[theory] What makes white ink so hard to print? Part 3

[blue moon]

so I had a chance to run about 10 tests on the white inks today and have gotten some pretty remarkable results. The testing was not conducted under controlled conditions, so all of it will have to be redone to get more objective measurements, but results are very solid and I don't expect them to change much. We will have to control the ink temperature better, give everything a little longer to settle down before taking the readings and run few more scenarios with the calibration fluid.

I was told by many ppl in the know about the change in viscosity as it relates to speed of printing, but the results were not what I expected.

So what does really happen with the ink's viscosity when it is stroked?
How does that impact our ability to print with it?

pierre

[Screened Gear]

Pierre,

I think what you are testing is going to be hard. To set each ink into your test in a controlled/neutral starting point you will need to mix them equally. Each ink will come to your shop under different circumstances. Some maybe brand new inks. Some inks could have sat for months. You can control the ink temp but you will have to make sure the ink is that temp all the way though. The starting points you pick will ultimately determine the winner. Lets say you pick 80 degrees for the starting temp. Then you mix each ink for 20 mins on some fancy mixing machine. Now you have a ink that is ready for testing. I would guess with a start like that most inks will perform very well. But what if you picked 100 degrees and mixed each ink for an hour. Now would you get a new winner? I am in Arizona and my inks in the summer are in the 100 to 105 degree range (maybe higher). What I am getting at is the conditions you are testing the inks are effecting your results. It is much like each shop preferring one white over another for some reason. The reasons are how they prep their inks. Some people add reducer, some mix them others don't do anything. Each one of these will result in a ink that performs differently.

Now for your questions

So what does really happen with the ink's viscosity when it is stroked?
It breaks down. Printing an ink adds force to the ink. This moving pressure stretches the ink and adds heat. Ink is much like taffy. Candy shops start with a hard brick of taffy and then work it until it can almost run off the table. If left alone long enough it will turn as hard as a brick again. Ink is the same way. Breaking those bonds makes the ink flow very easily.

How does that impact our ability to print with it?
Inks that flow well can be printed on higher meshes. They can also be printed faster with less pressure. This results in inks that can print on top of the shirt and clear the screen in one stroke. So bottom like less ink is printed. You get better coverage and you use less effort in the printing process. Down side is you have to prep your inks or know after a certain number of shirts you can adjust your printing style.

[abchung]

So what does really happen with the ink's viscosity when it is stroked?

Normal plastisol ink is Thixotropy, which will get thinner under shearing stress.
However, white ink is a dilatant fluid due to the Titanium Dioxide that makes the ink white. Dilatant fluid is shear thickening, which will become thicker when more pressure is applied.

How does that impact our ability to print with it?

I THINK we should print slower with less pressure for white inks. While for non-white plastisol we should use a fast stroke.

[Binkspot]

More of questions then comment.

I understand how heat will affect the viscosity of the ink but does the constant shearing of it reduce the viscosity. In other words if a new pail of ink has a viscosity of 1000 cSt (I don't know the actual just pulling numbers out of my a$$) then you run a job and due to the constant shearing is the viscosity reduced to let say 990 cSt. Then the next job more viscosity is reduced by shearing. If you load a screen with ink, run a job then scrape the ink out and put back in the bucket and mixing it is this reducing the viscosity of the whole bucket? Over time a 5 gallon bucket would go from 1000cSt to maybe 800 cSt, by the time you got to the bottom if would be essential a completely different ink. I know in high pressure hydraulic systems you should avoid using needle valves on the high side due to the nature of the needle actually cutting the oil reducing its viscosity and lubercating properties. Is the same happening shearing the ink across the screen?

If the ideal temp for a pallet is 140 and the ink 100 with the transfer of heat between the pallet and screen/ink it is almost impossible to maintane the optimal ink temp? Wouldn't it be better to make the inks to have ideal printing traits  at a higher temp like 140? Or better yet a multi viscosity ink that will be optimal at a range of temp and change accordingly.

How does the "slickness" of the emulsion effect the viscosity and shearing properties of the inks. If you have an emulsion that creates more drag on the squeegee then another will that affect the viscosity?

Sorry to ramble on and maybe none of it matters in the end, have lots more to over think on this topic.


 

[tonypep]

It appears we are once again stumbling upon the interdependent variables aspect of the screen printing process which closely resembles chaos theory. Opponents of chaos theory often argue that it is merely a result of extremely minute variables that cannot be accurately measured IE "We cannot study what we cannot measure.". True enough. True scientific analysis insists that we change only one variable at a time in order to accurately derive results. That said; with the variables bordering on the infinite, one could spend the better part of a lifetime examining this issue.
To that end, while I respect this endeavor, I feel it may lead to some false assumptions.
Waterbase is so much easier when applicable but you know I had to get that in!

[blue moon]

More of questions then comment.

I understand how heat will affect the viscosity of the ink but does the constant shearing of it reduce the viscosity. In other words if a new pail of ink has a viscosity of 1000 cSt (I don't know the actual just pulling numbers out of my a$$) then you run a job and due to the constant shearing is the viscosity reduced to let say 990 cSt. Then the next job more viscosity is reduced by shearing. If you load a screen with ink, run a job then scrape the ink out and put back in the bucket and mixing it is this reducing the viscosity of the whole bucket? Over time a 5 gallon bucket would go from 1000cSt to maybe 800 cSt, by the time you got to the bottom if would be essential a completely different ink. I know in high pressure hydraulic systems you should avoid using needle valves on the high side due to the nature of the needle actually cutting the oil reducing its viscosity and lubercating properties. Is the same happening shearing the ink across the screen?

If the ideal temp for a pallet is 140 and the ink 100 with the transfer of heat between the pallet and screen/ink it is almost impossible to maintane the optimal ink temp? Wouldn't it be better to make the inks to have ideal printing traits  at a higher temp like 140? Or better yet a multi viscosity ink that will be optimal at a range of temp and change accordingly.

How does the "slickness" of the emulsion effect the viscosity and shearing properties of the inks. If you have an emulsion that creates more drag on the squeegee then another will that affect the viscosity?

Sorry to ramble on and maybe none of it matters in the end, have lots more to over think on this topic.

all good questions!
here's what I can answer. Unit of measure for viscosity is centiPoise (cP). According to manufacturers, inks start at 100k+ cP and with shearing drops below that. White ink ALSO is thixotropic so it drops in viscosity as it is being sheared. Additional mixing does not change viscosity. It looks like once is stirred to certain level it stabilizes. Temperatures definitely change the viscosity and small change (10°) can make a 25% difference (on the readings we are getting)!

I don't know if the shearing (or printing) has any long term effect on the ink, but that would be easy to measure. It goes on my to do list.

If the ink was designed to print at 140°, you would have to flash all the time! We could have two different inks though, one for cold printing and one for hot.

so, how fast should we really print? Is there such a thing as too fast? What's causing the limits?

pierre

[IntegrityShirts]

We could have two different inks though, one for cold printing and one for hot.

This is what I am leaning toward now in my dumbed down view of all this. A summer ink and winter ink when shop temps vary wildly. When it's 90+ degrees in my shop all the white ink flows, but in winter when it's barely 60 in the shop and 70 in the screen room where the white ink lives, then it changes everything about what ink to use.

[ScreenFoo]

I think this is an interesting question: How fast should you print?   
I think it's the same answer to this question:  How fast CAN you print, and if it's slow, why?

If there's anything that screwed me up (and still does occasionally,) if you print white slowly, you put down more ink than printing a medium speed, but if you print quickly (with the proper variables under control) you can put down even more, as well as matting down the fibers better.


I'm really interested to hear what you come up with on the viscosity on new ink vs. some that's been printed on.  I always think it's getting thicker, but is it the printing, or the heat, or the age?  How many other things could make that happen?   

[alan802]

I've got some samples here that I'll send to Pierre and they are different speeds but same ink and screen and blade.  All I changed was the speed and slightly adjusted the pressure so that the ink would clear the screen with one stroke. 

These prints were printed through a 180/48 with a Dr J squeegee blade with the sharp edge.  Tension 24 newtons.  Speed and pressure was changed as stated earlier.  I'm not sure the pics will do the difference justice but we can send these samples from shop to shop if anyone else would like to see them in person.
Top print is 30"/sec, bottom is 4"/sec.



Opacity is better but the biggest gain I see when we print fast is the print finish.  It's so much smoother and has better fiber matte down making the top colors much better.  Maybe it's one of the reasons why we didn't see much/any real benefits with the Action roller squeegee when I tried it out a while back.

I wish I knew the exact answers to Pierre's questions and I'm learning as much as I can as fast as I can.  But at this point, we print as fast as we can, especially with white ink and we haven't had any issues with the ink breaking down or gaining too much viscosity on longer runs.  We run a very slightly modified Rutland Tidy white.  We mix in just enough cheap, longer bodied white ink to the Tidy to help it not be such a squeegee climber and I think it's as close to the best white ink on the market that we've tried.

[Screened Gear]

Alan,

Good example of what speed does. Correct me if I am wrong, a faster print will change the amount of downward force over a slower one. The angle will also change if your pressure is enough to bend the blade. I have seen good results over the last few years speeding up my prints.

[alan802]

Alan,

Good example of what speed does. Correct me if I am wrong, a faster print will change the amount of downward force over a slower one. The angle will also change if your pressure is enough to bend the blade. I have seen good results over the last few years speeding up my prints.

Joe has tried his best to explain it to me and there is some great info in the textile printer's bible explaining those dynamics but knowing how those things work enough to explain it here is not something I think I could do.  I know the goal is to have zero force being applied to the pallet and we all know that applying less pressure from the squeegee will help to reach that goal and then speed will affect it as well but to what degree I'm not too sure.  Blade selection, off contact, ink well, and a bunch of other things are involved as well, or perhaps not just involved but may be more important to the force on the pallet, that stuff is over my head right now.

[Screened Gear]

Alan,

Good example of what speed does. Correct me if I am wrong, a faster print will change the amount of downward force over a slower one. The angle will also change if your pressure is enough to bend the blade. I have seen good results over the last few years speeding up my prints.

Joe has tried his best to explain it to me and there is some great info in the textile printer's bible explaining those dynamics but knowing how those things work enough to explain it here is not something I think I could do.  I know the goal is to have zero force being applied to the pallet and we all know that applying less pressure from the squeegee will help to reach that goal and then speed will affect it as well but to what degree I'm not too sure.  Blade selection, off contact, ink well, and a bunch of other things are involved as well, or perhaps not just involved but may be more important to the force on the pallet, that stuff is over my head right now.

I like that you mentioned ink well. I think that is a over looked item. I honestly I still print with too much pressure but my prints are nice and thin and soft (for plastisol). My goal in printing is to make a comfortable product for the client. Some times a little force is needed to set that ink down.

[ScreenFoo]

Some times a little farce is needed to set that ink down.

Even though I suspect this was an honest spelling mistake, it gets my vote for best quote on the topic so far.  I agree 100%.