When I use to work in a glass container plant, we use to have a sort of private joke among ourselves whenever we were facing a problem in the line. In those – frequent … - occasions we would use to say that the problem fault was from the guy that was on the screen …
Commonly referred as the “screen”, this is a human visual inspection performed in the Cold End as the containers pass in front of an on line light box (light screen).
Usually this is done by an operator sitting in some sort of a chair on the Cold End, immediately after the last of the automatic inspection machines (although few production lines have light box screens additionally installed before the first inspection machine).
It seems pretty straightforward, right?
Well, the fact is that this is probably one of the inspection points in glass container plants that is most often misused and object of great misunderstandings.
It is very useful and plays a significant role in the Quality inspection plan for the line if correctly used.
But it can also be misleading, because it can give an erroneous sensation of effectiveness that it cannot provide for.
First let’s see what actually a good use of this inspection is.
The objective of this inspection is only to assess the visual quality of the containers and therefore check the effectiveness of the inspection machines regarding this particular aspect.
If in the screen inspections are detected defects that should be rejected by the inspection machines – that the machines have the capability and are adjusted to – then something is wrong with the inspection machine(s).
The detected defects should be run on the appropriate inspection machine checking for rejection repeatability.
It is also another point to collect information regarding the overall visual quality of the containers being produced and to forward this information to the hot end.
This is especially important for cosmetic type of defects that may run continuously or not and that should be corrected by the hot end operator.
Now, let's take a look at some of the misuses.
The cold end operator inspects at regular intervals – during a short period of time – the containers that are passing in the line (in front of the light box screen). It is common to find frequencies of inspection of each two hours during 5 minutes.
This is a very important point: the screen inspection is a special kind of a “sample inspection” and in normal situations should not be used as a “continuous inspection”.
Only in very particular situations – if the inspection machines cannot properly visually inspect the container due to its geometry (or engravings) or at the start-up moment when the inspection machines are not yet properly adjusted – this inspection can be used 100% of the time as a backup visual inspection but with obvious limitations.
This can only be considered as a last resource situation. In such situations it is probably more effective to adjust the final sampling plan by increasing sample sizes and / or frequencies due to limitations in automatic inspection.
Although it can give some comfort to the Quality responsible – “at least we have someone looking to the containers …. “ – the fact is that it has very low effectiveness.
I know. I have been there taking that very same decision many times …
In fact, if there are no abnormal limitations to the inspection machines’, using the screen inspection continuously is only a waste of human resources.
If used continuously at least care should be taken to frequently rotate the operators that perform this inspection due to visual fatigue.
In fact this is the main reason why this inspection cannot be considered to be effective when used continuously. There are obvious limitations regarding the accuracy and precision of the human eye to perform such inspection considering average line speeds.
Since using the screen continuously it is not a standard situation – I am speaking for the majority of the glass plants - when there is the absolute necessity to do it, usually we must resort to extra manpower.
Sometimes, if there are no available resources internally, the temporary manpower is recruited externally to the plant. Unfortunately in these situations, it is frequent to find operators – sitting on the screen – that have neither the competences nor knowledge to perform this inspection.
They don’t even know what a bottle or a jar is… Therefore the joke…