Guides / Process

Why a Fired Ceramic Part Comes Out 20 Percent Smaller

August 3, 2026

Every sintered ceramic part starts larger than it ends. Powder is pressed or cast into a green body that is maybe 50 to 60 percent dense, and firing removes the space between the particles. Volume falls by roughly half and every linear dimension shrinks 15 to 25 percent.

That is not a defect. It is how the material becomes dense and strong. But it is the reason as-fired tolerances exist, and the reason your drawing needs to say which surfaces are ground.

What happens in the kiln

1. Binder burnout. Organic binders that held the green shape are removed slowly at a few hundred degrees. Rushing this cracks parts.
2. Initial necks. Particles begin to bond at contact points as atoms move.
3. Densification. Pores shrink and close, particles pull together and the part contracts. This is where the dimensional change happens.
4. Grain growth. Held too long or too hot, grains coarsen, which costs strength. Kiln recipes balance density against grain size.
5. Controlled cooling. Too fast and thermal stress cracks the part.

A full cycle is measured in many hours or days, not minutes, and kilns are loaded in batches. That is why firing sets the lead time on a part that needs its own blank.

Where the residual error comes from

A shop scales the green geometry by a shrinkage factor measured from its own production. What it cannot fully remove:

• Powder lot variation. Particle size distribution and binder content shift slightly between lots.
• Green density gradients. Pressed parts are denser near the punch faces, so they shrink unevenly.
• Geometry. Thick and thin sections shrink at different rates, which distorts the part as well as sizing it.
• Kiln position. Temperature is never perfectly uniform across a load.
• Setter interaction. Friction against the setter plate restrains shrinkage at the contact face.

Add those together and you get the familiar figure: about 1 percent of the dimension on a well controlled process, sometimes better on small simple parts and worse on long thin ones.

What this means on the drawing

Do not carry a metal general tolerance across. A ±0.05 mm general note on a 60 mm as-fired dimension is asking for something the process cannot do.

Mark ground features explicitly. Anything with a real tolerance must be ground after firing, which is a separate operation with its own cost. See green machining or hard grinding.

Expect distortion on long parts. A 200 mm bar will not be straight as fired. If straightness matters, it gets ground.

Allow grinding stock. If a feature is going to be ground, the fired part needs extra material there. The shop handles this, but it changes the blank and therefore the price.

Why this drives the two route decision

Because shrinkage is proportional, the green route is excellent for shapes and poor for fits. Once you accept that, the design pattern falls out:

• Complex geometry, pockets, holes, contours: cut them green
• Fits, seal faces, flatness, parallelism: grind them after firing
• Everything else: leave as fired and do not tolerance it

Parts designed that way are dramatically cheaper than parts that are fully ground, and far more accurate than parts that are fully as fired. The numbers are in the ceramic tolerance and surface finish guide.

One line for your RFQ

“Features marked G are ground after sintering. All other dimensions as fired, ±1%.”

That single note tells a shop how to quote, and it usually removes a revision cycle. The rest of the buying process is in the buyer guide.

Send your drawing and we match it with up to 3 shops that run the route your part needs. Reply within 24 business hours.

Frequently asked questions

Keep reading

• HIP for Ceramics: What the Extra Furnace Cycle Buys You
• Glazing and Sealing: Making a Porous Ceramic Part Hold Up in Service