The nature of the plastics is to expand while heating and compress while cooling. It is called as shrinkage. And this shrinkage is different from plastics to plastics. A good mold maker understand the design of the part and will consider correct shrinkage
While doing the product design, avoid thin wall sections. Read the plastics material guide to understand the minimum capabilities of the plastic. If there are areas which are thinner that the advised allowance, there is a great possibility of deviation in dimensions
Thick wall sections. A good designer will always plan for even distribution of thickness in the plastic product. Different wall thickness lead to different cooling & shrinkage. If the wall thickness distribution is not even, then adequate cooling water system has to provided to compensate the thickness difference.
Good quality steel for core and cavity inserts
Usage of good CNC Milling machines for core & cavity machining