Impact strength is expressed in terms of foot pounds, as determined by standardized drop-ball tests using plastics of equal thickness. Scratch-resistance is expressed by the barcol hardness index, with the more resistant materials having higher index ratings. Resistance to wind load is measured by two methods. Using a specially designed plywood test-box, vacuum is exerted on a pan face installed in its sign cabinet. A basic principle of aerodynamics, that wind creates a low-pressure zone on the opposite side of the surface over which it passes, can cause plastic faces to be sucked out of signs in severe windstorms.
Plastic sheet is rated according to design wind load, which is expressed in pounds per sq.
By circumscribing the test panel in one foot squares, loading weight per square foot can be measured. Obviously, if a face blows out of the sign, the load-rating of the plastic becomes academic. In order for a rigid plastic to withstand its specified design load, the sign must have positive edge retention. Users should note that a vast difference exists, according to geographical area, in the amount of UV exposure plastics receive. Thus, the same face material that retains its color and flexibility after 10 years in Seattle or Pittsburgh may be destroyed much sooner by sunlight in Miami or Phoenix.