The hole in the hole defect refers to the pores that can be found on the transverse surface of the green body. Some are a nearly circular hole, and some develop into a through hole that almost penetrates the center of the green body. This is a common defect. The causes of the pores in different parts of the injection molded sample are different.
Generally, the pores in the middle part are relatively small, which may be caused by insufficient mixing of the feed material, gas trapped, and high injection temperature, causing the powder to separate from the binder. Correspondingly, it can be eliminated by adjusting the feeding quality, reducing the mold temperature and injection temperature. The pores at the bottom are relatively large. Some are even full-body or half-body through holes. The main reason for the occurrence of such holes is that the gas is trapped in the sample due to insufficient exhaust at the bottom during injection. Because the upper wall of the sample is thin and the bottom wall is thick, the dynamic feed flows from the top to the bottom of the die under the injection pressure during the injection process. When the feed fluid reaches the bottom, the cross-sectional area of the cavity suddenly increases. The feed will be progressively punched along the inner side through the bottom surface of the sample, so that the last place to be punched is not the bottom surface of the cavity, but the junction of the thin wall and the thick wall. Therefore, the exhaust holes on the bottom surface of the mold cannot be fully exhausted, causing the gas to gather and form a relatively large hole.