by Roger L. Jennings
www.rjennings.com
Reprinted with permission from IMPRINT CANADA
Capillary film can make a screen printing shop more profitable, improve the quality of printing, and solve some difficult printing problems. If you have not tried capillary film, you will want to get a sample to see how it can help your business.
Capillary film is liquid emulsion spread on a plastic sheet and then dried by the emulsion manufacture. The film is sold in different thicknesses to control the thickness of the ink deposit and to bridge the holes in the mesh. The film is applied to the bottom of the mesh so that the ink passes around the threads of the mesh. The image is created by pure stencil material without the influence of the mesh.
Images will have higher resolution when printed through a film product compared to liquid emulsion. Liquid emulsion encapsulates the mesh producing an image influenced by the mesh. Compact discs, for example, frequently are printed with a film product to produce better quality with fine details on the hard surface of the CD than is possible with liquid emulsion.
The next time there is an order requiring fine details, particularly words, the value of capillary film will be evident if printed first with liquid emulsion in the screen and second with a film product like 20 or 30 microns thick on a mesh in the 200s's or 305.
The film is sold in different thicknesses. Depending on manufacturer, the thickness can be something like 20, 30, 40, or 50 microns. Newer products from 90 to 1000 microns thick are now available to print the new high density inks.
Update: Here are some recommended films:
T-shirts - 50 microns
Pique, herringbone, etc., golf shirts - 100 microns
6-Panel caps - 150 microns or thicker
For example, when printing white ink on black shirts, 50 microns will allow a controlled thickness of opaque white ink to be deposited to achieve high opacity and a relatively soft hand. Printing one colour white ink on black shirts without flashing is possible when using a film stencil. Production should comfortably be 72 shirts per hour compared to 25-30 shirts when flashing is required.
The print-flash-print routine is used to achieve the opacity a film product can achieve better without flashing. (See IMPRINT CANADA, January-February 1999 for more details on Printing on Black at www.rjennings.com.)
A film product then, produces better definition to the edges of the image, plus allows controlling the thickness of the ink deposit. This is valuable information, because surfaces that are not paper smooth can be printed with high resolution and excellent opacity when the appropriate stencil thickness is selected.
Update: The dyed films, e.g. red, produce better quality than clear films. The thicker films, like 100 to 250 microns, are typically applied to coarse mesh like 60M, 86M or 110M, and most screen printers use the less expensive white mesh. White mesh refracts light. A thick film on white mesh may have ragged edges compared to a dyed film which cuts down on light refraction and under cutting of an image. Under cutting is where the image in the screen is slightly smaller than the image on the positive, and that creates problems registering colors when setting a job up.
Twills, piques, and other coarse materials, even corduroy, can be printed when a film product is used. Corduroy requires releasing more ink to the substrate to achieve acceptable opacity than a twill. So a thicker stencil would be used. Also, a corduroy requires a high density ink so that the image holds its definition when the support of the stencil is removed.
Film products are sold as capillary when only water is required to adhere the film to the mesh. If the film product is not capillary, then the screen must be coated first with liquid emulsion to act as an adhesive when applying the film. This is called the indirect-direct method.
One of the advantages of capillary film over films that require liquid emulsion is that no messy liquid emulsion has to be handled. If retensionable frames are used, for example, and liquid emulsion gets in the slots where the mesh is locked into the frame, the mesh can be difficult to remove. Capillary film does not create this problem.
Film products solve several other problems as well. If pin holes are a problem, capillary film will eliminate the pin holes. Pin holes result from dust in the air that settles on wet liquid emulsion. Printing causes the emulsion to break down where dust is present. Films do not have this problem, because film is applied to mesh with water that flushes off any dust. Also, film does not have the dust in the film to create a weak spot that could spring a leak.
When printing caps, sometimes customers want tall images on the cap. The image can be exposed as close as 1/16" to the inside of the frame. The film allows coating an even thickness of stencil material to the edge of the frame, and even overlapping the edge. Liquid emulsion is hard to coat close to the edge of the screen and will thin out close to the edge. Frequently the emulsion will break down near the thin side of the frame when liquid emulsion is used whereas a film product will not break down.
If the stencil breaks down, production time is lost repairing the screen, and mis-prints may have been created. The cost of these problems can be measured by the profit that could have been earned printing during the time production was lost repairing the screen.
Film products work well with gray positives, that is, images created by computer or photocopy machine on translucent paper. Typically these images are not dense black like positives from a camera. Light leaks through the image during exposure. Over exposure can make wash out difficult.
An exposure calculator, which is nothing more than a specialized film positive, allows determining how much light is required to expose the stencil material. Thicker film stencils require more light exposure. The exposure calculators are offered by the emulsion manufacturers and sold through their distributors.
Once the time required to expose film is determined with the exposure calculator, then every time that stencil thickness is selected the screen printer will know how long the light must be on to avoid over or under exposure. Scum and white foam that develops when washing out a screen can be avoided just like the problems of over exposure. Screens can be made satisfactorily the first time thereby avoiding wasting time and materials making screens a second or third time.
People have two complaints about film products. First they say it is expensive. Well, the films that are 20-50 microns thick cost about Can. $. 015 per square inch. So a cap or left chest print that requires 4" x 6" of film would cost Can. $ .36 per screen. So cost is not a good argument on small images.
Film could appear expensive on large images and short print runs. If small details, edge definition, and surface texture are not a problem, then liquid emulsion might be better to save money.
The other complaint people have is they have difficulty applying the film to the mesh. If capillary film is used, there will be no difficulty, if water is sprayed up the screen to coat the mesh completely with water just before the capillary film is rolled down the mesh. The film should be coiled to the diameter of a cigar with the emulsion side out so that even pressure is applied when rolling the film down the screen. When the indirect-direct method is used, the film can simply be laid over the coated image area. Thick films may have to be coated again with liquid emulsion from the ink side of the screen to strengthen the bond with the film.
The thicker films like 100 to 1000 microns are typically pure photopolymers rather than diazo emulsions. The pure photopolymers typically expose in 1/7th of the time required by a diazo emulsion of equal thickness. However, the increased thickness will require significantly longer time to expose. The time required will be determined by testing. If the film peels off during wash out, it was under exposed. Either a longer time is required, or the film can be laminated.
If florescent, halogen or other poor light source is used, a screen can be coated with a thinner film, such as 50 to 90 microns, exposed and washed out. The screen can be laid flat, and a second piece of film applied and adhered with water or liquid emulsion as appropriate. After the screen dries, the plastic liner can be removed from the film and the image mounted exactly to make this job easy.
Then the screen can be exposed a second time, washed out, and coated a third time. Laminating allows creating thick stencils to print high density inks.
Film stencils should be employed at least in limited circumstances, such as when the urgency of the job does not allow time to dry the screen before applying emulsion. They may not replace liquid emulsions, but can avoid wasted and messy liquid emulsion, low productivity and poor print quality. If you have not tried film, you can ask the manufacturers or their distributors for a free sample.
Once you try film, you will love it.
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Editor's note: Jennings has an MBA from The Wharton School, University of Pennsylvania, and was formerly a management consultant with the international accounting firm Coopers & Lybrand. As a consultant he has turned many poor performing companies into very profitable clients. Roger speaks at The Imprinted Sportswear Shows on ways to improve profitability, and operates R Jennings Manufacturing which makes screen printing equipment. If you need assistance, call him at (518) 798-2277 or fax at (518) 798-3172.
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To see capillary film applied, and to see the effect from using capillary film, see "High-Density Printing on Caps" in Impressions Magazine, March 1999, page 100, or look the article up at www.rjennings.com.
Ulano Corp. is recommended as the supplier of capillary film.
(C) R Jennings Manufacturing Company, Inc. 2002