There are a number of clipped Planchets, each describing the type of clip. The correct terminology is an Incomplete Planchet. Just an FYI.
The Blakesley Effect is a term used to describe inefficient metal flow, opposite a clip on a coin, when the rim is formed and the subsequent imperfect or incomplete rim formed at that position after striking.
Blakesley effect. When blanks are punched from the strip they have flat surfaces. If they were struck that way the centers would be strong and the coins would be weak around the rims and often not fully struck (the dies are slightly convex so they meet the planchet first near the centers and then working outward.) To counter this the edges of the blank are "upset". This is done by running them through a mill that applies pressure against the edges toward the center. As the blank goes through the mill the diameter decreases and the edge thickens. This allows the rim and periphery to form better during the strike. If the planchet has a clip, as it rotates through the mill, when it reaches that spot it is suddenly smaller in diameter than it is supposed to be and the pressure drops. With no pressure the upset rim doesn't form opposite the clip. Then when the coin is struck that section that is missing the upset edge results in weakness on the finished coin 180 degrees opposite the clip. (Blakesley effect) The reason why it is not always present is because if the strike is strong there is enough force and metal movement to fill the rim in that location anyway.
