Better process for tool set loading before hot pressing? 1

[italyct23]

Hello,

I work in a laboratory and we are experimenting with hot pressing raw powder (ceramics, metals, etc). Due to the nature of hot pressing and the expense of the powder, it is necessary for us to press near net shape. To do this, we load the powder into a graphite die ring in the shape of the final product. Once the powder is in the ring, it is necessary to "level" it so it presses uniformly and to ensure there are no divots or hills on the surface of the powder. We do this simply with a leveling hand tool.

I have two problems with this method. It takes a very long time to do and there is a great deal of room for error. The other problem is that it does not densify the powder before pressing. What I mean is, the powder has a low tap density going into the press thereby causing a need for higher temperatures and pressures to compact the powder to near 100% density. I assume a powder with a higher tap density will require less pressure and temperature to achieve the same results. Simply placing this powder in a tap density machine like this one:

http://www.quantachrome.com/Density/Autotap.htm

causes our powder to compact significantly which reduces the volume the powder takes up in the tool set among other things.

My question is, is there a machine out there or something I can build that can not only increase the tap density of the powder but also eliminate the need for the manual leveling step? I don't think a vibratory table is the right tool because research shows that the pre-compaction must only take place on the vertical axis, i.e.-exactly like the tap density machine. So, basically, I would need a giant tap density machine. The key is that the powder is densified in the tool set itself so that the tool set can be loaded directly into the hot press after this process. The problem this poses is that some of these tool sets will weigh up to 30 pounds with the powder in them.

Any ideas would be greatly appreciated.

Thank you,
Mike

 

[ornerynorsk]

Would there be economic advantage in going to 2 toolsets, one to produce a preform and the other to finish the pressing. Preform could be done in lower tonnage (IE lighter weight toolset) so that the tool could be placed on a vibratory plate for leveling/handling/filling. PM is definietely not my area of expertise, just some random thoughts.

 

[tripleZ]

What type of particle shape are we talking about here, and what is the approximate mesh size distribution of the powder? If you're working with metal powder, the shape of the individual powder particles will affect your apparent density. So I guess question #1 is what is the powder made of, and how is it formed (e.g. atomization, carbonyl, etc.)? And particle size/shape will play a large part in how dense the preform may be compacted to. For instance, you'll find you can get an irregular shaped powder mix to compact differently than a spherical shaped particle.

Question #2 pertains to mesh size. You may be able to achieve a better initial tap density if you screen out the larger particles. For example, a vibratory mesh screener using only -325 mesh powder might help you to improve your apparent density. To insure there are no divots in your powder, it would help to know what your die shape looks like. Assuming you have a flat ground top, a flat ground parallel block should suffice as a leveler, unless you're using a contoured die shape.

Question #3 pertains to lubricant. What sort of powder lubricant are you using, and in what concentration is it? Are you using a die wall lubricant, or actually blending in lithium stearate or acrawax?

There are a lot of variables which may contribute to your question, so I'm sorry if I'm not giving you a straight-forward answer.

 

[italyct23]

Thank you both for your information so far.

I'd like to answer the previous three questions.

1) We do both ceramic and metal powders in elemental form as well as alloyed. Most of the time, we press one elment of metal powder. It is usually raw powder from a mine that is spherical in shape and screened to a specific mesh size.

2) Many times we use a mesh range like -100/+325 mesh. Sometimes we use only one mesh size like 325 mesh. The die shape is an extremely simple cylinder with no contours whatsoever.

3) As far as lubricant, we line the sidewalls with either graphoil or boron nirtide spray depending on the powder type. We can't afford any contamination from a lubricant that is mixed in with the powder.

Thank you again.

-Mike

 

[EdStainless]

The range of particle sizes will greatly impact compaction. You need a good distribution of sizes (baseballs and pingpong balls) in order to get good fill and uniform contact.

You are using upper an lower punches, right? You may be able to vibrate your lower punch. The punch should not be real tight. I built a system once that had a tapper under the punch to make it vibrate vertically. A small rod comes up through the base plate to do this. When are pressing the load is all on the base plate.

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