PM Sample Approval: First Article, PPAP, and Capability - What to Expect

# PM Sample Approval: First Article, PPAP, and Capability - What to Expect

Bringing a new PM part into production requires a sample approval process before full-volume delivery begins. For automotive and industrial buyers, this typically means first article inspection (FAI), PPAP documentation, and a capability study confirming the process can hold critical dimensions consistently.

If you are specifying a PM part for the first time, or managing a PM supplier qualification, this guide explains what these processes look like in the context of sintered parts - including what PM-specific documentation looks like and where the timing and scope differ from machined-part qualifications.

First Article Inspection (FAI)

A first article inspection is a dimensional and material verification of the first parts produced from production tooling (hard tooling, not soft tooling or machined prototypes). The purpose is to confirm that the production process - tooling, press, sinter furnace, secondary operations - produces parts that conform to the engineering drawing.

What is measured

A first article report for a PM part typically covers:

Dimensional features:

• All drawing-controlled dimensions (bores, ODs, heights, radii, flatness, angularity)
• Measured with calibrated instruments: CMM, air gauges, plug gauges, height gauges, surface plates
• Reported as actual values against nominal and tolerance

Material properties:

• Sintered density (Archimedes method per MPIF Standard 42 or ASTM B962)
• Hardness (Rockwell or Vickers depending on hardness range and grade)
• Tensile strength and elongation if specified (usually on tensile test coupons sintered in the same furnace run)
• Chemical analysis if required (typically by ICP or combustion analysis)
• Microstructure (etched metallographic section - confirms sintering quality, pore structure, any carbide formation)

Surface treatment (if applicable):

• Coating thickness (electroless nickel, zinc plate)
• Salt spray test results
• Steam treatment appearance confirmation

PM-specific checks:

• Density gradient (if specified for uniform performance)
• Oil content (for oil-impregnated bearings, per MPIF Standard 35 requirements)

Number of parts in FAI

FAI sample size varies by customer requirement:

• General industrial: typically 3-10 parts, measured individually
• Automotive: typically 5-10 parts per PPAP requirements, with some dimensions measured on all parts and others on a subset
• The supplier's internal FAI before submission may include more parts

FAI timing

FAI is conducted after hard tooling is complete and production process parameters have been set. This occurs after:

• Tooling manufacture and acceptance (8-16 weeks from tool release)
• Trial production runs to set process parameters (1-3 weeks)
• Initial internal inspection by the supplier (1 week)

Total time from tool release to FAI submission is typically 10-20 weeks for a straightforward PM part. Complex parts or parts requiring heat treatment take longer.

PPAP (Production Part Approval Process)

PPAP is an automotive-origin framework (defined in the AIAG PPAP manual, 4th edition) that documents the full evidence package for a new production part. It is also used in non-automotive industrial supply chains, sometimes with modified scope.

PPAP levels

PPAP submissions are classified by level:

Most new PM programs at Tier 1 and Tier 2 are Level 3.

PPAP documents for PM parts

A Level 3 PPAP for a PM structural part typically includes:

1. Part Submission Warrant (PSW): The cover document summarizing the submission and affirming conformance.
2. Design documentation: Drawing and any referenced specifications current at submission.
3. Engineering change documentation: Change level confirmation if applicable.
4. Design FMEA: If design responsibility is with the supplier.
5. Process flow diagram: Step-by-step flow from powder receipt to final inspection.
6. PFMEA (Process FMEA): Failure mode analysis for each step in the PM process - compaction, sintering, sizing, heat treatment, inspection.
7. Control plan: Documents how each critical parameter is controlled in production: compaction force, sinter temperature, density inspection frequency, etc.
8. Measurement system analysis (MSA / Gauge R&R): For critical dimensions, demonstrates that the measurement system (gauge, operator, environment) is repeatable and reproducible.
9. Dimensional results: FAI measurements on the minimum required sample.
10. Material certifications: Heat/lot certificate for PM powder, showing compliance to alloy specification.
11. Performance test results: If the customer requires functional testing (pressure test, load test, fatigue samples), results are included.
12. Capability study (Cpk/Ppk): Statistical analysis of critical dimensions. See next section.
13. Qualified laboratory documentation: Certifications for any external test labs used.
14. Appearance approval report (AAR): For cosmetic parts only - rarely applicable to structural PM.
15. Sample parts: Physical parts retained at the supplier; sometimes submitted to the customer.
16. Checking aids: If specialized fixtures or gauges are made for this part, they are documented.

Capability Studies (Cpk / Ppk)

Capability is a statistical measure of how well the production process holds a dimension relative to its tolerance. It is reported as Cpk (process capability index) or Ppk (performance index for pre-production data).

Minimum automotive requirement: Cpk ≥ 1.33 on critical dimensions (some customers require 1.67 for safety-critical features).

What Cpk means for PM

For PM parts, Cpk is measured on:

• Critical bore diameters (typically the tightest tolerance features)
• Critical ODs for press-fit or mesh interfaces
• Part height / axial length on functional faces
• Density (if specified as a controlled characteristic)

A PM supplier running a capable process will typically show:

• Cpk 1.5-2.0 on sized bore features with good tooling
• Cpk 1.3-1.7 on as-sintered or lightly sized dimensions
• Cpk 1.0-1.3 on challenging geometry or dimensions near the edge of process capability

If a supplier returns a Cpk below 1.33 on a critical dimension, it does not mean the part is bad - it means there is not enough statistical margin to guarantee all future parts will be within tolerance without 100% inspection or process improvement.

Sample size for initial capability

Minimum sample size for a meaningful initial capability study: 30 measurements (individual part measurements). PPAP typically collects 30 parts from a minimum of two press setups or two furnace lots to capture inter-lot variation.

PM-Specific Nuances in Sample Approval

Sintering lot variation: PM mechanical properties vary somewhat by furnace lot (atmosphere consistency, belt speed, temperature profile). First article and capability samples should come from at least two sintering lots if possible, to capture this variation.

Density as a controlled characteristic: If density is a critical characteristic on the drawing, it must be included in the control plan and capability study. Density is typically measured destructively (Archimedes method), so it is sampled rather than 100% inspected. Control charts on compaction press tonnage are often used as a proxy for density in real-time production.

Heat treatment lot: For HT grades, mechanical property samples (hardness, tensile) must come from the same heat treatment load as the PPAP dimensional samples - not a separate run.

Secondary operation sequencing: If the part has secondary machining, sizing, or impregnation, the control plan and PFMEA must cover each step. Parts measured after the final operation must have passed all preceding steps.

What Buyers Should Prepare

To support a smooth PM sample approval:

1. Issue a complete drawing before tooling release. Dimensional and material requirements should be frozen - or changes after tooling start must go through engineering change control.
2. Define critical characteristics on the drawing. Mark dimensions with Cpk requirements explicitly (triangle with C, diamond, or customer-specific symbol). Critical characteristics trigger full MSA and capability.
3. Specify PPAP level required. If you expect Level 3, state it in the purchase order or quality agreement.
4. Provide functional test specifications separately if required. If the part must pass a pressure test, load test, or life test, specify the test conditions and acceptance criteria in writing.
5. Agree on timeline. PPAP for a new PM part typically takes 12-22 weeks from tool release. Build this into the program schedule.

Contact us to discuss sample approval requirements for your PM program. We can advise on typical capability levels for your critical dimensions and identify any drawing requirements that may need process alignment before tooling is released.