Best Glass for Screen Print Exposure

What Type of Glass to Put Over Screen Print Exposure?

If you’re setting up a screen-printing exposure unit or doing DIY exposure at home, one of the key elements you’ll need is a sheet of glass to hold your film positive tightly against the screen during exposure. But not just any glass will do. The type of glass you use can make or break the clarity of your exposed stencil, and ultimately, the quality of your print.

Let’s break down what type of glass is best for screen print exposure, why it matters, and what to avoid.

Why You Need Glass for Screen Exposure

During the exposure process, your aim is to make sure the film positive (your printed design) is pressed as flat and tightly as possible against the emulsion-coated screen. This ensures crisp lines and sharp edges. If there’s a gap however slight between the film and the screen, light can scatter and bleed around the edges, leading to blurry or underexposed areas.

A sheet of glass placed on top of the film and screen helps eliminate those gaps. It flattens the image perfectly and allows even UV light exposure across the whole design. But to do that job properly, the glass needs to have a few key qualities.

Use Clear, Untreated Float Glass

The best type of glass for screen print exposure is clear float glass. This is standard, plain glass that hasn’t been tinted or treated to block UV rays. It's the same kind of glass used in picture frames and greenhouse windows. It's affordable, easy to find and transmits UV light well which is exactly what you want during exposure.

Float glass has a smooth, flat surface which ensures good contact between your film and screen, reducing light refraction and allowing for sharper detail.

Avoid Tempered or UV-Protective Glass

Tempered glass is commonly found in furniture or safety applications. While it’s stronger and safer in some situations, it's not suitable for exposure units. Why? Because tempered glass often contains additives that reduce UV transmission. And UV is the very thing you need to harden the emulsion properly during exposure.

Similarly, avoid any glass that has been treated to block UV light such as glass used in energy-efficient windows or sunglasses. These will significantly reduce the effectiveness of your exposure, making it harder to achieve a clean stencil.

What Thickness Is Best?

A thickness of 4mm to 6mm is ideal for most exposure units. It’s heavy enough to press the film flat against the screen without flexing or cracking easily, but not so heavy that it becomes awkward to lift and handle.

If your screen is particularly large, you might want to go towards the thicker end for stability. Just make sure your exposure unit can handle the weight and that the glass is fully supported underneath.

How Big Should the Glass Be?

Ideally, your glass should be slightly larger than the area you’re exposing. It should cover the entire printed area of your film positive, pressing it evenly onto the screen. If it’s too small, you risk getting uneven contact and patchy exposure, especially around the edges.

Be sure to keep the glass clean dust, smudges and dried ink will block or distort light, and that can show up in your stencil.

Safety First

Glass is sharp, fragile and heavy. Always handle it carefully, wear gloves when needed and make sure the edges are either polished or taped to avoid cuts. If you’re building your own DIY exposure unit, be sure the glass is properly supported and can’t slip or slide during use.

A Final Thought

The right glass might seem like a small detail, but it plays a big role in screen printing success. Clear, untreated float glass gives you the best UV transmission and the sharpest stencil, helping your designs come out clean, crisp and ready to print. Whether you're working with a homemade lightbox or a professional exposure unit, investing in the right sheet of glass is one of the simplest ways to improve your results.