  26,000 3-inch photomultiplier tubes (PMTs) have been produced for Jiangmen
Underground Neutrino Observatory (JUNO) by the Hainan Zhanchuang Photonics
Technology Co., Ltd (HZC) company in China and passed all acceptance tests with
only 15 tubes rejected. The mass production began in 2018 and elapsed for about
2 years at a rate of $\sim$1,000~PMTs per month. The characterization of the
PMTs was performed in the factory concurrently with production as a joint
effort between HZC and JUNO. Fifteen performance parameters were tracked at
different sampling rates, and novel working strategies were implemented to
improve quality assurance. This constitutes the largest sample of 3-inch PMTs
ever produced and studied in detail to date.
