CA1057927A - Manufacture of thin walled plastics articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Plastics sheeting to be thermoformed into containers as heated to the thermoforming temperature by immersion in heated liquid. This ensures even heating and is exceptionally beneficial in the heating of polypropylene, which has a critical, narrow thermoforming temperature range. The machine for effecting heating is provided with apparatus whereby residual bath liquid can be removed from the sheeting after it leaves the heating bath and before thermoforming.

Description

1()5'79~7 1 Thi~ invention relates to the manufacture of thin walled plastics material articles which are for~ed from ~heet plastic# materials (which may be laminates).
Any suitable technique such as vacuum forming, pres~ure forming, plug assist or mechanical forming or any combination of ~uch techniques may be used in the forming of the articles, but in all cases the sheet material i3 heated to effect a degree of softening of the material ~o that it can be formed and therefore the forming of the articles will be referred to simply and generally as ~ther~oforming".
Thermoforming is of course well known and i~ widely practised, and there are many thermoformed articles in every day use in many countries of the world, such articles including domestic holloware and drinking CUps. t Certain materials, and in particular polypropylene, polyethylene and expanded plastics sheet require to be thermoformed at very accurate temper?tures in order to achieve high speed production and high quality product~.
Conventional methods of heating such as infra-red heaters do not provide sufficient accuracy of heating to achieve these desirable features even when the sheets being thermoformed are of uniform thickness which in commercial practice is not usually the case.
; Furthe D re, many types of plastics sheet wrinkle or sag when being heated which causes even further temperature variations in the heated sheet when the heat-ing is carried out by a radiation proces#.
The present invention seeks to provide a method of heating which is useful for a very wide range of pla~tics ~heets including polystyrene, A.B.S. acrylonitrile, nylon, ~()5 ~Z7 polyester, polyvinyl chloride,cellulose acetate and indeed any sheet which i9 capable of being thermoformed, and in which the aforesaid disadvantages are obviated or mitigated.
According to the present invention there is provided a method for the manufacture of thin walled plastics material articles, wherein the articleQ are thermoformed ina heated sheet of the plastics material and the sheet i8 heated by a heating process including immersion of the sheet in the heating liquid of a heating bath.
Heating by immer6ion in a liquid ensure~ that even heating of the sheet a~ross its width, and the sheet can at no point reach a temperature which is graater ~han that of the bath liquid.
The sheet may be in web form and may be fed into and out of the bath liquid.
The bath will preferably be a single bath and the web will be arranged to run therethrough and direétly to a thermoforming machine or unit at which the articles are formed. The construction of the articles i8 of course immaterial to this invention.
The web preferably travels out of the bath in an upwards ~preferably vertical) direction so that residual bath liquid can run back into the back. The web may be passed between the nip of a pair of sgueeze rollers, maintained at a temperature comparable with that of the web 80 as not to cool same, with this end in view.
It is desireable that the liquid in the heating bath should be of a type which does not wet or have a wetting effect on the web, 80 that there will be minimum or no carry over of liquid from the bath by the web.

1 The web may require to travel ~hrough a duct in it8 travel between the bath and the thermoforming unit. It is preferred that this duct be closed and flooded with a gas or vapour which does not effect the web chemically but which i8 at a constant and controlled temperature.
The bath liquid temperature will alsp preferably be under careful control, and to this end it may be circulated through a heat exchanger under the control of thermostat means.
Where the plastics material is polyp~opylene web, the heating liquid of the bath may be for example molten metal, such as woods metal, glycerine, polyethylene glycol, polypropylene glycol or ammonium nitrate.
Where the bath liquid tends to evaporate through being heated, a condensation unit may be provided to return condensed liquid back to the bath.
The temperature of the bath liquid will preferably be thermostatical~ly controlled in order to ensure that the web reaches the critical temperature. The minimum length of time which the web must be in the bath to reach the desired temperature will depend upon the dimen~ions of the web but in any case this time can be adjusted to suit the material being heated. There is no maximum immersion time as the temperature of the web cannot get higher then the bath liquid temperature.
We have carried out tests with polypropylene 3heet of melt flow 3.5 and thickness of 0.03" and we find that a 20 sec~nd immersion time in glycerine or polyethylene glycol is sufficient to raise the polypropylene sheet to the bath tempe~ature of 150 C. Where molten metal is u~ed, this time can be as little as 3 seconds.

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~QS7~'X7 1 Articles, such as domestic holloware, formed from polypropylene heated in this manner can be reliably produced in large quantities and taking large numbers on average they exhibit an excellent surface finish, toughness and clarity which are better than those characteristics in conventionally formed polystyrene articles.
It is not necessary that the liquid ba~h should comprise the whole of the heating of the web. Pre-heating and/or post heating such as steam heating, radiant heating or the like may be employed. This may be suitable where the plastics material web being heated is polystyrene, as the liquid bath could be used to perform the major portion of the heating of the web, and for example, conventional radiant heaters, which are inefficient, could be used for only the small post heating step of bringing the web up to forming temperature from the temperature at which it leaves the liquid bath. In any event, the heating by immersion in the liquid is always beneficial because of the even temperature distribution which is achieved.
The invention thus provides a simple method whereby plastics material webs can be heated evenly, enabling the effective use of plastics materials such as the polyolefins e.g. polypropylene, for thermoformed articles produced in large numbers; the cycle time can be made very short and in many cases a cheaper yet superior article can be produced.
The invention also provides apparatus for carrying out the method as aforesaid and articles produced according to the method as aforesaid.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying 3~ diagrammatic drawings, wherein:-Fig. 1 is a front view looking in the direction ofarrow 1 in Fig. 2 of a machine accordiny to and for carrying ~057927 1 out the method according to the present invention:
Fig. 2 is a sectional side view of the machine of Fig. 1, but showing only parts thereof;
Fig. 3 is a front view of part of the machine and showing a detail;
Fig. 4 is a sectional side view of a detail of the part of the machine shown in Fig. 3, the section being taken on line IV - IV of Fig. 3; and Fig. 5 is a sectional side view of a detail of the part of the machine shown in Fig. 3, the section being on line V - V of Fig. 3.
Referring to the drawings, the apparatus as shown in Fig. 1 comprises the casing 10 in which is contained a removable roller assembly 12. The casing 10 in fact forms a ; liquid bath and the normal liquid level in the bath is indicated by the reference numeral 14. When the assembly 12 is located ' in the casing 10, it is essentially located submerged in the liquid.
The assembly 12 comprises a pair of fork shaped 20 brackets 16 between which are rotatably supported two lower rollers 18 and 20, such rollers 18 and 20 being on the ends of the prongs of the fork shaped brackets 16 and a single upper roller 22. The rollers 18 and 20 are symmetrically disposed in relation to roller 22, and the axis of all three rollers 18,20 and 22 in the in use position, lie in vertical parallel planes.
Inside the casing there are two heated coils 24 and 26 which project between the prongs of the fork shaped brackets 16, and respectively extend from opposite sides of the casing 10.

Each coil 24 is supplied with electrical power from a power unit 26 which is thermostatically controlled by means (not shown) -1(~579'~7 1 influenced by the temperature of the liquid in the casing 10.
The roller assembly 12 is removably located in the casing 10 when in use, by having guide pins 28 engaged in slots 30 of guide bars 32 attached to the casing 10. This roller assembly can be lifted freely out of the top of the casing as desired. The casing 10 may have a removable cover should this prove to be necessary.
ln order to heat a web of synthetic plastics material to the temperature of the liquid in the bath the plastics material is fed in sequence around the rollers 20, 22 and 18, and in the arrangement shown in Fig. 2 a roll 34 of plastics web material passes initially over a guide roller 36 located to the front of the machine and above the bath defined by casing 10. This roller 36 has axially displacable collars 38 which can be positioned axially of the roller 36 to suit ; the width of web 34. The web travels vertically downwards from roller 36 and into the liquid in the liquid bath of casing 10.
Whilst in the liquid it passes round roller 20, roller 22 and then roller 18 as shown, and eventually the web 34 leaves the liquid bath in a vertical manner and passes into the vertical narrow duct 40 at the top of which is located a guide roller 42 of similar construction to roller 36, and having axially displacable collars 44 which are similar to collars 38. From this roller 42 the web passes horizontally, as shown in Fig. 2, to a thermoforming machine in which the articles are formed in the heated web.
As will be clear, the function of immersing the plastics web in the liquid bath is to heat the web evenly throughout its entire surface area, and to this end the liquid in the bath is maintained as accurately as possible at a 1057~'~7 1 predetermined temperature. The thermostatic control ensures that the temperature of the liquid is maintained accurately within predetermined limits.
It is desirable that the temperature of the web 34 should be maintained as it passes up to the duct 40, and to this end duct 40 is defined by a pair of space plates 46 and 48, the plate 46 having at the top end thereof, a horizontal extension 50. The sides of the duct 40 will be closed by suitable end closure plates.
In order to maintain the plates 46, 48 and 50 at a temperature matching the temperature to which the web has been heated in the liquid bath, the liquid from the bath is lead through bores 52 in the unit comprising plate 50 and plate 46, and through similar bores 54 in the plate 48. In fact, the liquid is lead from the heating bath to the first bore 52A in plate 50, and then is lead in turn, so that it follows a meandering path, through each of the bores 52 in plate 50, then each of the bores 52 in plate 46, until it is directed from bore 52B, the last bore in plate 46, to the first bore 52C in plate 48.
20 Again, the liquid follows a back and forth path through the -~
bores 52 in plate 48 in turn, until the liquid issuing from the last bore 52D in plate 48 is eventually returned to the liquid bath. This ensures that the plates 46, 48 and 50 are maintained basically at the same temperature as the liquid in the bath, and therefore the web 34 will be maintained at the even temperature over its area.
Indeed, it is the intention that the machine should be appropriately insulated, in order to minimize heat loss as far as possible, or completely. The entire machine will under these circumstances, run at the predetermined temperature . .

10579'~7 1 to which the web is to be heated for best results for mass production thermoforming.
As the web 34 leaves the liquid in casing 10, it may carry with it some residual liquid, and it is desirable that this liquid should be removed before the web reaches the thermoforming machine. It is to be pointed out that the liquid which will be used in the liquid bath will be of a nature as not to chemically attack the web, and will also be selected so that it does not tend to wet out the web 34. In other words, lO by the nature of the liquid andits compatability to the web material, it is expected that there will be very little carry over of residual liquid by the web 34.
The arrangement shown in Figs. 3, 4 and 5 however is designed to effect a wiping of the web to remove such - residual liquid, before the web reaches the thermoforming machine.
~ The means for achieving this wiping in the example ; illustrated, comprises four steam jet tubes 56, 58, 60 and 62.
These tubes are arranged in pairs, 56 and 58 and 60 and 62, one pair as shown being attached to the plate 46 inside duct 40 and the other pair being attached to plate 48 inside duct 40.
Each of the tubes 56 to 62 is provided with an axial slot 56A -to 62A, and each of said slots is arranged so that steam which issues therefrom is directed in a downwardly inclined manner towards the upwardly travelling web 34, so as to wipe from the web any residual liquid thereon and to cause it to fall back down the duct 40 and into the liquid bath. The tubes 56 to 62 which are of copper, are secured centrally thereof to the plates 46 and 48 by means of individual clamps 56B to 62B as shown in Fig. 4, and the ends of the tubes are held by brackets 64 and 66 in such a fashion that the tubes can expand axially lOS7~27 1 with heating and cooling in use from the grippec centre portions thereof.
Each of the tubes 56 to 62 is adapted to produce the curtain of steam directed towards the upwardly travelling ;-web 34, and in,order to achieve an even steam pressure in each of the tubes 56 to 62, inside each tube there is (not shown) a feed tube through which the steam is passed, and this feed tube has a plurality of parallel and shellically disposed slits therein which ensures that an even pressure pertains through the whole of the inside of each tube.
In some cases it may not be necessary to use all the tubes 56 to 62A, and the equipment has means whereby one tube of each pair can be cut off from the steam supply.
The steam supply for the tubes 56 to 62 will be such -that the steam temperature is comparable with the running temperature of the machine. Thus, the steam will not have the effect of overheating or overcooling of the web 34, which could be detrimental in the subsequent thermoforming of the articles in the web 34.
The tubes 56 to 62A are held in effective contact with the plates 46 and 48 in that the plates 46 and 48 have small acute groves in which the tubes 56 to 62 are respectively engaged. This good contact ensures efficient transmission of -heat from the plates 48 to the tubes 56 to 68, and vice versa.
The equipment shown is suitable for the heating of polypropylene sheet material having a melt flow index of 3.5, such melt flow index being that referred to ASTM values.
The equipment may be provided with a number of modifications not shown in the drawings. For example there --may be a tension roller in order to maintain the tension in the : ' '. ~
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lV579A~7 1 web 34 should this be necessary. Furthermore, there may be provided means for holding the edges of the web 34 as it travels through the machine, although it is believed that because in some cases it is not necessary to provide guiding means such as chains, for holding the edges of the material, this is a particular advantage of the present invention. The guiding of chains cannot take into account the lateral and lengthwise extension of plastics sheet material which usually takes place when the material is heated. The machine may also be provided with means whereby the length of path through the machine can be varied.
There will of course be a suitable means for circulating the heating liquid in the manner indicated, such means being a suitable pump for example.
Where the material 34 being heated is for example polypropylene of melt flow index of 3.5, it is preferred that the temperature of the bath be maintained at 158 centigrade, ; and that this temperature be accurately controlled by the -;

thermostats. The liquid in the bath may be of any suitable liquid as disclosed herein, although it should be mentioned that the temperature of the bath may require to be altered - depending upon the molecular weight of the polypropylene and/or its crystalline structure. In forming polypropylene, it is desirable that the polypropylene should be heated to a temperature which is under the melt temperature of the poly-propylene being heated, so that indeed the polypropylene is formed whilst still in a solid as opposed to a liquid condition. The selection of the temperature to which the web is to be heated by the liquid in the bath will of course be a matter of section depending upon the material to be heated.

10579'X~
1 In the apparatus described, the web is fed into the liquid by means of a pair of driven rolls 36A and pulled out of the bath by means of chains which hold the web as it is passed through the thermoforming machine. There may also be a dancing roller 36B between the feed rolls and the guide roll in order to enable periodic overfeeding of the web into the bath.
Any suitable other means may be used for feeding the web into -and through the bath. The drive means for feeding the web is preferably of a variable nature, so that the speed of the travel of the web through the bath is variable. When the web is fed directly to the thermoforming machine, it will be understood that the web will dwell in the bath whilst the thermoforming machine operates upon a previously heated section of the web to form the articles therein. It may also be desirable to arrange for each or any of the rollers 18, 20 and 22 to be drivable.
In the example described, the means for removing -excess liquid or residual liquid remaining on the web as it leaves the liquid bath comprises the pipes 56 to 62. Instead of or in addition to this arrangement, there may be other means for effecting removal of the excess liquid. In one arrangement, the excess liquid can be removed by creating a vibration on ; the material as it leaves the bath. This can be done by vibrating on the material directly by suitable means, or by creating turbulence on the top of the liquid in the treatment bath at the region where the web of material leaves such treatment bath.
Another possibility is to use flame jets. These jets are momentarily played upon the sheet after it leaves the bath.
f In another arrangement, the web may be caused to pass subsequently through a bath of a liquid which will not tend .
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~0579'~7 1 to adhere to the web such as a subsequent bath containing merc~lry of liquid metal. The web would therefore pass through two treatment baths, the first containing a liquid having a characteristic which makes it tend to wet out the web more than the liquid in the subsequent bath, so that the subsequent bath has a wiping effect on the material. In this case, because the subsequent bath has the function only of wiping the web, it may be made quite~ small. In a practical construction, a partition plate defines a small chamber for the liquid of the subsequent bath, such chamber having an entry side and an exit side, and any excess liquid from the first treatment bathl removed from the web in the chamber, would tend to flow to the surface at the entry side of the chamber, enabling same to be drawn off at will whilst the material travels from the exit of the chamber in dry condition. In another practical construction, a subsequent treatment bath for example containing liquid metal may be located in the treatment bath proper, and may have a very narrow slit through which the material can pass, such slit being on the base of the subsequent treatment bath. The slit would be so dimensioned that the mercury could not pass through same, but the web is enabled to pass through the slit and in so doing is wiped clean by the liquid metal. The subsequent treatment baths of the examples described above, would be kept at approximately the same temperature as the liquid in the main treatment bath.
Another means for use in removing residual liquid may be heated knives which actually contact the web as it leaves the liquid. Such knives would require to be heated, in order not to chill the web before it reaches the thermoforming machine. It is also possible to use squeeze 3~ rollers maintained at the temperature to which the web has 1~579'~
1 been heated in the bath. Such rollers can be heated by the passage of the liquid of the bath therethrough in the same manner in which the liquid is cycled through the plates 46 and 48.
The web itself may of course be treate'd with a liquid repellent substance, such as silicone oil or grease which would glve the web the characteristic of resisting liquid retention or wetting out of the web.
Normally, thermoforming machines are disposed so that the forming dies'operate by relative movément in a vertical direction. This arrangement requires the heated web of material to be turned to a horizontal position, if it-leaves the liquid bath in a vertical direction which is desirable. It is of course possible to arrange for the thermoforming machine j to be'located above the liquid heating bath, and for the dies to be arranged to operate by relative movements in a horizontal direction. This wo11ld enable the web to travel directly into a position between the dies, without regular variation from the vertical disposition. A composite arrangement such as this could be extremely advantageous, because if the thermo-forming machine and the liquid heating bath were formed as an '~ integral unit, then the retention of the heat in the apparatus, -the power required to maintain the apparatus at the desired temperature, would be improved.

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Claims (32)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for the continuous manufacture of thin walled plastics material articles, wherein the articles are thermoformed in a heated web of the plastics material fed along a path of travel to a thermoforming machine and the web is heated evenly in said path of travel by a heating process including immersion of the web in the heating liquid of a heating bath for such a period until the web reaches an even temperature across its width and the web whilst hot passes directly to the thermoforming machine, and wherein residual heating bath liquid is removed from both sides of the web after it travels out of the bath and before being thermoformed.

2. A method according to claim 1, characterized in that the temperature of the liquid in the bath is automatically controlled so as to remain within predetermined critical limits.

3. A method according to claim 2, characterized in that the web travels out of the liquid in the heating bath in an upwards, preferably vertical direction.

4. A method according to claim 1, characterized in that jets of gas, vapour and/or flame are directed onto the web for the removal of residual liquid.

5. A method according to claim 4, wherein the jets are directed onto the web in a downwardly inclined direction.

6. A method according to claim 4, characterized in that jets are of steam at a temperature comparable to that to which the web is heated by the bath liquid.

7. A method according to claim 4, characterized in that two jets, each acting as a knife, act on each side of the web.

8. A method according to claim 4, characterized in that the web is passed through a pair of nip rollers for the removal of residual liquid.

9. A method according to claim 4 characterized in that the web is passed through an additional bath of a different liquid which wipes residual liquid from the web.

10. A method according to claim 9, characterized in that the additional liquid is liquid metal maintained at the same temperature as the liquid bath.

11. A method according to claim 1 characterized in that the web after leaving the liquid bath enters a duct before passing to the thermoforming machine.

12. A method according to claim 11 characterized in that the means defining the duct are heated to maintain the duct at a temperature as near the temperature of the bath liquid as possible.

13. A method according to claim 12 characterized in that the means defining the duct are plates having passage means therein and through which the liquid from the liquid bath is circulated.

14. A method according to claim 1 characterized in that the liquid in the bath is heated by heating coils which are located in the bath, and which are thermostatically controlled in dependance upon the temperature of the liquid in the bath.

15. A method according to claim 1 characterized in that the web is of polypropylene.

16. A method according to claim 15, characterized in that the bath is maintained at a temperature of 158°C.

17. A method according to claim 15, characterized in that the web is immersed in the liquid bath for at least 20 seconds.

18. A method according to any of claims 15 to 17, charac-terized in that the liquid of the heating bath is molten metal, glycerine, polyethylene, glycol, polypropylene glycol or ammonium nitrate.

19. A continuous method for the manufacture of thermo-forming of thin walled articles from a length of polypropylene sheet comprising the steps of:
(1) causing the polypropylene sheet to travel along a path to a thermoforming station at which the sheet is thermoformed into thin walled articles, (2) causing the polypropylene sheet to be immersed in a heating bath of liquid during a portion of its travel along the path so that the web becomes heated to an even temperature across its width, (3) maintaining the temperature of the liquid in the heating bath within the range required to cause softening of the polypropylene sheet, (4) causing the evenly heated sheet in its travel toward the thermoforming station to move upwardly out of the liquid in the heating bath, (5) removing residual heating bath liquid from the sheet where it extends upwardly from the bath by directing jets of fluid onto both sides of the sheet to cause the residual liquid to flow back into the bath, and (6) thermoforming the softened sheet at the thermoforming station to produce articles therefrom.

20. The continuous method according to claim 19, wherein the jets of fluid directed onto the sheet to remove residual heating bath fluid are jets of gas whose temperature is identical with or approximately the same as the temperature of the bath.

21. The continuous method according to claim 20, wherein the jets of fluid are of steam.

22. The continuous method according to claim 19, further including the steps of:
(7) preventing the heated polypropylene sheet from cooling after leaving the bath by moving the sheet along a duct leading to the thermoforming station, and (8) maintaining the temperature in the duct at a temperature not less than that of the liquid in the heating bath.

23. The continuous method according to claim 22 wherein the temperature of the duct is maintained by heating the duct with liquid drawn from the heating bath.

24. A plant for the manufacture of thin walled plastics material articles, including a heating means by which a web of the plastics material is evenly heated to a thermoforming temperature, a thermoforming machine wherein the heated web is thermoformed into the articles, a feed means for feeding the web from the heating means directly to the thermoforming machines whilst still hot, wherein said heating means comprises a bath for containing a liquid which does not chemically affect the plastics web, means whereby the web can be immersed in the liquid for sufficient time to be evenly heated, and means for removing residual liquid from the web after it has left the heating bath and before being thermoformed.

25. A plant according to claim 24, characterized in that the feed means is adapted to remove the web from the bath in a vertical direction.

26. A plant according to claim 24, characterized in that said means for removing excess liquid from the web includes a pair of nip rollers.

27. A plant according to claim 25, characterized in that said means for removing excess liquid from the web includes a unit for creating jets of gas which impinge on the web.

28. A plant according to claim 24, characterized in that said means for removing excess liquid from the web, includes a bath for a different liquid through which the web can be passed by the feed means.

29. A plant according to claim 24, characterized by an exit duct along which the web passes in travelling from the bath to the thermoforming machine.

30. A plant according to claim 29, characterized by circulating means for circulating the heating bath fluid through passageways in said duct plates to maintain the web at the thermoforming temperature whilst in the duct.

31. A plant according to claim 24, characterized in that the heating means includes heating coils disposed in the heating bath.

32. A plant according to claim 31, characterized in that the bath temperature is maintained constant within limits by thermostats which control the operation of the heating coils.