Online Wet Film Measurement


__ The coil coating industry is increasingly facing tougher competition and a more demanding client base. Short production runs are more common and customer quality expectations have not relaxed, so there is a drive towards improved efficiency and cost savings. Increasing product yield by avoiding downgrades represents a real opportunity for increased profitability. When there is sufficient market demand, having additional capacity from within existing production shifts also provides an opportunity for additional profit.
___ Delivering the tools needed to meet these opportunities and challenges is what motivates Wolf Innovation. At Wolf Innovation we have recently launched a fourth new product for the coil coating quality control sector called WFM2™. This is a patented, automated, wet film thickness instrument that replaces WFM1™. In combination with PVS1™ (paint volume solids instrument) this provides a significant new opportunity for savings by eliminating out of specification product from the start of a production run. It can take several hundred metres of strip before a thickness related defect is detected and a correction is made. The reason is simple – it is not possible to defect a defect until painting has commenced.

WFM2™ Data and Modes of Operation
WFM2 Provides 3 thickness output values:
• Wet film thickness on the coater roll
• Wet film thickness on the strip, and
• Dry film thickness on the strip (in combination with PVS1™)
Primary measurements are made on the coater roll. Modes of operation include:
• Forward and reverse coating mode, and
• Measurement before and during painting.
The measurement range extends to thin films so measuring primer thickness is very reliable.


Coater Head Set-Up Procedures and Potential Losses
___ When it comes to coater set-up prior to painting there is a heavy reliance on experience, and only a few approaches are available. Replicating coater set-up conditions from run to run is a common approach, but this also has limitations. The variables that affect coater set-up, apart from the obvious ones, include coater roll diameter, roll hardness, paint volume solids, coater roll swelling (due to solvent absorption), and mechanical inconsistencies such as coater roll eccentricity. If any one of these is not in line with expectations, the initial film build target may not be achieved. Instances do occur where fully instrumented servo controlled coaters deliver film builds that vary to a significant level from one side of the strip to the other – this is both surprising and unexpected for this type of technology. The only sure way to hit target film build from the onset of coating is by direct film thickness measurement during coater set-up.
In common use, the wet film thickness wheel applied to the strip after painting commences provides a basic approach to measuring wet film-build on the strip. This is a destructive test that relies on operator skill. The method has a significant error band. Since the measurement is made after painting has commenced many metres of strip will have been painted before any correction can be made, that is, a correction to dry film build. Separate readings are needed on each side of the strip adding to the time required for the measurement whilst increasing the amount of lost product.
___ For a 100,000 tpa paintline operating at full capacity 5 metres of wet film wheel track lines every 20 tonnes equates to over 80 tonnes of lost product per year, excluding any out of spec product at the start of a painting run. This is a conservative estimate. Instances of ‘out-of-specification’ product are not avoided.
___ Another approach is ‘the test strip’ in which a section of coil is painted once the coater has been set up. The coil for the test strip is loaded onto the line between threader strips. A sample is cut from the test strip for measurement of film thickness, colour, and other paint physical properties. The cost of test strips is high and production cannot proceed while the line is stopped waiting for the test strip results. The advantage of test strips over the wet film wheel is that dry film thickness on the strip and colour can be assessed accurately in the laboratory.
___ For a 100,000 tpa paintline operating at full capacity 20 metres of test strip every 50 tonnes equates to about 130 tonnes of lost product per year. It also equates to about 500 hours of lost production time. That equates to a potential 10,000 tonnes of additional capacity within existing shifts.

WFM2™ – Paint Coater Set-Up Before Painting
___ WFM2™ uses an intrinsically safe confocal sensor to measure the surface of paint on the coater roll and the surface of the elastomeric roll at the same location. This data provides a measured value for wet film thickness on the coater roll. Coater and line speed set-up values are imported into WFM2™, which allows determination of wet film thickness on the strip. PVS1™ values of paint volume solids are then used to determine dry film thickness on the strip.
The three coating thickness values given by WFM2™ are shown in Figure 2.


To be of value the measured wet film thickness on the coater roll has to be correct and there are some hurdles to overcome to achieve the integrity of this measurement:
1. The paint wet film thickness on the roll before painting is not equal to the wet film thickness during painting – shown in figure 3. To overcome this WFM2™ uses a very soft pneumatically operated doctor blade to wipe a narrow band of paint from the roll upstream of the sensor – this mimics painting.


2. The wet film thickness on the coater roll has to take account of roll and bearing eccentricity and roll shape problems. The instrument measures and stores thousands of reference readings to characterise the roll shape and bearing wear in order to take these out of the equation.
3. Conversion of the wet film thickness on the coater roll to the equivalent wet film thickness on the strip requires a one-off calibration for each coater. During reverse coating not all of the paint is wiped off the coater roll onto the strip – shown in figure 3b, above. The residual paint on the coater roll after the kiss point is known as leakage. As part of the calibration process WFM2 characterises leakage for a coater and accounts for its effect. Figure 4, below shows the distribution curve for leakage values from a coater across 100 paint runs covering most topcoat types applied on this coil paint line.


4. Conversion of the wet film thickness to dry film thickness on the strip requires an accurate value for paint volume solids (PVS). PVS1™ reliably measures paint volume solids in a production environment. PVS1™ is the only PVS instrument and method to fully cure the paint to an equivalent degree as occurs in a paint line oven. This is particularly useful with plastisols that are cure-sensitive.
Accurate prediction of dry film build before painting commences has the added benefit of significantly reducing off color defect caused by out of specification paint thickness. The value of this benefit should not be underestimated.

WFM2™ – In Use
___ WFM2™ does not interfere with coater room operations including coater roll cleaning and change over. To achieve this:
• The sensor components are designed to slide into and out of position on a linear bearing usually mounted off to the side of the coater. The brackets are tailored for each coater design.
• The sensor has hands free operation.
• The small doctor blades can be cleaned quickly and easily.

Typical Performance – Repeatability Under Production Conditions
___ After setting up a coater ready for production WFM2™ was used to take 8 sequential film thickness readings without altering the set-up between readings. The purpose of the test was simply to check and illustrate repeatability of the reading. The results, given in table 1 below, speak for themselves. Deviation of WFT ROLL is consistently within +/- 0.012mils across all paint types.


Production Data – Comparing WFM2™ and Phaeton™ Dry Film Measurements
___ A random selection of nine results from a 16-day period of production compares dry film thickness measurements with Phaeton™ dry film thickness measurements. The results shown are typical for WFM2™ in terms of strip DFT results verses lab thickness results. The largest variance here is 0.03 Mils, and the least is 0.004 Mils. On average, results were within +/- 0.015 Mils of the lab measurement. This capability of measuring and setting strip dry film thickness before the commencement of painting provides a significant and new opportunity for improved prime product yield and for significant gains to the bottom line.
___ WFM2™ wet film measurement in combination with rapid in plant paint volume solids assessment has now been introduced on a coil line in Asia to determine dry film thickness before painting commences. The results from WFM2™ are in good agreement with independently measured values of dry film thickness determined with the off-line Phaeton™ DFT instrument. This provides validation for both WFM2™ and for Phaeton measurements.


PVS1 is the only paint volume solids instrument to cure a paint sample to the same degree of cure as occurs on a coil paint line. Consequently results from PVS1 together with WFM2 provide dry film build results with a high level of confidence.
___ Out of specification product from the early part of each new production run can now be effectively eliminated with this new and effective QC tool. When things do go wrong, start up losses in the order of hundreds of metres of strip can easily occur. Savings in substrate, paint, and a reduction of field and customer complaints, and downgrades will provide an attractive payback for WFM2™.
___ Improvement in production yield shows the real value of this ‘new-to-the-world’ leading-edge quality control innovation.