BobCDT
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Discs are not individually checked for shrinkage. They are batch tested and should all be the same within each batch. When you look at the disc manufacturing process, the powder used in each batch is all from a single batch. The powder press is set a the same huge pounds per square inch of pressing the powder into a disc form. The discs come out of this press and are all (100%) weighed to make sure the correct amount of powder dropped into the disc mold. Once pressed, each disc is individually shrink wrapped and then go into a cold isostatic press in most disc manufacturing facilities. This ensures even density throughout the disc. Uneven density can cause warpage but I would not necessarily conclude this to be John's problem. Not all companies use this second isostatic press step. Next, all the discs are pre-sintered in large sintering furnaces for 4-5 days with a max temp of about 1000C. This preshrinks the disc and burns out the binders that help hold the disc together during the first press cycle. It is this sintering cycle that determines Vickers hardness and thus tool usage and shrinkage of the batch. the hotter and or longer this cycle the less shrinkage we have and the harder the disc. There are other considerations for this sintering cycle such as managing this temp to reduce chipping when we mill the discs. Lastly, not all powders are created equal. Like everything else, there are high quality and low-quality powders readily available. Honestly, you tend to get what you pay for. Generally speaking, inexpensive discs are likely made from low-grade zirconia powders. They may work fine for single units but could be a contributing factor to John's problem. I don't really know what powder John's discs are made from but even Tosoh, the world leader in dental zirconia powder sells different grades of powders. I would suggest using high-end disks for large bridges to reduce potential problems.
