Powder Manufacturing Processes Machinery's Handbook, 31st Edition
1541
Gas Inlet
End Cup
Heating Coils
High-pressure Cylinder
Workpiece in Hermetically Sealed Capsule
End Cup
(a)
(b)
(c)
Fig. 17. Hot Isostatic Pressing: a) Container is Filled; b) Container is Evacuated and Sealed Hermetically; c) Schematic Illustration of Hot Isostatic Pressing
The main advantages of HIP are that it improves mechanical properties, produces com paction with almost 100 percent density, and increases workability, and that, when in- corporated as an integral part of the manufacturing process, it reduces scrap and allows frequent replacement of wrought components. Further, it maximizes material utilization by improving material properties, so pro- cessed parts exhibit higher reliability and longer service life, and parameters can be estab- lished to minimize subsequent heat-treatment operations. Disadvantages of HIP include that it is a relatively expensive process and that good dimensional accuracy of parts is not easy to achieve with this method. HIP applications include defect healing of castings, consolidation of metal powder (pre form on near-net-shaped parts); improving the structural properties of premium invest- ment castings, primarily for aerospace applications, for which the process was initially used; sintering of diamond and carbide tools; and fabrication of metal matrix composites in the aircraft industry. Powder Injection Molding.— Powder injection molding (PIM) refers to the processing of both metal and ceramic powders. When one is dealing with metals or alloying powders, the term metal injection molding (MIM) is used. Powder injection molding is an innova tive and cost-effective manufacturing process commonly used for complex, high-quality medical and dental components. It is a high-volume, high-quality, cost-effective process that helps eliminate secondary machining operations for metal part production. PIM is very efficient for manufacturing small, intricate, and complex parts with good mechani - cal properties and geometrical accordance. The most common used materials for MIM are stainless steel, chrome-nickel steel, iron, and titanium. The process is generally best suited to parts measuring less than 0.25 in. (6 mm) thick and weighing less than 100 grams. Typical parts are components for watches, precise mechanic instruments, surgical knives, and automobiles. Powder injection molding is a multi-stage process. A general outline of the process is illustrated in Fig. 18. In MIM, very fine powders ( ≤ 10 μm) are processed as follows: Mixing: Metallic powders are mixed with an organic binder. Regarding the maximum powder content, 60 percent is typically powder and 40 percent is binder. Powder content can be slightly increased by blending different binder particle sizes. Pelletizing: Granular pellets are formed from the mixture into feedstock. Molding: The feedstock then undergoes a process similar to die casting; it is injected into the mold at a temperature of 284–400°F (140–200°C) and under pressure of 445–725 psi (3–5 MPa). High pressure powder injection molding at pressures of 5,000 psi (34 MPa) allows molding of thinner features, albeit with increased mold wear. Debinding: The molded green parts are heated at a low temperature to burn off the binder, or a chemical catalytic is used to remove the binder. Sintering: The parts are sintered in a furnace at sintering temperature (generally within 70 to 90 percent of the melting point of the metal or alloy).
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online