If the mold has a hot runner system there can be locations that trap material leading to drag out over time. Areas of particular concern include the sprue opening, any 90° corners, mismatch between components, valve pins, spreader tips, and internal heaters. Any area that results in a hang up of material can cause defects long after a material or color change has occurred. Even if only transparent material is ever run through the mold, a poor material change in the barrel can put contaminated material into the manifold.