A mold includes a first section, a second section, and a weakened region that joins the first section to the second section. The weakened region is breakable, deflectable, or deformable in response to a first threshold force to enable the first section to be removed from a shell independently of the second section after the shell is formed over the mold. The first threshold force is less than a second threshold force that would damage or permanently deform the shell. The first section is a first mold of at least a portion of one or more first teeth of a dental arch and the second section is a second mold of at least a portion of one or more second teeth of the dental arch.