This invention relates to a method for laminating a transparent safety panel to the screen-bearing viewing window of a CRT (cathode-ray tube) and particularly, but not exclusively, to a large CRT, and to the product of that method. By "large CRT" is meant a television picture tube or an information display tube having a viewing window bearing a viewing screen of at least a 30V size.
In one prior type of CRT, a glass safety panel is laminated to the viewing window of the CRT in order to reduce the danger of implosion and also, should the CRT implode, to reduce the danger of injury to people and things nearby. Suitable structures and methods for laminating CRTs smaller than 26V screen dimension have been described previously, for example, in U.S. Pat. No. 4,204,231 issued May 20, 1980 to M. M. Permenter.
In one prior laminating method, a safety panel is positioned in a desired spaced-apart relationship with a CRT window, and then a strip of flexible pressure-sensitive tape is wrapped around the edges of the CRT and panel to define a closed cell between the window and the panel. Thereafter, the cell is filled with a catalyzed liquid resin and allowed to cure to a clear transparent solid which adheres to the facing surfaces of the panel and the window. During the filling step, vent holes are punctured in the tape to allow air in the cell to escape. After the cell is filled with resin, the holes are taped shut to prevent both the leakage of resin and the formation of bubbles in the cell during the curing step. A foam tape with adhesive on both sides adhered to the margins of the panel and the window may replace the tape in the foregoing method.
In the foregoing method, the cell is filled with the viewing window positioned in a generally horizontal attitude, with the window facing downward. The tape provides a temporary hydraulic seal for the cell, and is also sufficiently strong to hold the panel temporarily in the desired downward-facing, spaced-apart relationship while the cell is being filled and the resin cures. In practice, especially when safety panels are laminated to CRT windows larger than about 25V-size, the taping step is not efficient and many temporary seals leak resin, and some seals fail to hold the safety panels in place. Also, because the windows face downward, it is difficult to determine whether gas bubbles are present in the viewable area in front of the window.
Because of the size and weight of a large CRT as defined above, all of these problems are aggravated and these prior methods are not practical for laminating a safety panel to the window of a large CRT. For example, a 25V-size CRT weighs about 55 pounds, while a 33V-size CRT weighs about 130 pounds and therefore cannot be handled manually in the factory. A cell, formed as described above, on a 33V-size CRT is difficult to fill with liquid resin with the window facing down because of the greater weight of the safety panel and because the greater weight of the resin causes greater leakage of resin during the filling and curing steps, especially through the venting holes in the tape. The tapes used in the prior methods to form the cell are not reliable to hold an 11-pound safety panel and about 8 pounds of liquid resin in the desired spaced apart relationship during the filling and curing steps. Sagging and wedging of the panel position and leakage and "flow-out" of the resin occur frequently with prior methods.