Methods of forming appliances for arch expansion

A method of forming an appliance to expand an arch of a patient, the method comprising:

• receiving scan data of teeth of the arch of the patient;
• and
• determining a shape of the appliance to engage the teeth and expand the arch, the appliance comprising a shell and a force generating portion, the shell including teeth receiving structures configured to receive at least a portion of the teeth, the force generating portion connected with the shell and spanning a space between opposing posterior teeth of the patient and to apply an expansion force between the opposing posterior teeth of the patient,
• wherein determining the shape of the appliance comprises:
• determining a shape of the shell based on the shell being formed of a first polymer material having first degree of crosslinking, and
• determining a shape of the force generating portion based on the force generating portion being formed of a second polymer material having second degree of crosslinking different than the first degree of crosslinking, wherein the shell and the force generating portion are made from the same prepolymer material, and
• manufacturing the appliance according to direct fabrication instructions based on the determined shape of the appliance.