Ghost haze is a problem you can conquer when you arm yourself with the proper chemicals and employ sound techniques. This article examines products that help remove ghost images as well as methods you can use to reduce or prevent their occurrence.
All screen-cleaning products should be applied to both sides of the screen and brushed into both sides of the screen. Remember to remove these products from both sides of the screen with a highpressure wash . What some perceive as emulsion-haze problem can actually be a procedural issue—for example, pressure washing from only one side of the screen (Figure 4) .
Latent or phantom haze
This type of haze is virtually invisible on the screen when viewed head on. There is no apparent stain, and the screen mesh may appearl like new. Only when the screen is viewed at approximately 45° does one see this latent image from a previous job. Latent haze typically causes no problems for screen printers, but on occasion, it may show up in the most unlikely places, such as embedded in the design of a subsequent job. The printed image shows areas of lighter color anywhere the image overlaps a previous job’s latent haze. This is how the phantom image shows itself. There’s a restriction of the normal flow and/or transfer of ink through the screen and onto the substrate in these overlapped areas.
Changes to the mesh’s surface energy occur when photopolymer chemicals react and/or interact on the screen during screen exposure, printing, and reclaiming. This change in surface energy affects the rate of ink transfer through the mesh, which could lead to latent or phantom haze.
Use of solvents also may contribute to surface-energy variation by causing the mesh to swell and not fully recover— or they may cause the mesh to dry out. This may occur only in the image area, as mesh is protected by emulsion in the non-printing areas. Although polyester mesh is very resilient, these subtle changes in surface energy may be enough to cause inconsistent ink transfer on subsequent print runs.
Screen processing is yet another culprit. If screens are not processed correctly and carefully during each step, a thin film of chemical residue may contribute to a latent haze. For example, underexposure and improper developing cause difficulties in reclaiming and leave the stencil susceptible to other chemical reactions with inks and ink removers. These reactions can cause physical changes to the mesh that may contribute to latent haze.
Long printing runs, whereby the mesh loses its ability to fully recover or regain its original physical structure in the areas of the printed image, leads to a condition called mesh bruising that can, in turn, cause latent images to form. Thread compression differs in image and non-image areas, leaving a permanent impression of the printed image in the screen, even after the screen has been fully reclaimed. The image reappears in the subsequent job.
One might also argue that the physical wear on the threads not protected by the stencil results in increased surface area, which in turn reduces ink transfer in those areas during the next job. But how could we then explain mesh bruising when capillary films are used? Capillary films do not fully encapsulate the mesh, thereby exposing all the threads on the squeegee side of the screen. Even direct emulsions do not fully encapsulate the mesh when they’re underexposed. The answer may be in how the threads compress and recover from the substrate side vs. the squeegee side of the screen and uniquely in the open image area vs. the closed stenciled areas.
Possible remedies for latent ghost haze include keeping a close control over all screenmaking and cleaning procedures, using mesh-preparation products that rely on surface treatments and wetting agents to balance the surface tension of the mesh, and using a slow-acting, twopart haze remover that contains sodium hypochlorite and can be dried on the screen.
Lifting the haze
Understanding how to identify the differences between the types of ghosting, their underlying causes, and how best to treat them will make you better equipped to select the most effective haze remover for your specific requirements. By considering how the variables discussed here contribute to ghosting, you also can make procedural adjustments to your processing steps to achieve maximum control over all forms of ghost haze.
Dave Dennings is product manager for screenmaking products with KIWO Inc., Seabrook, TX, a manufacturer of stencil materials and screenmaking equipment. He is also responsible for running the company’s applications lab. Dennings has been involved with screen printing for more than 17 years and has served in technicalsales positions with both KIWO and Sefar America. He is a member of the Academy of Screen Printing Technology.
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