Zirconia discs are manufactured to mill easily, maintain low tool costs and enable a reasonable mill time to provide a favorable ROI for crown fabrication. That said, it is not necessarily a perfect product for milling as milling does come with a few problems. Chipping is likely the most common problem that occurs when milling zirconia. Interestingly, there are not a lot of reasons but many labs do encounter this from time to time while others find it to be a daily occurrence. Here is my short list of chipping problems and resolution to each of the problems.
1. Incorrect margin line offset settings: When designing a zirconia restoration it is critical that the margins are milled at 0.2 MM (200 microns). In order to reduce the cost of tools the discs are pre-sintered to a relativity soft Vickers hardness. Milling this soft disc with thinner margins than 200 micron margins will result in and increased likelihood of chipping. One may get some units to mill thinner but the predictability is not there and running an efficient milling operation is most effective when chipping is reduced to the bare minimum. This reduces both machine time, and wasted zirconia. In both 3Shape and Exocad there are settings to automate this design feature so all restorations designed will have or maintain minimum thickness.
2. Worn milling tools: All milling tools have a life expectancy. Once dull, all tools cause zirconia chipping. When purchasing, there are two primary considerations. First, purchasing tools from a company that supports your mill will likely lead to more rapid tech support resolution when troubleshooting. Next, when evaluating cost, it’s best to base the decision on “tool cost per unit.” Less costly tools may actually cost more in the long run. Lastly, unless you have software that tracks tool usage life, longer life tools will reduce chipping as you will have dull tools less often and thus less chipping.
For a more in-depth look at how to reduce chipping and optimize your digital workflow, download our eBook, 19 Tips for the Digital Lab.