Thermoforming Draw Ratio and Wall Thinning

The sheet has a fixed amount of material, and the areal draw ratio -- formed surface area over the sheet area it came from -- divides the starting gauge to give the average wall. Nothing in the process changes that average. Only the distribution is available. The depth-to-diameter ratio is the rough screen, easier to compute than the areal ratio and good enough to say whether a part is formable at all: unassisted female forming runs to roughly 0.5, and deeper than that needs plug assist or pressure forming. It is a screen and not a substitute, because two parts at the same H/D can have quite different areal ratios depending on their shape. THE AVERAGE IS NOT THE SPECIFICATION, AND THAT IS THE POINT. The bottom corners are the last material to arrive, formed from sheet that has already chilled against the sidewall it touched on the way down, and they run a third to a half of the average. A part quoted on its average wall is quoted on a number that appears nowhere on it. The corner is where the part fails, so the corner is what the starting gauge has to be chosen for -- and the sheet thickness that meets a corner minimum is materially heavier than the one that meets an average, which is where the part's cost actually comes from. Plug assist is the standard answer because it attacks the right variable. It pre-stretches the sheet and redistributes material toward the bottom, improving the corner substantially while leaving the average exactly where conservation puts it. Opening the corner radii does the same thing from the design side at no cost at all, and it is the change most often available and least often made. This estimates from an entered geometry, approximating the formed shape as a cylinder plus a flat bottom. It does not compute a real formed area from a three-dimensional part, model the actual material distribution (which depends on sheet temperature uniformity, the mould, the plug shape and the forming sequence), predict the corner fraction -- that is entered, and it varies widely -- select a forming method, address trim, sag, or the sheet's own temperature window, or handle male (drape) forming, where the material distribution is different in kind. The thermoformer's own trials and the sheet supplier's data govern.

areal draw ratio ADR = formed surface area / original sheet area, average wall = sheet gauge / ADR, and the sheet gauge for a corner minimum inverts it; the formed shape is approximated as a cylinder plus a flat bottom.

Thermoforming draw ratio and wall thinning as the trade states them. The corner fraction is ENTERED, commonly a third to a half of the average, because it depends on sheet temperature uniformity, the plug and the forming sequence and is not predicted here.

One area ratio and one division.

Estimate. AHJ and licensed professional govern.

Field names used by the API: opening_diameter_in, draw_depth_in, sheet_thickness_in, corner_fraction, min_wall_in, hd_limit, hd_ratio, formed_area_in2, areal_draw_ratio, average_wall_in, corner_wall_in, sheet_for_corner_in

• Cylinder plus flat bottom a real three-dimensional part has a different formed areathe part model
• Corner fraction is entered it varies widely and is not predicted herethe thermoformer's own trials
• Female forming male or drape forming distributes material differently in kindthe forming method