Screen Printing

 

Screen Printing

Screen printing is a technique that transfers ink through a mesh into a surface. People confuse screen printing as a complicated subject but it’s not. Screen printing is the easiest method to make an unlimited amount of copies of a single design.

The Basic idea of this is to somehow cover the negative surface of the design so the ink can go just through the design.

There are plenty of ways to do this covering up (Exposing the screen).

• Emulsion (light-sensitive chemical)
• Stencil
• Waterproof materials

Emulsion (Light-sensitive chemical)

This is the main way of doing screen printing. All the mass scale printers are using this method. Basically, we use a light-sensitive chemical that is mainly made with a Silver halide (AgI, AgBr & AgCl). The emulsion is more of a gel-like substance rather than a substance that's totally liquid. The silver halide crystals, which are light-sensitive, rest in the gelatinous material; they can be incorporated onto a variety of mediums such as paper, plastic, fabric, or glass during the manufacturing process. In photographs, the silver halide reacts to the chemicals used in the film developing stage, which produces the image from the film on the photo paper. It's a slightly different process when it comes to promotional products.

Best emulsion for screen printing

Several types of emulsions exist on the market. How do you decide on which emulsion is best for you? First, you must ask yourself a few questions: What experience level do I have with screen printing? Am I a beginner, intermediate or pro? What type of exposure unit do I have? What environment will I be washing out my emulsion in? Is it light-safe What kind of ink will I be using? Asking yourself these kinds of questions will help you save time and money in the long run. Once you have these answers, you can look at the pros and cons of each kind of emulsion and compare them to your resources and experience. There are two main categories of emulsion: Presensitized or Diazo (mixed).

 PRESENSITIZED

This kind of emulsion is highly sensitive, exposes quicker, has a longer self-life, and captures fine detail beautifully. The downside of this kind is that it is VERY sensitive. In fact, it is so sensitive that daylight on a cloudy day would start to expose your image. If you wash the screens out outside, the emulsion would overexpose even the stencil. When exposing a screen, you have a 10% window of error. For example, say you expose a screen for 60 seconds. The 10% window of error would mean you could go six seconds over or six seconds under before you'd start seeing issues with your exposed emulsion. If the screen is under-exposed, parts of the image will wash out. If the screen is over-exposed, you won't be able to wash out some of the finer details of the stencil. If you are a beginner, I would hold off on using this type of emulsion for a while. For this kind of emulsion, you will want an exposure unit with a timer and a darkroom equipped with a washout booth and supplies. This kind is great to work into, but you may run into more issues if you get into this type too soon.

PROS: 

• Exposes faster.
• Longer shelf life (up to one year).
• Great detail resolution.

CONS:

• Very sensitive — not for units without timers and needs to be washed out in a light safe area.
• If slightly over or underexposed, you will have a washout issue (remember the 10% window of error).
• More expensive compared to diazo-mixed emulsions.
• Limited resistance to water exposure. (Though Green Galaxy CryoCoat is an exception to this rule)

Examples of pre-sensitized emulsions are;

Cryocoat, HiFi Photopolymer, and SVP Photopolymer.

DIAZO (MIXED)

Diazo (mixed) emulsion requires the addition of a Diazo powder before printing. This must be mixed into the entire base prior to opening and printing for the first time and is often referred to as “sensitizing” the emulsion. When you mix the diazo, make sure to use water with a neutral pH balance like distilled water. You don't want to use tap water because the minerals within the water make interfere with the diazo.

Once mixed, the emulsion may last for two months if you store it in the fridge. Do not store it in an environment that's too cold because the emulsion can freeze. If you store the diazo emulsion on a shelf in your shop that's in the mid-70s, it could last for six weeks. If the shop is in the mid-80s, the emulsion could last 2-5 weeks. 

The diazo-mixed emulsion takes longer to expose but is more forgiving. The 10% window of error also applies to diazo-mixed emulsions. Since it takes longer to expose diazo emulsions, your window of error is larger. Say you expose a screen for 10 minutes, you could go over or under by a minute. 

Diazo emulsion works great with units that do not have timers and it can be washed out under UV light successfully (most of the time) without any issue, allowing you to wash out this emulsion outside. Diazo-mixed emulsions are also cheaper and less of a headache for those just getting into screen printing. This type is more forgiving, while still retaining high quality and detail.

PROS:

• Cheaper.
• Forgiving on under or overexposure.
• Can be washed out under UV light (as long as you have made the stencil wet before bringing the screen outside: sunlight can still expose the image).
• Offers good detail.
• User-friendly.

CONS:

• Shorter life span.
• Must mix to use.

Examples of diazo-mixed emulsions are;

Water-Based Hybrid Dual Cure, RXP Dual Cure, TX Discharge, and DCM Dual Cure.

EXPERIENCE LEVEL

Beginners, use diazo. Since it takes longer to expose, it's more forgiving to errors whether it's from exposing issues or light leakage. Remember the 10% rule, if it takes 10 minutes to expose a screen, you could expose a minute over or a minute under before you start running into issues. 

Check out our stencil cut process

https://www.youtube.com/watch?v=_k8YDT00S6E&ab_channel=SticknoBulls

we'll talk about this in the next article

Exposing the screen using with light-sensitive emulsion step by step

1. Making the screen
2. Cleaning the screen
3. Making the artwork of the design
4. Applying the emulsion onto the screen