Monitoring of metal powder-binder mixing process by eddy-current sensor

Abstract

Metal Injection Molding (MIM) is a technology by which metal products of high density and complex shape can be mass-produced in a cost-effective manner. The raw input to the MIM process is a compound consisting of metal powder and thermoplastic binder. The distribution of metal powder throughout the compound must be uniform if high-quality products are to be obtained.This paper describes a new method for monitoring the metal powder distribution using an eddy-current sensor. Experimental sensor readings were gathered using two different compounds, each with a different binding agent. Analysis of the data showed clearly that the sensor output was proportional not to the powder to binder ratio at the sensor location, as originally expected, but rather to the metal powder contents per unit volume. This phenomenon may have arisen from the fact that the density of the compound varies with the powder to binder ratio. A series of sensor readings taken at regular time intervals was used to monitor the overall distribution of metal powder within the compound.The merit of this technique is its reliance on a simple and inexpensive sensor probe. The drawback is that the sensor measurements are affected by the mixer's metal rotor and by temperature drift. The effects of temperature drift can be compensated by monitoring the temperature of the compound. The interference of the metal rotor was an obstacle to in-process monitoring in the present research. In the future, in-process measurements may be achieved by changing the shape of the mixing container from a “bowl” to a circular channel, in which the sensor will be able to measure the compound at a safe distance from the rotor.