GB2324054A

GB2324054A - Method of coating glass

Info

Publication number: GB2324054A
Application number: GB9703033A
Authority: GB
Inventors: Frederick William Mcdowell; John Thomas Hill
Original assignee: Pilkington United Kingdom Ltd; Pilkington PLC
Current assignee: Pilkington Group Ltd; Pilkington United Kingdom Ltd
Priority date: 1997-02-14
Filing date: 1997-02-14
Publication date: 1998-10-14
Also published as: GB9703033D0

Abstract

A method of applying paint to a surface of a sheet of flat glass (10) is disclosed in which the glass is presented to a paint curtain (12) by conveying means (14) and an ioniser bar (24) is provided to provide an electrostatic charge on the upper surface of the glass sheet. Electrostatic charges are generated by separation of the glass (10) and conveying means (14) due to friction and a resulting electric field is produced between the underside of the glass and the paint. The method of the present invention reduces the electric field to the extent that the presence of paint on the underside of the glass is substantially avoided.

Description

Improvements in or relating to the Manufacture of Glass This invention relates to a method of applying paint to glass. More particularly it relates to a method of applying paint to one surface of a sheet of flat glass whilst simultaneously ensuring that paint does not contact the other surface of the sheet.

In the manufacture of glass mirrors flat glass is coated on one side with a reflective coating and a protective coating of paint is then applied to the same surface. Large glass sheets coated with the reflective coating are transported along a series of conveyors with the surface to which the coating has been applied being uppermost. The conveyor comprises a number of rubber belts which run over conveyor support boards. The belts carry the glass from a washing and cleaning process through to a paint curtain and paint is applied to the uppermost surface of the glass as it passes through the paint curtain.

On standard silver mirrors the above process works well. However with uncoated glass and glass coated with nonconductive reflective coatings there exists a problem. It has been found that after these sheets of glass have been passed through the paint curtain strands of paint are present on the lower surface of the glass which trail back from the leading edge of the glass sheet. These strands must be removed from the glass and at present this is done manually with the paint being scraped from the surface with a blade. This method of removing the paint is both labour intensive and increases the cost of production, and in removing the paint with a blade the glass may be scratched making it unsuitable for commercial use and thereby reducing the yield.

It is an object of the present invention to provide a method of applying paint to glass which overcomes the aforementioned problem.

According to an aspect of the present invention there is provided a method of applying paint to a surface of a sheet of flat glass comprising the steps of presenting a sheet of flat glass to a paint curtain by conveying means; reducing the electric field between the underside of the glass and the paint prior to passing the glass through the paint curtain such that the presence of paint on the underside of the glass is substantially avoided.

It is believed that the presence of paint on the lower surface of the glass is a result of the electrostatic charge which is generated when surfaces are separated due to friction between the surfaces. The charge created results in the lower surface of the glass attracting paint from the paint curtain. In the case of silvered glass electrostatic charge is also generated because of the glass-conveyor separation. However the highly conductive silvered layer tends to charge by induction to the opposite sign of the charge on the underside of the glass sheet and the electric field couples between the lower surface and the silvered surface and does not extend towards the paint. As there is no field there is no resultant force of attraction between the paint and the lower surface of the glass sheet.

An embodiment of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a side view of conventional apparatus for applying paint to a surface of a sheet of flat glass.

Figure 2 is a side view of the apparatus used for applying paint to a surface of a sheet of flat glass according to an embodiment of the present invention.

Figure 3 is a plan view of the apparatus of Figure 2.

Referring to Figure 1 a sheet of flat glass 10, which may be uncoated or coated with a non-conductive coating, is presented to a paint curtain 12 by conveying means 14. The conveying means comprises two parallel rubber belts 16 which run over conveyor support boards 18. The belts 16 are driven by roller 20. The paint curtain 12 is produced by a slit feed system (not shown) and runs continuously. The paint falls under gravity from the slit, which is at least as wide as the sheet to be painted, into a trough 22 and is then recirculated back to the feeder by a pump.

As the glass 10 separates from the rubber belt 16, surface electrostatic charges are generated and these are represented as positive and negative signs in the drawings. Charge of one polarity is produced on the glass while charge of the opposite sign is produced on the belt. The glass is travelling at this point at approximately 80 metres per minute and the charge on the glass has virtually no time to relax before it arrives at the paint curtain 12. The electric field region 24 within the space between the glass 10, the trough 22 and the paint curtain 12 produces a force of attraction between the lower surface of the glass sheet and the paint, causing a wrap around effect where strands of paint adhere to the lower surface of the glass.

In Figures 2 and 3 the apparatus is essentially the same as that of Figure 1 with the addition of an ioniser bar 24. The ioniser bar is located above the glass sheet and extends across the width of the sheet and is connected to an alternating supply 26. In use it provides an electrostatic charge on the upper surface of the glass sheet and an electric field is set up between the upper and lower surfaces of the sheet. This electric field is represented in Figure 2 by the vertical lines between the upper and lower surfaces of the sheet. As the electric field does not extend towards the paint there is no resultant force of attraction between the lower surface of the glass sheet and the paint.

Although the use of an ioniser bar is disclosed as the means of providing an electrostatic charge on the upper surface of the glass any other suitable means may be used.

Trials were conducted where a sheet of flat glass of 3.2 metres width and 4 millimetres thick was conveyed at a speed of 80 metres per minute to a paint curtain. The apparatus providing the paint curtain is standard equipriient available from Bovone Elettromeccanica s.r.l., Regione Pernigotti, Italy which provides a paint curtain of 3.3 metres in width and delivers paint at a rate of 75 cc per metre width per second. A Haug ioniser bar available from Weko (UK) Limited, Surrey,was located 30 mm above the glass sheet and extended across the full width of the sheet.

The trials were performed on uncoated glass and glass coated with a non-conductive coating with and without the ioniser bar in operation. With the bar switched off the wrap around effect was observed on both the uncoated and coated glass. With the bar switched on the effect was not observed on either the uncoated or the coated glass.

In the above embodiment only the application of paint to glass is described, however any other suitable coatings are also envisaged by the invention.

Claims

1. A method of applying paint to a surface of a sheet of flat glass comprising the steps of presenting a sheet of flat glass to a paint curtain by conveying means; reducing the electric field between the underside of the glass and the paint prior to passing the glass through the paint curtain such that the presence of paint on the underside of the glass is substantially avoided.

2. A method as claimed in claim 1 characterized in that the electric field is eliminated by electrostatic means.

3. A method as claimed in claim 2 wherein the electrostatic means comprises an ioniser bar.

4. A method as claimed in claim 2 or claim 3 characterized in that an electrostatic charge is provided on the upper surface of the glass sheet such that an electric field is set up between the upper and lower surfaces of the glass sheet.

5. A method as claimed in any preceding claim characterized in that the glass is coated with a non-conductive coating.

6. A method of applying paint to the surface of a sheet of glass substantially as hereinbefore described with reference to Figures 2 and 3.