Planchet and Blank are sometimes used to refer to the same thing; a coin that has not been struck by the dies.
Most of the time the term for the round metal disc that has NOT been through the upsetting mill is a "blank." These are flat faced with no raised rim. This is also called a Type I when referring to the disc and also known as a blank.
After the disc has been processed by the upsetting mill it will have a raised rim around the outside. This is more often called a Type II when referring to the disc and also known as a "Planchet."
"Shine, shine, a Roosevelt dime All the way to Baltimore and running out of time" - Tom Waits 'Clap Hands'