There are lots of emulsions out in the world of screen printing. The most common two types are presensitized and diazo mixed, or dual-cure emulsion. Which emulsion is best? That answer depends on the type of shop and darkroom setup. Here’s a guide to both types of emulsion.
Presensitized emulsion does not use diazo. Diazo is an agent that makes emulsion sensitive to light. Presensitized emulsions like Baselayr Complete or Baselayr Plastisol are ready to go as soon as it arrives in a shop.
Presensitized emulsion is highly sensitive. It can capture intricate detail and burn quickly. This type of emulsion also has a longer shelf-life. The downside? Presensitized emulsion is extremely light-sensitive. A little bit of UV light exposure can cause issues with the emulsion. Darkroom conditions should be optimal for this type of emulsion. A vacuum exposure unit will also help increase the performance of presensitized emulsion.
When exposing a screen, each screen has a 10% window of error. Say a screen takes 60 seconds to expose. The 10% window means six seconds on either side before the emulsion will start to have issues. Presensitized emulsions burn at a rapid speed, so the window of error is small. For example, a printer using Baselayr Complete Emulsion on a 156 white screen on a Baselayr V3648 Exposure Unit would burn the screen in 25-35 seconds. Their window of error would be 2.5-3.5 seconds before issues would arise.
The underexposed emulsion will cause parts of the image to wash out. Overexposed emulsion won’t rinse out finely detailed parts of the design. Because of these potential risks, screen printers should have optimal darkroom settings: a UV-safe, warm, dry space and a vacuum exposure unit.
QUICK GUIDE FOR PRESENSITIZED EMULSION
Here’s a quick guide for printers using presensitized emulsion:
- Exposes faster
- Longer shelf life
- Great detail resolution
- Very sensitive: not for units without timers or darkrooms not dialed in
- A small window of error
DIAL IN THE DARKROOM
The darkroom needs air circulation, humidity control, a place to store screens, and yellow light bulbs. It doesn’t have to be a fancy setup. Here’s a list of supplies to create the best darkroom:
- Light-safe bulbs: yellow or red bulbs are the most common
- A dehumidifier to keep humidity below 40%
- A screen rack (homemade or purpose-built) to dry screens
- Fans to create airflow across the screens
- A hygrometer to monitor temperature and humidity in the darkroom
DIAZO MIXED EMULSION
Diazo mixed emulsion, also known as dual-cure emulsion, requires the addition of an agent called diazo. Diazo is a powder that makes the emulsion sensitive to light. Emulsions like Baselayr Long Lasting are dual-cure emulsions.
Dual cure emulsion is more forgiving and great for screen printers without dialed-in darkrooms. Once the diazo is mixed into the emulsion with pH-balanced water — distilled water, for example — it can last for two months.
Diazo mixed emulsion takes longer to expose and is more forgiving. A little bit of UV light won’t have the same damaging effect on dual cure emulsion as it will on presensitized emulsions. For printers with compression units or imperfect darkrooms, this emulsion works wonders.
MIXING DUAL CURE EMULSION
Mixing dual cure emulsion takes a few steps. Here’s a walkthrough:
- Grab a bottle of distilled water and let it reach room temperature. Find a stir stick like a popsicle stick or paint stick.
- Fill the bottle of diazo with the distilled water to the fill line. Replace the cap and shake well for at least 30 seconds.
- Once the diazo has dissolved into the water, pour it carefully into the bucket of emulsion.
- Using a stir stick, slowly mix from the bottom up until the diazo is completely combined with the emulsion. Take this process slowly to keep as much air out of the mixture as possible.
- Clean up the bucket of emulsion and replace the lid. Set the mixed emulsion aside for 3 hours to allow any air to settle out.
Once the emulsion has been mixed and settled, the coating process can begin.
QUICK GUIDE FOR DUAL CURE EMULSION
Looking for a quick guide to diazo mixed emulsion? Look no further:
- More forgiving in darkrooms without perfect light blockage or on exposure units without timers
- Offers good detail
- A larger window of error
- Shorter lifespan
- Must be mixed to use
Choosing the proper emulsion not only depends on the darkroom conditions but the exposure unit used for creating screens. The best type of exposure unit is a vacuum unit. This exposure unit uses vacuum suction to create the best positive contact between the light source, screen, emulsion, and film. Compression units require physical compression to create this positive contact.
LED light is the new standard when exposing screens. It optimizes the spectrum of light. Exposure time and correct emulsion curing are both heavily dependent on using light of the correct wavelengths. This light must be able to both penetrate the coating, as well as promote cross-linking.