What Causes Sink Marks & Warping? Sink marks and warping in injection molded parts is primarily caused by the differential cooling rates between the surface skin and the interiors of the part. All plastics undergo shrinkage when cooled. The outer surface skin cools and solidifies rapidly as it comes in contact with the mold. The plastic in the interior of the parts takes much longer to cool down. As the interior cools and shrinks it creates a vacuum inside. The surface skin suffers a dimple or sink as the semi molten plastic moves to the interior to neutralize the vacuum. Alok’s “Sink Free” masterbatches have been formulated to help the processors avoid expensive refits/redesigns to their molds/parts. Besides eliminating sink marks, “Sink Free” masterbatches also reduce the warping characteristics of the molded part. As a result the cooling time can be greatly reduced. This ultimately gives the processor the benefit of shorter cycle times and therefore increased production.