Injection molding metal is not the most common way to produce a three-dimensional metal part. The traditional methods – machining, forging, and, the most popular, casting – still hold sway most of the time.

But, molding metal is a sometimes-overlooked concept that can be a viable option for designers looking for ways to make a component that combines the benefits of metal’s mechanical properties with the design flexibility of an injection molding operation. This is especially true for high volume, complex parts that maybe too small or too difficult to do easily with the more conventional methods.

Within the umbrella of injection molding metals are different technologies. The most mature of these technologies is metal injection molding, often referred to as MIM, which debuted in the U.S. in the 1980s. Today, a number of custom molding companies offer this capability with materials such as low alloy steels, stainless steels, iron-cobalt-nickel based, and copper based metals.

These parts were made with the metal injection molding process by Phillips Plastics.

A decade after that arrival, a new molding technology emerged that focused exclusively on magnesium as a material. The magnesium injection molding process is also known as Thixotropic molding or “Thixomolding,” And was developed by Thixomat, Ann Arbor, Mich., which licenses the technology to other companies. Most recently, a third method hit the market that uses a special metal alloy developed by Cool Polymers, Warwick, R.I., that can be used in a standard injection molding machine without any preprocessing steps and does not require special equipment.

While metal molding methods are not as widespread as other processes, they are being used to make millions of parts in a variety of industries. Phillips Plastics, the Wisconsin based molding company with both magnesium and metal injection molding capabilities, has produced parts for appliances, consumer electronics, lawn and garden tools, medical devices, power tools, and military products, says Wade Cullen, plant manager for Phillips’ magnesium facility. The company has worked on a leaf blower for Toro, speech-generating devices for DynaVox Technologies, and housings for ruggedized military computers for VT Miltope, a Vision Technology Systems company. Some products used multiple technologies. For the computer, some components that needed to be the most durable were made using the metal injection molding process. Other computer components where weight and durability were paramount were made using the magnesium process. Magnesium’s lightweight and inherent shielding properties were the reasons that DynaVox turned to the material for its speech recognition systems.

Another custom molder that uses both technologies is Advanced Forming Technologies of Firestone, Colo. Applications included the housing and components of a reciprocal saw, which is made from magnesium, and internal components of the saw, which were made with metal injection molded parts.

Cool Polymer’s metal alloy can be injection molded in a standard injection-molding machine.

Perhaps the biggest applications for the Thixomat process are in personal electronics such as a cell phones and cameras, says Steven LeBeau, president of Thixomat. One of its earliest applications was the Mitsubishi ultrathin Pedion subnotebook. The magnesium-encased computer had a total thickness of 18 mm and weighed less than 3 lbs.

The versatility of applications is common to these three different methods. Despite their successes, one might ask why one would switch from the well-established methods for making three-dimensional metal parts. The answer varies by method, volume, part size, accuracy, and other factors. Matthew Bulger, plant manager for Netshape Technologies, Sellersburg, Ind., a company that specializes in metal injection molding, says that metal injection molding costs less than CNC machining, and produces better and more intricate feature detail, and thinner walls, than does die casting. He adds that this process is best for small parts with very complex shapes and thinner walls, which cannot be stamped, screw machined, or made with powdered metallurgy.

The magnesium process also compares favorably to die-cast magnesium parts, creating parts that have tighter dimensional tolerances, and better tensile strength, yield strength, elongation at break, and density.

Parts made with traditional thermoplastic and Cool Polymer’s Xyloy metal alloy.

The molding methods can create near net or net-shaped parts. But, other technologies can also do that. High-end CNC machines can also create very accurate, very intricate parts. In this case, the main advantage of molding complex parts is in the reduced processing costs; a CNC machine might be required to make many passes to create the part, says David Smith, plant manager for Advanced Forming Technologies, and still need to do some secondary machining operations.

The design flexibility afforded by a molding program is also another benefit as compared to other methods. A rule of thumb is that if you can make a plastic part on an injection-molding machine, that component could be converted to a molded metal part.

Parts made from lighter materials such as magnesium are also being used as a replacement for plastic. One appliance manufacturer has switched from a plastic housing to metal because it is more durable. In this case, the metal’s mechanical properties outweighed the economic justifications for plastic. While plastic costs were lower in this case, the metal parts’ advantages in terms of strength, inherent EMI shielding, thermal resistance and conductivity combined to sway the manufacturer to change the material in its product.

Enlarge this picture A comparison of the mechanical properties of parts made through thixomolding as compared to die-cast magnesium AZ91D. Source: Advance Forming Technology

Choosing between the three methods can be a simple matter, or one with a great many variables. If a part must be made from stainless steel due to thermal issues or other factors, than metal injection molding is the only option among metal molding processes. The other two molding methods are limited in material selection: the Thixotropic technique only uses magnesium and Cool Polymer’s alloy is a zinc and aluminum blend.

However, choices are not always so cut and dry. Cost is obviously a big concern and metal injection molding is generally more expensive than the other technologies. The cost of the metal powders, which can be as much as three times that of magnesium, as well as the metal injection molding process itself, can be cost prohibitive and typically requires higher volumes of parts to cover these extra costs.

Material costs are one reason that smaller parts are best suited to the metal injection molding process. As the size of the part rises, so do the extra material costs, as well as the fewer parts per batch that can be processed. At Phillips, the largest metal injection molded part was only 150 grams. Compare this to the magnesium process, which might be as heavy as 10 lbs. Cool Polymer’s metal alloy can be used on parts weighing a few grams to several pounds. The physical dimensions of a metal injection molded part depend on a number of factors including wall thicknesses. While each application differs, Pelke says that the length to width ratio should be no greater than 10 to 1. Because each application differs, he says the best rule of thumb is to work early in the process with the supplier to best optimize design.

Enlarge this picture Comparing thixomolding to die cast parts by factors such as complexity and part cost on a 1-5 scale (1=poor, 5=excellent) Source: Advance Forming Technology

Process steps and special equipment can also be a consideration as they vary from method to method. Cool Polymers’ process is the simplest of all. Its metal alloy can be used in any standard injection-molding machine, using the same tooling as that used for plastic molding. For its part, the magnesium injection molding process occurs inside a specially built molding machine that automatically softens the magnesium, flows it into the mold cavity, and forms the part.

Metal injection molding, on the other hand, requires several steps and can have higher cycle times and increased material handling. Material handling is typically done through an automated, robotic system, however. The metal injection molding stages include compounding metal powders, in particles measuring about 20 microns big, with a plastic binder in an approximately 60-percent metal to 40-percent plastic-binder ratio. The material is loaded into an injection molder in which the plastic melts and flows as in a traditional injection molding process. At this stage, the molded component is called a green part because it is in the shape of the finished part, but hasn’t been sintered into its final size and consistency and the plastic binder has not been removed.

The part still consists of a 60/40 metal-to-plastic ratio. To remove the binding agent, the green part is placed into a debinding machine, which extracts most of the plastic material. The extraction is typically done thermally in debinding furnaces, but can be done with solvents and degreasers.

Enlarge this picture The metal injection molding process allows for the use of many types of metals as seen in this material list. Source: Phillips Plastics

When removing the plastic, voids in the part are created. To close the voids, the part is put in a sintering furnace where it is brought near the melt temperature of the metal. (Stainless steel 17-4, for instance, would be sintered at about 2,100 DegF.) The high heat fuses the powder particles and helps to close most of the voids. Smith likens this process to two beads of water that are near each other and through surface tension becomes one bead of water.

During the sintering process, as the voids condense, the part shrinks. Shrinkage rates can vary, but commonly parts shrink about 20 percent. The shrinkage rates are predictable, however, and designers simply include this shrinkage rate into their part size specifications. The process is very similar to traditional powder metallurgy, also known as press and sinter, which also uses powdered metals and sintering. Compared to traditional powder metallurgy, however, metal injection molding can mold geometries that eliminate secondary operations, and offer superior density, corrosion performance, strength, and ductility.

The resulting finished part is in the form of a near-net or net-shape part, says Tony Pelke, engineering manager for Phillips Plastics’ metal injection molding facility. Pelke adds that the molding machine’s ability to do multi-cavity parts can lead to very complex parts. It also can help reduce a multiple-piece assembly, by eliminating individual parts that had to be mechanically assembled and replacing them with one fully molded metal part. In one case, a medical device went from a 9-piece assembly to a single-piece assembly. Previously, the parts were made by machining them one at a time and in succeeding operations are assembled using mechanical fasteners.

Enlarge this picture Comparing metal injection molding to other methods to make metal parts. Source: Advance Forming Technology

This flexibility is one reason that molding is popular for producing parts no matter what its material composition. A big advantage for metal injection molding, is the sheer number of materials with which it can work. The most commonly used elements in metal injection molding processing are iron, nickel, chromium, and molybdenum. Iron and nickel are two of the easiest elements to process because of their compatible melt temperature and ease of sintering.

While density rates can vary by material, metal injection molded parts on average are not as dense as with the magnesium process. With metal injection molding, the part can be between 94 and 99 percent dense. This can be improved through a secondary process called “hipping,” or a hot isostatic process, in which the part is placed in a large pressure vehicle, heated to near melt and subjected to extremely high pressures to compress out remaining voids.

The magnesium process makes parts that are about 100 percent dense right out of the molding machine. With the Thixotropic process there are no voids as with the metal injection molding process. Magnesium chips are heated to get the material to a thixotropic state, which is a gel-like consistency that becomes more fluid under force or pressure. Unlike die-casting, it does not require the handling of molten metals in a separate melting and transfer system. While in this gel-like state, the material flows into the tooling cavity for molding. The tooling is similar in cost and construction to plastic injection or die-cast tooling and lifters, slides and other actions can be designed into the tooling, says LeBeau.

Phillips built the external housings for the portable speech generating devices from DynaVox Technologies. Magnesium was used because of its durability, lightweight, and inherent shielding properties.

Pelke says that this technology is a good solution for companies looking to migrate their products from plastic to metal. By converting from plastics to magnesium, the finished part will be lighter, thinner, and stronger. Magnesium is a great choice, he says, for any manufacturer that is pushing the limits on plastics, in terms of higher temperatures, heat dissipation, heat conduction, heat resistance, EMI shielding, and other factors.

Because it is a magnesium part, it is lightweight, and strong. It can also create very thin walls, as thin as 0.5 mm. In one example, a computer housing that had a 1.5 mm thick plastic housing was replaced with a 1 mm thick magnesium housing and got greater strength during impact tests.

One drawback of the process is that it produces parts that can creep at elevated temperatures. According to LeBeau and others, creep can cause a magnesium component to deform under load at elevated temperatures (250 DegF) over extended periods of time, even if the stress applied is below the yield stress of the alloy.

The metal injection molding production line at Advanced Forming Technology.

The company is also working to expand its material selection. While currently Thixomolded parts are made solely from magnesium feed stock, the company is researching the potential for using zinc and aluminum metal materials.

As it happens, zinc and aluminum are the components that make up Cool Polymer’s metal alloy blend. What sets apart Cool Polymers’ metal alloy from the other molding processes is its ease of use. While similar to metal injection and magnesium molding, the Cool Polymers alloy requires no special equipment beyond a traditional injection molding machine and the tooling. Designs that currently use plastic or even die-cast metal are target applications.

According to Jim Miller, Cool Polymers’ product manager, what separates this method from the others is the fact that it uses a standard injection molder and the infrastructure and knowledge base of plastic injection molding is already in place. Because of this, Miller is confident that the alloy’s use will continue to grow.

This drive gear for a copy machine was developed using Thixomat’s magnesium molding process.

While there is a kinship with plastic injection molding, they are two different processes. For instance, design engineers should be aware of the thermal conductivity differences between plastic and metal, which can affect such things as cool down rates in the cavity and heat transfer in the molding machine’s barrel. These differences can cause flow and freeze-off problems, especially if the part to be molded is long and thin. Both the mold maker and the designer can play a part in helping to deal with this challenge. Designers can make for short sections or, in the case of a larger part length, design in supporting structures. The molder either has to get into the tool cavity quicker or the Delta T that exists between the melt temperature and the tooling temperature has to be increased.

When it comes to tolerances, Miller says that it is part dependent, but is on the order of thousandths of an inch, per inch. Relative to plastics, the M950 is more controllable because the shrinkage rate is predictable and low, about 0.5 percent, which is at the low end of what is seen from plastics. The material is isotropic, and will not undergo the residual stresses that can cause warpage in certain plastics. In the long run, Miller says this will create flatter parts.

Part complexity is similar to injection molding. It allows for thin walls and deep draws.

The mechanical properties of the new Cool Polymers’ material include a tensile strength of 325 MPa; yield strength of 250 MPa; elongation at break of 2 percent; modulus of elasticity of 90 Gpa; specific heat capacity of 0.1 BTU per pound DegF; and thermal conductivity of 110 w/mk. Thermal conductivity is important for heat sink applications as well as “any electrical product or device with electronic components that are likely to generate heat. The alloy can help manage the heat,” Miller says.

Phillips Plastics’ made this impeller for a Toro leaf blower using the magnesium, or Thixomat, process.

To this point, the Xyloy M950 is Cool Polymer’s only offering, but new materials are under research including light metals such as magnesium and aluminum. Miller says the company’s long-term plan is to sell the material, as other companies sell plastic resins to OEMs and injection molders. At this point, they are doing some manufacturing in house for customers, while shipping material to a few manufacturing customers. Miller adds that these are not beta sites, but full-fledged production operations.

Production operations any of these three molding methods may have been slow in coming, but with new materials and expanded molding installations, the pace of adoption is expected to quicken. What once was an overlooked capability may soon gather steam and be as well known as the traditional methods that came before it.

For more information, email:

Advanced Forming Technologies: DSmith@pcc-aft.com
Cool Polymers: jim.miller@coolpolymers.com
Netshape Technologies: mbulger@netshapetech.com
Phillips Plastics: leslie.lagerstrom@phillipsplastics.com
Thixomat: hpritzker@thixomat.com