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The History of Screen Printing. The Fundamental Principles of Screen Printing. Mechanisation of the Screen Printing Process. It is assumed that the reader has no previous experience, so we will be starting from scratch. We will also be dealing with various types of printing machine mechanisation and working environments. 1 A variety of applications. There are those who openly state that screen printing is a black art. THE HISTORY OF SCREEN PRINTING.
This stencil is still in use today and is known as the indirect type. We will say more about this later. Naturally a demand was created for better, faster methods. THE FUNDAMENTAL PRINCIPLES OF SCREEN PRINTING. A screen, comprising a frame upon which is stretched a mesh. A photo stencil of the required design attached to the mesh. A squeegee, comprising a holder into which is fitted a flexible, resilient blade. A secure base on which to position the component to be printed.
An operator to combine these five items. Loads can be very high, possibly exceeding 80 kg on each side of a 250 mm frame. Tables 1, 2 and 3 give the physical properties of many of these meshes. The most common of these are T and the thinner thread version S. 24, the ultra thin 325 stainless steel. Tension must now be considered. The number is the same but the units are different of course.
It must be remembered that the figure is theoretical. Screen tension will be discussed later. The great majority of screen printing falls into one of four classifications. If the mesh is elongated by 0.
There are three principle reasons for this. An emulsion of sensitised PVA is coated on the mesh. The direct emulsion technique is normally the cheapest. The Indirect provides an easier method of preparation but has a relatively short life. Its adhesion to the screen is comparatively poor. This is a hybrid of the other two methods. The next diagram shows the effect to too thin or too thick a coating. If the emulsion is too thin it will be relatively weak. Bring the screen into intimate contact with the surface being printed.
Force the ink into the open areas of the screen. Shear and remove the excess ink from the surface of the screen. Control the rate of release of screen. Very few surfaces are truly flat. The diagram shows how the angle of the blade affects the filling action. The screen separation should be by peeling rather than pulling away. 004 for stainless steel, . 006 for polyester and .
Larger gaps and less distance from the edge will require wider blades. The quoted width is the minimum. Uses of enzymes in chemical industry are two other types of squeegee. The material is available in a variety of colours. This is particularly apparent when printing fine lines.
The squeegee will tend to push it out. Under these circumstances printing adjustments must be made. Not require undue effort to force it into the open area of the screen. Be easily removed why are enzymes used in the industry the mesh by surface contact with the subject as the screen peels away behind the squeegee.