One of the major problems connected with fabric expansion joints has been the tendency to fold or pucker at the corners. The use of radius corners, on the order of 6″ to 8″, does relieve the problem somewhat, but it still remains. It is important to remember that one of the main ways to stress a rubber compound to failure is to bend it at a sharp angle and maintain that stress in the presence of heat and chemical attack. In other words, the exact environment of a fabric expansion joint.
Another area of concern is space availability for a conventional flat, “belted” type joint. Very often, there is limited space around the duct work to accommodate the preferred out-turned geometry of the flange adaptors. Another problem might be the breech opening. Often times there is not room to turn the flanges inward and still allow for the movements required of the joint.
The central idea of a molded corner design is to make flanges in the belt to serve as the attachment flanges usually made from metal and to remove the corner stress which this naturally creates. Previously, this required molding in individual corners, a tedious and expensive process which also created failure points at each of the splice points. The new method, illustrated above, requires only the insertion of small coupons of material in each corner flange. There is only one splice across the belt, eliminating the failures mentioned above. The solid corners are produced by taking a partially cured belt, putting it into a mold, and adding the coupons. Thus we give the corners the appropriate shape necessary to relieve the normal stress.
The two most popular polymers are EPDM for cost purposes and FKM for performance. A variety of reinforcing plies can be used, depending upon the service requirements. Bi-TEF molded corner expansion joints can also be made. They take advantage of the “melt-splice” characteristics of Bi-TEF to offer a much tighter, virtually 90 degree corner for even the smallest of spaces.