The door panel of an automobile is fabricated by a plastic hot-extrusion process resulting in the ribbed cross section shown schematically. Following a process involving air-cooling, painting and baking, the panel is ready for assembly on the vehicle. However, upon visual inspection, the rib pattern is evident on the outer surface. In regions over the ribs, the paint has an "orange pee]" appearance, making the door panel unacceptable for use. The apparent reason for this defect is the variable micro finish caused by differential cooling rates at the surface of the panel, which affect adherence of the paint. Your assignment is to estimate the panel surface temperature distribution as a function of cooling time.

Use the finite-element software FEHT to obtain the temperature distribution in the symmetrical section shown schematically. The panel is ejected from the extrusion press at a uniform temperature of 275°C and is allowed to cool on a transport table where the air temperature is 25°C and the convection coefficient is 10 W/m2 ∙ K. The thermo physical properties of the extruded plastic are p = 1050 kg/m3 ∙ c = 800 J/kg ∙ K. and k = 0.5 W/m ∙ K.

(a) Using the View/Temperature vs Time command plot temperature histories for selected locations on the panel surface. Is there noticeable differential cooling in the region above the rib? If so, what is the cause?

(b) Use the View/Temperature Contours command with the shaded band option to plot the isotherm contours. Select the From Start 10 Stop time option, and view the temperature contours as the panel cools. Describe major features of the cooling process observed from the display. Use other options of this command to create a 10-isotherm temperature distribution for a time that illustrates some of the foregoing important features. How would you redesign the ribbed panel to reduce this thermally induced defect, while still retaining the stiffening function required of the ribs?

Transcribed Image Text:

- Region of- nonuniform appearance T h Panel surface 3 mm 26 mm Rib Symmetrical section 12 mm ardr Rib -6 mm 2 mm-