The picture above is a cross-sectional view of one of the holes in the ring die made by plane polishing. Each hole in the ring die is composed of three parts. From the inside to the outside, they are: guide hole, compression hole, Release hole. The guide hole is also called the pre-pressure hole, and the material is pre-pressed through the guide hole; the compression hole: the material is shaped and conformed through the compression hole through pre-pressure, and then goes out through the release hole at the bottom. As shown in the figure, D is the hole diameter of the pressed pellet, and L is the effective compression distance, which is the length of the guide hole plus the length of the compression hole. The ratio between the aperture and the effective compression distance is the compression ratio:
Compression ratio = hole diameter / effective compression distance = D/L.