Injection molding shrinkage simply refers to how much the polymers will contract during the cooling phase. During the injection molding process, molten plastic material is injected into a mold cavity under high pressure, and when the plastic material cools, it shrinks as it solidifies. 

This shrinkage can cause the molded part to be smaller than the intended size, and have an uneven shape known as warpage.

Injection molding shrinkage can vary depending on the type of plastic material used, the design of the part, and the processing conditions. It is important to take into account the expected shrinkage during the design and tooling stages to ensure that the final part meets the required specifications.

To be safe, manufacturers often compensate for shrinkage by oversizing the mold cavity or adding additional material to the part, which is then trimmed or machined after molding to achieve the desired dimensions.