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             SPONGE RUBBER

SPGESTRIP.jpg (14053 bytes)                                   SPGELAMINATING.jpg (13055 bytes)

      SPONGE STRIPPING                                                  SPONGE LAMINATING

As with the other rubber mauufacturing methods sponge products can be formulated from a variety of elastomers with varying physical properties depending on your specific application.Sponge rubber can be had in sheets, rolls, molded and extruded parts.There are two types of sponge products listed below with their individual characteristics:

                                                                                      SPONGE TYPES

OPEN CELL

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.

Since the solid skin must stretch to cover the surface of the mold during the blowing of the sponge compound, there are practical limitations to designs which can be made by this process, as when skin stretches, the thickness decreases and may ultimately break through. In addition to the above method, an applied skin may be formed by dipping a molded and cured part in latex or cement and depositing a coating on the surface of the part, followed by suitable drying and curing. This coating may be built up to desired thickness by multiple dipping. Limitations on this method are those inherent in most dipping methods such as a tendency to bridge slots or holes, loss of detail of molding, and uneven thickness of skin.

EXPANDED CLOSED CELL

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.

Design of extruded or molded shapes (uncored or cored) radically affects the compression of parts and leads to greater or less apparent compression set values.

CELLULAR SILICONE RUBBER

Cellular silicone rubber in sheet, molded or extruded forms can be made by processes similar to those for other cellular rubbers, or by foaming a liquid silicone polymer. A post-cure period in a hot-air oven is usually used to insure complete vulcanization. Because dimensions can undergo some change during this postcure, wider dimensional tolerances must be allowed, particularly on molded items. Suggested dimensional tolerances for molded cellular silicone rubbers are given in Table 32, and for extruded cellular silicone rubbers in Table 33. Chemically blown cellular silicone is almost always produced with a closed cell structure. Cellular silicone rubber foamed from a liquid can be partially or completely open cell.

SPONGE-DENSE SEALING PRODUCTS

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.

Sponge-dense sealing products are almost always closed cell. These products have many of the characteristics of expanded (closed cell) rubber mentioned in previous paragraphs.

COMPRESSION SET TEST

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:

 

 

COMPRESSION SET COMPARATIVE TABLE

Open Cells Closed Cells
1. Air is free to pass through open cells. There is no effect of the 70 C (158 F) test temperature on the air pressure in the cells. 1. Air is not free to pass through the closed cells. The 70 C (158 F) test temperature causes an increase in air pressure in the closed cells.
2. All of the compressing pressure is on the rubber during the test. 2. Part of the compressing pressure is on the rubber, but part of it is on the air in the cells during the test.
3. There is no air diffusion effect through the cell wall structure. 3. During the time that the closed cell structure is under pressure, in the test there is some air diffusion through the thin cell walls. (This is the same diffusion effect that occurs when air pressure decreases in an automobile tire over a period of time, even though there is no specific leak in the tube. This effect is a basic characteristic of the rubber or synthetic polymer. It cannot be changed significantly by the cellular rubber product manufacturer.)
4. The rubber is free to recover immediately after the test. Air can go back into the open cells immediately. 4. The rubber is not free to recover after the test. Air cannot go back into the closed cells immediately.
5. The sample retains the compression set after the I test. 5. The sample continues to recover long after the test period is over.

6. The compression set test result indicates the state of cure of the rubber sample. An undercured sample shows a high compression set.

6. The compression set test result does not necessarily indicate the state of cure of the sample. It is more an indication of the amount of air that has diffused from the closed cells and has not yet diffused back. However, if all other variables such as density, thickness, recovery time, etc. are controlled, then compression set is a direct function of cure state.
7. On samples which are otherwise equivalent, the test results are not affected greatly by the thickness of the sample. 7. On samples which are equivalent in other respects, the test results are greatly affected by the thickness of the sample tested. This is because of the diffusion effect as noted above.
8. The compression set test result is not directly affected by the hardness of the open cell sponge. 8. The compression set test result is affected by the hardness of the sample, harder materials showing lower percentages of set. This is because in the harder material the rubber portion supports a relatively higher amount of the total pressure in comparison with the air cells.

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.

 

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