DE69720049T2 - Process for perforating a polymeric film and polymeric film - Google Patents

Abstract

Apparatus for the non-impact formation of perforations in a web of substrate material such as a polymer film (15) includes a static laser (20), movable beam deflector means, e.g. rotatable drum (30), carrying a plurality of optical reflectors (35) arranged to sequentially and selectively deflect the beam onto successive target sites on the web to perform perforation by ablation of the surface. Either the laser is selected on the basis that its emission wavelength coincides with a molecular absorption band at a resonant frequency or the material itself is doped to provide a suitable absorbance at the standard emission wavelength of the available laser.

Description

This invention relates to industrial processes for non-contact perforation ("punching") of polymer films considered thin or ultra-thin, and thin polymer films produced thereby.

Background of the invention

Ideally, an apparatus used in an industrial process for perforating materials should be able to provide variability of openings of desired shape without difficulty. In this specification, the words "perforation" and "hole" are used to indicate generally the formation of an opening or incision in a material, without any limitations being intended as to the shape so formed, unless otherwise stated.

Perforating web-like materials as part of an engineering process also requires delivery of openings through the material in a consistent and efficient manner at minimal cost. Often the material must be pierced to a high degree, however the use of mechanical impact methods can result in distortion or possible weakening of the material due to local stretching of the material or other damage resulting from contact with the piercing device. Very thin paper webs or polymer films are difficult to process mechanically. In addition, with mechanical devices, changes in the selected hole size usually require some downtime and manipulation of the hole punching equipment to change the sizes to be cut, spacing, etc.

A typical use of mechanical punching systems is to cut holes in polyethylene films. The general process details regarding the use of such mechanical punching perforation processes are given in Table 1 below. Table 1 Specification for on-line hole punching

In such known films, the holes are typically 10 mm diameter and arranged at 100 mm centers, with a 300 mm spacing on each side across the width of the film. The pattern is repeated at 80 mm intervals.

Currently known hole punching systems suffer from a number of limitations in terms of yield and production rate, choice of appropriate film materials and thicknesses, and a tendency of the device to "clog" by becoming contaminated in the material. Therefore, it is highly desirable to improve such systems and to investigate other methods, such as non-contact methods.

The concept of focusing electromagnetic energy as a particle beam or laser beam for cutting purposes has been investigated by others. These methods have proven successful on paper substrates, but their application to polymeric materials, especially relatively thin films (e.g. 17 µm or less), is particularly challenging. Problems observed in the art include uncontrollable burning or melting of the plastic material or insufficient penetration of the object surface, resulting in unreliable processing, which manifests itself in either irregular hole formation or non-formation of the desired perforation. Another disadvantage is that simultaneous perforation of a plurality of layers cannot be achieved when melting occurs because the corresponding layers may stick together.

The technical literature on industrial applications of lasers shows the use of lasers to perforate a range of materials (paper, plastics, ceramics, etc.) where the perforations are formed either by thermal melting or by ablation of the material. Ablation involves absorption of a resonant wavelength with the rapid and effective breaking of chemical bonds in the material and subsequent rapid vaporization of the material, providing a clean impression on the material. Unfortunately, this is a process that has been of limited industrial value to date as it relies on resonance at particular wavelengths, restricted to those materials that exhibit appropriate absorption characteristics.

Ablation processes are generally initiated by optical energy at wavelengths in the range of 0.2 to 10 um and the lasers commonly used are neodymium:yttrium aluminum garnet (Nd:YAG - wavelength ~1 um) and carbon dioxide (CO2 - wavelength ~9 to ~11 um). The wavelength for ablation of a particular material depends on the material type and composition. Kapton, for example, effectively ablates at 0.28 um, which coincides with an emission length of a U.V. excimer laser.

The use of laser based perforation and cutting equipment in industrial based process operations is well known and has found application in the perforation of high density polyethylene irrigation pipe, and in the cutting of integrated circuit boards ("CO₂ Lasers in Plastics Production", D Werth, Lasers & Applications, June 1985). The use of lasers in polymer Materials are discussed by SE Nielsen in a paper entitled "Laser Material Processing of Polymers", in Polymer Testing 3, 1983, pages 303 to 310, which describes the perforation of a polyurethane prosthetic venous device using pulsed laser light.

The use of lasers on thin substrates such as foils or fabrics has been investigated in the paper industry and is explained in the following documents.

As far as working on polymer films is concerned, EP-A-0 155 035 describes the perforation of a polyolefin film bag for air release purposes by the use of a laser to form the minimum achievable perforation, i.e., not more than about 150 µm, preferably 70 to 90 µm. These sizes are too small for present purposes. In particular, such a proposal is totally unsuitable for the purpose of producing perforated polyethylene films as achievable by mechanical punching processes referred to in Table 1 above.

A method for forming larger holes and/or slots is described in WO-A-96119313, possibly using a CO2 laser, given its effectiveness in converting radiant energy to heat in the material to be perforated. This involves additionally directing a gas, which may be enriched with oxygen, to burn the desired holes. Such a proposal does not provide ablation and cannot be applied to provide good hole quality through multiple layers.

DE 40 00 561 A1 (closest prior art) describes the addition of a special dopant (diphenyltriazine) to polymethyl methacrylate in the examples whereby flat bottomed holes can be created in a workpiece and also refers to previously published work using Tinuvin 328 (α2-(2'hydroxyphenyl)benzotriazole) as a dopant in polymethyl methacrylate, and suggests that two other organic substances, DADA and TPBD, yield CO and CO₂ upon photolysis. There is also speculation that other gas-generating substances may be used.

GB 1 255 431 describes the coating of workpieces with an organic agent prior to contact with a laser beam used to form holes in the workpiece. The organic agent is intended to form a barrier to the fusion of ejected material with the edges of the hole formed in the workpiece, thereby enabling the removal of the ejected material thereafter.

An object of the present invention is to provide improvements in or relating to the perforation of substrates on a commercial level. Another object is to provide an improved device for forming relatively large holes in thin web or sheet substrates, especially in plastic film substrates.

Thus, one object of the present invention is to provide for the perforation of web-like substrates in polymeric materials, such as polyethylene, by an ablation process using energy from a focused or coherent electromagnetic radiation source. Another object of the invention is to provide a successful method of applying energy emitted by a laser source device in an industrial process for non-contact perforation of web substrates.

EP-A-0 925 140 describes an apparatus for the impact-free formation of one or more perforations in a web of substrate material, the apparatus comprising:

Device for transporting and conveying the web of substrate material through the apparatus;

a concentrated or focused electromagnetic energy source for generating a coherent beam capable of ablating a substrate material, preferably a laser source device having a transmission wavelength that lies within an absorption band in a transmission spectrum of the substrate material; and

Means for directing the beam to the web of substrate material, whereby the beam is selectively and sequentially directed to perforation target points of the substrate in sequence, for example by periodically inserting a means for reflecting or deflecting the path of the beam to direct the beam and cause ablation and perforation of the web.

Preferably, the beam alignment means comprise an array of reflective surfaces mounted on a support means which is movable in operation to introduce each reflective surface sequentially through the laser beam path in a repeatable manner and thereby automatically switch the beam from one target point to the next according to a predetermined program of operation. The switching is effected by the controlled positioning of the array of reflective surfaces and by simultaneously controllably advancing the web of material substrate beneath the beam alignment means.

The laser may be operated from a static operating point position adjacent to the web transport system, essentially in continuous operation during a production run, with all the necessary switching of the beam being accomplished by the beam delivery mechanism, with a reflective surface or surfaces operative to re-orientate the beam of ablative radiation against the web of substrate material exactly where and when required, so as to effectively provide a means of directing the radiation with respect to the target point on the substrate. By appropriate control of the web feed in conjunction with the laser delivery mechanism, multiple passes over the same target point on the substrate may be possible. Such an arrangement may be controlled by a local microprocessor or by a larger computer in controlling the entire production run.

While it is desirable to operate with a minimum number of energy source devices, it should not be assumed that the invention as described herein is limited to the use of a single source device. Circumstances may arise where more than one source provides a process advantage. In general, however, the method and apparatus described herein eliminates the need to rely on a plurality of such source devices to perforate a web of substrate material.

In one embodiment described in EP-A-0 925 141, the means for applying or redirecting the ablating radiation comprise a body rotatable about an axis parallel to the direction of the radiation emitted by the laser, which body is provided with a number of reflecting surfaces which, during the rotation of the body, selectively and sequentially direct the laser radiation onto the path of the substrate material. This represents a system of few moving parts, which is technically not complicated and thus does not require maintenance and should therefore provide many hours of reliable operation.

In one such illustrative embodiment, the rotatable body comprises a drum, the rotation of which causes a reflecting device to interrupt the beam of a source device and to reorient it onto a band of material substrate sequentially in a repeatable predetermined pattern. Preferably, the drum is rotated by a motor, the operation of which is controlled by a computer or microprocessor to enable it to be synchronized with synchronized with the tape transport system.

In operating and using the device, consideration must be given to a number of factors. These factors include the type of actual substrate material involved, the thickness of the substrate material (single layer or multi-layer), the size of the perforations required, and the density of the perforations required. Of course, the operating characteristics of the laser itself will also affect the operation of the device and in particular the speed at which the web of substrate material can be processed using the device.

Summary of the invention

Taking into account the nature of the substrate material, there may be a suitable absorption band corresponding to the emission wavelength of the selected radiation source. Thus, where the source is a carbon dioxide (CO2) laser, which is a laser suitable for industrial use, the polymer film may be a polyimide, a Phillips type hot pressed polyethylene film (see Figure 4), a Corlona 800 Shell polyethylene, or another commercially available polyethylene rich in pendant methylene or other groups which give the material a suitable absorption characteristic. If the selected substrate material does not have a suitable absorption band to match the preferred laser source device or any other that may be available, then in accordance with an inventive aspect of this invention, contemplation should be given to modifying the selected substrate material to cause it to acquire the desired absorption properties. This aspect can also be applied to achieve perforations on a paper substrate in a similar manner to the processing of polymer films as specifically described herein.

This can be achieved according to the invention as claimed in claims 1 and 2 by introducing a dopant to the substrate material during its manufacture. Suitable dopants include inorganic materials such as opacifiers, fillers and pigments, e.g. polysiloxanes, silicates and titania, and organic materials compatible with the substrate, for example suitably absorbing polymer additives such as typical polyethylene compatible co-monomer, e.g. vinyl acetate, ethyl acrylate, menthyl acrylate, butyl acrylate, a vinyl ester or an ester of acrylic acid or methacrylic acid, and mixtures thereof to obtain a doping effect by forming a co-polymer with the base polymer. Such a polymer comprises in its matrix additional functionalities capable of absorbing at the desired wavelengths when irradiated. Of course, when selecting the dopant, care must be taken to anticipate or check for any adverse effects, its effect on the technical properties of the substrate material. Thus, for example, a polythene film may be adversely affected by the inclusion of excessive amounts of ethylene vinyl acetate (EVA) co-polymer, which has a significant effect on the physical properties of the film. Therefore, the appropriate dopant additive material to be selected and the amount of dopant additive material used will depend on a number of factors, such as:

(I) The required wavelength;

(II) the required mechanical strength of the polymer film; and

(III) the establishment of a removal rate appropriate for a practical production environment.

The dopants may comprise up to 20%, for example 1 to 10%, by weight of the film. In the case of an inorganic dopant, for example silicate material, the dopant may comprise less than 9% of the final film by weight, preferably 4 to 5%.

A significant advantage of the present invention is that it enables relatively large holes, larger than 1 mm (0.001 m) to be formed in thin polymer films, for example in thin films of polyethylene of, for example, 70 µm or less, up to about 400 µm thick. The invention can also be usefully applied in the simultaneous perforation of multiple layers, which by ablation enables rapid perforation cleanly through the layers, which remain separable. The invention also provides the capability of piercing very large holes by using a sequential, narrow perforation process.

In order to effectively ablate the substrate material, the wavelength of radiation emitted by the laser must be suitable for the substrate material, that is, of the correct order, otherwise the substrate material will not be ablated but will melt and burn. Therefore, the laser is selected or, if possible, tuned to emit a wavelength that coincides with or is substantially the same as the absorption wavelength of the substrate material to be ablated. However, since selecting and installing a laser source device is a major capital investment in any production line, it is preferred to consider prior modification of the substrate material.

Therefore, if the absorption wavelength of the substrate material is not in the correct range with respect to the available laser source equipment, it is preferable to adjust the absorption wavelength of the substrate material during the initial preparation of the material, i.e. during a preliminary melting step before film formation, or during a paper web forming step, by adding suitable dopant or additive materials such as pigments or co-monomers identified to provide the appropriate absorption properties as mentioned above.

A wide variety of laser source devices are available and it is believed that the invention can work with any laser device capable of delivering radiation of the correct wavelength at a sufficient power density to effect ablation rather than melting the substrate material. Preferred lasers are the high power monochromatic lasers such as excimer lasers and carbon dioxide lasers.

Short description of the drawings

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings:

It shows:

Fig. 1 is a schematic block diagram illustrating a generalized process of the invention for perforating a material using a laser;

Fig. 2 shows an optical absorption spectrum through a known 50 µm polythene film sample;

Fig. 3 is a graphical representation of the removal rate of a polythene film by an etch rate versus flux for the example in Fig. 2 using a 193 µm excimer laser with etch rates of 35 µm and 50 µm;

Fig. 4 shows the optical absorption spectrum of a Phillips-type hot-pressed polythene;

Fig. 5 the optical absorption spectrum of a Carlona 800 shell polythene;

Fig. 6 the optical absorption spectrum of a generic polythene rich in pendant methylene groups;

Fig. 7 is a schematic diagram of a preferred embodiment of an apparatus for forming a hole or holes in a polymer film according to the process of the present invention;

Fig. 8 is a schematic end view of the apparatus according to Fig. 7,

Fig. 9 schematically shows the parameters involved in hole formation using the invention;

Fig. 10 is a representation by graphical representation of the relationship between the height above the work surface and the drum radius for the device according to Fig. 7;

Fig. 11 optical absorption spectra (CO2 laser) for 3 types of polythene films (A, B and C) used in this invention;

Fig. 12 shows the results of the removal of a 50 µm thick polymer film type (A) doped with inorganic materials, the absorption spectrum of which is shown in Fig. 11;

Fig. 13 shows the results of ablation of a 150 µm thick film (C), including both an inorganic dopant and a copolymeric dopant in the polymer example whose absorption spectrum is shown in Fig. 11; and

Fig. 14 shows the results of the ablation of a doped, 200 µm thick foil of type (B), whose absorption spectrum is shown in Fig. 11.

Example

In Fig. 1 of the drawings, a general overview of the process of perforating a web of substrate material is given by means of a block diagram. In this example, the process is applied to a web of substrate material which is a polymer film 15, here a film of polythene.

With this general method as illustrated, the polymer film 15 is transported through a perforation to the operative zone, for example by means of a conventional sheet or film transport conveyor system 23. At a point in the operative zone, a laser perforation device 20 is mounted with respect to the transported polymer film 15. The apparatus includes beam shaping optics 21 to ensure that the laser beam is coherent and well defined, and beam delivery optics 22 to fire the beam accurately and precisely onto the surface of the polymer film 15.

From the illustrated embodiment it will be apparent that the beam delivery system is based on the technique known as beam time division multiplexing, due to the fact that the beam is switched sequentially to illuminate different target areas on the film 15. The alternative technique of beam split multiplexing has the disadvantage of reducing the energy of the beam by splitting. Therefore, greater care must be taken with this technique to ensure that the desired ablation is achieved. The inventive concept of hitting a specific absorption band can still be used provided that the split beam can still effect ablation. Where the energy density delivered to the target area is insufficient to effect ablation, the raw material will absorb the energy and just melt and hole formation will be random.

Of course, the disadvantage of beam energy reduction by division can be overcome by moving the laser device and associated optics relative to the target film surface or by providing multiple laser sources corresponding to the number of holes required on the film. However, such alternatives represent additional costs and complicating factors that suggest these options are not preferred in an industrial film production line.

With reference to Figures 7 and 8 of the drawings, the embodiment of a laser perforation apparatus according to the invention explained here will now be described in more detail.

The apparatus includes a laser 20 with optical means for beam shaping, optical mechanisms for beam delivery (non-contact perforation means) 25, and film transport means (not shown) for transporting the polymer film 15 through the processing equipment in the direction shown by the arrow, past the laser and under the beam delivery mechanism 25. In the specific embodiment, the laser perforation apparatus 25 is positioned so that the laser beam emitted by the laser 10 is transverse to the direction of transport of the polymer film 15.

The laser 20 used in the arrangement shown is selected with reference to the polymer film or films to be processed and in particular with reference to the materials to be processed or expected to be processed using the equipment. This is mainly due to the fact that in the treatment of polymeric films in particular the ablation of the material by laser irradiation is preferably carried out in such a way as to provide clean and well-defined perforations, as explained above, in order to avoid melting and burning of the material.

To ensure that the material of the film 15 is removed, the wavelength of the light emitted by the laser should be wavelength of the polymeric material. Since industrial lasers generally have a specific wavelength of emission, it may be necessary to adjust the wavelength of absorption of the polymer film material to ensure ablation as described above.

In this example, the apparatus uses a high output CO2 laser in the wavelength range of 9 um to 11 um.

The laser 20 provides the energy required to perforate the polymer film 15 and this is in the form of a focused, i.e. well defined, sharp laser beam of coherent radiation. Those experienced in the field of lasers will understand the principles of beam shaping and alignment. Briefly, after the beam is emitted from the laser, it passes through the laser shaping optics 21 (Fig. 1). The geometry of the beam is modified by the laser shaping optics 21 so that the flux (energy per unit area) in the plane of the film 15 is maximized and limited to the dimensions of the perforation.

The beam delivery mechanism uses optical devices, here mirrors 35, to realign and position the beam exiting the beam shaping optics for correct ablation from the surface of the film 15 to obtain the perforation. In practice, it is usually necessary to form a plurality of perforations at high speeds of the manufacturing operation. In this embodiment, multiple perforations are ablated rapidly and sequentially by beam time multiplexing. As a result, there is only one beam acting on the substrate at any single instant in time, but the speed of operation is such that this would not be obvious to an observer.

In this particular embodiment of the apparatus, the optical device 25 for outputting the beam comprises a drum 30 which is pivoted about an axis substantially rotatable parallel to the direction of laser radiation, but not coaxial therewith. The drum has an outer surface on which a number of mirrors 35 are mounted. The drum 30 is arranged transversely to the direction of movement of a tape substrate to be perforated and extends across its entire width.

The drum 30 is rotated about the axis of rotational symmetry by a belt drive device, shown here but unnumbered, wherein power from a motor, which may be an electric motor, is transmitted by a belt to a speed reduction drive shaft attached to the drum 30.

In operation, the perforation process requires that the polymer film 15 be transported to an appropriate target zone and that the drum 30 be rotated by the motor while the CO2 laser 20 emits a beam of radiation which passes through the beam shaping devices 20 before striking one of the mirrors 35 on the drum 30 and is deflected downwardly onto the polymer film 15 where it ablates the material of the film 15 and forms a perforation.

To determine the factors to be controlled for the operation of the laser perforation device, the following general points were taken into account:

The invention relies on the use of ablation, as opposed to melting, and this can only occur where the irradiated target material is capable of strong molecular absorption in resonance with the incident wavelength. Thus, the choice of laser is limited to the natural molecular absorption bands of the material. Furthermore, the ablation rate depends on energy density. In order to break bonds and eject material from a body of polymer, a threshold energy must be available, thus requiring a critical energy density. As the energy density increases, more material can be vaporized. Eventually, however, the rate saturates.

Ablation rate saturation, where an increase in the input laser energy density no longer increases the depth of material removal occurring, occurs around 1/e absorption depth, i.e. when the thickness of the material is sufficient to absorb approximately 37% of the incident signal. In general, polythene and similar plastics can be ablated by a typical excimer laser at a rate of 0.5-1 µm, with a saturation energy density on the order of 5 J/cm2.

The rate at which material is removed is determined, for a given energy density, by the material's absorption at the laser wavelength. Stronger absorption results in resonant absorption by a chemical bond, which then breaks by ablation, in preference to melting, and thus more material is removed in a single shot. If the band is weak, much of the pulse energy is converted to thermal energy and is therefore wasted. Tests performed on polymer material have shown that a 1/e absorption length of 1-2 µm results in saturated ablation at a rate of ~2 µm per shot for a typical excimer laser used for polymer ablation, which has an emission wavelength of 193 nm, an energy per pulse of 4 mJ, and a duty cycle of 5 Hz, 20 ns per pulse.

Furthermore, the energy density must reach a critical level over the entire shape to be ablated if a hole is to be formed in a single pass. Thus, the energy emitted by the laser must increase as the square of the diameter of the hole to be formed. The energies available for the typical excimer laser, as specified in the previous paragraph, are suitable for holes 200 µm in diameter, but for larger holes the beam must be scanned. Also, an excimer is not suitable for piercing large holes because the energy delivery rate is too low.

Once the ablation rate is saturated, increasing the input laser energy density does not increase the depth of material ablated in a single pulse. Therefore, to increase the rate of material removal, the pulse repetition rate must be increased rather than the energy density.

For effective non-contact processing, the laser source must be precisely matched to the material properties and process requirements. Experimental work was carried out to identify the most suitable source to cut large (10 mm) holes in material by ablation.

Ablation is the key to forming high quality holes in the polymer and only occurs when the incident energy input is in resonance with the sample absorption. A short list of such wavelengths can be obtained from absorptions of the optical absorption spectrum. Fig. 2 shows the optical absorption spectrum of a sample of known 50 µm polyethylene film (80% LDPE, 20% LLDPE), between 0.190 µm and 25 µm. The spectrum is corrected for background absorption. The bands are listed in Table 2 with lasers having emissions close to the wavelength. Table 2: Optical absorption bands of known polythene sample and lasers with resonant emission wavelengths.

There is no absorption compatibility with the two most commonly used lasers, CO2 at 10 µm or Nd:YAG at 1.064 µm. However, of the lasers listed, the excimer laser is the most commonly used for material processing.

The absorption spectrum of Figure 2 shows that the sample has an absorption of approximately 90% (10% transmission) through 50 µm at the excimer wavelength of 193 µm. This corresponds to an absorption length of 20 µm.

The absorption of polymers becomes stronger at shorter wavelengths, so that the absorption wavelength decreases accordingly. However, since the reliability of excimer lasers operating at wavelengths shorter than 193 nm is relatively poor, they are considered unsuitable for industrial applications.

The ablation rate was found to depend on the energy density of the incident laser beam at the absorption band. To confirm the importance of the laser operating wavelength, polymer samples were exposed to energy densities of 1 to 10 J/cm2 from sources operating outside an absorption band; CO2 lasers at 10.6 µm and 9.4 µm and an excimer laser at 251 µm. In each case, the polythene layer melted uncontrollably.

The data in Fig. 3 show the depth of test material removed per shot by a typical excimer laser as defined above. These results demonstrate that an energy density of at least 10 J/cm² is necessary to remove 0.5 μm depth.

The data confirm that the removal rate saturates around 10 J/cm2, so that simply increasing the energy density will not lead to a significant increase in the cutting rate. This rate of increase is in line with expectations due to the relatively weak absorption of the tape.

The two-layer removal was tested and confirmed to be clean without melting. The two layers did not melt together.

The area of material removed depends on the laser power. An energy density of 10 J/cm² is necessary to remove 0.5 um of material. However, the laser used here was a high power excimer laser, suitable for removing polymer, as follows:

Product: Lambda Physik LPX325i; Nova tube and Halo safe for high admissibility and easy use;

More than 1.5 billion shots for a single gas filling;

Emission wavelength: 193 nm; energy per pulse: 25 mJ; and

Duty cycle: 250 Hz, 20 ns pulse.

This laser emitted only 4 mJ per pulse, resulting in a maximum hole diameter, without scanning, of 225 µm. The maximum energy obtainable from this class of lasers is around 25 mJ, so the maximum hole diameter per pulse remains very small, around 564 µm. To meet special requirements, a multi-pass scheme with a laser operating at extremely high frequencies is required.

It was found that the total depth of material removed depends on the laser repetition rate. Regardless of the area to be removed, a large number of pulses are necessary to cut through the polythene film, provided that only 0.5 um is removed per pulse with an energy density of about 10 J/cm².

As stated in Table 1 above, the total film thickness typically varies between 70 µm and 400 µm; thus between 140 and 800 pulses would be necessary to ablate a hole of 225 µm diameter. Furthermore, the film can travel at either 40 m/min or 26 m/min in a repeating pattern every 80 mm. Thus, 225 µm (corresponding to the unit of ablation diameter) would be swept in 0.3 ms or 0.5 ms, respectively. Ablation should therefore occur at more than 467 kHz or 1.6 MHz to form just one 225 µm hole.

Because the perforation of polythene by ablation thus holds great potential, there is currently no laser source device having a transmission wavelength matching the absorption band of currently used polymer films. The solution proposed in this invention is to adapt a polymer that allows a practical laser source to be used. This is most easily achieved by creating an intense absorption band at the wavelength of an industrial type of laser.

A CO2 laser would be preferable as it is a highly reliable, inexpensive source with low maintenance costs and is widely used in industry. A typical system as shown in Table 3 offers high powers, 1 kW at high repetition rates, 5 kHz, and is highly reliable. Table 3: CO₂ laser specification.

The material modifications, likely removal rates, jet delivery system and resulting cutting speeds are considered below.

An intense absorption band between 9.2 µm and 10.6 µm, the CO₂ wavelengths, can be generated in one of a number of ways, such as the non-limiting examples of:

(I) Use of an alternative polymer such as polyimide

(II) Doping of existing polythene with, for example, a polysiloxane or a titania pigment.

(III) Selection of a polythene prepared by a different process which shows a suitable absorption. For example, Phillips type hot pressed film, Carlona 800 shell polythene and generic polythenes rich in pendant methylene groups which give some absorption in this range as shown in Figures 3, 4 and 5 respectively.

Modification of the material may have major implications for the production process, recycling, and cost and mechanical strength, all of which must be taken into account when considering the feasibility of modifying the material to match the spectral absorption properties to a preferred laser source.

An industry standard CO2 laser operating at wavelengths between 9.2-10.6 µm has been used successfully to ablate large holes in polyamide. Typical ablation rates of 10 µm for a saturated energy density of 1 J/cm² allow deep holes to be formed quickly. The high output powers facilitate large ablation areas.

The expected performance of readily available sources is summarized in Table 4 for the processing conditions specified in Table 1. Table 4: Expected removal rate and achievable hole diameter.

A polyimide ablation rate of 10 um from 1 J/cm² pulse results in a 400 um deep Hole, equivalent to cutting through 8 layers of 50 µm film, can be done in 8 ms. The diameter of the hole cut in this time with a standard 1000 W laser delivering 200 mJ per pulse is about 5 mm. Lasers operating at several times this power and repetition rate are not uncommon (see Table 3), although more expensive, so higher repetition rates are possible if required.

The design of the drum scanner and the practical operating conditions are determined by the time necessary to cut the holes, the steel geometry, the polymer transverse speed and the pattern repetition.

Time: The optimal time to expose the film is given by the time to remove the sample:

t = N/R

where R is the laser repetition rate and N is the number of pulses to remove the working thickness, which is given by:

N = T/Ta

where T is the total thickness to be machined and Ta is the thickness removed per shot.

The optimal time is the time for which a given segment of the polymer layer is exposed to the laser beam and is given by the time in which a single mirror is fully illuminated by the incident beam:

T = α/ω

where, with reference to Fig. 8, ω is the angular rotation rate and α is the angle traveled around the circumference in that time.

In the case where te represents the required exposure time, α represents the "useful" part of the mirror, and assuming that the angle is small,

α = (P - d)/Cs,

where P is the width of the mirror at the drum circumference, d is the diameter of the laser beam and Q is the radius of the scanning drum.

Beam geometry: This application requires that the laser pulse cuts through numerous layers of the polymer with repeated pulses, so that in order for the shape formed to be the same for each layer, the laser spot must be directed to the same location on the polymer surface throughout the exposure. The beam traversal and polymer velocities must therefore be matched. In such a case, the illuminated area is the circular beam area: Π/₄d².

Traversing speed: Vb can be related to the drum rotation rate and the height of the mirror above the working surface:

Vb = (Cp - Cs)ω

the polymer velocity, Vp, and the velocity Vb of the directed jet are set equal:

Vp = Vb = (Cp - Cs)ω

and the drum rotation rate is:

W = Vp/(Cp - Cs) = Vp/D

The working height of the drum above the moving surface, D, can be selected independently, whereby

D = Cp - Cs

is.

Pattern repetition: The number of required perforations, n, over the layer defines the number of mirrors that fit on the circumference of the working drum, thus

2ΠCs = XnP

where X is an integer ≥ 1 indicating that the pattern can be repeated across the drum to obtain a slower rotation rate.

The time of pattern repetition, tr, is given by the distance between the repetitions, Dr, and the velocity of the polymer, Vp:

tr = Dr/Vp

For the design to be successfully repeated, the pattern repeat time must be greater than the time to create all the perforations across the film:

tr ≥ n t

Drum geometry and operation: First, the feasibility of cutting the perforation in the allowable time for the specified working hours must be confirmed. By combining the required exposure time, the beam diameter and the required number of mirrors and the polymer velocity of the working surface, D, can be determined with reference to the height above the drum radius, Cs.

te ω = (P - d)/Cs

ω = Vp/D

2Π Cs = XnP

A combination results in

D = te V/(2Π/Xn - d/Cs)

Assuming that the drum rotation frequency is related to the height of the mirror above the working surface,

f = ω/2π

ω = Vp/D

Realistic values can therefore be determined for different numbers of pattern repetitions on the drum, given the time for one pattern repetition, tr = 1/f Limits can be specified on the drum rotation frequency in conjunction with those for the operating height above the working surface. Thus, the scanning parameters for the working beam can be specified.

Perforation cutting can be achieved for all the conditions defined in Table 1, for a given drum radius, Cs, and number of mirror sets, X, above the drum by varying the drum rotation frequency, f, and the height above the working surface, D. Realistic operating frequencies and working heights can be achieved with 5 mirror sets on the drum.

For X = 5 and Cs = 15 cm

For X = 5 and Cs = 20 cm

Increasing the working height is achieved by increasing the number of mirror sets, for example

for X = 10 and Cs = 20 cm

Not only is there the cost consideration of using a large number of mirror sets to meet all the necessary conditions, increasing heights and lower frequencies should be used. This will make the production process less reliable; optical losses through the air will reduce performance at the working surface, while a rotation frequency of 0.1 Hz can be relatively easily disturbed.

Claims (8)

1. A method for producing a non-contact perforated thin polymer film of 70 µm or less up to about 400 µm, comprising (a) selecting a laser source device capable of emitting a laser beam having a transmission wavelength that matches an absorption band of a transmission spectrum of the film, (b) adjusting the absorption properties of the polymer prior to forming a film therefrom by incorporating a dopant therein to provide increased absorption at the transmission wavelength of the selected laser source devices, the dopant being selected from an inorganic dopant and a comonomer additive compatible with the polymer film and capable of forming a copolymer therewith, (c) providing a movable array of guide means capable of reorienting the laser beam of the selected laser source means, (d) providing a transport device for conveying a film in a controlled manner through a zone for performing non-contact perforation on that film by ablation from the surface, (e) locating the selected laser source device and the arrangement of guiding devices in operative proximity to the perforation zone, and (f) actuating the laser source means and moving the array of guide means to reorient the laser beam sequentially and selectively to successive target locations on the film to effect ablation and perforate the film in accordance with a predetermined operating program to provide ablated perforations. 2. A thin polymer film of 70 µm or less up to about 400 µm having perforations ablated therein by a laser source device, the polymer film being a polyethylene-based film comprising a A dopant for providing increased absorption at the transmission wavelength of the selected laser source device, the dopant being selected from an inorganic dopant and a comonomer additive compatible with olefinic polymers and capable of forming a copolymer therewith. 3. A polymer film according to claim 2, wherein the inorganic dopant is selected from inorganic opacifiers, fillers and pigments, and is included in the film in an amount of up to 20% by weight. 4. A polymer film according to claim 3, wherein the dopant is selected from silicates, polysiloxanes and titania. 5. A polymer film according to claim 3, wherein the dopant is included in the film in an amount of 1 to 10 wt.%. 6. A polymer film according to claim 2, wherein the co-monomer additive is selected from vinyl acetate, ethyl acrylate, methyl acrylate, butyl acrylate, a vinyl ester, an ester of acrylic acid, an ester of methacrylic acid, and mixtures thereof. 7. A polymer film according to claim 6, wherein the co-monomer additive is present in the film in an amount of 1 to 10 wt.%. 8. A polymer film according to any one of claims 2 to 7, wherein the ablated perforations exceed 1 mm in diameter.