Sponge rubber is made by incorporating into the compound a gas-producing chemical such as sodium bicarbonate, which expands the mass during the vulcanization process. Sponge rubber is manufactured in sheets, molded strips, and special shapes. Sheets and parts cut from sheets will usually have a surface impression since sheets are usually molded against a fabric surface which allows air to be vented during the expansion of the sponge. Molded strips will have open cells exposed at the ends of the part unless otherwise specified. Die-cut parts will have open cells on all cut edges. On parts where open cell surfaces cannot be tolerated this should be so specified.

Trapped air, which may affect the finish, is a universal problem of sponge manufacturing due to the fact that sponge molds are only partially filled with uncured rubber, allowing for expansion to fill the mold. For this reason long and/or complicated cross sections may require vents or multiple splices to effect low reject percentages. To minimize trapped air, it is common practice to use a considerable amount of a chemically inert dusting agent such as talc, mica, or starch, which is difficult to remove completely from the surface of the finished part, although molded closed cell parts prepared by transfer molding need not have this disadvantage.

In addition to a normal mold skin surface, some parts are manufactured with an applied solid rubber skin or coating to give a more durable, water-resistant surface when exposed to weathering. This is usually applied by calendering a thin sheet of solid rubber compound (0.005 in.-O.040 in.; 0.12-1.0mm) and applying it to a sheet of sponge compound and placing this in a mold suitably parted to form skin on the exposed surfaces of the part.

Closed cell rubbers are made by incorporating gas forming ingredients in the rubber compound, or by subjecting the compound to high pressure gas such as nitrogen. Expanded rubbers are manufactured in sheet, strip, molded and special shapes by molding or extruding.

Closed cell sheets are generally made rectangular and of sufficient thickness to be split into several layers for die-cutting. From this use is derived, for economic reasons, the term "skin one side or no sides, our option". Closer tolerances can usually be maintained on split sheets (no skin surfaces) than on sheets with a natural skin. Unless otherwise specified, the presence of the skin on the top or bottom surfaces of sheet and strip expanded rubber is optional. Die-cut parts will have exposed cells on all cut edges. On parts where exposed cell surfaces cannot be tolerated (appearance or abrasion, etc.) this should always be so specified.

Extruded closed cell rubber is made by extruding the raw compound through a die which determines the shape of the section. The extruded stock is carried from the die by a conveyor system in a continuous process through vulcanizing chambers. As it moves through the vulcanizing chambers the heat causes the blowing agent to decompose to produce an inert gas which expands the extrusion. The gas generation takes place in the middle section of the vulcanizing process and the cure is completed as the extrusion completes its travel through the remaining chambers. On emerging from the vulcanizing chamber the extrusion is cooled to create dimensional stability. Hole punching, coating, dribacking buffing and cutting are additional operations which can be performed following the cooling. The extrusion can be placed onto reels in continuous lengths or cut to specific lengths depending on the needs of the customer.

Characteristics of Extruded Closed Cell Rubber are:

1. The surface of the extruded section has a natural skin formed during vulcanization.

. It is possible to produce the part in continuous lengths.

3. A great variety of complex and irregular shapes may be produced.

4. Air chambers or hollowed-out designs may be utilized, giving the advantage of reduction in weight of material. The design engineer, by properly designing the cross-section with maximum air chamber space, can generally achieve considerable advantage in terms of performance and compression deflection.

Molded closed cell parts are manufactured similarly to open cell molded sponge. They require venting of trapped air and possibly the use of inert dusting powder which is difficult to remove completely from the surface of the finished part.

Distinct advantages of closed cell products are their low water absorption characteristics, providing a tight seal and the ability to conform to curves, corners and varying cross-sections without bridging or creasing. This is attributable to the closed cells which do not collapse, losing air as in open cell sponge, and yet deform sufficiently to conform tightly to irregular surfaces. Its thermal value is utilized in insulation applications.

In recent years manufacturers of cellular sealing products have developed and are supplying a type of seal based on a co-extrusion of dense and sponge rubber. The majority of these types of seals are used in the automotive industry to seal doors, hoods, and trunk lids. The major components of this type of seal are cellular compound, dense compound, and reinforcing woven wire (or stamped steel) embedded in the dense portion. The continuous curing process usually requires two extruders with the utilization of hot air, molten salt or fluidized bed curing mediums.

A compression set test has been in use for a long period of time on solid rubber and open cell sponge rubber products - 50% compression of sponge, for 22 hours at 70° C (I 58° F). The set test is used to determine the quality of those products and their applicability to certain types of usage. Because of this extensive use of the set test on other materials, it is frequently applied to closed cellular materials for the same purposes, namely to determine the quality and applicability of the closed cellular material for general usage or for specific jobs. However, due to the special characteristics of the closed cellular structure, the compression set test has an entirely different effect on closed cellular materials and requires an entirely different interpretation The differences in application and interpretation of the compression set test on open and closed cellular materials are shown below:

It is because of this very great difference in behavior of open cell materials vs. closed cell materials in the compression set tests that ASTM D-1 056 and SAE J1 8 contain a modified set test (22 hours at room temperature, with 24 hour recovery) on these materials. For the same reason, several military specifications on closed cellular materials do not use the standard test as indicated above but have various special test requirements which take into consideration the differences of the properties of the closed cellular materials.