what causes 'chatter' breakdown?

[jvanick]

we had a set of screens break down today, and it looks like chatter marks on either side of the screen, like the squeegee was 'skipping' and then breaking down the emulsion.  The screens have roughly 6000 or so prints on them, so maybe it's just overuse for SP1400?

never have really seen this before, and wondering what's causing it.

Emulsion if it matters: SP1400
Approx squeegee pressure: 45-50 psi
20 degree angle
70/90/70 duro squeegee (relatively sharp)
Plastisol ink.

[ZooCity]

If it's just on the edges of the squeegees that you see the chatter marks, it's an easy fix- round your squeegee corners.  Also, top tape your squeegee drop/finish areas as well as the very edge where the sq corner hit the emulsion.

If the chatter marks are inside the image area it's just a matter of your blade/pressure/substrate/# of impressions/off contact, etc. being too much for the emulsion.  SP1400 is tough chit from what I hear but 6k impressions will start to get at most any emulsion, just a matter of friction over time.   You can combat break down inside the image area by face coating the ink side of the screen, putting a little more emulsion on there to resist the wear/tear.

But really, rounding the corners and top taping will solve most of this.  Good luck!

[jvanick]

If it's just on the edges of the squeegees that you see the chatter marks, it's an easy fix- round your squeegee corners.  Also, top tape your squeegee drop/finish areas as well as the very edge where the sq corner hit the emulsion.

If the chatter marks are inside the image area it's just a matter of your blade/pressure/substrate/# of impressions/off contact, etc. being too much for the emulsion.  SP1400 is tough chit from what I hear but 6k impressions will start to get at most any emulsion, just a matter of friction over time.   You can combat break down inside the image area by face coating the ink side of the screen, putting a little more emulsion on there to resist the wear/tear.

But really, rounding the corners and top taping will solve most of this.  Good luck!

we already round the squeegee ends, and tape where the squeegee drops are... plus tape the ink side where the edges of the squeegees ride.

This is actually IN the image area, not edges.

[ABuffington]

Are the chatter marks parallel or are the marks from stencil openings outward in any direction?

Al
Murakami

What was the ambient shop temp over weekend?

Alan

[jvanick]

They were parallel AND starting from the open area out in some cases, other cases, it was in the middle of the screen itself.

ambient temperature doesn't go below 60.  It *is* very dry in here... in the screen room it's about 22% RH right now, in the shop, around 30%.  (if that makes a difference)

Pic of  one of the screens attached.  The "line" you're seeing at about on the 1/3rd of the image is where the the tape is on the back side of the screen.

[ZooCity]

We see this with emulsions when it's dry sometimes.  Particularly with HVP.  I would try the face coat on the ink side or just plan on swapping out screens every 5k impressions. 

[Colin]

I have had this happen with any emulsion that is "Harder" than other emulsions.  I.E. non diazo Water Resistant....  This happens all the time with Murakami's HV, Saati's PHU, and other emulsion like this that are designed to be water/discharge resistant without needing a hardener.  The reason for the cracking is the mesh is "Flexing" or moving with each print stroke and causing the emulsion to actually delaminate/crack since the emulsion does not have the ability to flex with the movement of the mesh.  High pressure and hard squeegee's exacerbate this effect.

Now, is the above what happened to the SP1400?  I have no idea.  But the description is almost identical to what I and others have experienced on multiple occasions.

[Colin]

And now I see some pictures!

That is identical to what happens here.  Its related to the flexibility of the emulsion.

[Binkspot]

We run the PHU and have had the "cracking" in the past. At least for us when it gets cold and dry (winter)  I cut my exposure time down about 10 ltu across the board, it seams to help and I have not had any screens break down.

[ZooCity]

Alan Buffington and I troubleshot this issue awhile back and yep, the verdict was pretty much too dry.  There is a Murakami dual cure, I want to say SP7500?, that he suggested as it's more flexible.

[IntegrityShirts]

I have this problem often in winter. I expose screens one day and print later in the week. Would misting the screens with water to "rehydrate " them before printing work?

[jvanick]

We run the PHU and have had the "cracking" in the past. At least for us when it gets cold and dry (winter)  I cut my exposure time down about 10 ltu across the board, it seams to help and I have not had any screens break down.

so you're saying to try deliberately underexposing the screen?

hmm... and here I originally thought that it was because we were under-exposing... interestingly enough, it doesn't seem to be happening at least as much on our 200 and 230 mesh screens.

I'll give that a shot for tomorrow's print run and see what happens.

now, I wonder what 10 LTU is on a LED unit... maybe 1 or 2 seconds when I typically expose for 22 seconds?

[Binkspot]

They are not underexposed as far as I can tell. Still getting a solid 7 on the stofer strip, non of the other tell tail signs. I believe it has to do with moisture content when exposing. I started this season two weeks ago, no signs on press but when scrapping the ink out of the screen I could hear the emulsion cracking. Reduced my exposure time and it went away.

As a reference a 156 yellow coated 2/1 with PHU is 75l ltu in the summer, 65 winter on our 3140.

[Colin]

Hmm.  Using Saati PHU our 3140 has us exposing at 150 ltu for plastisol and 180 ltu for waterbase.  Yellow 150/54  sure lock panels.  Gets us a solid step 7 @ 150 ltu.

But we coat 2/2 sharp side.

[Binkspot]

I'm proberly doing it wrong. I have not had any issues with those times of screens breaking down. Biggest  run that we have done with this emulsion was 3k plastisol and a 600 straight discharge double stroked. I keep the exposure time the same for both but post expose twice the exposure time or set in the sun for a bit when doing WB or discharge.