Certainly, one of the most important parts of a pack is the foam that provides the padding between your body and the straps that carry the weight of the pack. In the early days of commercial pack design, the choices were pretty simple - open cell foam, various iterations of closed cell foams such as Ensolite, or no padding at all.

Open Cell Foam
Open cell foam is a material where the open-air chambers in the foam are interconnected. This makes for extremely soft and highly compressible foam. You can still find many camping mattresses made from this material, and it's likely the couch in your living room also contains this type of foam. While open-cell foam is very comfortable on your sofa, it's not very good as a padding material for packs. The high compressibility of the foam can cause pack straps to dig into your body, providing a minimum of comfort. While the straps may look good and feel comfortable at your local outdoor shop, they won't be when the pack's under full load. You can spot open-cell foam easily. Grab a pack strap and squeeze. If the strap compresses to 1/4 it's original size, it's open-cell foam.

Ensolite/Closed-Cell Foams
Ensolite is a closed-cell foam material where the open air chambers in the foam are completely surrounded by foam and not interconnected. As a result, Ensolite does not compress easily, yet provides good padding since the "air bubbles" in the foam are completely encapsulated. You can still find many outdoor shops carrying raw sheets of ensolite foam as sleeping pads. Since the air chambers in Ensolite are completely encapsulated and the foam is made from rubber, the material absorbs very little water.

For many years, Ensolite foam (and variations) cut into rectangles and sewn into fabric sleeves provided the chief source of shoulder and hip belt padding in better pack designs. The foam was dense, impervious to sweat, and comfortable. You can still find many packs made from this material today, and it's a perfectly good padding material for pack straps. You can spot closed-cell foam easily. Grab a pack strap and squeeze. If the strap hardly compresses at all, it's a closed-cell foam.

Dual-density Foams
Since open-cell foam is more comfortable next to the body (more compressible) and closed-cell foam is better next to the load (firmer support), several manufacturers utilize a dual-density design in their padding. Packs with dual-density foam attempt to capitalize on the advantages of both foams - they're more comfortable than closed-cell foam, yet support the load better than open-cell foam.

Compression Molded Foams
One manufacturing drawback to Ensolite, neoprene, and other basic closed-cell foams concerns the manner in which they're fabricated. The material is typically poured into a frame and cooked into flat sheets that are then cut to size. The problem is, what happens if you want to create an unusual shape? The cost of cutting and shaping foam is high and results in significant material wastage. A new process was needed for the exotic shaping, forming and customization of the next generation pack design.

Compression molded polyethelene foams originally appeared in European packs around 1988, and finally appeared in the Canadian and U.S. marketplace. Today, many of the better packs utilize this foam material in their hip belts, shoulder straps, and back padding. Compression molded foams utilize a different manufacturing technique. The outer nylon material is adhered to a block of polyethylene foam using a heat sensitive adhesive. Then the entire assembly is heated so the foam and adhesive soften. At this point, the assembly is compressed in a mold to provide shape. The shaped hip belt, shoulder strap, or back pad is then cooled to resolidify the foam and adhesive. The end product is a pad that is shaped into a custom form.

Compression molded pads are closed cell, pound for pound slightly lighter in weight than Ensolite or other older style closed cell foams, and kinder to the body since they are "shaped" without hard edges. As an added benefit, cavities can be formed inside the molded foam pads, and different densities of foam can be included to provide bendability in the final shape. Jack Wolfskin uses multiple densities of foams in their hip belts so they wrap around the waist easily for example. Polyethelene foams don't have the "memory" of other closed cell foams, but this is an advantage since over time, the foam will truly mold to your body shape.