Offset Press: Plotting a Course for Color Consistency
WHAT DOES it take to keep an offset printing press performing consistently? It's a question I have been asked more times than I can count — and with good reason.
I use the analogy of driving a boat to describe what a press operator must do to make a press match a proof over time. If you’ve ever driven one, you know that a boat moves in unintended directions, and you must constantly correct for the forces of wind and current to stay on course and reach your destination. Just like a boat captain, press operators need to make ongoing adjustments to correct for external forces and stay on course over the length of the press run.
There are far more variables affecting print consistency than the direction of a boat and, of course, the first step to staying on course is defining your goal. The second is understanding exactly how those variables will impact consistency. Finally, the operator must make ongoing adjustments to the press to compensate for their impact on the final print.
Whether using the G7 process or measuring against the custom standards you’ve set for your shop, proper benchmarking requires more than simply measuring ink density. A deeper dive into TVI, gray balance, overprint colors, etc., is needed to assess changes over time.
There are a number of tools that provide the detailed data needed to ensure your press is printing to spec on every job. Before we examine those, though, let’s start with the culprits: the variables that will constantly try to pull your press off course.
First, it’s important to understand what press operators have control over and what they don’t. Here are the basics:
What You Can’t Control:
- Weather (temperature and humidity), unless you are in a fully climate-controlled pressroom;
- Raw materials, like printing paper;
- Ink manufacturing consistency;
- Mineral content of the water; and
- Ink demand per ink zone of the job on-press.
What You Can Control:
- Water conductivity;
- Condition of blankets and rollers;
- Mechanical adjustment of the press (pressures);
- The ink keys on each unit; and
- The amount of water metering onto each unit.
Let’s break down the “Can’t Control” factors first. The impact each has on printing is critical. Luckily, there are ways to take control, if you know what to look for.
- Temperature and humidity fluctuations can affect both ink viscosity and water evaporation. Both have an affect on print consistency. Some of this can be compensated for by ink key and water control adjustments, but controlling the climate in the pressroom is even more effective.
- Then, there’s material inconsistency. For example, printing paper is not always designed around printability. Some papers absorb water, which is good for consistency. Others repel or don’t absorb much water, which can cause the press to destabilize — requiring the operator to chase fluctuating ink densities throughout the run. Steering customers toward papers that print with less difficulty is a mitigating step.
- Ink, the other raw material, will certainly cause changes in printing if batches are different. Quality control in the manufacturing process is key, so use ink from a known source with a track record of consistency.
- These days, many printing plants use Reverse Osmosis (RO) purifiers to remove minerals and impurities in the water. I live in an area where the water source changes twice annually. This caused problems for years … until local printing plants installed RO purifiers. Problem solved!
- Each job on-press has varying amounts of ink requirements across the printed sheet. Areas with low ink, such as text-only areas, tend to get too much water. This requires turning the ink keys low, which decreases control over the amount of ink being delivered to the sheet. Areas of high ink usage print with greater stability, as the ink keys are open wider and give the operator more adjustment latitude. Plus, less water is required to keep the plate clean, allowing for better ink/water balance.
Take the Wheel: Measurement + Analysis
Now, the good news: The physical aspects of printing — the press and its components — are all under the control of the operator. A regular program of press measurement, adjustment and maintenance is essential to attain print consistency, and a surefire way to reduce waste, decrease downtime and eliminate headaches.
The key ingredient to consistency is tracking changes over time. This is the most effective way to control the variables affecting print quality. Another important step is committing to regular calibrations. A press curve adjustment and/or recalibration should be done on at least a quarterly basis.
But how do you stay on course between calibrations? You guessed it. Data is key.
First, include a control strip on each press form and use a handheld spectrophotometer to collect color data. Current ink densities, overprint colors, TVI (dot gain), G7 gray balance and neutral print density are important data points. They help operators understand press behavior to take action immediately to remain in tolerance. You can’t control what you don’t measure, so printing control strips and having a measurement device to read them is essential.
Next, know that using the color bar across the sheet isn’t the best way to gather data. Variable ink usage and the inaccuracy of ink key settings can render that tactic useless. A control strip in the gutter, positioned to print around the cylinder in a single ink zone, is the best place to gather data for assessing printing conditions.
Finally, use analytical software to analyze the measurement data. Specialized software is a highly efficient and incredibly accurate way to control printing devices. It enables operators to use consistent, data-driven processes to keep their press within tolerance on every job.
The most important factor here is evaluating your data over time to address trends that are taking the press out of tolerance. Just a few weeks’ worth of insight can reveal press drift. Then, the operator can check mechanical conditions, replace a blanket or take other maintenance actions to bring the press back into alignment. Regular measurements can also indicate issues like the need for plate curve adjustments in prepress.Printing has always been considered a craft, and with craft-based manufacturing there are always inconsistencies in the final product. That’s why an increasing number of smart printers concerned with quality and efficiency are gravitating toward Statistical Process Control (SPC).
They realize it’s a necessary step to remain efficient, profitable, and competitive. Still, adopting an SPC program becomes a culture shift for the organization, which requires a focused, top-down effort — and time — to implement.
So, consider being on the leading edge with SPC. But, in the meantime, adopting best practices including regular measurement and maintenance routines will keep your boat on a course for consistency. Diligent steering will result in quicker makereadies with less waste, more flawless printing and happier customers.
- Bruce Bayne | Kacharagadla