MIM machining matchup

Oct. 20, 2005
Metal-powder injection molding stacks up well against other metalworking methods for cost and quality. A real-world example shows how.

Gregory M. Brasel
Vice President of Manufacturing & Product Development
Megamet Solid Metals Inc.,
Subsidiary of CIC Group
St. Louis, Mo.

Metal injection molding was key to letting the designer of a new lock meet stringent specs including a linear tolerance of ±0.13% on the lock striking face width and a straightness callout of 0.002 in. over 2.25 in. for its trunnion. Prior to MIM, machining from bar stock or a cast blank used to be the only viable alternatives for such complex and high performance designs.

In practice, the number of MIM alloys used to make precision parts, such as the lock, is far less than those used for casting and machining. For MIM, machinability is rarely an issue so a few MIM alloys (with their heat treatments) can cover the typical material selection trade-offs. (AS = as-sintered, CH = case hardened)

It's been decades since a manufacturing process has come along that can rival the "default" metalworking options of machining and investment casting. But one alternative — metal-powder injection molding or MIM — is increasingly the process of choice for precision components of generally small size made in medium to large quantities.

The thought process for deciding on the MIM route can get involved. An equation or a simple set of bullet points can't describe it. So a real-world example may help explain things. Specifically, the redesign of a new lock serves as a case in point. The pivotal question was whether or not lock parts could be built to stringent specs and still be competitively priced against existing designs.

Close dimensional control on the width of the lock striking face was key, as was the straightness of its trunnions. The parts also had to rotate freely (without spalling which could seize the lock) while staying close to the housing for tamper resistance.

Additionally, lock function depends on geometric relationships. Tamper resistance, for example, relies on parallelism between the rear of the striking face and the stopping face or "stop tabs." Both surfaces must also rotate true around the trunnions. And finally, an excellent surface finish spec not only assures corrosion resistance but also gives the lock a "high-value" look. Designers considered machining the parts completely from bar stock, machining subcomponents and assembling them, machining an investment casting, and MIM (with potential secondary machining operations).

It's important to look at material selection first and then weigh how machining will impact part cost. In general, material costs are strongly affected by machinability. Machinability is given as a percentage rating of how difficult a material is to machine relative to AISI 1212 steel.

Highly machinable alloys cost more. These so-called "free-machining" grades help defray other costs because tools last longer, cycle times are shorter, and fixturing is less involved. Moreover, there are fewer setups and the machine tools required can be physically smaller.

There are a number of machinable alloys and stainless steels (those containing less than 0.5% carbon) that can be through hardened to the same hardness level with roughly similar mechanical properties. However, "free-machining" comes with some pros and cons. So it's important to look at a few considerations before specifying an alloy:

Machinability and mechanical properties: Alloying elements, including carbon, that boost heat-treated strength degrade machinability. Ferrite strengtheners such as nickel and silicon cut machinability more than equivalent amounts of carbide formers such as chromium and molybdenum.

Elements such as lead and manganese added to improve machinability in high-strength steels have the side effect of producing nonisotropic mechanical properties.

Machinability and stainless steel: Stainless steels machine poorly. They work-harden quickly and their high strength gives them a "gummy" nature. They need slower machining speeds and more power, which shortens tool life. Martensitic stainless steels are brittle and machine more easily, but tool life suffers because of the hard carbides in the matrix. Precipitation-hardening grades are also an option. They come in machinable grades but need secondary heat treating.

Machinability and distortion: Cutting tools put compressive stress in part surfaces. Therefore, high precision components are often machined in the asheattreated state. Free-machining grades are prone to have high, often unbalanced stress concentrations from machining depending on the amount and speed of cutting. Nonuniformly applied and stored stresses vary martensitic transformation causing the parts to distort.

Other machinability factors: Stress cells develop when highly stressed regions act as anodes to the less-stressed cathodic areas. Anodic regions are those with finer grain size, higher grain boundary and dislocation densities, and more cold work (all factors affected by machinability).

Stress-corrosion cracking is another material property that needs to be eliminated. That's because stresses needn't be externally applied. Stored residual stresses (from machining) and a corrosive environment can cause catastrophic failure.

The lock designers considered three alloys based on mechanical properties and ease of machining. The first, 41L40, is a leaded, medium-carbon, low-alloy steel having a machinability factor of 85%. However, the alloy corrodes easily so parts need chrome plating that boosts manufacturing costs. But an added benefit of chrome plating is a more attractive finish. The two other options are the stainless-steel grades 17-4PH and 17-7PH. Both can be solution treated and aged to tensile strengths from 150 to 200 kpsi (1,030 to 1,380 MPa). But 17-4PH has better machinability and can be cut at the same speed as 17-7PH, but at twice the material hardness.

Comparing 41L40 to the 17-4PH stainless steel shows that the 41L40 is less expensive initially, more readily available, easier to machine, less costly to heat treat, and has higher tensile properties. The advantage of 17-4PH is that it doesn't need plating to be corrosion resistant. Plating, however, is relatively inexpensive compared to the total cost advantage that 41L40 brings to the table.

Machining completely from bar stock: Selection of the 41L40 alloy dictates material cost and tool life. Next comes determination of the machining steps, workholder requirements, number of setups, machine time, production rate, and labor costs.

members estimated total machining time at about 15 min/part. That equates to about USD 9 to 12/piece. Adding material back into the design was an option that would make the part easier to machine. But part dimensions could not increase, as that would negate the small size and low profile of the lock redesign.

Machining and assembling subcomponents: Machining can be a better option if the part needn't be from a single block and can be assembled from three subcomponents. Separate machines can cut the part if it can be made in three segments. This improves tool life and lowers the number of tooling changes. It also reduces the amount of cold work and material lost to chips.

Each component in the three-part assembly can be heat treated separately and subsequently assembled in a fixture to meet tight dimensional and geometric specs.

There's a downside, however, to machining in three segments. Complex locating features must now be added because-of the need to capture and lock the fingers. There's a need for additional tolerance control (precision fit of mating parts). There are more failure modes so quality must rise to compensate. And of course, there is more labor, fixturing, and part handling when three segments are mated.

These operations could take around 10 min/part not counting heat treatment, quality requirements, and the actual assembly operation. The cost of the machined assembly would be about USD 6 to 8/part.

Machining an investment casting: Here it's important to determine if a netshape casting is possible. If not, an alternative approach is to cast blanks that are then machined.

Investment-casting (IC) houses typically use published tolerance control specs and design guides from the Investment Casting Institute. Doing so would dictate a large machining allowance given the lock's 2.24-in. (57-mm) long trunnion with 0.002-in. (0.05-mm) straightness. An IC part would be out of straightness by as much as 0.01 in. (0.25 mm). Both ends of the part would also need machining to get the proper width tolerance.

Additionally, an angular tolerance of 2° means the back of the striking face and the stop tabs would need machining to meet the parallelism spec.

The tightest linear dimension is 1.5 ±0.002 in. (38 ±0.05 mm) over the width of the striking face. This will miss the nominal by as much as ±0.014 in. (±0.35 mm). And the casting will need final finish machining on the striking face to meet surface finish specs.

In short, virtually the entire part needs finish machining. Setups and fixturing will be identical to those for the machined wrought stock. But the primary advantage is that investment-casting blanks eliminate costly rough machining operations. Other than postmachining, direct labor content is the major issue with IC parts. IC involves many labor-intensive process steps. People perform some critical function at nearly every turn such as assembling wax moldings into the tree, dipping into investment, pouring metal, or removing and cleaning castings.

Another cost associated with IC is the additional machining to get the right surface finish. Typical IC surface finishes are in the 125- in. (3.2- m) range. With care they can approach 63 in. (1.6 m) but this is still well above the lock's 10- in. cosmetic surface requirement.

But a complex, near-net-shape investment casting can be economical despite rough machining costs. A 2 to 3-min savings in machine time can drop part cost by as much as USD 1.50. So a domestically produced IC and machined part could cost USD 8 to 10. The same part sourced overseas might be USD 4 to 5.

In general MIM parts need little if any machining because the process controls dimensions to a high degree. However, the dimensional specs on the lock were challenging. So the key to whether MIM would be a good candidate was the amount of machining needed after MIM processing. MIM typically maintains ±0.3% of a linear dimension. But careful processing can make MIM tolerances approach ±0.1%. The tightest dimensional tolerance on the lock part is ±0.13% over the width of the striking face. With adequate process controls, MIM can deliver this dimension without secondary machining. All other linear tolerances are within the ±0.3% range.

The straightness of the trunnions is also within normal MIM limits. Typical straightness value are 0.002 in. (0.05 mm) for lengths up to 2 in. (50 mm). So it's reasonable to expect that a carefully controlled MIM process can produce the 2.25-in. (57-mm) long trunnion with 0.002-in. straightness.

Another tight spec on the part is parallelism. Although controlling parallelism is different than controlling linear tolerances, MIM typically can hold parallelism to 0.002 in., which is within spec for the lock. A secondary bending operation might set the angle between the stop tabs and the striking face. But it should not need machining.

Its roots in powder metallurgy let MIM reproduce the chemistry of practically any wrought material. And only a few alloy chemistries (with their heat treatments) are needed to cover the entire spectrum of strength versus ductility for carbon alloy steels and strength versus corrosion resistance for stainless steels.

There are several MIM stainless and low-alloy steels that can produce parts with enough hardness (40 to 44 HRc) and tensile properties on the order of 150 to 200 kpsi (1,030 to 1,380 MPa). Tests of MIM 17-4PH H1025 at 98% theoretical density, for example, showed that 50 consecutively molded tensile bars had similar mechanical properties to those in published guidelines. Other commonly MIMed materials which attain enough hardness include MIM 4140 and MIM 4605.

MIM eliminates issues of machinability,-heat treatment, and surface treatment. Chrome-plated alloy steel is also acceptable for MIM. So material selection issues come down to cost and performance. The cost difference between the most expensive alloy (MIM 17- 4PH) and the least (MIM 4605) is nearly 35% (with MIM 4140 nearly in the middle). The heat-treat temper for 4605 will need tweaking to improve ductility.

In the lock example, the width of the face (±13% over 1.5-in. length) was one of the most difficult dimensions for MIM to control. The tight spec came from the limited tolerance control possible with traditional zinc die casting for the housing. Expanding the tolerance was a possibility if the MIM process could hold tighter tolerance control.

MIM also takes less time and fewer operations to produce cosmetic surfaces. Typical MIM surface finishes are around 25 in. before any finishing operations.

Feedstock price is typically the dominating factor for MIM. But MIM's low labor cost, better design freedom, and net shape can offset this cost. It's also important to not overlook all the overhead costs not directly attributed to producing a part. Countless models have devised many means of valuing and assigning overhead costs. One widely used model is the Theory of Constraints (TOC) a management approach developed to deal with issues of effective cost and quality management.

According to TOC the sintering operation is the bottleneck — the constraint on the critical path to shipping MIM parts. The sintering operation in MIM requires the most expensive equipment with the longest acquisition time. It is the most difficult to control and cannot be shortened by adding more equipment or manpower. And other processes cannot take place concurrently while the parts are in the furnace.

Adding sintering capacity changes the entire cost and throughput structure of a MIM plant, more so than adding capacity to any other step. As the bottleneck, the rate of MIM part shipments can't exceed the rate at which they are sintered; in likewise logic, it doesn't matter how quickly MIM parts can be molded or debound because the rate at which they can be sintered is essentially fixed.

The fact that sintering constrains the MIM production system gives a basis for allocating overhead in a MIM shop. There are a fixed number of furnace loads that can take place during a certain time frame. So overhead cost per furnace load is just the overhead costs during that time divided by the number of furnace loads. The number of parts per furnace load is then divided into that number to get the overhead cost per part.

This technique is of considerable utility: The resulting overhead cost per part describes how the MIM shop makes money. The calculation relies on throughput, which is a function of the efficiency, size, and the number of installed furnaces. Furnaces, in turn, impact the size of the engineering and administrative staff. The calculation thus scales with the efficiency and size of the operation.

Defined in this way, one can see how overhead bears on the price of the part. In the lock example, it is a large MIM part (by today's standards) and takes up a significant portion of furnace capacity. Attributed overhead must reflect these realities because the part uses more of the company's moneymaking ability than a smaller part.

Labor accounts for a small part of the MIM part price. In stark contrast to both machining (where price is based on billable hours) and IC (which requires 23 days of essentially manual labor to produce a run of components), the MIM process spans eight days under the same assumptions. Humans aren't involved in the direct operations other than staging (loading onto furnace supports), which is readily automated. And it uses less labor for material and work-in-process movement, and for quality assurance compared to other methods because there are fewer numbers of operations. All these factors bring the estimated price of the MIMed lock parts to USD 2 to 4 each compared to both domestic and foreign sources with the other metalworking methods.

Megamet Solid Metals Inc.,
(314) 739-4499, megamet.com

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