Powder injection molding (PIM) encompasses metal injection molding (MIM), ceramic injection molding (CIM) and cemented carbide injection molding (CCIM). All three processes combine the attributes of polymer injection molding with the engineering and performance properties of metals, ceramics and cemented carbides. MIM is cost effective in producing complex parts. The ability to mix different powders and binders allows engineering a part having specific thermal, wear, magnetic and strength properties. In addition, the process yields net-shape components requiring little or no secondary operations, which simplifies production, increasing yields and lowering manufacturing costs. A crucial step in the MIM process is to remove the binder used in the formation of the part prior to sintering to the required part density. This article addresses the problems associated with the thermal debind and sinter technology of MIM parts and their solutions.
In the MIM process, fine powders are mixed with various binders to create a thermoplastic feedstock, which can be injection molded. The parts are then debinded and sintered to full density to attain the desired mechanical and physical properties.
Advantages of the MIM process include:
- Excellent shaping possibilities (complex shaped parts, including parts with undercuts not possible with conventional PM parts, can be manufactured with no, or very little, secondary operations
- Excellent surface quality (superior to that of precision cast parts), which eliminates or greatly reduces finishing and polishing costs
- Excellent material properties (up to 96-100% of theoretical density is achieved)
- Excellent tolerances (parts are dimensionally accurate within ±0.05%)
- Excellent material selection (MIM becomes more advantageous to make parts that are difficult to machine)
Four main MIM manufacturing steps are the creation of the feedstock, injection molding cycle, solvent/chemical debind cycle and thermal debind/sinter cycle. During the thermal debind/sinter cycle, the part is subjected to undesirable powder-binder reactions, shrinkage and thermal stresses, all of which contribute to cracked, warped, incorrect chemical composition and poor density.
The goal of the thermal debind step is to remove most of the binder via evaporation at low temperatures, leaving a polymer "backbone" to hold the powder particles in place so the part can be sintered. The polymer evaporates at sintering temperatures, and the part densifies. It is critical to have even binder evolution and a sweep gas flow around the part to ensure no binder redeposition on the part. Complete binder extraction ensures producing high-density parts with no blisters or off chemical composition (e.g., carbon control in stainless steel).
In early MIM processing of wax/polymer parts, thermal debinding typically was done in air and vacuum, as well as atmospheric nitrogen or hydrogen ovens depending on the material and the binder. Today, the use of a polyacetal binder requires a specially designed oven in which the binder is catalytically removed via nitric acid vapor. The process works from the outside of the part to the inside, unlike that for a wax/polymer binder. However, debinding times for both binder types are a function of part thickness; the thickest part governs the total cycle time.
Problems associated with earlier ovens were uneven gas flow and large thermal gradients, both of which contributed to poor debinding, resulting in binder redeposition on parts and contamination on the furnace interior. While this is not a problem with polyacetal binders because debinding is a chemical reaction, it is not available for some metal powders, and requires a special catalytic debind oven. In addition, parts after catalytic debinding are fragile; that is, their brown strength is more of an issue than with wax/polymer binders.
Parts debound in thermal debinder or nitric-acid ovens must be handled to place them in a separate sinter furnace; a time consuming process involving heating to debind temperatures, cooling the part, removing from the debind oven and loading into a sinter furnace. The sinter furnace then goes through the same temperature cycle. Breakage of brown parts is a consideration due to the additional handling.
A furnace capable of operating with a partial pressure range of 1 to 760 torr and combining thermal debind and sinter cycles in one step was developed to overcome the problems of long process cycle times and part breakage. The design includes a built-in retort and gas management system (Fig. 1), allowing processing of MIM parts under laminar gas flow, which eliminates contamination issues.
The process gas flows across the parts from gas-distribution holes to the center of the retort through a gas manifold with inlet and outlet passageways (Fig. 2). Gas is preheated by heating elements, which ensures flow to the center of the retort. Three principal types of gas flow are turbulent, laminar and molecular. At an atmospheric pressure of 760 torr (typical for air and controlled-gas ovens), gas molecules flow at high pressure and velocity, colliding with each other, which creates uneven flow and shadow effects on the parts.
At a pressure of 1 torr or less (typical for vacuum ovens), gas molecules collide with each other randomly and gas flow becomes unpredictable. The gas flow also escapes to the cold walls of the oven, creating random flow on the parts and contamination inside the furnace.
At a pressure of around 300 torr, the gas molecules flow at sufficient velocity to flow smoothly and evenly (laminar flow) over surface irregularities, creating no shadow effects, much as if the parts were submerged in a liquid (Fig. 3).
Turbulent flow results in greatly separated flow at the back side of the part. This uneven flow creates an uneven temperature distribution on the part resulting in different debinding and sinter results. Lowering the density of the gas via partial pressure provides a greater chance of laminar flow with less separation at the backside of the part and a higher gas velocity, which creates a thinner viscous boundary layer, allowing greater thermal transfer. Ideal laminar flow guarantees even, continuous debinding of the part because the gas moves in a predictable way. It also moves the process gas to the center of the retort where the binder contaminant is pulled through the gas manifold to an easily cleanable debind trap.
A furnace equipped for partial pressure operation offers great flexibility. Operating profile parameters can be tailored for the specific concerns of the material being processed. These include slow ramping during specific temperature phases, variation of partial pressure and the mix of gas to obtain surface finishes, all of which contribute to the wide variety of materials that can be processed in such a furnace including carbon and alloy steels, stainless steels, irons, high-temperature alloys, tool steels, titanium and Ti alloys, and tungsten-base alloys.
The furnace is heated using radiation and convection, depending on the partial pressure and required temperature uniformity. Temperature uniformity varies greatly with different pressure levels, temperature ranges and type of gas (Table 1). The ability to change the partial pressure, gas type and flow allows processing any MIM material with any binder component. Some materials, such as copper and stainless steel, require a higher partial pressure during sintering to prevent their evaporation.
Laminar gas flow allows processing loads with identical or different sized parts in the same furnace envelope and in the same process run because gas flows evenly from both sides to the center (Fig. 4). There is no shadow effect under laminar gas flow (see Sidebar).
Batch furnaces with laminar gas flow capability provide complete debinding, ensure temperature uniformity during debind and sinter, eliminate evaporation of key constituents of the part, provides versatility and flexibility in producing a wide variety of PIM/MIM parts and eliminates a separate thermal debind step. The benefits result in improved properties of the part and reduced cycle time and energy consumption. IH
SIDEBAR: Versatile PIM/MIM manufacturing
Batch furnaces with laminar gas flow capability provide complete debinding and ensure temperature uniformity during debind and sinter, resulting in improved part properties and reduced cycle time and energy consumption, as illustrated in the following examples.
Processing a carbon steel electronic housing (Fig. A). The total cycle time to process an electronic housing that was being produced in a separate debind oven was reduced by 45 hours, considerably reducing energy and gas use. In addition, carbon control and part dimensional tolerances were greatly improved.
Processing a carbon steel ring mount (Fig. B). A carbon steel ring mount processed in a traditional manner had high porosity (7.04 g/cm3), high oxygen content (0.60%), poor carbon control (+/-0.02%) and poor dimensional control and concentricity. By comparison, processing in a partial-pressure furnace under laminar gas flow conditions increased the part density to 7.76 g/ cm3, reduced oxygen content to 0.11%, tightened carbon control to an acceptable level (+/-0.01%) and resulted in excellent dimensional control and concentricity.
Processing a stainless steel safety grip (Fig. C). Treating this part in the laminar gas flow, one-step processing furnace eliminated the need for a separate thermal debind step, reduced energy consumption and improved carbon control and dimensional tolerances .
Processing a copper-tungsten part (Fig. D). Sintering copper parts under partial pressure eliminated copper bleed out (loss of copper).