Laser welding of thin polymer ®lms to container substrates foraseptic packaging

N. Brown*, D. Kerr, M.R. Jackson, R.M. Parkin

Mechatronics Research Group, Systems Engineering Research Centre, Faculty of Engineering, University of Loughborough, Loughborough,

Leicestershire, LE11 3TU, UK

Received 8 February 2000; accepted 6 March 2000

Abstract

Keyhole laser welding of polymers is a subject well covered and researched, but relatively little information exists regardingthe welding of thin polymer ®lms, particularly to a heavier substrate. This paper presents the design of a suitable test apparatus

for laser welding thin ®lm to a heavier substrate, and shows the results of an investigation into the feasibility of laser weldingmulti-layer polymer ®lm lids to tubs for the manufacture of aseptic food containers. A consistent weld, free from defects, is thekey to process success. Typical welding defects have been synthesised in order to investigate, and consequently remove, theircause. The result is a reliable welding method based on even ®lm clamping. With careful attention to machine design, a seal of

high mechanical strength and chemical integrity is possible. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Laser; Welding; Polymer

1. Introduction

A need has arisen to investigate non contact sealingof aseptic food containers. The currently employedprocess is a contact-based method whereby heated topand bottom matched tooling is used to press togetherthe ®lm and container, for a ®xed time period, inorder to provide a seal by softening of material at theboundary layer between the ®lm and substrate. Amulti-layer construction is used for both the ®lm andthe containers, in order to provide a shield betweenthe item in the container (e.g. food, pharmaceuticals,sterile medical supplies), and the outside world. The®lm and substrate construction combine an oxygenbarrier layer, a water barrier layer, tie (or bonding)layers to prevent delamination, and external layers formechanical strength. Although a successful mechanicalseal produces a product with a long shelf life, whereby

usefulness of the multi-layer construction of the sealed

container is demonstrated, the process has three main

shortcomings.

Firstly, the process is a contact method, so any

foreign particles may become attached to the hot seal-

ing plates, such as splashed food. This could deform

the ®lm during sealing, producing a mechanical defect

in the seal. Some types of bacteria only require a 1-mgap to penetrate the seal, so such a defective seal

would mean abandonment of as many containers pro-

duced in the production run produced, before the

defect is found. A non contact-based method, using a

laser as a heat source, means that there are no mech-

anical parts in contact with the ®lm at the join to con-

taminate.

Secondly, since the hot shoe process depends on a

®xed time delay to make the seal, the process may not

be readily speeded up, and generally takes 3±5 s per

batch of four pots. Laser welding of thin ®lms has

been successfully undertaken at speeds of 50 m minÿ1

[1], o�ering a possibility of rapidly accelerating the

Optics & Laser Technology 32 (2000) 139±146

0030-3992/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0030-3992(00 )00033 -5

www.elsevier.com/locate/optlastec

* Corresponding author. Tel.: +44-1509-263-171.

E-mail address: n.brown@lboro.ac.uk (N. Brown).

sealing process, particularly when multiple weldingbeams are used.

Thirdly, each container design to be sealed requiresbespoke tooling to suit its size and shape. Customtooling adds to production costs, as does changeovertime between designs. Sealing of mixed products istherefore not feasible with conventional tooling.

CO2 laser welding was chosen as the preferred join-ing method, such equipment having been applied pre-viously to plastics welding [2,3] and also beingeconomical in equipment and running costs [4,5].Readily absorbed [6], and capable of welding at highspeed [1]. ND-Yag lasers have also been used to goode�ect for marking [7±9].

2. Welding process

2.1. Experimental apparatus

Experience gained from previous design of opticalrigs for laser materials processing within the mechatro-nics research group enabled rapid design of a test bedfor polymer welding. In order to understand fully thetechnology involved with the laser welding of asepticpolymer containers, this section illustrates the appar-atus used to carry out the experimentation. The devel-opment of a suitable piece of apparatus has formed acritical part of this research, a dedicated system forlaser welding being necessary to produce the exper-imental results. An overview of the apparatus used canbe viewed in Fig. 1, and the optical path detail isshown in Fig. 2.

The schematic shown in Fig. 3 shows the arrange-ment of the traverser, the optics, and associated safetyinterlocks and control hardware.

3. Weld inspection apparatus

Lighting at an angle of around 208 to the horizontal

was used to illuminate the weld surfaces, as shown inFig. 4. The e�ect of altering the lighting angle 230%was not found to have a detrimental e�ect on theimage quality. Extension tubes ®tted to the camerawere used to decrease focal length and increase magni-®cation. The system provides a resolution of 100 pix-els/mm.

Images are captured on a video grabber board, andprocessed using Image Pro software, running on astandard PC. These image ®les are then exported asFlat Files for processing in Matlab (.FLT). Flat Filesfor the purpose of this application are the same aseight bit raw image ®les (.RAW), the reason Flat Fileswere chosen as a storage medium being that Flat Filesmay be used to hold 16 or 14 bit colour informationshould the need arise. Pictures are also stored asJPEGS. JPEG images take up little space (typicallyless than 20% of that of a raw image), and are widelyused. JPEG Image viewer/conversion software is com-pact (<300 k) and is stored with the images to ensureforward compatibility of the data.

4. Initial experiments

The ®rst test group was used to establish initialwelding conditions for opaque (Nylon 66 outer layer)polymer ®lm. The white polymer ®lm could be madeto crudely spot-weld with a small degree of charring,but a travelling beam weld invariably would notmechanically join the ®lm to the container rim. Thereason for this was that upon heating, the opaque ®lmwould curl away from the rim. Consequently, heatwould be passed into the ®lm more quickly than itcould be conducted away, causing rapid thermal run-away and resultant charring. In practice, a mechanicalseal is made using the opaque ®lm involving defor-mation of the top surface of the pot and ®lm. Thee�ects of an attempted laser weld are interesting none-theless, and have indicated that a join was potentiallypossible. Tests continued using a thermoplastic multi-

Fig. 2. Laser optical path.Fig. 1. Laser cabinet overview.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146140

layer clear ®lm, which was not chosen for its trans-mission properties, the wavelength of the CO2 laserbeing such that energy would be absorbed at the sur-face of the ®lm, rather its extra thickness and ductilitywhen heated. However, the use of a clear ®lm providedseveral main advantages:

1. This ®lm was purposely designed to provide a heatseal with no mechanical interaction, and may bemore easily welded as such.

2. The ®lm does not re¯ect or conduct away much In-

frared (IR), consequently less laser power is

required to provide a weld.

3. The ®lm sags rather than curls when heated, provid-

ing enhanced contact with the tub rim.

4. Of course, being a clear ®lm, we can view the resul-

tant weld with ease, and may therefore achieve a

good understanding of which factors contribute

most to acceptable weld quality.

Fig. 3. Overview of laser welding rig.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146 141

5. Weld features

Fig. 5 shows the e�ects of the gaussian energy distri-bution across the laser beam. Considerable changes inthe surface geometry can be seen to have occurrednear to the centre of the weld, with reduced disturb-ance towards the edges.

5.1. Weld defects Ð voids

Two types of voids exist, melt-through voids wherean area of ®lm is vaporised, and internal voids wherethe ®lm is intact, which are generally much smaller.Fig. 6 provides a good representation of melt-throughvoids. Fig. 7 shows an example of internal voiding,which can be seen in the pictures as the darker part ofthe weld, seen as such due to internal scatter of illumi-nating light.

5.2. Inducement of defects

After viewing the internal and melt-through voids, itwas deemed wise to attempt to ®nd out more aboutthe cause of such defects. To this end, foreign objectswere introduced into the weld path in order to try toreproduce the same e�ects arti®cially. The experiment

was a success, and it was clearly seen that any breakin contact between the upper and lower weld surfacescan prove detrimental. The following pictures show thee�ect of introducing a foreign object into the weldpath. Fig. 8 is an example of the tests carried outusing air-jet textured yarn as a foreign object. It canbe seen fairly clearly that the net e�ect is to lift someof the upper ®lm clear of the tub surface, which a�ectsthe heat transfer capabilities of the whole system. Notbeing able to conduct the heat away as quickly as if itwere lying ¯at on the tub rim, the ®lm vaporises and amelt-through void is formed. Fig. 9 shows the e�ect ofusing thin wire to introduce a gap between mating sur-faces. Internal voiding is produced where contact ispoor, and where no contact is present, melt-throughvoiding may be seen.

6. Modi®ed welding process for rotary welding

In order to achieve a full periphery weld, a rotatingtable was designed and built by the industrial collabor-ators (Fig. 10), to hold a rotating container beneaththe beam. The rotary speed was measured using astandard frequency counter, and converted to revminÿ1. Control of the motor was achieved using aPulse Width Modulation (PWM) generator in the form

Fig. 4. Inspection rig.

Fig. 5. Gaussian weld e�ect; large section and beam pro®le.

Fig. 6. Edge of melt-through void.

Fig. 7. Image of weld with internal void.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146142

of a function generator with variable mark/space ratio,through a Field E�ect Transistor (FET) ampli®er. Theforeign object tests (outlined in Section 5.2) showedthat clamping of the ®lm during a full periphery weldwas as critical, if not more critical, than the methodused for linear welds. Insu�cient clamping wouldresult in a small `wave' of bunched up ®lm appearingjust in front of the beam path, which upon completionof the weld would form a small tunnel across the weldat its starting point. In order to avoid such weldbreaches, a series of weights were ®rst applied to theedges of the ®lm, but this was found to be time con-suming and not completely reliable. A solution was touse a perspex clamping plate, closely following the tuboutline around the weld periphery.

7. Results

7.1. Straight welds

Around 200 tests were carried out in order to estab-lish the best parameters for polymer welding. Early ex-periments showed that greater attention must be paidto clamping, ensuring that the ®lm is ¯atter against thelid rim by moving the clamping mass closer to theweld line. Initial tests were carried out using a lightmass to hold the ®lm in place, which was initiallytacked to the substrate with two 0.5 mm diameter dotsof solvent-based latex glue, some 20 mm from theweld ends.

Test Clamping Comments

5_6 Mass and glue tack Weld more consistent than5_3 and 5_4, no peeling

5_7 Mass and glue tack Weld more consistent than5_3 and 5_4, no peeling

5_8 Mass and glue tack Consistent weld5_9 Mass and glue tack Consistent weld5_10 Mass and glue tack Peeling, no weld achieved5_11 Mass and glue tack Consistent weld5_12 Mass and glue tack Slight internal bubbling 4

� 0.5 mm 2 internal voids5_13 Mass and glue tack Cutting of top layer, no

contact with tray, no weld

Referring to tests 5_8±5_11, we can see that theweld consistency was poor, due to less than 100%even contact between the ®lm and substrate. Leavingout the glue tack and using a spring clamp as the pre-ferred clamping method, it was possible to achieve100% repeatability for straight welds, the following

dataset showing just a small portion of the results ofthe tests carried out. The next step after proving theviability of producing a straight weld was to attemptfull periphery welding of aseptic containers to providea complete seal.

Test Comments

5_14±5_25 Clean weld, no voiding or discoloration

7.2. Full periphery weld long-term liquid permeabilitytests

The polymer ®lm features a water barrier layerwhose purpose is to prevent the ingress and egress ofwater from a welded container. Five pots were ®lledwith water and two with isopropyl alcohol, and werelaser welded in October 1998. These pots have beenstored upside down since that time such that waterpressure is greatest in the region of the weld. By Janu-ary 2000, there had been no seal failures.

7.3. Full periphery weld oxygen transmission test usingOxtran tester

The oxygen transmission tester (Oxtran) used by theindustrial collaborator was used to validate the integ-rity of the oxygen barrier layer in the welded polymer®lm. This layer ensures that no ingress of oxygen is

Fig. 8. Foreign object test Ð yarn.

Fig. 9. Foreign object test Ð 0.2 mm copper wire.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146 143

possible during product storage, thus preventing bac-terial growth.

Four samples failed the test; over a range of instru-ments.

One sample passed; 0.0001 cc/pkg/day (seems toolow and may indicate a fault in the test cell), and afurther sample passed; 0.0077 cc/pkg/day (reliableresult).

As a control, similar tests were done on proprietary®lms sealed mechanically on the pot.

Film sample 1 (standard nylon 66 ®lm plus EVOHbarrier layer): 0.0047 cc/pkg/day.

Film sample 2 (standard nylon 66 ®lm plus EVOHbarrier layer): 0.0027 cc/pkg/day.

Pot only: 0.0005 cc/pkg/day.These results were seen as encouraging, as two out

of the six samples did produce an e�ective seal. Itshould be noted that both samples involved a double-pass weld (two rotations of the pot), but had verydi�erent power settings.

7.4. Mechanical strength

In order to assess the mechanical weld strength, theforce required to peel apart the lid and the tub ismeasured, with the peel force applied at an angle of458 to the direction of the peel. Fig. 11 shows the peelforce for an ordinary heat sealed circular pot. As theforce is ®rst applied, a small peak in the peel force isgenerated due to lugs which are inserted into the weld,which provide a `lead in' to the main weld body. Thepurpose of this is to lessen the speed at which the lidmay be peeled, to avoid sudden removal of the lid andspillage of the contents. At 18 mm the peel force isincreased again, as the maximum weld surface area onthe edge of the circular weld is presented to the testapparatus. Some variability from the peel start to peel®nish is experienced, as is the case here, such that thepeel force at 130 mm is slightly higher than the peelforce at 18 mm.

The peel strength characteristics of a laser weldedpot are shown in Fig. 12. On ®rst appearance the peelcurve looks less clean than for a laser welded pot, andthis is because the curve shown here represents aninteresting sample, whereby several features of thelaser weld can be clearly seen. Weld-edge peaks, asdescribed for the conventionally welded pot at 18 and130 mm, may be seen for the laser welded pot at 5 and120 mm. The reason for this displacement is theslightly smaller diameter used for the circular laserweld. Separation is not clean with the laser weld, sincethe join is of such strength that the top layer of sub-

Fig. 11. Peel strength for conventionally heat sealed pot.

Fig. 10. Rotary welding apparatus.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146144

strate is actually removed during peeling, giving rise tothe jagged force pro®le between 20 and 60 mm. At60 mm a sudden rise in the peel force occurs due toweld overlap, which is the point at which the laserweld begins and ends. At this point, on one side of thetub, the energy from the laser has penetrated the ma-terial twice, causing a much stronger join, and a dee-per layer of substrate is removed. At 70 mm the forcerequired to peel the ®lm is so great that the ®lm rup-tures, and from 80 to 110 mm we can see the forcerequired to tear the ®lm. In general, the laser weld hasa signi®cantly higher separation force than for a con-ventional heat seal.

8. Discussion

The introduction of defects as a control shows thata defective weld can often result from uneven contactbetween the ®lm and substrate, facilitating uneven heattransfer between ®lm and substrate, sometimes meltingthrough the ®lm before it has had a chance to join (inthe case of melt-through voids), though more fre-quently leaving a gap between ®lm and substrate (as inthe case of internal voids). The introduction of arti®-cial defects to produce voids in the laser weld has beensuccessful in proving that even clamping of the ®lmaround the weld area is critical, and as such, led to thedesign of a successful modi®cation to the apparatuswhich facilitated rotary welding, in the form of a

specially designed clamping plate. The aim of futurework is to remove the need for such a plate so that thefull ¯exibility of a laser-based welding system can berealised. The water penetration test results are particu-larly pleasing, and strongly suggest that containershelf-life with regards to water permeability for laserwelded containers is competitive with those producedby conventional heat sealing.

The oxygen penetration tests show that the furtherre®nement of the laser welding process is required inorder to improve repeatability, although the mechan-ical strength of the seal is far greater than for conven-tionally sealed containers, closer examination of laserenergy boundaries within which a weld may takeplace, should produce an e�ective mechanical seal withminimal damage to the oxygen barrier layer.

Peel tests show that a laser weld is possible withgreater mechanical strength than a join made by con-ventional hot shoe sealing.

9. Conclusions

The work outlined in this paper was undertaken aspart of a feasibility study, and has demonstrated thefeasibility of multi-layer plastic welding by an IR laserbeam. It has also shown the feasibility of weldingaround the entire periphery of circular containersusing a rotational stage. Examples of these welds havebeen tested for strength and peel ability using industry

Fig. 12. Peel strength for laser welded pot.

N. Brown et al. / Optics & Laser Technology 32 (2000) 139±146 145

standard techniques. Results revealed adequate weldstrength, but a greater force was necessary to peelthem open. In most cases the weld strength proved tobe as great or greater than that of the partially heat-a�ected parent material adjacent to the weld seam.The peel therefore tended to take place along theedges of the weld, rather than by separation across theweld zone.

A small sample of welded tray containers was testedfor oxygen penetration using industry standard tech-niques. Although several samples failed these tests, asigni®cant number did pass, suggesting that further op-timisation of welding parameters should achieve satis-factory performance.

A short series of tests were carried out to investigatethe e�ect of bridging by foreign bodies. These wereintroduced into the weld region ahead of the laserbeam path, between the container and the ®lm lid.Materials, such as water, metal wires and textile threadproduced similar results in that melt-through zones orvoids were clearly visible in the weld path on examin-ation. Such voided zones are relatively easy to detectusing standard image analysis techniques, as they aresigni®cantly di�erent from images of a good weld. Thegroundwork has been laid for future projects whichcan re®ne the process in order to produce a laser seal-ing method to outperform the conventional process.

Acknowledgements

The authors would like to thank our project part-ners, RPC Containers Ltd (Corby), GainsboroughCraftsmen Ltd, and the Ministry of Agriculture, Fish-eries and Food. The project was funded under theMAFF link scheme, Project No. AFM 34.

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