Whether a plastic part is thermoformable is determined in the CAD model. Anyone who discusses geometry, wall thicknesses, and demoldability only after the first sampling pays for this loop with tool modifications, material waste, and lost weeks in the project timeline. This can be avoided during design: with a structured check that evaluates each model along this guide and makes typical sources of error visible early on.
Sarah Guaglianone
Updated on July 2, 2026
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In plastic thermoforming, a heated semi-finished sheet or film is drawn over a tool and brought into shape by vacuum or compressed air. The component is therefore not created by injecting material into a closed cavity (as in injection molding), but by stretching an originally uniform-thickness starting material over a mold. This creates wall thicknesses, radii, and geometric transitions directly from the subsequent material distribution.
For design, this has two important consequences. First, thermoforming is typically a single-sided tool-bound process. The forming contour is on one tool side. Subsequently, the component must be safely demolded from the tool. Geometries with undercuts, closed contours, or missing draft angles are therefore critical or not thermoformable. Even in the CAD model, it should therefore be clearly defined which side is the tool side and in which direction the component is demolded.
Second, local material accumulations cannot be built up arbitrarily. While in injection molding, ribs, domes, or massive functional areas can be specifically formed with additional material, a thermoformed part is created solely by forming and stretching the existing sheet or film.
Extreme material thickenings or highly material-intensive features are therefore only possible to a limited extent or not at all. The decisive factor is not only the drawn geometry, but also the question of how much the material thins out in deep, narrow, or sharp-edged areas.
Not every geometry suitable for injection molding, 3D printing, or machining can automatically be thermoformed. This is precisely where Design for Manufacturing comes in: The CAD model is checked early to see whether pull direction, tool side, wall thickness progression, radii, demoldability, and critical geometries match the thermoforming process.
The earlier this check takes place, the easier design adjustments can still be implemented. Especially in the development phase, small changes to radii, drafts, or component depths can later have major impacts on tooling costs, sample quality, and project duration.
When a CAD model is only evaluated for thermoformability after the first sampling, critical areas often become visible late. Typical consequences are:
A design-accompanying check reduces these risks because critical geometries are identified before tooling, inquiry, or sampling.
The following guide can be applied directly to a current STEP model: either manually with the standard functions of your CAD system or automatically via the formary DfM analysis.
This makes the most important influencing factors visible: pull direction, tool side, demoldability, radii, draw ratio, resulting wall thicknesses, and possible undercuts or narrow spots.
The following seven check points help to systematically evaluate a CAD model for its thermoformability. They show which aspects designers and developers can already check before the inquiry and at which points an automated thermoforming check creates additional security.
Before you check the content, analyze the model quality. Because a thermoformability check is only as reliable as the CAD model on which it is based. It is particularly important that the model realistically depicts the thermoformed component and does not contain unnecessary assembly information.
Therefore, first check:
Most CAD systems offer geometry diagnostics with which open surfaces, self-intersections, and faulty normals can be found before export. Run this check once before you pass on the model.
Thermoforming knows only one pull direction. This does not necessarily have to be already defined by the orientation of the CAD model, but should be defined before the actual thermoformability check. Only then can radii, draft angles, undercuts, and wall thickness progressions be meaningfully evaluated.
In practice, this means:
Only with a defined pull direction and specified tool side can all further checks be meaningfully evaluated.
The choice of forming method determines which side of the thermoformed part lies directly against the tool and can therefore be reproduced more accurately.
This side is particularly important when certain areas later fulfill a functional task, for example form nests in a tray or contact surfaces in an inlay. It should therefore already be clear in the CAD model which side must later be manufactured particularly accurately.
In a negative mold, the heated material is drawn into a cavity. The material is pressed against the tool-facing outside. As a result, the outer contour is particularly dimensionally accurate.
The inner contour, on the other hand, results from material thickness, draw ratio, and material flow. It is therefore only dimensionable to a limited extent. Negative molds are particularly suitable when the outer component geometry is decisive or when depressions, pockets, and cavities are to be depicted.
In a positive mold, the material is drawn over a raised tool contour. The material rests on the tool-facing inside. As a result, the inner contour can be dimensioned precisely. The outer contour results from material thickness, stretching, and flow behavior. Positive molds are particularly suitable when inner contours are functionally decisive, for example in trays, inlays, or workpiece carriers in which components must be safely guided, fixed, or positioned.
For design, this means: Before the thermoformability check, determine which side of the component will later be decisive. For trays, inlays, or workpiece carriers, the inner contours of the nests are often relevant because components must be securely fixed there. For housings, covers, or cladding parts, on the other hand, the outer contour may be decisive, for example because of installation space, optics, or adjacent assemblies.
Only when pull direction, tool side, and dimensionally accurate side are defined can draft angles, radii, undercuts, and wall thickness progressions be meaningfully evaluated.
Every surface parallel to the pull direction needs a draft angle, otherwise the component will jam in the tool. A rule of thumb in thermoforming is to demold inner surfaces more generously than outer surfaces, because the shrinking material would otherwise remain against the tool.
Most CAD systems offer a draft or draft analysis in which you specify the pull direction as the Z-axis and the software colors all surfaces below a defined limit angle red. Pay particular attention to walls perpendicular to the Z-axis on stiffening ribs, deep cavities, and webs.
Radii that are too small are the most common reason a component tears or thins extremely locally. The thermoforming process distributes the material over curvatures; sharp corners force the material into an area where it can no longer flow.
Check systematically:
Use the curvature or radius analysis of your CAD system to mark radii below your defined limit value in color.
This is the actual crux that distinguishes thermoforming from other processes. From a semi-finished sheet of defined thickness, a component is created whose local wall thickness depends on the draw ratio: the deeper the geometry at a location, the more the material thins out.
Important: A constant wall thickness is usually not possible with thermoforming. The material is stretched from an originally approximately uniform-thickness roll or sheet stock. The resulting wall thickness distribution is therefore not constant after forming, but depends on how much the material is stretched in the individual areas of the component.
These thickness differences can be reduced through design adjustments and process engineering measures, for example through larger radii, lower component depths, adjusted pull direction, or pre-stretching with an upper punch. However, they cannot be completely avoided. The exact subsequent wall thickness also remains process-dependent and can only be approximately assessed before production.
Practical checking:
Here you identify geometries that are theoretically drawable but practically not thermoformable:
Visualize this in CAD via sections along the pull direction.
A CAD check is only useful if the result flows documented into the next iteration. Record for each geometry area:
If you prepare your CAD model based on these seven points, you create a good foundation for further tool and material decisions. Many typical risks in thermoforming can thus be identified early, such as radii that are too small, missing draft angles, unfavorable draw ratios, or critical undercuts.
However, for a reliable assessment, a manual check is not always sufficient. Especially with more complex geometries, a digital DfM analysis can help make critical areas visible in the STEP model and evaluate thermoformability more systematically. The formary DfM analysis checks, among other things, radii, wall thicknesses, draw ratio, and demoldability and presents the results color-coded and in a PDF report.
This makes it easier to assess whether the component can go directly into the next project phase or whether design adjustments before tooling, sampling, or quotation request make sense.
The formary thermoforming check currently only accepts STEP. Export your model as .step or .stp, ideally as a single part without assembly overhead.