The role of the bearings of the dies in the control of aluminum extrusion

What is extrusion die

Aluminum Extrusion - an industrial technology of molding of long aluminum profiles with constant cross-section. This cross-section is formed by a hole - working channel - a tool steel, which is called the matrix. aluminum flow within the matrix is ​​controlled by complex phenomena tribomechanical, which depend on the pressure, flow rate and temperature of aluminum, and difficult mathematical modeling. Therefore, understanding the mechanics of aluminum pressing process is still very limited and design of extrusion dies is largely based on empirical rules, as well as the experience and intuition of a design engineer [1-3].


Figure 1 – Dies at the aluminum extrusion plant [AEC]

The reasons for the uneven flow of aluminum

One of the main problems in the design of the matrix is ​​aluminum leveling the flow velocity at the outlet of its working channel.

The main reasons for the non-uniformity of the metal flow through the outlet of the flat matrix are:

  • the impact of the container. For direct compression the frictional resistance on the border of the preform container slows metal flow near the surface of the workpiece. Therefore, the center metal workpiece moves faster, than its "peripheral" areas.
  • Differences in the profile thickness. The wider working channel matrix, the smaller the flow resistance it exerts aluminum and, Consequently, higher flow rate through the channel.

As the control over the aluminum matrix in a plane

The main methods for controlling the flow rate of aluminum through the working channel of a flat matrix are (Fig. 2) [1-3]:

  • change the length of the parallel working belts.
  • changing the angle of the working belts,
  • change the size and shape of the prechamber.


Figure 2 - Flat extrusion die with prechamber [3]:
1 - forkamera, 2 - running web, 3 – the output part

Types of working belts matrix

The figure 3 three types of working belts are schematically shown (figure 3):

  • parallel;
  • tapering along the metal flow ("slowing down") – English term: “choked”;
  • expanding along the metal flow ("accelerating") – English term: “relieved”.


Figure 3 - Types of working belts [3]:
a) parallel; b) slowing down; c) accelerating

The working belts with retarding or accelerating angles may be on both the working channel portion, so completely throughout the working channel.

Parallel running web

Parallel belts are working standard. In this case, the flow rate of aluminum through the working channel of the matrix is ​​adjusted by changing the length of the running web. Examples working belts of varying length flat die for smoothing the flow shown in figures 4 and 5.


Figure 4 - Control over the aluminum matrix in a plane
using different lengths of working belts [2]

Figure 5 – EXAMPLE working length distribution belts
in a flat matrix [1]

Slowing and accelerating running web

Slowing and accelerating the running web at the entrance and exit of the working channel shown in Figure 6. Typically the flow is used for slowing a better filling of the working channel matrix, for example, when pressing hard aluminum alloys (series 2xxx and 7xxx) [2]. It provides profiles of improving dimensional stability. Acceleration of the flow is used to equalize metal flow profile over the cross-section.


Figure 6 - Decelerating and accelerating corners of the working belts [2]

Adjustment of working angle belts

Changes in the working angles of the girdle is a popular technique in spotters matrices for the production of extruded aluminum profiles plants. This method makes it easy to adjust the flow of the metal in the new matrices, since even the removal of small amounts of the girdle of the matrix material gives a large effect in the acceleration or deceleration of the flow of metal.

Compensation of the elastic deflection of the matrix

Slowing the working angle of the girdle is used to compensate for the elastic bending matrix elements, such as, for example, "Languages" (figure 7). If workers belts "language" should be during compression parallel, they perform with a small angle slows down, that when the deflection under load working belts are parallel.


Figure 7 - Element "language" in a flat matrix [3]

Flow resistance in the working channel matrix

The work [3] presents the results of numerical simulation of the resistance of the working channel of the matrix, depending on the angle of the working belt (figure 8). established, that negative (accelerating) angle gives low resistance to metal flow. With an increase in the accelerating flow resistance value of the angle remains constant. When reducing the accelerating angle for a small fraction of a degree to zero, that is before, as the working channel is parallel, its pressure increases dramatically. In this case, pressure stabilization is achieved only at a positive (retarding) angle of about 0.5º.

Figure 8- The pressure at the inlet into the working channel of the die
width 2 mm and length 5 mm depending on the angle of the working belt [3]

Such metal flow behavior can be explained as follows:. In the accelerating (expanding) working belt, aluminum touches the walls of the working channel of the matrix only at its inlet and then loses contact with the working belts (Figure 9a). The increase of the angle, and also the length of running web, no longer influences the flow resistance of the working channel metal.


Figure 9 - Contact between aluminum and working belts [3]:
(a) - accelerating belt, (b) - retarding belt

The decelerating working belt ensures full contact of aluminum along the entire length of the working belt (Figure 9b). A slight increase in the angle, at which there is full contact, almost no effect on the rate of flow of metal. The transition from almost no contact to complete the contact occurs at the value of the angle about 0º. Thus there is a sharp increase in friction, which corresponds to the jump in the graph the pressure pattern 6. This behavior of the working belts was established and experimentally [3].

Recommendations for working corners belts

Based on the results presented above on the dependence of the flow resistance of various types of working belts in [3], the following recommendations are proposed for the application of the angles of working belts.

When to slow down running web

Slowing working belts used metal flow control, if necessary, increased demands on the quality profile surface. The magnitude of deceleration angle should be such, to ensure full contact with aluminum. In this case, flow resistance is predictable and stable, and the influence of irregularities on the working belt on the quality of the profile surface will be minimal [3].

Effect of the working angle of the girdle on the quality of the surface profile cm. also here.

When to accelerate work belts

Accelerating (expanding) working belts provide minimal resistance to metal flow and therefore can provide maximum pressing speed. They are used in the case of, When the control flow through the working band is not necessary, and there is no requirement for high quality of the surface profile.

Sources:
1. T. Sheppard, Extrusion of Alluminum Alloys, 1999
2. P. Saha, Aluminum Extrusion Technology, 2000
3. G. van Ouwerkerk, PhD dissertation, 2009