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Screen Printing Print Faults Identification by Tricia McAulay
Screen Printing Print Faults Identification by Tricia McAulay
This article is the first in a series taken from the MacDermid Autotype e-book ‘How to be a great Screen printer. In this, Tricia McAulay talks from her own experience about identifying many typical screen printing print faults, their root causes and solutions.
Tricia McAulay is a senior print technician at MacDermid Autotype, responsible for evaluating the print capabilities of speciality hardcoated materials manufactured for Film Insert Moulding and Membrane Touch Switch applications. Tricia has over 30 years of hands-on screen printing experience.
‘Like every screen printer, I sometimes see faults in my prints. To fix them I need to find the root cause. My favourite resource for finding the root cause is a set of ‘reference prints’ I made some years ago, under the wise tuition of Bill Appleton. These prints are very special because, I deliberately created a poor set up to induce the common type of faults you achieve when things go wrong.
I can’t give you a copy of my prints, but I can give you the next best thing which are pictures of many of those print faults, along with the explanation of what I deliberately did wrong to induce them. In no way is this a comprehensive guide to all screen printing faults, but hopefully you will find this a useful and practical guide for some of your own problem solving’ techniques.
The test prints
We used a variety of images for the test prints and in particular an image from the Serilor Log test suite. This was chosen as it’s very challenging to print.
Fig 1: Test print showing faults marked up
In addition to the faults marked up in the test print, we also included the following faults for reference; tape marks on the positive, a thumb print in the drying filler, filler breakdown, coating lines in the emulsion from using a nicked trough, flooding, filling in, etc. which all help to build up the reference library of faults.
The consequences of choosing the wrong stencil
To demonstrate what can happen when you choose the wrong stencil for high quality printing, we deliberately combined a high Rz (10µ) stencil with a high squeegee pressure.
You can easily see the massive dot gain that was induced by flooding as shown below.
Fig 2: Dot gain
When this print is compared with one taken from a low Rz stencil film, using the same press settings as above, the flooding disappears and the dot reproduction is far more accurate to the film positive.
Fig 3: Good dot reproduction
Wrong mesh tension
Slur occurs from a slack mesh, or too large a squeegee drag.
Figs 4: Print showing ink slur on the left side of the image and Fig 5: Print showing ink slur and squeegee judder
The two examples in the print above show different types of print slur that can occur, however the root cause is the same.
The squeegee direction is from right to left and because the mesh is too slack and/or the squeegee pressure is too large, the squeegee ‘drags’ the mesh along creating the slur directly connected to the main image. In extreme cases, as shown in the left image, the squeegee actually judders up and down, taking the stencil with it and also prints a ‘light ghost’ of the image.
Too fine a mesh
Here you can see the effects of using too fine a mesh for the ink used. The ink particles are larger than the mesh openings which leads to classic sieving where particles actually start to block the mesh holes.
Note that the particles aren’t extremely large, but if a hole is less than 3 x the width of the particles, then the mesh can block very quickly. Caution, you can also get a very similar effect to this if the ink dries in.
Fig 6: Print showing mesh blocked by ink particles
To show the effects of ink drying-in, we used an ink with no added retarder and deliberately paused after the flood stroke before printing. Naturally the ink had dried-in, blocking some of the mesh openings, giving the classic drying-in pattern. Drying–in problems are often more prevalent in the image at the start of the print stroke, this is because the ink that has remained in the mesh after the print, has had longer to dry-in before it was re-flooded.
Fig 7: Incomplete print due to ink drying-in problems
Wrong ink – Too much viscous drag (with a large off contact)
Viscous drag can be a problem when printing large block areas or negative images. To demonstrate this we deactivated the automatic lift-off and used a very viscous (non-reduced) ink. During the start and middle of the print stroke, the drag on the mesh from the viscous ink was so large that it caused the mesh to release slowly from the print.
Major problems occur if the mesh sticks and then releases too violently as the ink can literally fly off in all directions. In the image below you can see how these ink particles actually landed several millimetres away from the edge of a solid printed block.
This problem would be exacerbated by using a viscoelastic (“stringy/tacky”) ink but this would not give individual drops like these, but produce “cobweb” strings (an effect that is far worse on static prone substrates).
Fig 8: : Ink spatter
Although these were very easy to see on the print, it was impossible to get a good photo of them.
Fig 9: Schematic of squeegee showing how ink teardrops are formed
You’ve probably seen them yourself, random blobs of ink, a few millimetres in diameter, which are often seen in a line in the print direction.
In our test, we induced them by using a tacky/sticky ink and a worn squeegee blade. This combination meant that during the squeegee stroke, ink built up on the wrong side of the squeegee – partly through hydroplaning and partly because viscoelastic inks naturally “climb” under shear. After a while, there was enough ink build up on the squeegee that it dropped off and fell through the mesh onto the print – giving the characteristic teardrop shape.
The screen used for this test was deliberately cleaned poorly, to leave a very noticeable ghost image on the mesh.
Fig 10: Image blocked by ghost image on mesh
Under the microscope, the ghost image in the mesh can be seen as patches of unprinted ink and when we printed that screen there was a corresponding ghost image clearly visible in the print. Interestingly, in this test we also saw that mesh-marking was especially prevalent almost as if the ghost image magnified this effect. Ghost image interference effects are most prominent when printing transparent or pastel colours.
Fig 11: Dust contamination causing ‘hickies’ in print
To demonstrate this problem, we contaminated a screen during printing with some dust particles and, not surprisingly, created what is often called “hickies”. I doubt if this effect would be appreciated by the customer! For practical advice and tips on how to avoid contamination, read the ‘How to Guide - Cleanroom working’ on www.macdermidautotype.com
Hard to photograph but easy to spot with the naked eye. There are many causes of orange peel but basically these come down to the fact that the ink is generally unstable during drying. We induced an orange peel effect by printing two incompatible inks one on top of the other
Using the wrong, or too much, solvent thinner or retarder is another way. The cause of this type of orange peel is interesting and is sometimes called the Marangoni effect. As the more-volatile solvent evaporates it leaves behind a solvent mix with a different surface tension. Ink flows from high to low surface tension so you start to get instabilities. These instabilities work in 3 dimensions and you start to get regular cells where solvent is rising in one point and sinking in another. Under the right (or wrong!) conditions these cells can form perfect hexagons, the classic mark of the extreme Marangoni effect.
The cure is either gentler drying, more compatible solvents (less difference in volatility = less difference in surface tension) or a more effective surface active agent which swamps the differences in surface energy.
Again this is a problem that’s easily visible on the print, but hard to show in a photograph. You see a broad regular pattern on the print that coincides with the pattern of the belt that conveys your prints through the oven. This has occurred typically because either the oven temperature is too hot and causes the substrate to distort or through differential drying of the ink film on the belt. So it is important to check your oven temperature before each job.
I am sure that every printer has seen these at some point!
For our experiment, we used extreme climatic changes to induce our misregistration test, by deliberately drying out (<5% RH) a print on paper before printing the second image on top of it. The dried out print actually shrank by 2mm over a 400mm print (0.5%). The test was repeated, but instead of drying out, we humidified the print (>90%RH) and it actually expanded by 1mm (0.25%).
Such gross misregistration is easy to spot, however doing this test was a reminder that for precision printing, exact equilibration of the substrate between prints is of great importance. For paper, the effect of water is well-known. But the effects on plastic substrates are less well documented. The complete version of ‘How to be a great Screen printer’ e-book contains a section about precision printing requirements for plastic substrates. A downloadable version is available from www.macdermidautotype.com
Screen printing is a complex process and this list of print faults is in no way exhaustive; however, the creation of this ‘fault reference’ library has helped us with our internal problem solving and also it makes a great training aid for new employees.
Why not collect examples of print faults when they arise in your production? Documenting a brief description of the root cause and the actions that you took to solve them could form the basis of your own reference library to help you troubleshoot in the future.
If you found this article useful why not read more in ‘How to be a great Screen printer’ e-book available to download from here
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