To squeeze, or not to squeeze, that is an interesting question!

Squeeze testing, also sometimes known as simulated impact testing is a destructive test that can be performed on glass containers during the manufacturing process.

This is a test usually implemented in line and therefore to which all produced containers can be submitted.

It is common to see this test equipment installed in many plants of specific glass manufacturers groups but for many others glass manufacturers as well this is an equipment almost unknown or unheard off.

So it deserves that we look at its pros and cons.

Squeeze testing involves placing a glass container under stress to cause weak containers to break. Those “weak containers” have surface flaws or stress concentrators that lead to container breakage when squeezed.

The stress is caused by placing a load on one side of the container as the container rolls along a shoe or pad.

This method places a uniform stress all the way around the container during the test. Glass is a brittle material and like most brittle materials, it is strong in compression and weak in (tensile) tension. By loading the container in the described manner, the container walls receive an alternating stress. This stress causes the container to fail or break if the container has any structural flaws in its sidewall. "Good" containers withstand the stress and are undamaged.

When installed, it is the first equipment on the Cold End just after the annealing lehr. This helps prevent jams and damage to downstream inspection and handling devices.

The squeeze tester “rejects” - breaks! - all ware that is semi-broken or susceptible to brake in the inspection machines and therefore to damage or unsettle the inspection equipment.

So this not like the typical inspection machine that we have in the Cold End but rather more like an “in-line-destructive-test”.

In fact, perhaps – in my view – the biggest shortcoming of this equipment is the fact that it is not possible to collect the relevant data concerning the containers that are being “rejected” – broken! -by the squeeze tester.

Like I have referred in one of my previous posts: the task for the Quality Control / Cold End areas in a glass plant is not only guaranteeing that no defective containers reach the pallet and are not sent to the customer.

Also very important is to provide continuous feedback with reliable data for correction. That is, to provide information for correction of the manufacturing process, like: type of defect detected, affected mould numbers, occurrence percentages… and that you cannot get from a squeeze tester! - but you can from an inspection machine.

In the other hand, like any other inspection machine, we should have a challenge sample to assess the effectiveness of the squeeze tester at regular intervals. That is to verify if the structurally weak ware is being broken by the equipment.

Usually this procedure involves the simulation of a defective sample by merely damaging its internal surface with a scribe. The sample is then tested in the squeeze tester and checked if it is broken by the equipment.

Additional key checks shall be made to verify that the unit is set-up properly and that in fact is not a source of product contamination or damage.

Covers must be installed above the top of the finish of the containers to prevent that tramp (flying) glass – from breakages - does not gets inside them. The wider the finish of the container the greater the risk involved.

Any embedded glass – resulting from container breakages – should be removed from the pressure wheel so that subsequent containers will not be damage (frictive damage mechanism). Squeeze testers are equipped to prevent this but regular checks and maintenance should be in place to guarantee it.

Finally, a minimum air pressure set-point for the squeeze tester should be established and monitored regularly to assure that the defective ware is rejected – squeezed and broken! - appropriately.

Squeeze testers are not a consensual equipment in the container glass industry and there are arguments for and against its use.

Personally I have never had the opportunity to work on a daily basis with the equipment. So I do not have direct feedback to give.

But from what I know of the equipment, lead me to be a little bit jealous from my fellow colleagues that had the luck to count with the aid of such equipment in their lines.

I think that it is especially useful in situations that we are producing non-round containers or for containers where the standard automatic inspection is hampered due to the container geometry or special features.

In all situations I see it like a life insurance.

Let’s talk about something that it is not controversial!

And then, there are those things that are not controversial (confused with this beginning? Please check my previous post).

Those things that everyone agrees upon. Those topics that generate a broad consensus.

In a glass container plant, one of those – rare… - subject’s, concerns Critical Defects.

The definition of critical defect does not leave any doubt: “defects that could or are likely to result in hazardous or unsafe conditions for individuals using the glass container or be injurious to their health, under normal conditions of use as defined between the glass manufacturer and the filler.”

Or in other words: “A defective condition on the bottle (or jar) which can/may cause physical injury or even death to the consumer or our direct customer during normal use/operating conditions. Based on judgment and experience indicates that it could likely result in a substantial Customer or Consumer complaint.”

Bottom line: when they are detected every effort must be made to avoid that the defective units reach the pallet and are shipped to the customer- at all costs!

The examples of defects that are considered to be critical are not even arguable:

-Bird swing, spike (any plunger pull on the container inside surface), over press/wire edge finish, flanged finish (horizontal glass projection at guide ring and neck ring match area), chipped finish, internal stuck glass; internal loose glass; internal soft blister, sharp external stuck glass, sharp (or could be broken to become sharp) seam, etc...etc.

Basically we are referring to any defect that could result in a substantial Customer or Consumer complaint.

Off course there are some less obvious.

Here I would include for instance defects that can affect the sealing surface of the glass container and therefore cause the loss of vacuum and ultimately spoil the filled product (being that the case).

Upon detection, or even the suspicion of occurrence, actions must be undertaken immediately both in the Hot End and the Cold End.

The secret about any defect is to make sure we are in control, not the defect.

As long as we know when it happened and then can TRACE/TRACK it effectively, we will never have problems.

The most important is to immediately eliminate risk (reject at Hot End) and make decisions in CUSTOMERS’ favor.

A detailed report, as per procedure, should be required for each and every CRITICAL DEFECT we identify during production.

Independently if the defect is detected in the Hot End or in the Cold End the procedures usually establish redundant mechanisms.

These mechanisms usually imply that some final product is rejected – just to be safe – even though it is reasonably admitted that no critical defected was packed.

We find small variations in these procedures between glass manufacturers but we can safely say that the final goal in all is to make absolutely sure that no critical defect is packed and unintentionally sent to customers.

From my experience there are two situations of great concern in what regards the management of critical defects:

• ON and OFF situations (the defect occurs intermittently);

• The critical defect runs in the production (more of what is in the Lehr at time of detection).

Now this is a REAL issue when the defect is of difficult visual detection and the inspection machines struggle to reject it…

Let me give you the following analogy (with the courtesy of a very good friend of mine).

Let’s imagine that the plant floor is a football pitch (soccer pitch if you are in the US).

The operator that is the Hot End in the IS machine is the striker and the operator that is the Cold End (the Quality Inspector so to speak) is the goalkeeper.

The goalkeeper has the objective of stopping all the balls (defects) that the striker shoots at him preventing that they reach the goal (the final pack) - preventing a goal of being scored.

I have to say that off course the objective of the Hot End operator it is not to produce defects. It does not want to score a goal. This is just an image to illustrate a specific situation.

If the striker does not shoot too many balls at the goal keeper at the same time the goalkeeper can defend its goal and prevent the score.

But if the striker starts shooting various balls simultaneously in different goal directions the goalkeeper just it is not able to stop them all. Some will relentlessly go into the goal.

The best way to break down a system is to overload it!

Basically this means that while the Hot End works on the critical defect, it must be simultaneously rejected at Hot End.

And that rejection only must be stopped when we have absolutely sure that the critical defect root cause was found and corrected!

In the Hot End it is better to not jump to conclusions too early and allow sometime for confirmation.

Follow-up in the Cold End – after the defect was declared fixed - is critical!

Let’s talk about something controversial!

You know…

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.