No matter what we put on paper—inks, varnishes, coatings, foil or film—we must not forget to pay attention to one of the biggest factors in how well paper is printed and finished. That is, of course, the print shop environment.
For our purposes suffice it to say that relative humidity and temperature play a central role in the successful printing and finishing of any paper. But you’d never know this by the way we see some press rooms operate. I’m guilty too.
Many years ago I was running an AB Dick on a beautiful spring morning…in Florida. I decided to work with the delivery bay door wide open to the world outside. Then my boss arrived with fire in his eyes, greeting me with “What the hell are you doing?” and slamming the door shut. That was my first lesson in press room environment.
Paper is hygroscopic which means it will either absorb moisture or release moisture into its environment to reach the same state of humidity as its environment. When it stops releasing or absorbing moisture, it is at equilibrium. For best printing and finishing results, paper should be at equilibrium before printing. Paper should also be at pressroom temperature before printing.
But remember, any time the relative humidity or temperature changes, water absorption or release will start all over again until a state of equilibrium is once again reached. This can happen before, during and after printing and finishing. This is also another reason why it’s important to have a stable environment.
Temperature is fairly easy to control with air conditioning and heating systems. Relative humidity is not so easy. Relative humidity changes with the air temperature. Warm air holds more moisture than cool air. A 70 degree room will hold more moisture than a 55 degree room, hence the term "relative humidity." In the US, relative humidity (RH for short) can vary form 10-90% depending on location and time of year.
To add to the mix: humidity and moisture content where the paper is manufactured make a difference in how the paper will perform. To simplify things, most paper companies manufacture their paper so that the end user (the printer) gets best results at an RH of about 40 to 50% and a temperature of 72º. When the print shop environment strays from the recommended conditions, problems arise such as wavy or tight paper, static, drying problems, picking and delamination. (source: Sappi Paper On-Press Troubleshooting)
What does this mean to the aqueous coated print job? Put simply, the higher the relative humidity, the slower that inks and coatings will dry since coatings and inks dry through evaporation. When you slow down the process from what the ‘recipe’ recommends, you affect the chemical reactions taking place.
- Slower drying can change the hardness and slipperiness of the print surface.
- If offset litho inks dry at a different rate than the coating, or if aqueous is applied to inks that aren’t dry, you can get poor adhesion of the coating. I’ve seen it literally crack or pop off the sheet, similar to the way a sheet of ice will break. This can also be a result of the wrong coating/ink combination. There are hundreds of job-specific coatings available, which in itself is testament to the importance of getting all the right ingredients for your specific print job.
- Slow drying usually means the press operator increases dryer temperatures which in turn affects ink and coating properties.
If no standards and controls exist, a changing environment will make it dramatically harder to produce a complicated coated job with good results.
For instance, say your RH is 45% one day and 65% the next. How will that affect job print quality, aqueous coating and drying? A job that runs perfectly one day may be totally different and not register the next even though everyone will swear that ‘nothing has changed.’ The folding machine operator ends up fighting a "folder" problem that is really a relative humidity problem which caused variations in drying, tackiness, curl, etc.
Troubleshooting is more complex with each additional variable. But if your temperature and RH are the same every day, you have two less items to contend with. It becomes a bit easier to figure out which factor is the problem. And the truth is you will have fewer problems.
Ignoring the subject of pressroom environment will cost you tens of thousands of dollars over the years, not to mention endless aggravation. I always suspect environmental problems any time I get questions about strange happenings with what should be normal jobs.
Yet problems can arise no matter how careful we are and there can be defects in paper, inks and coatings. If you are stuck with a problem job, there are those folding remedies we mentioned in the previous article to help with register. Regarding the environment, several readers wrote to say that they use humidifiers and/or vaporizers to significantly boost the relative humidity in the vicinity of the folder. This probably puts it above recommended levels but we are now talking about an emergency fix. Some swear by the use of fabric softener in the vaporizer to help with register.
If you haven't already done so, a study of your pressroom and bindery environment is worth your time. There are plenty of troubleshooting books and studies available on the pressroom environment. Paper manufacturers are a good place to start, and there are usually very specific recommendations for preparing various papers for printing. Coating, film and varnish manufacturers also have very specific recommendations for hundreds of products.
The Printing Industries of America has an extensive bookstore and training curriculum. Sappi paper has a good library of technical studies here if you really want to get down to studying. The Graphic Arts Education and Research Foundation (GAERF) is also a comprehensive training resource.
If you simply monitor your print shop’s relative humidity and temperature you ensure that the ‘recipe’ your print job calls for has a significant chance of satisfying everyone involved. The odds are you won’t have to troubleshoot nearly as much as you once did. Unless it happens to be a beautiful spring day…
As always, we welcome your comments and experiences below!