Metal Injection Molding

Chapter 1: What is Metal Injection Molding?

Metal Injection Molding (MIM), also referred to as powder injection molding (PIM), is a manufacturing process used to create solid metal components through injection molding technology. Originally developed for shaping ceramic materials, this technique was adapted in the 1970s by Raymond Wiech to handle metal materials. Since the 1990s, MIM has become a widely adopted method in metal fabrication.

In metal injection molding, fine metal powders are combined with a plastic binder to create a feedstock suitable for injection molding. This mixture is melted, shaped, and cooled using a standard molding machine. After molding, the parts undergo binder removal and sintering to eliminate the plastic binder and enhance the metal's density and compactness. These subsequent processes ensure the final parts achieve the desired geometry, composition, and physical properties.

Metal injection molding is ideal for the high-volume production of small, complex parts with intricate details. It allows for precise manufacturing without the need for additional machining. This process is compatible with a range of ferrous and non-ferrous metals and is generally more cost-effective compared to forging, casting, and machining. Parts produced through MIM are widely used across various industries, including automotive, aerospace, electronics, telecommunications, medical, dental, sporting goods, consumer products, and defense.

Chapter 2: What are the stages of the metal injection molding process?

The process of metal injection molding involves the following steps:

Feedstock Mixing

The metal powder is combined with a thermoplastic binder to form an intermediate material known as "feedstock," which is suitable for injection molding. The binder is crucial in the metal injection molding process and serves the following functions:

• It modifies the viscosity and improves the flowability of the feedstock. High flowability is crucial in pushing the molten feedstock over the heating barrels and filling the mold cavities.
• It imparts cohesion and strength to retain the shape of the molded parts, even after debinding.
• It ensures good releasability of the molded parts from the mold.

The binder consists of a blend of organic polymers, carefully formulated by material researchers and manufacturers. Typically, a MIM binder includes a combination of three types of polymers:

• A polymer that imparts good fluidity and lubrication (e.g., polyethylene glycol, wax)
• A polymer that provides strength to the molded part (e.g., polypropylene, polyethylene)
• A surfactant that prevents agglomeration of powdered particles (e.g., stearic acid)

These components can adversely affect the metal's mechanical properties if left in the final product, so they are removed after shaping. A suitable binder for MIM should possess the following characteristics:

• A suitable binder must be water-soluble or easily decomposed by a solvent or a catalyst.
• It must be unreactive with the metal.
• It must allow recyclability of excess materials resulting from the injection molding step.
• It must be economical and environmentally friendly.

Feedstock preparation begins with combining powdered metal and binder. The metal powder used is typically 20 microns in diameter, finer than that used in traditional powder metallurgy. The materials are mixed to achieve a metal-to-binder volume ratio of 60:40. This mixture is processed at high temperatures to ensure uniformity, then granulated and cooled to form pellets suitable for injection molding.

Injection Molding

Metal injection molding occurs in standard injection molding machines, similar to those used for plastics. The homogenized feedstock pellets are melted and injected into molds, where they take on the shape of the mold cavities. As the molten material cools, it solidifies within the mold. To account for shrinkage during sintering, the mold cavities must be slightly larger than the final product's dimensions. The resulting products from this stage are known as "green parts," which are about 20% larger than the final components but retain the correct geometric shape. These green parts are essentially scaled-up versions of the final product. After a designated cooling period, the green parts are ejected from the mold, and any excess material from the feedstock flow is removed.

A standard injection molding machine includes the following components:

Clamping Unit

The clamping unit generates and applies the necessary force to keep the mold halves closed during injection and cooling. It includes the ejection system, which removes the green parts once they have cooled. Additionally, the clamping unit handles the opening and closing of the mold halves between cycles and ensures their alignment during operation.

Injection Unit

The injection unit is tasked with heating and injecting the feedstock into the mold cavities. It comprises the following components:

• Hopper: The hopper stores the feedstock pellets before feeding them to the barrel. It has an opening at its bottom at which the pellets pass through. This opening is metered to control the volume of the feedstock entering the barrel.
• Barrel: The barrel houses the injection molding screw and contains the feedstock being melted. It is jacketed with heaters that provide heat to melt the feedstock into its viscous liquid state. The feedstock becomes more fluid-like as it travels along the length of the barrel due to the combination of intense heat, high pressure, and friction.
• Injection Screw: The injection screw mixes the melted feedstock and pushes it across the barrel. These are accomplished by the simultaneous rotational and sliding motion of the injection screw.
• Nozzle: The nozzle introduces the molten feedstock into the mold. It is aligned with the stationary mold half adjacent to the injection unit. The volume of the feedstock introduced to the mold is called a shot.

Mold in the Molding Process

A mold is a tool that shapes the feedstock and consists of two halves. The front half is stationary and positioned next to the injection unit, while the rear half is mounted on a movable plate that opens and closes the mold, and is situated next to the ejection system.

The mold cavity is the space formed when the mold halves are closed. A single mold can contain multiple cavities, with the cavity dimensions determining the size of the green part. The injection unit fills the mold through the nozzle, with the feedstock flowing from the sprue to the runners and then to the gates, which introduce it into the mold cavities. After cooling, the sprue, runners, and gates are filled with solidified feedstock, which is later removed through trimming on separate equipment.

Air vents are included to release trapped gases, and a cooling system helps dissipate heat during the cooling and dwelling phases.

Key parameters in injection molding include clamping pressure, injection pressure, holding pressure, heating zone temperatures, and injection speed. Optimizing these factors is essential for producing defect-free parts.

Debinding

Debinding removes most of the organic binders from the green part, which can degrade its mechanical properties if left. The result of this process is known as the "brown part." This stage leaves a porous structure that is less dense than the final product but retains the geometry from the injection molding phase. The brown part still contains some binders that help hold the metallic particles together. These pores allow the remaining binders to escape during the sintering process through evaporation.

Debinding reduces the compactness and strength of the part, so it requires careful process and handling controls to prevent damage to the molded components.

Debinding can be achieved through several methods, often using a combination of techniques. The choice of method depends on the solubility and decomposition properties of the organic binder.

Thermal Debinding

Thermal debinding involves heating the green part in a controlled oven to eliminate the organic binder. The binder is either oxidized in air or pyrolyzed in nitrogen at the binder’s degradation temperature. This method effectively removes the binder without disturbing the metallic particles' arrangement, but it may leave residual binders that affect the part's properties and induce significant debinding stress.

While thermal debinding uses relatively inexpensive equipment, the process can be lengthy, often taking 24 hours or more to create the necessary interconnected pores in the brown part for residual binder evaporation.

Solvent Debinding

In solvent debinding, the green part is immersed in a solvent bath to dissolve the soluble binder. Common solvents include acetone, trichloroethylene, heptane, and water. While organic solvents are effective, they are often toxic and costly, and their recovery and recycling are common practices to reduce environmental impact and expense.

For binders that are water-soluble, such as polyethylene glycol, water is used as the debinding solvent. Aqueous debinding is advantageous due to its lower cost, ease of handling, and minimal environmental impact. However, it can be water and energy-intensive, may take longer, and produce significant amounts of wastewater.

Solvent debinding generally has lower capital and operating costs compared to other methods but can be slower than catalytic debinding.

Catalytic Debinding

In catalytic debinding, the green part is exposed to an acid gas, such as nitric or oxalic acid, which decomposes the binders. This process occurs at around 1200°C, a temperature below the binder's softening point to minimize thermal defects.

Catalytic debinding is advantageous due to its shorter processing time compared to other methods. However, it is primarily suitable for polyacetal-containing binders and may affect the chemical properties of the metallic particles.

Supercritical Fluid Debinding

Supercritical fluid debinding utilizes a supercritical fluid, often carbon dioxide, to extract organic binders by exceeding its critical point in temperature and pressure. This method is highly effective, economical, and environmentally friendly, with significantly shorter debinding times and reduced defects compared to other methods. It is particularly effective for dissolving non-polar molecules like paraffin wax but may not remove polar or high molecular weight binders. Additionally, the process can cause shrinkage of the green part due to high pressures.

Sintering Process

The sintering phase is the final stage in metal injection molding, where the "brown" part is heated in a specialized furnace. This furnace operates under an inert gas atmosphere to prevent oxidation. The temperature during sintering is carefully controlled to be close to the metal's melting point, promoting partial melting and bonding of the metal particles. Following sintering, the part might require additional processes such as heat treatments, surface finishing, or further fabrication techniques to achieve the desired properties.

The key goals of the sintering process include:

• It eliminates the pore structure in the brown part. Consequently, the part shrinks to 75-85% of its molded size, which is the size of the final part. The shrinkage occurs uniformly and can be accurately predicted.
• It causes the residual organic binders to evaporate from the brown part. The residual binders diffuse through the interconnected pore structure created by the debinding step.
• It imparts strength and compactness as a result of binder and pore removal. The sintered part is denser than the brown part.

Hot Isostatic Pressing

Hot Isostatic Pressing (HIP) is a subsequent process used after sintering to enhance the part's density to nearly its theoretical maximum. This technique aims to minimize porosity and rectify defects such as internal cracks, voids, and surface imperfections. Additionally, HIP improves the material's overall strength, fatigue resistance, and ductility.

During HIP, the sintered component is subjected to high pressure (ranging from 5,800 to 30,000 MPa) and elevated temperatures (up to 3600°F or 2,000°C) within a sealed chamber. The process typically operates at 70-90% of the material's solidus temperature. An inert gas, commonly argon, exerts uniform pressure on the component. This pressure induces plastic deformation, creep, and diffusion, effectively reducing internal voids and enhancing the material's density.

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Chapter 3: What are the advantages and disadvantages of metal injection molding?

Metal injection molding offers several benefits, including:

• MIM can support large production quantities of metal parts with complex geometries and details. It is best suited for high-volume manufacturing of small and precision parts with tight tolerances.
• MIM can accurately produce features such as internal and external threads, undercuts, teeth (e.g., gear teeth), slots, holes, fins, markings, and engravings without the need for secondary machining and fabrication processes.
• MIM imposes few restrictions on the part design. It gives freedom to manufacture a variety of shapes.
• MIM can produce parts with superior mechanical properties. The strength and hardness of MIM parts are comparable to machined wrought alloys.
• MIM gives a good surface finish, though it can be further enhanced.
• MIM can fabricate multi-component parts as a single piece.
• MIM produces less material wastes and scrap than a machining process, which is important for expensive materials such as refractory materials, titanium alloys, superalloys, and specialty metals. This process can convert 95-98% of the material into usable metal parts.
• MIM is less expensive than machining, investment casting, and stamping in the long run.
• MIM can be performed on a wide range of metals, which include:
  • Stainless Steel
  • Carbon Steel
  • Copper Alloys
  • Nickel Alloys
  • Tungsten Alloys
  • Titanium Alloys
  • Cobalt Alloys
  • Iron
  • Carbide
  • Cermet

Metals that are highly reactive, toxic, or prone to oxidation are generally unsuitable for MIM. Lead, magnesium, manganese, and beryllium are examples of metals typically avoided in this process. While some research indicates the potential for using aluminum alloys in MIM due to their lower melting points, they are not yet widely used in commercial applications.

Here are some drawbacks of metal injection molding:

• A MIM operation may require a high capital investment and processing costs. This is due to the acquisition and operation of several machines as there are multiple steps involved in MIM. The highest expense will come from the procurement of the injection molding machine and its mold tool. However, high returns may be achieved when high production volumes are fulfilled and delivered.
• MIM can be expensive for small production demands.
• MIM is suitable for small to medium-sized parts. Shaping large parts can decrease the capacity of the mold and furnaces, making processing costs higher. Few cavities can only fit in a mold if each cavity is large.
• MIM may be a complicated metal fabrication process.

Chapter 4: What considerations should be taken into account in metal injection molding?

When designing for metal injection molding, consider the following factors:

Ejector Marks

Once the part has cooled and solidified, it is ejected from the mold using ejector pins, which can leave visible marks on the surface. Therefore, it is important to strategically position critical features away from these marks during the design process. To reduce the depth and visibility of these ejector pin impressions, pin sleeves can be employed.

Parting Line

The parting line is the boundary where the two halves of the mold come together, and it leaves a visible seam on the surface of the part. This line can be either straight or curved, depending on the mold design. Since the feedstock flows more easily around the parting line due to air venting, all molded parts will have this seam. While a parting line is unavoidable, its placement should be considered carefully to ensure it does not impact the part’s functionality, appearance, or dimensional accuracy. It is advisable to position the parting line along the edges and to avoid placing critical features directly on it.

Mold Gating

A mold gate serves as the entry point for molten feedstock into the mold cavity. It is strategically positioned at the thickest part of the metal component to ensure that the feedstock fills the larger sections first. Since the gate creates a mark on the final part, it's important to evaluate how this mark might affect the part's performance and appearance.

Part Thickness

Maintaining consistent thickness throughout the part is crucial to prevent issues such as sink marks, warping, and uneven shrinkage during the sintering process. Thinner sections of the part will shrink before thicker sections, potentially causing distortion. It’s important to ensure that any changes in thickness are gradual to avoid these problems.

Metal injection molding is effective for creating parts with wall thicknesses between 0.1 mm and 10 mm. Opting for thinner sections can help shorten both the sintering and molding cycle durations.

Corners and Holes

It is advisable to avoid small holes, especially those near corners and edges of the part, as these areas can be prone to void formation. Sharp corners can impede the flow of molten feedstock, so incorporating rounded corners into the part design is preferred for improved fill and overall part integrity.

Producing Undercuts

Metal Injection Molding (MIM) allows for the creation of undercuts without additional machining. To achieve this, a cam mechanism is used during molding. This cam is inserted into the mold prior to closure and then retracted from the green part before ejection. It is important to avoid placing undercuts on internal bores, as this can complicate the molding process.

Sintering Process

Flat components can be easily positioned in conventional flat support trays during the sintering process. However, any hanging sections of the part might droop or collapse under gravity. Therefore, a specialized fixture is required to provide support for these sections.

Conclusion

• Metal Injection Molding or metal injection molding (MIM) is the process of manufacturing metal parts through injection molding technology.
• The stages of a MIM process are feedstock preparation, injection molding, debinding, and sintering.
• The binder is a thermoplastic additive added to the metal powder to make it suitable for injection molding. It is an intermediate ingredient that is removed after shaping the metal (during debinding).
• Metal injection molding takes place in a standard injection molding machine consisting of a clamping unit, an injection unit, and a mold tool.
• The debinding methods employed in MIM are thermal debinding, catalytic debinding, solvent debinding, and supercritical fluid debinding.
• A hot isostatic pressing (HIP) step may be employed to increase the part's density further and eliminate defects such as voids, cracks, and pores.
• MIM can support the high-volume manufacturing of geometrically complex parts with intricate details. It offers design flexibility, produces parts with superior mechanical properties, and can be performed on a wide range of materials. It may be less expensive compared to investment casting and machining.
• However, MIM may require a high capital investment and processing costs and is not suitable for small production runs. MIM can be a complicated metal fabrication process.
• Ejector marks, parting line, gating, part thickness, corners, holes, and undercuts must be considered during the design phase. Fixtures must support hanging sections of the parts to prevent sagging.

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