TABLE OF CONTENTS If the blanks (sections of wire or rods) were simply inserted into the moulding die and then pressed, the die would be made useless after only a few units. Galling would occur in the die because of cold welding between the workpiece and the die. As a result, burrs would form on the die which would make the pressed parts unusable. For this reason, the blanks must be carefully prepared before pressing. This preparation, which is summed up as surface treatment , includes
pickling, phosphating, lubricating.
The pickling process is intended to remove oxidic coatings (rust, scale) so that the surface of the press blank is metallically clean, ready for the actual surface treatment.
Diluted acids are used as a pickling agent, e.g. for steel, 10% sulphuric acid (percent by volume).
If grease, oil or soap were directly applied to a metallically clean (pickled) blank as a lubricant, the lubricant would have no effect. The film of lubricant would come off during pressing and cold welding and galling would take place.
Therefore a lubricant carrier coating must be applied first, forming a firm bond with the blank material.
Phosphates are used as a carrier coating. Phosphating applies a non-metallic lubricant carrier, firmly bonded with the base material of the blank made of
steel (with the exception of Nirosta steels)
zinc and zinc alloys
aluminium and aluminium alloys.
This porous layer acts as a lubricant carrier. The lubricant diffuses into the pores and can...
