I have not seen the initials IM used to refer to a patent or application. Pending applications are generally secret until formally published or issued. You can lawfully "design-around" a patent-pending article, but the risk can be high, especially when the patent claims have not been published. If you like the box design, then it is almost certainly easier and cheaper to simply go hire that box company for your products than to try and design around or, worse, get into a patent fight.
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I don't think IM refers to the patent number. "Patent Pending" means they filed a Patent application, but it is not yet issued as a patent and may not even be published.
You can search http://portal.uspto.gov/pair/PublicPair with this number ass the application number, but i doubt that's what it is.
You can also search http://assignments.uspto.gov/assignments/q?db=pat for the name of the company to see what patents and published applications you can find.
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The first thing the comes to mind with "IM" is instant messaging. I am not aware of people using an IM number in conjunction with a patent pending status though. Another possibility is that the "IM #" represents the companies internal product identifier (just a shot). If this is the case then you may have to approach the company to get information about the patent. Maybe some other attorneys see a connection to "IM" and patent pending that I am unaware of. I will let them discuss it. You can always try to search for the patent using other fields, such as: assignee, applicant, title and so forth. However this might be tediously frustrating if you are not 100% sure of those fields.
In the meantime, it may be better just to work with the company to use the box or develop a work-around. There is always risk with trying to knock off someone's design prior to issuance and prior to publication. Be cautious and engage a patent attorney's expertise when you come to know more of the details.
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No one who knows anything about patents -- and packaging professionals are pretty savvy about patents -- would include a patent application number on a product and, if they did, they would certainly not incorrectly preface the application number with "IM#." I think that acronym means "item management" number or some other packaging industry specific identification. I suggest you bring in a patent attorney to review the matter and perhaps reach out to the company who makes the package.
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IM does not refer to a patent. It is likely likely either "Inventory Management" (an inventory control number - otherwise known as a "serial number"), "Invention Memorandum" (an internal number for invention disclosures of a company), a part number, or a phone number ("instant message" or texting number), or it's "13" rather than "IM", as 13 is the start of many non-provisional patent application serial numbers or it's really "PN" for patent number or part number or it's for "Information Manual" to point you to the user manual. I have seen each one of those at one time or another on parts. You should show this to a patent attorney and find out which. Alternately, you could just call the manufacturer and ask them what the number means, and they will surely tell you as it is presumably there to tell you something.
Yes, you can search for that number on GAPS http://www.google.com/advanced_patent_search although I doubt the number will turn out to be a patent publication number. You might use another of the GAPS fields or some combination of them if you know that field's information.
Any registered patent attorney can quickly find out if there is a patent publication by that number
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: Be careful.
Before using a design that is subject to a patent application you need to consult with an attorney or patent agent who can guide you through the landmine you could be stepping into. Good luck.