Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
  • B29C55/02—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets
  • B29C55/04—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets uniaxial, e.g. oblique
  • B29C55/06—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets uniaxial, e.g. oblique parallel with the direction of feed
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
  • B29C55/005—Shaping by stretching, e.g. drawing through a die; Apparatus therefor characterised by the choice of materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C61/00—Shaping by liberation of internal stresses; Making preforms having internal stresses; Apparatus therefor
  • B29C61/02—Thermal shrinking

Definitions

• the present invention relates to thin plastic film, uses thereof, and to a method of preparing such film.
• Plastics packaging films are well known.
• a film of, for example, low density polyethylene (“LDPE”) (often blended with linear low density polyethylene, "LLDPE”) is made by extrusion.
• Thinner films can be made by adjusting the extrusion process, e.g. by reducing the distance between the die lips, but there is a limit to this.
• LDPE low density polyethylene
• LLDPE linear low density polyethylene
• Some materials are extruded and then thinned.
• the thinning may be carried out before they have set after extrusion (e.g. the "blowing" of high density polyethylene extruded as a tube), or with substantial heating of a cast film (e.g. the stretching and orientation of polypropylene).
• thinning to produce a re- reelable film has not been applied to the cheaper materials low density polyethylene (“LDPE”) and linear low density polyethylene (“LLDPE”).
• LDPE low density polyethylene
• LLDPE linear low density polyethylene
• Such materials have been stretched only immediately prior to use for stretch wrapping.
• the aim is not to produce a stable film, but to produce a film with a substantial tendency to decrease in length (“elastic memory”), so that it will grip a wrapped article. If such a stretched film were wound onto a reel, the force on the reel core would be crushing. It is thus desirable to provide a thinning process applicable to such materials.
• thin films (even including films of thickness under lO ⁇ m, even 5 ⁇ m) which may be suitable for packaging can be produced by extruding a plastics film and then thinning it under controlled conditions.
• the thinning is generally effected uniaxially, suitably by stretching in the machine direction.
• the film to be thinned may be cast or blown film which has not been further processed to orient it. Desirably, the film is stretched without substantial necking (i.e. reduction of the width of the film).
• the problem of elastic memory can be ameliorated or even solved by allowing a controlled relaxation prior to use or reeling.
• the invention provides a method of producing a film, which comprises extruding a plastics resin film, allowing the film to • cool sufficiently to become dimensionally stable; stretching the stable film so as to reduce its thickness and, prior to use or reeling, allowing a dwell time under conditions such that elastic memory effects are substantially reduced so that the film is capable of being reeled up for subsequent use.
• the thinning is effected by passing the film through an assembly comprising tension rolls, then around a control dance roll, before the film is wound for storage or use.
• the dance roll may be automatically displaceable to regulate the speed of the thinning assembly to control the tension applied to the film. This can be particularly important when the film is very thin (e.g.
• the technique may be applicable to thinning quite thick films down to a moderate thickness, and moderate films down to low thickness, e.g. under lO ⁇ m.
• a film can be given several successive thinning treatments to achieve a desired thinning.
• the step of allowing a dwell time to reduce memory effects may involve permitting some shrinkage of the film and/or holding it under controlled low tension for a relaxation period.
• controlled shrinkage film may be stretched by a high speed roller with a first linear speed S, and passed (preferably via a tension control unit, e.g. a dance roll) to a wind up roller with a lower linear speed, e.g.
• the film may be held under the lowest tension compatible with maintaining stable control of the travelling web.
• a festoon arrangement suitably between a dance roll and a winding station.
• the film is suitably maintained under control before the winding step, to provide the dwell time.
• the invention provides a thinning assembly comprising an array of tension rolls whereof a downstream roll has drive means; and a dance roll arranged downstream of said powered roll such that a variable film path may be provided, extending through the thinning assembly and then looping around the dance roll; and wherein the position of the dance roll is variable in response to variations in film tension to vary the size of the loop.
• a control means senses the position of the dance roll and/or its rate of displacement and acts to control the rate at which film is fed through the unit. To enable the dance roll to respond rapidly, it should be of low inertia and displaceable with low friction.
• the thinning assembly includes a wind up roller for receiving film from the last thinning roll (via the dance roll if present), the wind up roller being arranged to operate at a lower linear film speed than the last thinning roll.
• Film produced by the method or apparatus of the invention preferably shows a recovery of not more than 5%. That is, if a length of film is released from tension, its shrinkage is not more than 5%. For example, LDPE film of thickness 35 ⁇ m was stretched by 300% (i.e.
• the invention provides film
• films may be usable, e.g. polypropylene and coextruded films.
• the invention provides a method of wrapping an object in film, wherein the film is produced as described above and wrapped around an object, with or without an intermediate reeling step.
• Film embodying the invention which has been thinned and allowed to relax to reduce memory effects, can be used to wrap even quite delicate objects.
• the film can consist essentially of the usual plastics materials which are used for wrapping films (such as LDPE and LLDPE), so that it can have the usual desirable properties. Thus it can be heat-sealable, and can have good slip or cling properties and satisfactory water vapour transmission rates.
• the invention provides an object wrapped in such a film.
• Fig. 1 is a schematic view of apparatus for producing and using film embodying the invention
• Fig. 2 is a schematic view of the dance roll system; and Fig. 3 shows an air cylinder in more detail.
• Fig. 1 shows apparatus in which relatively thick film (e.g. 35 or 15 ⁇ m) 10 is converted to thin film (e.g. of thickness 10 or 5 ⁇ m respectively) 10', which is fed to a packaging machine or onto a reel or to a subsequent process.
• the thick film 10 may come direct from an extruder 12, or may first have been wound onto a reel 14. (The extruder is shown only schematically at 12. In practice it would include means for cooling the extruded film, e.g. a chill roll, so that film would be stable before being processed. )
• the film 10 passes first through a thinning unit 16. In this example, this employs three tension rolls 18,20,22. The downstream one of these (22) is driven by a driye motor 24.
• the others (18,20) may be driven from it via a gear train (shown schematically by the broken line 26) or held in fixed speed ratios by electronic controls, such that the rolls are driven at successively higher speeds in the direction of film travel.
• the middle roll 20 may be an idler.
• the upstream roll 18 may be driven by a separate motor 24A.
• the rolls 18,20,22 have high friction surfaces, so that they can apply a consistent level of traction to the film. This together with their narrow spacing means that neck down of the film is minimised.
• a preferred surface for the rolls 18,20,22 is provided by rubber with a multiplicity of small protuberances. Analogous "pimply" metal surfaces can also give good traction, but with a greater risk of tearing the film.
• a pressure roller 23 pressing the film against the final roller 22 to minimise film slip. If the film comes immediately from a reel 14, this is conveniently mounted on a pivot arm 30 such that a reel bar can be pivoted away from the thinning unit for a reel to be mounted, whereafter it can be pivoted back and urged against the upstream roll 18, suitably by a pneumatic cylinder assembly 32. The film reel 14 is then rotated by contact with the roll 18. In cases where it becomes necessary for the film to be in greater peripheral contact with the rolls 18,20,22 then shown in Fig. 1. A nip roll may be interposed between reel 14 and roll 18 to provide an extended film path around the rolls.
• the unit 16 works essentially under conditions of constant speed. Since the output film 10' may be very thin, large variations in tension cannot be tolerated. Therefore, there is a dance roll assembly 34 immediately downstream of the unit 16. This uses a displaceable dance roll 36 and a stationary roll 38, such that a variable film loop 40 is defined. The dance roll 36 is displaced by small tension changes, as will be described later.
• the film 10 will typically have been extended to about three times its original length (and, therefore, reduced to about one third of its original thickness) by its passage through the stretching unit 16.
• a film, particularly of an LLDPE/LDPE blend, which has been extended like this may show considerable elastic memory. That is, it has a strong tendency to return at least some way towards its original configuration. It is desired to make it lose a substantial part of this tendency. This is achieved by permitting controlled relaxation.
• the apparatus shown in Fig. 1 may incorporate a festoon arrangement designed to interpose a suitable delay between the thinning of the film and its eventual wrapping onto or about an object or reel.
• the festoon arrangement 42 may comprise a multiplicity of rollers 44 defining a serpentine path for the film.
• the rollers 44 must have a very low frictional drag, or substantial tension will be applied to the film. There may be a further dance roll assembly downstream of the rollers 44 for stabilising the tension.
• the festoon may be such as to delay the film 10' by several seconds. preferably at least 3 and suitably 7 or more.
• Fig. 1 shows the film 10' being rewound onto a core to form a reel 45.
• the usual variations in extruder behaviour will lead to random fluctuation in the input to the thinning unit. (This is true even if the unit 16 is fed from a reel 14, since the film on this reel will have thickness variations as a result of the behaviour of the extruder when it was produced. ) If it were not for the dance roll assembly 34, there would be severe variations in the tension of the film downstream of the stretching unit 16. Since the film 10' is there likely to be thin and travelling at high speed, such variations carry a serious risk that the film will break, leading to a shutdown of the apparatus.
• the dance roll assembly and the associated control apparatus are shown in more detail in Fig. 2.
• the dance roll 36 is mounted on a distal end of a pivot arm 50.
• This has a pivot 52 at its other end, and an intermediate pivotal connection 54 to the piston rod 56 of an air piston/cylinder assembly 58.
• the pivot arm 50 is also coupled at an intermediate region to a position transducer P (which provides an output signal indicative of the angular position of the arm 50, and hence of the position of the dance roll 36 and the size of the loop 40); and also to a rate transducer R, which provides an output signal indicative of the rate at which the arm 50 moves.
• the signals from the transducers P,R are fed to a differential amplifier A, which provides an output signal which controls the stretch drive motor M (24).
• the speed of this motor is determined by a tachometer T, which provides a further input to the differential amplifier A.
• the control circuitry is under feedback loop control to provide operational stability.
• the thinning unit 16 has to follow the extruder output speed (e.g. using electronic speed reference).
• the dance roll assembly and control apparatus may then act to control the rewind unit, and hence the rewind tension.
• the air piston/cylinder assembly 58 is preferably of the type shown in Fig. 4.
• the air cylinder 60 contains a piston 62 that is able to float relative to the piston rod 56 in a radial direction. There is a radial clearance between the piston and the cylinder wall, such that there are no rubbing seals on the piston which is supported on an air cushion or air bearing.
• Air at a predetermined pressure is fed to the cylinder 60 through a large diameter air pipe 64, fed from an air reservoir 66, which is supplied via a pressure regulator 68.
• the dance roll and pivot arm are arranged to be of low inertia.
• the nominal output velocity of the unit 16 is regulated to maintain the dance roll position within a convenient range for its pivoting movement.
• the control circuitry can automatically determine if a break in the film occurs (causing the dance roll to move rapidly to the extent of its possible travel in one sense), and may automatically shut down operation.
• control of relaxation may involve winding up the thinned film at a speed which is controlled to be lower than the speed at which is leaves the thinning unit 16.
• control of relaxation may involve winding up the thinned film at a speed which is controlled to be lower than the speed at which is leaves the thinning unit 16.
• Path Y in Fig. 1 represents the use of only this differential speed relaxation.
• the film is wound on a reel 45 at a (preferably constant) linear speed which is detected (and may be controlled) by a monitor roller 70.
• a monitor roller 70 Preferably this is coupled to a rate transducer R' (Fig.
• the film can pass around the dance roll with a low (and constant) tension, so it can undergo substantial recovery before it is rewound.
• a film must not have more than 5% recovery remaining, if it is to be reelable.
• the force exerted by the dance roll in its mid position was 23.5N, i.e. the tension in the film was 11.75N. • This is about half the tension conventionally used for reeling film of this width (about 50cm). Over its intended range of pivoting the dance roll force varies from 28N (at one end) to 12N (at the other end).
• the recovery left in the film was found to be from 2 to 3%.
• the material was found 'to be heat sealable, using a hot bar sealer or an impulse sealer.
• the samples for tensile testing came from the centre of the web.
• Table 3 compares the properties of linear polyethylene of 20 ⁇ m thickness produced by Teno AB, with the same material after passage through the illustrated apparatus (Fig. 1, path Y). The passage reduced its thickness to about one third of its original value.
• Table 3 Thinning of 20 ⁇ m LDPE Film
• the strength of the material is reduced only slightly in the transverse direction, while it is greatly increased in the machine direction.
• the tear resistance is also only slightly lessened in the transverse direction.
• the film still has some extendibility in the machine direction, and can be used for wrapping. Of course, it need not be used directly, but can be reeled and stored for subsequent use. Extruded films inevitably contain occasional small holes.
• any holes are enlarged and can easily be detected by known hole detection apparatus located between the thinning unit and the rewinding or utilisation station.
• the input film comes from a reel 14, it is easy to stop the process when a hole is detected and, for example, cut out the holed portion of film and possibly start winding a new reel 45. Thus it is possible to produce reels of film which are hole-free.
• Film produced by a method embodying the invention has many uses. For example, it can be slit into strands for use in knitting (e.g. of sacks). It can be fibrillated (generally after irradiation, usually with an electron beam) to produce fibres. These can be extremely fine, particularly if the film is very thin. Film may be used in the production of laminates, e.g. with paper or board, typically by means of a water-based vinyl acetate copolymer adhesive.
• the illustrated apparatus can give the following advantages:

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)

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• 1988
  • 1988-01-13 GB GB888800670A patent/GB8800670D0/en active Pending
• 1989
  • 1989-01-12 JP JP1501531A patent/JPH03502667A/ja active Pending
  • 1989-01-12 KR KR1019890701705A patent/KR900700270A/ko not_active Application Discontinuation
  • 1989-01-12 EP EP89901734A patent/EP0397740A1/en not_active Withdrawn
  • 1989-01-12 WO PCT/GB1989/000020 patent/WO1989006594A1/en not_active Application Discontinuation
  • 1989-01-12 AU AU29490/89A patent/AU2949089A/en not_active Abandoned

Non-Patent Citations (1)

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