More than any other parameter, the calcination process is capable of modifying the performance of the Cr/silica catalyst system (also known as Phillips-type catalysts) to generate activity and to influence the diverse active population in order to produce a wide variety of different polymer characteristics. This includes the polymer molecular weight, the shape and breadth of the MW distribution, the efficiency of comonomer and macromer incorporation, and the location of both short-chain and long-chain branches within the MW distribution. These fundamental polymer characteristics control how the polymer behaves during molding operations and they influence every physical property of the finished HDPE part. For this reason, a considerable number of calcination recipes have been developed over 60 years of commercial use to tailor the polymer characteristics for various HDPE applications. This is done by manipulating the relative amounts of differing active sites on the catalyst. This report attempts to explain the chemistry involved in the calcination process, to show how it can be controlled in various ways to achieve commercial ends, and to extrapolate these trends to predict the future of catalyst activation in the HDPE industry. (C) 2016 Elsevier B.V. All rights reserved.