WO2002090052A1

WO2002090052A1 - Abrasive blast machining

Info

Publication number: WO2002090052A1
Application number: PCT/GB2002/001785
Authority: WO
Inventors: Mark Christopher Turpin
Original assignee: Morgan Crucible Co PLC
Current assignee: Morgan Advanced Materials PLC
Priority date: 2001-05-03
Filing date: 2002-04-18
Publication date: 2002-11-14
Anticipated expiration: 2003-11-03
Also published as: WO2002090053A1; TW548871B

Abstract

A method of machining an article (506) comprising the steps of:-a) forming a pattern of apertures in a mask (504),b) placing the mask on or adjacent (506) the article and,c) directing abrasive materials (503) against the mask to remove material from the article in which the abrasive materials are directed towards the apertures to form a substantially conical abrasive blast locus converging towards the aperture.

Description

ABRASIVE BLAST MACHINING

This invention relates to an improved method of abrasive blast machining and is particularly, although not exclusively, applicable to manufacture of flow field plates for fuel cells and electrolysers (particularly, although not exclusively, for proton exchange membrane fuel cells and electrolysers).

Fuel cells are devices in which a fuel and an oxidant combine in a controlled manner to produce electricity directly. By directly producing electricity without intermediate combustion and generation steps, the electrical efficiency of a fuel cell is higher than using the fuel in a traditional generator. This much is widely known. A fuel cell sounds simple and desirable but many man-years of work have been expended in recent years attempting to produce practical fuel cell systems. An electrolyser is effectively a fuel cell in reverse, in which electricity is used to split water into hydrogen and oxygen. Both fuel cells and electrolysers are likely to become important parts of the so-called "hydrogen economy". In the following, reference is made to fuel cells, but it should be remembered that the same principles apply to electrolysers.

One type of fuel cell in commercial production is the so-called proton exchange membrane (PEM) fuel cell [sometimes called polymer electrolyte or solid polymer fuel cells (PEFCs)]. Such cells use hydrogen as a fuel and comprise an electrically insulating (but ionically conducting) polymer membrane having porous electrodes disposed on both faces. The membrane is typically a fluorosulphonate polymer and the electrodes typically comprise a noble metal catalyst dispersed on a carbonaceous powder substrate. This assembly of electrodes and membrane is often referred to as the membrane electrode assembly (MEA).

Fuel (typically hydrogen) is supplied to one electrode (the anode) where it is oxidised to release electrons to the anode and hydrogen ions to the electrolyte. Oxidant (typically air or oxygen) is supplied to the other electrode (the cathode) where electrons from the cathode combine with the oxygen and the hydrogen ions to produce water. A sub-class of proton exchange membrane fuel cell is the direct methanol fuel cell in which methanol is supplied as the fuel. This invention is intended to cover such fuel cells and indeed any other fuel cell using a proton exchange membrane. In commercial PEM fuel cells many such membranes are stacked together separated by flow field plates (also referred to as bipolar plates). The flow field plates are typically formed of metal or graphite to permit good transfer of electrons between the anode of one membrane and the cathode of the adjacent membrane. The flow field plates have a pattern of grooves on their surface to supply fluid (fuel or oxidant) and to remove water produced as a reaction product of the fuel cell. Flow fields may also be provided to supply coolant fluids. Various methods of producing the grooves have been described, for example it has been proposed to form such grooves by machining, embossing or moulding (WO00/41260), and (as is particularly useful for the present invention) by sandblasting through a resist (WOOl/04982).

International patent application No. WOOl/04982 disclosed a method of machining flow field plates by means of applying a resist or mask to a plate and then using sandblasting (or other etching method using the momentum of moving particles to abrade the surface, e.g. waterjet machining), to form features corresponding to a pattern formed in the mask or resist. Such a process was shown by WOOl/04982 as capable of forming either holes through the flow field plates, or closed bottom pits or channels in the flow field plates. The process of WOOl/04982 is incorporated herein in its entirety, as giving sufficient background to enable the invention.

The applicants have found that, with a conventional abrasive gun, variations in the flow of abrasive media can result in an uneven formation of channels. Additionally, the shadow effect of the mask can result in different channel profiles being formed at the centre of an article and at its edges.

The applicants have realised that these problems can be reduced, to give a more even cutting of a substrate material, by directing the abrasive in a substantially conical blast locus.

Accordingly, the present invention provides a method of machining an article comprising the steps of:-

a) forming a pattern of apertures in a mask b) placing the mask on or adjacent the article and, c) directing abrasive materials against the mask to remove material from the article in which the abrasive materials are directed towards the apertures to form a substantially conical abrasive blast locus converging towards the aperture. Further details of the invention will become apparent from the claims and the following description with reference to the drawings in which: -

Fig. 1 shows schematically in part section a part of a fluid flow plate incorporating gas delivery channels and gas diffusion channels formed by an abrasive air blast technique (sandblasting). Fig. 2 shows schematically a partial plan view of a fluid flow plate incorporating gas delivery channels and gas diffusion channels.

Fig. 3 is a schematic drawing illustrating one method of undercutting a channel;

Fig. 4 is a schematic view illustrating the use of a multiple headed gun in accordance with the invention. The following description will refer to the manufacture of flow field plates by abrasive blasting (sandblasting) through a resist, but aspects of the invention are not limited to this method of manufacture.

To form both gas delivery and gas diffusion channels a technique such as abrasive blasting may be used in which a template or resist is placed against the surface of a plate, the template or resist having a pattern corresponding to the desired channel geometry. Such a technique is described in WOOl/04982, which is incorporated herein in its entirety as enabling the present invention.

With this technique the plates may be formed from a graphite/resin composite or other non- porous electrically conductive material that does not react significantly with the reactants used. The type of abrasive blasting much preferred is the use of an air blast. Waterjet machining is generally found to be too aggressive for easy control, but with care and good control equipment would be possible. It is found with this technique that the profiles of channels of different width vary due to the shadow cast by the mask. Fig. 1 shows a flow field plate 101 having a narrow channel 102 formed in its surface. Because of the shadowing effect of the resist used in forming the channel the channel is exposed to sandblast grit coming effectively only from directly above. This leads to a generally semicircular profile to the channel and to a shallow cutting of the channel.

For progressively larger channels (103 and 104) the resist casts less of a shadow allowing sandblasting grit from a progressively wider range of angles to strike the surface of the flow field plate, so allowing both deeper cutting of the surface and a progressively flatter bottom to the channel.

Accordingly, by applying a resist with different width channels to a plate and exposing the plate and resist to sandblasting with a fine grit, a pattern of channels of different widths and depths can be applied.

Applying such a pattern of channels of varying width and depth has advantages. In flow field plates the purpose behind the channels conventionally applied is to try to ensure a uniform supply of reactant material to the electrodes and to ensure prompt removal of reacted products. However the length of the passage material has to travel is high since a convoluted path is generally used.

Another system in which the aim is to supply reactant uniformly to a reactant surface and to remove reacted products is the lung. In the lung an arrangement of progressively finer channels is provided so that air has a short pathway to its reactant site in the lung, and carbon dioxide has a short pathway out again. By providing a network of progressively finer channels into the flow field plate, reactant gases have a short pathway to their reactant sites.

The finest channels could simply discharge into wide gas removal channels or, as in the lung, a corresponding network of progressively wider channels could be provided out of the flow field plate. In the latter case, the two networks of progressively finer channels and progressively wider channels could be connected end-to-end or arranged as interdigitated networks, with diffusion through the electrode material providing connectivity. Connection end-to-end provides the advantage that a high pressure will be maintained through the channels, assisting in the removal of blockages. The question of interconnected channels vs. blind channels depends on which side of the electrode we are dealing with. Hydrogen ions travel from the anode, through the polymer, and are made into water at the cathode. All of the water is made on the cathode side (air or oxygen side) of the cell. The water generation on the cathode side means that the air side gas channels cannot be blind ended, as this would cause flooding. Interdigitated will also be tricky unless a GDL is used as the permeability of the electrode is not high. Interdigitated channels also restrict the removal of impurities from the supply gas. Accordingly, the model wherein the branched channels join end to end or drain to a larger channel is preferred.

Fig. 2 shows in a schematic plan a portion of a flow field plate having broad primary gas delivery channels 104, which diverge into secondary gas delivery channels 103 which themselves diverge into gas diffusion channels 102. Gas diffusion channels 105 can also come off the primary gas deliver channels 104 if required. The primary and secondary gas delivery channels may each form a network of progressively finer channels as may the gas diffusion channels and the arrangement of the channels may resemble a fractal arrangement. The primary gas delivery channels may have a width of greater than 1mm, for example about 2mm. A typical depth of such a channel is 0.25mm but depth is limited only by the need to have sufficient strength in the flow field plate after forming the channel. The secondary gas delivery channels may have a width of less than 1mm, for example 0.5mm and, using the sandblasting technique may be shallower than the primary gas delivery channels. The gas diffusion channels have a width of less than 0.2mm, for example about lOOμm and may be shallower still.

The flow field plates may be used with a gas diffusion layer, or the gas diffusion channels may be provided in a sufficient density over the surface of the flow field plate to provide sufficient gas delivery that a gas diffusion layer may be omitted. When acting as a fuel cell, the gas delivery chaimels deliver gas to the gas diffusion channels which disperse the gas across the face of the flow field plate. When acting as an electrolyser the gas diffusion channels act to receive the gas from across the face of the flow field plate and the gas delivery channels deliver the gas for collection. For the sandblasting technique, the limit on channel width is a function of the mask thickness used in the sand blast process. Image Pro™ materials (Chromaline Corp. US), are very thick at 125 micron. These masks limit track width to about 100 microns. Other mask materials can be spray coated onto the substrate and exposed in situ. These materials are much more resilient and hence can be much thinner. Chromaline SBX™ can be used to etch features down to 10-20 microns wide.

Various mask types may be used:- a) adhesively mounted sheet masks b) masks that are applied by painting, spraying, screen printing or any other such method to cover the desired surface of the article and then treated to selectively remove areas c) masks that are applied and re-used d) masks that are directly printed or applied to the surface (e.g. by ink blast printing) the invention is not restricted to any particular form of mask, but types b) and d) lend themselves most readily to mass production. Of course, the abrasive material used in the abrasive blast must have a finer particle size than the feature to be formed. However, finer particle size leads to a lessening in the abrasion rate. The applicants have found it useful to use a relatively coarse abrasive material in the blast (e.g. 50μm to 250μm diameter silica or alumina grit) to form the wide channels, followed by use of a fine abrasive material (e.g. 5-20μm diameter silica or alumina grit) to form the finer features. The coarse and fine materials may be mixed and applied in one step. The invention is not limited to any particular abrasive material.

Preferred materials for the plate are graphite, carbon-carbon composites, or carbon-resin composites. However the invention is not restricted to these materials and may be used for any material of suitable physical characteristics, with suitable choice of abrasive. The use of angled blasts of abrasive materials is advantageous. The schematic drawing of Fig. 3 shows a resist 1 placed against a plate 2. The resist 1 has a thickness d_r and has an aperture 3 of width w_r. Abrasive materials projected through the aperture 3 has abraded the material of the plate to produce a void 4 of depth d_v and width w_v. Assuming that no particles bounced back from the lower surface 5 of the void 4 with sufficient strength to abrade the reentrant surface 6, it can be seen that the minimum angle α of the re-entrant surface is determined by the lowest angle of approach of particles to the aperture 3. Therefore, for this configuration, the maximum void width can be calculated as:- w_v = w_r + 2*d_v*(w_r/d_r).

As examples, Table 1 below gives calculated void widths based on an assumed resist thickness of 0.125mm and varying sizes of aperture and desired depths.

In practice, sandblasting does not produce a mathematical point sized cutting tool and such would be required to produce a void of the shape shown in Fig. 3. Also, the angle of undercut produced is dependent upon the angle of incidence of the abrasive particles produced by the sandblast, provided that this is not shallower than α. If the angle of incidence of the abrasive particles produced by the sandblast is shallower than α then the plate 2 will be in the shadow of the resist and little or no abrasion of the surface by the sandblast will occur.

Fig. 4 shows a gun 501 with two heads 502 for simplicity, but it should be understood that the invention may be used with one or more heads, and preferably three heads. Each gun is mounted on the head such that the angles of incidence, P_A and ββ, of the jet of blast material

503 may be varied. To effectively abrade the substrate 506, this angle is limited by the thickness, d_r, of the resist 504 and the width of the aperture in the mask, w_r as described above. Voids 505 may be formed in the substrate 506 using this multiple head in two ways. Firstly, the multiple headed gun may simply be traversed across the substrate so that the jets of blast material are directed so as to form an undercut void, 505. For a multiple headed gun comprising three heads, the third is preferably directed at 90° to the substrate to ensure that a void with a flat bottom is formed.

Secondly, the multiple headed gun may rotate about an axis 508 out of the plane of (and preferably perpendicular to) the plane of the substrate 506, creating a substantially conical blast locus. This is particularly advantageous for ensuring that voids or apertures through the substrate are undercut uniformly. Of course, a single angled gun rotated about an axis out of the plane of (and preferably perpendicular to) the plane of the substrate 506 would achieve a similar effect but multiple guns mean that the rotational speed of the head may be kept lower.

If the point of the conical blast meets at the surface of the substrate then the abrasive particles may interfere with each other. If however the point is below the surface of the substrate then the blast will describe a circle or ellipse on the surface reducing such interference.

By providing a moving locus for the abrasive materials, the sensitivity of the method to variations in abrasive flow is reduced, since a given point on the surface will be exposed to abrasive several times, and from several directions, so that variation in flow will be averaged over those times and directions.

Claims

1. A method of machining an article comprising the steps of:- a) forming a pattern of apertures in a mask b) placing the mask on or adjacent the article and, c) directing abrasive materials against the mask to remove material from the article in which the abrasive materials are directed towards the apertures to form a substantially conical abrasive blast locus converging towards the aperture.

2. A method as claimed in Claim 1 , in which the abrasive materials converge to a point at the surface of the article.

3. A method as claimed in Claim 1, in which the abrasive materials converge to a point below the surface of the article.

4. An abrasive blasting gun comprising one or more guns mounted on a rotary head and angled with respect to the head so that on rotation of the head a convergent conical abrasive blast is formed.