Electrostatic performance of various lubricant powders in P/M electrostatic die wall lubrication

Electrostatic die wall lubrication is an advanced lubrication technique in the production of machinery parts of powder metallurgy. Electrostatic performance of lubricants is one of key factors for successful operation. Among electrostatic parameters, the charge-to-mass ratio of dry lubricant powders has been considered to be the most critical factor to make good adhesion between the dry lubricant powders and the inner die wall. This paper investigates electrostatic performances and potential dependences of the charge-to-mass ratios of various single lubricants and lubricant mixtures, such as zinc stearate, W-special wax, EBS wax, graphite and h-boron nitride, and some combinations of these single lubricants. The experimental results indicate that not all of the single lubricants can properly be charged by electrostatic charging. Zinc stearate, h-BN and EBS wax can easily be charged and the charge-to-mass ratios can be obtained up to the proper value (0.2 μC/g) high enough for die wall lubrication practice. For lubricant mixtures, wax combinations show satisfactory electrostatic response and both conducting and insulating lubricant powder additions influence the electrostatic performance of the mixtures significantly. Mechanism of charging is discussed and a quantitative estimation of the ratio is given. Processing variables influencing the charge-to-mass ratios are also discussed.