Spray Painting Glass

Easily Coloring Glassware By Spray Painting Glass!















History and principle

Spray painted perfume bottles
Spray painting was developed at the end of the 19th century in the United States. The first devices were devices nowadays known as air brushes. The use of a hand operated compressor tells something about the state of the technology at that time.
With an air brush small amounts of paint are atomized/nebulized onto relatively small surfaces which makes this technique a perfect tool to make artistic images, although the air brush was initially not invented to make art work.

The spray gun was derived from this air brush. Higher pressures were used for the atomization process and larger amounts of paint could be atomized in shorter time. This opened the way to coat larger surfaces like the bodies of ships and cars.

Atomization 

The definition of automization is 'the making of an aerosol, which is a colloid suspension of fine solid particles or liquid droplets in a gas'.

For spray painting this process of atomization can be described as follows:

When liquid is forced through a nozzle, the liquid pressure along with the geometry of the nozzle causes the liquid to emerge as small ligaments.These ligaments then break up further into very small liquid particles, droplets.
The liquid can be put under pressure and then forced through the nozzle. This is called airless spray painting.
But most of the time the liquid is atomized with the help of compressed air. The liquid is in a reservoir at atmospheric pressure. At the outlet of this reservoir air passes with high speed sucking the liquid into the air stream and then goes to the nozzle where the atomization takes place.

Different spray cones

Nozzle type

The geometry of the nozzle determines the form of the spray. 
There are nozzles which make a full cone spray, a hollow cone spray and a flat spray. 

The form of the spray influences the drop size. A full cone spray has the largest drop size, followed by the flat spray, followed by the hollow cone spray.
The diameter of the nozzle is of course also of influence to the drop size. 
The smaller the nozzle diameter, the smaller the drop size

The flow rate of the paint out of the reservoir also is of influence to the drop size. An increase in flow rate increases the drop size. The flow rate can be influenced buy the reservoir outlet diameter.
The pressure of the air has an inverse relationship to the drop size.
 An increase in pressure will reduce the drop size

Double sprayed glass bowl. First coat is Ormosol 'Metallic Silver' and second coat (partly) is Ormosol transparent color.

Other physical influences 

A higher viscosity increases the amount of energy required to atomize the spray. Hence, an increase in viscosity will increase the drop size. 
An increase of the surface tension of the paint will also increase the drop size. 

All these factors have to be considered while spraying paint, no matter if the spraying is done automatically or manually. 


Spray painting glassware in practise

The best way to understand what's happening during the spraying process, is to try to visualize what's happening:

While a droplet of paint is moving from the nozzle of the spray gun to the glass surface, some solvent of the paint droplet evaporates. 
When too much solvent evaporates, the droplets arrive at the glass in a stickier state. Droplets with less solvent content will not spread as good over the glass surface as droplets with more solvent content.  If too much solvent evaporates during the transport from spray gun to glass surface, a coating will not have the desired properties.
Spreading out of the paint droplets, which arrive at the glass surface, is important to make an even layer of paint of a certain thickness.

If there are too many droplets arriving at the glass surface, then there will be too much paint and you will see 'tears' of paint. 
The same can happen when the droplets are too big.
On the other hand, if the droplets are too small then they contain not enough paint to spread out evenly. 

Spray painted vase, tumbler and shot glass with OrmoGlass.

Another problem arises when too much solvent evaporates from the droplets during the transportation from spray gun to glass surface. Too much and too quick evaporation of solvents decreases the temperature of the droplet and hence increases the viscosity. An increase in the viscosity of the paint also influences the spreading of the paint on the glass surface. 

The size of the droplets and the evaporation rate of solvents can be controlled by the distance of the spray gun to the glass object, the air pressure, the flow rate of the paint from the reservoir, the type of spray (cone, flat, etc.), room temperature, temperature of the paint and temperature of the glass object.

Spray painting is a powerful tool for painting any kind of glassware. It is a quick decoration technique and it is easy to automate. 
One of the big advantages of spray painting over brush painting is that it is much easier for painting larger objects like glass vases and ligthing glass.


Manual color spraying

While spraying glassware manually, the spray gun is moved by the operator over the surface with slightly overlapping strokes to ensure an even or continuous coat. The operator must be skilled and at least must understand the factors (described above) which are of influence on the final result. This way of spraying is for flat surfaces.

Hollow glassware (drinkware, vases, etc.)  is usually placed on a turntable to ensure that during the spraying process, the coat will be even on all sides. The operator then moves the spray gun from top to bottom or bottom to top.


For OrmoSol and OrmoGlass we usually have the best results with a nozzle with a diameter between 0.5 and 1.2 mm. A good pressure for a nozzle of 0.5mm lies within 1.2 to 1.5 bar. Distance to the object is about 15 - 30 cm. Room temperature between 18 and 26 ºC. The relative humidity should be lower than 60%. 
It works best if the angle between the nozzle and the glass surface is held at an angle of 90 degrees at all times.

Glass bowl sprayed with OrmoSol
When the conditions are not optimal the operator has to make choices, about how to do the spraying based on the influencing parameters described above.
This might look as a very difficult process to start up with, but it is most of the time a matter of trial and error and adjusting, until the right way to spray in a certain situation is achieved. After that exercise the spraying can be done regardless of the number of glasses. Spraying a drinking glass is done in a few seconds and a skilled sprayer can do 1000 pieces per day during an eight hour shift.



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Spraying a glass is less efficient than hand painting a glass because more paint is lost. 
Nevertheless, are the material costs per item are low when you spray paint glassware with OrmoSol.
For spraying the foot + stem of a wine glass, cocktail glass or champaign flute, the material costs will be less than 0.02 usd.
To cover a whole champaign flute (stem+foot+bowl) will cost about 0.05usd.



Simple spray booth for manual spraying
Manual spraying can best be done in a spray booth. This doesn't need to be a spray booth which is totally closed, with a controlled temperature and relative humidity. It can just be a table model booth which functions as a working space with an exhaust, to avoid the inhalation of the spray and solvents.


Automatic color spraying

Spraying is a decoration technique which can be easily automated. The pressurized air used for the spraying can also be used for pneumatic movement of the spray gun.

There are a lot of companies who manufacture automatic spraying lines for glass companies. Since there are so many different paints it is important to check the performance of a certain spraying line with the paint which
will be used. Another point of importance is that the equipment is explosion safe, because even if a water based paint is used, a dust explosion may occur.



Since we have been involved in spray painting of glassware for many years we have developed a system together with a first class manufacturer of spray painting equipment. This fully automatic color spraying line is built and tested for OrmoSol and OrmoGlass.
 
An automatic sprayline in action
This spraying line is designed for glassware of any form and size and can be adjusted for each particular application.
While manual spraying delivers about 1000 pieces per operator per shift, this fully automatic spraying line does about 10 - 40 drinking glasses per minute. This is an equivalent of about 5000 to 20000 per eight hour shift! The number of glasses is of course depending on the size and shape of the glass and of the size and shape of the area to be covered.

The spraying line is further equipped with an inline curing facility. All you have to do is put the clear glassware at the feeding section and take the cured and colored items after the curing from the conveyor, ready to be packed.

If you want detailed information about manual spraying or automatic spraying then please Contact Us.