Plastic thermoforming is shaped by factors like material quality, temperature, pressure, speed, and tool geometry. With the right design principles, you can prevent errors in CAD and production. formary has gathered key tips to help you optimize your thermoformed parts.
Sarah Guaglianone
Updated on June 24, 2026
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The key design rules for plastic deep-drawn parts are:
If these design guidelines for plastic deep-drawing are followed, quality, costs and process reliability can be significantly improved. You will find more details on this in the following sections.
When designing plastic parts, it is important to always incorporate the largest possible radii in order to improve stretchability. Deep-drawn parts should always be manufactured with a minimum radius of 1.5 mm.
When it comes to the tool side, a minimum radius is required; as a very rough rule of thumb (since this depends on many parameters), this should not be less than the original thickness of the material.
If sharp edges are required, ensure that the design of the plastic parts includes a fillet radius that is at least as large as the original thickness of the material. If the fillet radius is set smaller, the parts cannot be deep-drawn, or can only be done so with great difficulty. Furthermore, costs will increase due to longer tool milling times.
The angle between the vertical part wall and the direction of demoulding is known as the demoulding angle. As the choice of demoulding angles affects the geometry of the deep-drawn part, the angles of the demoulding angles must be determined at an early stage and incorporated into the design data.
Design vertical walls with draft angles to ensure easy demolding without visible surface defects.
ℹ️ Recommended Draft Angles:
The forming ratio describes the relationship between the height and width of the formed area. Since the material is stretched into the mold, it becomes thinner during the process. The forming ratio depends on the tool geometry and final part shape.
The resulting wall thickness can be roughly calculated using the following formula:
d₂ = (F₁ / F₂) × d₁
Where:
ℹ️ More details on radii, draft angles, and forming ratios can be found in the tolerances section.
The further or deeper the material is stretched into the tool during thermoforming, the thinner the remaining wall thickness becomes. Typically, the wall thickness to be achieved is determined, and then calculated backwards (see point 3) to determine how thick the starting material must be (so-called "reverse engineering").
The stretch or draw ratio in thermoforming describes the length ratio of the material in the side profile (the initial thickness) and the thermoformed part. Generally: Positively formed parts result in a smaller and therefore more advantageous stretch ratio than negatively drawn parts with the same contour.
Due to the viscoelastic behavior of thermoplastics during stretching, some rules of thumb must be observed during the forming process when designing plastic parts:
With negatively formed parts, the weak point of the thermoformed part after forming is therefore in the base area, as material is stretched from the edge into the bottom of the tool cavity. The material thins out in the process.
With positively formed parts, the weak point of the thermoformed part is at the edge, as the material first touches the positive tool at the base area of the future thermoformed part, thinning out the edge area downwards. Initial thicknesses chosen too thin and a poor forming ratio, i.e., the relation of opening width to opening depth, further exacerbate the negative effect.
Tip: If you are unsure whether your component is designed for thermoforming, we are happy to support you in the design and optimization of your construction with our DfM analysis. Everything you need to know about DfM can be found in our whitepaper Design for Manufacturing: Manufacturing Feasibility at the Click of a Button.
For negative-formed cavities, the depth-to-width ratio should not exceed 1.5:1. A greater depth results in significant thinning, increasing the risk of rupture at the bottom edges.
The table explains the possibilities of different pull ratios for negative and positive shapes:
| Depth-to-Width Ratio | Positive Forming | Negative Forming |
| 0.3:1 | Possible | Possible |
| 0.5:1 | Possible | Needs punch assistance |
| 1:1 | Possible | Needs punch assistance |
| 1.5:1 | Material distortion at the limit | Material distortion at the limit |
| 2:1 | Must be tested; cannot be simulated | Must be tested; cannot be simulated |
As already mentioned: In thermoforming design with plastics, it is essential to avoid small radii. Especially with precisely fitting component cavities, nests can be designed to be much more thermoformable with softened contours while maintaining full functionality.
Figure 1 shows a cavity contour that is difficult to thermoform, while Figure 2 shows a softened contour that can be formed easily. By softening the draft angles and radii, the thermoformed part can be drawn more easily, which increases quality. Additionally, this minimizes the risk of tearing and guarantees higher reproducibility.
Ultimately, the cycle times are also shorter, which results in a generally welcome reduction in the unit price. Transitions from edge contours or stepped areas can also be smoothed out using chamfers.
If the radii are too small, there is always a risk of creasing during deep-drawing, particularly with positive moulds. To prevent creasing, ribbed transitions can also be incorporated into the plastic design to ensure that any creases that do occur during positive deep-drawing are at least controlled.
Undercuts are features that prevent the part from being easily removed from the deep-drawing die. Unlike in injection moulding, there is no second die half, so they make demoulding more difficult.
Tips:
Bear in mind that processing shrinkage is a key factor in the design of a plastic deep-drawn part, as it affects the final dimensions and shape. After demoulding, the part shrinks further due to the material’s behaviour. This shrinkage varies depending on the material and can continue for up to 24 hours even after the part has cooled. In the case of semi-crystalline thermoplastics in particular, post-shrinkage never stops and the part may continue to shrink over time.
Deep-drawn parts are designed for most industrial and packaging applications with a tolerance of +/- ~1 mm. This corresponds to the tolerance range according to ISO 2768-c for the 120 to 400 mm length dimensions commonly used in this product area.
Forming tighter tolerances usually involves additional costs. These are due to longer demolding times, greater manufacturing costs, and/or longer cycle times.
Fixing points are required to assemble various deep-drawn parts or assemblies. Many fastening technology options are used to securely close deep-drawn parts. These must be carefully considered from the outset to ensure that the design is suitable for plastics. Depending on the mechanical load, the frequency of opening and closing, and the stability of the connection, various options are available for achieving a force-fit connection.
In permanently force-fit connections, screws or rivets are often attached to the deep-drawn parts at the transition points between two parts in order to achieve a connection.
If your thermoformed part is to be stacked, this must be taken into account at an early stage of the design process to ensure that sufficient space is left for stacking. Several factors are key when selecting the appropriate stacking technique:
The choice of stacking method for a thermoformed part depends primarily on the part’s rigidity and the material’s sliding properties. The thicker the material (which is often the case with reusable trays), the easier it is to stack. The poorer the material’s sliding properties, the better the static friction, and consequently the easier it is to stack.
If the components are not allowed (or should not) be stacked on top of one another, but a gap of a few millimetres must be maintained, a higher-quality stacking technique is required. The reason for this is that the stack of thermoformed parts is self-supporting, meaning that the weight of the trays and their contents must be borne by the tray surface without any additional supports.
Complex stacking techniques involving movable stacking elements require the use of a flap mould. Manufacturing such a mould is more time-consuming than producing a relatively simple undercut stacking mould.
Softer plastics exhibit greater deformation and higher processing shrinkage after demolding. Materials with high hardness and rigidity are automatically in a better tolerance group in terms of accuracy.
If you require markings on the thermoformed part, you are welcome to have these engraved directly into the mould.
The cost of engraving can vary considerably, depending on the type of engraving required. As a general rule, a recessed engraving is relatively inexpensive, whereas a raised engraving is considerably more labour-intensive and therefore more expensive.
By following these basic design principles, potential sources of error can be avoided right from the design phase. This helps to reduce costs, minimises the need for iterative cycles and ensures safe production.
Would you like to ensure that your component is optimally designed for deep drawing? We would be happy to assist you with the design and implementation of your deep-drawn parts, and we recommend consulting our Design Guide for further guidance.
Radii facilitate deep drawing and prevent cracks and wrinkles. Minimum radius for radii: 1.5 mm; for sharp edges, at least the material thickness.