The sprue should be located at the thickest portion of the pattern to ensure a steady and consistent flow of molten material during casting. This placement helps maintain adequate molten metal pressure, reducing the risk of shrinkage defects and voids. It allows the metal to properly feed the casting as it solidifies, promoting uniform filling and minimizing internal stresses. Positioning the sprue at thinner areas or margins can cause premature solidification and incomplete casting. Therefore, locating the sprue at the thickest part ensures optimal metal flow and casting integrity.

Reference: Principles and Practice of Fixed Prosthodontics, Rosenstiel, Edition 4.