Some cartridge end caps are hard and others are soft.
Some catridges have one hard end cap and one soft end cap.
Why is this?
To determine the exact degree of shore hardness required, we first must look at the element sealing points. Sealing points both position the element and prevent water by-pass.
Historically, single cartridge systems all used cartridges with inside i.d. friction seals with hard end caps. This design provided both strength and pleat support. However, as filters became more sophisticated, so did end cap design.
Sta-Rite's Posi-Flow element, for example, uses a soft vinyl bottom to both seal the inside and outside edges and lock the cartridge into the filter base, while the top end cap is a hard compound for cartridge strength and integrity. Hayward's Star Clear filter uses hard end caps for integrity with a soft gasket to allow a knife edge top seal. Both the new American Predator and Jacuzzi CFR designs rely on a soft top and bottom to achieve an o-ring seal. These end cap configurations are excellent designs. Only the compounds change to meet the manufacturers' specifications.
This is another reason why Unicel builds its replacement cartridges to OEM design specifications, and why Unicel is number one in quality and reliability.