DRAW RATIO

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GUIDELINES:

The deeper or taller the part the heavier the starting gauge of sheet required. Allowing the part or any feature of the part to be narrower than it is tall will thin the sheet at a much quicker rate.

OVERVIEW:

The draw ratio is the key to understanding thermoforming processes. The part has a finite amount of surface area that needs to be covered by a flat two-dimensional sheet. When the sheet is heated and forced over or into a mold it must stretch to conform to that shape. As the sheet stretches it thins out. Local design features on the part may cause the sheet to thin at a greater rate than in adjacent areas.

SPECIFICS:

The draw ratio can be described numerically if the surface area can be calculated. The formula for expressing the draw ratio is as follows:

Draw Ratio = the Surface Area of the part / Footprint of the part

Example #1:

Assume a part is 10"x 12"x 2" deep. Therefore the Draw Ratio will
be:

Surface Area = 2(10" x 2") + 2(12"x2") + 10" x 12"
= 40" + 48" + 120"
= 208"

Footprint = 10" x 12"
= 120"

Draw Ratio = 208"/120"
= 1.7

If the desired ending wall thickness of the part is 0.100" use the draw ratio as follows to estimate the starting gauge of the sheet:

Draw Ratio x Desired Finished Gauge = Minimum Starting Gauge

1.7 x 0.100" = .170" Assuming perfect material distribution.

Example #2:

Assume a part size of 10" x 11" x 5" deep.

Surface Area = 2(10" x 5") + 2(11" x 5") +
(10" x 11") = 300"
Footprint = 10" x 11" = 110"
Draw Ratio = 2.73

If the desired ending wall thickness is 0.100" use the draw ratio value as follows:

2.7 x 0.100" = .273" starting gauge. Assuming perfect material distribution.

The above examples ignore the effect that a specific feature on the part (i.e.. a localized severe draw like a sharp corner) may have on the thinning of the sheet. The draw ratio is designed to get the part designer "in the ball park" when calculating the necessary starting gauge. Many part quotations will have two gauges specified (a high and low) because of the difficulty in predicting the proper starting gauge. Because the starting gauge is so critical to the cost of the part it is important to get proper feedback from the thermoformer when reviewing draw ratio. There are many thermoforming techniques and mold designs used to help the sheet stretch as uniformly as possible. A competent thermoformer will be able to implement them in a project with a difficult draw ratio.