# Extrusion of aluminum: heating of the pressing tool

## Pressing tool

Aluminum extrusion press tools include:

- extrusion dies, which directly generate the profile, and
- set of auxiliary pressing tools, which ensures the normal operation of matrices: bolsters, podbolstery, lining.

## matrices heating furnace

Correct heating of dies and press auxiliary tools is critical. Many matrices break down precisely because of overheating or underheating before working on the press. The following methods of working with a pressing tool for extrusion of aluminum and easily pressed alloys such as alloys of the 6xxx series, for example, 6060, 6063 or AD31, help eliminate these problems.

1) Control the hardness of the matrix – before each heating, check its Rockwell hardness.

2) Matrices are heated in an oven, which has the same temperature throughout its volume. Non-uniform heating of the tool must be excluded.

3) Matrix heating furnaces must have a screen, which prevents direct radiation heating of the instrument.

4) The recommended holding time at a given heating temperature is:

- for ovens without air circulation - 1 hour for every 25 mm steel thickness;
- for ovens with air circulation - 45 minutes per 25 mm steel thickness.

Remember, that heating above a predetermined temperature or an excessive holding time in an oven will reduce the hardness of the steel and may damage the die surface, especially on the surface of the belts.

5) The maximum limits for the duration of stay of matrices in the furnace after reaching the heating temperature:

- 260 ºС - not limited;
- 315 oC - 40 hours;
- 370 oC - 10 hours;
- 425 oC - 8 hours;
- 480 oC - 2 o'clock.

6) It is necessary to check the matrix heating furnace monthly for the accuracy of temperature control. It is recommended to install double temperature control on these ovens.

## Heating the bolster

Bolster for solid matrices must be heated to the same temperature, as matrix and lining or at least up to 205 oC. Bolster for hollow matrices is heated to the same temperature, as the matrix kit. Bolster should be delivered to the press with a matrix in the shortest possible time, to maintain the temperature required for pressing.

## Heating matrices

Critical solid matrix matrices, for example, with narrow tongues, needs to be heated to a higher temperature, than usual, non-critical matrices. The heating temperature of solid matrices is determined from the experience. The heating temperature of matrices such as porthole and cruciform should be approximately from 455 to 510 oC. Bridge matrices are heated on 55-110 ºС below, than the temperature of the workpiece depending on the size and shape.

## General Heating Recommendations

- Heat the matrix in the matrix heating furnace at a rate not exceeding 440 ºC per hour. For tubular (hollow) matrices, heating is performed more slowly and with a longer exposure.
- Warm up lining and bolster. It is necessary, so that their temperature is not lower than the temperature of the matrix.
- Follow, so that the matrix does not anneal by pressing too hot aluminum.
- In the case of tube dies and other hollow dies, care must be taken, so as not to overheat the die due to pressing too many blanks through it without changing the die. Since hollow dies cannot be cooled directly during their operation, then they need to be given periodic "rest". Some experts recommend pressing through one die no more than 50 blanks, after which it is changed to another matrix. This avoids annealing of the matrix - such cooling intervals extend its life.
- For hollow dies, the problem of removing aluminum after pressing is a rather serious problem.. Most factories use aluminum etching by immersing the matrix in a solution of caustic soda at a temperature of about 70 oC. It is strongly not recommended to immerse still hot matrices (230-260 ºС) into the caustic soda solution, so this significantly shortens the life of the matrices due to thermal shock.

Source: T. & W. Cockburn, Aluminum Extrusion Technology Seminar, Chicago, 1977.

See. also:

Aluminum extrusion: hollow die adjustment

Aluminum extrusion: pressing new dies