Dummy block for extrusion of aluminum
Dummy block
Press the washer is the most critical element of extrusion equipment, which is used in the process of extrusion of aluminum (figure 1). It affects the quality of extruded products, and the press overall performance. The Russian term for dummy block is "пресс-шайба" .
Figure 1 - Diagram of a direct extrusion press [1]
What affects the operation of dummy block?
Good work of dummy block depends on many factors, eg:
- centering (alignment) of the press as a whole,
- the distribution of temperature of the container,
- temperature and physical properties of the aluminum billet,
- proper lubrication,
- maintenance sсhedule.
Functions of dummy block
Functions of the press washer may seem very simple at first glance,. This detail, which is like a continuation of the stamp (punch), intended, to directly push the softened by heating an aluminum alloy through a die. The press washer should perform the following functions:
- Transmit the press force on the alloy in each pressing cycle at high temperature
- Rapidly expand to a predetermined limit under pressure and leave only a thin layer of aluminum on the container sleeve
- Easily separated from the workpiece after its pressing
- Do not capture any gases. The trapped gas may cause damage to the container or the sleeve itself press washer
- Contribute to solving small problems centering press
- Have a quick assembly / disassembly at the press stamp
The maximum wear of the press dies typically occurs on the front parts thereof. As a result, the ram requires replacement or repair. Therefore, they are made of steel with high wear resistance. Usually enough to change only the outer ring, while the remaining parts are still working. This is very important to reduce press downtime. [2].
Dummy block must operate freely inside the container, to provide maximum performance of extrusion press. This should be a good centering press. Deviation of movement of the stem from the centering axis press usually easily detected on uneven wear, which can be seen at a press washer ring. Therefore, centering (alignment) of the press should be done regularly and carefully..
Steel for dummy block
Dummy block is part of a pressing tool, which is subject to great stress. Dummy blocks are made of tool steel die - H13 by American classification. Domestic analogue of this steel - steel 4H5MF1S. Before you start, press press washer is heated to a temperature of not less than 300 oC. This increases the thermal stress resistance of the steel and prevents the formation of cracks due to thermal shocks.
Figure 2 - Press washer manufactured by Damatool from steel H13 [2]
Temperatures of billet, container and die
Overheated workpiece (above 500 ºС) will reduce the pressure on the press washer during pressing. This can lead to insufficient expansion of the ram and, Consequently, extrusion of a metal blank into the nip between the press washer and the sleeve of the container. This may reduce the period of the press-service washers and even to significant plastic deformation of its metal. Similar cases can be associated with containers, which have different heating zones.
Sticking of dummy block to billet is indeed a serious problem. This is especially common on long workpieces and soft alloys. Modern method to combat this is to apply a lubricant on the workpiece end face on the basis of boron nitride.
Dummy block maintenance
The washer must be checked every day:
- Possible aluminum adhesion is determined by visual inspection
- Check free movement of the rod and the ring, as well as the reliability of the fixing of all screws
- Every week dummy block must be removed from the stem and be cleaned in the bath for etching of dies.
- Excessive expansion can occur during operation of the dummy block. You need to control, that this expansion is not too high. An excessive increase in the diameter of dummy block can significantly reduce its service life.
Sources:
1. Saha P. K. Aluminium Extrusion Technology, ASM International, 2000
