Thursday, August 18, 2005
作者: Mark Coudray
Off Contact Influence on Print Quality
by Mark Coudray
Whenever the topic of print quality on press arises, the conversation will usually drift to the topic of off contact. This is one of the most
critical parts of the print equation. On the surface it appears to be quite understandable and not very complex. Like most parts of the screen-print
process, closer investigation reveals surprising complexity. In this month's column I would like to explore some of the obvious, and not so obvious
aspects of this part of the process, and how we can use this knowledge to improve the overall print quality of our images.
The purpose of off contact ultimately is to help maximize the sharpness of our print while controlling build-up on the back of the printing
screens. This is achieved by increasing the separation distance of the screen above the print surface. This distance typically varies
from 1/32" to about 1/4". What is the right off contact distance and how will we determine it?
Besides increasing print sharpness, off contact helps to minimize the about of ink "touch-off" or "build-up" on the backside of screens contacting
a printed part of the design. Wet-on-wet printing results in some transfer to subsequent screens. If the ink build-up is too great, creeping ink
can partially or totally block other parts of the printed image. This is especially true with halftone printing. Print quality in general suffers
when this happens. Proper off contact minimizes the contact time of the screen to the print surface. It also minimizes the surface area that comes
in contact with the print. The combination of these two factors helps to minimize any ink transfer.
In virtually every shop I visit, the universal solution to a poor print is to increase off contact and increase squeegee pressure. For simple
line work and solid fills this may be sufficient. In the long haul, it leaves a great deal to be desired, and is an inefficient approach to the
factors at hand. Before we can determine what the right setting will be, we must look closer.
Off contact works in conjunction with squeegee and ink conditions. Different durometers, edge conditions, and pressures will have varying effects
on the deposit. Likewise, the fluid characteristics of the ink will also have an impact on the application of off contact. We simply cannot consider
only raising the screen above the print surface as the solution. The other influencing factors must be accounted for as well.
Too much of a good thing
Too much off contact is as bad as too little. In the first place, increasing the off contact increases the fatigue cycle. This is the total
distance that the screen mesh must be stretched and relaxed during the print cycle. It is akin to bending a paperclip over and over. Sooner or
later, it will break. If off contact is one sixteenth of an inch, the fatigue cycle is one eighth of an inch. Increasing the OC to 1/8" will double
the fatigue cycle to one fourth of an inch. As the mesh continually cycles, the tension continuously drops while the mesh becomes increasingly
brittle. With a constant tension load on the threads, spontaneous failure is common. To minimize this, we must keep the OC as low as possible to do
the job for which it was intended.
The higher the off contact, the more squeegee pressure must be added to stretch the mesh to the print surface. Now the squeegee is being used as a
mesh stretcher in addition to shearing the ink. Often this causes the blade to bend. As soon as this happens, the squeegee begins to hydroplane over
the ink surface. You no longer shear the ink; you mash it through the mesh openings. Obviously, this is not good for print quality.
Besides the hydroplaning that takes place, the increased squeegee pressure also increases friction on the squeegee edge. This leads to rapid edge
degeneration and rounding, which further contributes to the mashing process. A dull and rounded edge has virtually no shearing capability. At best
it will slip and slide over the screen surface, lubricated by a thin layer of liquid plastic, our ink.
As the off contact increases, the mesh tension at the printing surface also increases, quite dramatically. This is one of the under lying principles
as to why OC yields a better print. Mesh tension increases at the print surface, delivering better ink release from the threads of the mesh.
Using off contact as a mesh stretching function is very poor. The problem lies in the distribution of tension across the screen. The closer the squeegee
is to the edge of the frame, the less elastic capacity there is for the mesh to move. This simply means that we can't stretch the mesh as much at the edge
as in the middle.
You can demonstrate this for yourself by holding a rubber band with 1/4" between your fingers. How far can you stretch it? Notice how much force
you must apply to stretch it as well. Now repeat the process with 1" between your fingers. Notice how much further and how much less force is needed
to stretch the rubber band. For us, this illustrates very uneven tension across the print surface. The closer to the edge of the frame, the less elastic
capacity exists, and the tension rises dramatically. This means that our ink will transfer at different rates, giving us an uneven print overall. In
process color reproduction, this principle can be disastrous to the color consistency.
The higher the mesh tension at the edges of the frame, the greater the danger of over stretching and damaging the fiber. This excessive tensioning
also creates very dangerous drag on the ends of the squeegee blade. If the blade is 75 durometer or higher, or you are using a multi-durometer blade,
the ends will resist the tendency to bend, and a hot spot will form. At the very least you will destroy the ends of your squeegee. Premature stencil
failure is common, and ripping of the mesh will be the ultimate failure associated with this condition.
High off contact also results in the pre-release of the ink through the mesh. The softer the ink, the less resistance there is. Ink is pushed through
the mesh opening before it comes into contact with the print surface. If you were to look closely at the backside of the screen, you would see a blob
of ink waiting to touch the surface. The size of this blob depends on many different factors, but in all cases, it will be larger than the stencil
edges can gasket. When the screen finally hits the print surface, the ink smooshes out all over.
You see this as one sharp print, followed quickly with decreasing print quality until you wipe the backside of the screen. The print returns to good
quality for one or two images, and then degrades. This will continue until you decrease the off contact. If you are printing halftone images, this will
totally destroy your print. The dot gain factor will be so high as to negate any tone carrying capability of the halftone. While too much flood bar
pressure can definitely contribute here, the high off contact is the primary culprit.
Machine set-up also plays a role in the off contact question. If your pallets are not parallel, and in the same plane, the off contact distance
will be different for each head. The same holds true for warped or distorted pallets. Any platen variation will result in variation in off contact
distance. Print quality will drift until equilibrium is obtained. In all cases, this stabilizing of the image will be less than optimum.
The Optimum scenario
The optimum press set-up would start with mesh tension as high as possible. The higher the mesh tension, the less work the squeegee has to do. This
means overall lower off contact, lower friction, and lower image distortion. Squeegee edges will last longer, which means faster printing and
My personal preference for off contact is for the screen to be no more than 1/16" above the substrate. There are two very easy ways to check this. You
can use the Nickel Method or the Quarter Method. Both are very low tech, but effective. The thickness of a US Nickel is .075" and a US Quarter .090". Use
the nickel for t-shirt set-up and the quarter for fleece set-up.
To use this method, place four coins on the platen, one in each of the four corners of the print area. Adjust off contact until the screen just touches
the coins when it is lowered to the print position. Check each of the press heads, and each of the printing platens. Make sure they are consistent.
With platen height and parallel established, the next step is the squeegee and print settings. I prefer a triple durometer 70/90/70 or 75/90/75
blade. The middle 90-durometer section keeps the blade from bending. This concentrates the shearing energy at the edge of the blade, in contact with
the screen surface. It is very much like concentrating the sun's energy through a magnifying glass. Relatively low energy quickly focuses until it can
start a fire. In our case, we only want enough concentration to shear or cut the ink film. If your blade bends, you are using too much pressure.
This highly concentrating shearing action delivers a quick print, without wasting energy or pushing the ink through the mesh ahead of the print. The over
image quality is excellent. You will maintain fine reverse printing, thin lines, and sharp halftones. Too much pressure and all of these desirable print
qualities will vanish within a print or two.
Finally, you want to keep the ink as stiff as possible. Higher viscosities resist flow. This means that the ink will print sharp and clean. As
soon as the ink is reduced, or softened, it will have a tendency to flow on its own. The result is poor print quality. For halftone printing, this is
essential. In fact, there are special halftone additives you can incorporate into your ink to increase the sharpness of the final print. The key here
is to make the ink work for you, not against you.
Off contact is a critical and necessary part of the printing set-up. You cannot do good work without some off contact. From our discussion here
today, it should be apparent that simply raising the screen above the print surface is not enough. Other factors come into play. All of them must be
accounted for, in a balanced manner, for you to successfully utilize a minimum off contact set-up.
Mark is founder and President of Coudray Serigraphics, a textile screen printing company and Coudray Graphic Technologies, a digital imaging and prepress supplier to the industry. He was inducted into the Academy of Screen Printing Technology in 1989 and has served as Chairman of the ASPT twice. In addition, Mark is the recipient of 18 SGIA Golden Image Awards, 2 Swormstedt Awards, Magnus Award, and the Parmele Award (2001). He is past SGIA Chairman (2000). He has served a total of 24 years as an SGIA Director. Mark is a noted industry author with Screen Printing, Impressions, Print Wear, Images, and other trade publications with over 275 articles, columns, and technical papers as well as an industry presenter at tradeshows, conventions, and technical symposiums. He is best known for his extensive work in controlling color halftone printing, quality, and production management of the screen printing process.