Yeap... vibration might be the key, The three critical factors would be amplitude, frequency and processing time. Assuming that humidity is not an issue...

A narrow partial size distribution with uniform particle geometry is critical - no needles or platelets.

 

Assuming that humidity is not an issue...

Last time I checked, silver powder is not hygroscopic, but Gopher would be the man for this one.

aaand...he beat me to the post.

 

aaand...he beat me to the post.

I forgot we had a chemistry guru in the forum...

 

Last time I checked, silver powder is not hygroscopic, but Gopher would be the man for this one.

aaand...he beat me to the post.

You are right, silver is not going to absorb moisture. Also, this is a pre-alloyed powder with tin and copper (plus other magical dust). All pretty inert metals and even more inert once alloyed. And even more inert once made into an amalgam.

I did lean something new with this post. The alloy is fairly soft and the mercury slowly penetrates the grain boundaries of the alloy in a solid liquid diffusio reaction. One the reaction is complete, you end up with two metallic phases. Mercury tin and a mercury silver phase (plus minor other phases with other elements). Once the diffusion is complete, the amalgam is hard and can no longer be worked by the dentist - about 30 minutes.


Anyhow, I am not fond of this post because I have always been suspicious of mercury and amalgam fillings. Note that the EPA is ok with fillings in your mouth (must have been a negotiated conclusion) but it conflicts with another EPA policy (which may also be a negotiated policy with no scientific basis). The conflict within the EPA is, anyone testing waste water for mercury must meet a particular criteria to avoid contaminating the sample. The requirments for mercury levels is so low that anyone with dental fillings in their mouth is not allowed near the sample. The mercury level in your mouth is several orders of magnitude higher than allowed as flow into some bodies of water (e.g. Great Lakes). And there are even crazy exceptions to that on chemical plants that have been grandfathered in unless those plants are no longer active - then any effluent from ground water contamination must meet the new requirements.

Luckily, I've only ever needed one filling. It is acryllic.

 

Drilled holes in steel plates is how most medication tablets are made. They use statistical quality control. The unit I saw had 100 holes around the perimeter of a steel disk. It was rotating very fast and they would take the 1001st pill for weighing and quality control of a single pill and they would weigh the 1000 for overall quality check.

The powder size is pretty minute ... 325 um mesh ... and its pretty dense as well.. so volume can be used in this case (this is what I think)
ps. when you say "you saw the unit" .. you meant the unit to fill Dental Amalgam capsules or something else ..

 

The powder size is pretty minute ... 325 um mesh ... and its pretty dense as well.. so volume can be used in this case (this is what I think)
ps. when you say "you saw the unit" .. you meant the unit to fill Dental Amalgam capsules or something else ..

As I said above, it was for pharmaceutical tabletting. Very fast. 20 to 30 per second.

Or you could use powder metallurgy equipment - see whole video (excellent) but specifically look at the 3:07 time.
Note that smaller parts can be made at much higher production rates than the larger parts shown.

To increase production rates, you could use a multi-cavity mould. Do not over-compact and eliminate interconnected porosity or your mercury will not penetrate. Soft, low melting alloys (brass, aluminum and, in your case, silver/tin) can achieve full density and near full strength without sintering.

 

As I said above, it was for pharmaceutical tabletting. Very fast. 20 to 30 per second.
Or you could use powder metallurgy equipment - see whole video (excellent) but specifically look at the 3:07 time.
Note that smaller parts can be made at much higher production rates than the larger parts shown.

To increase production rates, you could use a multi-cavity mould. Do not over-compact and eliminate interconnected porosity or your mercury will not penetrate. Soft, low melting alloys (brass, aluminum and, in your case, silver/tin) can achieve full density and near full strength without sintering.

Beautiful ... My initial idea was almost the same .. use two plates with drilled hole of calibrated volumes to required weight ratios. Place one on top of the other, but with an offset to create a temporary powder containing hole. pour powder in the holes in the upper plate and scrap off the excess using a moving blade, hence leveling the surface. The second plate underneath would hold the powder while we fill it. Once leveling is done, pull the plate underneath to make powder drop under gravity into already aligned capsules. Use vibration to make any particles that might still be hanging to the plates.

I guess I have to study more into how to accurately translate weight into volumes. Right ?

 

I guess I have to study more into how to accurately translate weight into volumes. Right ?

I think that in the end you're still going to need at least one certified weight scale for statistical analysis anyway... But this system looks promising, cheaper, and much faster.

 

I think that in the end you're still going to need at least one certified weight scale for statistical analysis anyway... But this system looks promising, cheaper, and much faster.

you're right. I am thinking on the same scale now. At the end , when the capsule is sealed completely, There can be a last weighing mechanism to weigh each capsule and accept or discard , depending on its deviation from the average weight.

And for weighing powder for volume, I think we need to use the old school method of mixing it in a non reacting liquid and find out the volume change the powder causes to the volume of the liquid.

 

@cmartinez , While we are at the topic, you mentioned you're in the dispensing industry. A question, what is a good way to dispense this metal powder, screw feeder or some pipette type (or something else) .. The thing is that this metal is quite fine (325 um mesh) and also very brittle, and its scratches the surface of the metal container if used over a long period of time. In screw feeder if powder and screw are moving in a tight position, it will damage the screw after all. What other kind of dispensing options can be there which doesn't involve much friction between the powder and the part ?

 

Very interesting question... I once designed a waterjet cutting machine that worked at 55Kpsi... I designed everything except the pump (which is called an intensifier) and a challenging part was designing the abrasive dispenser. In the end it all came down to a pneumatically activated sliding gate made from a sheet (a flap) of stainless steel. Friction is impossible to completely get rid of, but it's minimized this way, and it works wonderfully if it's properly designed and flatness tolerance is tight between the gate and the orifice. The feed rate was adjusted by rotating a plate that had an array of different diameter orifices, and the abrasive was delivered pneumatically, through a hose, from a pressurized container several meters away from the dispenser. Unfortunately, I don't have a very good picture of this with me at the moment, but you can see the dispensing unit in the following image, which is the transparent container located in the upper right corner, filled with the reddish powder.

 

@cmartinez , While we are at the topic, you mentioned you're in the dispensing industry. A question, what is a good way to dispense this metal powder, screw feeder or some pipette type (or something else) .. The thing is that this metal is quite fine (325 um mesh) and also very brittle, and its scratches the surface of the metal container if used over a long period of time. In screw feeder if powder and screw are moving in a tight position, it will damage the screw after all. What other kind of dispensing options can be there which doesn't involve much friction between the powder and the part ?

You will always have friction you just need a material that is harder than your abrasive.

Why don't you push the "easy button" and pay another company to pelletize your powder for you. Normally you send them a little bit of powder and they will test it and give you a quote. So much easier than inventing the wheel - again. It is called contract manufacturing or Toll manufacturing. So much easier when your supplier can provide all of the quality tests and data.

 

You will always have friction you just need a material that is harder than your abrasive.

Why don't you push the "easy button" and pay another company to pelletize your powder for you. Normally you send them a little bit of powder and they will test it and give you a quote. So much easier than inventing the wheel - again. It is called contract manufacturing or Toll manufacturing. So much easier when your supplier can provide all of the quality tests and data.

Of course that is on the table as well ... I am just gathering as much information as i can, the final decision would be with the high ups ...

 

Very interesting question... I once designed a waterjet cutting machine that worked at 55Kpsi... I designed everything except the pump (which is called an intensifier) and a challenging part was designing the abrasive dispenser. In the end it all came down to a pneumatically activated sliding gate made from a sheet (a flap) of stainless steel. Friction is impossible to completely get rid of, but it's minimized this way, and it works wonderfully if it's properly designed and flatness tolerance is tight between the gate and the orifice. The feed rate was adjusted by rotating a plate that had an array of different diameter orifices, and the abrasive was delivered pneumatically, through a hose, from a pressurized container several meters away from the dispenser. Unfortunately, I don't have a very good picture of this with me at the moment, but you can see the dispensing unit in the following image, which is the transparent container located in the upper right corner, filled with the reddish powder.


View attachment 77521

Thanks for sharing your experience. I was also thinking of a flap based model, rather than a screw based, made up of stainless steel or carbon steel which is harder than the powder.

 

@cmartinez , I need your expert opinion about dispensing. We need to dispense mercury in the same weight ratio as the powder (400 , 600 or 800 mg). The dental mercury dispensers we are carrying out our current experiments with are producing varying results, for example, if we gauge it to dispense 400 mg, it would dispense somewhere between 390 - 410 mg. We are using the normal dispensers available in the market, which use volumetric dispensing. Have you ever faced any such dispensing challenge ? What can be our starting point ?

 

@cmartinez , I need your expert opinion about dispensing. We need to dispense mercury in the same weight ratio as the powder (400 , 600 or 800 mg). The dental mercury dispensers we are carrying out our current experiments with are producing varying results, for example, if we gauge it to dispense 400 mg, it would dispense somewhere between 390 - 410 mg. We are using the normal dispensers available in the market, which use volumetric dispensing. Have you ever faced any such dispensing challenge ? What can be our starting point ?

Yes I have experience in a similar field. A few years ago I participated in a project involving dispensing a special conductive mix of adhesives for cell phone production, and it was done in the range of 5 to 10 grams, but 400mg shouldn't be that different.
Considering the properties of mercury, and assuming that you don't have wide temperature swings in your facilities (mercury's thermal expansion coefficient is more than 5 times that of iron), I would probably also consider volumetric dispensing for speed and simplicity. If you're missing ±10 mg out of a 400 mg charge (that would be a variation of ±2.5%) then it means that either your dispensing system has mechanical play present in its parts (because of fabrication tolerances being too large, or because there are elastic materials being used) or that the mercury is adhering to the inside of the hypodermic tube (or orifice) being used to dispense it due to capillary effects, adding yet another source of hysteresis in the system.
I would have to take a closer look at your equipment to give you a more thorough opinion...

 

Yes I have experience in a similar field. A few years ago I participated in a project involving dispensing a special conductive mix of adhesives for cell phone production, and it was done in the range of 5 to 10 grams, but 400mg shouldn't be that different.
Considering the properties of mercury, and assuming that you don't have wide temperature swings in your facilities (mercury's thermal expansion coefficient is more than 5 times that of iron), I would probably also consider volumetric dispensing for speed and simplicity. If you're missing ±10 mg out of a 400 mg charge (that would be a variation of ±2.5%) then it means that either your dispensing system has mechanical play present in its parts (because of fabrication tolerances being too large, or because there are elastic materials being used) or that the mercury is adhering to the inside of the hypodermic tube (or orifice) being used to dispense it due to capillary effects, adding yet another source of hysteresis in the system.
I would have to take a closer look at your equipment to give you a more thorough opinion...

There is no particular equipment though. Its a simple dental mercury dispenser purchased from the market, and you guessed right, there is a screw used to vary the amount of mercury dispensed. It is possible that the screw has a play and that is causing the variation?

 

There is no particular equipment though. Its a simple dental mercury dispenser purchased from the market, and you guessed right, there is a screw used to vary the amount of mercury dispensed. It is possible that the screw has a play and that is causing the variation?

Yes it is possible, and not only that, I can see that the dispenser's head is made of plastic, which is yet another likely source of elastic distortion.

 

Yes it is possible, and not only that, I can see that the dispenser's head is made of plastic, which is yet another likely source of elastic distortion.

We were just checking the dispensing mechanism to get the idea how these things work, by studying the solutions already present in the market. What in your opinion can be the crucial points to keep in mind here?