Picture trying to assemble a handful of tiny metal parts into an assembly that fits inside a golf ball, or keeping 0.003 in./in. tolerances while machining tiny spiral bevel gears. Imagine doing either a million times. Manufacturers faced with such daunting tasks often turn to plastic injection molding. It is hard to beat injection molding for replacing complicated, multipart, metal assemblies with a single plastic one. However, many assemblies cannot be converted to plastic because they must endure high temperatures, corrosive or abrasive environments, or need the strength and toughness of metal. In such cases, they can get many of the same benefits of injection molding by being made using metal injection molding, or MIM.

One biomedical company chose MIM to mold tiny camshafts, geared escapement drums, and levers for use in a device that delivers medication to asthma suffers. The delivery system is made less complicated because metal injection molding consolidates as many as 10 complex metal parts into only three.

Injection molding is usually associated with plastics. However, metal reaps many of the same benefits from the injection-molding process as those obtained by plastic. Metal powder mixed with polymer binders are injection molded to form a composite “green” part. A multistage powder metallurgy sintering process removes the plastic from the molded part, transforming it into solid, dense metal. MIM parts are capable of having many of the same dimensional features and surface finish qualities as injection-molded plastic parts. In addition, multipart assemblies can often be made as one unit.

Injection molding gives many design freedoms not easily matched by other traditional metalforming processes. MIM provides near theoretical densities resulting in mechanical properties superior to most other forms of powder metallurgy and approaching those of wrought material. Low alloy and stainless steels, soft magnetic or nonferrous alloys such as titanium, and those based on cobalt are all proven MIM converts.

MIM parts compete well in niche markets where components range in mass from 30 to 150 gm, have walls 0.25 to 6.35 mm thick, and are too complicated to manufacture by other methods. However, MIM technology is not a drop-in replacement for every small, complex component. MIM is often noncompetitive for applications currently using screw machining, zinc die castings, or stampings. Designers must evaluate each part individually. For instance, complex geometries requiring three or more tool setups may make them a good candidate for conversion to MIM. Investment castings or conventional powder metal parts needing secondary and finishing operations may also cost less if converted to MIM.

Another element which must also be factored into the conversion equation is the $10,000 to over $100,000 price tag typical of injection molds. If medium to high volumes (10,000 to over 1,000,000 parts annually) are anticipated, MIM is competitive. For smaller volumes, MIM may also reduce costs, especially for highly complex parts and those assemblies requiring extensive secondary machining or surface-finishing operations. Savings of up to 50% over machined parts or investment castings have been reported.

DESIGNER’S PRIMER
MIM technology is a union of thermoplastic injection molding and conventional powder metallurgy. Polymer binders mixed with fine metal powders are injected into molds similar to those used for plastic injection molding. Slides and cores, common tools in conventional injection molding, are able to produce more complex features than conventional machining allows or that can be made using castings.

Injection molding accurately duplicates mold surfaces. This results in parts having as-sintered surface finishes of <32 RMS (microinches). In addition, high-resolution features such as raised or subsurface lettering and logos are easily molded. Internal or external threads and knurled features, costly machined features, are essentially free with injection molding.

MIM uses many of the same design principles applied in molding plastic parts. The molds for injection molding have different characteristics than conventional investment molds or dies. Injection molding uses two mold halves to form the parts. Material is forced into the mold cavity through openings called gates. Once the parts cool and harden, the mold opens and the parts are ejected.

Gate locations are critical for proper mold fill but, like ejection or knock-out pins and mold parting lines, leave marks on part surfaces. The location of these molded imperfections can interfere with function as well as cosmetics. It may be to a designer’s advantage to work with an experienced MIM molder early in the conversion process. The biggest savings come from components that are specifically designed for MIM, where design objectives closely balance with process capabilities.

Conversion of machined components to MIM reverses the impact material removal has on part cost. MIM parts are more expensive when material is added rather than when it is removed. Material should only be placed where it is a structural asset. Unnecessary mass can be eliminated by adding thru-holes or blind holes using in-mold cores. Injection molding can make noncircular, tapered, or larger-than-normal thru-holes or passages not economically machined in conventional parts. This gives designers the ability to lighten components without impacting structural integrity. Mass is also reduced by removing fastener platforms and other features required for joining multipart assemblies. Similarly, reducing wall thickness or tapering them often slims down overall part size as well.

Designers must adapt their mold designs to allow for part shrinkage that can be as high as 20%. The polymer materials used as carriers are chemically, thermally, or catalytically removed during various processing or firing stages, leaving only metal. The metal retains molded features and is nearnet shape after reaching high temperatures in a furnace. The high temperature does not melt the metal, but instead fuses the fine metal-powder particles by a mechanism called sintering.

Flow characteristics of the soft polymer- metal mixture may also impact how molds are designed. It is helpful to use mold-flow modeling software to determine if internal mold radii, gate injection locations, and other internal features are all optimized and do not interrupt flow dynamics. Even with computer modeling, first article parts must be carefully examined to ensure that requirements for density and part uniformity are being met.

FEEDSTOCK BASICS
The ability to injection mold metal by means of conventional plastic injection- molding machines depends upon the properties of the powdered metal and the way it mixes with the polymer binder. The proper formulation of the powder metal and polymer mixture called the feedstock formulation depends on how the polymers are removed later in the forming process. The process is called debinding.

There are basic features fundamental to all feedstock systems. Metal powders for the MIM process are primarily obtained by either gas or water atomization. The atomization process transforms molten metal into fine droplets by spraying a liquid metal stream with water or blasts of inert gas. When the hot metal contacts the cold water or gas it forms metal droplets which rapidly solidify into individual powder particles. The particles are microingots of the base alloy and have fine microscopic grain structures. Gas atomization gives the best results. The formed particles are spherical in shape and have more controlled particle size distributions typically ranging from 10 to 20 μm, than irregularly shaped particles formed by water atomization. Other elemental powders such as iron and nickel fabricated using a combination of vapor deposition and condensation form particles with diameters from between 2 to 5 μm.

The metal powder particle size and distributions are critical parameters of the MIM process. The parts will be inconsistent if tight controls are not met. When the powder is combined with the polymer, it is heated and cooled then reground to form the feedstock. Heating the feedstock prior to molding softens the polymer binder, allowing it to carry the powder metal into the mold. Uncontrolled variations in particle-size distributions will affect the viscosity of the polymer-metal melt. This, in turn, impacts the rheology or flow characteristics of the mixture inside the mold. If the material does not flow inside the mold consistently, production parts will not have the same density. Parts may also contain voids if pockets of polymer-only material are deposited. These pockets will leave voids when the polymer is removed during the debinding stage, and will be present in the fully sintered part.

The selection of the thermoplastic binder depends on the debinding process used, but there are general principles common to most binders. During debinding the decomposition exhaust or by-products must not be toxic or corrosive. The polymer must also be easily removed from the component without distorting its general shape. Removal of the binder becomes a bigger issue as parts become larger and thicker. If thermal debinding is used, it cannot exceed molding or mixing temperatures of the binder. And finally, the molded “green” part must be tough enough for handling and storage.

PROCESS OBJECTIVES
The equipment used to produce MIM parts are specially equipped and modified injection-molding machines. However, molding technology varies across the industry. Factors which influence the technology include machine specifications, tooling methods, and feedstock types. Parts can be produced with extremely repeatable weights through the use of closed-loop feedback controls employing cavity pressure transducers. Combining these controls with consistent feedstocks minimizes as-molded density variations, leading to tight dimensional controls.

During debinding, 75 to 90% of the polymer binder is removed from the part. The residual polymer helps hold the metal particles together prior to sintering. MIM uses several different binder removal methods. These include thermal, solvent, and catalytic debinding. Thermal debinding processes employ a sequence of high temperatures to remove the binder. Solvent debinding removes the binder by dissolving it in chemicals or water. The catalytic process is the newest, most advanced binder-removal method. The introduction of a catalyst initiates a fast chemical reaction within the system which efficiently removes the binder from the part.

Thermal and solvent debinding with their high temperatures and chemical solutions often cause distortions. Catalytic debinding produces parts with excellent shape and good dimensional control because the process stays below the softening point of the binder. The catalytic debinding process is reported to remove binders from molded parts five to ten times faster than traditional thermal debinding processes. Catalytic debinding temperatures are only 120 to 130°C.

In the final step, the parts are sintered with a temperature and atmosphere profile specific to the alloy being processed. At the lower temperatures of the sintering cycle (300 to 400°C) the residual binder is removed. As the temperature increases, particles fuse, pore volume shrinks, and grain boundaries form at particle contacts. The grain size and part density depend on sintering time and temperature. The sintering temperatures in MIM range from 1,200 to 1,400°C. The combination of fine particle size (<20 μm) and higher sintering temperatures produces greater sintered densities than conventional powder metallurgy processes. MIM densities between 96 to 99% of theoretical are common.

Recent years have seen a shift in sintering technologies used by the MIM industry. Throughput and repeatability of continuous furnaces give advantages over conventional batch furnaces. Continuous sintering reportedly provides the most uniform temperatures and consistent processing conditions for a wide range of materials. If the processing equipment integrates debinding and sintering in one continuous operation, every part will experience the same time/temperature profile. This improves dimensional repeatability and lowers processing costs, especially for extremely large production runs.

While continuous sintering offers unmatched throughput and repeatability, batch furnaces are well suited for materials requiring vacuum sintering or for large parts that require special sintering cycles. Batch furnaces are generally used to process exotic materials such as titanium and cobalt chrome, and for large parts weighing over 120 gm.

Common MIM alloys are stainless steels, alloy steels, tool steels, soft magnetic alloys, and controlled expansion alloys. The Metal Powder Industries Federation (MPIF), Princeton, N.J., offers Standard #35, Materials Standards for Metal Injection Molding. The standard covers the most commonly used MIM materials with details on chemical and mechanical properties of each.

PART PROFILES SUITABLE FOR MIM
•Complex geometries — parts requiring three or more machining operations.
•Precision — ±0.003 to 0.005 in./in.
•150 gm or less in weight — parts weighing less than 30 gm work best.
•Small enough to fit in a tennis ball — ideally golf-ball sized or less.
•Length less than 3 in.
•Medium to high volumes (10,000 to over 1,000,000 parts annually).
•Exotic metal, low volume, and prototype programs in batch furnaces.
•Self-supported parts — otherwise fixtures are required.

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