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Metal Fabrication

Plastic Test

Choosing the right plastic for the job means understanding the fine print

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Manufacturers conduct various tests for product durability, but they fall roughly into three categories: weathering, impact and abrasion resistance and wind resistance.

Weathering

The plastic industry’s continuing research programs include actual outdoor weathering tests. To achieve comparisons in a shorter time-frame, however, the industry employs simulated weathering tests using two methods. The first method is the bulb type and the second method is the xenon-arc type. The bulb method is considered to be a harsher, more accelerated test, because it produces more damaging wavelengths. The spectrum produced by the bulb type device is considerably more severe than that produced by sunlight as measured in Miami, FL.

The xenon-arc device is roughly equivalent to the Miami level of natural sunlight. Plastic sheet is tested for yellowing, color stability and brittleness after being exposed for a specified number of hours. These tests confirm the value of UV stabilizers, which act as sunscreens, for outdoor plastics.

Impact and abrasion resistance 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.

Wind resistance

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Resistance to wind load is measured by two methods. The first method is vacuum testing, which measures negative or suck-out wind-load resistance. 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. ft. (PSF). Ratings for commonly used sign plastics are indicated as follows: 20 PSF load = 75 mph wind velocity 30 PSF load = 90 mph wind velocity 40 PSF load = 100 mph wind velocity 50 PSF load = 130 mph wind velocity

The second method for wind load testing is the sandbag test which measures positive "push-in" pressure on the windward side of signs. By circumscribing the test panel in one foot squares, loading weight per square foot can be measured. Most outdoor signs carrying wind load ratings are rated at 30-35 PSF.

Edge retention ("putting on the dog") is an important aspect of a sign’s wind-load rating. 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. For this reason, national sign programs are more likely to specify materials that are more resistant to UV radiation.

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