Understanding IPA and IPA substitutes, part III

To conclude this, we need to understand one important factor, which is ‘dosing’. Whilst this factor is a key differentiator, when one is attempting IPA reduction and/or elimination. But unfortunately, it remains as most misunderstood, and probably, therefore, one sees casualness when handling this on the shop floor. Dosing requires precision.

Isopropanol dosing: electronical vs mechanical

Mechanical dosing is where a mechanism triggers the addition of Isopropanol (IPA) to the dampening water when the specific gravity (SG) of the dampening water rises above a certain value.  

IPA has a low SG and when mixed with water, which has a SG of 1.00 @ 20 degrees Celsius the SG of the mixture falls. SG is measured mechanically by means of a hydrometer, a weighted float which rises when the SG increases and falls when the SG falls. In a mechanical dosing system, the hydrometer (float) will fall to a point where it mechanically triggers the addition of extra IPA.

Electronic dosing is a loose term for a variety of sophisticated IPA concentration measuring methods that are non-mechanical. Perhaps the most common is the Technotrans ‘Alco Smart’ system. This works on the principle of transmitting a beam of sound through the IPA and water mixture to a receiver, the frequency of the sound received is designed to vary in sync with the variation of SG of the mixture and the amount of IPA dosed into the system is then metered accordingly.

Why use ‘electronic’ dosing?

When the concentration of alcohol is relatively high (6-12%), there is a significant difference in SG between the mixture and that of plain water, this gives the float a significant degree of movement. Furthermore, the dosing accuracy of the mechanical system, if well maintained may be around +/- 1%. This means for a setting of 10% the dampening water will contain between 9 and 11% a 2% difference on a 10% setting equating to a variation of 20%.

When using very low levels of IPA, the difference in the actual SG of the different mixtures is much smaller and beyond the abilities of a purely mechanical system to accurately control. However, electronic systems are typically accurate to +/- 0.1% meaning that if the IPA level is set to 3% the variation will be from 2.9 to 3.1%, a variation of less than 10%.

If a mechanical doser were to be used for this level of alcohol, the best accuracy that could be achieved would be in the region of +/- 2% which would equate to between 1 and 5% IPA in the dampening water, a variation of 500%! Such a large variation in the amount of alcohol would give corresponding variations in the lithographic properties of the dampening water.

Why is it important to accurately dose IPA?

Precise control of all press variables is the key to consistency of print output, the goal of all quality conscious printers and a major determinant of profitability. IPA has the property of conferring a wide latitude of press settings when used at high percentages (>8%). This property of IPA we have seen in detail in the first two parts of this series.

When the percentage of IPA is reduced, the latitude of press settings become more important and any changes in the percentage of IPA in the dampening water will have a proportionately stronger effect on:

1 Dampening water transport to the plate surface

2 Ink /water emulsion form and stability

3 Tolerances in roller settings

The result will be either a cyclic variation in the quality of the printed output or more work for the press minder or both.

Can a printer reduce and/or eliminate IPA without using electronic dosing?

The answer is ‘yes’. IPA can be reduced to around 6% with mechanical dosing so long as the doser is kept clean and well maintained. When IPA is reduced to 0% then no alcohol dosing (obviously) is required. However, running at 0% IPA is difficult and not all dampening systems are suitable for this process.

Many a consultant or manufacturers or users look at low usage if IPA as zero. Low usage = < 5%, the primary reason for this is, the analogous hydrometer is designed to detect % presence of IPA of > 5% only. Certain type of jobs may still need to use IPA though in less percentages.

Due to very high percentage of margin of error, I recommend using only electronic doser when attempting to either reduce or eliminate IPA usage.