I wanted to prepare a pellet from a powder using a uniaxial press and a matrix attached to a pump, but with the pressure and the holding time that I have chosen, the pellet obtained breaks up.
Are there a way to determine the correct pressure and holding time before launching tests?
I don't think there is a fool-proof way of doing that. The amount of pressure you need to apply is heavily dependent on what kind of material you are dealing with - in my experience the lighter materials are harder do compress into a pellet.
If your pellet is breaking I have two suggestions: 1) Try applying a little less pressure, too much pressure may induce segmentation in the pellet which leads to breaking, not to mention you are quite likely losing sample isotropy. 2) Maybe removing adsorbed water from your sample might help - try leaving your powder in a vacuum desiccator overnight, and making the pellet right after you break vacuum. Good luck!
The best results are obtained by successive trials and errors, and they depend on your material(s). In a general way, too high presures are to be avoided, as they produce shear stress in the sample, leading to delamination.
I agree with Francisco that it is always better to condition the samples before. Otherwise the repeatability willnot be achieved.
Alain
Do try using a lower pressure but as it has been said, you can only get the right condition by trying different ones
Making pellets at low pressure will lead to porous sample.To get the optimum pressure, you can start with low pressure, measure porosity. If it is too high, increase the pressure further next time till your porosity reaches lowest possible. Stop this process when your pellet starts cracking at higher pressure. Porous samples are quite prone to moisture, hence porosity should be as small as possible. In conclusion, only by trial one can find optimum pressure to get lowest porosity pellet of a particular material.
Thank you for your answers .The element that I use is a granulated aluminum, which makes it difficult for a pellet prepared.
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