Stamping is a cold forming process, but that does not mean that heat plays no role. With every press stroke, energy is released as heat, and that heat causes internal stresses that affect dimensional accuracy, tool life and the mechanical properties of the final product. In this article, we explain how thermal stress during stamping occurs, which factors amplify it and how you can manage it.
What thermal stress in stamping entails
Stamping is a cold-forming process in which metal is pressed into a die without an external heat source. Yet heat is generated, and that heat plays a greater role than many manufacturers expect. With every stroke of the press, the material deforms plastically, and part of the required energy is released as heat. This phenomenon is inextricably linked to every metalworking process using a punch press and therefore deserves targeted attention.
The harder the material, the greater the resistance and the more local heat is released. This effect is most pronounced in high-strength materials such as martensitic stainless steel or cold-rolled structural steel. The temperature in the material rises locally, and then the component cools down rapidly. This rapid cooling creates internal stress differences, known as residual thermal stresses.
A concrete example: when stamping a mounting clip in 1.5 mm cold-rolled steel at a high stroke frequency, the strip gradually builds up heat. The pieces at the top of the stack cool down more slowly than the pieces lying directly on the cool production table. The result is subtle dimensional deviations between the first and the five hundredth part in the run.
Why the stroke frequency amplifies the voltage
The speed of the press partly determines how quickly heat builds up. At a low stroke frequency, the material has more time to dissipate heat via the strip and the tool. At a high stroke frequency, the heat accumulates before dissipation can take place.
This is particularly an issue in progressive stamping with high production speeds. Suppose you are processing a copper contact strip with a stroke frequency of 400 strokes per minute. Copper conducts heat well, which is advantageous. However, with less conductive materials such as stainless steel or certain aluminium alloys, the temperature in the tool and the strip rises rapidly.
At Eurotechniek, when setting up a new tool, we take into account the optimal stroke frequency for the specific material. That is not an assumption, but a result of measurement data from previous runs with comparable materials and geometries. Producing too quickly does not save time if you end up with rejections or corrections afterwards.
The role of the tool in managing tension
The tool itself has a major influence on how thermal stress develops and spreads. A die that cools poorly or dissipates heat unevenly exacerbates the problem. The contact zone between the material and the punch side is the critical point, because that is where plastic deformation takes place and where the heat is concentrated.
Tools with a good surface finish and the correct coating reduce friction and therefore also heat generation. A TiN coating or a comparable hard coating lowers the coefficient of friction while simultaneously protecting the punch edge against premature wear. That has a dual benefit: less heat production and a longer tool lifespan.
For complex geometries, such as parts with deep draws or sharp bends, the deformation concentrates in small zones. Those zones heat up the fastest. In the tool design, you account for this by intentionally choosing radii at those positions slightly larger than the minimum drawing requirement. This spreads the deformation over a larger surface area and locally reduces the peak temperature.
Lubrication as an active management tool
Lubrication is standard in many stamping processes, but the choice and application of the lubricant determine how effectively the thermal effect is limited. A good lubricant reduces friction, cools the contact surface and prevents the material from sticking to the punch side.
The lubricant film must be homogeneous. If the film is too thin in certain positions, friction increases locally and a hotspot is created in the tool. If the film is too thick, it affects the dimensional accuracy of the stamped geometry. The correct viscosity depends on the material type, the stroke frequency and the geometry of the component.
At Eurotechniek, we look at the right lubrication specification for each project. For a customer in the automotive sector, we deep-drawn aluminium draw parts for a sealing system. The initial runs showed microscopic cracks in the draw rim. After adjusting the lubricant and reducing the stroke frequency by fifteen percent, the defect disappeared completely. The dimensional accuracy remained stable over the entire production run.
Material behaviour and the influence of recrystallisation
Not every metal reacts the same way to the heat generated during stamping. The microstructure of the material plays a decisive role. Cold-rolled steel already has internal stresses due to the rolling process. Those stresses are in equilibrium, but the stamping process can disrupt that equilibrium.
With materials that have a low recrystallisation temperature, such as certain aluminium or magnesium alloys, the heat released during stamping may already be enough to locally alter the microstructure. This affects the mechanical properties of the final product, even if the component looks visually correct.
For critical applications in, for example, the aviation or medical sector, this is a serious design consideration. In that case, you specify not only the chemical composition and hardness of the base material, but also the thermomechanical condition. Eurotechniek asks for these kinds of specifications before we design a tool. It prevents surprises after approval of the initial production parts.
Measuring and controlling residual stresses after the tamping process
Thermal residual stresses are not always immediately visible. A component that is dimensionally correct straight from the press can deform after some time when the internal stresses are released. This is called relaxation and it occurs during storage, transport or during the initial thermal load in the application.
There are various methods for measuring residual stresses. X-ray diffraction provides an accurate picture of the surface stress. Contour methods reveal deeper stress profiles. For most industrial stamping processes, a combination of dimensional inspection directly after pressing, after storage, and after a thermal cycle is sufficient.
When residual stresses are too high, you can reduce them via stress-relieving annealing. This is a heat treatment where the component is briefly brought to a temperature below the recrystallisation limit. The internal stresses even out without the geometry or hardness changing significantly. At Eurotechniek, we apply this if the customer specification requires it or if measurement data gives cause to do so.
Frequently asked questions about thermal stress in stamping
How do I notice in practice that thermal stress is a problem?
The most common signs are dimensional deviations that become larger as the production run progresses, parts that are slightly warped after storage, or small cracks that only become visible after some time. Sometimes you also see that the pieces at the beginning and the end of a run measure differently at the same critical measurement points. These are indications that heat build-up in the tool or the strip is playing a role.
Are all metal types equally sensitive to thermal stress during stamping?
No. Materials with a low thermal conductivity, such as austenitic stainless steel, are more sensitive than materials that dissipate heat quickly, such as copper or aluminium. The harder the material and the greater the deformation per step, the more heat is released. When choosing the base material, you take this into account in the design, especially with high stroke frequencies or complex geometries.
Can thermal stress be completely prevented during stamping?
Complete prevention is not realistic. Every plastic deformation is accompanied by heat generation. However, the effect can be managed to a level that does not jeopardise the quality requirements. The right combination of stroke frequency, lubrication, tool coating and, if necessary, post-treatment such as stress-relief annealing is the standard approach for this.
Grip of your tamping process starts with the right knowledge
Thermal stress is a real phenomenon that plays a role in every stamping process. Controlling it requires knowledge of materials, tool design and process parameters. Anyone who sets this up properly prevents rejections, extends tool life and delivers consistent quality over the entire production run.
Contact Eurotechniek if you have any questions about your stamping process or if you want to have a drawing assessed for risks regarding thermal stress and dimensional accuracy.