Small Part Reject with air blast

ndzied1

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Aug 2002
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Hello:

This started as a private message with Eric Nelson but I figured more people might be interested in the topic.

Recently, Eric had discussed his spice cap inspection problem on this forum. He posted a video showing the rejected parts being blown off the conveyor with an air blast. I now have a possible job where we would need to blow reject parts off a conveyor and wanted to know what kind of air valve he used which wasn't discussed in the previous topic.

... from Eric Nelson ...
Hi Norm,

I usually use SMC valves on all our machines, but on this particular machine the customer requested Mac valves. I have never been a fan of Mac as they used to be quite unreliable. They have improved since them, so I can recommend them. They actually have one of the fastest response times in the industry, so in your application they just might be ideal (not bad prices either!).

The valve I used for the reject was a simple Mac 35 series 3-way. The reject system had no trouble keeping up with a throughput of 400/min. Scan time becomes an issue as you start getting fast, so consider interrupts to trigger the blowoff at the correct position. At 100 ft/min, a conveyor travels farther than you'd think in only a few milliseconds! Also be sure to use a higher resolution timer to control the blowoff time. A timer with a 0.1 second time base just won't be repeatable...

Good luck with the project!...

Some other questions I have would be
  • how do you come up with the on time for the air blast
  • should I use any specific type of nozzle... (I'm thinking a small oriface to avoid spreading the air stream and affecting other parts)
  • what pressure do things like this usually run at?

For my application, the part length will be about 12mm with approx 24mm between parts (thus, 36mm from the leading edge of one part tot he leading edge of the next). My target is 4 parts per sec. so the part will pass by a stationary point in 83 msec.

Will probably have a micrologix 1500 processor on the machine.

Any answers or other suggestions greatly appreciated.

Thanks
 
It's not worth figuring out the ON time for the air blast ahead of time. If the part is light, it may blow off with a pulse of just a few ms, or you may need to keep it on longer for a heavier part. I made mine adjustable from the HMI.

Since it didn't need to be adjustable, the air jet on the spice cap machine was just a small hole in a piece of aluminum with the back tapped to accept an air fitting. Usually we use small adjustable air nozzles with a built in flow control. I think they're from McMaster-Carr but I couldn't find them in the catalog. I'll find out tomorrow where we get them from.

You could put a separate regulator on the blowoff valve, but I've never had a problem just using the regulated air that goes to the other valves (usually ~80PSI). The air blast doesn't use much air. I think it's more important the have control over the flow rather than controlling the pressure.

beerchug

-Eric
 
Norm:

I just would like to ditto Eric: I also am using SMC, but MACs, with all their shortcomings, have one advantage: they are fast!

As far as the parameters concerned, I think it is much easier to spend some time doing trial and error, rater than trying to analyze it mathematically... it is quite a task and the results are quite imprecise anyway...
Needless to say, you got to have dry non-lubricated air supply for these things.
 
Eric Nelson said:
...I'll find out tomorrow where we get them from.

Helluva thing when you have you quote yourself... :rolleyes:

Sorry Norm, I forgot to check on them today. I'm not sure who's working this weekend (besides ME of course), so worse case I'll have the info for you Monday afternoon.

beerchug

-Eric
 
I agree with all of the above. I am using a 1500, Mac valve and air manifold to eject parts off the conveyor of a printing press when needed. The ppm is not as great in my application but I think you’re on the right path. I think we get our flow controls and air jets from SMC.

My two cents
Bob
 
Hey Norm,

I did try and find the part that Eric had described, on mcmaster.com, but was unable to... there's just too many things to find electronically. Maybe you or Eric would have better luck... Online, it's about page 850, but don't think these are quite what Eric had in mind.

On that thought, there's quite a few ways to do what you are attempting, with Eric's idear being a good one. Just another thought to throw out there... maybe a bumper to knock the stuff off the conveyor? Working in the bottling industry, we use 'ejectors' that are basically pneumatically controlled bump pads that just bump the part onto another conveyor. We have had success up to 1600 ppm (27 pps, approx...) and I'm sure that we could go higher if needed. The only difference is we use a seperate control for inspection and rejecting (filtec, if you're interested... tried their site, filtec.com, but the bumpers may be too large for your application).

Other thoughts... you may wish to take some time and decide how you want others to control this system. SMC and mac are great components, as suggested, but you could set this up in a myriad of ways. Do you wish to make the hose for air rejection adjustable? I believe smc carries 'modular' snap together air hose, which isn't a bad way to go, but with the speed that you will be running, do you want to take the chance on someone adjusting out of position, letting bad parts go by and rejecting good parts? The HMI is a great way to go, but what is the skill level/dedication of the operators? Or do you wish to lock this out and have only maintenance/QA be able to adjust the parameters? I would definitely suggest the air setting being adjustable, since there are a ton of issues you can have go wrong, like low air pressure in the plant, parts deteriorating over time, etc...

I guess I'm just trying to say that you are on the right path with your speeds and ideas. The 1500 has a great scan time, and it can be done. The bugger is in the details... ;)

Hoot
 
Since a picture's worth.....
[attachment]
I couldn't find the nut that clamps the nozzle to the fitting, but you get the idea. The nut also has a notch in it to allow the nozzle to exit at a 90° angle (the nozzle tips can be formed as well). It's not clear in the photo, but the black block is tapped 10-32 on the end and has clearance for two #10 screws cross drilled in the body. IOW, it's VERY universal when it comes to mounting. (y)

I doubt we're the only ones using these, so someone else may know of a source. I STILL think they're from McMaster... :D

beerchug

-Eric

nozzle.jpg
 
The air blast doesn't use much air. I think it's more important the have control over the flow rather than controlling the pressure.

The air blast reject system we use blows bread dough (approx 770 gms weight)off a small conveyor, and we found it necessary to have a small pressure vessel (i.e. accumulator) mounted close to the air valve, as when the reject valve turned on, there was insufficient flow from the factory air supply.

I suppose it will depend upon the weight of your product :)
 
Pneumadyne
612.559.0177

This company has parts that look just like the above pic.
Hope this helps.
 
Thanks to all

Thank you all that replyed to this thread. It's like I got to go to a seminar on the topic without leaving my chair (well, I did have to go to the little boys room :p ).

As I said, we're only in the quoting stages right now but hopefully we get an order out of the thing. If so, I'm sure I may have some more questions for you all at that time.

Cheers :site:
 

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