Casting Study: Grid Module

Mosaic mimic panels form a display system that provides an overall picture of complex operations and gives technicians the ability to monitor and control events. The system’s main advantage is its modular design, providing flexibility for easy modification and expansion. The heart of the modular design is the grid module. The grids are designed to be mounted in any 90-degree orientation and accommodate various size display tiles. Each grid is capable of being joined at any point without disturbing or affecting adjacent grids. Additionally, they can be cut to any dimension or have sections cut out and restored later. The grids are extremely intricate Zamak 3 die castings. The 11-1/4 in. (288mm) square grid has 144 square holes (7/8 in., 22mm) and 121 cored holes (0.125 in./3 mm I.D.). Each cored hole has a thin-wall collar, 0.020 in. (0.5 mm) thick, on either end which seats the plastic display tiles. The grid’s critical parameters are flatness, squareness and outside dimension. The panel’s perimeter dimensions must be the same along the entire length and width of the panel, and the length can be as long as 100 ft. For both function and aesthetics all lines and planes must be parallel. Because of this, dimensional control is critical. Outside grid walls are 0.062 in. +/-0.003 in. The cored holes have centre-to-centre tolerances of +/-0.002 in. Centre-to-centre tolerance for two mating grids is +0.0025, -0.003 in. Flatness is also critical for good fit, with a maximum allowable variation of 0.004 in. After casting, the grids are trimmed in a two-stage press and then dry blasted, producing totally flash-free surfaces. The grids are then shipped for assembly without any further operations or finishing. Designers chose zinc die casting to produce the grids because of its ability to maintain tight dimensional control. Aluminium die casting does not have the flow characteristics to properly fill the thin-wall sections. Investment casting would cost significantly more. Engineers ruled out stamping because forming the honeycomb grid would also stretch the material, sacrificing dimensional control. Although plastic moulding can hold tight dimensional tolerances, the plastic material would not have enough rigidity for the large panels.