Exposure & emulsions

Where does exposure fit in the screen printing process?

What emulsion should I use?

How long can I keep emulsion?

How do you get rid of pin holes?

How long should I dry my screen?

 

Let’s go back to basics. Screen printing is also a stencil method of print making in which a design is imposed on a screen of polyester or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced into the mesh openings of the mesh by the fill blade or squeegee and onto the printing surface during the squeegee stroke.

Emulsion is the means of making impermeable substance. Light sensitive, thick liquid which coats the screen. When the screen is exposed the emulsion hardens and unexposed areas drop out to leave the stencil.

So you have your positive artwork

Making Your Artwork Positive.

Now this process is one of the most tricky in screen printing and needs to be done thoroughly and carefully. Never skip on any step especially drying.

The first step is to degrease your screen

It is very important to degrease the screen prior to coating the screen with emulsion and exposing the artwork. A screen that is not degreased will have problems exposing and increase the chance of pinholes, the emulsion will not adhere to the screen properly and might washout before the image can be seen. The emulsion could also not wash out at all. Make sure you let it dry thoroughly before you coat it with emulsion.

Choosing your emulsion

There are many emulsions on the market and every printer will have their preferred type and brand. Some emulsions come pre sensitised and ready for use and some will need mixing. Our emulsion is a 2 part emulsion, its comes with sensitizer.

Shelf life is different depending on when the sensitizer is added . Separately both parts can last from 12-24 months. Once mixed the emulsion lasts 6-8 weeks (its life can be prolonged slightly if it is stored in a cold place, like the fridge).
Always make sure you have chosen the right emulsion if you are using Plastisol inks / Solvent inks you need a Solvent resistant emulsion like Ulano Proclaim or if you are using Waterbased inks you need a water resistant emulsion like Ulano 925wr. We also sell Autosol emulsion, which is a good dual purpose emulsion, it can be used with both waterbased and solvent inks.

Location

Make sure you are doing all parts of the emulsion and exposure process in a light safe environment with NO outside or bright light. This includes mixing your emulsion, coating your screens, drying screens, exposure, and washout (as soon as screen is exposed as long as you wash it out straight away this is not a problem)..

A costing trough is the best way to get an even coverage of emulsion. Always check to make sure the coating trough has a straight edge and no damage otherwise you will get poor coverage and you can snag the mesh.

Here are a few things to watch out for if your images are not washing out correctly:
That you are not coating your emulsion too thick. One coat both sides should do it, with a nice even spread. There are sometimes where you may need some extra coats e.g. printing transfers and printing white onto a dark garment but you will need to adjust the exposure time.

Coating Your Screen

That your film positive is very opaque and dark. If you hold it up to a light and can see through it, you need to double print your film to achieve a more opaque image.
That there is positive contact between your screen mesh and your positive film. If the film is not pressed completely against your mesh then you will get light reflection between your positive and your screen which will result in a blurry and not clear image. That’s why the glass is so important.
That you are exposing your screen for the correct amount of time, which will depend on the exposure system you are using. If you are unclear on exposure times please feel free to ask us. The simple run of thumb is – if the emulsions washes off too easily and you start to lose the stencil then the screen is underexposed and if after prolonged washing the stencil does not come through they you are likely to have overexposed the screen.
After you coat your screen, you want it to be COMPLETELY dry  before you expose the screen.

Exposing the image

Exposing Your Screen

To transfer your image onto your screen you will need to use an exposure unit. There are many different exposure unit set ups on the market and each has a different light source. Each having different light sources. UV, halide and halogen are very popular in the UK. Our Wicked exposure lamp has a 1000 watt halogen light source and is provided will all waterbased and Plastisol kits.

We also sell Actinic and Metal Halide exposure units which are able to expose quicker (typical exposure time is 2 minutes for an Actinic unit and 16 minutes for the lamp depending on size of the screen) than the cheaper lamp solution. Some units include built in drying cabinets making the whole process a lot faster.

A very large part of your decision if you are new to screen printing will be your budget and space. Exposure units vary dramatically in price. The WPS Lamp is small and compact,

After exposing and rinsing out your exposure, you want your screen to thoroughly dry (and ideally harden your screen, by exposing it to the sun or your exposure unit) before inking and printing. This process is often called double baking.

Before inking you need to use blocking tape (can use brown tape or professional blocking tape) to cover up areas on the screen not covered by the emulsion. Also fix any pinholes either using blocking tape or using screen filler which is a liquid emulsion, make sure you choose the right one as there are different versions for solvent and water resistant stencils.

Now you are ready to print

Registering Your Print

When you get bigger there are other options like the Revolutionary Riso QS200 Designed for the Professional Printer which prints directly from your computer onto a screen. Then the whole process takes under 5 minutes!

Have you got problems with poorly exposed screens?

Do you have pinholes, stencil breakdown, or poor quality prints?

Are you wasting production time and materials?

Also see our blog post Creating a screen using traditional exposure methods

Following our post last week about exposure timings taken from our FAQs we were asked for more details. Transfering your artwork onto a screen is the most complicated part of the screen printing process, do it incorrectly and you will create problems for yourself. Here we have an example of a design with pin holes.

Design

The principle of the screen is that there is a mesh with little holes. The ink is forced through the little holes on to the substrate (t-shirt, paper, cap) underneath. We only want the ink to go through in certain places, in this case the red lettering. So we create a stencil where our design is black and the background clear and we use red ink. We put emulsion on the screen and expose the screen using the stencil. The emulsion hardens and blocks the little holes in the mesh where it reacts to the UV light. The stencil blocks the UV light in the areas of the design. If the exposure is not correct we can end up with some of the little holes not blocked at all (pinholes) or not to a correct depth or hardness. Then when the squeegee comes along and scrapes the ink along the screen, as well as forcing the ink through the mesh it also scrapes of a little bit of emulsion and makes more holes. This picture shows some pinholes just above the e.

So correct exposure is the key to getting a completely covered, really hard, good quality careen that will last for a long print run.

Most exposure lights and chemicals give you recommended exposure times but these can require fine tuning. The UV light has to harden the emulsion on BOTH sides of the screen. It is a chemical reaction between the emulsion and the UV light that hardens the emulsion. Where the emulsion is hardened the mesh is blocked. The stencil protects the emulsion from the light so when the screen is washed that part of the stencil allows the ink through the mesh.

If your screen is under exposed there is not enough emulsion is on the screen. Then as the squeegee rubs on the screen it removes the emulsion leading to problems.

In this example above the stencil is the red lettering. Therefore the clear area of the screen must be complete blocked by hardened emulsion. But we have some pinholes which means stencil filler or tape are required to block them. Correct exposure will stop this occurring.

Lamps age and whilst it might look like they are doing their job they are not producing enough UV to properly expose the screen

Most manufactures give timings for your lamp/emulsion but it is worth doing a step test at 2-3 month intervals to check for any degradation and so you can fine tune your exposure times.

A step test is a graduated exposure of your screen to see which time has the best results

You can get a step wedge or a Step Transmission Gray Scale,, but you don’t really need one. You can use a piece of thick cardboard to test exposure time. Start by burning the whole screen 1 minute less than the recommended time for your unit/emulsion. Then cover about an inch of the emulsion with the cardboard. Every 30 seconds, cover another inch of the emulsion by sliding the cardboard forward. Once the screen was fully covered by the cardboard, the test is complete. Wash out the screen to find which exposure time produced the cleanest result. Keep a record of this and then you can compare results in 2-3 months time.

A simple inexpensive unit like the WPS exposure unit has all the features you need.

Light Source: 1000 watt halogen light.  More efficient than two x 500 watt light units, where the light can overlap and cause uneven exposure.

Adjustable Height: For adjusting the exposure unit

For 12″ x 16″ screens – 10 inches and expose for 6 minutes

For 20″ x 24″ screens – 20 inches and expose for 15 minutes

For 31″ x 24″ screens – 23 inches and expose for 20 minutes

Important: Foam is charcoal in colour to absorb the light and not to reflect the light as lighter colours will do. This will prevent sharp toothing on fine line work and text when exposing.

 

You will need to supply your own sheet of glass. The glass needs to be at least 6mm thick double glazed or 10mm thick either single/double glazed.  The thicker the sheet of glass, the more heat from the lamp it will be able to withstand so you can reduce the distance of the lamp from the screen to reduce your exposure time.

 

This unit will do the same job as a unit costing £500.00+ it just takes a little longer to expose the larger screen.

 

For the professional printer requiring larger volumes the Heavy Duty Exposure Unit is more sutable.WPS sells a  range of  heavy duty exposure units manufacturer in the UK which come in three standard sizes and a number of variations to suit customer requirements. The three standard models have all the same features, powerful suction pump, touch up lights, safety switch etc. There are a number of power options available 1, 2, 3, 5 & 6 K/w versions.

 

Two controller options are available, a timer system which comprises a digital timer, and switch for the vacuum and fluorescent tubes as well as a power switch, or our Magellis PLC control system this incorporates an LCD display, time settings and all programable features are accessed from the screen. This unit can also incorporate a light integrator which compensates for lamp degradation over time and automatically adjusts the length of the exposure.

 

Each unit is fitted with fluorescent tubes for artwork positioning and stencil touch up. The powerful suction pump ensures intimate contact between artwork and screen giving perfect exposures every time.

 

Each exposure unit is suitable for direct or indirect films and can be used in an open work room.

 

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