random thoughts toward a successful paste dispenser.

These are RANDOM thoughts out loud, right now….there are a few things that need to be done to make the paste dispenser work, I have studied the various syringe/ auger constructions, their claims over others etc etc extensively.

rough drawing……

Visio-dispenser

Visio-dispenser_2

simpler Visio-dispenser

MK1 will be direct syringe pressure type.
the syringe types need a bit of work- a combination of the below :
0) needs temperature stability/control
1) no air piston- direct piston driven by lead screw/stepper. no air anywhere .
2) pressure in the chamber needs to be monitored . have to figure this one out. something on the wall that provides pretty reasonable consistency. I am thinking resistive or similar sensor on the walls of the pressure chamber.
3) pressure into the chamber by a piston is easy to increase by pushing the piston down, but on the withdrawal which is important has to be done without introducing air.

A withdrawal is required as part of the cycle. Many system have problems with voids created. A vac created may not pull back the fluid reliably and as desired, so that needs to be worked on. there may need to be the vac created by the retreating piston PLUS another means to pull the fluid back up the needle. Perhaps an opposing (with hole in the middle) piston at the bottom of the chamber to push the fluid back up. Surface tension will permit the fluid in the nozzle also to assist pulling the fluid back up (and avoid any voids- that’s the idea- no voids are to be created.
There are other means to push/pull the fluid.   Constricting a channel of sorts (squeeze) or expand- the fluid needs to be pulled up the nozzle.
It’s hydraulics, also.  Hydraulic systems are using push , pull…..Must remember. if only the piston moves back, we are relying on the vac and the surface tension to pull everything up. the question is for example, if the pressure chamber was full, that’s alot of fluid to pullback on vac and surface tension. So I think there needs to be assistance of sorts. maybe there are ribs or something on the walls that move, I dunno, but I will start thinking.  if there is pullup (by vac due to piston pullback) AND pressure from the bottom of the chamber UP (by another piston) then we can avoid voids in the main chamber and only the small quantity in the nozzle needs to be pulled back up the nozzle, not a large volume up the chamber.
Perhaps…. deforming the walls , changing the shape of the entire chamber with a crush, or a expansion would provide useful methods, also,

IE use a piston for movement  of fluid to ensure the chamber stays pressurized, but also use deformation of the chamber for dispensing. Epiphany : If  a large surface area of the chamber deforms  this is acting on a much larger  surface area than the end of a piston.
such a deformative wall though is a difficult thing to build . The pushing piston is required to keep the deformation chamber  full, as the deformation chamber has finite breathing ability.  so the deformative chamber for dispensing is a  hard but interesting option.

Either the chamber (stock) or the applicator (needle ) could be deformed..There is a good supply of piezoelectric actuators around, time to learn about them…
Air or fluid could be used to deform a container, also, like a bladder or sealed container inside a container to squeeze or open the contents.

now we have good pressure and release control and pressure sensors, when the dot is deposited, best performance is a precise delay before the precise pullup.
Distance to Z=0 is going to have to be known to at least half the dot size- at least 0.1mm , so there will need to be a very precise distance sensor and height control of the head.

The 32 gauge auger system that is designed for 01005s and it is successful, uses a 0.1mm nozzle internal diameter.  I will start with that size.
There will need to be multiple devices to be able to deposit with reasonable speed on a mixed PCB- IE different nozzle sizes. Watching on my air dispenser dispensing through a 0.1mm nozzle, there is at least a 1/2 of a second for the dot to be let go- IE takes a while for the small amount of surface tension, presumably because the area is very small, takes time to let go. Pull up too fast and the blob is not reliable, the head pullup needs to be slow. So the smaller dots end up being quite time consuming to put down.

I read that the surface tension and behaviour  changes quite a bit with temperature so the chamber paste AND the PCB surface will need to be temperature controlled.  No use dropping carefully controlled paste onto an icy PCB.

To summarize :
VERY accurate control of Z height  with better than 10um movement resolution .
Piston compression from one end of the chamber
Piston compression from the other end of the chamber..
Deformation of the chamber or applicator (needle)

another idea is that once the paste is moved into the applicator (needle) this area could be sealed off from the stock chamber.(rotating / sliding ) , constriction of the applicator etc) and the applicator could then be deformed by a fast pulse (IE a piezo actuator or solenoid) . This way the only way for the paste to go is out the needle.

ideas…ideas.. a solenoid smacking into a deformable wall would work…
if there was a sub chamber from the main stock chamber like a hollow block that had the applicator on it and it was sealed from the stock chamber, an deformation actuator could be one or more of the walls of this stock chamber. I think for success, the pushing force needs to be applied over a very large area  to volume ratio. Not just over a small area of the volume.
Additionally, if there is a separate deformation chamber to the stock chamber, a pull-back motion in the stock chamber may not be required.

In fact, ipso facto, if the paste is pumped in, there might really be no need of a piston/ construction bladder for the stock chamber.

Constriction of the duct  between the stock  and delivery chamber can be simplified. A duck valve. (thanks Robert)

OR just constrict the stock chamber (for delivery and pullback)  and have no separate delivery chamber.This solves the problem if refilling the delivery chamber without ooze.
The thing is, the small chamber removes a whole lot of fluid the pulse has to be transferred through.. so perhaps another valve is required on the delivery needle.

My guess if the bigger the volume getting squeeze, the more  low-pass the delivery behaviour. and the harder to control over temperature and paste variation…

Might use ultrasonics to measure density, voids, pressure.
Like three transducers in a 120 deg  arond… 1MHz transducers…

 

No need to go crazy with piezo though, voice coils, solenoids they all work.
golly there are all sorts of piezo things

http://kineticceramics.com/piezomotor_act.html?gclid=CKWTweL12swCFQwnvQodQ0MN4Q

https://www.americanpiezo.com/piezo-theory/actuators.html
http://www.physikinstrumente.com/products/integrated-piezo-actuators.html
http://www.cedrat-technologies.com/en/mechatronic-products/actuators.html

 

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