when washing out emulsion from an exposed screen, I'm having a hard time getting accurate detail.
The result most of the time is that the edges of the emulsion begin to come apart and lift off the screen in an almost fraying effect. It is really hard to get detail, especially halftones to wash away properly
The weirdest result is when parts of the same screen wash out with accurate detail and other parts wash out inaccurately
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-im using a 20x24 wooden automated uv exposure unit (Item Number: RXP2024), some have said that the bulbs are spaced too far apart?
-i have tried exposing at different times (5min - 6 min), but have not found any consistency
-i have washed out using all types of washer heads: power spray, light drizzle etc.
-i am using ulano procliam pink, brand new
-i am washing out only a minute after the screen has been exposed
-the screens have been 100% dried
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any help would be greatly appriciated,
thanks
Emulsion Fraying
Moderator: Moderator Team
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X_Autotype_#1
- Intern
- Posts: 29
- Joined: Fri Aug 18, 2006 7:21 am
Well "kids...", there are a whole lot of variables that come into play when exposing screens, and many more when imaging halftone stencils. The first thing you need to be confident about is the UV density of your positives. If the UV density is at least 3.0 or above, that will eliminate that variable in your positive.
Your light source contains a bank of 8 fluorescent tubes, at a distance of about 10". Halftone imaging is extremely difficult to control when using a multiple point light source. To get the best possible image, a single point light source should be used. Light undercutting the positive is a common problem caused when using a multi-point light source in stencil imaging. Compare it to your shadow outside if there existed 8 suns lined up in the sky. Your emulsion is being exposed by light coming in from 8 different angles, which will make high resolution imaging impossible. Additionally, your light source does not have a vacuum frame to ensure direct contact of the positive to the emulsion across the stencil area. If there is any minute variance in the contact between your positive and the stencil, you will fail at high resolution imaging. Although somehow many printers are successful in imaging halftones on this type of light source (there are some reading this response), your light source is unquestionably not the best type for halftone imaging. A single point light source and an operational vacuum draw down frame is best suited for detail imaging.
Other factors also need to be considered, including the mesh you are using. Mesh color is important in obtaining good stencil resolution. A white fabric can actually channel UV rays down through the fibers in a similar fashion to fiber optics and cause crosslinking of the emulsion behind the positive. Colored fabric will help to eliminate this problem. Your mesh count needs to be at least 4x - 4.5x the lpi of your halftone positive. A 65 line halftone should be imaged on at least a 260 lpi mesh count or higher, and colored mesh (as mentioned) is preferred for best results.
Your emulsion coating method also plays a very important role in the results. ANY variation in emulsion thickness will cause problems with exposure of your stencil. Thicker areas will be less exposed, while thinner areas will cause over exposure. This is one of the great benefits of using stencil films (capillary or indirect) for your halftone imaging opposed to direct emulsion. Coated films provide very little to no variation in stencil thickness across the image area, and the carrier film that is removed creates a very flat stencil surface for better contact with your printing substrate.
Screen preparation is also important. Although you are making sure that your emulsion is completely dry, are you sure that the fabric was completely de-greased with a good commercial screen cleaner before you coated it? If not, any areas containing finger oils, solvent residue, or other contaminants will give you problems that you won't see until the washout phase of stencil making. Obviously by then it is too late.
Your washout should be a good spray of water similar to a decent shower head, not too hot. Although a properly exposed stencil will likely hold up to a power washer, you should avoid excessive water pressure when imaging halftones.
These are some of the variables that come to mind regarding your question. If you keep as many of these variables under control, you stand a much better chance of creating a printable image. The next feat will be to actually print the halftone, which will involve a whole additional set of variables to control.
It never stopped the old timers, so keep on experimenting to nail down a procedure that give you the best results with your equipment.
BEST OF LUCK!
X
Your light source contains a bank of 8 fluorescent tubes, at a distance of about 10". Halftone imaging is extremely difficult to control when using a multiple point light source. To get the best possible image, a single point light source should be used. Light undercutting the positive is a common problem caused when using a multi-point light source in stencil imaging. Compare it to your shadow outside if there existed 8 suns lined up in the sky. Your emulsion is being exposed by light coming in from 8 different angles, which will make high resolution imaging impossible. Additionally, your light source does not have a vacuum frame to ensure direct contact of the positive to the emulsion across the stencil area. If there is any minute variance in the contact between your positive and the stencil, you will fail at high resolution imaging. Although somehow many printers are successful in imaging halftones on this type of light source (there are some reading this response), your light source is unquestionably not the best type for halftone imaging. A single point light source and an operational vacuum draw down frame is best suited for detail imaging.
Other factors also need to be considered, including the mesh you are using. Mesh color is important in obtaining good stencil resolution. A white fabric can actually channel UV rays down through the fibers in a similar fashion to fiber optics and cause crosslinking of the emulsion behind the positive. Colored fabric will help to eliminate this problem. Your mesh count needs to be at least 4x - 4.5x the lpi of your halftone positive. A 65 line halftone should be imaged on at least a 260 lpi mesh count or higher, and colored mesh (as mentioned) is preferred for best results.
Your emulsion coating method also plays a very important role in the results. ANY variation in emulsion thickness will cause problems with exposure of your stencil. Thicker areas will be less exposed, while thinner areas will cause over exposure. This is one of the great benefits of using stencil films (capillary or indirect) for your halftone imaging opposed to direct emulsion. Coated films provide very little to no variation in stencil thickness across the image area, and the carrier film that is removed creates a very flat stencil surface for better contact with your printing substrate.
Screen preparation is also important. Although you are making sure that your emulsion is completely dry, are you sure that the fabric was completely de-greased with a good commercial screen cleaner before you coated it? If not, any areas containing finger oils, solvent residue, or other contaminants will give you problems that you won't see until the washout phase of stencil making. Obviously by then it is too late.
Your washout should be a good spray of water similar to a decent shower head, not too hot. Although a properly exposed stencil will likely hold up to a power washer, you should avoid excessive water pressure when imaging halftones.
These are some of the variables that come to mind regarding your question. If you keep as many of these variables under control, you stand a much better chance of creating a printable image. The next feat will be to actually print the halftone, which will involve a whole additional set of variables to control.
It never stopped the old timers, so keep on experimenting to nail down a procedure that give you the best results with your equipment.
BEST OF LUCK!
X