When it comes to creating crisp and detailed stencils for screen printing, one of the most important steps is the exposure process. This involves shining UV light through a film positive and onto a coated screen, hardening the emulsion everywhere that light passes through. To achieve sharp results, the film must sit flush against the emulsion side of the screen. This is where a sheet of glass plays a key role. The right type of glass helps maintain contact and prevents blurring or light bleed.
In this article we will explore what type of glass you should use over your screen during exposure, why it matters and how to ensure the best results.
Why Glass Is Used in Screen Exposure
During exposure, the film positive must stay perfectly flat against the screen mesh to ensure a clean transfer of the design. Any air gaps, wrinkles or lifted areas can cause unwanted light to hit the wrong sections of the emulsion. This results in fuzzy lines or blocked stencil areas.
A sheet of glass is placed over the film to press it evenly against the screen. It acts as a weight, holding everything still while light passes through. The clearer and flatter the glass, the better the exposure quality will be.
The Best Type of Glass to Use
The ideal glass for screen exposure is clear float glass. This type is commonly used in picture frames and windows. It has a smooth surface, consistent thickness and high clarity which allows UV light to pass through without distortion. It is also affordable and widely available in DIY stores.
You should choose a glass that is 3 mm to 6 mm thick. Thinner glass may not apply enough pressure while thicker glass can be unnecessarily heavy and harder to handle. The size should be large enough to cover your entire design area with a little room around the edges.
Avoid Tinted or UV-Blocking Glass
It is important not to use tinted glass or glass that has been treated to block UV rays. These include some types of safety glass or coated display glass which are designed to reduce fading from sunlight. While they are useful for other purposes, they will interfere with the exposure process by filtering the light needed to cure the emulsion.
Look for glass that is labelled as standard clear glass or float glass with no added treatments.
Should You Use Tempered Glass?
Tempered or toughened glass is stronger than regular glass but it is not usually recommended for screen printing exposure. During manufacturing, tempered glass can develop slight surface distortions that affect the evenness of light transmission. These subtle patterns may not be visible to the eye but can affect the detail in a fine stencil.
For most exposure setups, basic float glass offers the best balance between clarity, flatness and cost. It is easy to clean, cut to size and replace if needed.
Acrylic vs Glass for Exposure
Some printers consider using clear acrylic or Perspex instead of glass. While acrylic is lighter and less likely to break, it can scratch more easily and may not apply as much pressure as glass. In addition, some types of acrylic slightly reduce UV transmission depending on their thickness and quality.
If you do use acrylic, make sure it is high-clarity, uncoated and at least 4 mm thick. Keep in mind that glass remains the preferred choice for most experienced screen printers.
Keeping the Glass Clean and Safe
Whichever glass you choose, it must be spotless during exposure. Dust, fingerprints or smudges can block light or create shadows on the emulsion. Clean the glass before each use with a soft cloth and a gentle glass cleaner.
Handle the glass carefully and always place it down slowly to avoid cracking the edges of your screen frame. You may want to sand the edges or use tape to avoid cuts when working with bare glass.
Using Glass in DIY Exposure Units
If you are building your own exposure setup at home, you can fit the glass into a wooden or metal frame above your light source. Some DIY printers rest the glass directly on top of the screen with a weight or clamps to hold it steady. Others build a simple press-style exposure unit that sandwiches the screen and film tightly between a foam base and the glass.
In all cases, the goal is to keep everything flat and still while UV light does its job.
Final Thoughts
The type of glass you use during screen exposure has a direct impact on the quality of your stencil. Standard clear float glass is the best option for most setups as it is affordable, flat and highly transparent. Avoid glass with coatings, tints or UV protection, and make sure the surface is clean and large enough to cover your design. With the right glass and careful preparation, you can achieve sharp, professional screen-printing results every time.