Zirconia can indeed be milled.
This process is typically done using CAD/CAM systems.
These systems are used for creating various dental prosthetic restorations like crowns and veneers.
Zirconia blocks used in these systems come in two forms: fully sintered and pre-sintered.
1. Pre-Sintering Process
Pre-sintered zirconia, also called "green-state" zirconia, is initially soft and malleable.
It is not suitable for milling until it undergoes a sintering process.
This process involves heating the material to a specific temperature.
This hardens it into a chalk-like state, making it suitable for milling.
The material density in the pre-sintered state is about 40% to 50% of its maximum theoretical density.
2. Fully Sintering Process
Fully sintered zirconia has a lower volume fraction of pores.
It has greater strength and improved resistance to hydrothermal aging.
It can be milled directly to the final desired dimensions without further heat treatment.
However, the high strength of fully sintered blocks leads to longer milling times and rapid wear of machining tools.
3. Advantages of Pre-Sintering
Despite the need for post-milling sintering and accounting for sintering shrinkage, pre-sintered blocks are commonly used in CAD/CAM systems.
This is due to their shorter milling times, easier processing, and higher productivity.
4. Types of Milling Machines
Milling machines for zirconia can be categorized into wet type, dry type, and combined type.
Wet type machines use coolant or water to remove excess material and reduce heat during milling.
They are suitable for hard materials.
Dry type machines use air for material removal and are ideal for softer materials like zirconia.
Combined type machines offer the flexibility to use both wet and dry methods.
They provide versatility in processing various dental restoration materials but require a higher upfront investment.
5. Sintering Process
After milling, zirconia undergoes sintering.
This is a critical process that fuses materials together without liquefaction.
It significantly affects the final mechanical strength and properties of the zirconia product.
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