In reference to the patent: US5991151 is this patent still valid and upheld or has it lapsed?
The term of the patent for an application filed after 1995 is 20 years after the priority date, subject to terminal disclaimers and/or extensions under 37.CFR 154.
In this case the priority date is May 30, 1997 based on an earlier filed Italian patent, there is no terminal disclaimer (from the first page of the patent, none listed), and there is no extension of time (from PAIR). Google lists that the maintenance fees have been paid. So the patent is in force until June 1, 2017.
most patents that I have seen tend to be for a 20-year period. however, I am unaware of the law behind how that works exactly. that being said i assume unless renewed (if that can be done) otherwise. this patent is only 18-19 years of age and is still valid
Since this patent was filed after 1995, it should expire 20 years from its filing date which is May 19th of 2018. However, Google lists its priority date as May 30, 1997 so it may expire then. I'm not a lawyer and expiration dates can be tricky, but it look like this patent is still in force and will expire either this May or next year in May.