Guide
MIM Part Inspection: What Do You Check for Density, Hardness, Tensile Bars and CMM?
MIM part inspection is the set of lot-release checks that show a sintered metal injection molded part has the density, mechanical properties, hardness and dimensions the drawing asks for. Unlike a machined part cut from mill-tested bar, a MIM part gets its properties during sintering, so the proof has to come from the lot itself.
By Saola Manufacturing engineering team · Published · Updated
On this page
What does a MIM inspection plan have to cover?
Four things: sintered density, mechanical properties from tensile (and unnotched Charpy) test bars, hardness where the lot is heat treated, and dimensions measured on the sintered part. Each one needs a named method and a report from the plant.
The material standard for ferrous MIM is ASTM B883-24. Its scope covers ferrous metal injection molded materials made by mixing metal powders with binders, then injection, debinding and sintering, and its abstract says the materials are subjected to a tensile test and an unnotched Charpy impact energy test. It lists ten compositions, among them MIM-4605, MIM-4140, MIM-316L and MIM-17-4 PH, and its chemistry table covers copper, chromium, molybdenum and nickel. The numeric limits sit behind the ASTM paywall and in MPIF Standard 35-MIM, so this guide does not quote them; it shows what to ask for.
The reason the plan differs from a machined-part plan is where the properties come from. MIMA, Process Overview describes the sintered end metal as having properties similar to annealed parts made by conventional metalworking, and says sintering is what closes the voids between powder particles. Density, strength and hardness are therefore results of the plant’s molding, debinding and sintering run, not of a bar-stock mill test report. If the report is missing, you have no evidence for the lot.
Dimensions follow the same logic. MIMA, Complex Designs with MIM states that the mold cavity sets a limit on dimensional control of the sintered product and that little capability remains to adjust dimensions after ejection except at extra cost. So the CMM report is the check on whether the tool and the sintering run landed the part where the drawing needs it.
How is sintered density measured, and which ASTM method applies?
Archimedes (water displacement) measurement: ASTM B962 for PM products that have surface-connected porosity, and ASTM B311 for impermeable materials with less than two percent porosity. Ask the plant which one it uses and why.
ASTM B962-23 describes a density method for powder metallurgy products based on Archimedes’ principle. It applies to green compacts, sintered parts and test specimens, and it deals with surface-connected pores by sealing them, for example with oil impregnation, so water does not enter. The same page states that impermeable PM materials, which do not gain mass when immersed in water, are covered by B311 instead. ASTM B311-22 is scoped to PM materials with less than two percent porosity, also by water displacement.MIM sits on the boundary between the two. MIMA, Process Overview says it is common to reach 96 to 99 percent of theoretical density in a controlled sintering atmosphere, and MIMA, Designing with MIM rates MIM at about 98 percent. Two percent porosity is 98 percent density (derived: 100 minus 2), so a lot at the dense end of that range is a B311 material and a lot at the lower end is not. This is an inference from the two standards’ scope lines, not a rule written in either, so do not hard-code one method on the drawing without asking the plant what its lots actually measure.
What matters for acceptance is that the report states the method, the measured density in g/cm³ and the theoretical density used to turn it into a percentage. The theoretical value depends on the alloy chemistry of the lot, and two plants quoting 98 percent against different references are not quoting the same part.
What does a MIM inspection plan look like in one table?
A short list of characteristics, each tied to a method or standard and to the document you want from the plant. The table gives the structure; acceptance values come from the material designation on your drawing.
Use the table as the skeleton of a first-article and lot-release request. It deliberately carries no acceptance numbers, because the figures belong to the alloy and heat-treat condition you specify and are published in paywalled standards.
| Characteristic | Method or reference | What to ask the plant to send |
|---|---|---|
| Material designation and chemistry | ASTM B883-24 designation list (for example MIM-4605, MIM-316L, MIM-17-4 PH) | Designation on the report, chemistry result for the lot |
| Sintered density | ASTM B962-23 (surface-connected porosity) or ASTM B311-22 (impermeable, under two percent porosity) | Method used, measured g/cm³, theoretical density used, number of parts measured |
| Tensile properties | Tensile test required by ASTM B883-24 | Bar or part test report: yield, ultimate, elongation, and which furnace lot the bars came from |
| Impact energy | Unnotched Charpy test required by ASTM B883-24 | Charpy report for the same lot as the tensile bars |
| Hardness | Rockwell or other scale named on the drawing; surface or core stated | Hardness readings, scale, location, and the heat-treatment record |
| Dimensions | CMM on sintered parts, one report per cavity | Full CMM report against the drawing, with cavity identification |
| Appearance | Drawing note for gate vestige, parting-line witness line and ejector marks | Photographs of the first articles, or an approved sample |
Why test bars, and what do they prove about the part?
Test bars prove the material and the process window: they show the sintered alloy meets the mechanical properties in the specification. They do not prove that a thick or thin section of your actual part reached the same density.
ASTM B883-24 makes the tensile test and the unnotched Charpy test part of the material requirement. The standard’s page does not say how bars must be made, so write the arrangement into the RFQ: ask whether the bars are molded from the production feedstock lot and sintered in the same furnace run as the parts, or produced separately. Bars from a different run tell you what the plant can do, not what your lot did.The gap between bar and part comes from geometry. MIMA, Complex Designs with MIM warns that non-uniform wall thickness gives non-uniform shrinkage and can cause distortion, internal stresses, voids, cracking and sink marks. A part with a heavy boss and thin webs can therefore differ from a uniform bar even in the same furnace load. That is why the plan pairs bar tests with a density measurement on the actual part under B962 or B311, rather than relying on one or the other.
For first articles, ask for the tensile and Charpy result and the part density together. For repeat lots, agree a lighter schedule in the RFQ, such as density and CMM every lot and bars on a stated interval. The ASTM pages do not set a sampling frequency, so the frequency is a commercial agreement, and it should be written down.
When does hardness matter, and how do you call it out?
Hardness matters when the lot is heat treated or carburized, or when the part wears. Name the scale, the location (surface or core) and the condition (as-sintered or heat treated) on the drawing, or the reading is not comparable.
MIMA, Secondary Operations says MIM parts get the same heat-treatment operations as their wrought counterparts, with quench and temper given as an example, and that heat treatment and hot isostatic pressing can improve strength, hardness and wear resistance. It also notes that surface carburization gives a high surface hardness. It publishes no hardness values, so a hardness requirement has to come from your drawing and the material standard, not from the association.One trap carries over from conventional powder metallurgy. PickPM, Engineering Properties describes apparent hardness for PM parts as a combination of particle hardness and porosity, and it separates that from microhardness of the matrix measured by Knoop or Vickers tests. MIM is denser, so the gap is smaller, but a lower-density lot can still read softer on a macro scale than its matrix. Ask the plant which type of reading it reports.
For case-hardened parts, the hardness reading is only half the requirement, because case depth carries the load. Our guides on effective case depth and Rockwell versus Brinell hardness cover how to write those callouts. For MIM, add the heat-treatment record to the list of documents you request.
How do you run a CMM check on sintered dimensions?
Measure every cavity, on the sintered part, to the drawing, and keep datums away from the gate and the parting line. Expect the process to hold a percentage of the dimension, not a fixed band, unless a secondary operation tightens the feature.
MIMA, Secondary Operations gives the average tolerance of the MIM process as within plus or minus 0.3 percent, which is why many parts are sintered to final dimensions. As a derived example using that typical average, 0.3 percent of a 10 mm feature is 0.03 mm (10 × 0.003); the band grows with the feature size. Where a feature needs better, the same page lists coining and machining among its secondary operations and says MIM materials can be sized or ground like their wrought counterparts, and those features then become the focus of the CMM report.Cavity control is the part buyers overlook. MIMA, Complex Designs with MIM says that part numbers and cavity identification marks can be molded in place, and it describes multi-cavity tools that must deliver an identical amount to each cavity. In practice that means a first-article report should identify which cavity each measured part came from, and a lot-release sample should cover all cavities, so one drifting cavity does not hide in an average.
Datum choice matters for the same reason. MIMA, Complex Designs with MIM notes that the parting line normally transfers to the part surface as a witness line and that MIM gates are generally much larger than in plastic injection molding. Choose datum features that are not on the gate vestige or the witness line, and say in the drawing note what cosmetic condition is acceptable there. Our guides on CMM inspection and first-article inspection from an overseas supplier cover the report format in general.
What should the RFQ ask the MIM plant to send?
A named material designation, a density method, a bar or part test report from the same lot, a hardness report if heat treated, and a cavity-identified CMM report. Put the list in the RFQ so the plant prices it.
Write the plan as a short list on the drawing or in the quotation request: material designation from the ASTM B883 list, density method and reporting basis, the tensile and Charpy test arrangement, hardness scale and location, the CMM report format, and the sampling plan for first articles and for repeat lots. Plants price inspection the way they price anything else, and an item that is only mentioned after award becomes a change order.
Saola places MIM work through a partner plant it manages and does not name, so the inspection package is agreed per quote and the quote states the production site and country of origin. If the part also needs machining after sintering, for example a finished bore or a thread, that machining is a separate step with its own inspection; the lead CNC plant’s CNC turning and CNC milling are the usual route, and the MIM report should state the stock left for it.
Frequently asked
Does every MIM lot need tensile and Charpy bars?
The ASTM B883-24 page says the materials are subjected to tensile and unnotched Charpy testing, but it gives no lot-by-lot frequency. Treat the frequency as a commercial agreement: full bar data on first articles and then a stated interval, written into the RFQ.
Is a density number enough to accept a MIM lot?
No. Density shows how well the lot sintered, but it does not show strength or dimensions. Pair it with the tensile and Charpy report and a CMM report from the same lot.
Which density standard should the drawing name, ASTM B962 or B311?
Ask the plant. B962 covers PM products with surface-connected porosity and B311 covers impermeable materials with under two percent porosity, and MIM lots can fall near that line. Many drawings simply say that density is measured by water displacement and reported with the method named.
Can MIM parts be hardness tested like machined parts?
Yes, with the same kind of testers, but state the scale, the location and the condition. The Rockwell scale suits a heat-treated surface, while a soft as-sintered part or a thin section may need a different method.
What if the CMM report shows a feature outside tolerance?
Per MIMA, features that need tighter control than the sintering process holds can be handled with coining, sizing or grinding, at extra cost. Raise it at first article, before the tool is changed, and ask the plant which route it proposes.
Sources
- ASTM B883-24, Standard Specification for Metal Injection Molded (MIM) Materials, ASTM International store page (B883-24 listed as current, updated 2024)
- ASTM B962-23, Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM International store page (updated 2023)
- ASTM B311-22, Standard Test Method for Density of Powder Metallurgy (PM) Materials Containing Less Than Two Percent Porosity, ASTM International store page (2022 edition)
- MIMA, Secondary Operations, Metal Injection Molding Association Design Center, accessed October 2026 (2026)
- MIMA, Process Overview: MIM, Metal Injection Molding Association Design Center, accessed October 2026 (2026)
- MIMA, Complex Designs with MIM, Metal Injection Molding Association Design Center, accessed October 2026 (2026)
- MIMA, Designing with MIM, Metal Injection Molding Association Design Center, accessed October 2026 (2026)
- PickPM, Engineering Properties, MPIF powder metallurgy design center, accessed October 2026 (2026)