AUTOMOTIVE

GLASS

SCREEN

PRINTING

PROCESS

REVIEW

A U T O M O T I V E G L A S S

Safety Glass

Experts:

The screen printing of automotive glazing might seem

as a process that is easy and simple to carry out

and does not require a lot of attention. However we

have come to learn that manufacturers often require

guidance in perfecting their process as much as in

any process.

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terms of waste produced. Washing machine defects in printing usually cause problems such as peeling, pinholes and transparency. Furthermore, poor qua-lity washing can also cause op-tic transmission defects, as the remnant contamina-tions are burnt on the glass surface. Washing machine performance, water qua-lity, brush type and speed, maintenance and process set-up parameters are ex-tremely important to recog-nize. Without good glass washing, it is impossible to reach perfect printing quality. The key parameters in washing are water qua-

lity characteristics such as conductivity, temperature, to name a few, but also line speed, brush speed, brush bristle distance and many more. During our site work we have learned that too often the washing process is not given the necessary attention, in part because of insufficient know-how of the key parameters.

PRINTING MACHINESThe global automotive glass industry uses many different kinds of machi-nery. Screen printing ma-chinery can be quite old and it can be a challenge to produce high quality

products when glass sizes are constantly growing. Screen printing machines are usually semi-automatic or automatic machines. Both types of machines have their advantages, but let’s look first at the semi- automatic printing process.

SEMI-AUTOMATIC PROCESSESSemi-automatic processes always need an operator to do the job. The printing table has a single size with rollers and a glass collar. Glass centring is carried out manually by the opera-tor. Set up time is usually fast and the operator will

I f we look at the recent history of screen printing we will find out that the

application was trending up in 1990 and today can be considered a standard pro-cess in windshield manu- facturing. OEM quality standards have raised the level of screen printing qua-lity requirements conside-rably for automotive glass, defining various details of performance. Successful quality production requires suitable equipment, quality tooling, know-how and en-vironment, to name a few. This article will review the principles of the screen printing process for auto-motive windshields.

GLASS CLEANLINESSTo begin the process it is essential to make sure that the printed glass is clean before printing. If printed glasses are dirty or dusty, production will face lot of problems during bending. In fact, defects that are caused by screen printing can usually be detected too late in the process chain. These defects can therefore have a remarkable impact on yield if they are reco-gnized only after printing. Washing quality is gene-rally underestimated and the importance of glass washers is not really recog- nized. It is easy to save in costs when buying low cost machines or by upgrading old machines for the production line but in the long run it may re-sult in additional costs in

terms of waste produced lity characteristics such as products when glass sizes

44 Glass-Technology International 4/2013

are good, but the picture cell size in the printer can limit image sharpness, but there is no film needed, just print and that’s it. Cri-tical screen parameters are simply: screen stretch, art-work, emulsion thickness and mesh size. There are also a few different ways to increase screen life using tape, clear film, foam and even cardboard.

PRINTING ROOM CONDITIONSPrinting room condition is a critical point in screen printing processes. Rooms should be kept clean at all times and only contain re-

way is to print film for the printed area after the film goes onto the screen sur-face. Then the screen goes into the lighting room and is exposed to light for a few minutes. The exposu-re light must be between 5000W up to 8000W and the lighting time needs to be controlled. Digital lighting machines work like printers and there is no need for film. Digital machines expose printed areas straight to the screen and after that the emulsion can be washed away from areas that were not expo-sed. Both of those methods have advantages. Printers

vation in the field of glass screen printing. However this equipment is more sui-table for the architectural-, white goods- and interior glazing sectors, as it does not need cycle times when printing simple automotive figures. At SGE, we are, of course, hoping that this technology and others will continue to improve and develop further.

SILK SCREENSSilk screens consist of se-veral important parame-ters. First of all, the frame profile needs to be suitable for printing. It needs to be hard enough to withstand force. Screens must under-go stretching during prin-ting and standing in the printing machine without bending. If the frame is too flexible it causes problems for stretching and prin-ting. Printing artwork is becoming more and more challenging in large prin-ted areas such as panora-ma roofs. The small raster sizes in sun visor areas in windshields are also more difficult to make. Emulsion coating either by automa-tic device or manually is a critical process step. Emul-sion thickness will increase wet ink thickness and can cause print defects on the printed area edges. Detai-led logos and small dots can also be difficult to print if emulsion thickness is not controlled. Screen exposure can be carried out in two diffe-rent ways. One very proven

inspect all glasses on a white light board – here a modern LED-table is ir-replaceable as it provides better inspection results. Using a semi-automatic machine, the printing, ma-chine control, and quali-ty control are continuous work. Automatic printing machines have centring systems, the printing table consists of small pieces, and glass inspection is not required for 100 per cent of the production pieces all the time. Generally, a uniform table is difficult to adjust and it causes prin-ting challenges in transpa-rency and the variations of the wet thickness of the printed enamel. Printing machine set up for both types of machi-nes is the same – Off con-tact, Peel Off and Printing speed adjustments follow the same principles. To highlight the main diffe-rences in these two exam-ple pieces of equipment, the semi-automatic printer needs 100 per cent inspec-tion and the operator needs to touch the glass and this way can potentially cause defects, whereas in the case of automatic machines there is no need to touch the glass and no requi-rement for 100 per cent inspection. This certainly makes a difference, but without correct opera-tion even the automatic line will not give good end results. At the time of writing this article, laser printers are the latest inno-

A U T O M O T I V E G L A S S

d b h ii i fil f h

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levant equipment. Daily floor cleaning and weekly ‘spring-cleaning’ are a good way of maintaining the ideal level of cleanliness in the printing room. Some- times manufactures use sticky mats at the printing room entrance and also clean room clothing for those who might be opera-ting in the print room, but have conveyors that are du-sty and dirty or machines covered by ink. For many reasons, the print room temperature and humidity must also be controlled as they can cause problems with enamels. Temperatu-res that are too cold can increase enamel viscosity and too dry air can incre-ase static electricity charge in the screen and machine.

PRINTING QUALITYPrinting quality inspec-tion for automotive glass is usually carried out by visual inspection. Opera-tors inspect glass on-line or manually using light boards for the glass. This is an acceptable way of doing it and very often inspec-tion is like this. Sun visors, rasters, unprinted area ink spots and trademark inspection can be done against the light but when inspections need to include printed surface inspection or transparency and pinho-les inspection it is not easy to do against the light.Printed glass transparency is usually caused due to the thickness of the wet ink being too low. During

glass bending, the furnace burns all the media chemi-cals from ink, and then the ink enamels onto the glass surface during burning. For this reason, the printed sur-face thins down during the bending process.Printed surface inspection can be done only when light hits the printed surfa-ce straight on. Inspections are usually carried out ma-nually after drying with a hot light source. Pinholes in printed areas are typi-cally small in diameter af-ter printing, and increase in size during the bending process. And this is why inspections should be car-ried out using a flashlight. Inspection conditions need to apply various conditions of lighting and background colouring to enable easier identification and elimi-nation of defects after the printing process.

CONCLUSIONSScreen printing in the au-tomotive glass sector has become a standard pro-cess all over the world. Screen printing can be a great challenge for wind-shield manufactures as the importance of this pro-cess and its pre-proces-sing conditions, such as washing, are not often re-cognized, or even under-stood. Automotive glazing dimensions are increasing and, therefore, the screen printed areas along with it.Production metrics such as flow rate, yield and waste need to be at a sustainable

Safety Glass Experts

International Oy. Ltd.

45

Postitorvenkatu 16

33840 Tampere - Finland

Tel: +358 – 400 – 979300

Fax: +358 – 2 – 65182539

E-mail: info@sge.fi

www.sge.fi

level for all process steps, so that unnecessary work can be minimized in all process steps.Screen printing includes many fundamental para-meters to be controlled, and only after mastering the process it is possible to expect results.Safety Glass Experts pro-vides a full range of theo-ry and hands on training packages for pre-proces-sing of safety glass: cutting, grinding, drilling, washing, printing and powdering.