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Pushbuttons, knobs and plastic faceplates in automotive interiors are constantly exposed to chemicals such as sunscreen. In some instances, structural degradation of the plastic may occur with specific chemical agents and degradation can also be accelerated by high temperatures. A study of the propensity of commercial polycarbonate (PC) to craze and ultimately crack when exposed to sunscreen is outlined in this paper. Three-point bend testing per ASTM D3929 results in permissible bending strain limits for prolonged exposure to sunscreen at elevated temperatures. The paper outlines implementation details of these strain limits applied to a button surface, including finite element analyses identifying strain at the observed failure locations. Design considerations for related parts and products are also outlined.

In this paper, hoop stresses in the hubs of plastic knobs used in instrument panels are analyzed. The objective is to provide analytical formulas for stresses due to knob assembly and temperature cycling. Insertion forces are estimated using simple beam equations to model spring deflection. These forces are used to predict stresses in the D-shaped hubs of the knobs, using equations for thick pressure vessels. Solutions for stresses due to thermal expansion mismatches between the plastic hub and metal spring are derived using a press-fit analogy. Finite element analysis (FEA) is used to supplement and validate the analytical results. The proposed simple calculations are useful for up-front design assessment, enabling the engineer to select appropriate dimensions for knob hubs, springs and shafts.