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Internal friction of air hammer

release time:
2019/12/3 10:05:31
number of times read:
378

Internal friction of air hammer

In order to distinguish the internal friction between the powder particles, the friction between the powder and the mold wall is usually called the external friction. The direction of external friction is the opposite of the pressing pressure. Therefore, the downward movement of the die punch must overcome this frictional force, which causes a reduction in the pressing pressure, which is called pressure loss. That is, the pressing pressure applied to the die punch is not all used for compacting the powder, but is gradually consumed during the downward transmission of the pressing pressure during pressing. It is conceivable that when the pressing pressure of the lower powder drops to zero, the powder cannot be compressed, that is, for a certain powder, not parts of any height and any cross section can be pressed. If long products are pressed with iron powder, horizontal pressing is most advantageous, and vertical pressing is difficult to form. Due to reduced pressure loss. It is related to the friction coefficient between the powder and the mold wall, so by reducing the bonding tendency between the green compact material and the mold material, reducing the surface roughness of the mold wall, the friction coefficient can be reduced. When the lubrication conditions are different, that is, between the compact and the mold wall The friction coefficient curve and the loss of the relationship between the pressure of the curve and the total pressing pressure are only when pressing a certain round cross-section of pure iron powder, without adding lubricant, and the forming height is the ultimate height.

The force required to release the compact from the mold is called the ejection pressure. The ejection pressure is related to factors such as pressing pressure, powder properties, compact density and size, mold and lubricant. One uses zinc stearate to lubricate the mold wall; one uses molybdenum sulfide to release the pressure, if the compact does not change, the demolding pressure should be equal to the friction between the powder and the mold wall. However, the actual situation is that after the pressure is released, the compact is stretched along the height and the side pressure is reduced. Therefore, the powder is related to the mold wall loose ratio. When designing the mold, the unit compaction pressure (ie compaction pressure) is generally taken into account. The characteristics of the powder body and plastic deformation, and assuming that the lateral expansion is within the elastic deformation range, the influence of the elastic deformation of the mold wall are not considered, which are not in line with the actual situation. Therefore, there is a certain error in the calculated value of the lateral pressure, which can only be used as a reference when estimating the magnitude of the lateral pressure. It must also be noted that the above-mentioned side pressure is an average. Due to the influence of external friction, iron powder compacted specimens at different heights of the compact are lubricated in the unidirectional mold wall; a non-lubricated static friction coefficient of the powder against the female mold wall, and the unit compaction pressure of the iron-based compacted compact. In the range of ejection, ejection pressure powder or brittle materials, such as hard gold and oxide powder, which are necessary for pressing compaction.