It seems that one of the most common problems new screen printers run into is printing white ink. I've heard a lot of questions like: Why do all my other colors print fine and white is so tough? Why is it that my white ink is so thick? Why can't I get a smooth white ink? It's always rough and seems to have texture to it. Is there anything I can do to thin my white ink to make it easier to print? Also comments like: Our white ink is bad, it comes off after only a couple of washes. There's something wrong with my press, it prints fine when I am printing on white shirts, but when I print on dark shirts the white ink is very blurry.
Unfortunately, the answers to these quarries usually lie in the printer themselves. No, your white ink isn't defecting, your press probably works fine, and you shouldn't need to thin the ink. Yes, you probably need to change some variables and techniques during the printing process. The truth of the mater is, the answer is in the question all along.
So why does the screen printing process work fine for a while and then as soon as you start printing white ink, smash, a road block? The truth is, you may have been doing the wrong things the whole time, it just didn't show up yet . It's like swimming, sure you can dog paddle and flounder around the pool all day. However, when it comes to swimming a long distance, without the proper techniques you'll work twice as hard and get half as far.
Printing on light garments is much easier than printing white on dark garments. On a light garment, not as much ink is required to achieve good coverage. Also many times you're working with thinner inks. Ink varies in viscosity according to how much pigment it has in it. For instance, a black ink is almost always going to be printed on lighter colors, the pigments needed for the black to show up are considerably less then the pigments needed for a white ink to cover a dark garment. Thus, you're black ink is much thinner and easier to work with. You can do almost every technique wrong when printing black ink on a white shirt, and there's still a good chance that your final print will look pretty good. Since the ink is thinner, it passes through the screen mesh much easier, with less force, and clears the screen better. (For those unfamiliar with the term clearing the screen, this defines the point when all ink is cleared out of the screen mesh and properly transferred to the shirt.)
So basically when setting up a screen printing job that requires black ink on a white shirt, you can use the wrong screen mesh, have no off contact, and print with the completely wrong squeegee technique and still get an OK looking final print. However, if these same wrong techniques are tried when printing white ink, uh oh, game over! The unfortunate thing is, many printers start out this way and teach themselves completely wrong. In fact, this is how I learned. It's not the end of the world though!
Let's talk about making a screen for optimal coverage on a dark shirt. There are two main factors that come into play, those are mesh count and stencil (emulsion) thickness. For optimal coverage on dark you'll want to use a 110-156 mesh (156 mesh for more detailed designs). If you would like to print an extremely thick coat of ink (on a jersey for instance) you may consider using an 86 or even a 60 mesh. For most prints, a 110 mesh will work great! One of the main tricks to get great in coverage on a garment is stencil thickness. The thickness of your emulsion directly dictates the thickness of ink laid onto the garment. Picture your screen stencil as a well, it only holds as much ink as the well is deep. With one pass, the maximum ink coverage you will achieve will be the thickness of your emulsion. Coating the screen multiple times on the shirt side or outside of the screen will create a thicker stencil and thus yield more ink and better coverage. You must buildup your emulsion on the outside of the screen, if you coat the inside of the screen, then you'll have to push through the stencil and work against yourself. To make an optimal screen for white, coat the screen once on the outside and once on the inside, let dry, coat again on the outside, let dry, and coat one final time on the outside. Also it is best if you use the round end of your scoop coater to lay more emulsion down. Keep in mind that since you are using a thicker emulsion, you'll have to increase your exposure time in order to thoroughly expose the screen. We recommend using a dual cure emulsion for the easiest results and post hardening the screen after washout.
Another thing you'll have to understand and come to terms with is the fact that white ink is probably going to be the thickest ink you'll ever use. To achieve a bright white image on a black shirt, the ink has to be opaque which in turn means thicker. Sure you can find thinner white inks, or try to reduce the thicker ones, however you're defeating the purpose and you'll end up printing twice as much to try and achieve the same result. On the other hand, if you learn how to print correctly, then printing white is like taking a walk in the park.
Over the past several years, every ink manufacturer in the country has been on the hunt for the best white ink on the market. Yes, they've come a long way. In fact twenty or thirty years ago white on black looked more like a light tan or gray than white. White ink has to be loaded with pigment which means it will always have a high viscosity.
SSo what's the trick? How can you print white ink like the pros.? The answer usually lies in a few simple changes to your setup and technique. Apply these changes and really, it's not that hard.
Before we delve into techniques, let's first discuss a little bit about pre press and setup.
To properly setup a job using white ink you first must understand a little bit on how to select the proper mesh size. Because white is thicker, you want to print through the lowest proper mesh size that applies to that print. If you try and print white through a higher mesh screen which has much smaller holes in it its going to make it much harder to push the ink through the screen. Its also not going to allow as much ink through the screen as a lower mesh would.
Typically you want to print white through mesh sizes ranging from 110-156. Granted sometimes the image dictates a high mesh count. For instance, since a half tone or fine line drawing cannot hold on a lower mesh screen, you'll have to use a high mesh frame and apply more passes to achieve a bright white. For the most part however, you'll want to use the lower meshes so that a larger amount of ink is deposited on the shirt.
The second part of setup is to insure the screen is adjusted properly on the press. You want to be sure that you have a proper off contact of about 1/8-1/16 of an inch. Since white is a little thicker, you may want to go with a slightly higher off contact than normal, perhaps around 1/8 of an inch. Off contact is the height between the screen mesh and the substrate you are printing on.br />
Proper off contact also allows the ink to be cleared from the screen mesh easily by releasing the mesh upward directly after the print stroke leaving all the ink smoothly on the shirt. One thing that you also want to be sure to apply when printing white ink is an off contact tab. This is a small piece of material (ie. a penny, cardboard, paper, a piece of plastic) that is the height of your off contact. This tab is placed on the end of the screen frame where it hits the top of the neck platen and not the shirt. By using an off contact tab, you insure that your off contact will remain through the entire print stroke.
The tab keeps the end of the screen up although you are pushing down on the mesh during the stroke. Without an off contact tab, you may find that you have a harder time clearing the mesh because the screen wants to stay flat against the shirt with ink stuck in it.
IIf you don't have the proper off contact a few problems could arise. If your off contact is too low then you will not be able to clear the screen properly. You should see the screen mesh bounce off the shirt and be able to see the ink and the shirt through a cleared screen mesh. If your ink stays in the screen mesh and the screen mesh sticks to the shirt, when you pull the screen up your ink will look very rough and parts may pull back off the shirt. If you're having this problem you may need to raise the off contact a little bit. Also, this problem could arise from your actual print stroke which will be discussed later.
On the other end of the spectrum, if you're off contact is too high; you'll have to push down extremely hard on the squeegee in order to get all the ink down onto the shirt. I've been out to shops to help customers who literally had almost a half of inch of off contact. If your off contact is too high, not only will it be hard to transfer the ink onto the shirt, but there's a good chance that your screen won't come down in the same spot every time. This could cause a blurry image or double images after a flashing. A proper off contact is extremely important for a good looking image and an easy print.
Next let's take a look at technique. The majority of the technique in the screen printing process involves the print stroke. This includes squeegee angle, pressure, speed and release. That standard tendency of a new printer is to literally smash the ink through the screen by pulling the squeegee across the screen with an extremely low angle.
When using thin inks, this improper technique could work because the ink passes through the screen so easily. However, if tried with white, you'll not only have trouble passing the ink through the screen but also have very blurry images because you're pulling the screen mesh so hard. Your squeegee should actually be about an 80-85 degree angle facing toward you. You should also focus your pressure over and down on the print and not on pulling the squeegee against the mesh.
Many new printers stand back from the press, you actually need to stand over the press to focus your pressure down and not back. When your pressure is focused down, then you simply move the squeegee back over the print. With the proper squeegee angle, all of the ink will properly shear through the image and over your screen. In fact you should be able to run your finger over the design after its printed and barely any ink should show up on your finger.
Now lets talk about the speed of your print. When we instruct printers to use the proper squeegee angle and pressure, the first impulse they have is to move the squeegee about the speed of a turtle. If your speed is too slow you won't be able to properly release and clear the screen. After a print stoke, many beginners tend to sit back and look at their print, then slowly pull up on the squeegee not allowing the screen mesh to bounce back up leaving ink still stuck in the screen. After a faster stroke, you need to almost give the screen a little pop in order to get the mesh to bounce back up leave a smooth crisp print on your shirt. This is achieved by a simple flick of the wrist, the same flick you would make to insure that you pick up all the ink on your squeegee blade. Only to create this pop and release the screen, the flick must be done a little faster.
Finally, when flashing in between passes, you must wait until the shirt cools down a little until you print again. This is much easier to achieve on a multiple station press because it allows the pallets to cool down. On a single station press you may have to wait a few moments and wave your hand across the pallet to cool it down.
If the ink is too hot from the previous flash, you may have trouble clearing the screen because the ink will want to stick instead of release the mesh. It could also cause ink to partially cure in the mesh. Since white ink is thicker and you apply more layers of it, you MUST insure a proper final cure.
White ink tends to reflect heat away instead of absorbing it in. This means that the ink and the entire print area must be cured from top to bottom at 320 degrees for about 45 seconds. Obviously the best way to cure white is in a conveyor belt dryer, however if you are using a flash dryer you want to be sure that the whole design gets the same amount of heat.
One of the best ways to insure a proper cure is called the stretch test. Since your white image will be a little thicker, this test is easy to read the results. After your shirt is cured, simply take a small section of your design and stretch it slightly between your fingers. If the design sticks together like plastic and stretches well, then it is cured and you can continue with those curing parameters. However, if the ink cracks or breaks apart, this means the base of the ink is not cured properly and you need to increase your curing time for the shirt.
By following these steps and techniques you should find printing white ink a little easier. Granted, because there are so many variables involved in screen printing, you may have to work a little at it and modify slightly. However, after a few times printing the right way, you'll start seeing better results and have fewer problems.