screen printing > Separations

Choosing LPI For Output

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bimmridder:
Thanks. I have HD Clear on my shelf. Guess I better see if I can squeeze someone for the additive.

drdot:

--- Quote from: tonypep on May 12, 2011, 10:44:44 AM ---Mark...... I'm pleased you mentioned blade sharpness. Quite often one of the most overlooked interdependant variables. I know of many flatbed printers that dispose of the squeegee material when finished with the run and simply replace it just for the secure knowledge that it is a stable variable. Correct me if I'm wrong; I know of no tool to measure this.

--- End quote ---

Tony, you bring up an interesting point. I have never trusted brand new squeegee material. The reason is not the uniformity of the edge, but rather, the ability to clamp the new blade so it is absolutely flat.  I used a Svecia squeegee sharpener for years. I rigged up a dial indicator that allowed me to parallel the guide rails to .001". This was mostly for very high res graphics printing (133 lpi and 150 lpi). The blade had to be perfectly sharp and perfectly smooth.

To determine if your blade is sharp enough, drag your finger tips lightly across the edge of the blade. If you can feel the individual ridges of your fingerprint, the blade is good to go. If you can't, the blade is too dull.

On another note, we sharpened our blades at the very least weekly. This meant that we were only taking off .001" -.004" of material. It was very fast and we could do it in 1 or 2 passes. Kind of like a butcher using a steel to bring the edge of the knife back. If you wait too long, you have to take too much material off to return the edge to printing condition. It also takes too much time and this is the main reason printers do not want to sharpen their blades.

blue moon:

--- Quote from: squeegee on May 11, 2011, 10:42:25 PM ---Pierre (bluemoon) mentioned this somewhere else and I've been intrigued by the idea ever since but haven't tried it yet.  If it is the case that 330/30 might allow a printable dot a couple percentage points lower (or higher) how much differnce could it make visably on a print?

I'd love to hear Pierre's take on this as well.

TIA

--- End quote ---

well, here is the deal . . .kick back, it will be long!

Before trying the S mesh, I was printing using static frames. They are in pretty decent shape for statics, and I buy more on regular basis. The ones I have go through some pretty harsh cleaning and coating, so they tend to delaminate over a period of time. They surprisingly do not lose tension very much as we print with very low off contact (about a 64th of an inch) and rather low pressure (if we can). So decent statics vs. the EZ frames with "S" mesh. Statics at 15N-18N and EZ's in mid 20's. This for 305 statics and 330/30 smart mesh.

After a couple of visits by Dan, 5% was solid on statics. Some days are better than the others, but I can count on 5% without much trouble. 4% will open, but not all the way. It seemes that I am hitting some kind of a wall at 4% and anything lower just does not open. This is at 55lpi.

For the competition print, I re-leveld the press, used new emulsion, new platten tape, new squeegees and a new bulb in the exposure unit. Also all the screens were shielded from the light before exposing (I walk through the production area with unexposed screens on my way to the exposure unit. Normally I just walk through quickly and let the light hit the screens. In this case, screens were double bagged on the way through the lit area). We also did a comparison between two RIPs and concluded that the Ghostscript halftones were much darker and cleaner shape. We  used 58 lpi for this print. 65 was having some issues and 60 divided evenly into 330 so we had to chose between 58 and 62. 58 seemed like a better bet.
So there were many changes between the regular prints and the high end stuff. But here is the kicker!!! The 3% dot was completely open with no visible interference from the mesh. 2% was open better than 90% and 1% was open better than 50%!!! Unlike the regular screens, there was no sudden drop off point and all the dots were opening. The obvious issue with the 1 & 2 percent was the mesh interference. We checked with a microscope and could tell that some of the dots were blocked. In most cases the blockage was not complete, but rather partial, but since it repeated at predictable interval, it generated a moire.

Another interesting (read: worthless trivia) part was that there were various levels of 1% in the art. Anything below 1 was not visible on the screen, but there were at least three different shades that were less than 2%. Setting the color wand on 0 tolerance, I could go and pick different shades within 1%.

We actually spent two days discussing how to get rid of the 1 and 2 percent halftones as they were clearly printing on the shirts and were showing a really bad jagged patterns. We could not and still do not know how to eliminate everything under 3% cleanly and had to resort to masking tape to block those off and create smooth edges. Dan thought I was crazy for doing it!

And now to throw another curveball, knowing what I know now, the 3 percent dot that we are holding is actually a lot bigger when printed. Even though the dots on the shirt are so fine (they are barely visible), it is nowhere near 3%. Yes, the photoshop calls it so, but if I had to guess, they are probably around 10%.

As far as the difference on the shirt, I don't know if it is visible in this particular example. I think holding a 5% with a good screen would have yielded and impressive print, but I also feel that the 2% difference is probably what made it an award winner rather then just an outstanding print. I don't think, actually I am pretty certain that an untrained eye would not know the difference.

That 2% does come into play with 4CP though. There the art is heavily dependent on the mixing of inks and many colors require a very small quantities of other inks to achieve a certain color. The example I often cite is a print for one of my customers that has a gold medal in it. The gold color is 100 yellow and about 3-4% magenta. Before getting the new screens, I had to bump the percentage up to 5-6% so it would open and this in turn would make the medal rose gold rather than yellow gold. By being able to hold the 3% I can actually make it be the correct color. We don't print as much 4CP any more, but if we do now, it always goes on the new screens and it prints beautifully with very soft transitions and pretty good color.

pierre

Dottonedan:
You had to tell everyone about the masking tape.      Ha!  I forgot about that.  Probably intentionally.

DouglasGrigar:

--- Quote from: drdot on May 06, 2011, 11:22:32 AM ---This is a very, very common question and there's usually some "machoism" associated with it. You know, I can print 85 lpi all day, and so on. Here are some things to consider. Just because you can print a finer dot, doesn't mean you should.

--- End quote ---


Not only do I agree, we have additional points to consider.

Anyone who thinks printing on garments with HTD above a range of 40-60 (at least doing it well) may well be fooling themselves.

Here is a photo of a popular brand (6.1 100% cotton) in black with white plastisol ink of 42 LPI 62 angle (microscope with 50 power magnification)



None of the dots pictured here are even close to 2% and that is not the issue but rather the capability of the substrate to hold the dots in position.

This has not been washed, it was directly out of the dryer.

Note the left circled dot - broken (again this is not a washed garment).

Now check out the right circle - only a very fine thread shows any ink... the dot is there, it exists on the screen and ink was pumped out... Where did it go?

It is actually stuck to the face of the screen stencil.

When will that ink get deposited - we do not know, and when it is finally deposited a ?double hit? of ink (or more) will end up in that position. There are several examples of missing dots that fell into the ?thread caverns? and of course stayed on the face of the stencil.

Now the print is far from perfect and is not intended to, it is simply a sample of something you would see in any HT print from ?joe printer? to examine the effects of the substrate on HTDs.

Basic stuff - but points out the massive numbers of variables any printer even ?joe? has to contend with.

Change the substrate and we change the possible or potential dot.

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