Printing unit comprising two or more stereolithography machines and at least one data processing unit (3, 31) configured in such a way as to manage the functions of each one of said stereolithography machines for printing three-dimensional objects through a stereolithography process, wherein each one of said stereolithography machines comprises first coupling means for the installation of at least one recharge cartridge of the base material to be used for making said three-dimensional objects, said recharge cartridge being provided with a univocal identification code, said printing unit comprising a validation system of at least said one recharge cartridge, said validation system being provided with: - said at least one data processing unit (3, 31) comprising reading means (32, 321) configured to read said univocal identification code of said recharge cartridge and transceiver means configured in such a way as to transmit said univocal identification code to a remote server configured to communicate with said data processing unit (3, 31) through an Internet connection; - said remote server comprising storage means having a predefined list of univocal identification codes, considered authentic and active, related to said recharge cartridges stored therein and said remote server having a memory allocation in said storage means that is reserved to each one of said stereolithography machines, said remote server being configured to: - receive said univocal identification code related to said recharge cartridge from said data processing unit (3, 31); - compare said univocal identification code with said predefined list in order to verify that said univocal identification code is authentic and active; - send a signal of validity of said univocal identification code to said data processing unit (3, 31) in the case where said univocal identification code is authentic and active; said transceiver means of said data processing unit (3, 31) furthermore being configured to receive said signal of validity, and, after receiving said signal of validity, to transfer said base material from said recharge cartridge to a vat arranged on one of said stereolithography machines and said remote server furthermore being configured to: - calculate the value of the total quantity of base material available for each one of said stereolithography machines by adding the quantity of base material transferred during said transfer operation to the total quantity of base material previously associated with each one of said stereolithography machines and stored in said memory allocation; - associate said calculated value of the total quantity of material with each one of said stereolithography machines, storing said value in said memory allocation; - modify the state of said univocal identification code related to said recharge cartridge from active to inactive in said predefined list, in the case where said base material was completely transferred from said recharge cartridge into said vat. - said at least one data processing unit (3, 31) comprising reading means (32, 321) configured to read said univocal identification code of said recharge cartridge and transceiver means configured in such a way as to transmit said univocal identification code to a remote server configured to communicate with said data processing unit (3, 31) through an Internet connection; - said remote server comprising storage means having a predefined list of univocal identification codes, considered authentic and active, related to said recharge cartridges stored therein and said remote server having a memory allocation in said storage means that is reserved to each one of said stereolithography machines, said remote server being configured to: - receive said univocal identification code related to said recharge cartridge from said data processing unit (3, 31); - compare said univocal identification code with said predefined list in order to verify that said univocal identification code is authentic and active; - send a signal of validity of said univocal identification code to said data processing unit (3, 31) in the case where said univocal identification code is authentic and active; - receive said univocal identification code related to said recharge cartridge from said data processing unit (3, 31); - compare said univocal identification code with said predefined list in order to verify that said univocal identification code is authentic and active; - send a signal of validity of said univocal identification code to said data processing unit (3, 31) in the case where said univocal identification code is authentic and active; - calculate the value of the total quantity of base material available for each one of said stereolithography machines by adding the quantity of base material transferred during said transfer operation to the total quantity of base material previously associated with each one of said stereolithography machines and stored in said memory allocation; - associate said calculated value of the total quantity of material with each one of said stereolithography machines, storing said value in said memory allocation; - modify the state of said univocal identification code related to said recharge cartridge from active to inactive in said predefined list, in the case where said base material was completely transferred from said recharge cartridge into said vat.