Shape Retention During Molding And Debinding Of Injection Molded 17-4 PH Stainless Steel Compacts

17-4 PH stainless steel is one material that has an important place within thepowder injection molding (PIM) industry. This material is used for many applications including dental brackets, gun parts, surgical instruments, and aerospace devices. The properties of this material can vary significantly depending upon the specific processing conditions. On the other hand, PIM has become an attractive process among the manufacturing routes to precise parts with complicated shapes. The removal of the binder from the powder compacts (debinding) can be a slow step and a source of problems. So, the problem with the inability of the compacts to retain shape during debinding is one of the most serious problems, which restrict the further growth of this technology.In this work, a comparison between the requirements of moldability and the compacts shape retention has been made. It was found that, there is often a contradiction between the requirements of flowability and the compacts shape retention. The most important factor affecting compacts shape retention during debinding is found to be the designation of the feedstock and binder system, next comes the compact stresses due to molding conditions.The compact stresses due to viscoelastic behavior of the powder injection molding feedstock have been approximated by an equation developed from modificationsof existing equations as follows: τ = τ0 Exp. - (G/η)tc where (G/η) = 1/tr, tc is the cooling time and (tr) is the relaxation time. The results indicated that, if the relaxation time is greater than the cooling time the compact will contain stresses due to the viscoelastic behavior. So, a long cooling time is desirable to minimize retained stress and to avoid distortion during debinding. The effects of thermo-mechanical properties of the PIM feedstock on the compacts shape retention during debinding process have been discussed and explained from practical point of view in this study. The feedstock with lower heat of fusion cools rapidly after molding, allowing more opportunity for largethermal stresses. The binder component with high heat of fusion and high-decomposed temperature is more effective as the second binder component for the compact to retain its shape during debinding.The higher the powder loading, the lower the moldability index and the lower the distortion during debinding. An empirical relationship between the distortion and powderloading could be obtained: = 7 x 106 E-0.23 where = the distortion and =the powder loading. This equation fits the results with an R-squared value of 0.918. The best powder loading with respect to better moldability index and lower distortion has found to be 64% for this material.The effect of molding conditions represented by molding temperature and pressure on the compact shape retention has been studied. The higher the molding pressure and temperature, the lower the distortion that occurs during debinding up to a certain level followed by increase in distortion with increasing molding pressure and temperature due to the appearance of residual stresses. The higher the molding pressure and temperature, the lower the dimension shrinkage after sintering. An empirical relationship between the distortion and molding pressure could be obtained:where = the distortion and P = the molding pressure. Thisequation fits the results with an R-squared value of 0.9032. Other empirical relationship between the distortion and molding temperature could also be obtained:where = the distortion and T = the molding temperature.This equation fits the results with an R-squared value of 0.9009.Incorrect debinding could lead to crack, pore, bubble and dilatory response of powder compacts. These defects that occur through the evolution of the pore structure of the compact during debinding were explained clearly in this study. Binder decomposition,pore structure evolutions, dimension changes and powder particle rearrangements that cause distortions during debinding have also been studied.