Screen Printing on Glassware

History and principle

Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The stencil forms open areas of mesh through which the printing inks can be pressed to the substrated (e.g. glass) creating a sharp-edged image. The mesh can also be partly blocked by an emulsion but in either way the screen shows an image built up of closed parts and open parts. The closed parts block the printing ink from the substrate and the open parts make it possible that the printing inks will go onto the substrate. 

In the schematic drawings below the basic process is shown:

    In the right figure:
    - A is the printing ink most of the time a paste material, 
    - B is the so called squeegee, the blade which is pressing the ink throught the open parts of the mesh,
      while it is moved over the mesh from one side to the other. 
    - C are the open parts of the mesh through which the ink is pressed. 
    - D is the mesh, also called ‘screen’. 
    - E is a frame holding the screen. 
    - F is the print which is the result of the ink pressed through the openings in the mesh onto the substrate. 

Traditionally the process was called silkscreen printing because silk was used as the material for the screen. 
Screen printing first appeared in a recognizable form in China during the Song Dynasty (960–1279 AD). 
Screen printing was largely introduced to Western Europe sometime in the late 18th century, but did not gain
large acceptance or use in Europe until silk mesh was more available through trade from the east.

Serving set with screen print decoration (Ocean Glass)

Screen printing, State-of-the-Art

Currently, synthetic threads are commonly used for making the mesh in the screen printing process. The most popular mesh in general use is made of polyester. But there are many different mesh materials nowadays ranging from nylon to stainless to glass. The properties of the screen need to be much better than the old silk screens due to the requirements from the industry. Screen printing in the industry, like the glass industry, is a high volume and high speed process rather than the slow process in the old days.

A print in one color
A significant characteristic of screen printing is that a greater thickness of the ink can be applied to the substrate than is possible with other printing techniques. This allows for some very interesting effects that are not possible using other printing methods. Because of the simplicity of the application process, a wider range of inks and dyes are available for use in screen printing than for use in any other printing process. 

Inks suitable for glass fall into the categories: 

Inks or enamels containing finely powdered glass or frit, heated to temperatures exceeding 600°C 
Inks containing inorganic pigments heated to temperatures exceeding 600°C 
Inks containing organic colorants and resins heated to 200°C; 
Inks containing organic colorants and resins cured by UV radiation. 

Multi-colored print with gold from Rastal

The last two categories have been developed to meet the demand for inks free of heavy metals (environment) and low cost firing (energy!). 
Inks containing organic colorants (cured by heat or UV) have further the big advantage that the colors are not restricted anymore to the limited range of colors coming from metal oxides, but that almost every color is possible due to the vast range of available organic colorants. 
Another advantage of inks with organic colorants is the possibility to make transparent prints. 

Available too, are inks containing precious metals so gold and platinum decorations, like texts and logos, can also be screen printed. 

The squeegee has also been subject to many improvements. Many factors such as composition, size and form, angle, pressure, and speed of the squeegee, determine the quality of the impression made by the squeegee. At one time most squeegees were made from rubber which is prone to wear and edge nicks and has a tendency to warp and distort. While squeegees continue to be made from rubbers such as neoprene, most are now made from polyurethane which can produce as many as 25,000 impressions without significant degradation of the image. 

The rate of screen printing production was once dictated by the drying rate of the screen print inks. Due to improvements and innovations, the production rate has greatly increased making screen printing the most powerful decoration technique in the glass industry for printing logos, texts and graphics.

Some specific innovations which affected the production rate and which also have increased 
screen print popularity include: 

    1. Development of automatic presses versus hand operated presses which have comparatively slow production times 
    2. Improved drying systems which significantly improves production rate 
    3. Development of the rotary screen press which allows continuous operation of the press. 
        (This is one of the more recent technology developments.)

The T83 (Fermac, Italy), a fully automated screen printer dedicated for glassware. 8 Different colors, 90 glasses per minute!

Screen printing in the glass industry

In the glass industry  some automatic screen printing equipment can print up to 90 pieces per minute in 8 different colors. These fully automated machines have the possibility that the glass is arriving on a conveyor belt and via a feeding table placed in the screen printer. After the printing the glass is automatically transported to a curing oven or a UV-curing device. 

Screen printing is a very versatile decoration technique for the glass industry but also for glass workshops. If you don’t have much prints to make you can choose from many manual or semi-automatic screen printers. For large volume production there is a vast array of machines available.
Screen printing is much done on drinkware and perfume and beverage bottles. Besides logos the technique is also used to make designs. The inks are dishwash proof so most glassware factories embrace this decoration technique.

There are as many applications for flat glass as for holloware and maybe even more. Glass coasters and mirrors are frequently decorated by screen printing. Appliance glass is often screen printed. Glass walls and dividers come nowadays in screen printed special designs and also when these large glass objects come in one even color the technique used is often screen printing. 

Glass sheets screen printed in one color
Screen printed glass wall