The production of permanent plastic scales-I: photo-impregnation of complicated and precise scale pattern on 'PERSPEX'

Abstract

This paper deals with the production of 'Perspex' protractors where the image pattern becomes integral with the base by a process based on photo-impregnation principle. This process involves building up of an impervious photographic stencil in a resistant material usually termed a 'Resist' (a coating that protects the area it covers from chemical action) on the "perspex" surface followed by chemical attack on the exposed portions of the plastic through the unmasked areas of the stencil with a tinted solution of an adequate solvent preparation. The image is rendered visible by the combined softening, penetrating and staining effects initiated by the etching cum dyeing mordant on the base. In course of the treatment the dye gets entrapped and deeply embedded into the material of the base. The tender dyed characters are allowed to harden and set by the evaporation of the solvents. The composition and action of the dye incorporating medium is so governed that it does not chisel by forming image wise cavities due to removal of material from the "perspex" as is usually associated with all commercial photo-mechanical etching processes.