Some Considerations on the Packing Structure of Powderlike Materials during Dry Pressing Procedure (1964)

Some experimental and theoretical considerations have been performed on the packing properties of powderlike materials observed during dry pressing procedure.Many works have hitherto been reported on the packing characteristics when pressing forces are induced to the powder bed, and then, the relationship between the strain and the compressive stress is represented in a simple hyperbolic curve as illustrated in Figure 3 when fine sorts of powder are used.However when comparatively stiff kinds of powder possessing low“powder diffusibility1)”are used as raw material, such simple relationship is not obtained, but their relationship between the strain and the compressive stress is observed as many simple hyperbolic curves in a super-positional form as shown in Figure 12.The mechanism of this phenomenon is thought by the authers as follows; when the compressive force is induced to the packed bed, the bed initially contracts as an elastic body, that is, all the particles of the powder in the bed are contracted elastically mainly in the contact points. This elastically contracted structure is maintained to the point where slippage takes place at the weakest point. Once this slippage takes place, the equilibrium of the whole system is broken, and the slippage flow takes place successively. This slippage flow is observed as plastic deformation of the whole system.The equilibium is restored for the whole system after the successive processes of plastic deformation have been accomplished. This re-newed equilibrium state means the seoond elastic configuration. Further compressive force deforms the second elastic body, elastically in the first instance, and plastically after slippage is caused. This elastic and plastic deformation observed one after another constitutes successive hyperbolic curvels as illustrated in Figure 12.When comparativey brittle and crushable kinds of powder are used as raw material, similar successive hyperbolic curves are observed though with linear type in some places as illustruted in Figure 11 and Figure 8. The former figure is the case when fractures take place in the contact points of the powder, and the latter bigure is the case when fractures are pronounced in the whole volume of the powder. The mechanical configuration is changed discontinously or suddenly in both cases, after fractures have taken place.In the process of plastic deformation, the relationship expressed in equation (1) is established noticeably by many invesigators between the stoain ε and the deformation degrel dl/l3)4)7)11), ε=∫l'olodl/l=lnl'o/lo (1)and the relation of equation (6) is also established experimentally between the stress σ and the strain ε4)7)11).σ=kεn (6)In the authers theoretical view this equation will be expressed in equation (12), σ={ec·ε}1+a (12)where, c is the integral constaut in the mathematical treatment, and a is the factor showing the degree of change of the mechanical structures. This factor is, in the authers' view an important variable in the pressing procedure of powder beds. Some considerations on ec and a are performed based on the experimental observations.