Ink slump is the common term used to describe the fact that a printed drop, or line/track of ink starts off at a certain height/width ratio then “slumps” to give a wider line with less height. As a screen printer this can affect the colour balance of a four colour print, or worse, change the functionality of a printed electronics circuit.
Right at the start we need to kill off a common myth about ink slump. As far as screen printing goes, it has nothing to do with gravity. Our dots and lines are far too small to be affected by gravity and your prints will slump just as much upside down.
So if ink slump has nothing to do with gravity, what is the cause?
It is simply the tendency of your ink to wet the substrate. If you put a drop of water onto the substrate it might do a variety of things. At one end of the scale, if the substrate is Teflon then the drop will just sit there with no slump (A). However, if the substrate is a very clean glass, then the water spreads out, driven by surface tension, until the drop has become very thin (S). When printing a functional ink onto a typical polymer substrate the slump will be somewhere in between.
The angle the drop makes to the substrate at any time is the ‘contact angle’. You start with an ‘initial contact angle’ and end up with the ‘equilibrium contact angle’. A typical drop of water on glass might start with a 60° angle then slowly slump down to 0°.
The theory of spreading (often called Tanner theory) is so complex that it needs a computer model to work out what happens, but the basic (and approximate) rule is straightforward and can be summarised in the table below:
The effect of time is amazing. If it takes 1 second for a drop to grow to a certain diameter, it will take over 1,000 seconds to grow to twice that diameter! This is because as the drop grows, the contact angle decreases and the spreading speed decreases even faster.
For screen printing inks typically the surface tensions are low and viscosities are high. So you would think that slump should not be a significant problem. But as soon as you go to fine lines, ‘significant’ takes on a new meaning. Even with very high viscosities, if the printed line/track has an initial high contact angle, then within a few seconds you can easily spread the line by 25μm on each side. So a 50μm line becomes a 100μm line before you’ve had a chance to dry it or UV cure it.
Even with very rapid drying of this conventional Silver conductive ink, this 50µ track becomes a 65µ-75µ track, as can be seen in the image above.
At the other extreme, slumping can cause a different problem.
Some specialist applications require ink deposits which have to achieve a high ink film thickness specification. If, for example, you are trying to print a 500μm wide track with a 100μm ink film thickness using a reasonably viscous ink (~100 Poise) then within 2 seconds the line has already grown to 700μm wide and slumped to only 82μm thick.
The computer model shows that you would have to have cured that ink within 0.4 seconds to avoid it slumping below 90μm!
The Drop Spread software models the 100µ thick x 500µ wide track
So what can you do about slump?
The table gives you good indications, but there are lots of complications. Changing the surfactant levels can reduce the surface tension and therefore reduce slump, though this isn’t always the case in complex formulations. However, surfactants can interfere with the functional aspects of the ink formulation, so this isn’t always possible.
The initial contact angle is largely a function of your ink deposit. A thin ink deposit (fine mesh, low-EOM stencil) will give you less slump. As slump speed is proportional to the cube of the initial contact angle, even modest reductions in ink thickness can give large reductions in slump. In the thick ink example, reducing the starting thickness to 90μm reduces the width of the slumped line by 20μm.
The equilibrium contact angle is often ignored, but it can be a vital part of your solution. The ink will stop slumping when the contact angle reaches the equilibrium value. So if you tuned your substrate surface energy so the theoretical equilibrium angle was equivalent to the initial angle you wouldn’t get any slump at all. This trick has been used in the world of fine-line inkjet printing where they have very low viscosities and therefore very large slumps.
As we are aware “viscosity” is not a simple concept, so it’s important to know which aspect of viscosity is important for slump. A perfect ink has a low viscosity during the shearing action of the mesh coming out of the ink, followed by a rapid recovery to a high viscosity to avoid slump. In the ceramic conductor industry they can often come close to this ideal as their formulations don’t contain polymers. Polymeric inks tend to have less of a reduction in viscosity with shear and are slower to recover, hence the battle with slump is much more difficult.
Ceramic conductive inks are printed onto ceramic substrates. These substrates are often microporous and they rapidly suck the solvent away at the leading edge of the slumping ink. This sends the viscosity skyrocketing and the slump comes to a halt. Crude porous substrates (such as paper) are obviously not a good idea as they destroy edge quality. However, microporous materials (holes in the μm range) do not have a big effect on edge quality. There are some debates about whether micro-roughness can slow slumping.
It’s clear that if you have a solvent that flash evaporates, your slump will also be reduced. The downside is that the ink will dry in to the mesh. Finally, if you can cool the substrate relative to the ink on the mesh (either by having a heated ink/mesh or a cooled substrate) then the ink viscosity increases and the slump decreases. An extreme example of this, is the printing of thermoplastic inks through a heated steel mesh which solidify almost as soon as they are printed.
Slumping on the beach
Have you ever noticed a ‘beach’ effect around your printed line or dot? It’s an ultra-thin bit of something that lots of us have never been able to analyse or explain. It turns out that the science of slump offers some insight. Spreading of a liquid is impossible without a ‘precursor film’. This was at first thought of as a mathematical device to do the calculations, but these films, perhaps only 0.1 micron thick can be seen under the right conditions. There are hints that the polymers in the ink can have difficulty entering the precursor film; if they can’t get in then the ink can’t spread. This correlation between beach and precursor film is only speculative, but it might be possible for an ingenious ink designer to take advantage of this effect and produce a low-beach, low-slump ink.
In a perfect world a cross-section through a printed track or dot would have vertical sides and a flat top with well-defined square ‘shoulders’ and if viewed straight on, the track or dot would faithfully reproduce the artwork.
In reality though, screen printed images do have a tendency to show up the influences from the mesh leading to a ‘saw-toothed’ effect in the print. A common ‘fix’ in the industry is to increase the stencil thickness to create a flatter stencil surface. Although this can be effective, it causes a secondary problem with the ink deposit at the image edges, where the printed track can actually have an ‘M’ shaped cross section.
Going back to the effects on slump, we have seen that modest increases in ink deposit exacerbates the slump issue, so increasing the stencil thickness to improve the 2D image quality can worsen absolute image reproduction.
The answer to this problem is to adopt a thin and flat (low EOM/low Rz) stencil system which will avoid the ‘M’ shaped deposit, yet at the same time give excellent image quality.
‘Ink slump’ might seem to be a bit discouraging as it has an impact on every screen print, however it is predictable and quantifiable. The secret is therefore to control the factors that have the biggest effect in order to minimise them and to compensate for the slump in the original artwork. This, of course, has been done for decades in correcting four colour process.
Whilst the latest generation of performance conductive inks are being formulated to minimise the effect as much as possible this is effectively outside the control of the printer. Conversely, the printer DOES have control over probably the most important parameters, which are ink deposit and image quality. Specifying a thin, flat stencil is the key to minimise and control the effects of ink slump.
This article was originally authored by Professor Steven Abbott who was R&T Director at MacDermid Autotype from 1992 to 2009. For more information on the science behind dot growth go to www.stevenabbott.co.uk/practical-coatings/drop-spread.php.
Although there are many papers on the science of slump, the work of Professor Glen McHale at Nottingham Trent University is especially insightful. His papers on spreading of drops and cylinders are the basis for the computational results shown here and his help is gratefully acknowledged.