the other end of the tool body 101. By arranging the resistance element deflected at a deflecting point or deflecting means at an angle away from the central part of the resistance means intended for application against the work piece the electrical connec¬ tion means will require only a small space so that a plurality of heating means can be arranged at a small mutual distance parallel with each other.

Figures 12-19 show similar embodiments of tools which are suitable for use according to the invention. An opening 114 for inserting a temperature measuring means e.g. a thermistor, is also disclosed on the figures. Furthermore, a coating 115 intended to prevent adherence to the work piece, e.g. made from Teflon glass fabric or a similar material is arranged on said resistance strip 102. Figure 17 shows a group of tools arranged in parallel, the tensioning means 106 on adjacent tools being directed alternating so that they form a larger angle QL~ of the means 106a and a smaller angle cL- of the means 106b, resp. , in relation to the central part of the resistance strip. By alternating arrangement the connection of the electrical lines 112-113 is facilitated.

Figures 118 and 119 show sections through such groups of tools 101 provided with heating means clamped in retaining means 117.

Below a number of embdiments are disclosed which comprise reference numerals corresponding to the drawings.

1. Electric forming tool provided with resistance heating means especially a welding tool (welding clamp or welding jaw) for welding plastics and similar, characterized in that said tool comprises a body 101 acting as heat insulat¬ ing substrate, said body preferably having lower heat conductivity than metals (steel, brass) , and a resistance heating means 102 arranged at the intended working surface of said tool.

2. A tool according to embodiment 1, characterized in that said substrate consists of a body of plastics or elastics, especially a polymeric material, which preferably is enforced with an enforcing filler, such as glass fibres, c fabric material or similar material, e.g. a phenolform- aldehyde resin reinforced e.g. with a fabric or glass fibres.

3. A tool according to embodiment 1 or 2, characterized in that said resistance heating means consists of metal, such as a metal strip, or an electrically conducting

r _C PI_ polymeric material, especially elastics material, such as rubber, with conducting filler, such as carbon particles.

4. A tool according to any of the preceding embodiments, characterized in that said heating means on the surface facing the work piece is coated or covered with a material 115 with good release characteristics in relation to the treated material, e.g. a fluorine containing organic poly¬ mer, e.g. polymerized fluorinated olefin, such as polytetrafluoro ethylene.

5. A tool according to any of the preceding embodiments, characterized in that the electrical resistance heating means 102 is maintained under tension with a spring means 109 excerting a tension force in the longitudinal direction of said heating means, said heating means consisting of an elongated body, preferably in the shape of a strip and preferably with rectangular cross-section.

6. A tool according to embodiment 5, characterized in that said tool comprises an elongated substrate body 101 and arranged on said substrate body an elongated heating means, at least a part 104 of said heating means at least at one end of said heating means being directed in an angle in relation to an adjacent, central part of. the heating means, the surface or the plane along which said heating means contacts the material being heated, and combined with a spring means 109 maintaining said heating means under tension, said end part of said heating means preferably also being connected to an electrical connecting means 112 for supplying heating power.

7. A tool according to embodiment 5 or 6, characterized in that the end part 104 of said heating means or the connecting means 106 connected to said end part extends through an opening 107 in said substrate body, with a compressed spring means 109, preferably a helical spring means, urging against the end of said heating means or a connecting means connected to said heating means, preferably against a protruding flange like part 108 or nut 110 on said connecting means, so that said heating means hereby is maintained under tension.

8. A- tool according to any of the preceding embodiments, characterized in that said substrate body consists of an elongated body 101 with preferably essentially square or rectangular cross-section, said heating means 102 being arranged on one side of said substrate and forming a flat or curved working surface facing the work piece, the end of said substrate body being provided with a protrusion 108 or a flange with a recess or opening 107 through which the end part of said heating means 102 or the connecting means 107 extends, said protrusion or flange protruding from a part of said substrate spaced from the side of said substrate facing said heating means, the end part of said heating means and/or the connecting means and optionally said opening or recess in said flange or protrusion and preferably also said protrusion or flange being directed in an oblique angle o, ,Q.„ against the working surface of said heating means, preferably in an angle of 15-75 .

Claims

1. A device for hot working, preferably hot -forming or welding of plastics, comprising electric heating means for said hot working and a control device for supplying electric heating power to said heating means, characterized in that said control device comprises a) power restricting means (3) , b) means (9) for setting a maximum heating power, said setting means (9) being connected to a control unit (4) which controls said power restricting means, c) means (6) for measuring the heating temperature, said means being connected to said control unit (4) , the signal from said temperature measuring means (6) acting through said control unit (4) on said power restricting means (3) to continuously or in a plurality of steps, preferably at least 3 steps, which preferably are essentially of equal size, restrict the heating power when exceeding a parti¬ cular, pre-selected temperature, and d) pulse control means (8) arranged for maintaining the heating power switched-on for an adjustable period of time during at least one continuous pulse period.

2. A device according to claim 1, characterized in that the device comprises connected to said pulse control means (8) a pulse repeating means for repeating a selected heating pulse, and means (10) for pre-selecting the number of pulses, and means (11) for pre-selecting a pause time period between each pulse in the hot working sequence.

3. A device according to claim 1 or 2 , characterized in that said power restricting means (3) is a thyristor and in that said control unit (4) supplies the thyristor with control or trigger pulses for phase control.

4. A device according to claim 2, characterized in that said power restricting means (3) consists of a thyristor working as a static switch which is acted upon by said temperature measuring means through said pulse control in a manner such that the pulse time is continuously reduced when exceeding a particular pre-selected temperature.

5. A device according to any of the preceding claims, characterized in that the means (6) for measuring the heating temperature is a thermistor.

6. A device according to any of claims 1-5, characterized in that said control unit comprises periphery means for adjusting the derivative of the output power versus tempera¬ ture when exceeding a particular pre-selected temperature.

7. A device according to any of claims 1-6, characterized in that said temperature measuring means is arranged in the immediate vicinity of said heating means, preferably at a distance of at most 1 mm, especially at most 0.5 mm.

8. A process for hot working, especially hot forming or welding plastics, with the aid of electric heating means for said hot working, especially resistance heating means, in which one controls the supply of electric heating power to said heating means during at least one continuous puls period, characterized by controlling the heating power, supplied by said heating means, continuously or in a plurality of steps, preferably at least three essentially equal steps, with the aid of temperature measuring means arranged to measure the temperature of the heating means or of the plastics subjected to hot working, said control being performed from a pre-selected upper controlled outset effect or starting power at a pre-selected lower starting temperature T, to a lower controlled end power T at a predetermined final temperature T . 23

9. A process according to claim 8, characterized by performing said control of the supplied heating power with the device according to any of claims 1-7.