LIST OF POTENTIAL ERRORS DURING PAINTING ON GLASS, DRYING AND TOUGHENING

1. Process of panes washing prior to painting
Washing of panes with nitro thinners causes the lack of paint adherence to the substrate during toughening. On the other hand, if the dirty or oily panes are not washed, it can result in incorrect covering of glass with paint, e.g. stains, etc.

2. Preparation of paint
The paint should be stirred with a mixer, paying attention to the fact that pigments which have different specific gravities may often fall onto the bucket bottom or rise to the surface. The paint often contains several pigments. A special attention should be paid to mixing of paint right on the bottom and on the joints of bucket’s wall with the bottom. Incorrect mixing will cause:
a) change of colour,
b) that the paint is not melted into the glass after toughening (in case when the ceramic frit, being the basic raw material, is heavier than pigments and that the pane will be painted almost with pigment itself),
c) very high transparency (in the case when the ceramic frit is lighter than pigments and the pane is painted with too much amount of this raw material),
d) improper selection of viscosity.
On the average for the body colours the viscosity of about 120 seconds in the Fords cup no. 6 is applied, and 70-90 seconds for satin paints (if a roller for satins is applied). Lowered viscosity may influence on colour change and will increase the transparency.
Increasing of viscosity will cause the strengthened covering with the paint, but it can also influence on the aesthetics of application (bubbles, craters, visible lines from the roller or so-called starry sky may appear).
After mixing, take as much paint as it is needed, so as not to store the thinned paint
(the delamination of paint is quicker when the paint is thin).
Remember that the measurement of viscosity should be conducted in the same temperature of paint (about 20-22°C). The viscosity will increase with the decrease of temperature (in winter, when
the paint is cool and its temperature is, for example, about 10°C, the viscosity can increase by over 100%). An opposite process will take place when the paint is warming up.
Application of improper medium – in this case the creaming of paint can take place and the transparent punctures, craters and bubbles will be visible on the painted surface. In order to store the paint for a long time, the bucket should be tightly closed after painting and the paint should be mixed from time to time (e.g. once a month). The paint can be thinned with the same medium which was used for the production of paint or with a thinner (e.g. DPM, TPM). However, too strong thinning with a thinner will cause the weakening of paint adherence after the drying process.

3. Painting on the roller coating machine
a) incorrect setting of roller prior to work
Prior to work (especially when the roller was changed), the roller should be levelled in relation to the belt which moves the glass. It is usually done with the method of “sheets of paper”. Incorrect setting of the roller will cause that the pane may be improperly painted (depending on which side is more inclined). In this case the lacks of paint on the pane may occur, or contrarily – too thick layer of paint on a part of the pane – it can cause even cracking of the paint.
b) pouring the paint directly onto the rollers
The paint, which during the painting works in the cycle with passing through the filter, is more stable, because the temperature does not increase significantly and the filter catches all impurities, including the dried paint.
If the paint is applied directly onto the roller, it cannot be cleaned, and the temperature cannot be controlled. Too big loss of viscosity may cause deviation from the standard assumed at the start.
We propose also to pass the paint through the screen (e.g. such screen as for coffee – paper screen for the pourover coffeemaker or screen for silk-screen printing, etc.) after its mixing and prior to placing it in the machine.
c) speed of the belt conveyor
On the average, the speed of the belt conveyor is 2-5 m/min. A significant acceleration of the belt conveyor may cause the deterioration of the quality of the glass being painted. The acceleration of the rubber roller with the belt (they should have the same speed) is recommended if we want to increase the thickness of paint application, for example we have 130 g/m2 of paint in settings, and we want to have e.g. 150g/m2 – then we can delay the metal roller, or (if the metal roller cannot be further delayed), we should accelerate the rubber roller along with the floor belt (we will, for example, increase the speed from 3 m/min. to 3.5 m/min for the rubber roller and the belt). The belt conveyor and the rubber roller should have the same speed.
When the roller is after regeneration, the roller’s radius if often smaller than it was before. In such the case, it is good to be equipped with an instrument for speed measurement, in order to measure the speed of the roller and conveyor belt, and set the same speed manually. Too big difference of speed between the roller and the belt conveyor may cause the application of thicker layer of paint on the end of the glass and even flooding of the belt or insufficient painting.
The thickness of paint application can be regulated not only by the speed of the rollers, but also by the adjustment of the gap between the first and the second roller.
When the roller is after regeneration, the roller’s radius if often smaller than it was before. In such the case, it is good to be equipped with an instrument for speed measurement, in order to measure the speed of the roller and conveyor belt, and set the same speed manually. Too big difference of speed between the roller and the belt conveyor may cause the application of thicker layer of paint on the end of the glass and even flooding of the belt or insufficient painting.
The thickness of paint application can be regulated not only by the speed of the rollers, but also by the adjustment of the gap between the first and the second roller.
d) worn out rubber roller
It can cause:
- non-uniform application of the paint on the glass,
- wearing of rubber grooves will cause improper application of the paint. In such situation, most frequently the paint viscosity is lowered to obtain aesthetic performance, which causes increased transparency and results in loss of the correct colour.
We should remember that if we want to obtain the repeatability of colour in the same project, then we should apply the same settings for the roller coating, the paint thinned to the same degree and the same glass.
e) painting with the same paint on the glasses with different thicknesses and from various manufacturers
If the customer does not specify the thickness of the glass, then the paint manufacturer matches the paint colour for the glass thickness of 4 mm. Due to the fact that for the flat glass the colour is watched through the glass in most cases, it should be assumed that the glass thickness will influence on the shade of the paint (assuming that the float glass has more or less green shade, depending on the manufacturer).
In order to not to make an error while placing an order, it is good to specify the thickness and manufacturer of the glass, or alternatively send the samples of the glass so as the manufacturer of the paint carry out the tests at its factory. The paint may vary even for the same thickness of the glass, if the glass from various factories is applied. If the colourless glass is to be painted (e.g. Opti-white), then such information must be absolutely provided to the manufacturer of the paint.
f) painting on improper side of the glass (tin)
In the case of the float glass painting on different sides will cause obtaining different colour effects for the same glass. Most frequently it is visible in the case of light colours (white, beige, light grey, etc.). The differences can be so visible, that the party which orders the glass may be convinced that it received the panes painted with different paints. It is a very common error and conflict with the manufacturer of paints, who assumes, when selling the paint, that the manufacturer of the toughened glass, as a professional, will apply the paint on the proper side.

4. Glass drying
The paints may be produced on various media. At present PRINT-GLASS produces the paints on six media: ZK-18, ZK-19, MS-33, MS-337, MS-40 and MS-25. The most popular medium is MS-40. It is a slow-drying medium. It is used when the Customer owns a drying tunnel. The paint based on this medium withstands well drying in high temperatures, even over 200°C. It is suitable for drying in short tunnels (e.g. 4 metres), when the temperature is high from the very beginning (e.g. 200°C). The MS-33 medium is a quick-drying agent. It is used when the Customer buying the paint has not a drying room or the drying room works in low temperatures (e.g. 60-80°C). It is not recommended to use the paint based on MS-33 medium in the case of short tunnels, where the temperature at the entry is very high (e.g. 200°C). In this case a thermal shock and breaking of the paint may occur. The correctly dried paint is such a paint which will not come off from the surface of the glass even after strong rubbing with hand. It often happens that the Customers who dried the paint only on the surface (it is dried outside and wet inside) assert that the paint has no adherence to the glass. In such the case the drying temperature and/or drying time should be increased.
The MS-337 medium is an agent with a significant amount of dry matter which makes it very resistant to friction and scratching after the drying tunnel (it is suitable even for transportation of painted and dried glass over long distances, e.g. to the hardening furnace). A disadvantage is that the white colour from the paint based on MS337 medium, measured on the spectrophotometer, is minimally darker than in the case when MS-40 or MS-33 is applied.
The MS-33 and MS25 media are the most suitable for painting with airbrush.
If the paint is not dried completely, it can cause settling of tarnish on the glass in the hardening furnace, even from the side of the glass (this effect can be reduced by washing the glass again after toughening).
The glasses painted with water based paints and solvent based paints should not be mixed in the hardening furnace.

5. Glass toughening
Attention should be paid to the fact that when the paint is applied for one project, it is best to work on one furnace. It often happens that a different shade is obtained when the toughened glass with the same paint applied is produced on several furnaces. In the case of some paints (e.g. dark – black) it is an insignificant difference, whereas for some paints this difference is significant enough that it can influence on the aesthetic effect of the whole (beige). Generally, it should be said that if a given batch is produced on one hardening furnace, then exactly the same parameters should be also applied. Change of temperature/time of firing causes a change of colour and glossing or tarnishing of the paint surface. When the temperature or time is reduced, the “starry sky” effect or improper adherence of the paint to the glass may occur (in extreme conditions the paint can be scratched off with a sharp object, e.g. key or knife, after toughening).
REASONS FOR PAINT PEELING OFF FROM THE GLASS AFTER TOUGHENING
- incorrectly selected temperature or time in the hardening furnace,
- paint is not adapted to toughening (e.g. for glass fusing or bending),
- paint was mixed incorrectly during preparation for painting,
- washing of panes with nitro thinner during the process of preparation for painting,
- thinning the paint with a thinning medium (nitro).
If the paint is not to meet other requirements (e.g. anti-slip paint, satin or matt effect, etc.), then it should be glossy after toughening.
If the paint is not glossy, it means that some mistakes were made during the process of painting or toughening. In such the case we ask for analyzing the correctness of the process, or preferably for contacting us. When we sell the paint, we always leave a sample of the paint and a sample of the paint on the glass. It can help to find and eliminate an error.