The stretching and preparation of the screen fabric is critical. There are many problems in screen printing that can be traced to a loose or undertensioned screen. Some of those problems are uneven coatings of emulsion on the screen, poor adhesion of film stencil materials, and even poor register of your printed designs.
Stretching Fabric
The method that you select for stretching the fabric onto the screen is very important and is directly related to the type of frame that is used, and the job that is to be printed. It may also be related to the accessibility of certain tools and the costs involved too. There are several different types of frames used for screen printing:
Wooden Frames - Wooden screen frames are popular due to their low cost. There are several considerations in selecting a wooden frame. The frame members must be constructed of at least 1 1/8" x 1 1/8" material. This size is actually a bit on the weak side and should be used for hand stretched screens only. If used with mechanical stretching devices, the wood members will bow and warp. A better size frame member would be constructed of 2 5/8" x 2 5/8" or larger. The fabric is attached to the frame using a cord and groove method, stapling, or gluing.
Metal Frames - Metal frames are usually constructed of aluminum or stainless steel. They are almost exclusively stretched by a mechanical stretching machine and can handle much higher tension of the fabric. The fabric is glued to the frame with a special adhesive that utilizes a catalyst. They are much more expensive than wood.
Self-Tensioning Metal Frames - These are one of the best frames that can be used for critical registration (lining up) multi-color jobs. The fabric is fitted into a special holding mechanism in the screen. It is stretched (or "tensioned") using wrenches that turn the mechanism in the frame. The fabric is stretched very tightly. If the fabric should begin to loosen, it is easily "re-tensioned" using the wrenches again. These are very costly.
Once the frame is selected, the method of stretching the fabric begins. There are four basic methods of stretching fabric:
Cord Method - In this method, a rope-like cord is inserted into a groove in the wooden frame. As the cord is driven down into the frame, it creates tension on the fabric. This is an economical method since the cord is inexpensive, and the frames may be used again and again.
Staple Method - This is one of the oldest methods of stretching screens. The fabric is held by hand or with a special pliers. The fabric is pulled as tightly as possible and then stapled. This is an economical way to stretch fabric, but usually results in wrinkles along the edge of the fabric. It also damages the wood, reducing the times that the frame may be reused.
Pneumatic Stretching Device - In this method, the fabric that is held in a special clamping device. A special tool that uses a "hydraulic car jack" type of assembly pushes the screen frame up against the fabric, resulting in a very tight screen. Heavy wooden frames or metal frames must be used with this method. It also requires a special glue and catalyst. This is a very good method for stretching screens, but it also requires a large investment and high screen production costs.
Self-Tensioning Frames - This is one of the best methods of stretching screen fabric. The frame contains a self-tensioning mechanism in the frame members. A wrench is used to tighten the fabric. Excellent tensioning can be achieved. This results in better stencil application and registration of colors. The fabric can also be retensioned after printing to result in longer printing runs. The drawback to this method is the high cost of the frames. Jobs can not be saved on the frame, so it becomes necessary to remake stencils if a customer requests more shirts at a later time.
Preparing Fabric
After the fabric is stretched, it is necessary to prepare the fabric to accept the stencil material. Since most stencils are water-based, it is critical to be sure the fabric is clean and free of foreign materials.
There are specific reasons for preparing the fabric, regardless of whether it is a natural fabric or synthetic. Fabric manufacturing usually results in chemicals such as starches and sizing that can contaminate the surface of the fabric and cause problems with stencil materials. Additionally, natural fabrics may contain oils that will conflict with water-based stencils. There are two basic steps to fabric preparation:
Degreasing the Fabric - Fabric is degreased by using a detergent-based material. After the fabric has been stretched, it is wet and the degreasing material is applied with a brush. It is worked into a soapy lather and left on the screen for several minutes. Then the screen is rinsed clean. It should not be touched with your hands after it is degreased, since the oils in your skin can also cause stencil problems.
Abrading the Fabric - An "abrasion" is a scrape on the skin. Abrading the fabric does the same. It roughens the surface of the fabric. It is especially important for synthetic fabrics. Polyester and nylon are made up of many smooth threads. The surface of these threads is very smooth. In order to get the stencil material to stick to the fabric, you must "roughen" or "abrade" the surface. A granular material or "grit" is used. It usually consists of "pumice". Pumice is a stone that is the result of molten lava that has cooled. It is very glass-like and sharp. By sprinkling this pumice grit on the surface of the fabric, and working it around with a brush, the surface is roughened. The excess grit is rinsed off just like the degreaser.
There are materials sold that combine both a degreasing material and abrading material in a paste form. They are called abrader/degreasers. These materials are convenient and easy to use. They cut the preparation down into one step.
Now that you've completed reading this material on Stretching and Preparing the Fabric, it is time to put this knowledge to work preparing your screen. Once that is done, you are ready to create a stencil. More information on stencils is contained in the next section.
Review the page again to be sure that you understand the material. REMEMBER, YOU WILL BE TESTED ON THIS MATERIAL!
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