If used properly (including selecting the proper size of cylinder), no heel block is required. The purpose of the cylinder is to lock the piston rod (and sliding core attached to the piston rod) in a forward position. This means the sliding core can be retracted before the mold is opened. This has a benefit when the B-side sliding core is passing through A-side insert while the mold is closed. Note: If the cylinder size is improperly chosen for the intended application, it is possible to have even slight movement on the end of the sliding core, so please take care when selecting an HLCP cylinder size. If you find that the load capacity of the desired cylinder size is very close to the estimated load requirement in the mold, then it is recommended to select a larger HLCP cylinder size. Information on load capacities (with and without preload) is given in the DME catalog as well as in the packing slip.