GB2260091A - Spray painting using a mask and a shielding air flow - Google Patents

Abstract

A method of spray painting an article, such as a housing for electronic equipment, having areas which need to be masked-off comprises fitting a mask (6) such that a spacing (2) is left between the periphery of the mask and the edge of the area to be masked-off, and passing a flow of air (A) through the spacing counter to the direction of paint spraying (S) to prevent entry of the paint into the spacing. The mask may be in the form of a pin-type protrusion (6) for masking the inside wall (14) of an aperture, or in the form of a cover (see dotted lines, figure 3) placed over the masked-off area (e.g. an area of the electrical circuit boards 22 & 28, figure 3). The problem of providing a close-fit around the masked area is alleviated. <IMAGE>

Description

SPRAY PAINTING The present invention relates to a method of spray painting articles which have apertures therein in order to prevent the paint spray entering the aperture and becoming deposited on the walls of the aperture. It also relates to masking areas of an article, particularly where it is difficult to achieve a good fit between the mask and the edge of the masked area. The invention also provides a mask for use in the method.

In many industrial processes, there is a requirement to spray paint the inside or outside of an article whose walls have apertures therein. For example, in the electronics industry there may be a need to spray the inside of a housing having apertures to accommodate components, such as switches, displays, speakers, locks etc. Very often, the insides of such articles are sprayed with a conductive coating comprising a metal (such as silver, nickel, copper or palladium) in a lacquer in order to provide electromagnetic shielding. The requirements for such electromagnetic shielding are becoming increasingly severe in order to minimise interference between adjacent pieces of electronic equipment. The housings are often formed of injection mouldings of non-conductive plastics material, which is then sprayed on the inside with a conductive shielding coating.In order to provide acceptable aesthetic appearance, it is a requirement that the paint spray should not enter the walls of the apertures and so be visible from the outside of the finished article. In some cases, this problem may be approached by providing a mask on the inside of the aperture which leaves a small margin around the aperture free from paint spray. However, this solution is not always acceptable and it may be specified that the spray coating must extend right up to the edge of the aperture.

Where the sprayed coating must extend right up to the edge of the aperture, it is conventional to provide an insert which is fitted into the various apertures in the housing from the front side so as to mask the aperture walls. This necessitates a very close fit between the inserted mask and the housing. In turn, this requires that the mask inserts be made to a very fine tolerance in order to leave as little clearance as possible between the walls of the aperture and the mask insert. In practice, a polymer film may be squeezed between the mask insert and the housing so as to provide a good seal against spray. A further problem, is that there may be variations in the tolerances in the housing itself, leading to difficulties in effectively fitting and removing the mask inserts.All these problems mean that the use of mask inserts to mask the walls or apertures is expensive and time consuming, leading to production difficulties and high costs.

It is an object of the present invention to mitigate these problems.

Generally speaking, the present invention envisages use of a mask where a clearance is deliberately left between the mask and the aperture wall, spray deposition being prevented by passing a countercurrent flow of gas through the clearance spacing.

Specifically, the present invention provides a method of spray painting an article having area therein which needs to be masked, which comprises: - providing a mask adapted to mask the area; - fitting the mask so that a spacing is left between the periphery of the mask and the edge of the area which is to be masked; - passing a flow of gas through the spacing; and - paint spraying the article; the arrangement being such that the flow of gas through the spacing substantially prevents ingress of paint into the spacing.

In one embodiment of the invention where the area to be masked is the inside of an aperture in the article, the mask is in the form of a protrusion which enters the aperture and extends at least partially through the depth of the aperture.

Ideally, the gas flow through the aperture, which runs counter to the direction of the paint spray, should be laminar flow so as to minimise turbulance at the rear end of the aperture, which might lead to paint deposition in undesired places. Preferably, the gas flow through the spacing pushes the paint spray to the side of the aperture and substantially prevents entry of the paint spray into the aperture. Thus, the spray direction is usually substantially anti-parallel to the direction of air flow through the gap. In this way, the paint coating can be built up right to the edge of the aperture without substantially entering the aperture itself. The spacing is preferably of a width which provides substantially laminar flow. Preferably, the width "a" of the spacing is less than 30%, preferably less than 15% and more preferably less than 5% of the depth "b" of the aperture.For normally encountered aperture depths, the spacing is usually in the region 1 to 0.01 mm, especially 0.05 to 0.05mm.

The width of the spacing may vary around the periphery of the aperture in order to provide suitable gas flow conditions, and generally speaking this may be determined on an empirical experimental basis.

It may be advantageous to taper the side walls of the protrusion so as to provide an increasing spacing towards the rear of the aperture, so as to assist maintenance of laminar flow. For example, in the case of a pin-shaped protrusion, the end of the pin may be domed, conical, frustoconical, pointed, parabolic, or bullet shaped.

The protrusion may, depending on the gas flow conditions, extend part way or the full way through the depth of the aperture. The protrusion may also protrude through the aperture beyond the reverse side of the article. Preferably it protrudes at least lmm beyond the end of the gap.

The rate of flow of gas through the spacing will generally be determined to deflect the particles of spray paint away from the aperture. The actual gas velocity will depend on many factors including the temperature and density of the gas, the momentum of the paint spray particles (which in turn depends on the density of the paint spray and the particle size, and the spray velocity). Usually, the present invention will be applied to gas-borne paint sprays, though it may also be applicable to spatter-type non-gas spraying procedures.

In a particularly preferred embodiment of the invention, the process is carried out with the use of a plenum chamber into which the article is placed. Masks carrying the protrusions are then moved inwardly from the walls of the plenum chamber into engagement with the apertures in the article to be spray painted and so as to provide the necessary spacings around the apertures. For this purpose, appropriate location lugs may be used. The edges of the article are then sealed to the edges of the plenum chamber and the plenum chamber is pressurised with air or other gas so as to establish gas flows through the various spacings prior to spray painting the inside of the article. After spray painting has been finished, the paint is allowed to dry at least partially before the masks are removed and the article is withdrawn from the plenum chamber.This procedure may be carried out semi-automatically.

In a further embodiment of the invention where an area of the article is to be masked off during painting, the mask may be in the form of a cover placed over the area. A clearance spacing is left around the edge of the mask and this spacing is substantially sealed by an air flow as described above from within the mask cover.

This enables difficult areas, particularly non-planar or irregular shaped areas, to be successfully masked.

A benefit of the present invention is that the masks do not have to be machined to fine tolerances, and also variations in the tolerances of the article can be easily accommodated. The masks are also more easily fitted and removed.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings wherein: Figure 1 is a part sectional enlarged view of an aperture having a mask protrusion therein; Figure 2 is a plan of an item to be painted having a mask thereover; and Figure 3 is a side view thereof.

The arrangement shown in Figure 1 illustrates an aperture 2 formed in the wall 4 of an article to be sprayed. A protrusion 6 having a domed head 8 and mounted on an apertured mask backing plate 10 is inserted into the aperture from the front side (relative to the reverse side 12) of the article which is to be painted.

The thickness of the wall 4 is "b" and the width of the spacing between the protrusion 6 and the inside wall 14 of the aperture is "a". As shown, the ratio a:b is about 1:5.

The reverse side of the article is sprayed with spray S so as to deposit a coating of paint 16 up to the edges of the aperture.

Spray painting is carried out as follows. Firstly, the mask and protrusion are inserted so as to provide a clearance as shown in Figure 1. Then, air under pressure shown by arrows "A" is passed from the front side of the wall through the spacing 2 and exits from the rear end of the aperture. The flow of air is substantially laminar and this laminar flow is maintained beyond the rear face 12 due to the presence of the domed section 8. The article is then spray painted by the application of spray "S" so as to deposit paint coating 16. The flow of air through the spacing tends to push to one side the paint spray in the area of the aperture and substantially prevents impaction of paint spray on the inside wall 14 of the aperture. Before or after the paint has set, the mask and protrusion are removed. They may then be cleaned and reused.

Figures 2 and 3 show the use of the invention to seal against paint ingress a difficult masking arrangement. A first plate 2 carrying electrical contacts 4 is provided with an upstanding wall 6 to assist masking during spray painting. Similarly a second plate 8 has a wall 10 surrounding the contact area. As shown in Figure 3, the plates are on different levels and a gap 12 inevitably exists between the adjacent ends of the walls. A mask 20 as shown in dotted lines is electroformed so as to fit over the contact area and around the walls 6,10. However, under normal painting conditions there is a danger of spray penetration around the edges of the mask and particularly through the gap 12. To minimise this, an air flow is passed into the mask as shown by the arrow.The air exits through the gap left between the mask and the plates 2,8 and minimises paint spray penetration under the edges of the mask.

Example 1 Tests were designed to quantify air flow parameters. A standard aperture insert was manufactured which consisted of eight cylindrical pins of equal size and diameter set vertically at 45 degree arc spacings round a circle in a base holder block. The pins at the 180 and 360 degree positions were of extended length to act as locations for the test aperture panels. The aperture panels had holes drilled at eight positions concentric with the pin positioning on the standard aperture insert. The 180 and 360 degree position holes were sized to give interferance fits on the correspondingly positioned pins on the insert fixture. All the remaining six holes on each aperture panel were of a size to give an air gap clearance round the insert pins.Six sets of test panels were provided, each set having different hole diameters to give varying air gaps when the test panels were superimposed over the set pins. Peripheral gaskets of various thicknesses were used to space and seal the gap between the base inserts block and the test aperture panels.

This allowed the insert pins to be set at varying projections beyond the sprayed surface of the test panels. The thickness of the test panels was 2mm.

Dimensional details of the test panels were: Table Outside diameter Area Air gap Gap/depth (microns) (mm2) (microns) Insert pin 2975 6.948 Aperture Set A = 3250 8.292 137.5 7% B " B 3315 8.627 170.0 8.5% I' I C 3412 9.139 218.5 11% " " D 3575 10.033 300.0 15% " " E 3705 10.776 365.0 18% ri " F 3900 11.940 462.5 23% The base block of the standard aperture insert was provided with a central hole to allow an air flow to pass through to the test apertures. The underside of the base block was in turn bonded to a rubber sleeve attached to the top outlet of a 1 litre Buchner flask acting as a plenum chamber. A De-Vilbiss Co.Ltd. air spray gun was used for the tests, having aircap No.30, jet AV-IS-Ex-1.8, and needle JG4-402 D-Ex.

(A) A conductive silver paint was chosen for the first series of spray testing as it gave good colour contrast with the black anodised test panels to aid visual assessment of coating penetration into the test apertures. Details were as follows: Paint density = 1.16 + 0.02 kg/l " solids content = 45% w/v " " form = colloidal particles of Ag.

" vehicle = vinyl composition Spray gun pressure = 3 to 5 lb/in2 (0.21-0.35 kg/cm2) Spray Form = aerosol mist.

* as diluted 1:1 with M.I.B.K. for spraying.

Following a series of spray tests to investigate the various parameters it was found that best results were obtained by pins projecting a minimum of 1 mm. above the sprayed surface and at air gaps over 300 uM along with air flows of 7 to 10 litres per minute. The pressure in the plenum chamber was about 201b/in2 (1.40 kg/cm2).

i.e. 7 and 10 L per min / 10.033 x 6.948 sq.mm.(3.085) = 2.27 to 3.24 litres / sq.mm. / minute flow range.

Air turbulence again began to disturb the spray pattern at these flow rates and 462.5 uM gap widths.

(B) Conductive nickel paint (Electrodag 440AS) was selected for a second set of tests utilising the same insert fixture and test panel sets A to F. Paint details were as follows: Paint Density : 1.53 + 0.02 kg/l.** " Vehicle : acrylic composition Spray gun pressure : 15 to 25 lb/in2 (%05-1.76 kg/cm2 Spray Form : aerosol mist.

** as diluted 1:2.5 with methyl isobutyl ketone (M.I.B.K.) for spraying.

Best results were again obtained with pins projecting a min. of 1 mm above the sprayed surface and at air gaps over 300 uM along with air flows of 15 to 20 litre per minute i.e. 15 and 20 L per min / 3.085 sq.mm.

= 4.86 to 6.48 litre / sq.mm./ minute Some disturbance of the spray pattern was observed at the 462.5 uM gap width but only at the higher contra air flow rates. The plenum chamber air pressure was about 20 lb/in2 (1.40 kg/cm2).

The following conclusions were drawn from these tests.

The air masking method was viable for low range spray pressures up to 5 lb/in2. against contra air flows in the nominal range 2.25 to 3.00 litre per minute per sq.mm. and air gaps of say 300 to 400 microns. At normal spray pressures in the 15 to 26 lb/in2 range contra air flows of 15 to 20 litre per minute through air gaps of 300 to 400 microns were effective. Masking failures did not occur in all air gaps below 300 microns. Observations suggested that these anomalous results arose from failure to achieve reasonable centering of some insert pins in these tighter tolerance holes, this in turn disturbing the laminar air flow.

Claims (14)

1. A method of spray painting an article having an area thereof which needs to be masked, which comprises: - providing a mask adapted to mask the area; - fitting the mask so that a spacing is left between the periphery of the mask and the edge of the area which is to be masked; - passing a flow of gas through the spacing; and - spray painting the article.

2. A method according to claim 1 wherein the area to be masked is the inside wall of an aperture in the article, and the mask comprises a protrusion which enters the aperture and extends at least partially through the depth of the aperture.

3. A method according to claim 2 wherein the front end of the protrusion facing the paint spray tapers such as to provide an increasing spacing towards the rear of the aperture.

4. A method according to claim 3 wherein the protrusion is pin-shaped and the free front end of the pin is domed, conical, frustroconical, pointed, parabolic or bullet-shaped.

5. A method according to any of claims 2 to 4 wherein the free front end of the protrusion protrudes beyond the end of the spacing.

6. A method according to claim 6 wherein it protrudes at least lmm.

7. A method according to claim 1 wherein the mask is in the form of a cover placed over the area on the article to be masked-off, and the spacing is left between the edge of the mask and the article.

8. A method according to any preceding claim wherein the width of the spacing is less than 30% of the depth of the spacing.

9. A method according to claim 8 wherein the width is less than 15% of the depth.

10. A method according to any preceding claim wherein the gas flow through the spacing is substantially laminar non-turbulent flow.

11. A method according to any preceding claim wherein the paint is sprayed in a direction which is substantially anti-parallel to the direction of gas flow through the spacing.

12. A method according to any preceding claim wherein the gas flow is from a plenum chamber and the mask forms part of a wall of the chamber.

13. An apparatus for use in spray painting an article having an area thereof which needs to be masked, which comprises: - a mask adapted to mask the area such that a spacing is left between the periphery of the mask and the edge of the area which is to be masked; and - means for passing a flow of gas through the spacing.

14. A method or apparatus for spray painting an article having an area which needs to be masked, substantially as disclosed in conjunction with Figure 1 or Figures 2 and 3.