Optimisation of shrinkage in the design of compaction tooling for WC-Co
Date
2008-02-29T07:58:51Z
Authors
Blaski, Krzysztof
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Abstract
Abstract
Tungsten carbide-cobalt powder is pressed before sintering into a compacted form using
punches and a die cavity. After the powder has been pressed to a specific shape, it is
sintered and shrinks a certain amount to a final size. To accommodate this shrinkage, the
pressing tools are designed to a certain “shrinkage percentage” and thus the pressed
component or compact is larger than the sintered component by that percentage amount.
During the pressing process, there is a large amount of friction between the powder being
compacted and the die cavity wall. To counter pressing friction, a lubricant is pre-mixed
with the tungsten carbide powder. In the past at Powder Industries, the powder was
mixed with wax and all of the tools were designed to a 20% shrinkage. In recent times,
the wax in the powder has been replaced by PEG (polyethylene glycol) by most
manufacturers as this increases the quality of the final product and is easier to remove in
the furnaces. As a result of the new PEG lubricant, the tool wear rate at Powder
Industries increased and because a higher pressure had been necessary to achieve powder
pressing to the same shape and form, often the pressed components exhibited cracks or
were not pressed ideally.
On account of the problems introduced by PEG, correct tool design for the shrinkage was
obtained by a ‘trial & error’ process. This project has been motivated by the need of
establishing pressing and/or design ‘rules’ that would do away with trial and error when
designing compaction tooling.
The project has consisted of investigating the physical properties of 23 grades of WC-Co
powder (with or without TiC and TaC) and of performing a series of pressing tests for
each grade. A relationship between the apparent density of a powder and the ideal green
density of the green compact pressed from the same powder has been found. Using this
relationship, an equation has been derived between ideal shrinkage, powder apparent
density, component sintered density and powder volatile content. Since the last three
parameters are known to the tool designer, this equation can be used to calculate the ideal
shrinkage when designing new compaction tooling. This method of calculating shrinkage
is now in general use at Powder Industries and many successful sets of compaction
tooling have already been manufactured
Description
Keywords
WC-Co alloys, shrinkage, compaction