Dummy-block

Fig. 6.7 Equaling-up for the StarLam mixer series by varying the number of microstructured platelets [27. To have the same volume filled within the recess of the housing, a dummy block with inner channel is placed on top of the platelet stack for...

This process. Fig. 15.7, is used for the majority of extruded products. A heated billet is positioned in the container with a dummy block placed behind it. The container is moved forward against a stationary die and the ram pushes the metal through the die. After extrusion, the container is moved back and a shear descends to cut off the butt end of the billet at the die face. The process is... 

Therefore, care must be taken when the thermodynamic model is changed in a flowsheet. It is recommended that a dummy block is introduced between... 

When extruding cast billets, of which only the faces are machined, a shell forms in the container, the thickness of which is about 2 to 3 mm depending on the billet diameter. It containes oxides, dross and other casting impurities and eliminates the need for machining the billets. If this shell were to be pulled out of the container after the extrusion by means of the ram, a very high pullback power would be required. Therefore a loose dummy block, whose diameter is slightly larger than that... 

Eig. 69. Separator press for separation of discard and dummy-block... 

Within a short distance of the die a transition cone in the form of a hollow funnel is formed towards the die aperture, which is inclined by about 20 to 45° towards the ram axis. The material in the dead corners participates in flow to hardly any extent. The material which flows back in the surface layers of the billet, is diverted at the faces of ram and/or dummy-block towards the die aperture. An inflow funnel is thus formed which - towards the end of extrusion - is liable to be occupied by fragments of dross and oxide drawn out of the skin ) (see Fig. 109 and 110). This disadvantage is obviated by leaving part of the billet unextruded, the thickness of this discard corresponding approximately to that of the dummy-block. 

Fq = face area of dummy-block pressing on billet, cp — = extrusion ratio,... 

Presses. Wear Tools Extrusion dies, extrusion mandrels and dummy-blocks... 

If thick-walled tubes with small inside diameters, such as hollow copx)er bars for stud bolts, are to be extruded from the same container, tools with a pointed mandrel, as per sketch b, are employed. The operation is the same as that described above. The stripper stroke is somewhat longer, because the advance motion of the mandrel in this case is shorter than that in sketch a by the length of the mandrel point projecting over the dummy-block. 

In order to minimize the wear of the mandrels, they are well lubricated like all of the other tools which come into contact with the billets, i.e. die, container bore and dummy-block the lubricant being in most cases a paste made of superheated steam cylinder oil and flaky graphite. 

Table 16. Dimensions of Dummy Blocks (Suggested Standards) )...

Dimension t is the length over which the die protrudes into the container and s the thickness of the dummy-block which is chosen to be 6 = 0.4 to 0.6 d. Empirical values of dummy-block thicknesses are tabulated in Table 16 the series to be chosen depends on the thermal stress. 

A dummy-block is placed against the face of the extrusion ram. For the extrusion of rods the dummy-block is centered on a bolt being inserted in the solid ram and for the extrusion of tubes by means of a hollow ram, on the mandrel. The transition from the cylindrical part of the ram to its flange takes a tapered form. The bearing surface of the flange is dimensioned for a permissible specific unit pressure of about 4,000 kg/cm-. The area of the thrust ring which transmits the ram power to the cross-head, is dimensioned for a surface loading of about 1,500 to 2,000 kg/cm. On account of its stress-concentration index the surface and the central... 

When extruding billets without shell , extrusion rams to which the dummy-block is rigidly attached, are often used, in which case there is only little clearance provided in the container bore. The dummy-block... 

Fig. 148. Ram with screwed-on dummy-block for extrusion of billets without shell...

The extrusion ratio is to be at least p = P// = 15, where P = cross-sectional area of dummy-block and / = cross-sectional area of the extruded tube. 

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