Date：2025-06-04 Views：1010
Gas and Water-Assisted Molding
Creating Large Structures with PIM
Controlled Porosity Structures
Powder Injection Molding (PIM) is evolving with novel processes that leverage powder structures to create innovative designs and material combinations. One significant advancement is the ability to combine multiple pieces into a single component. Initially, this was achieved by assembling individually molded pieces of the same material before sintering. Newer variants extend this concept to combinations of different materials using single-step co-molding technology.
Green machining and green assemblies are also emerging as valuable processes in PIM. Green machining involves adjusting dimensions and features of the molded compact before debinding or sintering. This process is useful for trimming gates, parting lines, or drilling holes. Although machining is slow, the green body is soft, resulting in little tool wear. However, green machining is limited to features that are large compared to the particle size, and high cutting speeds with low feeds are necessary to achieve accurate dimensions.
Green assembly, on the other hand, involves combining components after molding but prior to sintering. This can be done by molding the first component and using it as an insert for molding the second component, sequentially molding two feedstocks into a tool cavity that expands between the first and second feedstocks, or assembling two separately molded components in the green state. This approach allows for cost reductions by using costly materials only where necessary.
Two-material PIM allows for the creation of components with customized functionality. This technology, adapted from plastic two-color molding, involves molding two different materials sequentially into a tool cavity. The process ensures that materials densify and bond during sintering without defects. Applications include magnetic-nonmagnetic combinations and components requiring high strength and wear resistance.
The two-material PIM process uses a molding machine with two barrels, each containing a different material. The multiple cavity tool rotates between the two nozzles, allowing simultaneous filling with both feedstocks into two different sets of cavities. The component is formed in two steps: the first step molds the more "inner" material, followed by a second molding step to form the second material over the first. Through proper feedstock design, the materials will densify and bond during the sintering cycle without defects or warpage.
This technology has been demonstrated for ceramic-metal combinations, such as alumina-molybdenum, to tailor electrical conduction pathways in an insulator. Other demonstrations include magnetic and nonmagnetic structures, and areas where function changes with position in the structure. Two-material PIM is particularly beneficial in fuel systems where demands for wear resistance and fatigue life require high-strength, hard materials, while fuel compatibility dictates resistance to sulfidation and environmental attack.
For components where surface contour is crucial, gas and water-assisted molding techniques have emerged. These fluid-assisted molding methods use internal pressurization of the feedstock against the mold walls. Gas injection uses air pressure up to 30 MPa, while water injection relies on hot water. These techniques offer better control over flow and wall thickness, making them suitable for large-size, low-mass, thin-wall structures.
These fluid-assisted molding techniques are particularly useful for components where a surface contour is important, but bulk properties are not critical. The warm PIM feedstock behaves similarly to bubble gum. At a slow rate of pressurization, a feedstock bubble can be filled and expanded as a film against the walls. Gas injection uses air pressure up to 30 MPa (4.3 ksi), and water injection relies on hot water. The latter is less compressible, so there is better control over the flow and wall thickness.
These technologies are still at the demonstration level but offer interesting options for large-size, low-mass, thin-wall structures. Early demonstrations include a ceramic spoon fabricated with a gas-filled handle and hollow golf club drivers with thin walls of just 0.4 mm (0.016 inch). Special binders help create a "superplastic" behavior for the feedstock, allowing considerable shape change without fracture.
To enhance the strength of PIM products and compete with forged alloys, a novel pneumatic isostatic forging process has been developed. This process involves heating sintered PIM components and placing them into a pressure chamber backfilled with liquid argon or nitrogen. The rapid pressure rise presses the compact on all surfaces, creating a forged substructure with improved mechanical properties.
From a metallurgical perspective, sintering is an annealing process, making it difficult for PIM alloys to compete with the strength of forged alloys. Forging builds up a dislocation substructure that increases strength. The pneumatic isostatic forging process aims to improve strength with less focus on removing final porosity. The process offers short cycle times and eliminates tooling for the forging process.
The sintered PIM components are heated and placed into a pressure chamber backfilled with liquid argon or nitrogen. When the liquid hits the hot component, it boils to form gas, pressurizing the working chamber. This rapid pressure rise presses the compact on all surfaces to create the desired forged substructure. The process has shown significant potential for components requiring high reliability and high properties, such as in biomedical, oil field, electronic, or aerospace applications.
PIM has traditionally focused on small components due to cost and production quantity considerations. However, new concepts combining PIM with rapid prototyping have enabled the creation of large structures. Soft silicone rubber molds are used to form complex shapes, which are then filled with PIM feedstock. This process offers excellent surface finish, short lead times, and suitability for short production runs.
Manufacturing large structures with PIM involves creating a physical prototype of the design, which is then used to form a soft silicone rubber mold. Once the rubber is set, it is stripped from the master, creating an inexpensive tool cavity with considerable complexity. The hot PIM feedstock is filled into the soft tool using low pressures, and heat is extracted to harden the binder. The component is then ejected, debound, and sintered.
This process allows for dimensional control within the ±0.2% range when temperatures are precisely controlled. It offers advantages such as excellent surface finish, short lead time, and suitability for short production runs. Most importantly, it enables the production of shapes that would be difficult to fabricate with any existing sintering technology. Commercial uses so far have been for tool steels, bronze, and stainless steels.
At the other end of the spectrum, PIM is expanding into microminiature components. This technology combines new plastic micromolding machines with small powders sized below 0.1 µm. Applications include actuators, sensors, and biomedical devices. Microminiature PIM components exhibit superior mechanical and wear properties, making them highly valuable in various industries.
Microminiature PIM is starting to penetrate the component size range below 1 mm, known as pPIM. This technology combines the attributes of new plastic micromolding machines and small powders sized below 0.1 µm. The small powders are necessary to form the small objects, while new molders are required to control die cavity filling. Tooling is fabricated using ultraviolet lithography techniques developed for semiconductor fabrication. Part handling is automated, and debinding and sintering are rapid.
Applications for pPIM are expected in actuators, sensors, portable consumer products, military projectiles, electronic assembly tools, oxygen analyzers, filters, and healthcare instruments. One driver for this growth is the perception of value for smaller, lighter devices. Very small components are highly valued, with examples such as fiber optic ferrules, laser guide tubes, printed circuit drills, microelectronic manipulators, wire bonding tools, and orthodontic components selling in the $4,000 to $20,000 per kg range.
Several new materials are under investigation for PIM applications. These include transformation-toughened zirconia, titanium alloys, and refractory metals like rhenium and niobium. Additionally, PIM is being used to create composites with enhanced properties, such as wear-resistant materials incorporating hard ceramic particles and high elastic modulus cermets.
Transformation-toughened zirconia is a good example of a novel material fabricated via PIM. The retention of the tetragonal crystal structure by stabilization additives gives a stress-induced phase transformation in the plastic zone ahead of an advancing crack. This results in the stress at the crack tip becoming compressive and inhibits crack propagation. These materials are now being fabricated via PIM and, in some cases, include colorants to make a tough, colored ceramic for decorative purposes.
Interest also exists in titanium, gold alloys, sterling silver, and refractory metals such as rhenium, niobium, and tantalum. Some new composite materials are envisioned based on the unique capabilities of PIM. For example, ceramic-ceramic composites can be formed using whiskers or particles to reinforce the ceramic matrix. Hard ceramic particles like titanium carbide (TiC), tungsten carbide (WC), chromium boride (CrB2), and titanium nitride (TiN) are added to steel to improve wear resistance.
Controlled porosity structures are useful for a variety of applications, including fluid control surfaces, filters, sound absorbers, air bearings, spargers, batteries, insulators, and capacitors. Pores can be open or closed, with closed pores being particularly useful for energy absorption in automotive safety applications. Pores are created by dissolving gases like nitrogen into the binder or using core formers that decompose during sintering.
Controlled porosity structures can be created by two routes. The first involves dissolving nitrogen (or other soluble gas) into the binder. During molding, the gas is nucleated as bubbles. If the bubbles are large, they remain during sintering to become closed pores. Usually, the bubbles are in the 10 µm size range, and the porosity is adjustable from 10 to 40%. However, the pores are only present on the component interior, so the surface is dense and the pores are closed. Applications include lighter weight stainless steel watch cases and precious metal jewelry.
Another route involves using core formers, which are usually inert particles whose size and amount dictate the porosity and pore size in the product. Particles that are easily decomposed during the sintering cycle are desirable. Examples include ammonium carbonate, camphor, stearates, and nylon. With high concentrations of polymer fibers, such as chopped nylon fishing line, alignment is useful since the pores are monosized and fluid flow is directional.
PIM allows for the creation of anisotropic materials with differential flow, heat transport, or mechanical responses. These materials can be designed to have properties like negative Poisson’s ratio or optimized thermal expansion and conductivity. Applications include electronic modules where heat flow management is critical.
One powerful use of anisotropic materials is in electronic modules. Heat flow away from the computer chip is highly desirable. PIM structures can be designed to place different materials at different positions to optimize heat flow. For example, Kovar is often used for its glass-metal sealing attributes, while W-Cu provides locally high thermal conductivity for heat dissipation.
PIM also enables the creation of structures with unusual mechanical responses. All structural materials shrink as they are stretched or grow as they are compressed, a property described by Poisson’s ratio. However, via PIM, it is possible to reverse this behavior by embedding simple machines in the structure. Examples include creating nonlinear elastic behavior with a leaf spring, instilling a negative Poisson’s ratio with curved walls, extending deformation strain during impact, and using different materials to obtain anisotropic strains during temperature changes.
The future of PIM is bright with continuous advancements in technology and materials. As PIM moves beyond just cost-effective component replacement, it is becoming a process that enables novel applications and innovative designs. With the ability to create complex geometries, customize material properties, and produce both large and microminiature components, PIM is poised to play a significant role in shaping the future of manufacturing across various industries.
Early PIM successes came from replacing existing components at a lower cost with improved structures. Today, PIM is moving into a phase where designers use the process to their advantage. This shift in thinking leads to novel applications where different manufacturing capabilities can be combined. For example, PIM is being used to create unique designs and materials that meet specific thermal expansion and conductivity requirements, which are critical in semiconductor packaging.
The W-Cu material system, processed from a mixture of copper and tungsten powders, offers a unique combination of high thermal conductivity and low thermal expansion. This makes it an ideal solution for applications requiring compatibility with silicon and other semiconductor materials. The microstructure of PIM tungsten-copper material consists of a sintered tungsten grain network with a high thermal conductivity copper matrix.
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