The field of the present invention is screens which are tensioned for use in separating and the like along with the apparatus, process and materials for the manufacture of such tensioned screens.
Metal screen cloth as well as many other suitable materials have long been employed in tensioned conditions for the processing of materials, the stripping of liquids and the like. Such tensioned screens often include both the screen cloth and a structural screen frame to which the screen cloth is strongly attached. The attachment of the tensioned screen cloth to the screen frame has conventionally taken many forms including mechanical interlocking elements, upper and lower frame elements either welded or bonded together with the screen in between and simple bonding of screen cloth to a screen frame without additional mechanical restraint.
A primary difficulty which has been encountered in the manufacture of such tensioned screens has been the maintenance of appropriate tension in the screen cloth throughout fabrication. Simple mechanical interlocking elements may be employed to tension a screen for some uses. However, such systems are expensive, complicated to assemble and adjust and have joints and cavities which are difficult to clean. Welding screen in place in a tensioned condition tends to create a lack of tension uniformity. The screen cloth between welding points is able to deform and slacken. Further, the use of a top frame element again creates joints and cavities which are difficult to properly clean, particularly if the screen is to be employed in the food processing industry.
Another structure difficult to properly tension is the bonded screen. A structural screen frame is provided which has an upper bonding surface. Screen cloth is tensioned, adhesive is placed on the bonding surface and the screen cloth is brought against the bonding surface until the adhesive is cured. Fabrication of screens in this manner have achieved proper post fabrication tension using adhesives which cure at room temperature. However, it is generally believed that adhesives perform best when, during their use, they do not operate at temperatures in excess of the cure temperature used in the bonding of the screen components. As tensioned screens find applications in a variety of uses which require elevated temperatures, bonded screens have not proven satisfactory.
To create a bonded, tensioned screen where the bonding occurs at an elevated temperature, the fabricator must overcome or ignore substantial problems. The tensioning of screen cloth becomes exceedingly difficult when the screen cloth is subjected to substantial temperature changes. Frequently used screen materials include metal screen cloth and metal screen frames. When either is heated, it expands. However, there is a vast difference between the heat transfer characteristics of screen cloth and screen frames. Screen cloth can be heated very rapidly while screen frames require substantial time and heat input. For the same reasons, screen frames cool much slower than screen cloth when placed in the same environment. As a practical matter, when placed in air, the screen cloth will immediately cool and attempt to contract to its normal dimensions. The frame on the other hand will only slowly cool and remain expanded long after the screen cloth has attempted to contract. Thus, if adhesives are cured at elevated temperatures in the fabrication of screens, the screen cloth is likely either to relax once heat is removed or to exceed the yield strength and permanently deform. In either instance, the resulting screen assembly lacks the appropriate tension and may suffer from other defects. A screen which is not properly tensioned can have buckled or sagging screen cloth. Buckling can cause poor flow of material being screened. Sagging inhibits transfer of vibrational energy and promotes fatigue failure.
Certain bonded screens have been fabricated with the bonding occurring at an elevated temperature. Thermal stress problems have been avoided in these instances by only heating the adhesive and an overlying ring. In such situations, the frame material is a poor thermal conductor, typically fiberglass. Adhesives, a tensioned screen, and a ring are positioned on the frame. Heat is then directly applied to the overlying ring until the adhesive is cured. Under these circumstances, the screen frame and the screen cloth are effectively not heated. In this way, thermal stresses between the two is avoided.
In addition to the foregoing, tensioned screens have found substantial utility in the food processing industry. Substantial requirements are placed on products in this industry. No toxic materials may be employed, the resulting products must be easy to clean and the products must not interact with the substances being processed. Many tensioned screen designs have difficulty meeting these requirements because of the selection of materials and adhesives or because of the creation of cavities and joints which are not easily cleaned. Abrasion and bond deterioration under heat can specifically affect bonded screens in use.