This article takes an in-depth look at Metal Injection Molding.
Read further and learn more about topics such as:
- What is Metal Injection Molding
- Stages of a Metal Injection Molding Process
- Advantages and Disadvantages of Metal Injection Molding
- Considerations in Metal Injection Molding
- And Much More…
Chapter 1: What is Metal Injection Molding?
Metal Injection Molding is the manufacturing of solid metal parts utilizing injection molding technology. It is also known as metal injection molding (MIM) and powder injection molding (PIM). It was first developed for shaping ceramic parts. In the 1970s, this technique was developed by Raymond Wiech to allow the processing of metallic materials. Metal fabricators have been widely adopted since the 1990s.
In metal injection molding, finely powdered metal is mixed with a plastic binder to produce a feedstock suitable for injection molding. The powdered metal and binder mixture is melted, shaped, and solidified in a standard molding machine. The molded parts are then subjected to binder removal and sintering processes to remove the plastic binders and increase the density or compactness of the metal parts. These post-molding processes also ensure the metal part’s right geometry, composition, and physical properties.
Metal injection molding is used in the high-volume fabrication of small, geometrically complex parts. It can precisely produce parts with delicate and intricate details without machining. It can be used with a variety of ferrous and non-ferrous metals. This process is also more economical than forging, casting, and machining processes. The parts produced from the MIM process are used in numerous industries, such as automotive, aerospace, electronics, telecommunications, medical, dental, sporting goods, consumer products, and weapons.
Chapter 2: Stages of the Metal Injection Molding Process
The steps involved in the metal injection molding of metal parts are the following:
The metal powder is mixed with a thermoplastic binder to create an intermediate product called the “feedstock” suitable for injection molding. The binder plays a vital role in metal injection molding that 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 is made of a mixture of organic polymers, which materials researchers and manufacturers formulate. Typically, there are three types of polymers blended in a MIM binder:
- 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 degrade the mechanical properties of the metal if it stays in the product. Hence, it is removed after shaping the metal. The characteristics of a suitable binder for MIM are the following:
- 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.
The feedstock preparation starts with the powdered metal and the suitable binder. The size of powdered metal is 20 microns in diameter, which is finer than those used in conventional powder metallurgy processes. Precise amounts of these materials are combined to achieve a metal-to-binder volume ratio of 60:40. These materials are mixed at an elevated temperature to produce a homogenous feedstock. They are then fed to a granulator and cooled to form the feedstock pellets acceptable by the injection molding machine.
Metal injection molding takes place in standard injection molding machines similar to those used in manufacturing plastic parts. The homogenized feedstock pellets are melted and injected into a mold. The melt takes the shape and volume of the mold cavities. The molten feedstock cools and solidifies inside the mold cavities. However, the volume of the mold cavities must be larger than the size of the final product to compensate for the shrinkage during sintering. The products from the injection molding step are called “green parts.” The green parts are 20% larger than the final part, but they are geometrically comparable. The green part is like a “scaled up” version of the final part. The green parts are ejected from the mold after a sufficient dwelling time. The excess materials resulting from the flow of the molten feedstock are finally separated from the part.
The standard injection molding machine consists of the following units:
The clamping unit is responsible for generating and applying sufficient clamping force to keep the mold halves closed during the feedstock injection and part dwelling. It houses the ejection system, which removes the green parts after dwelling on the mold cavities. It is also responsible for opening and closing the mold halves between molding cycles and maintaining their proper alignment during operation.
The injection unit is responsible for heating and injecting the feedstock into the mold cavities. The injection unit consists of 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 gives the feedstock its shape. It is divided into mold halves. The front mold half is stationary and adjacent to the injection unit. The rear mold half is attached to a movable plate that opens and closes the mold and is adjacent to the ejection system.
The mold cavity is the space created when the mold halves are closed. A single mold tool can have multiple mold cavities. The dimensions of the mold cavity give the dimensions of the green part. The injection unit fills the mold with the shot. From the nozzle, the viscous feedstock flows to the sprue, to the runners, and finally to the gates. The gates introduce the feedstock to the mold cavities. After the molding cycle, the sprue, runners, and gates are filled with the solidified feedstock. The excess materials formed due to the feedstock flow in these channels are removed by trimming. Trimming takes place on stand-alone equipment.
Air vents remove the entrapped gasses inside the mold. A cooling system is also present to dissipate heat during cooling and dwelling.
The critical parameters in injection molding are clamping pressure, injection pressure, holding pressure, heating zone temperatures, and injection speed. They must be optimized to ensure defect-free parts.
Debinding decomposes and removes most of the organic binders from the green part. The binders degrade the mechanical properties if left on the metal part. The resulting products from this step are called the “brown parts.” The brown part has an interconnected pore structure and is less dense than the final part. However, the geometry obtained from the injection molding step is retained and not altered. The brown part still has some binders that hold the metallic particles together. The porous structure allows the escape of the remaining binders during sintering through evaporation.
Debinding decreases the compactness and strength of the part. Hence, it must be performed with the careful process and handling controls to avoid damage to the molded parts.
Debinding is accomplished by the below methods. A combination of these methods is sometimes employed. The method is chosen based on the solubility and decomposition properties of the organic binder.
In thermal debinding, the green part is heated in a temperature-controlled oven to remove the organic binder. The part is oxidized in the air or pyrolyzed in nitrogen at the degradation temperature of the main binder. The advantage of this method is that the binder is removed without disrupting the packing of the metallic particles. However, it can leave residual binders that degrade the part's properties. It can also induce high debinding stress.
Though it only requires a less expensive oven, thermal debinding can take a long time (which lasts 24 hours or more) to create the interconnected pores on the brown part necessary for the evaporation of the residual binder.
In solvent debinding, the green part is immersed in a solvent bath to dissolve the soluble binder. The common debinding solvents are acetone, trichloroethylene, heptane, and water. However, organic solvent binders are toxic to the environment and expensive. The solvent is often recovered from the spend debinding solution and recycled for another debinding process.
Water-soluble binders (e.g., polyethylene glycol) use water as the debinding solvent. Aqueous debinding is preferred because of its cost, ease of handling, and minimal effects on the environment. However, it may consume a lot of water and energy, take longer, and generate substantial amounts of wastewater.
Solvent debinding has lower capital and operating costs than other debinding methods. However, it usually takes longer than catalytic debinding.
In catalytic debinding, the green part is exposed to an acid gas (e.g., nitric acid, oxalic acid). The acid gas decomposes the binders present in the part. The decomposition is carried out at around 1200C, which is below the softening temperature of the binder, to reduce the thermal defects on the part.
Catalytic debinding has a much shorter processing time compared to other debinding methods. However, it is only suitable for polyacetal-containing binders. It can also affect the chemical properties of the metallic particles.
Supercritical Fluid Debinding
Supercritical fluid debinding is the process of extracting the organic binders using a supercritical fluid. A supercritical fluid is formed by increasing the pressure and temperature of a specific fluid higher than its critical point. Carbon dioxide is commonly used as the supercritical fluid because it is economical and environmentally friendly. Supercritical fluid debinding is highly effective and claimed to have a much shorter debinding time than other methods and reduce the debinding defects. It can dissolve non-polar molecules such as paraffin wax, a widely used primary binder. However, it cannot remove polar and high molecular weight binders. Moreover, the green part may shrink since it is subjected to high pressures.
Sintering is the final step of the metal injection molding process, in which the brown part is heated in a furnace. The furnace has an inert gas atmosphere. The sintering temperature is set near the melting point of the metal to induce partial melting (liquid phase sintering). The sintered part may be subjected to additional heat treatment, finishing, and other fabrication processes like cast and wrought parts.
The objectives of the sintering step are the following:
- 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 secondary process after sintering. It is employed to increase the density of the part up to 100% of the theoretical density of the material. It further reduces the porosity of the part and eliminates defects such as internal and external cracks, voids, and pores. It also increases the strength, fatigue resistance, and ductility of the material.
In this process, the sintered part is compressed at high pressure (5,800 to 30,000 MPa) and high temperature (up to 3600 °F [2,000 °C]) in a gas-tight chamber. The HIP temperature is 70-90% of the solidus temperature of the material. An inert gas, usually argon, compresses the part. The gas pressure acts uniformly on all surfaces of the material. The densification mechanisms during HIP are plastic deformation, creep, and diffusion. Since the gas pressure is higher than the yield strength of the material, the part will undergo plastic deformation, and its internal voids will collapse. Creep and diffusion mechanisms close the remaining pores in the part.
Chapter 3: Advantages and Disadvantages of Metal Injection Molding
The advantages of metal injection molding are the following:
- 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
Metals with higher melting temperatures (e.g., stainless steel, carbon steel) are ideal for MIM. However, metals that are toxic, easy to oxidize, reactive, or volatile must be avoided. The MIM of lead, magnesium, manganese, and beryllium are avoided. Some studies suggest the feasibility of MIM of aluminum alloys, which melts at a relatively low temperature, but it still has not been employed in commercial mass production.
The disadvantages of metal injection molding are the following:
- 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: Considerations in Metal Injection Molding
The following are some design considerations in metal injection molding:
After the part has solidified and dwelled, it is removed from the mold by ejector pins. These pins leave marks on the surface of the part. Hence, the location of critical features in the part must be considered during the design phase. The critical features must be away from the ejector marks. Ejector pin sleeves may be used to minimize the depth and appearance of the mark.
The parting line is a separation line that indicates the plane where the mold halves meet. A visible line is imprinted on the part's surface, which coincides with the parting line. Depending on the mold tooling design, it may be a straight line or a curve. The molten feedstock tends to move out of the parting line because air is easiest to vent on that location. All molded parts have a parting line, and having one is inevitable. Having a parting line may be harmless, but its impact must be assessed if it affects the part's functionality, form, and geometric tolerance. A parting line may be concealed by placing it on the edges of the part. Locating critical features on the parting line must be avoided as much as possible.
A mold gate is an opening wherein the molten feedstock is introduced to a mold cavity. The gate must be placed on the portion of the metal part with the largest cross-sectional area so that the thicker sections will be filled with the molten feedstock first. The gate also leaves an imprint on the metal part; hence, the impact of this imprint on the functionality of the part must be assessed.
As much as possible, uniform part and wall thickness should be maintained to avoid sink formation, warpage, and shrinkage during sintering. Thinner sections are sintered first before the thicker ones, resulting in distortion of the part. The change in part thickness must be gradual.
MIM is suitable for parts with wall thickness ranging from 0.1 mm to 10 mm. Molding of thinner parts can reduce the sintering and molding cycle times.
Corners and Holes
Small holes and holes located near the corners and edges of the part must be avoided. Sharp corners can favor the formation of voids as the molten feedstock may not reach these portions; hence, this feature must be avoided. Rounded corners in the part design are preferred.
Undercuts can be readily made through MIM without the need for machining. A cam action is required to produce undercuts. It is placed in the mold before it closes and slides away from the green part before it is ejected from the mold. Putting an undercut on internal bores must be prevented.
Flat parts may be readily placed in standard flat support trays during sintering. Hanging sections included in the part can sag or collapse due to gravity. Hence, a custom fixture to support these sections is necessary.
- 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.