What Affects Zirconia Shrinkage During Sintering?

 

When a dental laboratory changes from one zirconia supplier to another, one of the first things that needs to be checked is the shrinkage factor.

The reason is simple.

Two zirconia discs can have the same diameter, the same thickness, and even similar strength and translucency specifications, but they may not contract by exactly the same amount during sintering.

This is not unusual.

Zirconia shrinkage is affected by what happens to the material before it enters the furnace, the composition of the zirconia, the density of the pre-sintered blank, and the thermal conditions used during final sintering.

For a laboratory, this means that "20% shrinkage" should not be treated as a universal number for all dental zirconia.

For a manufacturer, the issue starts even earlier. The shrinkage behavior is partly established before the zirconia blank reaches the dental laboratory.

I often look at the problem this way:

The furnace controls part of the shrinkage behavior, but the furnace does not create the shrinkage value by itself.

The material already has a history before it is loaded into the furnace.

The First Factor: Pre-Sintered Density

One of the less obvious factors affecting shrinkage is the density of the zirconia before final sintering.

A pre-sintered zirconia block is not fully dense ceramic.

It is a compacted body containing zirconia particles and internal pores. During final sintering, the structure densifies and the dimensions decrease.

If two pre-sintered blocks have different densities, they do not necessarily have the same amount of space available to be eliminated during sintering.

This relationship has been demonstrated experimentally.

A study on zirconia compacts found that linear shrinkage depended on the density of the pre-sintered block. As pre-sintered density increased, linear sintering shrinkage decreased. The researchers also found that different areas of the same experimental block could show different shrinkage rates. The top section showed the lowest shrinkage, while the bottom section showed the highest in that particular experiment.

This finding is relevant when evaluating zirconia materials.

If a supplier gives you a shrinkage value, it is worth asking how the pre-sintered material was controlled before that value was measured.

The shrinkage number is not independent of the blank's starting condition.

Powder Compaction Has a Direct Effect

Before zirconia becomes a CAD/CAM blank, the powder needs to be formed into a compact body.

The pressure used during compaction affects the structure of that body.

A particularly useful study from the dental materials field investigated this relationship.

Researchers compacted nanosized zirconia powder at different isostatic pressures and examined the resulting sintering behavior.

In their tested system, increasing compaction pressure from 50 MPa to 400 MPa reduced measured linear shrinkage from 24.54% to 20.9%.

The researchers also found that higher compaction pressure increased the sintering rate and lowered the temperature required for sintering. They reported that a compaction pressure range of 200–300 MPa gave the most favorable combination of sintering behavior and machinability in that experiment.

These values should not be copied directly as manufacturing targets for every commercial zirconia block.

The study used a particular zirconia powder and experimental manufacturing conditions.

The useful lesson is the relationship:

different pre-sintered structures can produce different shrinkage.

For a zirconia manufacturer, controlling powder forming is therefore part of dimensional control.

For a distributor, this is one reason why asking only for the final shrinkage percentage does not provide the complete technical picture.

Particle Packing Also Matters

Compaction pressure is only one part of the pre-sintered structure.

The characteristics of the zirconia powder also matter.

Particle size distribution, particle morphology, agglomeration, additives, moisture conditions, and the way the powder is processed can influence how particles pack together.

Imagine two containers filled with small balls.

If the balls are all roughly the same size, there can be more empty space between them.

If different particle sizes are distributed appropriately, smaller particles can occupy some of the spaces between larger ones.

The same basic idea applies to ceramic powder packing, although real zirconia systems are much more complicated than this simple example.

The starting pore structure influences how the material densifies during sintering.

As a result, two manufacturers can use zirconia with the same nominal yttria content and still obtain different processing behavior.

Chemical composition is important, but it is not the only variable.

The manufacturing route matters as well.

Zirconia Composition Can Change Shrinkage Behavior

Another factor is the zirconia itself.

Dental laboratories now use several types of zirconia, including conventional 3Y-TZP and more translucent 4Y- and 5Y-containing materials.

They differ in yttria content, phase composition, microstructure, and often in their intended balance between strength and translucency.

Those differences can also be associated with different dimensional behavior during sintering.

A recent study examining seven dental zirconia CAD/CAM materials found that material type had a very large effect on measured shrinkage. The reported partial eta-squared value for the material effect was 0.893, indicating a strong contribution of material type to the variation observed in the study.

The same research reported typical linear shrinkage of around 19–21% for 3Y-TZP materials, while some 4Y- and 5Y-based materials showed values around 18–19% under the tested conditions.

That does not mean every 3Y material shrinks by 20% or every 5Y material shrinks by 18%.

It means that zirconia composition and formulation can influence dimensional change, and the value needs to be measured for the actual material.

This matters for distributors who sell several zirconia grades under the same brand.

A laboratory should not assume that the CAM compensation factor for one product automatically applies to another product simply because the discs have the same 98 mm diameter.

What About the Pre-Sintering Process?

There is another stage that can be overlooked.

Before a zirconia blank is supplied to the dental laboratory, it normally goes through a controlled pre-sintering process.

The purpose is not to produce the final dense ceramic.

The blank needs to remain machinable.

However, the pre-sintering temperature affects the condition of the material that the milling machine receives.

A study on 3Y-TZP investigated the effect of compaction pressure and pre-sintering temperature. The researchers found that pre-sintering temperature had a clear effect on machinability. In their tested material, the best cutting surface and edge integrity were obtained when the pre-sintering temperature was between 800°C and 900°C.

These temperatures should not be treated as a universal recipe for commercial zirconia.

The thermal history of the block starts before the dental laboratory's furnace cycle.

When the block reaches the laboratory, its density, strength, pore structure, and machinability have already been influenced by the manufacturer's production process.

Does Sintering Temperature Change Shrinkage?

Yes, but the relationship is not as simple as saying:

higher temperature = more shrinkage

Sintering temperature affects densification and grain development, but the actual dimensional response depends on the material and the sintering schedule.

A study comparing different sintering temperatures and times tested zirconia at:

1510°C for 120 minutes

1540°C for 25 minutes

1580°C for 10 minutes

The study examined both conventional and highly translucent zirconia materials.

Interestingly, the highest flexural strength in the tested groups was found with the 1580°C / 10-minute condition, while the differences between some of the lower-temperature groups were not statistically significant. The crystalline structures were also similar between the tested conditions.

This does not mean that 1580°C for 10 minutes is a suitable program for every zirconia.

It demonstrates something more useful:

temperature and time interact.

A furnace program cannot be evaluated by looking at peak temperature alone.

The same material can experience different thermal histories depending on how quickly it is heated, how long it is held, and how it is cooled.

Holding Time Can Change the Material After It Has Reached the Target Temperature

Once zirconia reaches its target temperature, the process is not finished.

The material continues to develop.

Holding time affects grain growth and the microstructure of the sintered ceramic.

An earlier study of two commercial dental zirconias compared holding times ranging from 20 minutes to 40 hours.

The measured grain size increased substantially with longer sintering conditions.

For one zirconia, the reported mean grain size ranged from approximately 347 nm to 1,512 nm.

For the second material, it ranged from approximately 373 nm to 1,481 nm.

The study also found that the longest holding condition produced grain-boundary changes and lower light transmission compared with shorter conditions.

This does not prove that longer holding always produces excessive shrinkage.

It does show that the material continues to evolve during the holding stage.

A laboratory should therefore not modify the holding time simply because the furnace can accommodate a longer cycle.

The manufacturer's recommended program is based on the particular zirconia formulation.

A Recent 4Y-PSZ Study Gives Another Example

A 2026 study on monolithic 4Y-PSZ examined several sintering parameters, including peak temperature, holding time, and heating rate.

The researchers found that mean grain size increased with longer holding time.

At 30 minutes, the reported mean grain size was approximately:

0.503 ± 0.037 μm

At 180 minutes, it increased to:

0.730 ± 0.041 μm

The same study also examined heating rates between 3°C/min and 10°C/min. Grain size remained relatively similar across that tested range, from approximately 0.701 μm to 0.727 μm, with no significant difference detected among the heating-rate groups.

This example shows why general statements about sintering need to be handled carefully.

A heating rate that makes a significant difference for one zirconia system may have a much smaller effect within a particular range for another.

The material and test conditions always matter.

Heating Rate: Important, but Not in Isolation

Heating rate is one of the parameters that attracts a lot of attention when laboratories compare conventional and fast sintering.

A faster heating rate changes how quickly the restoration passes through different temperature ranges.

But faster heating does not automatically mean greater shrinkage.

Nor does a slow heating rate automatically produce more accurate restorations.

The effect depends on the material, restoration geometry, furnace characteristics, and the complete program.

For example, one study comparing different zirconia sintering protocols used conventional conditions of 1510°C for 120 minutes, a speed condition of 1540°C for 25 minutes, and a super-speed condition of 1580°C for 10 minutes. The mechanical results were not simply determined by total cycle time.

For a laboratory considering fast sintering, the more useful question is:

Has this specific zirconia been validated under this specific fast-sintering program?

That is more meaningful than asking whether the furnace can heat at 100°C/min.

Cooling Is Part of the Thermal History Too

Heating and holding usually receive most of the attention.

Cooling is often treated as the final step that does not matter very much.

That is too simple.

Zirconia continues to undergo thermal changes during cooling, and the cooling profile can influence the final state of the material.

The exact significance depends on the zirconia composition and the temperature range involved.

For laboratories, the practical point is not to invent a universal cooling rate.

It is to avoid modifying the manufacturer's cooling profile without a reason.

If a material supplier has validated a complete program, changing only the cooling stage means the laboratory is no longer using exactly the same process under which the material was characterized.

Pre-Sintered Density Can Vary Within a Block

This is particularly interesting from a manufacturing perspective.

A zirconia block may look completely uniform from the outside.

But if the internal density is not uniform, different regions can respond differently during sintering.

The study on uniaxially pressed zirconia compacts found that different sections of the same block showed different linear shrinkage rates. The bottom section showed the highest shrinkage and the top section the lowest in the experimental specimens.

This does not mean that commercial dental zirconia blocks necessarily have a large top-to-bottom shrinkage difference.

Modern manufacturing processes are much more controlled than a simple laboratory pressing experiment.

The study does, however, highlight a real manufacturing issue:

density uniformity matters.

For a zirconia manufacturer, this is one of the reasons pressing, drying, pre-sintering, and quality-control procedures cannot be treated as separate issues.

They are connected to final dimensional behavior.

Multilayer Zirconia Makes the Question More Complicated

A conventional zirconia disc may already have a complex manufacturing history.

Multilayer zirconia adds another variable.

A multilayer disc can contain different zones with different compositions or mechanical and optical characteristics.

If the composition changes through the disc, the sintering response may also vary.

Recent research on commercial zirconia materials has included strength-gradient multilayer products and examined shrinkage at different locations.

This type of testing is useful because it moves beyond the assumption that a disc can always be represented by one perfectly uniform material.

For a dental laboratory, this leads to a practical question:

Is the published shrinkage value based on one position in the disc or on measurements across different layers?

If you are buying multilayer zirconia in volume, I would ask the supplier this question before making the material part of a standardized CAM workflow.

Restoration Geometry Can Affect What You Measure

Another factor is the shape of the restoration itself.

A rectangular test specimen is easy to measure.

A crown is not.

A crown contains:

thin margins;

thicker occlusal areas;

internal surfaces;

rounded transitions;

cement space;

sharp or curved anatomical features.

A long-span bridge adds connectors and multiple units.

A 2017 Micro-CT study examined sintering shrinkage of Y-TZP copings and compared manufacturer-reported shrinkage with experimentally measured values.

The researchers found statistical differences between reported and measured shrinkage values, as well as differences between geometric test specimens and actual copings. They also reported that coping shrinkage was not completely uniform and that experimentally measured internal fit differed from the internal space specified in the CAD/CAM software.

This has practical relevance for laboratories.

A shrinkage value measured on a simple bar or cube is useful for material characterization.

But it does not describe every detail of how a complex crown will change.

The Shape of the Restoration Can Matter More Than Expected

A 2020 study investigated six different zirconia coping preparation designs and compared standard and fast sintering.

The researchers measured linear and volumetric dimensional changes at different stages of fabrication.

The sintered copings showed average linear changes of approximately 1.32% under the standard protocol and 1.54% under the fast protocol, with volumetric changes of approximately 3.82% and 3.90%, respectively, relative to the initial design parameters.

The authors concluded that preparation design had a greater influence on the dimensional changes than the difference between the standard and fast sintering protocols tested.

These values should not be confused with the approximately 20% linear shrinkage of the zirconia blank itself.

The researchers were measuring dimensional change of the fabricated coping relative to its design at different stages.

That distinction is important when reading technical papers.

A paper may use the word "shrinkage" while measuring a different part of the production chain.

Why Thickness Distribution Matters

Consider two restorations made from exactly the same zirconia.

The first has relatively even wall thickness.

The second contains very thin areas next to much thicker sections.

Even if the material has a known average shrinkage factor, the thermal response of the two restorations may not be identical.

The thicker areas contain more material and have a different thermal mass.

The geometry also affects how heat reaches different parts of the restoration.

This is one reason that laboratory technicians should be cautious when applying a fast sintering program validated on a single crown to a large bridge or a restoration with substantially different geometry.

The question is not only:

"Can the zirconia be fast-sintered?"

It is also:

"For which restoration size and geometry was the program validated?"

What About Different Zirconia Grades From the Same Manufacturer?

This is a common purchasing issue.

A manufacturer may offer:

high-strength zirconia;

high-translucency zirconia;

multilayer zirconia;

anterior zirconia;

or different 3Y/4Y/5Y formulations.

The discs may have identical dimensions.

They may even use the same shade system.

But the CAM compensation factor should not be assumed to be identical.

If the formulation or manufacturing process changes, the shrinkage behavior may change as well.

This is why the product code matters.

When a laboratory creates a CAM library, each zirconia material should have its own verified parameter rather than being grouped only by disc size.

Can Sintering Temperature Be Used to Correct Shrinkage?

This is a tempting idea.

Suppose a laboratory finds that a new zirconia is slightly too large after sintering.

One possible reaction is to increase the sintering temperature in an attempt to make the material shrink more.

I would not recommend using temperature as a substitute for a correct CAM compensation factor.

Changing temperature does not only change dimensional behavior.

It can also change:

grain growth;

phase composition;

optical properties;

mechanical behavior;

and other aspects of the microstructure.

Research has repeatedly shown that changes in sintering temperature and holding time can alter zirconia microstructure.

If the problem is an incorrect shrinkage factor, the first thing to check should be the material data and CAM setup.

The furnace program should not be used as a simple dimensional correction tool.

A Practical Example: Two Materials, One CAM System

Imagine a laboratory receives two zirconia materials.

Material A has a measured linear shrinkage of:

20.0%

Material B has:

19.0%

The final target dimension is:

20.00 mm

For Material A:

20.00 ÷ 0.80 = 25.00 mm

For Material B:

20.00 ÷ 0.81 ≈ 24.69 mm

The difference is approximately:

0.31 mm

That difference comes from only a 1-percentage-point difference in linear shrinkage.

The example is simplified, but it shows why a laboratory cannot safely assume that all zirconia discs can use the same compensation factor.

The CAM software needs to know which material is being processed.

What Can Cause a Laboratory's Shrinkage Results to Change?

When a laboratory notices that restorations no longer fit the way they used to, I would not immediately blame the furnace.

There are several things to check.

The zirconia material changed

Was a new supplier introduced?

Was the product code changed?

Was a different translucency or strength grade used?

The batch changed

Is the new disc from a different production lot?

Does the supplier provide batch-specific shrinkage information?

The CAM parameter changed

Was the compensation factor entered correctly?

Was the CAM library updated?

Was a parameter accidentally overwritten?

The sintering program changed

Was the holding time changed?

Was the peak temperature changed?

Was a speed program used instead of the conventional program?

The furnace program was copied from another material

This happens more often than people expect.

A program that works well for one zirconia does not automatically become correct for another.

The loading pattern changed

Are more restorations being processed at once?

Are large bridges being placed in the same cycle as small crowns?

Has the tray arrangement changed?

The furnace condition changed

Has the furnace been used for a long period?

Has calibration been checked?

Have the heating elements aged?

None of these possibilities should be ignored.

What Should a Zirconia Manufacturer Measure?

From the manufacturing side, shrinkage control should ideally be checked at more than one point.

A basic dimensional-control program can include:

Pre-sintered dimensions

Measure a defined test specimen before sintering.

Sintered dimensions

Measure the same specimen after the validated sintering program.

Linear shrinkage

Calculate dimensional change in the required direction.

Density

Check the pre-sintered and sintered density.

Batch comparison

Compare results between production lots.

Dimensional distribution

Measure more than one location when the product structure requires it.

For multilayer zirconia, this can be particularly useful.

A single average number can hide variation between different areas of the disc.

The exact test method will depend on the manufacturer's quality system and product specification, but the basic idea is straightforward:

If dimensional compensation is important to the customer, dimensional behavior needs to be measured during manufacturing.

What Should a Distributor Ask Before Buying in Volume?

If I were evaluating a zirconia supplier for distribution, I would ask questions beyond the product catalogue.

For shrinkage, I would ask:

1. What is the measured linear shrinkage?

Not just "approximately 20%."

Ask for the specified range if available.

2. How is it measured?

Ask about the specimen and test procedure.

3. Is the value batch-specific?

This matters if the product is supplied in large quantities.

4. Does each zirconia grade have a separate CAM factor?

It should not be assumed that all products share one value.

5. How is multilayer shrinkage characterized?

Ask whether measurements are made at different disc positions.

6. Which sintering program is used for validation?

The shrinkage number should have a defined thermal condition behind it.

7. Has the dimensional behavior been checked on actual dental restorations?

A geometric specimen and a crown do not behave in exactly the same way.

8. What happens when the recommended sintering program is changed?

This is especially relevant if the distributor's customers use fast-sintering furnaces.

These questions are useful because they reveal whether the supplier treats shrinkage as a controlled material property or simply as a number printed on a datasheet.

What Should a Dental Laboratory Record?

For laboratories processing a large number of zirconia restorations, it is useful to keep a simple material record.

For each zirconia product, record:

manufacturer;

product name;

zirconia type;

disc size;

batch number;

CAM compensation factor;

recommended sintering program;

furnace used;

date introduced into production;

and any dimensional verification results.

This does not require a complicated quality-control system.

Even a simple internal record can help when something changes.

If a laboratory receives a new batch and suddenly sees a difference in fit, the team can look back at the batch number and processing conditions rather than trying to remember what happened several weeks earlier.

Shrinkage and Sintering Are Connected, But They Are Not the Same Thing

It is worth separating two ideas.

Shrinkage is a dimensional result.

Sintering is the material-processing process that produces densification and changes the microstructure.

The two are closely related, but they should not be treated as identical.

Changing the sintering temperature may affect shrinkage, but it can also affect grain growth and other properties.

Changing the pre-sintered density can affect shrinkage without changing the laboratory's furnace.

Changing the zirconia composition can change shrinkage even when the same furnace program is used.

Changing the restoration geometry can influence the dimensional result even when the same material and program are used.

Investigating shrinkage therefore requires looking at the entire production chain.

A Simple Way to Understand the Main Factors

If I were explaining the issue to someone new to zirconia manufacturing, I would divide the factors into three groups.

Before the furnace

This includes:

powder characteristics;

composition;

particle packing;

compaction pressure;

pre-sintered density;

pre-sintering conditions.

These factors establish the starting structure of the zirconia.

Inside the furnace

This includes:

peak temperature;

heating rate;

holding time;

cooling conditions;

atmosphere;

and the furnace's actual thermal performance.

These factors determine how the starting structure develops during sintering.

After sintering

This includes:

final dimensions;

density;

grain structure;

phase composition;

and the fit of the restoration.

The laboratory sees the result here.

But the result was influenced by everything that happened earlier.

The Most Important Point for Laboratories

If a zirconia supplier gives you a shrinkage value, do not treat it as an independent number.

Ask what it is connected to.

For example:

20% shrinkage under what conditions?

That is a much better question than simply:

What is the shrinkage?

Was the material tested using the recommended furnace program?

Was the sample a simple bar or an actual restoration?

Was the measurement taken on one batch or several?

Was the material a conventional 3Y-TZP, a 4Y material, a 5Y material, or a multilayer zirconia?

These details help determine whether the number can be applied to your production workflow.

Final Thoughts

So, what affects zirconia shrinkage during sintering?

There is no single answer.

The starting condition of the zirconia matters. Pre-sintered density, powder characteristics, compaction pressure, and pre-sintering conditions can all influence how much the material contracts.

The zirconia formulation matters as well. Different zirconia systems can show different dimensional behavior under the same thermal conditions.

Then there is the furnace cycle.

Peak temperature, holding time, heating rate, and cooling conditions affect the material's thermal history. Research has shown that changes in sintering conditions can alter grain size and other properties, while studies on zirconia compacts have demonstrated that pre-sintered density and compaction pressure can influence linear shrinkage.

Finally, there is the restoration itself.

A crown is not a cube.

A bridge is not a bar specimen.

The geometry, thickness distribution, and internal surfaces can affect the dimensional result that a laboratory actually observes. Studies using real zirconia copings have found differences between geometric test specimens, manufacturer-reported shrinkage values, and experimentally measured restoration behavior.

For a dental laboratory, the practical conclusion is not to search for one "correct" zirconia shrinkage percentage.

The better approach is to use the shrinkage data for the actual material, keep the correct CAM compensation factor, follow the material manufacturer's validated sintering program, and check the result when introducing a new material or batch.

For a distributor, the same principle applies when evaluating a supplier.

A useful zirconia datasheet should tell you more than strength and translucency. Shrinkage data, CAM compensation information, measurement conditions, and recommended processing parameters are all relevant when the product will be used in a digital dental workflow.

From the manufacturing side, shrinkage is not something that begins when the dental technician presses "Start" on the furnace.

It starts much earlier, with the powder, the forming process, the density of the pre-sintered blank, and the way the material is manufactured.

The furnace then becomes one part of that chain.

Consistent zirconia production therefore requires more than checking the final sintered restoration. The dimensional behavior has to be controlled from the beginning of the material's manufacturing process.