Uniaxial press: 1 ram vs 2 rams

[Metallus]

Assume to have 2 hydraulic presses for compacting powders with the following designs:

Press A has 2 rams that apply force in opposite directions, pointing towards the powders in the middle
Press B has 1 ram that applies force in one direction, towards the base of the press

My colleagues keep telling me that Press A is "symmetric", has different stress distribution and will lead to a more uniform compacted pellet, whereas press B will have a "gradient" compacting because of the non-uniform stress (keeping the overall applied force constant).

Why should it be different? Even if I'm applying a load only on one ram, the other end will apply an equal force in the opposite direction by reaction (3rd law), since it's the same material anyways. It should be the exact same thing. I'm a chemist and they are engineers, so I always assume to be in the wrong in these matters, but this just sounds too wrong to my brain.

What am I missing? Can you help me?

 

[Dr.D]

In the absence of friction, your contention that A and B should be equivalent is certainly correct.

With friction (particularly side wall friction), there will be some difference.

 

[Metallus]

In the absence of friction, your contention that A and B should be equivalent is certainly correct.

With friction (particularly side wall friction), there will be some difference.

If you are referring to friction between the ram and the walls of the sample holder, it should be negligible since everything is lubricated. If you are referring to the powders pressing against the walls horizontally and generating attrition when they are pressed vertically, then what would change? On the other end we have the same situation, don't we?

And what about their claims? Are they indeed fictitious or have any basis? They always say that the stress distribution isn't the same. What do they actually mean?

 

[Mech_Engineer]

I would tend to think there won't be any difference between A and B unless the pellet under compression has unusual material properties, but as it turns out, this is a common problem in material sintering presses. According to my quick research, double-ended presses are used to minimize material density variation in sintered powder parts.

http://thelibraryofmanufacturing.com/pressing_sintering.html

When manufacturing a thin part from a powder, (high diameter to thickness ratio), [density gradients] may not present a problem [when using a single-end press]. With thicker parts, significant density variations might occur relative to the distance from the punch. To mitigate this type of problem of density-pressure variation within a compacted powder, two opposing punches are usually employed.

 

[Dr.D]

In case A, you have the prospect of relative motion between the ram and the container walls at both end. In case B, this prospect exists at only one end.

 

[Metallus]

I would tend to think there won't be any difference between A and B unless the pellet under compression has unusual material properties, but as it turns out, this is a common problem in material sintering presses. According to my quick research, double-ended presses are used to minimize material density variation in sintered powder parts.

http://thelibraryofmanufacturing.com/pressing_sintering.html

So it is indeed as they say, using two rams will overcome the density gradient. But why does that happen? Isn't the powder being pressed from the other side as well simply by reaction to the applied pressure? And what if I used press A but only applied pressure on one end? Would that still lead to a density gradient?

Damn, my brain just refuses this.

In case A, you have the prospect of relative motion between the ram and the container walls at both end. In case B, this prospect exists at only one end.

What consequences does this have?

 

[Tom.G]

It would seem the problem arises because the powder is not an ideal fluid. If you were compressing say water, then the force would be uniformally distributed throughout the volume. Going to the other extreme, if you compress a rock the force will be axial (at least until it shatters.) In the intermediate case of a powder, I would expect the lines of equal force to be somewhat divergent from the ram to the sides of the die, due to the powder grains being forced between each other. This divergent force would introduce a somewhat conical shape to the lines of equal force in the lower part of the material.

 

[Lord Jestocost]

...What am I missing? Can you help me?...

Have a look at [PDF]Powder Technology H.Hofmann, P.Bowen - LTP | EPFL for a simple model (chapter: 1.1 Fundamentals of Compaction). The pressure decreases with depth in the powder bed due to wall friction. Thus, you get different density distributions when using 2 ram or 1 ram presses (for a schematic diagramm see: http://www.keramverband.de/brevier_engl/4/1/4_1_3.htm).