One of the most important considerations for a converter, skiver or die cutter, after the proper selection of product has been done, is to assure the material is properly handled insuring the finished product performs and fits as designed.

Stability of the cellular material is essential if the design dimensions are to remain dependable.

Shrinkage is always a consideration with all cellular materials.When materials are expanded, the positive expanding forces of the nitrogen gas contained within the cells is constantly trying to normalize its pressure to that of the outside. It leaks through the cell walls slowly (nitrogen gas is used because of its molecular size and slow permeation rate) as the gas pressure normalizes, the elastic memory in the rubber component contracts and is constantly trying to return to its original unexpanded size.

Equilibrium is reached when the elastic memory and the gas pressure balancesand the material stabilizes.An exception occurs if the temperature is elevated significantly to soften the material, then the memory that remains takes over again and further contraction takes place.

Notes:

Elevated temperature of the slit sheets for a period of time will normalize the materials quicker. The length of time and the temperatures will vary with the type of material and the environment which the finished product must withstand.

The higher the quality of materials, for example, those which having high rubber content and lower amounts of fillers and extenders are typically more difficult to normalize, due to the thicker and less permeable cell walls than that of a material that has been expanded to low density and has much more filler. Examples: Buna N, Epichlorohydrin, medium to higher density Neoprene and SBR materials.

Its easy to visualize the negative effects of having shrinkage occur after one has applied pressure sensitive adhesive, die cut or stripped, only to have it contract and be unusable. Therefore all efforts and materials are lost, so it is wise to invest in normalization methods to assure this does not occur.

The effects of shrinkage must be taken into consideration when slitting materials from thicker buns or sheets. The degree of allowance from the target must be built in to assure the normal contraction in thickness does not result in out of tolerance finished goods. Once again this varies with the type product and the converter should educate self to the normal level of contraction that occurs.

Adhesive Application

A high percentage of cellular materials converted to strips, seals, and pads may require that a peel and stick adhesive be applied to them. There are many manufacturers that offer fine products that will perform with almost any type of expanded material and most companies are willing to recommend the most compatible materials for the product and application.

Adhesive Types (PSA)

There are various types of commercially available pressure sensitive films and tapes from a large number of manufacturers. Some are Mylar film supported with PSA on both faces. In some applications with certain rubber types it is advisable to tailor the PSA chemistry in the layer that bonds to the cellular material to offer high compatibility with the rubber type.

Generally acrylics perform better in this mode than rubber based film. Consult the adhesive manufacturer for more assistance.

Application

Equally important as the control of material stability is the application of adhesive films. Regardless of whether the film is a supported membrane (tissue or Mylar file) or if its simply a cast film. The film must be applied in a fashion to absolutely minimize any stretching that might occur from pressure rolls or conveyors or even hand rolling that might introduce stretch or distortions in the foam. In time the materials will creep and return to their unstressed sizes, similar to natural shrinkage and the product is lost. Product must be allowed to reside in a relaxed state as the film is applied. Pressure rollers should be set to a minimum pressure required rolling out trapped air, but never allowing them to compress and “squeegee” the rubber due to pressure.

Utilizing minimum pressure letting sheets or rolls sit for a rest period will assure the bond builds between the foam and the adhesive.

Rolls: If adhesive is applied to sheets that have been spliced onto roll goods, it is good practice to roll the unlaminated rolls with sufficient slack to allow normalization, but not wrinkling and to allow a droop loop before applying PSA thus assuring again the material’s relaxed state.

It is recommended to roll the goods on a medium diameter core with the paper rolled inside. If rolled outside a certain amount of creep between the PSA and the rubber will occur and when unrolled the release paper and the adhesive film will exhibit wrinkles. This should be investigated on a case by case basis as the effect varies with rubber thickness.