Artwork is the photographic tooling used in the fabrication of thin metal parts. It is generated by a series of graphic and photographic steps designed to produce a faithful image of the desired metal part, properly sized and compensated for the processing condition, on a piece of film or a glass plate. Compensation of the artwork is carried out mathematically before the artwork is generated to determine the exact dimensions desired in the photographic image. The photographic processes applied to a metal sheet are designed to leave a protective layer on the desired metal while leaving the unwanted metal unprotected. However, when etching occurs, the etchant not only dissolves the metal in unprotected areas, but also undercuts the protective layer of photo resist, as in Figure 1.
- dp = part dimension
- da = artwork dimension printed on the photoresist
- dc = amount of compensation
- t = metal thickness
Generally, artwork etch compensation is about one-half of the metal thickness: dc = 1/2t. So as a rule, the width of a slot or diameter of a hole etched into the metal will be greater
than the image on the artwork by about the thickness of the metal. A drawing is computer-generated with dimensions corrected for compensation and used as a basis for
generating the photographic master artwork.
What Is A Phototool?
A phototool or pair of photographic reproductions is made by contact printing from the master. Of each pair, one is the image of the master and one the mirror image, so that when the pair is sandwiched together, emulsion side to emulsion side, the images in the emulsion coincide. Alignment of the two halves of the phototool and correspondence of dimensions in the image are extremely important, because the image must be printed on both sides of the metal sheet, with etching to proceed simultaneously from each surface. Misregistration can mean distorted parts, parts out-of-size, or ragged walls in the etched area which are angled with respect to the surface, as in Figure 2.
Since the phototool itself is two-dimensional, the parts which can be made from the tooling will generally be thin parts fabricated into two-dimensional configurations. A third dimension can be produced by stacking of parts, by bending, or by step-etching (multiple etching).
What Is Chemical Etching?
Chemical etching is the fundamental step of fabrication. An etching solution capable of dissolving the metal being processed is sprayed onto sheets in a piece of equipment specially designed for the purpose. The operator controls the final dimensions and etch quality by controlling the variables, such as temperature, spray pressure, etchant concentration, etchant age, and time in the tching sprays.
During the etching process, the processed metal sheets are laid on PVC rollers and carried through the spray chamber by a variable speed conveyor. In general, the etchant temperature and concentration are specified for the work being done. The etching operator adjusts the speed so that the time in the etch chamber allows just enough reaction to etch out a part of the correct size.