Heating dies for aluminum extrusion

An important part of press equipment

Extrusion dies and auxiliary pressing tools (shims, bolster, podbolsters) must be heated to a suitable temperature before, both mounted on the press. This is necessary in order, to avoid fracture of the matrix and to minimize the time, required to achieve stable extrusion parameters. The temperature of the pressing tool is one of the critical parameters, who need to tightly control, to achieve stability, and repeatability of aluminum extrusion. Therefore, good furnace for heating the matrices are important equipment for any extrusion press.

Figure 1 - Extrusion press tool [1]

Extrusion die and another pressing tool made with high accuracy from a heat-resistant tool steel at a fairly complicated technology. Therefore, they are quite expensive and must be handled carefully and with them, to provide them with the most long life. Moreover, if the matrix is too cold or too hot, this could be the cause of unproductive press operation to form an excessively large amount of marriage.

Furnace types: traditional and others

The most widespread traditional furnace chamber with an upper hinged lid. They take up little space and can be installed as close as possible to the press. Another common type of furnaces are the single furnace. They are especially convenient to use for matrices of large size. These furnaces require additional production space.

Less often, primarily, on special productions, other furnace designs are used: pull-out, rotary, inert atmosphere, induction, infrared.

Furnace top loading

The most common type of furnaces for heating matrices are furnaces in the form of a rectangular chamber with a hinged lid in its upper part (Figure 2). This type of furnace is the cheapest, the easiest to install and takes up less space in all.

Figure 2 - Chamber many-heating furnace matrices manufactured Tecalex [3]

Each furnace is heated from both 10 to 20 matrices, depending on their size.

To ensure a more rapid heating and increase the temperature uniformity in these furnaces typically circulates hot air. In this direct radiation from the heat source to the tool is prevented by means of special screens.

When working with these furnaces is necessary to monitor, so that the newly loaded cold die does not take away too much heat from neighboring dies [2].

Single oven

Improvement single chamber furnaces are furnaces. Several of these furnaces rank in the shop near the press. Each of them is designed to heat only one matrix. Due to the small size of the heating chamber heating is performed without the use of air circulation.

These ovens have the following advantages:

The oven can not be opened for loading and unloading of other units of the pressing tool
Next to the heated matrix is never cold matrix
Heating matrix takes less time
Preventing overheating matrices due to easier control the duration of heating of each matrix.

Figure 3 - Single oven heating of the matrices Tecalex [3]

Stoves with drawers

These furnaces have 3-5 Drawer, each of which is placed 4 or 5 of dies. Each section has its own temperature control. Loading and unloading of die assembly have a lesser impact on the other matrices, than traditional chamber furnaces.

Figure 4 - Furnace for heating matrices with pull-out sections [2]

rotary kiln

This type of furnace has a turntable, which can be rotated so, to the desired matrix is opposite the window of loading and unloading. This design is more expensive and requires more space. However, it provides improved uniformity of temperature of the matrix because the loading and unloading of the matrix does not affect the temperature of other matrices.

Figure 5 - Rotary furnace for heating matrices [2]

Inert atmosphere furnace

Gaining popularity furnace with an inert argon or nitrogen atmosphere, to reduce the oxidation of the matrix. Each matrix is located in a separate sliding section, which provides very precise temperature control, reduction in the duration of heating and energy savings. These furnaces require higher expenses for their control and maintenance.

Figure 6 - Furnace for heating matrices in an inert atmosphere [2]

gas furnaces

In direct gas heating is more difficult to control the temperature, and the products of the combustion gas, including, water vapor, may adversely affect the metal matrix. Therefore, if necessary, only indirect (indirect) gas heating using radiant tubes is used..

Electrical resistance heaters

In most furnaces heating matrices applied electric resistance heaters. These heating elements are robust and well regulated to ensure uniform temperature. Electrical components must be placed in an oven so, not to create local overheating of the metal matrix.

Induction electric heating

These furnaces can heat the matrix to the operating temperature for a few minutes. Since the heating occurs within the matrix, then holding at a temperature not required. An important advantage is the absence of harmful long stay matrices at a high temperature - higher lifetime, less oxidation and damage to workers belts. However, these furnaces have a high initial cost and high maintenance costs.

infrared heating

In these furnaces for heating the matrices applied infrared radiation. Suppliers of these furnaces claim, These furnaces provide quick and accurate heating of the matrices.

Heating matrices and another pressing tool

For proper operation of the heating furnace matrix must be heated pressing tool to a desired temperature irrespective of, in which part of the furnace it is. The duration of stay of dies and other tools in the furnace should be strictly regulated [1]:

If prolonged heating is insufficient matrix may break
If heated for too long, the matrix may lose its hardness (which will also lead to fracture), and running web matrix strongly oxidize and covered with sores.

It is known, that many aluminum presses do not heat the auxiliary pressing tool at all (shims, subbolsters and bolsters) [1]. However, experts strongly recommend not only to warm themselves matrix, but all of them supporting a pressing tool. Each time a new matrix cold lining, subbolstery consuming and bolster her heat stabilization process and lengthen the extrusion process.

General rules for heating the pressing tool

The solid matrix. The heating temperature of the matrix is determined empirically for each matrix. In the absence of experimental data the temperature recommended by 425 to 455 oC. Matrix critical "tongues" to warm to a higher temperature.
Hollow matrices porthole (porthole) and cruciform (spider) are heated to temperatures from 455 to 510 oC.
Bridge matrices are heated to a temperature of 55-110 ºС below the temperature of the workpiece, depending on the ratio of pressing (drawing) and the shape of the profile.
Bolster for solid matrix must be heated to the same temperature, as the matrix and lining, or to a temperature of at least 200 oC.
Bolster hollow matrix must be heated to the same temperature, and that the hollow matrix assembly. At the same time bolster must be transmitted to the press for the minimum time, to keep the temperature, required for extrusion.
After reaching the predetermined temperature on the surface of the tool should be maintained in the furnace for 45 minutes per 25 mm total thickness thereof, to achieve uniformity of temperature in a circulating air oven.

The temperature drop in the heating furnace matrices

Difference in matrices heating furnace temperature should be not more 15 oC. "Hot" and "cold" locations in the furnace are removed by installing additional heat shields for changing circulated air flow.

Accuracy of temperature in the furnace

The accuracy of the actual temperature in the oven controlled at least once a month by comparison with a control thermocouple readings recorded. It is recommended to have two furnace device for temperature measurement in case of failure of one of them.

Sources:

Aluminum Extrusions From Start to Finish – Presentation / Randy Thomure, Sapa Extrusions, 2013.
That Ovens – The Extrusion Press Maintenance Manual, 4th Edition / Al Kennedy.
http://www.tecalex.net/en/furnaces.html