AU616076B2 - Welding of plastic films - Google Patents

Description

COMMONWEALTH OF AUSTRALIA 6 6 0 FORM PATENTS ACT 1952 COMPLETE SPECIFICATION FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: R pelated Art: 4 Name of Applicant: HUNTER DOUGLAS LIMITED Address of Applicant: 395 Newman Road, Geebung, Queensland 4034, Australia Actual Inventor: Thomas John Dekker *.Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney Complete Specification for the Invention entitled: S* "WELDING OF PLASTIC FILMS" 0 The following statement is a full description of this invention, including the best method of performing it known to us:- -1- I i4 i 1 v- 'Iiir-Ci*rl 15 -2- 2- The present invention relates to a method and apparatus for the high speed, high frequency welding of organic thermoplastic films in either a continuous or intermittent operation. The process is designed to produce uniform, strong welds without waste or damage to the plastic film. The machine typically operates at a line speed 1-2,000 metres per hour and is primarily designed for the continuous packing of elongate bodies such as tubes. The welding principle involved however has much wider applications which will become apparent as the sees concept upon which the machine depends becomes clear.

SWhile the high frequency joining of plastics has been 0000 0000 o known for many years, there are some materials that do not :00 lend themselves readily to this type of welding.

fees Thermoplastic polymers have been joined in a variety of ways over the years. U.S. patent 2,724,416 describes a heating cycle in which a current is passed through a strip *0 S of resistive material in contact with the material to be joined. Unfortunately the hot surface, which remains in ooo contact with the plastic, cools too slowly to allow oooooo effective cold clamping of the joint.

O o The problem is overcome in U.S. patent 3,047,911 where current is passed through a Nichrome wire in close proximity to the plastic surfaces to be joined. The temperature of the surface is controlled with reasonable accuracy because the thin wire heats and cools quickly allowing the material to be fused while hot and then held under pressure during the cooling cycle.

i1 -i -3- Polyolefins such as polyethylene are not suited to welding by external application of heat and pressure, through heated bars or rollers. Such methods can result in wrinkling, puckering, charring, tearing or sticking of the plastic to the heat source. These problems become more pronounced as the speed of the process increases.

For many years plastics have been welded using high frequency electric fields generated between two electrodes. The field heats the plastic by dielectric loss in the parent material. Dielectric heating of this type is caused by molecular disturbance in polar materials when they are inserted into the field. The advantage of S.o.

goo the process is the rapid heating of the dielectric material. Heating occurs throughout the dielectric see* simultaneously, rather than progressing inwardly from the surface.

High frequency heating is suitable for many plastic materials. There is, however, a large group of materials for which this method of heating is ineffective. These ooooe 26 materials have such low power factor and excellent electrical insulating characteristics that high frequency fields are unable to create a molecular disturbance which will generate heat. These polyolefins which include polystyrene, polyethylene and polytetrafluroethylene materials are termed low dielectric loss materials and they experience insignificant heating in high frequency fields. i The amount of heat generated in a material in a high

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4 -4frequency field is determined by the frequency, the applied voltage, the dimensions of the material being heated, and the dielectric constant and loss factors of the plastic being joined. The high frequency field can operate at 20-200MHz and more typically between 20-60MHz.

In high dielectric loss materials high frequency bonding is superior to other types of joining processes in the mechanical strength, flexibility, and transparency of the join.

High frequency welding devices relying on roller or bar electrodes are not always successful in continuous packaging lines as they have low production rates, even in *materials with high dielectric loss factors. Low dielectric heat loss materials have normally been heated oo' o 'indirectly in these machines by using dielectric plates of a high loss material to generate heat.

Non-polar, low heat loss plastics such as polyethylene, polypropylene and polystyrene are difficult

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and expensive to weld using high frequency methods because 20 of the level of power required to generate internal heating of the materials. Satisfactory joins require frequency generators of much higher frequencies than is o o. required with high loss materials.

Collins states in U.S. Patent 2,570,921 that materials that do not fuse in high frequency fields have power factors below 0.001 whlen measured at 60Hz, 1KHz and

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Even though difficult to join, polyolefins are I

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I l 1 r l^ 17 5- extremely useful materials because of their physical properties. In medical applications bags made frompolyethylene can be much thinner than those made from PVC because of their greater resistance to permeation by fluids. Non-polar polyethylene is approved for colostomy bags in hospitals. The thinner polyethylene also offers cost advantages over thicker polyvinyl chloride.

Polyethylene films can be used in disposable articles forred from multiple layers of plastic, but no suitable method of welding or sealing has yet been developed.

Since these materials are also used for packaging, the speed and economy of any joining process are important o00 factors. Other non-polar materials, such as acetates, do son not require petroleum in their manufacture, and would provide a cost advantage in an energy conscious world.

While direct fusing of non-polar, low heat loss plastics by external heating has not been successful, 00 0 S o several indirect processes have been used. In U.S. patent 2,606,856, Hurrey describes the use of thermo-setting ooooe S27 adhesives, having high dielectric loss factors, to join 000000 polyethylene. The technique is limited in usefulness as it often cannot reproduce the cohesive strength of the original material. Adhesive may also cause contamination if used for food packaging, or in sterile conditions.

Other methods make use of secondary materials such as high heat loss pastes or liquids that promote secondary heating of the parent material. These methods may also contaminate the parent material. In Australian Patent

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i 6 420,764 Heller uses a secondary agent for heating. This agent is essentially a solid. An Australian patent application No. 61,688/86 by Tetra Pak International AB of Sweden, describes a method of indirect heating wherein the plastic is heated via an aluminium foil in the plastic.

U.S. Patent No. 4,268,338 describes a method of joining non-polar plastics via the heating of a high heat loss dielectric plate. High frequency current is applied to an electrode plate and die.

All of these processes are said to function by indirectly heating the non-polar plastic by way of a high 0 heat loss intermediate. The intermediate may be a paste or a liquid which contains high heat loss polymers.

Alternatively, the intermediate may be a solid such as a i metal or a plate or film of high heat loss material.

Since earlier workers have shown the direct heating of polyolefins did not produce satisfactory fusion, it is 0 suggested that the processes described here are examples of fusion through the combined effect of heating and high 21 frequency current.

This concept was first suggested by Collins in U.S.

Patent No. 2,741,296 where he attempted to increase the dielectric loss factor of plastics by heating to allow fusion dielectrically. He describes the manufacture of plastic tubes from sheet material by the application of pressure in a high frequency field. Collins claimed that it was virtually impossible to fuse plastic materials having a low dielectric loss

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I -7factors by the application of external heating alone. He suggested that when these plastics are heated by an external source the material changes, increasing the dielectric loss factor and allowing fusion of the material by high frequency fields. The higher the increase in temperature, the higher the dielectric loss factor. High loss factors reduce the need for high power output radio frequency generators, allowing the material to be welded readily in fields of moderate power and frequency.

Collins suggested that it was preferable that heating and the application of the field should be simultaneous.

His invention provided heating contact between the plastic and an external belt made from a high heat loss material S which was heated by the same electric field used to fuse the material.

Collins also tried external heating by infra-red soot electric heater with high frequency electrode, being a Do roller, in direct contact with the plastic. He claimed that heating electrically required less high frequency oooo 20 power than the direct contact method. The joining process ooooo S was said Lo be less effective, as the heat should ideally be applied at the point where the plastic is experiencing S5 the electric field.

In U.S. patent 2,457,659 Graham reported improved high frequency welding of plastic films and increased operating speeds in continuous welding lines when the high frequency electrodes where held at elevated temperatures C-80 C) during the welding process.

&8 S 'i 8 The main object of this invention relates to improvements in the art of high frequency welding thermoplastic materials that have low dielectric loss properties, although the principle also applies to high loss materials.

Another object of the preferred form of this invention is to provide a process in which heating of the film occurs only in the area where the weld is to be made, while avoiding all physical contact with the heated surfaces being joined.

So A further object of the invention is to make a stable joint which will resist chemical and physical attack, and to have a joint that has physical properties similar to sob the parent materials. The joint is flexible, waterproof and elastic, and is made in a speedy and economic manner.

It will become apparent that the invention as described satisfies these requirements and is practical 0* *and low in cost as it uses existing materials and technology.

00*0:* 240 This invention seeks to overcome the difficulties of simultaneously heating non-polar plastic materials while subjecting them to a high frequency field. All previous methods have made use of direct contact processes or non-contact heating that is applied outside the zone of the electric field. Because of these inefficiencies, the high frequency power requirements are much higher than necessary.

S- Ill. 5 L46~cf( 9 The present invention in one form makes use of the high frequency field itself to generate a corona discharge to heat the plastic undergoing fusion. The result is a substantial reduction in the frequency of the field and the power requirements of the process.

Corona discharges have been applied to the treatment of plastics in the past. These have been confined to the surface modification of materials to promote better adhesion for subsequent treatments, such as painting or adhesives. One such process is described in Australian Patent No. 535,509 where the corona is obtained by the discharge of high frequency current between two *000 electrodes, the base electrode preferably being provided with a dielectric as an insulating material which assures Goof a surface cGvering discharge.

High frequency generators are well known. In U.S.

0000 patent 2,741,296 Collins describes an early device capable 00. of generating typically 300-400 watts at frequencies up to 200MHz. More recently, high frequency generating *2 equipment with electron tubes achieve frequencies of 2MHz S" with typical welding frequencies of 1.5MHz. These devices are progressively being replaced with solid state high frequency generators which are limited to frequencies of approximately 200KHz.

According to one aspect the present invention provides a method of welding or joining thermoplastic materials comprising the steps of applying a high frequency field to the area of the material to be joined or welded, cz, 0 S o 7) 10 forming a corona discharge simultaneously at the point of application. of said high frequency field.

For preference, a source of high frequency and high voltage is used to form said corona discharge simultaneously with said high frequency field.

According to a second aspect the present invention provides apparatus for welding or joining thermoplastic materials comprising, a pair of spaced spark electrodes, the material to be welded or joined being placed or passing between said electrodes, said electrodes being connected to a high frequency high voltage source and being spaced so as to create a corona discharge therebetween, said spacing being sufficient to allow the material to be joined or welded to pass between said electrodes without making physical contact therewith.

One advantage of the embodiments of this invention is the ability to make use of lower power, solid state z 'o devices rather that the older, vacuum tube high frequency generators. The packaging line device in the embodiment of the invention described below is capable of welding low L* e*heat loss materials, such as polyol 7 nr using a machine with voltages in the range of 2-20K' medium frequencies in the rage of 5-2OKH.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 11 Figure 1 shows a schematic circuit diagram of a high frequency welder for use with the sleever; Figure 2 shows a pictorial perspective view of the in-line welder according to the invention; Figure 3 shows a schematic of the general arrangement of an in-line sleever according to the invention; Figure 4 shows a schematic section of the in-line sleever over the welding head; Figure 5 shows a detailed sectional view of the welding head; and Figure 6 shows a similar view to Figure 5 with a

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I different product being welded.

Referring to Figure 1 the welder according to this embodiment comprises a pair of spark electrodes 10, fc *fed from the secondary winding 11 of high voltage, high frequency transformer 12. The voltage output of this winding is typically between 2ki and 20kV at a frequency Sof between 5kHz and 20kHz. The primary winding 13 of the transformer 12 is coupled to a feedback controlled DC-AC inverter 14 which produces a voltage typically between 0 and 100 VAC at a frequency between 5kHz and 20kHz. The feedback controls allows for changes in speed of material through the welder, variations in material thickness and i changes in power requirements.

Figure 2 shows in detail the welding electrodes 10, mounted in insulating blocks 15 and 16 and connected to the transformer 12 via cable 17. The material to be welded passes along the plane 18 through the opening 19 with the edges to welded (not shown) passing between the i'| electrodes 10,

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si 12 The in-line sleever shown in Figure 3 illustrates one application of the invention. The product 20 enters the machine on the in-feed conveyor 21 to the right of the drawing, and is covered above and below by the packing film provided from rolls 22 and 23 respectively. The product 20 and film 24, 24' enter the in-line welding head where the two films 24, 24'are fused along their edges to produce a cylindrical package enclosing the product.

The packed product leaves the machine on the run out conveyor 26 where it is separated from the continuous packaging tube being produced by a cutter 27, and labelled oO o oo by coder 28.

oo .Figures 3 to 5 show in detail the in-line sleever o: welder which consists of two sets of electrodes 30, and 31, 31' mounted in a welding head 25 and positioned each side of the product to be packaged. A high frequency field is generated across the electrodes by the welder shown in the circuit diagram of Figure i. As the plastic filr,. 32, is being drawn between the electrodes, the corona 20 discharge heats the plastic, increasing the heat loss e factor of the non-polar material, which in turn fuses ooo under the influence of the high frequency field causing the discharge. It should be noted that this process is not limited to non-polar plastics. High heat loss materials can also be welded in the same way, and with the same savings in power during the fusing process.

The in-line packaging process described produces a high quality continuous tubular pack at high speeds in 13 very thin materials. It is able to do this because the forming and joining processes do not apply any retarding forces to the passage of the film through the device.

More conventional machines using high heat loss materials produce the join by contact or indirect heating methods which reduce the speed of the process. Low heat loss materials, which may be less expensive, or stronger, are unable to be joined using these processes.

While this invention describes in detail a packaging line making use of the principles of corona discharge high frequency welding, it must be appreciated that the e principle may be applied to any process in which tne csimultaneous application of heat and high frequency fields will result in fusing of materials in the field. Earlier •c the benefits of using polyethylene in medical applications was discussed. The products, which could generally be described as bags, are difficult to make because joining o' processes have not been developed to handle the non-polar films involved. It will now become apparent that the 2Q invention described here can solve these difficulties.

0 While the embodiment of the invention described is 000000 for an in-line packaging machine it is obvious that such a process is not confined to fusing plastic films in straight lines. Any device would allow the film to be moved between the welding electrodes, or would allow the electrodes to move over the film, can produce complex i joins more suited to bags. Devices in which the electrodes are shaped into curves or pockets could allow 14 the necessary welds to be made while moving only the film.

In some medical applications sterile conditions are important to the quality of the product. The invention described allows fusing of polar and non-polar materials without physical contact, or the use of intermediate heat generating materials which might be non-sterile. The corona discharge process, described in the invention, may itself act as important method of decontaminating the plastic surface during the welding procedure.

Sometimes, intermittent welds are required. It will be apparent that the invention described here can produce .*go both continuous and intermittent welds by controlling the 0o 0 power supply.

o The invention is not restricted to the fusing of thin .0 plastic films. Non-polar materials of a range of a.0.

thicknesses can be joined by suitably designing the field generator.

In thicker materials where cold clamping cycle is required to consolidate the weld zone, or to ensure well 20 formed joints, the process allows post pressure applications outside the welding zone. These can be extended as required by increasing the run off time after welding. It is important to note that as this process S separates the welding cycle from the cold clamping cycle, the production rate is not influenced by the cold clamping dwell time, In summary, the invention ensures that the heat which increases the loss factor, and the field which fuses the 11 15 plastic, are applied simultaneously, and without directly contacting the surfaces being joined.

It will be readily appreciated by those skilled in the art that the welding process described is not limited to using a high frequency generator of the type illustrated. The main object of the claim is that any circuit that will produce a corona discharge in a high frequency alternating field between two electrodes will satisfy the requirements of the invention. It will also be apparent that a uniform distance between the electrodes is important in controlling the quality of the weld S* produced.

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Claims (9)

1. A method of welding or joining thermoplastic materials comprising the steps of applying a high frequency field to the area of the material to be joined or welded, forming a corona discharge simultaneously at the point of application of said high frequency field.

2. A method according to claim 1 including passing said material to be welded or joined between a pair of electrodes having formed therebetween said corona discharge and said high frequency field.

3. A method according to claim 2 wherein said material is passed in non-physical contact between said electrodes.

4. A method according to claim 2 or 3 wherein a single source of high frequency and high voltage is used to form said corona discharge simultaneously with said high frequency field between said electrodes.

5. Apparatus for welding or joining thermoplastic materials comprising, means for applying a high frequency field to the area of the material to be joined or welded, means for forming a corona discharge simultaneously at the point of application of said high frequency field.

6. Apparatus for welding or joining thermoplastic materials comprising, a pair of spaced spark electrodes, the material to be welded or joined being placed or passing between said electrodes, R i l;j ;:iii I ~ii~c i; WY- ~1 LV- TaY- 17 said electrodes being connected to a high frequency high voltage source and being spaced so as to create a corona discharge therebetween, said spacing being sufficient to allow the material to be joined or welded to pass between said electrodes without making physical contact therewith.

7. An in-line welder comprising on or more apparatus according to claim 6, further including means for feeding material to be welded or joined between said electrodes.

8. An in-line welder substantially as hereinbefore described with reference to Figures 6 and 7 of the accompanying drawings.

9. An in-line sleever substantially as hereinbefore beep described with reference to Figures 2 to 5 of the accompanying drawings. Dated this 22nd day of November, 1989 HUNTER DOUGLAS LIMITED Attorney: PETER HEATHCOTE Fellow Institt. 7 t Aiorneys of Australia 0