How to overcome the challenge of deep draw parts

Draw ratio is the key to understanding thermoforming. The goal is to produce a part with close to uniform walls, so you need to start with the right sheet thickness. However, the dynamics of parts with deep draw present unique challenges.

What is draw ratio?

Every plastic part has a finite surface area that needs to be covered by a two-dimensional sheet, stretched to conform to the shape. Draw ratio quantifies how much the sheet’s surface area must expand to create the final part. 

Draw ratio is a simple calculated number that describes the surface area of the finished part divided by the footprint of the flat plastic sheet.

You can then estimate the required starting sheet thickness by multiplying the draw ratio number by the desired final wall thickness. 

Learn how to calculate draw ratio in our in-depth article.

Why draw ratio matters especially for deep parts

While there’s no clear line between normal and deep draw, anything beyond a 3:1 ratio is generally considered a deep draw. 

Parts with low profiles (under 12”) don’t stress the material much. When the part is taller, a heavier starting gauge plastic is required. Allowing the part to be taller than it is wide causes the sheet to thin out at a much quicker rate. 

It often depends on what aspect of the part is most important for the customer. Sometimes a single feature can cause an otherwise okay part to become difficult. The plastic can be extended non-uniformly. 

Failure to plan for deep draw undermines the cost-effectiveness of the thermoforming process. The sheet is more likely to cool unevenly, causing stability and shrinkage problems. Webbing and even tearing from over-stretched material negatively impacts productivity.

Extra deep requires extra effort

The key to handling deep draw is in the up-front engineering of the equipment, tooling, and the process. Proper tooling also depends on whether the mold is positive or negative. 

Positive Mold: A positive vacuum forming mold is installed above the plastic sheet. The material below is pre-stretched (sometimes called the bubble-blow process) then vacuum snaps it back against the mold.

Negative Mold: The mold is held below the sheet. A temperature-controlled plug assist helps push the material into the mold to maintain more consistent thickness. 

Plug assist is a mechanical form of pre-stretching which makes sure the material stretches more evenly, rather than only at the deep end. Heating the plug keeps the temperature even, so that parts of the plastic sheet don’t chill during forming. 

Only one in 10 thermoformers have this capability. Be sure to work with a thermoformer who has the engineering and right equipment to accomplish what your part needs.

More ways to plan for deep draw

Adjusting the part design may also be required. Increasing corner radius and draft (3-5°) help to reduce internal stresses. Additional ways to address deep draw problems focus on the plastic sheet itself. 

You can simply increase the sheet gauge. A thicker starting sheet gives you more material to stretch. Alternatively, you can start with a larger sheet size for the same reason. The problem with these solutions is that they increase the cost of raw materials. 

Selecting the right material plays a role in dealing with deep draw. Some resins stretch more and provide design flexibility. Other plastics with more polymer memory may be more likely to shrink after cooling, making them less likely to hold their shape.

Industrial step stool example

Thermoformed step stool utilizes deep draw techniques

This 3-step plastic step stool illustrates a functional deep draw part. Many industrial step stools are fabricated with metal parts, but this unit was thermoformed as a single piece. Profile achieved the 26” x 39” x 30” high dimensions using the techniques described in this article. Even though some wall thinning is visible at the hand holes, this product still offers a 500 lb weight capacity. 

Conclusion 

Because the starting gauge is so critical for an accurate final part cost, it is important to get proper feedback from your thermoformer when reviewing draw ratio. Their material expertise is also critical. Be sure to work with a thermoformer who has the knowledge, experience, and equipment to handle deep draw projects.