AU626075B2 - Manufacture of finely perforated sheet material - Google Patents

Description

1 Y_ OPI DATE 05/03/90 APPLN. ID 40633 89 AOJP DATE 12/04/90 PCT NUMBER PCT/AU89/00336 PCr INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: WO 90/01393 B23K 26/10, 26/00 Al (43) International Publication Date: 22 February 1990 (22.02.90) (21) International Application Number: PCT/AU89/00336 (74)Agent: HALFORD, Graham, William; Halford Co., 49-51 York Street, Sydney, NSW 2000 (AU).

(22) International Filing Date: 4 August 1989 (04.08.89) (81) Designated States: AT (European patent), AU, BE (Euro- Priority data: pean patent), CH (European patent), DE (European pa- PI 9745 4 August 1988 (04.08.88) AU tent), FR (European patent), GB (European patent), IT (European patent), JP, LU (European patent), NL (European patent), SE (European patent), US.

(71) Applicant (for all designated States except US): COMMON- WEALTH SCIENTIFIC INDUSTRIAL RE- SEARCH ORGANISATION [AU/AU]; Limestone Published Avenue, Campbell, ACT 2601 With international search report.

(72) Inventors; and Inventors/Applicants (for US only) BRANDT, Milan [AU/ AU]; 3 Monastery Place, Cherrybrook, NSW 2120 (AU).

CRANE, Kenneth, Cecil, Allan [AU/AU; 18 Kooloona Crescent, West Pymble, NSW 2073 6 2 (54) Title: MANUFACTURE OF FINELY PERFORATED SHEET MATERIAL (57) Abstract A method of perforating a sheet material (11) comprising removably supporting the sheet material (11) on a support surface of a support body exposing the sheet material to a laser beam (16) to form perforations in the sheet material (11) and removing the resultant perforated sheet material (11) from the support body The support surface has a low reflectivity to the laser beam such that the geometry of the perforations is not significantly affected by reflected radiation.

I W) 90/01393 PCT/AU89/00336 -1- MANUFACTURE OF FINELY PEk 'ORATED SHEET MA'TERIAL BACKGROUND ART This invention relates to the production of finely perforated sheets, for example screens or sieves used in centrifugal separators or filters. It includes the manufacture of sheet materials in which many fine perforations are involved, particularly if one side of the sheet as perforated is required to be smooth and the lips of the perforation at that side are required to be fairly sharp. For example, the invention has applications to the production of separating screens for use in so-called centrifugals used in sugar production for the separation of sugar crystals from massecuite sugar syrup, and will be described here in that context. It is to be understood, however, that the invention is in no way limited to that particular application. Other applications include sieve bend screens used in the food processing and mineral processing industries.

A screen typical of those presently employed in the sugar industry is formed from a number of sections each in the shape of a segment of an annulus, which together form a frusto-conical screen when supported within ':he centrifugal by means of a supporting basket or mesh. The screens are between 200 and 350 microns thick, and are conventionally formed by the electrodeposition of nickel with the use of a metal matrix master to define the slots through which the molasses passes from the centrifugal.

These slots are typically 60 microns wide and 2000 microns long on the working side of the screen, and have sides which taper outwardly to larger dimensions on the outer side of the screen from which the molasses exits, to reduce the incidence of blockage o4 the slots by sugar crystals.

The slots are distributed uniformly over the surface of the screen, and are provided in various spacings to give open areas of between 6% and 15% of the area of the working side of the screen. The total area of such a screen is typically 1.2m for which an open area of 8% SUBSTITUTE SHEET s

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I I WO 90/01393 PCT/AU89/00336 -2requires 800,000 slots. In the case of screens presently employed a layer of chromium 15 to 20 microns thick is applied to provide a surface which has sufficient hardness and resistance to corrosion.

The useful life of such screens is limited by the gradual enlargement of the slots which occurs due to corrosion and erosion. The principle mechanism involved has been identified as arising from cracks in the chromium coating, which allow the hot molasses into regions between the chromium and the nickel. This leads to galvanic corrosion, eventually resulting in pitting of the nickel and flaking of the chromium. Once the softer nickel is exposed, it erodes or corrodes quickly, and the slot width increases. Increasing slot width significantly decreases the separation efficiency of the screen, which eventually V must be replaced. The useful working life of such screens in continous centrifugals may be approximately six weeks.

The tensile strength of these screens must be sufficient to avoid stretching of the screen and its deformation into the gaps in the supporting basket. Such deformation may cause an increase in the size of the slots, and will also create resistance to the flow of material along the working surface of the screen.

It is important that the inner or working surface of the screen should be smooth in the immediate vicinity of the slots, and this must be taken into account in the choice of production method.

As mentioned, such screens or other sheet materials with fine perforations may be produced by an eiectrodeposition technique. Alternative techniques include mechanical punching, chemical etching (also known as photochemical machining), electrochemical machining, electrodischarge machining, and the use of focussed electron beams or laser beams. The most appropriate method for a particular application depends on many complex factors, but in the context of the present invention, it is significant that the use of a laser has some generally-accepted advantages. The perforation of :r sheet materials by laser beams is a common practice in manufacturing industries throughout the world.

Advantages of this approach include the ability to form very small perforations (holes with diameters of 60 microns are not uncommon), the good compatibility with computer-numerical-control (CNC) equipment for moving the workpiece sheet or the beam, the lack of tool wear, and the applicability to a wide range of materials (both metallic and non-metallic). The use of a laser enables perforations with a larger rat.o of depth to width than those practicable with chemical etching, and consequently the use of a laser is not restricted to the production of very thin perforated sheets.

However, lasers have not been widely used to produce sheet materials in which a large number of fine perforations are required. Some screens or products similar to screens which have involved a laser manufacturing technique have been reported. The present invention involves a manufacturing technique which enables perforations of high quality to be produced, and which in many cases is expected to be more economical than alternative laser techniques.

Particularly in the case of thin sheet materials (e.g.

steel less than imm thick) it enables very severe problems usually associated with the use of a laser perforation technique to be overcome. These problems will become evident in the description below.

SUMMARY OF THE INVENTION An object of the present invention is therefore to provide a method which enables such screens to be made from materials which combine adequate strength and hardness with corrosion resistance, such as stainless steel or titanium. Stainless steel cannot be electrodeposited, and electrodeposition of titanium is expensive, so a method has been sought which will enable the perforations to be produced in a slheet of the material by some form of cutting, drilling or j 6h" r -3/1punching. By the use of the techniques of the present invention, r---c Yo

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S laser drilling or laser cutting may be used for this purpose.

A major problem to be overcome in the successful use of laser drilling and cutting for the production of perforations having the geometry described above and traditional for fugal screens is the achievement of adequate perforation quality, often while working with an extremely thin workpiece. In the case of sugar screens, the screen must be thin, both in order that it will fit into the fugal, and in order that the i depth of the slots will not be great enough to produce Sunacceptable drag as the molasses flows through. It is therefore desirable that the thickness of the material used for manufacturing the screen sections for sugar screens be in the region of 200-350 microns.

SFor other applications, the screen may be typically to 2mm thick and approximately 1 square metre in Sarea.

Such a thin workpiece must be supported during the laser drilling, and, as will be evident from the description below, it is often necessary to fix the workpiece to the support body to substantially prevent.

the workpiece from buckling or warping. In the practice of the present invention, the screen sections are fixed onto a support body for which the supporting surface has a low reflectivity to the laser beam. In this way the screen section is continuously supported without the sharp-edged and smooth character of the perforatibn on the beam-exit side of the screen section being marred by reflected radiation.

Thus, in one form, the present invention provides a method of perforating a sheet material comprising: removably fixing said sheet material onto a support surface of a support body in order to substantially prevent buckling of the sheet material induced by the action of perforation; exposing said sheet material to a laser beam to form C) -Li perforations in said sheet material; and 'L A removing the resultant perforated sheet material

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Fr T. ~I Li -4/1from said support body.

Preferably said support surface of the support body has a reflectivity of less than about 20% to said laser beam, more preferably less than about In another form of the present invention, the reflectivity of the supporting surface to the laser beam is sufficiently low to prevent the perforation geometry

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i -T being significantly affected by reflected radiation.

The material of the support body preferably has a low absorption coefficient for the laser beam employed to minimize the rise in temperature of the material. The material of the support body may be substantially transparent to the laser beam, with the 0.1mm thickness of the support body adjacent to the supporting surface absorbing less than about 10% of the laser beam. If a highly absorptive material is used problems may thus arise from the consequent heating and perhaps melting of the lips of the perforation by conduction from the support body, or the loss of support due to local vaporization or melting of the support body. A highly absorbing material can of course be used where these disadvantages are tolerated or overcome.

The support body should present a firm supporting surface, and should preferably be compatible with adhesives which may be used to adhere the screen section to it, as described below.

In a preferred form of the invention, the support body is also substantially transparent to the visible spectrum to facilitate microscopic examination of the perforations during production.

A suitable material has been found to be an acrylic resin, such as polymethylmethacrylate, which is substantially transparent to many types of laser beams and which has a low reflectivity of approximately 4 to 6% per surface to such laser beams and which if raised to a high temperature by conduction from the workpiece or by heated glue, will depolymerize and ablate cleanly rather than melt against and adhere to the workpiece. However, this invention is not resticted to the use of acrylic as the support body. Other suitable materials may include polycarbonates, glasses and quartz.

The means for supporting the sheet on the o support body should preferably maintain the sheet in intimate cntact with either the support body or with -sli- I -5/1an adhesive or other filler substance. The filler substance may be provided at i 'i :1 I -II the interface between the workpiece and the support body to eliminate voids and provide the intimate contact. The filler material should have a low reflectivity to the laser beam. Voids have been found to have a deleterious effect on the precision with which the perforations may be formed. The number of voids which is permissible will be dependent on the desired open area of the perforated sheet. It has been fond that the intimate contact results in the molten and gaseous material produced by the incidence of the laser beam on the sheet being ejected in a direction away from the support body and that very little material resolidifies in the perforations or on the smooth face of the perforated sheet.

A fundamental problem which must be overcome if satisfactory perforated sheets of thin material are to be manufactured by the method described herein, arises from the tendency of the workpiece sheet progressively to buckle as the making of sucessive perforations proceeds. The tendency of the sheet to buckle can be quite severe in many cases, particularly if the sheet is large and/or the areal density of perforation is high. If not fixed to a support body over much of the area to be perforated, thin workpieces may form ripples or pucker. If fixed to a support body, severe stresses can then be transferred to the support body -during-the-process of perforating, tending-to-make the support body distort from its preferred flat form.

Breakage'of the support body may even by induced. The mechanism responsible for the tendency of the sheet to buckle is thought to be thermal expansion in the immediate vicinity of the slots. In any case it is found that, unless suitably prevented, the workpiece progressively distorts to an extent which exceeds the depth of focus which may practically be realized using a laser beam and which may also exceed the ability to control adequate depth of focus via automatically controlled tracking of the lens. (Such tracking by o C, means of a sensor which provides information as to the -6/1- S position of the surface at any instant may be used in the practice of the present invention). It will be appreciated that the problem is exacerbated as the number of perforations for a given size of perforated >i.Vd 5Q44 kh r WO90/01393 PCr/AU89/00336 -7sheet is increased.

This problem for thin workpiece sheets is solved by another aspect of the invention in its preferred forms, whereby the workpiece is adhered to the support body by means of an adhesive which can be applied over the support body for the entire area that is to be perforated.

The use of such an adhesive has the further advantage of taking up small irregularities in the surface of the support body, thereby increasing the flatness of the workpiece during production of the perforations.

Preferably, the adhesive should provide sufficient bond strength to prevent the sheet separating from the support body during the perforation process and have sufficient shear strength to prevent buckling. The adhesive should preferably either ablate cleanly leaving little or no residue in the perforations or be nonabsorbing of the laser beam such that the adhesive is unaffected by the laser beam.

In the case of a stainless steel workpiece and an acryrlic support body, a suitable adhesive has been found to be a high strength epoxy resin. For large workpiece sheets, two large fixed rollers with a suitably adjusted gap between them may be used to squeeze the sheet and the support body together.

The method of removal of the workpiece from the support body will of course depend on the materials used.

In the case of stainless steel and acrylic with epoxy adhesive, it has been found that the perforated sheet and the support body may be separated by mechanical means simply by peeling or prising apart. Alternatively, the workpiece aid the support body may be separated by heating or cooling to affect differential expansion or contraction.

For example, it has been found that the use of liquid nitrogen is often advantageous in this respect. The epoxy may subsequently be removed from the stainless steel by immersing it in a tank of a mixture .f toluene and ethanol, sold under the registered trade mark "Eposolve- The epoxy may be removed from the acrylic by L F -8light sandpapering, and the acrylic may then be reused. The ability to re-use the support body, and the fact that only a thin layer of adhesive is required, have important implications for the economy of the techique. It has been found that using these techniques, fugal screen sections can be manufactured which require no post-machining. Other methods sometimes suitable for supporting thicker workpieces on the support body may include clamping, suction and magnetic attachment.

BRIEF DESCRIPTION OF THE DRAWING i Preferred embodiments of the present invention shall now be further described with reference to the i 15 accompanying drawings, in which: SFigure 1 is a schematic cross-sectional representation of the method of the present invention; and Figure 2 is an end view of a preferred method of removing the perforated sheet from the support body.

As shown in Figure 1, a sheet material (11) to be perforated is attached to a laser-transparent support body (12) by a layer of adhesive The support body is in turn supported by an added support body (14) and the assembly is mounted on a work table (not shown) for controlled translation in the plane of1: the .sheet-material The support body may be attached to the added support body by means of bolts which pass through the added support body into threaded, blind holes in the support body.

The added support body need not be non -reflective of the laser beam and may be made of a metal such as aluminium as the laser beam (16) has its focal point near the sheet material and the beam diverges past the focal point. Providing a reasonable thickness of support.body, for example 10mm, is interposed between the sheet material and the added support body, any radiation reflected from the added u J/c4 support body will be of quite low intensity. The 0 cC C za /i Ia ii- Si id

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w I i ,Y -8/1added support body may not be required for applications where the tendency to buckle the sheet and the support body is low, or a thicker support body may be used to eliminate the need for the added support body. Alternative means of attaching the support body to the

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_I I~ i i WO 90/01393 PCT/AU89/00336 -9added support body may be used.

A laser beam (16) is directed onto the sheet material by beam-focussing optics and the sheet moves with respect to the beam in order to perforate the sheet. The material removed from the sheet by the laser beam is blown clear of the path of the laser beam by a gas jet (18) and the optics are protected by a sacrificial plate (19) which is substantially transparent to the laser beam and which is periodically replaced.

The support body and adhesive are preferably substantially transparent to visible wavelengths and the perforated sheet may be inspected by microscope with a light source (21) being located below the support body.

As illustrated in Figure 2, the perforated sheet (11) may be simply removed from the support body (12) by inserting a knife edge (22) and peeling apart.

Screens having circular or rectangular perforations have been produced using the method of this invention, and other shapes may be used by suitable adjustment of the beam optics.

As has been discussed above, in the case of sugar fugal screens it is necessary that the slots taper outwardly from the working side of the screen to the molasses exit side, and it is therefore necessary to produce this taper with the laser beam. This can conveniently be achieved if the laser beam is directed at the molasses exit side of the screen material, and the laser beam parameters are adjusted appropriately using existing techniques.

While the invention has been described herein in detail in relation to a specific embodiment, it will be understood that this has been for the purposes of facilitating the description of the invention, and is not to be taken as implying that the principles of the invention are narrow in their application. Many variations in the techniques described above are possible within the scope of the invention as defined by the claims. For example, while the preferred material for the WO 90/01393 PCI'/AU89/00336 screen is stainless steel of suitable composition, there may be applications where titanium or other metals, or non-metallic materials, may be preferred.

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Claims (7)

1. A method of perforating a sheet material comprising: removably fixing said sheet material onto a supporting surface of a support body in order to substantially prevent buckling of the sheet material induced by the action of perforation; exposing said sheet material to a laser beam to form perforations in said sheet material; removing the resultant perforated sheet material from said support body; and wherein the reflectivity of said supporting surface of the support body to the laser beam is sufficiently low to prevent perforation geometry being significantly affected by reflected radiation.

2. A method according to claim 1 wherein said supporting surface of the support body has a reflectivity of less than about 20% to said laser beam.

3. A method according to claim 1 wherein said supporting surface of the support body has a reflectivity of less than about 10% to the laser beam.

4. A method according to claim 1 wherein the absorption coefficient of the material of the support body for the laser beam employed is sufficiently low that the temperature of the support body adjacent the sheet material does not rise to a temperature sufficient to affect the edges of the perforations or sufficient to cause vapourization of the material of the support body. A method according to claim 1 wherein a 0.1mm thickness of the support body adjacent to said supporting surface absorbs less than about 10% of the laser beam which is transmitted through the supporting surface.

6. A method according to claim 1 wherein said o suppo polyc said

14. body

16. perfo body t materi contre materj ell 3IVQ ~11 J -12- support body is substantially transparent to visible wavelength radiation to facilitate visual examination of the perforated sheet. 7. A method according to claim 1 wherein said support body is formed from an acrylic resin or polycarbonate or glass or quartz. 8. A method according to claim 7 wherein said support body is formed from polymethylmethacrylate. 9. A method according to claim 7 wherein said support body is formed from polycarbonate. A method according to claim 1 wherein the sheet material and the support body are separated by a layer of material which is capable of filling voids between the sheet material and the support body. 11. A method according to claim 1 wherein said sheet material is attached to said support body by a layer of adhesive material. 12. A method according to claim 11 wherein said adhesive material is an epoxy resin. 13. A method according to claim 1 wherein said support body is formed from an acrylic resin or a polycarbonate, and said sheet material is attached to said supporting body by a layer of adhesive material. 14. A method according to claim 13 wherein said adhesive material is an epoxy resin. A method according to claim 11 wherein the perforated sheet material is removed from the support body by mechanical separation. 16. A method according to claim 11 wherein the perforated sheet material is removed from the support body by heating or cooling the support body and sheet material to affect differential expansion or contraction of the support body and the sheet material. I 11! I 434 V&\ 0 ~L'1 NV LB