In reference to the patent: US5779129
is the patent valid coz its more than 20 years now.Can we use the design for our production
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You are correct that for patents filed after 1995 that the expiration date is 20 years after the filing or priority date. In this case the filing date is January 24th, 1997 so this patent should have expired on January 24th, 2017. I looked in the US Public Pair site to see if there were any patent term extensions and didn't find anything. Extensions are sometimes issued when there is an excessive delay in processing a patent application. You should be okay making a product exactly reproducing the invention described by this patent. I would caution that there is always the potential for other patents that might be refinements that are still in force. A good way to start looking for these would be to look at the forward citations for this patent. As DonQuiKong points out in his comments, there is at least a theoretical possibility that an older patent with a patent term extension might be relevant to your freedom to market this invention. I think this is highly unlikely but not impossible.
Please understand that I am not a lawyer and this is not a legal opinion. As always, I suggest consulting with an actual patent attorney.