Me again. So, I just realized that I'm not using Ulano 925. They gave me Autosol 7000. It says it's direct emulsion for general purpose solvent and water resistant, dual cure emulsion. Maybe this is my problem. Anyway, can someone recommend how I should coat my screens and which emulsions are the best for high detailed graphics, water based inks and durable stencils? Thanks again.

I would try and bring the light closer to the screen. i'm using ulano tlx i get an exposer time of 2.5min with 40watt fluorescent tubes and the same with 500watt halogen. how big of an area are you exposing?

Your exposure time is too long. Try one thin coat of emulsion on both sides of the screen. If you have a scoop coater, use the thin edge. Remember -- thin coat. Also, what is the mesh count of your screens. A 110 mesh count takes longer to expose than 156, and 156 takes longer to expose than 205, etc.

I have a homemade ultraviolet exposure unit, that works really well.

Yo,
sounds like the main problem is the emulsion being the wrong one. Ulano products have always given me good results and before I had a light setup I would even expose using sunlight. It would work fine but is alot quicker than using a light.
Are you using a professional contact frame or just taping to a piece of glass..maybe the glass has UV protective coating/tinting.
Doing a step exposure test as tedious as it is will give you all set uop you will need for future screen making.

thanks for your replies. the thin coat worked pretty good for reducing exposure time. plus I think the emulsion I'm using is part of the problem too.

I have a similiar problem, i dry my thinly coated 18x22 screens in a dark drawer then run over to the next house during the day, and put it on a piece of glass with my stencil underneath it. I have a piece of glass on top connecting the screen to the stencil, and my exposure time is just taking Way too long. Im doing it in an open garage. could it be im just exposing it too long which would be hardening the chost image so it wont wash out? and if i at all see a ghost image on the screen should i right away try and spary it out? any advice would be awesome!
Thnx

if i was going to guess, i would say that this has to be a spectrum problem, since what youre doing when youre exposing a screen is creating a chemical reaction with light. the wattage is not as important as the right kind of light. i dont know much more about this, since i randomly found a combo thatworked, and have been using it ever since. you might just try a different bulb?

Okay, you have lots of "guesses" as to what your exposure problem might be, but no real answers. Here are some simple rules from one of my recent articles…

Rule 1 – Properly Store Emulsion
You must use emulsion that has not been allowed to reach a temperature higher than 100 degrees, nor lower than 32 degrees Fahrenheit. Emulsion can polymerize and lose its sensitivity when stored above 100 degrees. It will not polymerize and will simply wash out from the screen after your exposure attempts.

Emulsion that has been allowed to freeze may also lose it’s sensitivity. If it has not become frozen for too long, you can slowly bring it back to room temperature and attempt an exposure. The exposure will be different than it was before you allowed the emulsion to freeze.

Storing emulsion in a refrigerator is never a good idea. It must be brought back to room temperature each time. This will affect the thickness of the emulsion and thus the coating ability.

Rule 2 – Coating Technique
You must coat your screen properly to achieve the correct thickness of ink, the best edge definition and one that will hold up through the press run. No one will ever be able to "tell" you how many passes it will take to coat a screen. Absolutely no one! They do not know your parameters. Emulsion will flow differently depending on:

consistency of the emulsion
temperature of the emulsion
speed of the coating
angle of the screen
size of the mesh opening
mesh count
tension level of the mesh

The correct way to coat a screen is never by counting the passes that one must make, i.e. one over one or two over two, because of these variables. You really can’t even say this in general. Yes, I know what the Speedball directions state, but then they are not screenprinters, are they? Here is the only, correct way, to assure that you are getting the correct amount of emulsion on your screen given all the variables.

Begin with the scoop coater on the bottom of the screen, i.e. the print side. Make one pass and then look at the top of the screen or the squeegee side. If the squeegee side of the mesh is not shiny with emulsion, you will need to make a second pass. Continue making passes on the bottom of the screen until you have pushed the emulsion through to the top of the screen and the mesh on the squeegee side is shiny. Then make a single pass on the top or squeegee side of the frame to put the emulsion back on the bottom or print side of the screen. This will give you the correct amount of emulsion for the mesh count, the mesh opening, the tension level, and most importantly the consistency of the emulsion.

Rule 3 – Dry the Emulsion
Emulsion must be dry before exposure. Emulsion must move from it’s liquid state of 60 to 70-percent water to less than 2-percent water, i.e. 98 percent dry or the moisture will keep the polymerization action from correctly happening. A thoroughly wet stencil could be taken into the direct sunlight and it would not harden the stencil.

It is a common misconception that heat will help evaporate the moisture from the emulsion. This is not true. Actually, heat can activate the sensitizer in the emulsion and cause it to polymerize prematurely and thus ruin any chance of correct exposure. Thus, the rumors of using heated fans, heat guns, paint stripping guns, the flash unit or even running it through the dryer are all very bad idea. Just don’t do it! Heat holds moistures and does not cause it to evaporate. The only way to evaporate moisture is to remove it from the area. A dehumidifier works exceptionally well for this. An air conditioner works well, but never use heat to do anything other than keep the screens at room temperature or from freezing.

You need a dry, dehumidified room with 45 to 55 percent relative humidity. You can purchase a small, inexpensive digital hygrometer from Radio Shack online, which will show you the relative change in humidity. You start at your normal Rh and then as you introduce the wet emulsion into the room the Rh will rise. As your dehumidifier removes the moisture from the coating area, the Rh will come down until it reaches your normal Rh for your area. While you may not reach 45 to 55 percent Rh overnight, it will be the correct Rh for your area.

You must dry the emulsion throughout, not just on the surface. This normally takes at least 8 hours to fully acclimate the emulsion to the ambient humidity of the area. Emulsions are polyvinyl acetate and polyvinyl resins, which are the same compounds used in carpenter’s wood glue. The rule of thumb is the same – Allow to sit overnight in a dry area.

Still having problems? Why not use a capillary film, which dries in less than an hour in a dehumidified room and produces a stencil with manufactured consistency.

Rule 4 – Correct Energy Source
You must use a lamp that is in the correct spectral range. It has to throw enough UV energy at the emulsion to create a chemical polymerization or hardening of the emulsion. The correct spectrum is 350 to 420 Nanometers of UV energy. The Halogen bulb is not going to throw enough UV energy in the right spectrum. All screenprinting emulsions are formulated to polymerize in the same spectrum.

The best sources for exposing screenprinting emulsion are metal halide and mercury vapor and they are the most commonly available exposure sources today, though carbon-arc lamps are also still in limited use outside of the United States where their use has been outlawed by OSHA. While some high intensity fluorescent tube arrays are adequate for exposing emulsion, they do a poor job of providing the pin-point light that is needed by the film positive to hold the edge definition.

Every lamp and lamp type has a different output frequency and power. It is entirely possible that two different 1,000 watt metal halide bulbs will need a different amount of time to expose the same screen. This has been proven by covering one side of a coated screen and exposing with “Bulb A” and then exposing the second side with “Bulb B.” The difference wasn’t dramatic, but it was different enough that the correct exposure time, according to the exposure calculator was 25-percent longer for one over the other.

Obviously, the higher the wattage within a given lamp type, the quicker the exposure. A 1,000 watt metal halide will take twice as long to expose a given screen as a 2,000 watt metal halide will, all other things being equal. But even then one bulb may create more energy in the 350 to 420 Nanometer spectrum and thus will expose the emulsion much quicker. Therefore, wattage alone is an unreliable indicator of a lamps ability to expose emulsion.

Just as in all things, price usually denotes superior products. Thus a more expensive lamp will expose emulsion must better and faster. Price is also an indicator of power consumption and longevity as well, in most cases. The life of most lamps is dependent upon how much you paid for the lamp. While you may not want to hear that, it is true with most manufacturers – you get what you pay for.

Rule 5 – Correct Distance
There are two considerations here, whether you are using a point light source or a fluorescent array to expose your screens. The correct distance for a fluorescent array is to have the screen the same as the distance from the center of one bulb to the next. Usually this distance is about 4 to 6 inches.

The distance from a point light source to the stencil is equal to the diagonal of the stencil to be exposed. Measuring from one corner to the diagonally opposed corner will give you the correct distance. If the lamp is to close, it will result in a hot spot in the center and cold spots on the outward edges of the stencil. While this doesn’t optimize the intensity of the lamp or the consistency, it is considered to be the best balance between the two.

Rule 6 – Lamp Age
All lamps age and as they do so, they lose power. A new lamp that can expose emulsion in 2 minutes will, after a few months of use, take up to 4 minutes to expose the same emulsion thickness. Thus, all lamps must be replaced as they age in order to lower the exposure time and maintain efficiency in the screenroom. As a general rule, most screenmakers change their lamps when the exposure time has doubled, although in a large shop where they may expose as many as 800 to 1200 screens a day, they may replace them when the exposure time increases as little as 20-percent. A light integrator keeps track of this aging and adjusts the time accordingly. Fluorescent lamp arrays never have an integrator as each lamp ages independently of the others.

I hope this helps in some way with your screenmaking efforts. You can learn more by reading the articles available at the Screenprint Store (screenprintstore.com) or by attending the School of Screenprinting (schoolofscreenprinting.com).

Bill Hood
Screenprint Consultant
School of Screenprinting