In present times, hand screen printing can be considered a slower method as compared to flatbed screen printing. It is done commercially on long tables of up to 100 yards. Printers, who are standing on both sides of the table, move the screen with great care, one frame at a time, until the entire length of cloth is printed.
A screen is made of a piece of mesh stretched over a frame. A stencil is formed by blocking off parts of the screen, forming the negative image of the design to be printed. Thus ink will appear on the substrate in the open spaces . Screen frames for commercial use are usually made of steel, or a lighter metal, with a hollow cross-section to provide rigidity with minimum weight. The coarser screen mesh fabrics are usually woven from polyester multifilament yarns, which are cheaper than the monofilaments preferred for fine-mesh screens. Screen fabrics used for printing on to paper, in the graphics industry, are sometimes calendared to reduce the open area and thickness. These are often coloured red or orange to reduce the level of light scattered during exposure, but this degree of sophistication is rarely necessary for textile printing.
Before printing occurs, the frame and screen must undergo the pre-press process, in which an emulsion is ‘scooped’ across the mesh and the ‘exposure unit’ burns away the unnecessary emulsion leaving behind a clean area in the mesh with the identical shape as the desired image. The surface (commonly referred to as a pallet) that the substrate will be printed against is coated with a wide ‘pallet tape’. This serves to protect the ‘pallet’ from any unwanted ink leaking through the substrate and potentially staining the ‘pallet’ or transferring unwanted ink onto the next substrate.
Next, the screen and frame are lined with a tape. The type of tape used for this purpose often depends upon the ink that is to be printed onto the substrate. These tapes are generally used for UV and water-based inks due to the inks’ lower viscosities.
The last process in the ‘pre-press’ is blocking out any unwanted ‘pin-holes’ in the emulsion. If these holes are left in the emulsion, the ink will continue through the stencil and leave unwanted marks. To block out these holes, materials such as tapes, specialty emulsions and ‘block-out pens’ may be used effectively.
The screen is placed atop a substrate while the Ink is placed on top of the screen, and a flood bar is used to push the ink through the holes in the mesh. The operator begins with the fill bar at the rear of the screen and behind there is a reservoir of ink. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a rubber squeegee to move the mesh down to the substrate and pushes the squeegee to the rear of the screen.
The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount, i.e. the wet ink deposit is proportional to the thickness of the mesh and or stencil. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (called snap-off) leaving the ink upon the substrate surface.
Textile items printed with multicoloured designs often use a wet on wet technique, or colours dried while on the press, while graphic items are allowed to dry between colours that are then printed with another screen and often in a different colour after the product is re-aligned on the press.
While the public thinks of garments in conjunction with screen printing, the technique is used on tens of thousands of items, including decals, clock and watch faces, balloons, and many other products. The technique has even been adapted for more advanced uses, such as laying down conductors and resistors in multi-layer circuits using thin ceramic layers as the substrate.
The choice of screen fabric depends on the fabric and design to be printed.
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