OK, where do I start? I've been printing coroplast for many years. I don't do it anymore, because not only does it kill brain cells, but well,... it kills brain cells. Here are the critical point of printing coroplast:
1. As you guys have already mentioned, your off contact is critical! You need to have enough to allow the screen to pull away from the mesh as you print, but not too much or you won't be able to get enough pressure.
2. Buy a whole case of screen opener. Screen opener will also kill your brain, but it is absolutely the only way to print with air dry ink. It's better than thinner because you can flood your screen, and lightly spray your ink to loosen it up as you print. It is also great to clean out your screen at the end of the day.
3. Use emulsion, your screen will last longer! Blockout is not tough enough.
4. Don't use tape to fill pinholes. Air dry inks eat through tape adhesive and it will only cause you problems later down the road. Use emulsion.
5. When you burn your screens, make sure that you position your images so that you don't have to move your template.
6. 155 to 195 mesh screens! I have had many suppliers recomend over 250 mesh, and you will fight it just like the guy on this post! Your ink is thin, but not that thin, and you will not have to work so hard if you at least use a 195 mesh. I don't think suppliers have ever really tried to manually print using a 250 mesh screen. 250 works great for automated printing machines, but not so great for us manual guys.
7. One post on this forum recomended using a hard squeegee. DO NOT USE A HARD SQUEEGEE! Use a 60 durometer squeegee which is the softest that I have in the shop.
8. Most of you do not grab your squeegees properly. You need to grab the sqeegee with the center of your palms directly in the corner of the squeegee, with your thumb on the back and your other fingers on the front. This gives you even preassure all across the squeegee blade, which is very critical when printing plastic. If you have a large image that covers almost the entire screen, you may need to grab another person and two man your squeegee.
9. As you pull the sqeegee, watch your mesh you will see when you have missed areas. If you miss an area, do not flood the screen again, just try to dry print it.
10. You only want to print the image once if possible. You print pulling toward yourself, then lift the screen slightly and flood the screen pushing back away from yourself.
11. Always make sure that before you pull the squeegee that you have some ink in front of the sqeegee.
12. Once you start printing, make sure you keep your screen flooded. It will allow you to keep printing for long periods of time. If you need to clean your screen, print a junker and then don't flood the screen.
13. Once you start, don't stop! Give yourself enough time do an entire color all at once. You can do both sides of the sign if you remember where you put your first print and start with it.
14. If your image bleeds a little bit, instead of trying to clean the back of the screen, just take some junkers and print three or four. That will usually take care of the problem, and you won't risk spreading a thin layer of wet ink all over the back of your screen.
15. If your image bleeds severly, and you need to clean the back of the screen, then print the image one more time on a junker, but this time don't flood. Take your screen-opener lightly spray it on the bakc of the screen. Wipe first on the bottom of the screen, then the top, then spray your cotton rag and do the bottom again. You will want to print a junker just to be sure.
16. When printing, watch the ink behind your sqeegee. If you are dragging little streams of ink behind the sqeegee that is bad. Unlike printing textiles (for the most part) this will put more ink in those areas and will show up on your sign. You want to scrape the ink nice and clean. If you are not applying consistent preassure, then you will see a "stuttering" effect both in your screen and in your print. This shows up as light lines in your print.
17. I like to reclaim my screens immediately after finishing a job. This way I know that I am getting all of the ink out of the screen. You absolutely need to clean your squeegee with screen opener and get all of the ink out of everywhere, especially in the corners.
18. Use a chemical mask! Don't kill brain cells.
Let me know if you are having any other problems, or if you have any other questions.