Coating continuity detector

A method and apparatus are disclosed for detecting the continuity of nonconductive coatings on conductive web materials, such as rolls of metallic foils. The coated side of the sheet material web contacts a conductive surface and electrical resistance of the coating is measured. The apparatus for measuring the resistance of the coating may be adjusted to permit small gaps in the coating to be bypassed while being triggered by major gaps in the coating.

BACKGROUND OF THE INVENTION 
Metallic foils, such as aluminum foil, tin foil and the like, are useful 
materials in forming laminated structures. These materials may add 
strength to such laminates, act as light and/or vapor barriers, or merely 
act to give a "quality look" to packaging materials. 
When forming laminated structures which include one or more metallic foil 
layers, it is often necessary to coat or prime the metallic foil prior to 
lamination. This is true, for example, when bonding aluminum foil to 
plastics resin film layers, such as polypropylene. Such coatings as 
epoxys, vinyls, polypropylene dispersions, nitrocellulose, ethyl cellulose 
and others are thus routinely applied to metallic foils. 
During production, these coatings are applied to the metallic foils by 
unwinding the foil in web form from a roll, coating the foil, drying 
and/or curing the coating and rewinding the foil onto another roll. At 
production speeds, no mechanical device has been heretofore known which 
was capable of detecting gaps in the coating on the metallic foil over a 
substantial length of the foil. Thus, these gaps were detected visually by 
an operator of the coating apparatus as the coated foil passed his 
operating station. 
The accuracy of such manual coating continuity detection is limited for 
several reasons. First, many of the coatings placed on metallic foils are 
transparent or nearly transparent. Thus, accurate visual inspection is 
highly difficult. Even when applying coatings that are relatively easy to 
inspect, distractions of the operator, and the sheer boredom of 
continuously watching coated foil pass the operator station, can lead to 
the failure of the operator to detect a substantial coating gap. 
It is desirable, therefore, to provide a method and apparatus for 
mechanically detecting the continuity of coatings on metallic foils which 
is free from operator judgment. 
While major gaps in such coatings cannot be tolerated, complete coverage of 
even thickness on the foil of the coating material is not always 
completely necessary in commercial practice. Thus, minor pinholes or 
thickness variations in the coating may often be tolerated. It is also 
desirable, therefore, that a mechanized coating continuity detection 
method and apparatus be capable of differentiating major gaps in the 
coating requiring corrective attention from minor acceptable gaps, such as 
pinholes, minor thickness variations and the like, which are not a cause 
for concern. 
THE PRESENT INVENTION 
By means of the present invention, these desired results are obtained. The 
method and apparatus of the present invention comprises electrically 
grounding a conductive sheet material web, such as a metallic foil, prior 
to coating thereof, applying a nonconductive coating to the sheet material 
web, contacting the coated side of the web with a conductive surface which 
is electrically insulated and detecting electrical resistance between the 
web and the detection surface caused by the coating material. The 
detection means may be connected to an alarm system, or may be connected 
to deactivate the coating operation or mark the web at the defect when a 
gap in the coating is detected. The detection apparatus also is adjustable 
to respond only to continuous gaps in the coating of a time sufficient in 
relation to the speed of the web past the detecting means corresponding to 
gaps of a size warranting corrective action.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning to the FIGURES, a web 12 of a conductive sheet material, such as a 
metallic foil, including aluminum foil, tin foil and the like, or a 
laminate including a metallic foil surface, is supplied from a roll 10 of 
the sheet material carried upon reel 14. As the web 12 leaves the roll 10, 
it passes over a guide roll 16. The guide roll 16, besides performing its 
guide function, may be electrically grounded, thus electrically grounding 
the conductive web 12 prior to coating, for reasons which will be 
explained below. Alternatively, the frame (not shown) carrying roll 14 and 
web 12 may be electrically grounded to provide the grounding function for 
web 12. 
After leaving the guide roll 16, the web 12 passes through the nip between 
a pair rollers 18 and 20. Rollers 18 and 20 may be driven rollers, to 
supply driving force to the web 12. However, these rollers 18 and 20 may 
also be free-wheeling rollers with the driving force for the web 12 being 
supplied by a takeup reel 44. Typically, roller 20 is a gravure roll, 
however, roll 20 may have a smooth surface. Roll 18 is typically a rubber 
roll. 
Roller 20 is partially submerged within a tank 22 containing coating 
material 24. This coating material is of a nonconductive nature, and may 
be formed from materials such as epoxies, vinyls, polypropylene 
dispersions, nitrocellulose, ethyl cellulose, and other similar 
nonconductive coatings routinely applied to conductive webs, such as 
metallic foils. As the web 12 passes between rollers 18 and 20, the 
surface of the web 12 contacting roller 20 is thus coated with the coating 
material 24. 
The web 12 leaves rollers 18 and 20 and passes over a free-wheeling guide 
roller 26 and into an oven 28. Within the oven 28, the coating material 24 
is dried and/or cured, depending upon the nature of the coating material 
24. For some coatings, oven 28 is not required, with air drying of the 
coating material 24 being sufficient. 
The web 12 next passes over another free-wheeling guide roller 30 and 
contacts coating continuity detection means generally shown at 32, the 
operation of which will be more fully described below. Web 12 may next 
pass between an optional additional pair of rollers 34 and 36, over 
free-wheeling guide rollers 38 and 40 and finally onto a take-up roll 42 
mounted upon the driven reel 44. 
As previously mentioned, the driving force for the web 12 could be supplied 
entirely by reel 44. In that case, rollers 16, 18, 20, 26, 30, 34, 36, 38 
and 40 will all be free-wheeling rollers. If, however, driving force for 
the web 12 along its path is desired, driven roller pairs 18 and 20 and 34 
and 36 operate at the same speed as take-up reel 44, to maintain uniform 
tension in the web 12 throughout its travel. 
Looking now more closely at the coating continuity detector 32, its 
operation is more fully illustrated in FIG. 2. 
As the web 12 passes from guide roll 30, it contacts a free-wheeling roll 
50. Roll 50 is formed of a conductive material, such as stainless steel, 
or aluminum, and is free-wheeling upon its supporting shaft members 52 and 
54. Preferably, this roll 50 is covered with a conductive polymer coating. 
This serves to protect roll 50 from corrosion, is easily cleaned and 
provides a practical and convenient way of passing electrical current 
without excessive sensitivity. Roll 50 is electrically connected through 
line 64 to a coating continuity detection apparatus 66. Detection 
apparatus 66 is electrically grounded to ground 70 through line 68. As 
previously mentioned, web 12 was electrically grounded at roll 16, and 
since all elecctrical grounds are common, a completed circuit has thus 
been formed. In order to maintain this circuit, insulators 56 and 58 
electrically isolate roll 50 from an electrical ground, such as the frame 
members 60 and 62 upon which roll 50 is mounted. 
When applying an extremely low voltage, said voltage being in the range of 
about 1.15 to about 11.5 volts DC, to prevent sparking, to the electrical 
circuit formed, the non-conductive coating material 24 causes electrical 
resistance to occur between the conductive web 12 and roll 50. The 
detection apparatus 66 senses this resistance. Should a gap occur in the 
coating upon web 12, the electrical resistance in the circuit falls 
significantly, and may fall practically to zero, and a reduction in or 
lack of voltage resulting therefrom is sensed by the detecting apparatus 
66. This gap detection causes detection apparatus to signal the operator 
that a gap has occurred, to shut down the coating line for corrective 
action, or to mark the web 12 at the location of the defect. Such marking 
may be accomplished by marking means 46, which may be an ink jet marker, 
tape marker or the like and is timed to mark web 12 when the defect seen 
at detector 32 reaches roll 44. 
As previously mentioned, not all gaps in the coating of web 12 are causes 
for alarm. Small pinholes and the like are acceptable in many instances. 
Yet, any gap in the nonconductive coating upon web 12 could produce a 
reduced or near zero voltage reading. Thus, detection apparatus 66 may 
include a timing means which is adjustable, based upon the speed of the 
line and the minimum size gap to be measured, to vary the sensitivity of 
the apparatus 66 such that only gaps of a minimum time span will trigger 
the alarm or shutdown mechanism. Thus, the detection apparatus 66 may be 
fine tuned to the specific coating operation taking place. Variations of 
the level of applied DC voltage may also help provide the fine tuning of 
the sensitivity of apparatus 66 necessary. These sensitivity controls may 
be used separately or in conjunction. 
Looking again at roll 50, collars 72 are illustrated adjacent the side 
edges of web 12. Should there be a region along web 12, such as its side 
edges, where coating either does not take place or where the continuity of 
the coating is not significant, collars or masks 72 are positioned upon 
conductive roll 50 at the corresponding locations around roll 50 where 
these coating discontinuities in web 12 are permitted to occur. The 
collars 72 are of a nonconductive material, and thus will operate to cause 
resistance between web 12 and roll 50 in the same manner as if the coating 
material were in place. The collars 72 may be plastic collars, or may be 
any other non-conductive shielding material, including such non-conductive 
tapes as teflon tape, masking tape and the like. 
From the foregoing, it is clear that the present invention provides an 
automated method and apparatus for determining the continuity of coatings 
upon conductive webs which permits such detection to be accomplished 
without operator intervention. 
While presently preferred embodiments of the invention have been 
illustrated and described, it is clear that the invention may be otherwise 
variously praticed and embodied, within the scope of the following claims.