Extrusion press container: stress and temperature

Aluminum extrusion

Aluminum extrusion is a plastic deformation process, wherein the aluminum material is heated preform, usually in the form of a circular cylinder, squeezed out of the container through the die hole. Moreover, the cross-sectional area of ​​this hole is much smaller, чем у исходной заготовки (рисунок 1). Under the influence of the force of the ram, the workpiece interacts with the container and the matrix in this way, that high compressive stresses arise throughout its volume, which suppress the formation of cracks in the material of the cast billet during its plastic flow through the matrix. In turn, in the pressing tool - press washer, контейнере и матрице – также возникают очень высокие напряжения [1].

Figure 1 – Принцип экструзии (прямое истечение) [1]

The container of the extrusion press

Контейнер является самой дорогой частью прессового инструмента экструзионного пресса (рисунок 2). Container components - body and inner sleeve, and often, intermediate middle sleeve - made of special heat-resistant tool steel. The service life of container components has a large impact on the overall economic performance of the press.. Container body and its inner sleeve, and, often, another one, medium, втулка (рисунок 3) должны иметь конструкцию и материалы, which provide them with a long service life at high temperatures and high cyclic loads with each pressing cycle.

Figure 2 - Container extrusion press company Tecalex [2]

Figure 3 - The main components of the container
современного экструзионного пресса (Tecalex) [2]

Stresses in container components

Напряжения в компонентах контейнера имеют две составляющие:

  • shrink-fit stress of the sleeve when assembling the container and
  • voltage, that occur in the pressing cycle.

Extrusion ram and die are axially loaded. The bushing of the container carries an axial load due to friction between the workpiece and the container. Moreover, втулка (и, Consequently, контейнер в целом) испытывают термические напряжения, which arise during its pressing and extrusion. Осевое усилие прессования вызывает очень высокие тангенциальные и радиальные напряжения во всей конструкции контейнера [3].

When assessing the stresses in the components of a container, the method of elastic calculation of the stress state of a thick cylinder when it is loaded with internal pressure is usually used. Axial symmetry of stresses in the container is assumed, as well as the axial symmetry of the temperature distribution along its entire length. Local thermal stresses, as well as longitudinal stresses from the assembly of the container and friction between the workpiece and the container are neglected. It is assumed, that all materials have the same modulus of elasticity and all stresses are elastic.

More bushings - less stress

In figures 4 and 5 показаны расчетные напряжения в контейнере для двух типов контейнеров:

  • с одной внутренней втулкой (рисунок 4) and
  • со средней втулкой и внутренней втулкой (рисунок 5).

Both containers have the same inner and outer diameters, and also the same load inside the container - 63 kg / mm2.

Figure 4 - Container with one inner sleeve.
Maximum von Mises equivalent stress - 100 kg / mm2 [2]

Figure 5 - Container with inner and middle sleeve.
Maximum von Mises equivalent stress - 77 МПа [2]

These calculations show, that the use of two sleeves instead of one gives a significant reduction in stresses on the inner sleeve of the container. При этом напряжение на наружной стороне контейнера незначительно возрастает [3].

Container temperature control

To minimize thermal stresses and to prevent loosening of the inner sleeve, the container and inner sleeve are heated to operating temperature for about 8 hours or even more with limiting the temperature rise to 50-60 ºC per hour. The temperature of the container and inner sleeve rarely rises higher during pressing 450 oC. If the elements of the container overheat, then tempering of heat-hardened steel occurs, from which they are made, and a decrease in its strength properties, including, hardness.

Sudden heating or cooling of the housing or bushing will result in excessive thermal stress. This can cause cracks in these container components or shearing of the sleeve.. To minimize thermal stresses, need to control the temperature. Для этого минимизируют градиент (перепад) температуры в элементах контейнера.

When considering temperature gradients in a container, it is necessary to consider how radial, and axial temperature distribution.

Radial temperature distribution

The radial distribution has a much steeper gradient and is therefore more difficult to control.. During the pressing process, the inner hole of the sleeve is inevitably the hottest part of the container with temperature, which approaches the temperature of the workpiece. The temperature distribution in the working container has the form, shown in Figure 6.

Figure 6 – Радиальное распределение температуры в контейнере [3]

Inside the container, the main type of heat transfer is thermal conductivity. The temperature of the outer surface of the container body determines the heat loss due to radiation. The temperature of the inner surface of the sleeve depends on the temperature of the workpiece and the energy, which turns into heat during the pressing process.

Under normal conditions, the container body is usually not hotter than the sleeve. This can happen, when the control thermocouple is too far from the heaters and the bushing cools. In this case, the full pressure of the main plunger can destroy the sleeve..

Axial temperature distribution

Controlling the relatively small axial temperature gradient in the container is also very important.. The temperature graph along the bushing axis has the form, shown in the picture 7.

Figure 7 – Осевое распределение температуры во втулке контейнера [3]

The loss of heat from the ends of the container leads to the fact that, what's the hottest container in the center, than at the ends. Moreover, the end of the container near the die is hotter, than the entrance to the container, since the workpiece is near the die much longer, than at the entrance to the container.

Container heating system

Why heat the container

Temperature control inside the container is important for several reasons.. This not only reduces stress in the container and, thereby, increases its service life, but also makes it possible to increase the productivity and quality of pressed products.

Система нагрева контейнера выполняет две важные функции:

  • container preheating
  • maintaining the temperature of the container during extrusion.

However, since the pressing process generates a lot of heat, it does not require much additional heating for the container, and often it is not required at all. In some cases, only a little heating and / or cooling is required to achieve optimal conditions.

Uniform temperature along the container axis

Maintaining a constant temperature distribution in the axial direction of the container usually requires only small amounts of energy in selected areas within the container.. for instance, to maintain a uniform axial temperature distribution, add extra heat to the end of the container, which is adjacent to the matrix and at the entrance to the container. Since different ends of the container have different temperatures, then the container heating system must distinguish between them and compensate for this difference.

Separate heating of the top and bottom of the container

Moreover, it may be necessary to separate the heat between the top and bottom of the container. The top half of the container is usually always hotter, lower than. The main mechanism of heat transfer in this case is thermal conductivity. Nonetheless, due to convection, heat extraction from the bottom of the container can be higher, from the top of it. Therefore, the container heating system must be able to heat the lower part of the container independently of the upper part..

Heating the container before the press

Heating the container before starting the press, and maintaining its temperature when the press is stopped can be quite difficult. The container needs to be heated, to minimize thermal shock, which occurs at the beginning of pressing. Moreover, the container needs to be heated quickly and efficiently.

Many older containers had external heaters, which supplied heat from the outside of the container body. This heating forms a temperature distribution, which has the form, opposite to, который возникает в ходе прессования: наружная поверхность контейнера является самой горячей, and the inner surface of the sleeve is the coldest. This type of heating and such a temperature gradient in the container body and sleeve is unacceptable for pressing.. The solution to this problem is to place the heaters closer to the center of the container., to heat directly the bushing.

Cartridge heating elements

Картриджные нагревательные элементы дают возможность подавать тепло в заданное место внутри контейнера (рисунок 8). They are located close to the hub, which reduces the time, which heat is required to warm up the sleeve to the specified temperature.


Figure 8 – Картриджные нагревательные элементы контейнера [2]

Sources:

  1. Aluminum extrusion technology / Pradip K. Saha - ASM International, 2000
  2. TECALEX Extrusion Press Containers, Tecalex, 2017
  3. Thermal Control of the Extrusion Press Container / D. From Dine, N. Gilada, P. Robbins, VI. Johannes, S. Takagi, Aluminum Extrusion Technology Seminar, Chicago, 2004