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Technology

How Does Screen Printing Work

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Screen printing is an exciting and versatile art form, which allows the printer to create an image, and then reproduce it over and over again. Screen printing is a printing technology which is widely used. So does the screen printer. If you have ever done stencil art, the concept is the same. Paint, or in printing, ink, is forced through the open areas of the stencil onto the surface below.
For t-shirt printing, we start out with a wooden or aluminium framed screen. The frame has mesh stretched across it, similar to a painter’s canvas. The mesh that t-shirt printers commonly use, is known as 43T and is made of a polyester monofilament.
 The mesh is coated on both sides with photographic emulsion. This is a light sensitive substance of glue-like consistency, which is drawn evenly up the screen so that the mesh is covered by a thin coating. To get an even, controlled coating, screen printers use a scoop coater, (sometimes known as a screen coater or applicator) which is made of aluminium and holds the emulsion ready to coat. The coated screen has to dry in a dark, dust free area. It is also advisable to dry the screen in a horizontal position, so that the liquid does not run down the screen and dry unevenly.
Meanwhile, a film positive should be generated from the artwork being used. A film positive is essentially a black or opaque rendering of an image on a clear film. For more details on the principles of film positives and the various methods used to create them, click here. (page coming soon)
The film positive is then positioned on the baseboard or printing area and marked with registration points. The dry screen can then be registered to the printing area and the registration points transferred to the screen. The film positive can then be taped to the screen in the exact desired position. The film positive is placed so that right reading is possible from the inside of the screen.
With the film taped to the screen, it is now ready for exposure. Photographic emulsion reacts to UV light by hardening, so the opaque areas of the film positive act as a block out, protecting the emulsion underneath from hardening. After adequate exposure time, the screen is rinsed, and those areas left unexposed to the light remain water-soluble while the rest of the screen has hardened and cannot be washed away. The emulsion in the unexposed area softens quickly and rinses out of the screen revealing a stencil of the image.
The screen is then allowed to dry before being checked and taped up ready for printing.
Next, the film positive is returned to its position on the baseboard, and the screen is placed in the head of the carousel. To learn more about the role of the carousel, please click here.
The screen is then lowered over the film positive, manouvred into position directly over the design and locked into the carousel head. The design position should be marked on the baseboard in some way to indicate where the print will fall. This aids in the repeated positioning of t-shirts or items to be printed. The film positive is then removed, and an appropriate squeegee selected. (The squeegee should be wider than the design you intend to print so that the ink is distributed evenly across the fabric.
A squeegee is what printers use to force ink through the open areas of the mesh. They come with a wooden or aluminium handle holding a thick strip of rubber. The rubber is available in various grades of thickness and flexibility.
Next comes time to select an ink. At Strumalum we use water based inks which have their own advantages and disadvantages. For more information on inks, please click here.
A test print is always advisable before the real deal. The printing area is prepared with a light covering of spray glue, this will ensure that the fabric doesn’t move while printing, especially important if a second coat is required or if there are multiple colours to be printed. The screen is then lowered into position, ink is laid on the screen below the start of the design, and the squeegee is used to draw the ink over the open areas of the stencil.
After a print, the ink needs to be dried before a second coat or another colour can be added. A flash cure unit can be used to dry the print. We have a fan forced unit that uses ceramic elements to heat the area. This method takes about 10 – 20 seconds to dry.
When the print is completed, the item is removed from the baseboard and the next item to be printed is put down.
When the print run is finished, it is important for water based users to clean the screen immediately so that no ink dries in the screen and hinders future use. Screens, squeegees and spatulas wash up with water. Care should be taken to ensure the screen is cleared of all ink, front and back. Squeegees should also be inspected for ink lurking in the crevices near the handle.
Before printed fabric can be ready for use, it must be heat cured. For water based inks, the curing process draws water out of the ink leaving only the pigment to bond to the fabric. This is achieved by applying heat usually by running the garment through a tunnel dryer which is like a pizza oven. Other methods are discussed here.(page coming soon) Water based inks generally cure at around 180 degrees celcius for about 3 minutes. , (different inks have different curing times and temperatures) Flatbed printer is a brand new digital product on the market. We always called it digital flatbed printer. Flatbed printing is a fashionable and advanced technology today.
When it’s all done, the stencil can be stripped from the screen and re-used for the next design. Also, by retaining your film positive (and keeping it in good condition), you can always re-expose the stencil for a re-print anytime in the future. You can choose the screen print if you like.