Patents are territorial, thus assuming no patent was issued in the USA you are free to manufacture and sell the embodied product.
You must remember that if the embodied product is just an improvement over a previously patented product, then you may need a license from the owner of the patent over which the improvement was made. How do you know?, you need to contact a Registered Patent Attorney and have her/him look up the matter.
There are no international patents anywhere in the world.
Your only concern is if you export the embodied product, and there is a patent in the country where you export the product, then you will be a patent infringer in that country, not in the USA.
USPTO Registered Patent Attorney, Master of Intellectual Property law, MBA I am neither your attorney, nor my answers or comments in AVVO.com create an attorney-client relationship with you. You may accept or disregard my free advice in AVVO.com at your own risk. I am a Patent Attorney, admitted to the USPTO and to the Florida Bar.
From what you described, it appears that there is no patent either in the US or in any other country that covers what you would like to do. You should consider discussing your product with a patent attorney to be sure that there isn't some aspect of the product that could be patented. Also, a patent attorney can conduct a prior art search and advise you on any art or granted patents that may affect what you want to do.
First of all, there is no such thing as an international patent. Each country (or group of countries such as the European Union) issues its own patents. Second, patents don't cover mere ideas. They cover tangible, useful inventions that meet the standards of patentability---useful, novel, non-obvious, and "enabled" (i.e., described in patent specification with sufficient specificity as to enable others skilled in the art to make or use the invention). Third, patents cover not only products, but also methods (or processes) for making them and methods for using them. Even though the idea for your product may have been in the public domain for decades, this does not mean that there are no patents that cover your proposed product, or methods for making or using your proposed product. Your first step should be to retain counsel to conduct a patent clearance analysis. I never cease to be amazed at how often my clients are mistaken when they express the view that their new proposed product does not violate patents owned by others. You will only know the answer to your question when you retain counsel to conduct a patent/IP clearance analysis. If you file a properly drafted U.S. utility application you will have the benefit of a one year grace period before filing foreign counterparts, but you need to be sure that your application is properly drafted, Because we have moved to a first to file system, it is essential that you expedite preparation and filing of your patent application if you have reason to believe that one of your competitors may do so.
I don't draft and prosecute patent applications---but I defend or challenge them regularly in litigation where parties expend hundreds of thousands and often millions of dollars in legal fees to vindicate their positions in the marketplace. It never ceases to amaze me how often people screw up their patent protection by trying to cut corners and save on legal fees. If you are bringing a product to market with significant commercial potential, you owe it to yourself and your family to expend the resources necessary for a proper patent clearance analysis and preparation of a well-written patent application. This is not the place to cut corners.
Two question areas block a secure assurance that you are 'safe'. (1) The difference between the raw idea, and the actual product. Flying machines -- excuse me, 'heavier-than-air flying machines' were talked about and tested for years before the Wright brothers flew one 112 feet at Kitty Hawk, NC; but the patent for their aeroplane was considered effective nonetheless as they'd solved a number of problems, including power-to-weight and in-flight-maneuvering, which had been only generally discussed. The prior disclosures must be of all the critical details -- those which are otherwise 'novel' and 'unobvious', not just generalities of goal or principles. Secondly, there is the 18-30 month 'gap' between foreign and U.S. applications through the PCT, plus the 'silent period' within the US -- where an applicant may have opted for no foreign priority and kept their application unpublished.
Also, as already mentioned, there is always a concern whether existing patents (or pending patent applications in their finally-issued form) will claim enough of what you need to do for your patent to function, that you must obtain a license to practise your invention from those prior applicants.
Those details matter, and should be discussed with a patent (prosecution) attorney, in confidence.
However you could on the alternative go ahead, manufacture and use and sell, and simply be ready to either license, or close down, the line(s) of business based on your product. All a patent ever gives the holder is the right to sue to enforce a monopoly. Whether this is a profitable enough effort depends on the specifics of business operations, where patents, litigation, and future uncertainties are the norm, not the exception. Good luck!
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Be careful here. There are international patent applications and international patent publications and the time periods involved are much longer than 12 months before they are necessarily found in a routine US patent search. The reason is that international patent applications are filed in their home country, which is not the USA except in the case of a US applicant. An international patent application will be published 18 months after it is filed, just like the US national application, but is not published US patent database since it has not yet entered the US national phase which does not occur until the 30th month following filing. So, to be sure you catch any international patent application, you need to search a database that includes international patent publications by WIPO. One such patent database is the one run by WIPO itself, called PATENTSCOPE that can be accessed at http://patentscope.wipo.int/search/en/search.jsf
or alternatively by searching http://www.freepatentsonline.com/ [which requires a free registration and also includes the European and Japanese patent databases] or several other patent databases. If you are just using GAPS (Google advanced patent search) at http://www.google.com/advanced_patent_search , you need to realize you will not get the international patent publications. Accordingly, as noted, be careful! Better yet, be smart and hire a registered patent attorney who knows to search WIPO as well as the US database.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Short Answer: An idea that has not been patented and is in the public domain is available for use by anyone. Re international protection, the same answer. Patents are territorial, so, if, for e.g., the idea is patented in Canada but not the U.S., then you won't be able to use it in Canada, but will be able to do so in the U.S.. Good Luck.