This invention relates to a quality control system for
monitoring a parameter of holes formed in a film, especially a flexible packaging
Perforated packaging films are used to package produce,
such as fresh produce, cheese, flowers, etc. The size and frequency of the perforations
in the film is selected to suit the produce being packaged. In particular, the shelf
life of fresh produce can be extended by packaging in film with perforation size
and frequency such that, as the produce respires and gas and water vapour is exchanged
with the surrounding atmosphere, the atmosphere within the package is modified and
becomes oxygen depleted and carbon dioxide rich, which has the effect of suppressing
respiration and slowing deterioration of the produce. Such packaging is termed modified
atmosphere packaging MAP.
For MAP to be effective, it is important that the hole
size and frequency is selected to match the produce; these parameters vary with
different produce and quantities of produce packaged. Therefore when forming perforations
in MAP film it is important to include a quality control process to ensure that
perforation parameters are maintained within predetermined limits. To date, this
involves taking samples of the processed film and measuring hole size and frequency.
Typically, sampling is done at the end of processing a reel of film, and if the
perforations are found not to meet the required quality standard, the reel is rejected.
Only a small sample size is analysed, typically, for a reel 1100 metres long, with
110,000 perforations, only between 10 and 20 holes are measured, therefore the percentage
of holes analysed is only 0.009 %. Also, the sampling process is off-line from the
perforation forming process itself, and samples are taken after the whole reel has
been perforated, so if the film is found to be defective, then this reel of film
is wasted at considerable cost of material and processing time. Also, rectification
of perforation problem can be time consuming and wasteful of material due to the
need for continual adjustment of the perforator and starting and stopping of the
machine to produce a sample for inspection. Also, although the perforation process
is considered reliable and controllable, if the equipment is not operated correctly
then irregular perforations will be produced either throughout the process or at
irregular intervals. The known sampling procedure at the end of processing a reel
will not necessarily detect these irregularities.
Further, the current sampling procedure is carried out
by an operator and is therefore open to human error, which limits operating speeds
An object of the invention is to provide an improved system
for monitoring perforation quality, preferably in real-time throughout the processing
of a film.
According to the invention a digital camera is provided
to form successive images of the holes in the film while the film is fed past the
camera; these images are captured and parameters of the holes are analysed in an
analyser so as to determine whether or not the parameters meet predetermined quality
standards, and a warning is triggered by the analyser if the quality standards are
A strobe light illuminates the film to form the images
of the hole, and the light and camera are operated at intervals to form successive
images. Typically, a digital camera and strobe light are configured to capture good
resolution images of holes in the film at film speeds up to 8 metres/second, with
hole sizes between 10 and 600 microns diameter; at a frequency between 1 and 1000/
The warning signal may serve to alert an operator of the
apparatus, and may trigger marking of that portion of the film analysed as sub-standard.
Also, the hole size and frequency monitoring system is used in conjunction with
a film perforator that forms the hole in the film, then the warning signal from
the analyser may be used to monitor quality in real-time during the perforation
process, and may be used to control the perforator so as to correct for quality
variations in hole size and frequency on-line. The control system of a film perforator
can also be used to trigger operation of the camera so as to correlate hole perforation
with hole inspection.
The distance of the light source from the film and orientation
of the light beam relative to the film are both important parameters in obtaining
clear images. Also, in the case of a light transmitting film, which will polarise
transmitted light, polarisation filters are preferably used to obtain optimum image
contrast between the hole and the film.
The tension of the film being analysed is also important
to obtain clear images and for timing capture of an image. Preferably, a driven
roller system is provided, additional to the main film drive, to tension the film
while it is analysed. Also, a high resolution encoder is preferably provided to
monitor the speed of the roller system, and hence the film, so that the position
of the sample of the film/ perforation being analysed can be accurately determined
relative to the field of view of the camera.
It is important to maintain the film at the focal distance
of the camera lens, for example, by ensuring that film drive rollers rotate uniformly
by reducing or eliminating vibration of the camera using suitable mounting means.
Test results indicate that the hole diameter as measured
by the camera decreases as film speed increases. It has been found that this is
due to blurring of the image of the hole. However, it has also been determined that
it is possible to compensate for this effect by increasing the gain setting of the
camera with increasing film speed. Increasing the gain setting increases the intensity
of the captured image. Conversely, the gain setting is reduced with reduced film
speed to reduce the intensity of the captured image.
The quality of the captured image is also found to vary
with different film substrates having different optical properties, and this requires
adjustment of camera gain, and the light threshold parameter, where the threshold
parameter sets the level of light at which the hole is defined.
The invention will now be described by way of example with
reference to the accompanying schematic drawing of film perforation apparatus which
includes a quality control system for monitoring hole size and frequency.
A web of flexible plastics packaging film 1 is fed continuously
between a pair of guide rollers 2, 3, and a perforator 4 is located between the
rollers to perforate the film as it passes. The perforator 4 comprises a laser head
which directs a laser beam at the film to form individual holes, and a laser controller
5 which controls repeated operation of the laser head in accordance with the film
speed to form successive equi-spaced holes along the length of the film.
The speed of the film is controlled by a machine controller
6 which feeds a speed signal into the laser controller 5, which in turn feeds a
control signal to the quality controller 7. The quality controller 7 feeds an output
control signal to a digital camera 9 and a light source 10.
The camera 9 is located on the opposite side of the film
1 from the light source 10 downstream of the laser perforator 4. The camera has
an extension tube 11 of 120mm in length connected to a telecentric lens 11a that
gives a +/- 1mm tolerance on focal distance to allow for web movement laterally
of its feed direction. This configuration produced a field of view of 1.3mm by 1.5mm.
The light source 10 is positioned directly opposite the
camera lens 11a. Alternatively the light source is displaced laterally and a mirror
is provided to reflect light from the source onto the film, the mirror being orientated
at 90 degrees to the film surface,
The light source 10 is a strobe light and its operation
is controlled at a frequency determined by the quality controller 7 in accordance
with the film speed signal so that an image of each hole is captured after it has
been formed by the laser perforator 4.
Image data from the camera 9 is fed to an image analyser
12 which analyses each image to determine whether the diameter of the hole is within
a predetermined tolerance and whether the frequency of the holes in the film is
also within a predetermined tolerance. Hole diameter is determined by measuring
hole area and then deriving the diameter mathematically. The hole frequency is calculated
by analysing the known repetition rate of the laser and the known web speed of the
film to hence calculate the frequency of holes.
A display or warning device associated with the image analyser
12 may be set up to warn an operator of quality defects, such as unacceptable size
holes or missed holes. Alternatively, the analyser 12 may generate an error signal
that is fed back as a control signal to the laser controller 5 to correct for perforation
errors automatically. The analyser may also control a marker to mark defective portions
of the film, or an operator may do this manually, for example, using metalised tape.
The system as described is able to analyse holes between
10 and 600 microns diameter, although it is preferably used to analyse holes 40
to 150 microns in diameter. Analysis is possible at film speeds up to 8m/second
with at least 20 inspections/second, but use on stationary film is not precluded.
An analysis of the hole size and frequency can be made in terms of oxygen transmission
rate which the holes will support, this being a critical parameter for MAP film.
The illustrated system can be readily calibrated on line.
The camera and light source are moved over to the extreme end of the machine where
there is a mounted section of film that has a known, calibrated hole in the centre
of the sample. The camera is focused onto the film and hence the hole, and an image
is captured for analysis to determine the hole diameter. This measured diameter
is then compared to the known diameter, and if found to be incorrect, then the system
is re-calibrated. Once calibration is complete, the camera and light source can
then be repositioned over the perforation area.
In an alternative embodiment of the invention, operation
of the camera 9 instead of being controlled from the laser controller to capture
an image of each hole or predetermined periodically spaced holes, is controlled
by the quality controller 7. The quality controller 7 can store programs for different
specifications of film perforation according to their intended use with different
The illustrated apparatus shows the use of a quality control
system with a perforator 4, but it will be appreciated that the quality control
system can be used on pre-perforated film specifically for quality control or in
conjunction with another finishing process such as slitting the film, printing the
film, forming bags from the film or laminating the film.
In order to capture a clear hole image, the strobe light
10 is preferably selected with a 100 Watts power rating and an extremely fast on-off
operating speed of 10 microseconds. The position of the strobe light relative to
the film 1 is also important, the optimum distance being set at 130mm ± 10mm,
with the light beam orientated at 90 degrees to the film surface.
Also, two polarisation filters 16 are provided to ensure
optimum contrast between the film and the holes, with the light emitted by the strobe
light 10 clearly visible through the hole. The optimum contrast is established by
rotating one of the polarisation filters until the definition contrast between the
light levels within the hole and the surrounding film is at its greatest.
The tension of the film 1 is important to capture a clear
image and for capturing the image at the appropriate time as the holes pass the
light and camera. The two rollers 2, 3 are located between the nip rollers (not
shown) and the rewind mechanism (not shown), and create an additional zone of tensioned
film there between. Both rollers 2, 3 are motor driven and are slaved off the main
drive (not shown).
A high resolution encoder 13 , typically generating 1024
pulses per revolution resulting in a resolution of 0.245mm per encoder pulse, is
fitted to one of the driven rollers 2, and monitors film speed accurately, thereby
allowing the position of a hole to be determined accurately relative to the perforator
4 and the field of view of the camera 9. This arrangement is able to determine hole
position to within ± 0.245mm at web speeds up to 6 metres/second.
The rollers 2, 3 are adapted to rotate uniformly within
a tolerance of ± 0.025mm so as to maintain the film within the focal distance
determined by the camera lens 11a. Also the camera 10 is preferably mounted so as
to reduce or eliminate the effect of machine vibrations that would effectively cause
the focal plane of the camera to move relative to the film. Rubber mounts are provided
in a connecting bracket 14 which connects the camera to the machine. Also, the mounting
bracket preferably comprises an adjustable mount 15, such as a dovetail linear mount
not shown) which allows the camera to be adjusted accurately in both the x and y
axis to +/- 0.02mm.
The hole diameter measurements determined by the analyser
12 at different film speeds from static up to 8 metres/second indicate that the
hole diameter progressively reduces with increasing film speed. This effect is due
to blurring of the image at increased speeds, and is compensated for in the analyser
12, preferably by adjusting the gain setting of the camera. The gain adjustment
within the software basically effectively varies intensity of the light source by
changing the light level of each pixel to make the captured image appear brighter
To evaluate the accuracy of hole measurement a number of
systematic trials have been conducted to understand the potential error in measurement
at different web speeds. Initially static hole images are analysed for diameter
in a population of 500 holes within 1000 metres of a standard plastic OPP film.
The same film is then passed through the field of view
of the camera at different operating speeds and the hole images are analysed again
and compared against the static measurements. The results can be seen below:
Average Hole dia microns
Typical Standard deviation
The table above clearly shows that the average hole diameter
measured reduces as the web speed increases, but by adjusting the gain settings
the results can be maintained within an acceptable tolerance. See table below:
Average Hole dia microns
Typical standard deviation
Gain setting %
The table above clearly shows that an accurate hole measurement
can be taken at varying web speeds, by adjusting the gain setting and therefore
adjusting the level of light of the illuminated hole.
Also, different film structures create different levels
of contrast between the light seen through the hole and the light seen through the
substrate of the film. Therefore, for each substrate, different polarisation positions
are required to obtain the maximum contrast between the light seen through the hole
and the light through the material. The gain setting also needs adjustment to ensure
the optimum level of light is transmitted through the hole compared with that through