The size of the printed item is smaller than my model data.

When the size of my 3D data is 18mm, the size of the printed item is only 17mm or smaller. I am confused. what is the reason?
I use gray V4 resin.
I searched for a long time, but did not find the answer.

Are you fairly new to printing on the Form 3? Also, when did you take your post-print measurement, was it immediately after printing, after the wash, after sanding, after your cure process, etc? You’ll get slightly different measurements at each stage. And keep in mind, measurements involving the support-side of your print will be the least accurate due to the often fuzzy-nature of the support side. Similarly, if you print without supports there’s often a little extra resin on the build platform side. Shrinkage in the direction transverse to the build platform is often a little different (usually larger) than in the other directions.

It would be nice to hear from Formlabs on this issue, but I’ve noticed different amounts of shrinkage depending on the resin type, the PreForm version, or maybe the firmware version on the printer, hard to tell these two apart as they often come together.

How do I know this? Well, if you tell PreForm to print in black, but you actually give it clear or Tough resin, you’ll get different shrinkage than if you tell PreForm honestly what you want to print. I seem to notice different amounts of shrinkage with PreForm updates, so I suspect the printer accounts for the local shrinkage factors of each resin types, to try to minimize problems for the end-users. The laser intensity is different for each resin type, so depending on which direction one goes in this experiment one can get under or overcuring, which contributes to the issue as well.

That said, if you want the prints to come out as close to your specs as possible, you just have to get used to the characteristics of how the printer prints each resin. I find I get very accurate prints in the standard modelling resins and several of the engineering resins, especially the ApplyLabWork Black resin. But I haven’t been getting terribly accurate prints in the Flexible 80a. That said, often you don’t need exact when you are printing Flexible 80a.