Moulding Anisotropy

Abstract

Abstract It has long been an accepted fact that the properties of vulcanized rubber articles are not uniform in all directions. Vulcanized rubber is, in general, an anisotropic material, and the results obtained from tests vary, to a greater or less extent, with the direction in which the strain is applied or the measurements are made. In much experimental work elaborate precautions have been followed whereby replicate samples have been taken in the same directions from the rubber material. The cause of anisotropy has always, as far as the authors are aware, been attributed to the methods of processing or shaping the raw stock. This is the view taken by the authors of one of the most recent publications. As calendering is an operation that produces an extreme distortion of the raw stock, such processing effects are considerable, and the resulting anisotropy has been known for many years as calender grain, or in appropriate cases, as extruder grain. It has been evident, however, for some years that the theory that the method of preparation of the raw stock is the sole or even the major cause of anisotropy is not adequate to explain certain phenomena. Some products exhibit pronounced anisotropy, although their method of preparation does not involve much distortion of the raw stock, and in other cases, such variation in properties as does exist does not give any correlation with the direction of processing. Consideration of cases such as these have led to the conclusion that the movement of the stock during moulding is an important factor in the problem, and one that, as far as can be seen from the literature, has been hitherto neglected.