One of the most important aspects of screen printing is selecting the right screen print fabric for the job. The screen print fabric serves two key functions. First, it supports the stencil. And second, it allows for and regulates the flow of ink onto the substrate.
In fact, the screen print fabric is critical in determining the quality of the finished product. No other variable affects the resolution of the print, the thickness of the ink deposit and ink consumption as much as the fabric.
While there are many products on the market, the selection process need not be complex. In selecting a screen printing fabric, the primary consideration is the ink system that you will use. The ink manufacturer’s technical data sheet will provide you with the mesh specifications. For screen printing large format vinyl graphics and signage, a plain weave, monofilament polyester fabric is recommended.
The technical bulletins will also spell out in detail everything that you really need to know about printing with their inks, such as:
- The substrates for which the ink is formulated;
- Which type of squeegee to use;
- What types of reducers and other additives along with the type of screen wash should be used;
- Safety considerations.
Below I have listed some popular inks used by sign makers along with the ink manufacturer’s recommendations:
Nazdar 59000 Series Enamel Plus Gloss Inks: Use 200-280 tpi PW (plain weave) monofilament polyester mesh. 59000 Series is a solvent-based enamel ink, which is ideal for outdoor applications, such as printing on aluminum sign blanks and glass.
Nazdar GV Series Inks: Use 150 – 350 tpi PW (plain weave) monofilament polyester mesh. This gloss vinyl ink works well when printing on pressure-sensitive vinyl films used for outdoor applications.
Nazdar 9700 Series Inks: Use 230-355 tpi PW (plain weave) monofilament polyester mesh. Nazdar 9700 is an exceptional general purpose screen printing ink. You can use this outdoor durable ink for printing on pressure-sensitive vinyl films, such as RTape VinylEfx®, Tyvek and plastics.
Nazdar 3500 UV Series Inks: Use 355-390 tpi PW (plain weave) monofilament polyester mesh. Nazdar 3500 inks are a popular choice for printing on pressure-sensitive vinyl films, such as RTape VinylEfx® metalized films. These flexible UV curable inks provide exceptional performance for fleet graphics applications, allowing for elongation when films are applied over rivets and other irregular surfaces.
Screen Print Fabric Variables.
While mesh count is a primary consideration in fabric selection, it is not the only variable. In selecting screen printing fabric the key considerations include mesh count; thread diameter; type of thread (monofilament or multifilament); and type of weave (plain or twill).
The mesh size indicates the number of threads per square inch. That number is important because it determines the size of the holes between each thread. A coarser mesh provides a greater mesh opening (a larger open space between the threads) allowing more ink to deposit on to the substrate. As the number of threads per inch increases, the size of the holes decreases.
The size of these holes is important for a couple of reasons. Inks comprised of larger particles require larger holes. For example, glitter inks used in textile printing, which are comprised of large particles, typically require a coarse screen print fabric with a mesh count between 25 and 40.
When printing with a solvent-based ink on vinyl, a heavy ink deposit is generally desirable. Fabrics with a mesh count of 200 to 280 are often recommended for solvent-based inks. Solvents, flow agents and other additives affect the volume of the ink as well as the thickness of the ink after it has dried.
The size of the holes also affects the resolution. Fabrics with high mesh counts produce images with finer detail, but also restrict the deposit of ink. Fabrics with lower mesh counts, by comparison, allow for more ink passage but with less print definition. In printing halftones, 4-color process, small type or very detailed graphics, you would need to use a screen fabric with a high mesh count.
UV inks also require fabrics with very small holes. When using UV inks, fabrics with high mesh counts in the range of 355 to 390 are more suitable. UV inks consist of 100% solids. Compared to solvent or water-based inks, UV inks contract very little after they are cured. That means that you need less ink deposited on the substrate. For this reason, fabrics used for UV inks are sometimes calendered, which reduce their thickness and result in less ink deposited.
The edge of screen fabric is usually marked with a code, generally consisting of two numbers, such as 340/35. The first number indicates the number of threads per inch. (In Europe, the number typically refers to threads per centimeter.) The second number refers to the diameter of the thread in microns.
The diameter or thickness of the thread is important because it affects the thickness of the ink deposited on the substrate. One reason is that the ink deposit can never be thicker than the mesh itself. If you use a thinner fabric, the layer of ink will be thinner. A thicker thread will yield a thicker deposit of ink.
With a thinner fabric, the open area of the mesh however is smaller. The smaller open area typically produces prints with better detail and quality.
Whatever mesh you use, to maintain consistency of print results, you should always use a fabric with the same mesh count and thread thickness. This is especially important if you are running a repeat order. Without using the same mesh, you won’t produce the same results.
In addition to the two numbers printed on the edge of the fabric, you should also notice either the letter P or the letters TW. Screen fabrics can have different weaves. The “P” in the product code stands for plain weave; “TW” indicates twill weave.
Most of what is sold is plain weave. The weave pattern can influence the thickness of ink deposited. Twill weave fabrics are typically thicker than plain weave fabrics of the same mesh count and thread diameter. Consequently, twill weave fabrics deposit more ink.
Thread Type: Monofilament or Multifilament.
Sign makers and large format screen printers typically use monofilament polyester fabrics. These screen print fabrics use single strand threads. Multifilament threads, on the other hand, are comprised of multiple strands. The advantages of monofilament fabrics are that the threads are smoother and stronger. As a result the fabric lasts longer in production, produces finer detail and is easier to reclaim than multifilament fabrics.
By comparison, multifilament threads, which had been used for printing garments, are coarser which provide more area for emulsions to bond to. The coarseness of the threads makes these screens more likely to clog while printing.
Material choices include polyester, nylon and metal. Polyester fabrics, which are typically recommended for inks used in the signage industry, absorb very little moisture compared to nylon fabrics. A low swell rate is key to maintaining print consistency, because if the fabric swells with increased humidity, the open area of the fabric is reduced. This reduces the amount of ink passing through the screen. The low swell rate of polyester also makes it a much better choice for printing with water-based inks.
Screen printing fabric comes in a variety of colors including white, yellow and orange. For fine detail work, colored material is often used because it absorbs light, preventing it from scattering. While you can produce better quality stencils, a downside is that exposure times for colored fabric can be significantly longer than exposing screens using white fabric.
White fabric by comparison can disperse or scatter light, which can destroy any detail in the stencil. Nevertheless, white fabric is often be used for simple signage applications. An advantage of white fabric is that shorter exposure times are required.
Ink deposit and print quality depend greatly on the product characteristics of the screen fabric, including mesh count, thread diameter and open area. In addition to the recommendations in ink manufacturer’s technical bulletins, a good screen print supply distributor can help you select the right materials for your needs.