Note: This is the third of a three-part series covering metal injection molding (1) trends, (2) material/process advances and (3) applications.

Metal injection molding (MIM) relies on a thermoplastic polymer blend filled with about 60 percent by volume of a small metal powder. This mixture of polymer and powder is injection molded to form a complex shape. Once molding is completed, the polymer (binder) is extracted, and the small powder is sintered.

Sintering is a high-temperature heat treatment designed to induce densification of the particles. Accordingly, the final product is typically 15 percent smaller than the tooling, but densified to a level where the mechanical and physical properties rival wrought metal materials.

Although numerous complex geometries can be fabricated via MIM, only certain component characteristics prove cost-effective. The small powders are more expensive than wrought materials, so there is an initial material cost penalty.

Early identification of designs that match well with the MIM technology helps ensure economic success. Typical considerations involve material, properties, component size and shape, tolerances, production cost, production quantity, and design features. For example, MIM excels at forming shapes with dead-ended holes, dovetails, slots, threads or curved surfaces.

As a starting point in considering MIM, the upcoming figure summarizes the typical, minimum and maximum attributes. Some explanation is in order.

In practice, there are many technology variants — powder types, binder formulations, debinding techniques and sintering furnaces. Such variation affects what is possible in terms of each company's capabilities. Accordingly, there are significant producer-to-producer differences, largely dependent on the age of the equipment.

MIM capabilities.

Let's start by looking at a cellphone hinge barrel, knuckles and cam MIM electronic application development. Parmatech Corporation's cellphone hinge barrel/knuckles/cam for end-user Motorola is an assembly of four parts that function in the flip assembly of an older Motorola Model V60C cellphone.

The parts have thin walls with highly complex geometries making them difficult to manufacture economically by any process other than MIM. They are made to a density of 7.6g/cm³ (grams per cubic centimeter).

The cam and knuckles are made from 17-4PH stainless steel powder and have a minimum tensile strength of 793 MPa (MegaPascals,115,000 psi), minimum yield strength of 648 MPa (94,000 psi) and 4 percent elongation. The hinge barrel, made from 316L stainless steel powder, a nonmagnetic alloy, has a minimum tensile strength of 448 MPa (65,000 psi), minimum yield strength of 138 MPa (20,000 psi) and 40 percent elongation.

Parmatech Corporation
Cellphone hinge barrel, knuckles and cam.

The right and left knuckles are assembled at the opposite ends of the hinge barrel to form the flip assembly mechanism. The oval hole on the right knuckle houses a light pipe, and the slot on the left knuckle is a conduit for wiring between the cell phone base and flip assembly.

Except for the cam, the knuckles and hinge barrel manufacturing systems are made via high-volume flow of parts through batch solvent, debinding, sintering and post-sintering. The cam and knuckles are made to a net shape. The length and slot diameter of the hinge barrel are coined, and the slot-end tab is twisted via an automated device.

Several hundred thousand parts are produced monthly under stringent process control conditions while maintaining a quality standard rate of cp of 2.0 and cpk (process capability) of 1.5. The length and slot diameter of the hinge barrel are coined, and the knuckles are buffed to a Class A surface finish. The cam and hinge barrel are nickel-Teflon plated for lubricity and wear resistance.

Next, a needle drive and distal clevis assembly is a good example of a MIM medical application. Smith Metal Products' needle drive and distal clevis assembly for end-user Intuitive Surgical is a needle driver and distal clevis made from 17-4PH stainless steel powder.

Smith Metal Products
Needle drive and distal clevis assembly.

The parts function in a minimally invasive endoscopic daVinci robotic surgical system. The high-precision robotic system performs complex surgical maneuvers, as dynamically controlled articulations provide the dexterity of the human wrist at the instrument tip.

The needle driver, while inserted into the distal clevis, sutures incisions during general laparoscopic surgeries. The two halves of the blank grip are mated and machined by the customer to the desired shape. Actuating cables are inserted through holes above the pivot point of the needle. The distal clevis is supplied as a net shape except for a final surface finishing operation.

The parts are made to a density range of 7.68-7.72g/cm³ (grams per cubic centimeter). The distal clevis has 35 - 38 HRC hardness and an elongation of 10 percent. Tensile yield strength is 1100 MPa (160,000 psi). The needle driver has a 38 - 42 HRC hardness range and an elongation of 8 percent. The tensile yield strength is 1070 MPa (155,000 psi).

Metal injection molding offered a cost savings of 90 percent compared to CNC machining the parts from bar stock.

Continuing, a good industrial example is illustrated by a pump body and cavity plates. Philips Medisize's industrial pump body and cavity plates for end-user Honeywell consists of complex 316L parts that provided improved reliability and simplified assembly over previously machined pumps.

Phillips Medisize
Industrial pump body and cavity plates.

Using metal injection molding minimized fluid sealing interfaces down to one O-ring seal for the basic pump. Molding flow passages of any cross-sectional profile allowed maximizing the flow area within a confined space, minimizing outlet and inlet flow velocities and pressure losses in the body, reducing overall pump dimensions.

MIM provided a surface finish of 16-32 RMS (micro inches). Produced to a density of 7.85 g/cm³ (grams per cubic centimeter), the parts have yield strength of 200 MPa (29,000 psi), an ultimate tensile strength of 550 MPa (80,000 psi), 70 HRB hardness level, and an elongation of 50 percent. Cost savings for the MIM pump body over the machined part exceeds 35 percent.

In conclusion, MIM is attractive because it produces consistent, complex-geometry components for high-volume, high- strength and high-performance applications. The driving force behind MIM's growing popularity in the injection molding community is that it allows molders to broaden the services and capabilities they offer end users.

Experts say molders can sometimes be frustrated trying to meet customers' needs with only thermoplastic materials and processes. These customers may require better properties than plastics alone can offer.

If their existing customer base requires MIM-type parts and plastic molders are not afraid of the capital outlay, steep learning curve and increased operational cost, metal injection molding can be an interesting possibility. A processor may want to consider buying a metal injection molding operation, or a joint venture with a current supplier.