Thermoforming Draw Ratio

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The goal of heavy-gauge thermoforming is to produce a part with as close to a uniform wall thickness as possible. As a part designer, you’ll need to specify a target wall thickness so that your thermoformer can select a plastic sheet with the right starting thickness. The more the sheet is stretched, however, the thinner it becomes. If the sheet’s starting gauge is too thin, the part walls won’t be thick enough. If the starting gauge is too thick, you’ll use excess material and add unnecessary costs and weight.

There are other considerations as well. For example, if your plastic part is designed to be flame retardant and meet the UL 94 standard, the target wall thickness needs to be thicker than the minimum amount that’s required for strength. If your part has sharp corners or other features that require a deeper draw, thinning in the walls may occur. The geometry of the mold over which the plastic sheet is stretched is also a factor. So, how does your thermoformer know the sheet thickness, or starting gauge, that you need?

What is thermoforming draw ratio?

Draw ratio is a calculation that your thermoformer uses to determine the minimum thickness of the plastic sheet to produce your part. It divides the surface area of the mold by the footprint, or top surface, of the heated sheet. Your thermoformer uses this calculated value to help achieve the minimum wall thickness for your ideal part – and with a uniform wall thickness.

Sometimes, you’ll receive a quote with two sheet thicknesses or starting gauges: a higher one and a lower one. That’s because draw ratio is an estimate that’s meant to get you in the ballpark rather than in a specific seat behind home plate. Since draw ratio isn’t something that you need calculate yourself, what else do you need to know about it? The following sections explain.

How do you calculate draw ratio?

Here’s the formula that’s used to calculate draw ratio.

Draw Ratio = Surface Area of the Mold / Footprint of the Heated Sheet

Now here’s an example you can follow.

The part is 10″ Width x 12″ Length x 2″ Height
The target or required wall thickness is 0.100″

The individual steps are listed below.

Step 1: Calculate the Surface Area

2(10″ x 2″) Width + 2(12″ x 2″) Length + 10″ x 12″ Top Surface

= 40″ + 48″ + 120″

This equals the approximated surface area of the mold (i.e., 208″)

Step 2: Calculate the Footprint

The footprint of the heated area of the plastic sheet (10″ x 12″) is 120″

Step 3: Calculate the Draw Ratio

208″ / 120″

The draw ratio is 1.7

So, the minimum estimated sheet thickness is 1.7 x 0.100″ = .0170″

Now that you know the draw ratio, you can use this number (1.7) and the desired ending wall thickness (0.100”) to calculate the minimum starting gauge for your plastic sheet.

How do you use draw ratio to calculate the minimum starting gauge?

Here’s the formula to use.

Draw Ratio x Desired Finished Gauge = Minimum Starting Gauge

Assuming a perfect material distribution, this is one-step calculation.

Step 1: Calculate the Minimum Starting Gauge

1.7 x 0.100″ = 0.170″

Final Thoughts

Your thermoformer uses draw ratio to estimate the minimum gauge of the plastic sheets that you need. Draw ratio is also used to calculate corner radii and draft. If the draw ratio is in the ballpark, you’ll get accurate material costs up-front during the design stage. If the draw ratio is too far afield, your cost estimates will be too low if there are design challenges such as sharp corners and draft angles.

Profile Plastics is an experienced thermoformer that understands the importance of draw ratio and how to calculate it. With our industry experience, we can help you to achieve a consistent wall thickness that’s based on what you need. Depending on your application, you might be able to use a thinner sheet than you thought.

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