To determine the exact degree of hardness of shore required, we must first examine the sealing points of the item. The sealing points at the same time positioning the element and prevent the water from drifting.
Historically, the systems of cartridges simple have all made use of cartridges with the inner sealing ring, friction with the tips of end hard. This design provided the strength and support of fold.
However, as the filters become more sophisticated, it is the design of the caps. The element Posi-Flow Sta-Rite, for example, uses a background of soft vinyl to seal the edges inside and outside, and lock the cartridge in the filter base while the top cap is a compound hard to the strength and integrity of the cartridge.
The filter Star Clear Hayward uses caps hard to ensure the integrity with a flexible sealing joint to enable a superior seal to the edge of a knife.
The two new models American Predator and Jacuzzi CFR are based on a top and a bottom flexible to get an o-ring. These configurations of the end cap are of excellent designs. Only the compounds change to meet the specifications of the manufacturers.
This is another reason why Unicel builds its replacement cartridges according to the design specifications OEM and why Unicel is number one in terms of quality and reliability.