It is known to compress metallic powder and also powder of other material into compacts with a view to sintering the compact subsequently. It is specifically tools of sintered carbide which are manufactured in the sintering process, e.g. reversible cutting blades for milling. Powder presses operate either as eccentric presses or hydraulic presses. Hydraulic presses are preferred because the hydraulic press cylinders can be controlled better with regard to the pressure and path. In a hydraulic press, an upper press cylinder and a lower press cylinder each are connected to the upper ram and lower ram which are associated with a die-bore. The powder is filled in by means of a charging shoe when the lower ram has been moved into the die-bore. Precise proportioning is accomplished by causing the lower ram to move up by a certain amount subsequently, whereupon any powder projecting beyond the die-plate surface is removed by stripping. Subsequently, the powder is compressed by means of the upper ram, in which process the lower ram may be shifted accordingly.
The factor crucial for the quality of the compact is that the powdered material be as homogeneous as possible in its density. It is known to enhance homogeneity by actuating the charging shoe in a predetermined manner, e.g. using different speeds in the forward and backward strokes. It is further known not to move the charging shoe to and fro only linearly in a single direction, but to overlay this motion by at least one lateral motion. It is further known to cause the charging shoe and the die-plate to oscillate by means of an appropriate vibration device of a predetermined frequency and amplitude to improve the homogeneity of the powder in the die-bore. Finally, it is also imaginable to vibrate the lower and upper rams, particularly during the fill-in procedure and at the beginning of the compression procedure. In case of compacts which are of different width extensions in an axial direction care has to be taken that the rams do not travel against an edge so as to be damaged. This will naturally damage the die-bore, too Hence, it is also known to associate the rams with distance-measuring transducers which ensure that the rams can be moved to predetermined positions.
When compacts are manufactured according to the method described it is further essential for the compacts to exhibit approximately equal densities. This requires that the maximum compression force which is achieved in the end position of the upper ram, for example, should remain as equal as possible. However, the compression force is dependent on different factors. When the filling volume is varied the maximum compression force produced will naturally vary, too. A different compression force will also result when the powder is distributed inhomogeneously.
It is further known to arrange a load cell between the press cylinder and the ram, by which the respective maximum compression force can be recorded. In operation, care has to be taken that if the maximum compression force differs too much from a predetermined value an approximation to the desired compression force be obtained by modifying certain parameters.