Sizing and Coining for Powder Metallurgy Parts: A Practical Guide

What Happens During Sintering (And Why Correction Is Needed)

When a green compact (freshly pressed PM part) is sintered, the metal powder particles bond by solid-state diffusion. The part shrinks as necks form and the pore network partially contracts. Typical linear shrinkage is 0.5–2% depending on alloy, density, and sintering atmosphere.

The problem is that this shrinkage is not perfectly uniform:

• Geometry variation (thin sections shrink differently than thick sections)
• Furnace thermal gradients (front vs. back of furnace belt)
• Part orientation effects
• Compact-to-compact density variation

The result: parts sintered in the same batch have dimensional scatter—typically ±0.1–0.3 mm on critical dimensions, depending on part size and geometry. For many functional applications (bearing fits, gear mesh, valve seating), this scatter is too large. Sizing and coining reduce this scatter to usable levels.

Sizing: Dimensional Correction by Cold Re-Pressing

What it is

Sizing is a cold pressing operation performed after sintering. The sintered part is placed into a precision sizing die and pressed at controlled force. The die surfaces cold-work the part surfaces that contact the tooling, correcting dimensional variation and improving the surface condition.

The sizing die is a precision tool—typically made from carbide for long tool life—with dimensions set to achieve the target part dimension after springback.

What it achieves

These are representative ranges. Actual tolerances depend on part geometry, alloy, density, and tooling condition. Tighter tolerances are achievable for favorable geometries (short aspect ratios, simple profiles); geometry-challenging parts (tall parts, thin walls) show more variability.

How sizing tooling works

The sizing die is designed similarly to the compaction die. For a bore sizing operation:

• A precision mandrel is pushed through the bore
• The outer surface of the part is contained by a die that controls OD simultaneously
• Top and bottom punches control axial height and face flatness

All surfaces that contact tooling during sizing are dimensionally corrected. Surfaces that do not contact tooling are not corrected. This is the fundamental limitation of sizing: it only controls what the die touches.

Sizing allowance

Before sizing, the PM part must be sintered slightly oversized in the dimensions that will be sized. The sizing operation removes this allowance by cold-working the surface. Typical sizing allowance:

• 0.05–0.20 mm on OD and ID features
• 0.05–0.15 mm on axial height

The allowance ensures there is material to remove; if the sintered part is already at or below the target dimension, the sizing operation has nothing to correct and the result is inconsistent.

Coining: Density Improvement and Surface Enhancement

What it is

Coining uses higher re-pressing forces than sizing, with the goal of densifying the material locally—not just correcting dimensions. Where sizing works the surface at modest load, coining cold-works the material to the point of significant plastic deformation, closing pores and improving local density.

Coining is used to:

1. Achieve very tight tolerances on flat surfaces (face coining)
2. Improve fatigue strength at critical cross-sections by closing surface pores
3. Achieve very smooth, dense surface finish on bearing or sealing faces
4. Produce features that require better-than-sintered density at specific locations

When coining is used instead of sizing

Use coining rather than sizing when:

• The surface finish requirement is below Ra 0.4 µm on a face
• The fatigue requirement demands near-zero surface porosity at a critical feature
• The dimensional tolerance is tighter than sizing alone can achieve for the geometry
• A sealing face must be dense enough to mate against an O-ring or seal

Coining requires higher tooling loads and accelerates die wear compared to sizing. It is specified when the application demands it, not as a default.

Full coining (restrike)

Full coining re-presses the entire part to near-full density across its cross-section. This is a more intensive operation used to achieve very high density (>98% theoretical) in PM parts without hot isostatic pressing. It is uncommon in normal structural PM but used in specialty applications where maximum density is required and HIP is not available.

The Sizing Die: What It Controls and What It Doesn't

The single most important concept for engineers specifying PM parts is that sizing only controls what the die touches.

Features that can be controlled by sizing:

• Bore diameter (mandrel contact)
• OD (die contact)
• Top and bottom face flatness and height (punch contact)
• Tooth or spline profile on OD (profiled die)
• Gear pitch diameter (profiled die)

Features that cannot be controlled by sizing:

• Cross-holes (holes perpendicular to press axis)
• Lateral slots or grooves
• Angled features
• Features on internal steps or shoulders not contacted by the tooling
• Concentricity of features not simultaneously in contact with the die

For features outside the sizing die's contact zone, tolerances remain at as-sintered levels—or machining must be added.

Sizing and Tolerances: Design Guidance

Specify tolerances by process. When drawing a PM part, identify which features will be as-sintered, which will be sized, and which will be machined. Assign tolerances accordingly:

• As-sintered: ±0.1–0.25 mm typical (part-dependent)
• Sized: ±0.013–0.075 mm typical (feature-dependent)
• Machined: ±0.005–0.025 mm typical (operation-dependent)

Applying machined tolerances to sized features, or sized tolerances to as-sintered features, either wastes money or creates a drawing that cannot be met by the process.

Allow for sizing stock. The drawing should either specify the sintered pre-sizing dimension or allow the PM supplier to define it. If the drawing specifies both sintered and finished dimensions, ensure the sizing allowance is positive (sintered part is larger than finished, so material is available to size down to target).

Consider multi-level geometry. On a stepped part with multiple diameters or faces, not all levels may be contacted by the sizing die simultaneously. Review the sizing die design with your supplier to confirm all critical features are contacted.

Sizing, Coining, and Heat Treatment Sequence

The relationship between sizing/coining and heat treatment affects the final result:

Size before heat treatment: Achieves better dimensional tolerance in the soft material. However, heat treatment may distort the part slightly after sizing. This is the common sequence for parts where raw dimensional tolerance after sintering is the driver.

Heat treat, then machine: For parts that will be case-hardened or through-hardened, machining tight features after heat treatment avoids distortion effects. This adds machining cost but achieves heat-treatment-resistant dimensional accuracy.

Size or coin after heat treatment (induction): Induction-hardened teeth or bore surfaces can sometimes be size-coined to correct minor distortion from the induction process. This requires harder tooling and careful load control to avoid cracking the hardened surface.

For most automotive gear and structural programs, the standard sequence is: sinter → size → (optional) machine selected features → heat treat (if required) → final inspection.

Practical Decision Framework

Use this framework to decide how to control key PM part dimensions:

Quality Checks After Sizing

Production quality checks for sized PM parts typically include:

• 100% dimensional gauging on critical features (bore diameter, OD, height) using air gauges, plug gauges, or CMM
• Go/no-go gauging for high-volume bore and OD features
• Surface finish measurement (profilometer) for sized faces on precision applications
• Hardness spot-check if sizing changes local work-hardened hardness in a meaningful way

Statistical process control (Cpk) on critical dimensions is standard on automotive programs and increasingly expected in industrial supply chains.

Common Mistakes to Avoid

Assuming all dimensions improve after sizing. Only dimensions in contact with the sizing die improve. Features not in the die remain at as-sintered tolerances.

Specifying tolerances tighter than the process can achieve. Very tight tolerances (±0.005 mm or less) on large features, tall parts, or features with high aspect ratio are difficult or impossible to achieve by sizing. Discuss with your supplier before finalizing these callouts.

Ignoring oxide growth from surface treatment. Steam treatment or other surface oxidation after sizing will add a thin layer that shifts dimensions. Account for this if parts are sized and then steam-treated.

Not specifying sizing on the drawing. If a drawing specifies only the final dimension and does not indicate "sized" on the relevant feature, the PM supplier may treat it as an as-sintered tolerance, which it cannot meet, or may add machining to cover the difference—at additional cost.

Summary

• Sizing is cold re-pressing after sintering to correct dimensional variation. It achieves IT6–IT8 class tolerances on features contacted by the die.
• Coining uses higher force to improve local density, surface finish, and tightness beyond what sizing alone achieves.
• Both operations only control surfaces in contact with tooling—features outside the die zone must be specified at as-sintered tolerances or machined.
• Specify which features are as-sintered, sized, coined, or machined on the part drawing to avoid ambiguity.

For questions about sizing and coining capability for a specific geometry or tolerance requirement, contact our engineering team. Reviewing the drawing before tooling release is the most cost-effective point to optimize the tolerance plan.