From the Aug. 31, 2015 edition of Coin World:
A commonly encountered planchet error is the split planchet. Such errors involve a blank or planchet cleaving spontaneously along the horizontal plane. The weights of the two hemi-planchets can be subequal or very unequal; it all depends on how far beneath the surface the plane of cleavage is located.
Split planchets are similar in both appearance and etiology to lamination errors. The latter typically manifest as superficial peeling and cracking. Both errors are believed to be caused by the presence of contaminants in the alloy. They can also be caused by a poorly mixed alloy.
A planchet can split before the strike or after the strike. The former are often referred to as “split-before-strike planchets” and the latter “split-after-strike planchets.” The latter error would be more properly referred to as a “split coin” since a planchet is transformed into a coin when it is struck. However, the term “split coin” has never caught on.
Whether it occurs before or after the strike, split planchets are characterized by the presence of coarse striations on the internal face. The roughness of the internal surface varies considerably.
Planchets are much more likely to split before the strike. A 1961-D Lincoln cent provides a typical example of a split-before-strike error. Coarse striations dominate the reverse face of this 1.4-gram example.
A representative example of a split-after-strike error is provided by both halves of a 1944 Lincoln cent. The direction and texture of the striations on the obverse half match those on the reverse half. This observation, along with the fact that the combined weight of the two halves matches that of a normal cent (3.1 grams), demonstrates that this is a genuine mated pair.
Split planchet errors are almost completely restricted to solid-alloy coins, like copper-alloy cents and copper-nickel 5-cent coins. Clad-composition coins are generally not susceptible to such errors, except in the case of rare “split core” errors.
Sometimes cleavage of a coin or planchet is only partial. The coin ends up with a cleft in the edge that extends a variable distance into the body of the coin. This error is known as a “clamshell split.”
The thinner side of a clamshell split will sometimes break off along its “hinge,” producing a planchet or coin with an exposed partial split. If this breakage occurs before the strike, the coin ends up with a well-struck portion and weakly-struck portion with coarse striations.