GB2137554A

GB2137554A - Plastic web for moulding

Info

Publication number: GB2137554A
Application number: GB08408153A
Authority: GB
Inventors: Gunther Kiefer; Hans Kohler
Original assignee: Adolf Illig Maschinenbau GmbH and Co KG
Current assignee: Adolf Illig Maschinenbau GmbH and Co KG
Priority date: 1983-04-02
Filing date: 1984-03-29
Publication date: 1984-10-10
Also published as: GB8408153D0; FR2543878B1; GB2137554B; DE3311988C1; FR2543878A1

Abstract

Heating apparatus, for the heating of a strip (18) of thermoplastic synthetic material before thermal moulding in an intermittently operating thermal moulding machine (14) comprises rotatable heating rollers (3, 4) around which the strip is guided, and a drive (6) for transporting the strip by way of a transport roller (21) arranged downstream of the heating rollers. The transport roller (21) is continuously drivable by means of the drive (6), which includes a regulating gear (23), and the size of a reserve loop (27), formed in the strip (18) between the heating apparatus and the moulding machine, can be scanned by means of a light barrier (26) for control of the transport roller (21) in dependence on the size of the loop. <IMAGE>

Description

SPECIFICATION Heating apparatus for heating a strip of thermoplastic sheet material The present invention relates to apparatus for heating a strip of thermoplastic sheet material, especially plastics film prior to thermal moulding thereof in a downstream intermittently operable moulding machine. After heating the strip for the thermal moulding machine, the strip can be deep-drawn through differential pressure in a known manner into articles of various forms.

The main advantage of a preheater lies in the processing polypropylene film. Such a film needs a substantially higher quantity of heat in order to reach its deformation temperature of about 1 60 C than, for example, a polystyrol film of like thickness.

Moreover, the deformation temperature must be met fairly accurately in the case of polypropylene film and substantial temperature variations, for example in the interior of the film, are not acceptable. It is therefore necessary to heat for a sufficiently long time in order to bring the interior of thicker films to the necessary temperature.

When it is desired to use an automatic thermal moulding machine, which is designed in terms of heating for polystyrol or polyvinyl chloride, with a polypropylene film, the film is preferably preheated by a preheater to about 80 to 1 00 C. Three advantages then arise: a) the heater of the automatic thermal moulding machine is then sufficient to heat up to the final temperature of the polypropylene film without the machine cycle rate having to be lowered; b) the supplied heat flows, during an equalisation path, into the interior of the film; and c) before the film is seized by transport chains of the automatic thermal moulding machine, it is preheated and has expanded by a certain amount.

The expansion arising during the remaining heating in the clamped-in state can be managed more easily than when the entire heating takes place in the clamped-in state, because in the former case the expansion is only about half as great.

Heating with tempered rollers, which can also act as a preheater, is known from DE-OS 25 44 042. In the description of this device, the problem of how the film is transported through the preheater and how matching takes place with the intermittent transport of the thermal moulding machine, is not entered into in any detail. It can be inferred from the description on pages 5 and 6 that the heating rollers are to be moved cyclically. It can also be inferred from the description in the middle of page 10 that the rollers 30, 32 and 34 can run "at slightly higher speed". However, it is not stated in respect of which speed. It is beyond doubt that such an influencing of speed for compensation for longitudinal expansion of the film is expensive and laborious. Consequently, the expert can infer from this specification only a device which has cyclically driven rollers.Matching with a respective transport of the thermal moulding machine is not referred to.

Since the rollers are of metal and must have a smooth surface, the danger of film slippage is present, so that the advance does not agree with the predetermined value. Since the rollers must have a large diameter in order for sufficient heating to be possible at high cyclic rates, they have a great mass.

In order to drive and brake these rollers during each advancing step, thus per cycle, a high amount of energy is required. Also, inaccuracies in advance inevitably occur in this case. In the illustrated device, moreover, the insertion of the film is not entered into, which is made more difficult by the fact that the rollers are hot and danger of burning exists.

A further disadvantage of the device according to DE-OS 25 44042 is that the heating rollers are heated by circulating oil. Expensive tempering devices are necessary for this as the regulation of the temperature is very sluggish. The oil must be fed to the rotating rollers through rotary joints and the danger of the leakage always exists. Heating at the start of operations takes a rather long time.

US-PS 43 06 856 discloses a preheater, in which the film strip partially extends perpendicularly. Heaters are arranged at both sides of the strip. The preheater does not have drive of its own, i.e. the transport of the thermal moulding machine draws the film intermittently through the preheater. This is disadvantageous insofar as the transport should operate as rapidly as possible in order to keep the cycle time of the thermal moulding machine as low as possible and the preheated strip portion must then take over the entire tension during the acceleration of the following strip and during turning of the strip roll. The consequence is film distortion which is unacceptable, for example in the case of preprinted film.During standstill of the thermal moulding machine, the radiant heaters, which have a substan tially higher temperature than the preheat tempera- ture of the film, must be driven into a rest position in order to avoid overheating. This means an action of the control of the thermal moulding machine into the control of the preheater. This thus cannot operate self-sufficiently, which would, however, be desirable in, for example, the subsequent equipment of a present thermal moulding machine with a preheater.

Moreover, operation of the thermal moulding machine is feasible without preheater in the case of re-equipment, i.e. a preheater should be able to be pushed against different thermal moulding machines without adaptation of control.

It would thus be desirable to provide a preheater, with retention of the principle of heating by way of rollers, which if required can be disposed as a compact unit in front of a thermal moulding machine without the preheater control having to be connected to the control of the thermal moulding machine. In that case, a reliable and accurate transport of the strip material to be processed should take place through the preheater without danger of the slippage of the strip and without distortion of the strip, and adaptation to the intermittent mode of operation of the thermal moulding machine should be possible. Moreover, a uniform heating of the strip should take place and the introduction of the strip should be easily possible without having to approach the hot rollers.Seen as a whole, the preheater should be relatively favourable in price, be able to be brought rapidly to operating temperature, and the danger of leakage of oil should not exist.

According to the present invention there is provided heating apparatus for heating a strip of thermoplastic sheet material prior to thermal mould- ing thereof in a downstream intermittently operable moulding machine, the apparatus comprising a plurality of rotatable heating rollers for guidance of the strip therearound, a transport roller arranged downstream of the heating rollers and drivable to transport the strip in the apparatus to a delivery side thereof, drive means comprising regulating gear means and operable to so drive the transport roller as to cause a loop to be formed in the strip between the heating rollers and the delivery side of the apparatus, and optical sensing means arranged to monitor the size of such loop.

Preferably, the transport roller runs continuously at settable speed and is disposed, under pressure, in direct circumferential contact with a downstream one of the heating rollers.

The drive of the transport roller is preferably constructed to be regulable and its speed is either manually settable or variable through a setting motor, in which case the intermediate loop of the strip can be scanned between heating rollers and thermal moulding machine by way of two light barriers and rotational speed of the drive for the transport roller correspondingly adapted through appropriate control.

For the introduction of the strip, the heating rollers can be provided with metal guide plates which extend therearound, and freely rotating or drivable guide rollers can be provided, preferably at the end of the metal guide plate, in the case of the last heating roller in order to be able to guide the start of the strip around the last heating roller and up to the transport roller.

In practice, the heating rollers can be heated up to about 11 0 C and the strip surface assumes about the same temperature. If the thermal moulding machine has to be turned off, then the drive of the transport roller can be turned off on attainment of a certain size of the strip loop. Otherwise, the strip remains in contact with the heating rollers, since overheating cannot occur. Moreover, starting of the thermal moulding machine is then possible at any time.

On turning-off the transport roller of the apparatus, the danger may arise that the strip lifts off a lower one of the heating rollers and hangs freely, i.e.

no longer has contact with the lower part of the roller. This is due to the strip expanding during heating and then, because longer, hanging freely downwards.

In order to avoid this, it is proposed to transport the strip for a certain short time in transport direction after the turning-off of the transport roller in order to compensate for the elongation of the strip and to bring this back into intimate contact with the heating rollers until the entire strip assumes the roller temperature. As one means of achieving this, a few seconds after switching-off of the drive it is switched back on again for about one second so as to bring the strip tautly back into contact. This operation can be repeated two or three times. This operation is controlled by way of time switch mechanisms in the control and thus takes place automatically. Alternatively, the drive of the transport roller can be allowed to run on at substantially slower speed for a short time after the switching-off.

Another possibility of compensation for length change would be to draw the strip against the direction of advance, but this is more costly from the mechanical aspect. It then has to be ensured that the strip is not drawn off the roll, i.e. clamping of the film roll or of the film web must be provided.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic front elevation of first heating apparatus embodying the invention; Figure 2 is a cross-section of a heating roller of apparatus embodying the invention; Figure 3 is a schematic elevation of second heating apparatus embodying the invention; Figure 4 is a schematic elevation of one form of strip guide arrangement in apparatus embodying the invention; Figure 5 is a schematic elevation of another form of strip guide arrangement in apparatus embodying the invention; Figure 6 is a schematic elevation of third heating apparatus embodying the invention, showing running-on control means for a transport roller of the apparatus; and Figures 7to 9 are diagrams of different running-on patterns provided by the control means.

Referring now to the drawings, there is shown in Figures 1 and 2 heating apparatus comprising a frame 1 which is mounted on wheels or rollers 2 and carries all working parts of the apparatus. These parts include two heating rollers 3 and 4 and a rubberised transport roller 21 with a drive 6. The construction of one of the heating rollers 3 and 4 is apparent from Figure 2. The roller is hollow inside and individual tubular heater elements 7 are mounted in bores uniformly around the circumferential wall of the roller. In addition, at least one temperature sensor 11 is arranged in the circumferential wall. Discs 8 close off the end openings of the roller and carry bearing spigots 9, which run in bearings 10. Wiper ring bodies 12 and 13 are mounted on the spigots 9 and a current feed to the heater elements 7 and to the sensor 11 takes place by way of brushes 44 and 45.

The two heating rollers 3 and 4 are arranged approximately one above the other so that the apparatus can be pushed over the normal roller take-up 15 of a thermal moulding machine 14.

Provided in about the middle between the two heating rollers 3 and 4 is a deflecting roller 17, which is pivotable away from the heating rollers and which ensures that the heating roller 4 is adequately looped around by a strip 18 of thermoplastic film from the rollertake-up 15. The roller can be pivoted awayforthe introduction of the strip 18.

Arranged at a small spacing from and extending partly around the heating rollers 3 and 4 are two metal guide plates 20 and 19, respectively, which prevent cooling of the heated strip 18 and serve for guidance of the strip during the introduction.

The transport roller 21 is pressed with bias, for example by way of springs, againstthe lower heating roller 3. It can be lifted off by way of an eccentric lever 22 and the strip 18 is then guided through the resulting gap. The transport roller 21 is driven continuously by way of a chain 24 from a drive 6.

In respect of the drive 6 of the transport roller 21, there are several possibilities. One possibility is a direct current motor, the rotational speed of which can be varied through a variation of the voltage. This drive is relatively expensive. Another possibility is a mechanical regulating gear, either with a normal three-phase motor or with a motor with switchable poles.

A regulating gear 23 with a manual adjustment of the rotational speed by way of a handwheel 25 is illustrated in Figure 1. Present in this embodiment is a light barrier 26, which monitors an intermediate loop 27 of the strip 18 formed between the heating apparatus and the thermal moulding machine 14.

Since the machine 14 operates intermittently and the heating apparatus transports continuously, the loop 27 does not have a constant sag. The rotational speed of the regulating gear 23 is set so that somewhat more strip is drawn off than the machine 14 processes. When the loop 27 interrupts the light beam of the light barrier 25, the regulating gear 23 is stopped until the light beam again becomes free, i.e.

during the next advancing step of the thermal moulding machine.

The drive is then restarted after a short time delay of, for example, one second. This time delay effects the required shortening of the loop 27. This short stopping period will always take place after a plurality of cycles.

The embodiment shown in Figure 3 is more expensive, but more reliable. In this case a regulating gear 28 with a setting motor 29, which is operated electrically, is employed as the drive. Two light barriers 30 and 31 check the upper and lower position of an intermediate loop 33. When the light beam of the lower light barrier 31 is interrupted, then a regulating device 34 provides a short pulse to the setting motor 29 in order that the output rotational speed of the regulating gear 28 is set to be slower.

On interruption of the light beam of the upper light barrier 30, a pulse is provided so that the rotational speed is set to be somewhat faster. An automatic adaptation of the output rotational speed of the regulating gear 28 to the required advance thus always takes place on change in the cyclic speed of the thermal moulding machine 14. A light barrier 32 in this case serves to switch the regulating gear 28 off entirely when its light beam is interrupted, for example on stopping of the machine 14. Correspondingly, the regulating gear 28 is restarted when the light beam of the light barrier 32 is freed. The light barriers 30 to 32 are settable in height and thus adaptable to requirements.

For the introduction of the strip 18, the deflecting roller 17 is pivoted outwardly. The strip 18 can then be pushed into the gap between the heating rollers 3 and 4 and the guide plates 19 and 20. It has proved that the strip 18, when it is somewhat thicker, can be pushed without problems almost up to the transport roller 21. Difficulties can arise in the last stage, however, when the strip 18 must be pushed upwardly, In order to circumvent these difficulties, it is possible to arrange additional guide rollers in this region, at least in the middle of the strip 18.

Figure 4 shows an arrangement of two easily rotatable guide rollers 35 and 36, which prevent too strong a friction between strip 18 and metal guide plate 19.

In Figure 5, it is illustrated how the introduction of the strip 18 can be facilitated by a pivotable, rubberised guide roller 38 rotatable by way of a hand wheel 37. After the introduction of the strip 18, the guide roller 38 is pivoted until it presses against the strip, and the roller 38 is then turned by hand until the strip is guided between the transport roller 21 and the heating roller 3.

It is also possible to drive the guide roller 38 from the drive of the transport roller 21 and then, if required, to simply pivot it inwardly so that it pushes the strip 18 forwardly.

When the thermal moulding machine 14 is turned off, then the drive of the heating apparatus is stopped - with delay - when the light beam of the light barrier 26 or 32 is interrupted. If the apparatus stands for a longertime, i.e. longer than a few seconds, then the strip 18 can lift off the lower region of the lower heating roller 3. This is due to the strip expansion during heating, since the strip 18 is on the one hand clamped by the stationary transport roller 21 and on the other hand rests on the heating roller 3 and againstthedeflecting roller 17. Lifting off of the strip is disadvantageous for the reason that a different strip temperature results in the lifted-off region, which can lead to rejects or to greater film consumption during the further operation of the thermal moulding machine.

In order to avoid these problems, the heating apparatus can be operated as described in the following with reference to Figures 6 to 9.

A first possibility is that time switch mechanisms 40 and 41 are incorporated in the control 43. A rest time T1 is set at the time switch mechanism 40 and a running-on time T2 at the time switch mechanism 41. T1 in that case is about 3 to 6 seconds and T2 about 0.5 to 1 second. The rest time T1 starts after the switching off of the drive 6 by way of the light barrier 26. The longitudinal expansion taking place during this time is compensated for by running-on during a running-on time T2.

It is possible to include a preselector switch 42, by way of which a repetition of the running-on can be set.

The running-on can in that case take place at the usual draw-off speed (see diagram of Figure 7) or at slower rotational speed (see diagram of Figure 8) in accordance with the construction of the drive 6.

There is a further possibility when a direct current motor or a regulating gearwith pole-switchable motor is used as the drive, a switching-over of the rotational speed thus being possible. It is then possible to operate with a time switch mechanism 39, which controls a running-on with the running-on time T3, which then takes place at lower rotational speed (see diagram of Figure 9). The running-on times T2 and T3 are in practice simple to determine by experiment.

Claims

1. Heating apparatus for heating a strip of thermoplastic sheet material prior to thermal moulding thereof in a downstream intermittently operable moulding machine, the apparatus comprising a plurality of rotatable heating rollers for guidance of the strip therearound, a transport roller arranged downstream of the heating rollers and drivable to transport the strip in the apparatus to a delivery side thereof, drive means comprising regulating gear means and operable to so drive the transport roller as to cause a loop to be formed in the strip between the heating rollers and the delivery side of the apparatus, and optical sensing means arranged to monitor the size of such loop.

2. Heating apparatus as claimed in claim 1, wherein the transport roller has a resilient circumferential surface and in use is resiliently pressed towards the circumferential surface of a downstream one of the heating rollers to convey the strip between said surfaces.

3. Heating equipment as claimed in either claim 1 or claim 2, the regulating gear means being manually adjustable.

4. Heating apparatus as claimed in either claim 1 or claim 2, comprising an electric setting motor controllable by the optical sensing mens to adjust the regulating gear means in dependence on the size of the loop, the optical sensing means being arranged to provide two light paths each for detecting a respective one of two different sizes of the loops.

5. Heating apparatus as claimed in any one of claims 1 to 3, comprising time switch means switchable to cause the drive means to operate at a speed substantially lower than a normal operating speed thereof, the drive means being switchable before switching off of the drive means by the optical sensing means.

6. Heating apparatus as claimed in any one of claims 1 to 3, comprising first time switch means to terminate drive by the drive means for a first predetermined time period in response to detection by the optical sensing means of a predetermined size of the loop and second time switch means to restore drive by the drive means for a second predetermined time period following the first time period.

7. Heating apparatus as claimed in claim 6, comprising preselector switch means operable to cause the drive means to provide drive for said second time period a selectable number of times.

8. Heating apparatus substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

9. Heating apparatus substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

10. Heating apparatus substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

11. Heating apparatus substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

12. Heating apparatus substantially as hereinbefore described with reference to Figure 6 and any one of Figures 7 to 9 of the accompanying drawings.