Why Ultraviolet UVA Blacklights Are Recommended for Screen Printing

Screen printing is a printing technique used to transfer designs onto a variety of surfaces, including fabric, paper, wood, and metal. The process involves creating a stencil, referred to as a "screen," and using it to apply layers of ink onto the printing surface.

Screen printing is widely used for making T-shirts, posters, and signs because it allows for vibrant colors and durability. It is also efficient for producing large quantities of prints at once.

There are several steps in the process.

Stencil Creation. A fine mesh screen (often made of polyester or nylon) is coated with a light-sensitive emulsion. A design is then printed onto the screen, blocking the emulsion in certain areas, which creates the stencil.

Exposure to Light. The screen is exposed to light, which hardens the emulsion that is not covered by the design. This leaves the areas of the mesh where the design is exposed to light soft and removable.

Washing the Screen. After exposure, the screen is washed. The areas that were blocked off by the design wash away, leaving an open mesh through which ink can pass.

Printing. The screen is placed on the material to be printed, and ink is spread over it using a squeegee. The ink only passes through the areas where the screen is open, creating the design on the surface below.

Drying/Curing. The printed material is then dried or cured to set the ink.

Ultraviolet UVA bulbs are commonly recommended for screen printing, especially for the Exposure step in the process of making the screen, for several reasons.

Proper Exposure of the Emulsion

UVA bulbs emit ultraviolet light in the UVA part of the spectrum (typically between 320 to 400 nm), which is crucial for exposing the light-sensitive emulsion on the screen. The emulsion is made to harden when exposed to UV light, and the UVA range is the most effective at curing or hardening it. Without the correct wavelength, the emulsion would not properly set, leaving a blurry or incomplete stencil.

Some screen printers have found an improvement in this process by switching from cool white (4100K) fluorescent tubes to unfiltered UVA blacklights, due to the increased radiant energy of ultraviolet lamps.

Controlled Exposure Time

UVA light has the right intensity and wavelength to allow screen printers to control exposure times more precisely. If a bulb emits too much or too little UV radiation, it can either overexpose or underexpose the screen, which can negatively affect the quality of the stencil and ultimately the print. UVA bulbs provide a balanced, predictable light that makes it easier to achieve consistent results.

This is one reason that UVA lamps are used, rather than UVB or UVC. In addition, UVA lamp exposure to humans is not nearly as dangerous as UVB or UVC, which can cause burns to the skin or eyes much faster.

Faster Process

Using UVA bulbs speeds up the exposure process. The hardening of the emulsion under the UVA light occurs faster compared to other types of light. This efficiency is crucial for high-volume production environments where quick turnaround times are necessary.

Because UVA lamps come in a variety of shapes and sizes that mimic regular fluorescent lamps, they are usually as easy to change out as a fluorescent bulb. They are often held in place at each end in a lampholder, and a bad lamp can be switched out in less than a minute.

UVA bulbs are often noticeably less expensive than equivalent UVB or UVC lamps of the same length, base, and wattage. They also have long lifetimes, and the ballasts that regulate power to the lamps are still widely available and inexpensive, making maintenance of the process economical.

For commercial processes, it may be a good idea to invest in a slightly more expensive ultraviolet ballast. These offer extra features, such as an LED light on the ballast to indicate whether a lamp has gone out, as well as optimized output for a range of UV bulbs they have been tested with.

Reduced Heat Emission

UVA bulbs emit less heat than other light sources like halogen or incandescent bulbs, while emitting more ultraviolet radiation. This is important because too much heat can warp the screen or distort the emulsion, leading to imperfect stencil creation. The lower heat emission of UVA lights helps prevent such issues.

Fluorescent-type lamps often run fairly cool, with the greatest heat source of the system coming from the electrical ballast. The ballast is most often located in the housing of the blacklight fixture, but it could be moved further away.

Durability of Stencils

UVA light ensures a strong, durable stencil. When the emulsion is properly exposed, it hardens thoroughly, creating a stencil that holds up over multiple print runs, producing clean and precise prints.

UVB or UVC lamps would cause more damage to the stencil. UVC especially can lead to degradation of materials with long or repeated exposures to the light source.

Filtered or Unfiltered?

Unfiltered UVA blacklights are most often recommended for screen printing. These are blacklights bulbs that have a white phosphor applied to them, making them look very similar to regular fluorescent lamp. They usually indicate a "BL" in the part number printed on the bulb to designate them as unfiltered blacklights.

What most people consider a fluorescent blacklight is actually a filtered UVA lamp, with a bluish-blackish appearance of the outside of the bulb. These can have the same ultraviolet emission spectra of the unfiltered version, but the black phosphor reduces the amount of visible light emitted from the bulb.

The UVA is the most important part for screen printing, so either a filtered or unfiltered lamp can be used in most settings, but there is little reason to filter out the visible light which can also contribute slightly to the hardening process. This is why unfiltered UVA lamps are typically used.

For more information, we've written an entire article on the difference between BL and BLB blacklights, although that article doesn't address screen printing directly.

Bottom Line

In short, UVA bulbs are used in screen printing because they provide the optimal exposure for emulsion, ensuring high-quality, durable stencils and efficient, consistent results.

In addition, they are still quite inexpensive to obtain and all parts of the system can be maintained and replaced relatively easily.