Help burning screens

[baobag]

Hi everyone!

We have problems burning the screens, and right now we are stucked and don’t know how to proceed. I did made the burning test and we’ve determined which is the best time to burn the screen (77 mesh counted in cm per square) and the result was much lower than expected, we were burning the screens always at 190” and the test with halftones was like 25” for perfect exposure. Now the screen is perfect for the first printing but the emulsion goes away partially after the first and second cleaning, also there is plenty of pinholes that appear espontaneously. Of course the screen seems to be underexposed but if we burn it for few more seconds the halftones are not revealed.
We use an exposure unit from ryonet with 8 fluorescent tubes (30w per tube) and a diazo emulsion named Variocoat 10915 WR.

Best regards from Barcelona!

[Chuggins]

Hi from the States!

Lets get some details - can you tell me:
1) Your relative humidity where you are drying the screens
2) what you are using to output your film
3) are you using a RIP to create your films?
4) what is the LPI you are printing your halftones at? Angle?
5) Do you have the 21 step greyscale calculator? Also known as a Stouffer strip - if you have it, what are your results?
6) What ink / ink type are you using?

My first reaction is that you are definitely underexposed. But knowing more about your emulsion, film and the ink you are using goes a long way to determining whats going on.

[baobag]

Yes, it seems to be underexposed.

1- Don't know about relative humidity, but where we coat the screen is inside a dark room.
2- We use transparency film. The artwork printed is not full opaque but it's very dark and black.
3- Don't know what is a RIP. We ue photoshop to prepare the designs.
4- For this specific artwork (first time doing four colours halftones) we used 40 LPP at these angles: 0; 15; 45; 75.
5- we don't have that calculator, but we did the test with a printed file with varied lines, halftones and text in different sizes that has 10 exact copies, we did the test by covering parts of the design and exposing it at different times.
6- We use water based inks. I don't know how to say it in english but we separate them in two different groups; "acramines" for inks (usually dark colours) that are printed on white (or so) shirts and "laca" for inks printed (usually white colours) printed on black (or dark) shirts.

Yesterday we did a few tests, it is possible that the different layers of coating determines the amount of time you need for burning the screen? We burnt two screens one with 1 coating print side and 1 coating squeegee side andthe other one 2/2 coating, the first one just needed 25" but the second one was good from 30" to 70".

Thank you very much! We discovered a whole new world and learning in this web/blog we didn't know before.

[Chuggins]

Yes, it seems to be underexposed.

1- Don't know about relative humidity, but where we coat the screen is inside a dark room.
2- We use transparency film. The artwork printed is not full opaque but it's very dark and black.
3- Don't know what is a RIP. We ue photoshop to prepare the designs.
4- For this specific artwork (first time doing four colours halftones) we used 40 LPP at these angles: 0; 15; 45; 75.
5- we don't have that calculator, but we did the test with a printed file with varied lines, halftones and text in different sizes that has 10 exact copies, we did the test by covering parts of the design and exposing it at different times.
6- We use water based inks. I don't know how to say it in english but we separate them in two different groups; "acramines" for inks (usually dark colours) that are printed on white (or so) shirts and "laca" for inks printed (usually white colours) printed on black (or dark) shirts.

Yesterday we did a few tests, it is possible that the different layers of coating determines the amount of time you need for burning the screen? We burnt two screens one with 1 coating print side and 1 coating squeegee side andthe other one 2/2 coating, the first one just needed 25" but the second one was good from 30" to 70".

Thank you very much! We discovered a whole new world and learning in this web/blog we didn't know before.

To much moisture in the emulsion will make it difficult to keep details. The emulsion will be "gummy" and not release fine detail. Make sure your screen drying area is under 40% relative humidity for several hours before exposing, make sure you have fans moving dry air over the screens that are drying.

RIP's are used to create halftone dots when printing on film. A RIP will also "hack" your printer to make sure your films are dark enough to stop all UV light. If your films are not actually stopping enough UV light, you will always loose fine detail. One way to check for this is to print a small solid square, expose for your normal time and check the edge quality of the exposed area. if you see any "tearing" of the emulsion on the edges or its difficult to spray out the whole image - the image area is kinda solid - your film is not stopping UV light.

40 lpi is kind of low, but if that works for you go for it! Try 22.5° angle for all films next time when printing on garments.

The "step wedge" test will only show you the level of detail you can keep at certain exposure times. it DOES NOT tell you if your emulsion is fully exposed.

If your emulsion is underexposed, you will experience your screen falling apart after a couple of prints. A properly exposed screen will last for hundreds if not thousands of prints - This 100% depends on using the correct emulsion for the ink you are using. Some Water Based inks have a LOT of solvents in them. This makes choosing the correct emulsion VERY important.

Exposure times are directly related to emulsion thickness. Thicker stencils will take longer to fully expose.

Hope this helps!