The image to be printed is created on the printing plate, normally by chemical etching. The plate is generally steel or a photopolymer material. There are other materials and methods of image creation that will be discussed later. Etch depth is approximately 25-30 microns. The thickness of the dried ink film varies between 2 and 20 microns. Compared to screen printing this is a thin film process.
The etching is filled with ink; the action of the silicone rubber printing pad picks up ink from the etching and transfers it to the object to be printed. Filling the etched portion of the plate can be done in various ways, but the mechanism of picking up the ink from the etched portion of the plate and transferring it to the object to be printed is always the same in conventional pad printing machines.
Whilst the pad moves towards the object to be printed the solvent continues to evaporate from the ink on the pad, and the outside surface of the ink becomes tacky. As the pad makes contact with the object and compresses the print surface of the pad rolls across the object and the tacky surface of the ink attaches itself to the surface of the object. The pad then lifts and the printing surface of the pad rolls away from the surface of the object and releases the ink leaving it laying on the surface. Whilst this is happening the etched portion of the plate is recharged with ink, and the pad returns to pick up another image from the plate.
The rolling motion of the pad caused by its shape is important as it squeezes out the air over the plate and the ink surface. The pad surface must come in direct contact with the surface of the ink in order to pick it up and so carry it to the component.
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