Some of the most impactful display graphics owe much of their appeal to the painstaking process of distortion printing. This article uses an actual job to describe the demanding workflow.
To start the wood pattern, we digitally print a full-size paper proof of the customer-supplied artwork. We sketch areas on the proof to indicate raised plateaus on the finished piece. Individual plateau heights are also indicated. This hand-drawn map is sent to the pattern shop, along with a digital art file for pattern fabrication. Individual shapes matching the full-size map are cut out of wood and assembled. All vertical faces of the pattern are angled-in (draft) a minimum of 3° towards the center of the pattern. Patterns with draft angles less than 3° create potential problems during the mold’s casting process. Similarly, molds with draft angles less than 3° may provide sticking points that, during the vac-form cycle, impede the cooled part as it is pulled off of the tool.
The mold for our Kentucky Lottery sign panel is a sand-cast aluminum plate, mounted to a 4-in.-deep, wooden vacuum box. After the wood pattern is mounted to an oversized piece of plywood and painted with wood sealer, it is moved to the foundry, where it is pressed into a container of fine sand (called a flask). The casting sand is compacted around the pattern, which is then carefully removed, leaving a mirror-image cavity of the pattern behind. To keep the casting hollow, a plug (called a core) is inserted into the cavity to prevent it from filling up with aluminum and becoming solid.
We heat aluminum in a kiln while preparing the flask. The molten metal is poured into the cavity prepared in the flask. After the aluminum cools, the flask, which is constructed in sections, is disassembled, the sand knocked away, and the rough casting is ready for finishing.
We polish and sand the casting’s rough surface until it is smooth, much of which is done by hand. Our Kentucky Lottery sign-panel casting is basically a large, square plate with several raised plateaus arranged over its surface. This is a shallow part—about 1.5 in. from lowest to highest point. Sanding and polishing this mold is fairly easy. Next, tiny vacuum holes are drilled through the casting. In general, the holes are about 0.025 in. in diameter. They’re small enough to prevent hot plastic from drawing into them during production, which would create dimples on the plastic’s surface.
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