Doctor blades should never be the primary reason for a machine stop! There are absolutely no “ifs, ands or buts” about it!
Such circumstances are inexcusable. Proactive action, coupled with early intervention, should render any and every imaginable scenario, where doctor blades take exclusive blame, avoidable. It’s that simple.
Essential moves, dictated by often complimentary doctor blade analysis, will identify misaligned chambers and excessive doctor blade wear. Benefits, like extended lifetime and reduced leaking, are delivered immediately. Pressure points can be eliminated. The good news is that every action required is relatively simple to orchestrate.
All images courtesy of TKM
Issues—chief among them, leaking chambers, UV ink spitting, back doctoring, premature blade wear and dot gain—while, at times challenging, can be managed and controlled. Perfect solutions, matched to individual presses, are readily available.
Here is a rundown of five major problems that result from neglect and mismanagement of doctor blade materials, selection and setting; as well as an outline of the steps to take to avoid the threats they represent—especially the potential negative impact they can have on productivity, production and profitability.
Leaking Chamber
Typically, doctor blades can be responsible for chamber leakage in the following ways:
- Doctor blade width is not matched correctly with the end seal
- The blade is cut to the wrong length
- Blades are wearing out too quickly
Your blade width should always be matched to the end seal (see Image 1). Blade length should always be cut so that the ends are aligned to at least the center of the end seal (see Image 2). Leaking can also be the result of a misaligned or damaged chamber (see Image 3) or utilization of incorrect end seal material or shape (see Image 4).
Often, a leaking chamber is the result of excessive pressure used to obtain proper seal. This over-pressure then causes excessive and premature wear—not only to your doctor blades and end seals, but also to the anilox roller.
The simple solution entails ensuring that the doctor blade dimensions are correct. Suppliers like TKM offer customized end seals, specific to individual press requirements. A customized seal results in lower pressure. And lower pressure prevents leaking—at the same time, it promotes increased lifetime of the doctor blade.
Whenever leaking is caused by normal doctor blade wear, various doctor blade material options can be applied to extend the lifetime and further reduce leaking.
UV Ink Spitting
UV ink has a much higher viscosity than water- or solvent-based inks. The high viscosity causes ink to hydroplane or to pass under the tip of the doctor blade, especially at higher press speeds. Never use doctor blades that were optimized for lower ink viscosities on UV-cured jobs. To prevent hydroplaning, we recommend the use of a thicker base material in conjunction with a fine tip.
For example, TKM’s solution is the Stable UV doctor blade. It combines a thick base material (up to 0.012-in.) with a fine bevel tip, resulting in a clean wipe and no ink spitting, even at a faster press speeds.
Back Doctoring
Back doctoring is a common problem in wide web printing. This issue is caused by the containment blade preventing ink from returning to the chamber, and results in dripping across the entire chamber into the ink pan, particularly when the containment blade is positioned at the bottom of the chamber.
Typical causes of back doctoring and what to do about it list out as follows:
- Too much pressure on the chamber. Lower the pressure; if not possible, then check end seals and chamber alignment
- Wavy doctor blade. Check if the chamber and clamp are clean and free of dried ink and burrs
- Tighten the clamps from center outward, but do not overtighten
- Blade too thick. Use a thinner containment blade
- Chamber misaligned. Manufacturers like TKM will analyze your used doctor blades to help confirm chamber alignment
- Containment blade width. Follow the manufacturer’s recommendation
- Ink viscosity. Make sure it is within specifications
Blade Wear
Once again, we must reiterate one point—doctor blades should never be the primary reason for a machine stop! Signs and risks of premature doctor blade wear include:
- Unnecessary downtime (machine efficiency)
- Ink contamination (metal slivers and particles; risk of scoring lines)
- Safety concerns (extra handling of the blades)
- Increased chamber pressure (more wear on all components—including end seals)
The first step to avoid premature doctor blade wear is to run your chamber at the lowest pressure possible.
Therefore, you must make sure that your end seals are correct and that the chamber is perfectly aligned.
Additionally, the correct choice of doctor blade materials and tips is very important to avoid premature doctor blade wear.
A very popular plasma-coated blade, TKM’s DuroBlade, can be used in white ink. This blade can last up to 10 million feet to 15 million feet. For expanded gamut (EG), we recommend our PowerPrint material. This blade typically lasts one week, thereby avoiding an unnecessary doctor blade change before your scheduled maintenance.
Doctor blade manufacturers offer a wide range of materials, technical support and blade analyses to overcome any premature doctor blade wear obstacles. Help is available. You can find the perfect solution for your individual presses.
Dot Gain
Dot gain means your dots increase during the printing process. This issue will increase the tone value over time, resulting in darker or fuzzy print. Many reasons can attribute to dot gain. The main reason for dot gain, as it relates to doctor blades, is the contact zone of the doctor blade is too large, or it increases during the printing process.
Let’s look at the causes and solutions:
- Too much pressure on the doctor blade will bend the blade. Flatten out the wear angle and increase the contact zone
- The tip of the doctor blade is too thick. Use a thinner doctor blade (if rounded edge) or use a Lamella/preground tip
- If you are using a plastic doctor blade, the blade usually must be thicker to prevent bending. A thinner steel blade could improve dot control. To eliminate dot gain, our recommendation is to use a steel doctor blade with a long and thick Lamella
There you have it—simple steps to make a printer’s life less stressful, more easily manageable, far more enjoyable and mishap free as well.
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