If thickness must change, use a gradual taper or fillet (ideally no more than a 15% change) rather than sharp steps.
Draft is the taper applied to the vertical walls of the part to facilitate ejection. Without draft, the part creates a vacuum against the mold wall, causing it to stick or warp during ejection. injection mold design guide
Sam finally pulled up the —not a dusty PDF, but a living document forged by decades of melted nylon and broken ejector pins. He flipped to the first true page, handwritten in the margins: If thickness must change, use a gradual taper