There are those subjects that when come up during a conversation between friends are bound to give much discussion.
I would say that: politics, religion and sports are perhaps among those subjects that frequently have the effect of stir up the spirits.
In a glass container plant there is one particular matter that has a similar effect (OK! Among others…)
And that concerns visual or cosmetic defects. Or more precisely the actions or procedures to take when there is the detection of such type of defects.
To put the question in very candid terms usually the discussion turns around: if detected we should reject or not such type of defects. After all, the glass producer is “tolerated” to produce a certain percentage of those of defects, isn’t it????
Let’s take a step back for a while and see where it all begins.
Unfortunately, producing glass containers it is not an activity free of producing defective units. There will be defective units in virtually every production batch. And this is true even after the producer has checked each individual product (trough automatic inspection machines)!
Is there anyone who can guarantee that automatic inspection is 100% reliable?
When the defect does not result in life or death outcomes, the supplier it is not expected to deliver defect-free goods. I am not saying the glass filler will not always demand that from the glass supplier. And I know that there is not a single glass supplier that will approach its customers saying: “most probably in this batch of bottles that I am sending to you, you will have defective units”.
I am just saying that there is the need in the supplier/customer relationship to set the limit between acceptability and refusal in a way that can be agreed upon and measured.
That limit is called the “AQL” – Acceptance Quality Limit.
Defects are classified in three main groups: Critical Defects, Major Defects (sometimes subdivided in Major A and Major B) and Minor Defects. Each glass defect is then framed accordingly with this classification.
For each one of the former defect categories are established AQL levels (quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling). These AQL levels (given in percentage) will ultimately translate in acceptance and rejection numbers for determined sample sizes and type of defect.
Usually there are standard AQL levels that the producer uses for its standard catalogue products and sometimes for exclusive products – sold exclusively to a single customer – are agreed specific AQL levels.
So, in a way, AQL’s seem to respond to the question concerning the percentage of visual/cosmetic defects to reject or accept.
But then there are some curious variations…
Like different sampling plans with different AQL levels for glass containers destined to the local market and the export market…
It makes one wonder if the customers are more or less demanding depending of their geographical location… (Just being ironic!)
AQL levels are a tool for regulating – commercially – the relation between producer and supplier. But operationally – on the glass plant shop floor – it poses a couple of challenges. Most notably in terms of communication of Quality Standards.
Quality Inspectors like any other “inspection equipment” need to be “calibrated”. Their criteria must be in line with the criteria defined by the Quality Manager. So every caution must be taken when communicating the concept of AQL and its implications to the quality inspections.
It cannot transpire the idea that it is allowed to produce a certain quantity of defects. AQL is not a Desired Quality Limit.
Also we cannot allow that it is perceived that some glass containers must have better quality than others depending if they are destined to customer A or B or market Y or Z.
The actions undertaken by the Quality Inspectors must have the ultimate goal of preventing that defective units are packed. Independently of the defects severity or AQL classification.
So this brings us to a hot topic.
Glass manufacturers are very much focused on efficiency. In fact – efficiency – is perhaps the most important KPI (Key Performance Indicator) in a glass plant. Even more important than Quality related KPI’s like a Quality Complaints index.
Now that is a controversial statement!
So everything that hurts efficiency in a glass plant is took very seriously and object of great discussion. So it is not rare to find moments of tension and eyebrows raised when:
A visual/cosmetic defect is detected in an inspection at the end of the lehr and rejected with the Mould Number Reader;
A visual/cosmetic defect is detected in a light screen inspection and it is not possible to adjust the appropriate inspection machine (the rejection it is not consistent) to reject the defect. Finally the defect is rejected in the Mould Number Reader.
The argument is: why reject with 100% efficiency a defect that is “allowed” in a certain percentage? We are losing more-or-less-good bottles (or jars)!!!!
Things have – are – changing.
Most of the times because of customers or market demand and pressure.
In fact, ourselves, the final consumers, we are the driving force behind the ever increasing demand in terms of Quality.
Having clear instructions known and understand by all players at the plant floor level it is a great help.
The procedures usually take in consideration the occurrence frequency – if it is continuous or intermittent – and severity of the visual defect, establishing the correspondent action:
Immediate rejection by MNR and information for correction to the Hot End;
Information to the Hot End, allow one lehr time for correction and if not corrected rejection by MNR.
If the procedures are clear and people know what they are doing that usually works.
I believe there is also a word for that: training.