Development of high accuracy classification system for small particles

为了提高焊球等功能颗粒的性能，需要进行高精度的粒度分级。为实现球形颗粒的高精度分级，研制了一种双辊分级机。由于两个辊子以一定的倾角布置，具有较小的张开角度，颗粒沿辊子之间的间隙运动，并在间隙宽度超过颗粒直径的地方落下。为了弄清颗粒沿缝隙运动的动态行为，进行了显微观察，并对高速数码相机(1/8000fps)拍摄的图像进行了分析。质点速度与滚子的角速度成正比，与倾角的切线成正比。粒子不仅在滚动，而且还在轻微跳跃，而且是连续的。小的跳跃是由颗粒与滚子之间的摩擦引起的，而颗粒的向前运动是由于重力的轴向分量。质点速度可以用滚子的周向速度、倾角和摩擦系数来表示。此外，颗粒的运动取决于操作条件，即滚子的方向和旋转速度，而不是颗粒的直径，如果颗粒是几百微米的话。对于较小的颗粒，颗粒与滚筒之间的粘附会影响颗粒的运动。辊子表面粗糙度小，降低了附着力，即使在粒径为100μm的情况下，仍能将98%的颗粒进行分级，分级精度在1μm以内。

High accuracy size classification is required to improve the performance of functional particles such as solder balls. In this work, a size classifier with twin rollers has been developed to realize high accuracy size classification of spherical particles. Since the two rollers are arranged at a certain inclination with a small opening angle, particles move along the gap between the rollers, and fall down at a point where the width of the gap exceeds the diameter of the particles. To clarify the dynamic behavior of the moving particles along the gap, microscopic observation was carried out, and images recorded with a high-speed digital camera (1/8000 fps) were analyzed. The particle velocity is proportional both to the angular velocity of the rollers and to the tangent of the inclination angle. The particles are not only rolling but also jumping slightly but continuously. The small jumps are caused by the friction between the particle and the rollers, and the forward movement of particles is due to the axial component of the gravitation. The particle velocity can be formulated with the circumferential velocity of the rollers, inclination angle, and friction coefficient. In addition, the particle movement depends on the operational conditions, i.e. directions and rotational velocities of the rollers, but not on the diameter of the particles if they are several hundred micrometers. For smaller particles, the adhesion between a particle and the roller affects the particle movement. The small surface roughness of the rollers lowers the adhesion and 98% of particles were able to be classified even though the particle diameter is 100 μm, and the accuracy of the size classification was within 1 μm.