Nanosecond and sub-nanosecond pulsed laser ablation of thin single and multi-layer packaging films

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Nanosecond and sub-nanosecond pulsed laser ablation of thin single and multi-layer packaging films Adrian H.A. Lutey a,∗ , Michele Sozzi b , Simone Carmignato c , Stefano Selleri b , Annamaria Cucinotta b , Pier Gabriele Molari a a

Università di Bologna, viale Risorgimento, 2, Bologna, Italy University of Parma, via G.P. Usberti, 181/A, Parma, Italy c Università degli Studi di Padova, Stradella San Nicola, 3, Vicenza, Italy b

a r t i c l e

i n f o

Article history: Received 6 May 2013 Received in revised form 12 July 2013 Accepted 13 August 2013 Available online xxx Keywords: Laser ablation Aluminium Polypropylene Polyethylene Paper Multi-layer films

a b s t r a c t Translating single and multi-layer packaging films are exposed to 0.5–0.8 ns laser pulses of wavelength 1064 nm and 10–12.5 ns laser pulses of wavelength 515 nm. Ablation depths and threshold fluences are reported for single-layer polyethylene (PE), polypropylene (PP) and aluminium of thickness 20–50 ␮m. Interaction and cut widths are reported for the same single-layer films and for four multi-layer films comprising aluminium-polypropylene and aluminium-paper. Ablation of the PE and PP films is only possible in the tested parameter range with 0.5 ns, 1064 nm pulses. Though a one order of magnitude reduction in the ablation threshold of aluminium is observed with 0.5–0.8 ns, 1064 nm pulses, the efficiency of material removal for fluences >8 J cm−2 is superior with 10–12.5 ns, 515 nm pulses. Multi-layer film response is found to be heavily dictated by the thickness of metallic layers. For multi-layer films with aluminium layers of thickness 7–9 ␮m, adjacent layers are removed by inter-layer heat conduction from the aluminium layer, in some cases leading to very large cut widths. For multi-layer films with aluminium layers of thickness <0.1 ␮m, direct ablation of all layers must take place for complete film penetration. The study provides quantitative results regarding process efficiency and quality for application of pulsed laser sources within the packaging industry. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Despite the maturity of laser materials processing in industrial settings, the high-speed laser incision and cut of thin single and multi-layer packaging films has seen limited research activity [1]. Such films often comprise combinations of aluminium (AL), polypropylene (PP), polyethylene (PE) and paper. Their cut via laser exposure offers advantages over mechanical techniques in terms of precision, flexibility and long-term cost reduction, as is seen in other developing thin-film applications [2]. The combination of layers with largely different thermal and optical properties, together with the presence of inter-layer thermal conduction, leads to material responses that may be fundamentally different to those of the individual materials subject to laser exposure as single-layers. Both short-pulse ablation and longer-term vaporisation or thermal degradation may occur simultaneously in different layers. In order to fully characterise the underlying phenomena and report useful data for packaging applications, the material response of both

∗ Corresponding author. Tel.: +39 0512093426. E-mail address: [email protected] (A.H.A. Lutey).

single and multi-layer films must be investigated. Laser pulses of duration 0.5–20 ns are of interest, as good process quality may be achieved at a realistic investment cost for the packaging industry. When subject to nanosecond laser pulses of sufficiently high intensity, metallic targets are heated above the equilibrium boiling temperature, toward the critical temperature (Tc ). Miotello and Kelly [3], following the earlier work of Martynyuk [4], argued that material removal under such conditions takes place due to explosive boiling or “phase explosion”. In this scenario, the surface temperature of the target reaches 0.9Tc , at which point the nucleation rate of vapour bubbles rises dramatically leading to a rapid transition from a superheated liquid to a mixture of liquid droplets and vapour [5–10]. Lorazo et al. [11–13] instead studied the thermodynamic pathways to laser ablation in metals and semiconductors using molecular dynamics simulations, concluding that phase explosion is in fact limited to pulses of duration <10 ps. For longer pulses, including nanosecond pulses, they argued that material removal is the result of “trivial” fragmentation: near-equilibrium expansion leading to dissociation of supercritical matter into liquid droplets. In both scenarios, the practical outcome is a step-wise jump in ablation depth at a discrete threshold fluence. Such a threshold for aluminium has been experimentally

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established as between 4.3 J cm−2 and 5.2 J cm−2 for 5 ns laser pulses of wavelength 1064 nm [14]. Other experimental studies present pulsed laser ablation data for aluminium in the wavelength range 193–1064 ␮m and pulse duration range 0.5–20 ns [15–19]. In general, there are large variations in reported ablation depth per pulse, with up to one order of magnitude difference seen between studies conducted with similar laser parameters (e.g. Refs. [14,19]). The variation in published experimental measurements necessitates further study to establish reliable data for metallic thin-film cutting operations. The pulsed laser ablation of polymers differentiates itself from that of metals by the onset of chemical change, the exact nature of which depends on the target material and laser parameters [20–26]. In some cases, there is still no general agreement as to whether the predominant mechanism of material removal is photothermal or photochemical [21]. Few experimental studies present pulsed laser ablation data for PE and PP in the nanosecond range: a sharp increase in ablated mass per pulse with repetition rate has been demonstrated for both plastics with high fluence 5 ns pulses of wavelength 1064 nm [27], while pulses of duration 400 ps have also been investigated for ablation of ultra-high-molecularweight-polyethylene [28]. Investigations into the nanosecond pulsed laser ablation of paper are rare; however, some studies present data for wooden targets [29,30]. Experimentally, this material is treated in the same way as polymers, with the ablation depth approximated as a logarithmic function of the laser and threshold fluences. To the authors’ knowledge, an investigation into inter-layer interactions during laser exposure of multi-layer packaging materials has not yet been presented. Laser exposure, in this case, presents the possibility of both direct ablation via laser absorption and evaporation or thermal degradation via thermal conduction from adjacent layers. Determination of which phenomena are responsible for layer removal is of interest, particularly for films containing layers of high thermal conductivity, as differences in cut and interaction width between adjacent layers may affect cut quality. Where such differences are visible to the naked eye (>300 ␮m), the process quality declines for packaging applications. In light of these issues, the present work is concerned with the characterisation of single-layer aluminium, PP and PE and four typical multi-layer packaging materials comprising aluminiumpolypropylene and aluminium-paper. Picosecond near infrared (NIR) and nanosecond green laser sources have been employed with both single and multiple-pulse exposures undertaken. Samples have been analysed using an optical microscope and a 3D optical profiler to determine the ablation threshold and ablation depth of the single-layer films and the interaction and cut widths of all films under the tested conditions. The presented data provides useful quantitative information regarding the efficiency and quality of laser cutting operations in the packaging industry.

Table 1 Tested film compositions and layer thicknesses. Film

Layer 1

Layer 2

Layer 3

Polyethylene Polypropylene Aluminium Duplex Triplex Metall. paper Alufoil

PE (50 ␮m) PP (20 ␮m) AL (20 ␮m) PP (20 ␮m) PP (20 ␮m) AL (<0.1 ␮m) AL (7 ␮m)

– – – AL (<0.1 ␮m) AL (9 ␮m) Paper (69 ␮m) Paper (69 ␮m)

– – – PP (20 ␮m) PP (20 ␮m) – –

Table 2 Laser characteristics under test conditions. Test Group

A

Laser model Wavelength Approximate costa Wall-plug efficiency Repetition rate Pulse duration Beam quality M2 Lens focal length Spot size Rayleigh range Average powerb Pulse energyb Maximum fluenceb

Helios IR 1064 nm $US 50,000 ∼4% 30 kHz 0.5 ns <1.2 70 mm 42 ␮m 1300 ␮m 4.05 W 135 ␮J 20 J cm−2

B

C

D

70 kHz 0.8 ns <1.2 70 mm 42 ␮m 1300 ␮m 4.83 W 69 ␮J 10 J cm−2

Boreas G15 515 nm $US 85,000 ∼2% 30 kHz 10 ns <1.2 75 mm 30 ␮m 1300 ␮m 3.78 W 126 ␮J 37 J cm−2

100 kHz 12.5 ns <1.2 75 mm 30 ␮m 1300 ␮m 5.91 W 59 ␮J 17 J cm−2

a Approximate costs based on values as of June 2013, including power supply, laser heading, cooling system and basic 2D galvanometric scanning head and controller. b Values at sample surface.

2.2. Laser sources

2. Experimental setup

A NIR and a green laser source were utilised for the experiments: the Helios IR (Innolight GmbH), with a pulse duration in the range 0.5–0.8 ns, and the Boreas G15 (Eolite Systems), with a pulse duration in the range 10–12.5 ns. The Helios IR was attenuated externally by a half-wave plate and polariser, while the Boreas G15 was attenuated by an internal device provided by the manufacturer. For both the Helios IR and Boreas G15, the horizontal beam was directed vertically onto the sample by a 45◦ mirror and focused onto its surface with a lens. The lasers were mounted on two different machines equipped with x − y translation stages on which the samples were mounted. Samples were held horizontally above the stage under a slight tension. No contact between the sample and the stage was present for at least 30 mm in the direction of translation in the exposed area. Each laser was operated at two different repetition rates, 30 kHz and 70 kHz for the Helios IR and 30 kHz and 100 kHz for the Boreas G15, yielding four test groups, A–D, for each material. Table 2 presents the characteristics of each test group. The average power at the sample was measured using a Coherent LabMax-Top power meter. This parameter was then utilised to calculate the pulse energy and fluence at the sample surface under the focused spot. The power meter was also employed to calibrate the attenuation systems.

2.1. Tested films

2.3. Procedure

Single-layer PE, PP and aluminium and multi-layer Duplex, Triplex, Metallised Paper and Alufoil packaging films have been tested. Their compositions are giving in Table 1. The considered multi-layer films were chosen so as to demonstrate the influence of metallic layer thickness on multi-layer film response to laser exposure. Duplex and Triplex, and Metallised Paper and Alufoil, are of the same respective structures; however, the former in each pair has a much thinner aluminium layer.

Exposures were performed by translating samples under the focused beam at velocities in the range 50 mm s−1 –1 m s−1 , changing the attenuation with each test. Changing the velocity allowed testing under single-pulse and various multiple-pulse conditions. The relationship between test velocity and the number of pulses is given in Section 3.3. All test groups were utilised at translation velocities of 50 mm s−1 and 1 m s−1 for aluminium and 50 mm s−1 , 200 mm s−1

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Table 3 Optical profiler data.

3

scope of allowing comparison between the per-pulse ablation rates obtained under single and multiple pulse conditions.

Objective lens magnification

20×

100×

Numerical aperture Field of view (␮m) Spatial sampling (␮m) Optical resolution (␮m) Vertical resolution (nm)

0.45 636 × 477 0.83 0.31 <20

0.90 127 × 95 0.17 0.15 <2

3.2. Ablation depth in plastics A theoretical description of ablation of plastics is given by Lippert [21], in which the following equation is employed to express the ablation depth: d=

and 1 m s−1 for PE and PP. The additional tests at 200 mm s−1 for the plastic films came about after no interactions were observed at 1 m s−1 . Group C was utilised at translation velocities of 50 mm s−1 , 200 mm s−1 and 1 m s−1 for all multi-layer films except for Alufoil, which was only tested at 50 mm s−1 . Alufoil was limited to the lowest velocity as its ablation behaviour at higher velocities replicated that of single-layer aluminium, with no observed interaction in the paper layer. Groups A and B were utilised at maximum fluence for all multi-layer materials at translation velocities of 50 mm s−1 , 200 mm s−1 and 1 m s−1 . Both Alufoil and Metallised Paper were subject to laser exposure from the aluminium side only for all tests, as no direct interaction with paper was observed under any of the tested conditions. The ablation threshold fluences of the single-layer films were determined by observing the laser incisions under an optical microscope. The onset of ablation was taken as the point at which crater-type formations, corresponding to the onset of material removal, were observed. Values were obtained by calculating the average of the minimum tested fluence at which ablation was observed and the tested fluence immediately below. Ablation depth measurements were obtained with a 3D optical profiler functioning in confocal mode [31]. Two different objective lenses were utilised with magnifications of 20× and 100×, respectively. The main characteristics of the instrument are summarised in Table 3. Ablation depth was taken as the difference in measured profile between the lowest section in each cut profile and the level immediately outside the area of laser interaction. For multiple pulses, ablation depth was measured in a number of sections along the cut axis then averaged. For single pulses, the depth was measured at the lowest section of each crater along the cut axis then averaged. All measurements were averaged across several data points to minimise singularities and experimental error. Interaction and cut widths were determined by observing the laser incisions under an optical microscope. The interaction width is defined here within as the maximum of either the optical modification width, crater width or observed melting width, while the cut width is defined as the span over which no material is present.

3. Theory 3.1. Ablation depth in metals A numerical simulation for the short-pulse ablation of metals, based on the Miotello and Kelly model, has been presented in another work [10]. In the simulation, material removal due to both normal vaporisation and phase explosion are accounted for during a single laser pulse based on the one-dimensional temperature distribution. An experimentally established shielding coefficient accounts for absorption, reflection and scattering of the incident beam by the ablation products, a mixture of vapour and liquid droplets. Laser intensity at the target surface during the laser pulse is reduced with this parameter according to the inverse exponential power law applied across the time-dependent ablation depth. The numerical simulation has been utilised to produce theoretical ablation depth curves for single-layer aluminium, with the

1 · ln ˛eff

F 

(1)

Fth

where d is the ablation depth per pulse, F the fluence, Fth the threshold fluence and ˛eff an experimentally established effective absorption coefficient. This equation has been utilised to produce theoretical ablation depth curves for the single-layer plastic films, with the scope of allowing comparison between the per-pulse ablation rates obtained under single and multiple pulse conditions. 3.3. Pulse overlap A simple theoretical representation of pulse overlap with a translating laser source is necessary for the useful description of ablation depth, which is typically reported on a “per-pulse” basis. At low velocities, the degree of pulse overlap is high. As the target translates relative to the laser beam, a given point on the surface is exposed to a number of pulses of different fluence. In this case, the total ablation depth may be approximated as the sum of the singlepulse ablation depths resulting from all incident pulses above the ablation threshold. At a given point, the distribution of pulse fluence is discrete; a function of the beam waist radius, ω0 , repetition rate, frep , and translation velocity, V, in correspondence with the Gaussian intensity distribution of the laser:





F±i = F · exp −2

Vi frep ω0

2 

,

i = 0. . .n

(2)

where F±i is the fluence at the given point for pulse numbers i to either side of the central pulse at i = 0, where the laser axis is directly in line with the given point. F is the on-axis laser fluence and n is the largest integer for which F±n ≥ Fth . 4. Results and discussion 4.1. Qualitative outcomes High quality incisions were obtained for most of the tested films within certain parameter ranges. Interaction and cut widths were generally limited to <300 ␮m for all films except Triplex, where values of up to ∼500 ␮m were observed for some layers. Microscopy revealed some variability in ablation at low fluence, particularly for the single-layer PP and PE samples. This effect was most likely due to small variations in laser pulse energy (instability) and material properties (inhomogeneity), their effects accentuated near the ablation threshold of each material. The first interactions seen in PP and PE with increasing fluence were optical modification and intermittent material removal. Raising the fluence further saw sharp onset of efficient material removal, with the Helios IR producing a cut width of similar dimensions to the laser spot size (Fig. 1). Such ablation was not observed with the Boreas G15, only optical modification. This device therefore yielded no useful results for the plastic single-layer films. Aluminium showed evidence of material removal at fluences above the ablation threshold for both lasers. Two 3D profile measurements of the aluminium film are presented in Figs. 2 and 3. In the former, conducted at 1 m s−1 , individual craters may be seen,

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Fig. 1. PP following 50 mm s−1 exposure to Helios IR laser, test group A, at 12.5 J cm−2 : cut width CW = 42 ␮m and interaction width IW = 120 ␮m.

Fig. 2. 3D profile showing the interaction of single 10 J cm−2 pulses, test group A, with an aluminium target translating at 1 m s−1 .

Fig. 3. 3D profile showing the interaction of multiple 20 J cm−2 pulses, test group A, with an aluminium target translating at 50 mm s−1 .

Fig. 4. Duplex following 1 m s−1 exposure to Boreas G15 laser, test group C, at 5 J cm−2 : aluminium cut width CW(AL) = 73 ␮m.

while in the latter, conducted at 50 mm s−1 , the incision is a continuous line. The aluminium layer of Duplex could be removed at low fluence with little visible interaction with the PP layers (Fig. 4), while a complete cut was only possible with the Helios IR. In this case, heat conduction from the aluminium layer appeared to be negligible, as direct ablation of the PP layers was necessary for a complete cut. A full cut of Triplex was possible with the Boreas G15 but not with the Helios IR. In the former case, the crater and cut widths of the aluminium layer were much smaller than those of the PP layers at high fluence (Fig. 5). Removal of the PP layers was due to heat conduction from the aluminium layer, as the cut widths of the PP layers were much larger than the laser spot size. The aluminium layer of Metallised Paper could be removed at low fluence with no visible effect on the paper layer. A complete cut of this film was not possible under any of the tested conditions. A complete cut of Alufoil was achieved at low velocity with both lasers, the cut width of the paper layer irregular but with little charring and of similar dimensions to the laser spot size (Fig. 6). This layer was removed by thermal conduction from the aluminium layer, as no direct laser interaction with paper was observed under any of the tested conditions.

Fig. 5. Triplex following 50 mm s−1 exposure to Boreas G15 laser, test group C, at 37 J cm−2 : PP cut width CW(PP) = 270 ␮m, PP interaction width IW(PP) = 470 ␮m, aluminium cut width CW(AL) = 25 ␮m and aluminium interaction width IW(AL) = 80 ␮m.

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Table 4 Measured ablation thresholds of single-layer films, where within maximum laser fluence, for all tested translation velocities and laser test groups. Film PE PE PE PP PP PP AL AL

Velocity −1

50 mm s 200 mm s−1 1 m s−1 50 mm s−1 200 mm s−1 1 m s−1 50 mm s−1 1 m s−1

Group A J cm−2

Group B J cm−2

Group C J cm−2

Group D J cm−2

6.6 ± 0.8 6.6 ± 0.8 – 6.6 ± 0.8 7.9 ± 0.8 – 0.5 ± 0.15 0.5 ± 0.15

7.7 ± 0.8 9.6 ± 0.8 – 6.2 ± 0.8 9.6 ± 0.8 – 0.5 ± 0.15 0.6 ± 0.15

– – – – – – 6.0 ± 0.8 6.8 ± 0.8

– – – – – – 5.2 ± 0.8 5.8 ± 0.8

The Helios IR induces ablation in aluminium at a fluence of approximately 10 % of that required with the Boreas G15. This is the result of a reduction in thermal heat transfer during 0.5–0.8 ns pulses compared to that during 10–12.5 ns pulses. As with the plastic films, laser wavelength is not considered the primary factor, as light of wavelength 515 nm is more readily absorbed by aluminium than that of wavelength 1064 nm [33]. A slight increase in ablation threshold is observed at 1 m s−1 for test group B compared to group A, further demonstrating dependence on pulse duration. The results with the Boreas G15 (test groups C and D), however, display the opposite behaviour: a decrease in ablation threshold with an increase in pulse duration. This effect may be the result of greater sample preheating with test group D, where the average beam power is approximately three times that of group C at threshold fluence due to the higher repetition rate.

4.2. Single-layer ablation thresholds The measured ablation thresholds of the single-layer films, where ablation was observed within the maximum laser pulse energy, are presented in Table 4. The measured thresholds of all films show dependence on the beam characteristics and, in some cases, the translation velocity. This indicates dependence on the pulse duration, beam wavelength and the pulse overlap or number of consecutive pulses. The variation in threshold fluence with velocity for the plastic films is due to dependence of this parameter on the pulse overlap or number of consecutive pulses. The fluence necessary to induce ablation is greater when the number of pulses is reduced. This effect is due to incubation, where material removal begins after several incident pulses. Such behaviour is consistent with the observations of other studies for PE and PP [27]. While the Helios IR induced material removal in the PP and PE films at moderate fluence, the Boreas G15 was not capable of inducing short-pulse ablation in either. There is therefore a strong dependence of ablation behaviour on pulse duration for these films. Laser wavelength is not considered the primary factor, as both PP and PE exhibit high transparency to light at both 515 nm and 1064 nm [32]. An increase in ablation threshold is also observed in most cases for the plastic films with test group B compared to group A, further demonstrating dependence on pulse duration. As with the plastic films, the variation in ablation threshold with velocity for the aluminium film is due to dependence of this parameter on the pulse overlap or number of consecutive pulses. In this case, the driving effect is target preheating in the vicinity of the focused spot at low translation speeds. An increase in target temperature increases superheating effects, reducing the threshold fluence. The effect is more accentuated for test groups C and D than for test groups A and B due to the greater difference in average beam power in the former case.

4.3. Single-layer ablation depths The measured and calculated total ablation depths and the calculated per-pulse ablation depth of PE for test group A are presented in Fig. 7. The calculated total ablation depth is derived by summing the values obtained in Eq. (1) for all pulses in the fluence distribution in Eq. (2). Both the total and per-pulse ablation depths are higher at 50 mm s−1 than at 200 mm s−1 , corresponding to an increase in ablation efficiency with the degree of pulse overlap or number of incident pulses. As with the ablation threshold, this behaviour is due to incubation, where material removal begins after several incident pulses. While interaction was observed with the optical microscope for test group B, no measurable ablation depth was obtained with the 3D optical profiler due to low quantities of material removal. 15 Exp. 50 mm s−1 Calc. 50 mm s−1 Exp. 200 mm s−1 Calc. 200 mm s−1

Ablation Depth (µm)

Fig. 6. Paper side of Alufoil following 50 mm s−1 exposure to Boreas G15 laser, test group C, at 37 J cm−2 : paper cut width CW(Pap) = 45 ␮m.

−1

Per−Pul. 50 mm s 10

Per−Pul. 200 mm s−1

5

0 0

5

10

15

20

−2

Fluence (J cm ) Fig. 7. Measured and calculated total ablation depth and calculated per-pulse ablation depth for PE subject to test group A.

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6

6

−1

10

Exp. 50 mm s , A

50 mm s−1, A 1 m s−1, A

9

Sim. 50 mm s , A

8

Sim. 1 m s−1, A

7

Exp. 50 mm s−1, B

Exp. 1 m s−1, A

Ablation Depth per Pulse (µm)

Ablation Depth (µm)

−1

Sim. 50 mm s−1, B

6

−1

Exp. 1 m s , B −1

Sim. 1 m s , B

5 4 3 2

5

50 mm s−1, B

4

1ms ,B −1 50 mm s , C −1 1ms ,C

−1

3

50 mm s−1, D −1 1ms ,D

2

1

1 0 0

5

10

15

20

−2

Fluence (J cm )

The measurement of ablation depth was not possible for the PP film, as complete cuts took place before measurable ablation depths could be obtained. Complete cuts of this film occurred with test group A at velocity 50 mm s−1 and fluence 12.5 J cm−2 , and at velocity 200 mm s−1 and fluence 18 J cm−2 . The total ablation depth under these conditions is therefore ≥20 ␮m. The measured and calculated total ablation depths of aluminium for all test groups are presented in Figs. 8 and 9. The calculated perpulse ablation depths under the same conditions are presented in Fig. 10. The calculated total ablation depths are derived by summing the values obtained from the numerical simulation (Section 3.1) for all pulses in the fluence distribution given in Eq. (2). The total ablation depth for all test groups is higher at 50 mm s−1 than at 1 m s−1 ; however, the calculated ablation depth per pulse is markedly lower. In contrast to the behaviour seen with the plastic films, the ablation efficiency of aluminium is therefore lower for multiple pulses than for single pulses. This is due to greater shielding of the incident beam by the ablation products (Section 3.1). Complete cuts of this

22 20

Ablation Depth (µm)

18 Exp. 50 mm s−1, C −1 Sim. 50 mm s , C

14

−1

Exp. 1 m s , C −1 Sim. 1 m s , C −1 Exp. 50 mm s , D

12 10

−1

Sim. 50 mm s , D Exp. 1 m s−1, D Sim. 1 m s−1, D

8 6

2 5

10

15

20

25 −2

10

15

20

25 −2

30

35

40

30

35

Fig. 10. Calculated ablation depth per pulse for aluminium subject to all test groups.

film took place with test groups C and D at 50 mm s−1 with fluences of 25.5 J cm−2 and 10 J cm−2 , respectively. Despite the lower threshold fluence of aluminium subject to NIR pulses of duration <1 ns, the per-pulse ablation rate is superior with green, 10–12.5 ns pulses for fluences >8 J cm−2 . This implies lower laser shielding by the ablation products at 515 nm than at 1064 nm. At 50 mm s−1 , the test groups with shorter pulse durations for each laser, A and C, lead to greater ablation depths per pulse; however, the total ablation depths are lower due to the lower repetition rates. Despite these differences, the variation in per-pulse ablation rate between different multiple-pulse scenarios is far less pronounced than that between multiple and single-pulse exposures. At 1 m s−1 , the per-pulse ablation depths with group A are greater than those with group B, while the per-pulse ablation depths with group D are greater than those with group C. The higher ablation rate of group D may be the result of sample preheating, as discussed in Section 4.2. The aluminium ablation depth per pulse for test group C at 1 m s−1 is within 20% of the values presented by Colina et al. [15] for single pulses of duration 10 ns and wavelength 532 ␮m. The perpulse values for group A at 50 mm s−1 are instead approximately one order of magnitude greater than those presented by Porneala and Willis [14] for multiple 5 ns pulses of wavelength 1064 nm. This misalignment may be due to the difference in pulse duration; however, the same per-pulse values from the present study are within 20% of those presented by Stafe et al. [19] for multiple 4.5 ns pulses of wavelength 1064 nm. Though conducted with a laser of different wavelength, the per-pulse values for test group C at 50 mm s−1 are within 25% of the values presented by Horn et al. [17] for both multiple 12 ns pulses of wavelength 193 nm and multiple 6 ns pulses of wavelength 266 ns, for fluences >10 J cm−2 . 4.4. Single-layer interaction and cut widths

4

0 0

5

Fluence (J cm )

Fig. 8. Measured and calculated total ablation depth for aluminium subject to test groups A and B.

16

0 0

40

Fluence (J cm ) Fig. 9. Measured and calculated total ablation depth for aluminium subject to test groups C and D.

Above the ablation threshold, the interaction width of PE subject to test groups A and B was in the range 16–48 ␮m at all tested velocities, with an approximately linear dependence on the fluence. A complete cut of this film was not observed under any of the tested conditions. The interaction width of PP behaved similarly below the onset of film cut, with values in the range 19–50 ␮m, before a discrete increase to 122–135 ␮m at onset of complete film penetration. Cut widths of 42 ␮m and 51 ␮m were observed with

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80

500

50 mm s−1 AL Cut

50 mm s−1 AL Cut 450 400

7

50 mm s−1 AL Int. −1

PP Cut

−1

PP Int.

50 mm s 50 mm s

70

50 mm s−1 AL Int. −1

50 mm s

Paper Cut

60

Width (µm)

Width (µm)

350 300 250 200

50 40 30

150

20 100

10

50 0 5

10

15

20

25

30

35

40

−2

test group A for PP at 50 mm s−1 and 200 mm s−1 with fluences of 12.5 J cm−2 and 18 J cm−2 , respectively. The interaction width of single-layer aluminium displayed approximately logarithmic dependence on the pulse fluence, rising sharply at the onset of ablation and less so at high fluence. Maximum values of 65 ␮m and 80 ␮m were observed for the Helios IR and Boreas G15, respectively. A 30% reduction in interaction width was also seen from 50 mm s−1 to 1 m s−1 for fluences >15 J cm−2 . Cut widths of 7 ␮m were observed at 50 mm s−1 for test groups C and D with fluences of 25.5 J cm−2 and 10 J cm−2 , respectively. The interaction and cut widths of the single-layer films were generally limited to the area exposed to the laser beam. This reflects the underlying mechanism of material removal: ablation where the local laser fluence exceeds the ablation threshold. 4.5. Multi-layer interaction and cut widths The measured interaction and cut widths, where observed, of all layers of Triplex for test group C are presented in Figs. 11 and 12.

250 200 mm s−1 AL Int. 200 mm s−1 PP Cut

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Fig. 11. Interaction and cut widths of Triplex subject to laser radiation of test group C at 50 mm s−1 .

1ms

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Fluence (J cm−2) Fig. 12. Interaction and cut widths of Triplex subject to laser radiation of test group C at 200 mm s−1 and 1 m s−1 .

Fig. 13. Interaction and cut widths of Alufoil subject to laser radiation of test group C.

At low fluences, interaction is only seen in the aluminium layer. At the onset of PP layer removal, the interaction and cut widths of this layer rise quickly, becoming much larger than those of the aluminium layer at high fluence. A full cut of this film is seen at 50 mm s−1 for fluences >25 J cm−2 , the PP layer cut width approximately ten times that of the aluminium layer. These cuts are of relatively poor quality to the naked eye, the difference in layer cut widths visible. The onset of PP layer cut is delayed at 200 mm s−1 with respect to 50 mm s−1 , while it is not observed at 1 m s−1 . A full cut of the film does not take place within the maximum laser fluence for velocities ≥200 mm s−1 . With the Helios IR at maximum fluence (test groups A and B), a full cut was not observed under any of the tested conditions; however, the cut and interaction widths of the PP layers were 450–700 ␮m and 160–340 ␮m at 50 mm s−1 and 200 mm s−1 , respectively; greater than those seen with test group C at both velocities. This was due to the lower ablation efficiency of aluminium with the Helios IR laser. The interaction and cut widths, where observed, of all layers of Alufoil at 50 mm s−1 with test group C are presented in Fig. 13. The cut width of the paper layer was irregular within the range ±50%, though negligible charring was observed. The interaction width of this layer has therefore been considered equal to the cut width. The characteristic behaviour of this film is analogous to that of Triplex. At low fluences, interaction is seen in the aluminium layer only. At the onset of paper layer removal, the cut width of this layer rises in a linear manner, approaching the aluminium layer interaction width. These cuts are of good quality to the naked eye, as there is virtually no perceivable difference in the layer cut widths. With the Helios IR at maximum fluence (test groups A and B), a full cut was observed at 50 mm s−1 , the cut width of the paper layer 62–68 ␮m and that of the aluminium layer 27–39 ␮m. At all other velocities the film behaved as single-layer aluminium. The aluminium layer cut widths of Duplex and Metallised Paper subject to test group C are presented in Figs. 14 and 15, respectively. The aluminium layer interaction widths have been considered equal to their respective cut widths, as the observed cut edges were clean for the aluminium layers of both films. No interaction was seen in the other layers. In all cases, the aluminium cut width is much larger than the laser spot size and increases with fluence. For Duplex, this increase is linear, while for Metallised Paper, it is approximately logarithmic. Interestingly, the aluminium layer cut

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becomes significant and the interaction and cut widths of the nonmetallic layers grow rapidly. The characteristic behaviour of the films with aluminium layers of thickness <0.1 ␮m is however, fundamentally different from that of the films with aluminium layers of thickness ≥7 ␮m. Though short-pulse ablation is still responsible for removal of the aluminium layer, inter-layer heat conduction is negligible. Short-pulse ablation of all layers must therefore take place for complete film penetration. This is confirmed by the complete cut of Duplex with the Helios IR, the only laser source with which direct ablation of single-layer PP could be achieved.

250 50 mm s−1 AL Cut 200 mm s−1 AL Cut

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Fluence (J cm−2) Fig. 14. Cut widths of Duplex subject to laser radiation of test group C.

width of Duplex is lower at 200 mm s−1 than at 1 m s−1 . A full cut is not achieved with this laser for either film. As with test group C, the aluminium layer cut width of Duplex with test groups A and B at maximum fluence was lower at 200 mm s−1 than at 1 m s−1 : 78–98 ␮m compared to 93–104 ␮m. A full cut of this film was observed at 50 mm s−1 with test group A, the cut widths of both the PP and aluminium layers 250 ␮m Complete penetration of this film was not achieved at maximum fluence with test group B. Under no conditions was a full cut observed in Metallised Paper, though aluminium layer cut widths were in the range 190–270 ␮m. It is evident that cuts of the PP layers of Triplex and the paper layer of Alufoil are driven by thermal conduction from the metallic layer. The aluminium layers are instead removed by short-pulse ablation. As a result, the aluminium layer interaction and cut widths are confined to the area directly exposed to the laser beam, while this is not necessarily the case for the other layers. The onset of PP and paper layer removal is delayed from that of the aluminium layer, as sufficient heating of the film must first take place. With increasing average beam power, however, conductive heat-flow

300 −1

50 mm s

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Fluence (J cm ) Fig. 15. Cut widths of Metallised Paper subject to laser radiation of test group C.

The present experimental investigation provides data and observations regarding the laser incision and cut of some typical packaging films. The threshold fluence and ablation depth data presented for single-layer aluminium, PE and PP widens the parameter range for which the pulsed laser ablation of these materials is reported experimentally. It has been shown that 0.5 ns pulsed NIR laser exposure at fluence levels ≤20 J cm−2 can be used to efficiently ablate single-layer PP and PE films at velocities in the range 50–200 mm s−1 , with the resulting interaction and cut widths of similar dimensions to the beam spot size. Increasing the number of incident pulses was found to improve the ablation efficiency for PE. Though a one order of magnitude reduction in the ablation threshold of aluminium was observed with 0.5–0.8 ns NIR pulses with respect to 10–12.5 ns green pulses, the efficiency of material removal for fluences >8 J cm−2 was found to be superior in the latter case due to lower levels of laser shielding by the ablation products. Unlike the case of plastic films, the ablation efficiency of aluminium was found to be higher for single pulses than for multiple pulses, while less significant variations in this parameter were observed between different multiple-pulse conditions. For multi-layer films, the effectiveness of one particular laser type depended on the thickness of the metallic layer, with the mechanism of material removal for non-metallic layers governed by the degree of thermal energy transfer from the metallic layer. Triplex and Alufoil multi-layer films with aluminium of thickness 9 ␮m and 7 ␮m, respectively, were processed most effectively with the green laser due to the greater ablation efficiency of aluminium with this source. For these films, the other layers were removed by thermal conduction from the aluminium layer, with cut widths in some cases much larger than the laser beam spot size. A Duplex multi-layer film with aluminium of thickness <0.1 ␮m required pulses of duration 0.5 ns to achieve a complete cut via direct ablation of all layers, while a Metallised Paper multi-layer film with aluminium of thickness <0.1 ␮m could not be cut by either source due to lack of absorption by the paper layer. These cases saw negligible thermal conduction from the aluminium layer to the others due to the low thickness of the metallic layer. It is clear that there is a strong dependence of the optimum laser cutting strategy on the composition of the film under consideration. The pursuit of an optimum single laser source capable of executing the majority of cut and incision operations in the packaging industry requires further investigation. On the basis of the present study, however, a green laser with a pulse duration in the range 0.5–1 ns would be an appropriate starting point, exploiting the advantages seen separately here within for both wavelength and pulse duration while remaining within the range of realistic investment cost for the packaging industry. Nonetheless, it has been shown that the laser sources investigated may be utilised for some specific operations with good process efficiency and quality.

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Please cite this article in press as: A.H.A. Lutey, et al., Nanosecond and sub-nanosecond pulsed laser ablation of thin single and multi-layer packaging films, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.08.054