Bringing Metal Parts to Life with Complex Geometry and Precision Tolerances

Phillips-Medisize CorporationApril 2016

Bringing Metal Parts to Life with Complex Geometry and Precision Tolerances 2

IntroductionMetal injection molding (MIM) is an effective way to produce complex and precision-

shaped parts from a variety of materials from low to extremely high volume capabilities

which can be produced cost effectively and with significantly reduced lead-times.

MIM Process OverviewMetal Injection Molding (MIM) is the process of producing a complex, net shape metal

components using injection molding technologies. It involves converting metal powders

to behave like a plastic by mixing them with polymer binders to form a feedstock which

is a pelletized blend of ~60% metal powder and ~40% polymer powder by volume,

and is molded in a machine and auxiliary equipment in a process very similar to that

of plastic injection molding to provide a “green” part. The green part is then processed

through de-bind and sintering process in which the polymer powder is removed, result-

ing in 14 – 22% linear shrinkage and a theoretical density of 97 – 99%. The shrinkage is

consistent to .3 – .5% of each specified materal. De-binding and sintering may be done

in batch systems, typically for smaller volumes, larger parts or less common materials,

or in continuous systems for larger volumes or common materials.

In many cases, MIM can be used to produce parts much more economically than with

the CNC machining or investment casting methods; in addition, parts have improved

surface finish and metallurgical properties over competing metal forming processes. The

ability of the MIM process to support mid-to-high volumes of mid-to-high geometric

complexity has resulted in significant growth of applications in the medical, automotive,

consumer, industrial and defense industries, especially in the last five years.

Common MaterialsMIM is suitable for a variety of materials ranging from low-carbon steels, stainless steels,

soft magnetic alloys, to other specialty alloys. While some MIM processors procure powders

and compound the feedstocks in-house, others utilize production-ready feedstocks from

sources such as the Catamold® line-up by the industry’s longstanding innovator, BASF.

Competing ProcessesWhen selecting a manufacturing process, MIM is most often compared to CNC machining,

investment casting, or conventional “press & sinter” powder metallurgy. MIM offers the

following advantages compared to these processes:

• CNC machining: MIM has greater ability to provide harder materials, is more cost

effective as volumes increase, and results in lower weights due to the additive

nature of injection molding vs. the subtractive nature of CNC machining.

• Investment casting: MIM allows for thinner wall sections, and provides a better

surface finish; it requires less secondary machining and is better suited for higher

volume manufacturing.

• Powder metallurgy: MIM allows for greater part complexity and a thinner wall

section, and provides higher density, higher strength parts with better corrosion

resistance.

Metal injection molding can

produce relatively small, highly

complex geometries with excellent

surface finish, high strength, and

superior corrosion resistance.

Bringing Metal Parts to Life with Complex Geometry and Precision Tolerances 3

While MIM is cost competitive on mid-to-high complexity shapes and for volumes as

low as 2,000 pieces annually, the process does not lend itself to competition with parts

that can be stamped or screw machined, or parts with a simple geometry designed for

CNC machining.

Secondary OperationsPhillips-Medisize can provide secondary operations to meet an array of specific

requirements. With typical tolerances for the MIM process within 0.003 to 0.005

inches per inch, (0.3-0.5%), many parts are sintered to final dimensions. If tighter

tolerances are required in certain areas, secondary machining operations can be

applied. Tapping operations can produce internal threads with tolerances tighter than

can be achieved via the metal injection molding process. Tumbling and polishing can

provide an aesthetic surface. Parts can be heat-treated, coated, and plated in similar

fashion to investment cast or machined parts. Suppliers with ITAR (International

Traffic in Arms Regulations) registration should be considered for firearms and

defense programs.

Applications for MIMMIM has experienced rapid growth in medical, automative, consumer, industrial and defense

applications over the past 5 years. This growth has been driven by two main factors:

1. Increased emphasis on cost reduction, without sacrificing quality has led

companies to seek alternatives to traditional processes such as CNC machining.

2. Increased understanding of the MIM process by designers, who have subse-

quently designed new products to leverage the MIM process and its inherent

advantages.

Today, MIM can be found in many areas such as:

• Defense: firearms

• Automotive: turbochargers, fuel pressure regulators, fuel injectors, transmission

components, rocker arms

• Consumer: cell phone hinges and clips, hand tools

• Industrial: punch down tool, bobbins, door locks, analyzers

Manufacturers using the MIM process expect to see continued future growth in all of

these areas as designers gain more experience and become more comfortable designing

for MIM. However, the greatest growth is expected to be in areas with volumes which

support the tooling investment required for MIM.

Metallurgical capabilities allow for

the maintenance of tight control of

all aspects of the MIM process.

MIM Process

Bringing Metal Parts to Life with Complex Geometry and Precision Tolerances 4

Factors of Successful MIM ApplicationsWhen developing products, designing for the MIM process requires specific knowledge,

similar to the way in which products are designed for plastic injection tooling and mold-

ing. Design considerations determined up front during the initial product development

will ensure the part is optimized for the MIM process and tight tolerances, which may

be beyond the capability of the process, are minimized.

The most common design considerations for MIM are:

• 0.1 – 30 g finished part weight – Generally speaking, MIM parts are in this size

range. Larger parts are less suitable and may be more cost effective to produce

with an alternate metal forming process.

• Part geometry that fits inside a tennis ball – Generally speaking, MIM parts

are of this order of size. Larger parts are less suitable and may be more cost

effective to produce.

• Uniform wall sections of .03 – .25” – This is variable. A consistent wall section

is critical. Wall sections can be thicker or thinner based on the size of the part

and where the thick or thin wall section is located near the gate.

• Draft angles of 0.5 – 1 degree to aid part ejection – This aids part ejection and

minimizes part distortion during the molding process.

• Dimensional tolerances of 0.5% to achieve capability – In other words, a toler-

ance over 1” would be .005” to achieve a capability of 1.33 CPK. A two inch

dimension would require .010” to achieve the 1.33 CPK. The tightest tolerance

we can achieve within capability is .0015” for any given dimension to ensure

capability. Anything tighter would require secondary machining operations.

• Corner radii to reduce stress – Generous radii at transitions eliminate stress

and distortion potential.

• Geometries which support the component through the high temperature sintering

process, in order for the part to shrink and densify to the final dimensional require-

ments. A flat sintering support is beneficial for dimensional stability and control. It

is not necessarily required for very small components of less than 1 gram.

• Threads formed by unscrewing cores in the tooling are a possibility, and can

result in significant cost and lead time savings by eliminating secondary tapping

operations. This option is dependent upon the volume and adds a significant

amount of cost to the tooling. It is therefore only practical for higher volume

applications.

• Annual volumes as low as 2,000 units, but more commonly 5,000 to several

million units annually. The higher the volume the quicker is the pay back for

the customer-purchased tooling. Generally speaking, 10,000 pieces annually is

at the low end.

The greatest advantage can be derived from the MIM process by properly designing

from the outset, which, after molding, de-binding, and sintering, achieves a net shape

part and thereby eliminates the need for secondary machining operations. This results

in a low cost and low manufacturing lead time solution.

As with all molding and casting processes, the most favorable outcome is achieved

by involving the manufacturing source early in the design of the system – not just the

Continuous debind and sintering

furnaces provide quality processing

and large volume capacity while

maintaining consistent quality.

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components – in order to obtain Design for Mouldability and Assembly (DFM/DFA) input,

eliminate part count, reduce assembly steps, and achieve a balance between tooling

investment and part costs.

SummaryMIM is an alternative manufacturing route which can offer solutions for metal parts

that in the past have either been very difficult or too costly to produce. When properly

designed for MIM, the process provides the design flexibility typical for plastics com-

bined with the material properties of metal. MIM offers numerous advantages over die

casting, investment casting, and machining and is changing the face of metal produced

components as we know them. Today, Phillips-Medisize is reshaping the face of MIM by

offering new material options, advanced design capabilities, processing options, and

the ability to serve a variety of markets that may not have originally considered this

capability.