Vibratory Grinding

Vibratory or slide grinding is an alternative to the conventional rotary barrel process. The workpieces are placed in a container which is subsequently filled with a mixture of granulated zinc, water, alumina as polishing medium, and a welling agent or anti-rust compound until the pieces are completely covered- Then the container is set into vibrating motion. This presses and thoroughly mixes the mixture against the walls of the molds. Thus, a kind of wiping action occurs that smooths the walls. A distinct disadvantage of this technique is pronounced abrasion of protruding edges. These have to be covered for protection. Limitations on this process are imposed by the size and weight of the molds.

Sand Blasting (Jet Lapping)

Sand blasting is of the best known and most common procedures. For mold making, it is modified such that the blasting medium is a water-air mixture containing fine glass beads. Mold surfaces are treated with this mixture under a pressure of 500 to 1000 kPa. This level out any unevenness, such as grooves. The attainable surface quality is not comparable to that of surfaces treated mechanically. The roughness height is about 5 um. The application of this technique appears lo make sense only for flat parts. Disadvantages are non-reproducible removal and relatively low dimensional stability.

Pressure Lapping

This process is a variant of jet lapping and also known as “extrude-honing”. It is limited to the treatment of openings. As the name indicates, it has found special significance in the fabrication of profile-extrusion tools where arbitrarily shaped openings with the lowest of cross sections have to be polished.

The procedure uses applications a pasty polishing compound of variable viscosity that contains silicon carbide, boron carbide or diamond grits of various sizes depending on the dimension of the opening. The compound is moved back and forth and average roughness heights of R^ = 0.05 mm are achieved in no time. The process is done automatically and requires only a short set-up time.

Electrochemical Polishing

With electrochemical polishing, or electro-polishing in short, the top layers of a workpiece are removed .The process is based on anodic metal machining and therefore qualifies as a “cold” process. Thus, the workpiece does not become thermally stressed. The process works without contact between workpiece and mold, so no mechanical loading occurs. Since removal only occurs at the workpiece, the workpiece is subjected to virtually no abrasion.

Through the removal of material, leveling of the surface of the workpiece occurs. High dimensional and molding accuracies, as well as good surface properties，can be achieved by electrochemical polishing* The aim is often to remove impurities introduced into the outer surface layer during preceding machining processes. Further advantages of the operation are reproducible removal and the resultant high degree of automatability.

Defects in the steel, such as inclusions and pores，are exposed. Therefore, the materials to be elecirochemically polished must be of high purity. Various steels, especially the usual carbon steels, cannot be optimally electrochemically polished.

Electric-Discharge Polishing

Electric-discharge polishing is not essentially a new or independent procedure. It is an extension of electric-discharge machining and immediately follows erosive fine finishing. Thus, erosion and polishing are done on the same equipment using the set-up. Consequently, to an extent depending on the level of surface finish required, it can replace time-consuming and costly manual postmachining.

In electric-discharge polishing, the discharge energies are very much reduced, e.g. through lower discharge currents, relative to electric-discharge fine finishing. As a result, removal rates are low and so electric-discharge polishing is also a time-consuming finishing process. Because electric discharge polishing works on the principle of removal by heat, thermal damage is done to the outer zone. The outer zone can be minimized but it can never be removed completely.

The structure of surfaces after electric-discharge polishing characterized is by rows of adjoining and superimposed discharge craters similar to that of electric-discharge machining. Here, however, they are shallow, largely circular and all of about equal size. The surface roughness of so polished molds is about R_a = 0.1 to 0.3 un with a diameter of the discharge craters of about 10 pm. These patterns are in the range of finely ground surfaces and meet the requirements of mold making in many cases. Thus, it is possible to forgo manual polishing, which is difficult with complex geometries. The necessary time is 15 to 30 min/cm², the exact pattern depending on shape and size.

Hence, electric-discharge machining allows molds to be machined completely in one set-up by means of roughing，prefinishing, fine finishing and polishing. However, the workable area is limited in this process. Furthermore, electric-discharge polishing is very time-consuming. On account of the thermal removal principle of electric-discharge machining，a thermally damaged outer zone always remains on the workpiece. This can be minimized by electric-discharge polishing, but can never be removed completely.