The Problem of Ink Micro-Foaming
Although improvements in inks have reduced many ink foaming issues, continued increases in press speeds have worked against the elimination of this as a consideration press-side, especially with many process and metallic inks.
Analysis as to the cause of this microfoam formation suggests that the air bubbles become entrapped as the anilox roll comes in contact with the ink inside the chambered doctor blade assembly. The microfoam formation may be due to the high shear at these high press speeds, the high-speed entrapment of the air from the anilox cells, or, possible, air carried on the land areas of the anilox rolls under the doctor blades.
The generation of these small air bubbles into the ink puts great demand on the ink antifoam additives. So much area is created that there is insufficient anti-foam to effectively control the microfoam formation.
Steps to Take
Anti-foaming additives are intended as "preventive medicine," whereas defoamers break existing foam bubbles. Most of these materials are greatly affected by high shear and high heat conditions.
In the event that some amount of foaming presents itself, several steps can be taken quickly to alleviate the problem with traditional water-based inks.
- Spray a small amount of a defoaming additive into the ink sump. Spraying or "misting" allows the droplets of defoamer to disperse more rapidly and evenly than simply pouring the additive. Avoid spraying defoamer directly onto printing plates as some of these can cause swelling. Care must be taken when adding glycol or anti-foaming agents. Too much can cause a pin-holing effect in the solid print areas.
- Reduce pump speed and the amount of agitation in the ink sump. Obviously the closer the ink supply is to the printing station, the less pressure is required to pump ink to the ink chamber. Those installations that use large pump housings (typically on the bigger presses) are more prone to create excessive foam. In these cases, frequent replenishing of the ink is the most favorable approach, if not always the most practical.
- Splashing from the ink return line is another source of microfoam induced viscosity rises. Limit the amount of splashing that occurs by re-directing it so that the returning ink flow is buffered as it enters the pail. Redirect the ink flow to the side of the supply container disperses the micro foaming much the same as tapping beer to the side of a glass when running a draft beer reduces foaming. Although submerging the ink return below the ink surface also reduces splashing, it can really add to micro foaming as the air bubbles are forced through the ink to the surface, once again foaming at the top. Keeping the supply kit at fuller levels also helps to reduce the foaming as there is more mass to get to foam.
- Small additions of a suitable diluent (a blend of water and isopropyl alcohol is commonly used) can be a good aid to keeping surface foam at bay. Check with your ink supplier as to the recommended diluent for your ink. It can be sprayed into the ink pan to break surface bubbles as they form, and will effectively lower the surface tension and viscosity of the ink, allowing air to escape more easily. Once again, be careful not to add too much of any additive too quickly or you can "over correct" and cause other problems.
- Replace your supply with fresh ink, particularly if microfoaming has become uncontrollable. This is usually a last resort, but may be necessary if the ink has gone unchecked for some time. Once out of the press, the old ink may return to a usable state, with time and some gentle stirring.
- Assess your ink pump situation. Selecting the right pump, hose and chamber inlet size / location for your application can be critical to minimizing any foaming issues.