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Photolithography: 1 (and only) concern
It is one of the essential elements in the screen printing process. The photolitho's function is to block the UV light that hits the screen during the exposure process and thus engrave a final artwork/design on that same frame.
⧗ reading time - 8 to 10 minutes ⧗
What does photolithography actually consist of?
It consists of a transparent/translucent sheet, printed or drawn, with the desired final artwork. Bearing in mind the previous explanation and its contextualization in the recording processIn this case, we can conclude that the aim is to block the UV light that hits the canvas in order to obtain a matrix. In this sense, it's important to have a transparent/translucent sheet, and secondly, our final artwork will have to be printed or designed in such a way that the light doesn't hit the emulsified canvas.
While the transparent sheet can be acetate, transparent polyester or tracing paper, enough to let the light through and harden the emulsion on the canvas, it is important that the final artwork/drawing is as dark and opaque as possible.
In the image, a photolitho printed by inkjet in our workshop.
Screen printing engraving process
In order to better understand the function of the photolithograph, it's best to explain the process of exposing a screen, putting it into context. The engraving of a screen is achieved through a process of exposure to light and one of the steps is to apply photosensitive emulsion to the screen. Once dry, the emulsified canvas is exposed to light, in our case UV light, together with the photolith (which is placed between the light and the emulsified canvas), thus allowing the engraving of a final artwork, i.e. a mask or matrix that blocks the passage of ink, but lets the ink pass through the areas we want.
This recording is possible because in the exposure process, there is an element that will block the light. Since emulsion is extremely sensitive to light, the light that hits the screen will harden the emulsion, making it impermeable. On the other hand, if there are protected areas to prevent light from hitting the emulsion, it will not harden on the screen and can be removed during the washing phase. In this sense, once applied to the screen, the emulsion that is not subject to UV light will not harden on the screen and can be removed with water.
One and only concern: Maximum opacity
One and only one concern may be an exaggeration, but we can't stress enough the importance of maximum opacity in the photolithograph. The opacity of the ink, whether the photolitho is printed or drawn, can compromise the entire exposure process and in the end the only solution will be to remove the emulsion and repeat the process.
A photolitho, whether printed or drawn directly onto the sheet, that is not dark and opaque enough will allow light to pass through the photolitho. This will make it difficult to wash and remove the emulsion you want to remove. This is because the incident light will begin to harden the emulsion, making it very difficult to remove, precisely in the areas you want to remove.
In the end, it won't engrave the design in the best way and you'll lose detail in the matrix.
Also in analog form, any element that is thin and has maximum opacity will work as a good mask. An example of this is cardboard.
How do you check the quality of a photocopy?
An effective way to check the quality of a photolithograph is to observe it against the light (ideally in a light box), to detect its opacity, but also flaws that can be corrected. Sometimes you don't realize how poor the quality of a photolithograph is until you compare it to a higher quality photolithograph. You realize that one has a homogeneous, dark spot, guaranteeing a high percentage of opacity, while the other has a heterogeneous spot with different shades of grey.
Is it important to have a top quality photolithograph?
right: laser printing in a print shop / left: inkjet printing at FICA
Attention
We shouldn't generalize between the two types of printing on the basis of the images above. There will certainly be higher print quality when it comes to laser. Just as there will be lower quality in inkjet printing.
To do this test, we went to the first (ordinary) print shop we found, which printed on acetate. While the inkjet was printed at FICA.
In practical terms, with a poor quality photolitho, after exposure to UV light and when the screen is washed, you won't be able to create the matrix with the desired definition. No matter how hard you try to wash the screen, there will be areas, depending on the complexity and fine details of the final artwork (the number of threads can also have an influence), that you won't be able to engrave.
Any ink that is not sufficiently dark and opaque will allow light to hit the emulsified screen and thus start the process of hardening the emulsion, making it difficult to remove. A photolitho with superior quality (and the right exposure time), at the time of washing, will allow the emulsion to be removed more easily, creating a well-defined matrix.
This doesn't mean that a photolitho that isn't too opaque and uniform will work. It will depend on the exposure time and the type of image we want to expose. An image with very fine details probably won't record in its entirety due to overexposed areas where the emulsion has hardened where it shouldn't have.
At FICA, we have a screen-opening press, set to a certain duration for engraving matrices with top-quality photoliths.
Recording a screen under UV light is a long subject (and one that varies depending on a number of factors), which you can read about in another article.
What is screen printing?
Digital photolithography
Therefore, when it comes to printing a digital photolitho, the choice of printer and its quality is very important. At FICA we print our photolithographs on an ink jet printer - Epson Surecolor P800 - with the necessary characteristics to guarantee superior quality. It is suitable for photographic printing, so it is compatible with transparent polyester sheets and prints with ultra-high black density.
However, a printer of its own won't do the job by itself, and requires that the file we're going to print has the characteristics indicated for printing. To do this, it's important to work on the file in such a way as to make all the spots and lines as black as possible.
Equally important is the transparent sheet, our advice, compatible with inkjet printers, is transparent polyester sheets. They're recommended if you're looking for professional quality. With these acetates you'll get excellent results due to the quality of the polyester, which is specially coated on one side. This guarantees very high ink coverage and opacity, so that your final artwork is printed with very good definition and sharp details. The choice of this element alone does not rule out the quality of the printer.
There are specific acetates for laser and inkjet printers, due to the characteristics of each type of printing. So make sure you use the correct acetate for each printer.
For laser-printed photoliths, there is a product to darken the print. The blackening spray converts gray print into opaque black.
Photolitho printing
At FICA Oficina Criativa, we print photolithographs with superior quality. The price of the photolitho varies depending on the size and percentage of printing (-50%/+50%).
Acetates
As well as screen printing services, you can find a range of materials and consumables in the store, including acetates for inkjet and laser, in A4 and A3.
Analog photolithography
You can also create analog photolithographs without creating digital files and using a printer. You can draw directly onto a transparent sheet, as long as you use the right elements, always remembering that it must be as dark and opaque as possible. To draw, you can use certain elements (and test their characteristics), such as India ink, fountain pens and posca (black), oil pastels or even 8B charcoal pencils.
These elements allow you to draw and create detailed designs, but you can also use other materials, such as Ulanoand other less obvious materials, such as plant leaves, which guarantee an excellent level of opacity. You can use any opaque, thin material you have at hand, the most common being cardboard.
Analogically, you can spread out plant leaves and cut cardboard into geometric shapes, creating final artwork without drawing. But it's also possible to draw, with the right elements, on plant leaves or acetates. These can be used directly in the screen printing process.
Analog to digital?
You can also draw by hand, scan and print on an acetate. Once scanned, you'll need to edit the image and increase the black levels to make it as dark as possible, in contrast to the white that corresponds to the void.
When you edit the image, once it has been digitized, try to clean up the noise around the drawing, i.e. all the points that may exist and you don't want to be recorded.
Alternatives?
It sounds absurd, but an alternative is possible with vegetable oil. You can draw on a sheet of lightweight copy paper (80-90gsm) and brush on vegetable oil evenly. When the sheet soaks up the oil and takes on a translucent appearance, you can use it as a photolithograph. It has its limitations, but it's a curious way to do it without using acetates, transparent/translucent sheets and digital printing.
Conclusion
In this article, we discuss the importance of the photolithograph in the screen printing process. The photolitho is a transparent or translucent sheet, printed or designed with the desired artwork, whose function is to block UV light during screen exposure. This blockage allows the final artwork to be engraved on the emulsified screen, and it is crucial that the photolitho is dark and opaque to guarantee the quality of the process. And this can also correspond to an opaque 100% card.
We reviewed that the engraving process involves applying photosensitive emulsion to the screen, followed by exposure to UV light along with the photolitho. The quality of the photolitho is essential, as an insufficiently dark photolitho can compromise the exposure and result in areas of the emulsion not hardening properly, making it difficult to remove in the wash phase. Conversely, it results in the creation of a matrix capable of selectively blocking the passage of ink
The importance of the opacity of the photolithograph is crucial, highlighting the need for a careful approach when choosing and preparing the material. Observation against the light, preferably in a light box, may be necessary to assess the photolithograph if there are doubts about its quality. Maximum opacity and homogeneity are determining factors, as a failure in opacity can compromise the exposure, making it challenging to remove the emulsion efficiently in the washing phase.
Obtaining a well-defined matrix, capable of faithfully reproducing the final artwork, also depends on these apparently subtle details, but which have a major impact on the final result.
For this reason, we stress the importance of choosing the right printers and acetates when printing digital photolithographs. In our case, we use an inkjet printer, the Epson Surecolor P800, which has 4 black ink cartridges (matte black, photo black, light black and light light black), and transparent polyester sheets, suitable for the type of printer, to guarantee printing with superior opacity and black density.
In addition, there are analog alternatives for creating photolithographs, such as drawing directly on transparent sheets using materials such as India ink, posca pens or charcoal pencils. As well as using opaque elements, whether they are suitable for this purpose, such as Ulano sheets, or opaque sheets such as cardboard. For example, we've already tested using plant leaves, which worked perfectly, blocking out the light remarkably well.
However, it is also possible to go from analog to digital. To do this, you need to scan a drawing and ensure that the black levels are at their maximum (you need to edit the image in software such as photoshop or similar), and to print on an acetate is another solution.
In short, there are many different options for how we can create a photolithograph and you can choose for various reasons, be they aesthetic, convenience or financial. The important thing is that the photolitho doesn't compromise the engraving process and that we don't have to repeat it again.
It's part of a whole. Of a multi-stage process, not an isolated act. It's essential that all the other steps, whether it's the choice of materials, the application of the emulsion or exposure to light, are aligned and well executed, in order to define the final quality of the matrix.