What is Silk Screen Printing?
By Pete Apo
What exactly is silk screen printing and what are the advantages and disadvantages for this type of printing? In this article we will explore these questions and also explain the process of screen printing and the basic equipment needed.
Screen Printing or Silk Screen Printing or Silk Screening is a printing process that forces ink through a mesh screen to create a design on apparel or other substrates. It is the oldest form of printing still in use today and by far the most popular, especially for t-shirt and garment printing. When walking through any department store apparel section today it is likely that 80 to 90 percent of any printed garments you see are screen printed. Other printing methods like Dye-Sublimation, Embroidery and Heat Vinyl Transfer are a much distant second in popularity of use.
Though silk screen printing is widely used, it demands a good deal of set-up or “pre-press” work before a single print is ever produced. New technology, equipment and techniques are always evolving to make the process faster and simpler, but the basics of screen printing have really not changed very much.
Below is an overview of the silk screen printing process:
Silk Screen Printing Artwork
If you start with good artwork you will increase your chances of having a great final print. When working with customers who might not know much about artwork and how it applies to silk screen printing, education can go a long way to improve their final product.
The first step in screen printing is to create a black and white image which is later printed on transparent film or vellum paper. The black portion of the image is where the ink will ultimately be printed. Since most images are no longer hand drawn but created on a computer and then printed on an ink jet or laser printer, it is helpful to understand the two main types of digital images: raster artwork and vector artwork.
Raster artwork or pixel-based artwork is the most common form of digital imagery found on the Internet, known by familiar files names such as JPEG’s, TIF’s, BMP’s or PNG’s. Pixel artwork consists of lots of tiny pixels, that when viewed up close, have a single color in each pixel. Although this is the most common type of image, it can pose difficulties in the screen printing process. Many times the pixelized image a customer submits has low resolution, and cannot be cleanly used for a screen print. Resolution is represented by DPI (Dot Per Inch). This number indicates how many pixels a particular image consists of in a one-inch space. Images, if saved directly from the Internet, will have a resolution of 72 DPI, which is often times too low to be blown up and work well on a t-shirt or other print.
The other problem with raster or pixelized artwork becomes apparent when attempting a multi-colored print. Since screen printing uses one screen for every color printed, each color will have to be separated from the other and this is often too hard to do with a pixel image. For multi-color prints, most images will need to be re-created into a vector image in order to be color separated. To avoid having to pay additional artwork charges it is best to provide your printer with vector artwork or at the very least a high quality (300 DPI) raster image.
Vector artwork is more technical in nature, but simply put it is a type of graphic image that uses mathematical algorithms. The algorithms allow the image to be scaled or modified without loss of quality or resolution. These images are easily resized or scaled in vector computer software and are of a higher quality graphic than raster images. They also offer superior color separation systems which makes them ideal for screen printing. Vector images are created in vector software like Adobe Illustrator, Corel Draw or Sketch.
Often times screen print customers attempt to take a regular raster image, open it and save it in vector software, and assume that this makes it a usable vector image. This is a common error, as the image will still be a raster image when opened by the art department. In order for images to become vectorized they have to be created or traced in vector software. You must then save them in a format that saves the vector information – a native software format such as Encapsulated Postscript (.eps) or Portable Document Format (.pdf).
Screens are the essential component of screen printing. Printed images are set or “burned” into the screen, and ink is pushed through onto the print surface. Screens come in many sizes, and screen size is chosen based on the size of the print desired, what is being printed and/or the print machine (press) being used.
The most notable part of the screen is the mesh. The mesh is a special type of fabric which is tightly stretched over a metal or wood frame. This mesh comes in different sizes, and commonly ranges from a mesh count of 65 to 305. These varying mesh counts are used for different print styles, artwork requirements or print surfaces. The mesh count has to do with the size of the holes in the screen where the ink will be forced through. A 65 mesh count, for instance, will be much more “open” than a 305, and it will be easier to push the ink through on a 65 mesh count screen. By the same token, a 65 mesh count screen will not be able to hold the level of artwork detail that a 305 mesh count screen will.
Another thing to note about screens is that one screen is used per color. For this reason screen printing is priced based upon the amount of colors in a particular print. Screen printing takes a lot of up front setup time, but then becomes more cost effective as the number of prints go up. Ask your printer about their minimum quantities along with the amount of colors they are able to print. Typical print shops will only be able to print six to eight colors, but other larger shops with more expensive equipment can run up to 12 or even sometimes 24+ colors.
As mentioned earlier, traditional screen printing requires a lot of pre-press time. This set-up time involves artwork, screen cleaning, screen “burning,”screen and press prep, and a test print. Artwork time, as explained previously, is where artwork is printed on transparent or translucent film. Before you actually put this image on a screen, the screen needs to be prepared to receive the image. On a previously used screen, as screens can be used multiple times, the old image and ink need to be removed. This process involves the use of various cleaners to break down the old ink and emulsion on the screen, then typically the use of a high pressure washer to clean the screen with water.
After the screen is set aside to dry, light sensitive emulsion will be applied using a scoop coater. Emulsion is a light sensitive liquid with a molasses type of consistency. A thin layer of this liquid is applied to both sides of the screen, then the screen is set aside flatly in a dark room on a screen rack until fully dried.
Once the emulsion is fully dried the artwork film is “burns” the image on the screen. The process of “burning” the screen starts when you take the film with a black image on it to a light table and expose the screen to UV light. The emulsion is light sensitive, so when exposed to UV light the emulsion will harden and not be able to be washed out without the use of special chemicals.
Next, you bring the screen to a washout booth and rinse with water all the emulsion that was not exposed to light. Once this is complete and the water is dried once again, you will have a screen that is ready for final prep and printing setup. Tape is typically used to cover any parts of the screen that you do not want printed, like registration marks, description markings and/or any mistakes or pin holes. The screen is then placed on the press, positioned accurately and secured.
Next you are ready to apply ink to the screen with a squeegee and conduct a test print. Test prints are essential to verify that the image looks correct on the garment, to line up multiple colors, and to ensure that you are ready for production printing.
Once the screen is prepared and the test print looks good, production printing begins. There are many types of screen printing machines, or “presses,” both manual and automatic. Each press has varying capabilities to handle different print quantities at one time, as well as screen locations which enable multiple colors. (Though more rare, some printers simply place a screen on top of a printing surface without a machine and just hold it as they print.)
Screen printing machines offer speed, quality and consistency to prints. Machines are loosely categorized by their number of print stations and print heads. Print stations are the number of garments or print products the machine can hold at one time and the heads are the number of screens or amount of colors the machine can do. It is common for larger print shops to have automatic machines, which rotate and print using hydraulic pressure. Manual printers, however, rely on human effort to rotate and print each color. Automatic machines are faster and produce a more consistent product but are more expensive to own.
Inks & Curing
Ink is the substance used in screen printing to create the printed image. Ink is pressed onto the print surface via the screen, and allowed to dry and permanently adhere to the final product. There are several categories of ink used in the screen printing industry. Inks vary based on the finished feel or “hand” desired for the product, or based on the surface you will print on. Here are a few different types of popular screen printing inks and a short explanation of their use:
- Plastisol Inks are the most common type of ink used in the industry for apparel, with many variations. Some are used for cotton or certain fabric blends and others are designed to be used on polyester or nylon fabrics. This popular screen printing staple comes in a vast amount of colors and requires a final heat cure of around 350 degrees, depending on product.
- Water Based Inks are a good alternative if you are looking for a great feel or a soft “hand” for the final print. Water based inks are also less detrimental to the environmental. They do not require a temperature cure but air dry on their own. For this reason a major concern is their tendency to dry in the screen prematurely. They must stay wet in order to be used without problems.
- Discharge Inks are also a water-based ink, but are mixed with an activator and used for printing on darker or colored garments. This ink lightens the fabric color while depositing pigment so the print color is visible. Discharge inks only work on 100% cotton fabrics and can be rather unpredictable depending on the treatment and dye lot of the fabric.
For the purposes of this article I will focus on plastisol inks, since they are the most popular and widely used ink for screen printing. When printing on a black or dark shirt using plastisol inks, it is often necessary that a base coat of white be printed first on the shirt and then the various other colors printed on top of this. This is to ensure a bright and lasting print.
Since plastisol ink only dries when a specific temperature is reached, a flash cure unit is needed. A flash dryer is a heating element that is placed over the print in order to cure its surface so that other colors may be printed over it. After the final print is finished, the shirt is typically placed on a conveyor dryer where a certain temperature is achieved for a final cure. Usually a final cure of plastisol ink is achieved when it reaches a temperature of 350 degrees for approximately thirty seconds.
When choosing the ink and final curing method you will use, it is critical to consider all the variables involved and the material you are printing on. Consider the final use and physical environment of the printed product (for example, a printed sign may be outside in the elements all day), do research and know the specs of the ink you are choosing.
Below is a list of all the basic standard equipment needed for screen printing:
- Screen and Frame (a mesh fabric stretched over a frame
- Photo emulsion
- A dark room
- Transparent material to print the design (film or velum)
- A lamp or light table
- High-pressure washer or pressurized water
- Dryer and/or Flash Unit
- Palettes or Print machine (Press)
Benefits of Silk Screen Printing
Bright & Vibrant Prints– Silk screen printing produces multiple and vibrant colors and can be used on many types of materials from t-shirts and hats to banners and signs.
Speed and Efficiency- Once the screen has been made and the pre-press work is completed the screen printing process is quick and easily replicated.
Big Order Friendly- Higher quantities result in a lower price per piece. Quantity pricing is applied by shops because of the ease to replicate prints once the press is set up. For instance, some automatic screen printing machines can print up to 4600 t-shirts in one hour.
Good Quality and Long Lasting– Screenprints have been known to keep their vibrancy for many years, and if cured correctly they will sometimes outlast the garment or sign they are printed on.
Versatility- There are a lot of different types of screen printing inks that create various effects and textures. It is hard to find a printing method as versatile as screen printing. It can be done on almost any surface as long as it is flat – fabric, wood, plastic, and even metal.
Straightforward- It is a basic process that does not change regardless of whether it is automated or done by hand. The tools are not hard to replace and won’t become obsolete as fast as other technologies.
Cons of Silk Screen Printing
Set Up Costs- The time people spend actually printing is much less than the time they spend preparing the design and creating the screens. Unfortunately, this added prep time translates to setup costs. These costs increase when ordering low quantities, which are not advised with screen printing.
Unsustainable- While efforts have been made to create more eco-friendly inks and screens, it is still a reality that screen printing wastes a lot of water. Excess water is used to mix up inks and clean the screens, and harsh chemicals are necessary to break down and clean the inks and emulsions.
Costs Add Up- Creating a screen for each color can be a hassle. Colorful designs complicate the process and make it more expensive, which is why it is better to limit designs to as few tones as possible and speak to your printer about using halftones.
Relative Complexity- It might sound like a contradiction, but silk screen printing can be quite complex depending on the design and project because it has more steps than other methods. Because of the number of variables in the process, it is highly beneficial to choose a print shop that has extensive experience.
Always do your own research first to assess the best printing process for any given project. If you have several of the same size prints you want on a large number of items, like 1200 concert tees, most likely silk screen printing will be the best option. If you are looking for 13 custom, full-color soccer jerseys with an oversized print design, with the player’s name and number on each jersey, most likely you will want a dye-sublimation print. If you need 48 polo shirts for a business convention with a company logo printed only on the left chest, it might be best to consider embroidery rather than screen printing. With so many different print options available, it is best to find a local printer with solid industry experience and communicate clearly the needs of your project.
Pete Apo is the owner of Impact Northwest located outside Eugene, Oregon in Creswell. He has over 17+ years of experience in the garment and promotional item printing industry and is the founder of the clothing and product line The Rusty Hawaiian. Impact Northwest is a provider of custom screen print, embroidery, dye sublimation, and promotional items to Lane County and the surrounding communities.