What Is a Draw Ratio?

Linear

• The linear draw ratio refers to the length variation between a line drawn between two points on a diagram of a form, and the measurement between the same two points on the physical form. The physical measurement tends to be longer because of imperfections on the surface of the form that do not exist on the diagram.

H:D

• H:D or height-to-diameter draw ratio is applicable where the form produces a cup or other cylinder. H:D is a comparison of the length of a line encompassing the profile of the part, to that of a perfect cylinder with the same height and diameter. The difference in the measurements can be quite large in the case of a tapered or stepped cup, where the diameters of the top and bottom are not the same.

Areal

• As its name implies, the areal draw ratio deals with the area of the overall form. This ratio is more complicated than linear and H:D ratios because it contains two dimensions. The areal ratio can also be quite large because the incorporated H:D and linear differences add in both dimensions.

Importance Of Draw Ratio

• Engineers should account for draw ratio when specifying the dimensions of the plastic sheet to use with a particular form or mould; otherwise the sheet will either not fill the form, or it will stretch and create areas where the material is thinner than the specification.