The guy on the screen

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…

A real challenge to the inspection machine

In a glass container plant – automatic inspection machines – are considered to be the basic pillar of the quality control system.

The inspection machines are able to inspect 100% of the glass containers that are produced. The others inspection and controls are based on sampling – stratified or random accordingly with a sampling plan - and therefore less powerful.

Being such an important tool it is crucial that are implemented control measures to assure that the machines are always performing accordingly with the best set-up.

I am often questioned about the capabilities of different models of inspection machines: which ones are the best? Are the ones that I have OK?

Perhaps politically correct, but nevertheless - I believe - so true, I always reply that the most important thing it is not that the machine model that you have but what you do with it.

Nowadays in the market are available inspection machines that are very similar in their inspection capabilities. So independently of the machines that are installed in the Cold End, the key factors for success are the practices and procedures implemented that assure that the machines are always working correctly with the best possible set-up.

Working correctly here means that the machines are rejecting only defective units and accepting only containers that are not defective, accordingly with the defined set-up. The set-up will define what containers are good and what are not.

Although never a desired situation, in some particular cases where the optimum set-up is difficult to obtain, from a Quality point of view, it is preferable to have a – small - percentage of false rejects (good units rejected) just to guarantee that the equipment is able to surely reject defective units if they appear.

A challenge sample is a – or rather the - tool to guarantee the adequate operation of the automatic inspection machines in the Cold End of a container glass plant. It is the sample considered the rejection limit for a determined defect or characteristic, used for the adjustment of the automatic inspection machine and efficiency tests.

Any automatic inspection equipment that is being utilized on a production line must have an appropriate challenge sample for that equipment.

When equipment is set up to detect different inspection zones on a glass container, each inspection zone shall have its own challenge sample. The test location on the challenge sample shall correspond to the setting on the test equipment. Equipment includes all Cold End inspection equipment and other similar units.

There are 3 requirements (or rules, if you prefer) for validating a challenge sample:

• Has only one defect for one type of detector;

• Has a defect with the smaller detectable size possible (the sample must be a real challenge to the inspection machine!);

• Must be kept clean all the time, avoiding possible false rejections due to dirtiness.

If any of the challenge samples does not fulfills any of these requirements or if it is found a sample that fulfills in better conditions, then the initial sample should be replaced.

Records used to challenge inspection equipment shall list each function being challenged. For each function, it should be recorded if:

• The container was not inspected for this condition;

• The sample is broken or lost. (New sample is needed).

All challenge samples shall be clearly identified either by writing the name of the defect on the sample or through a documented alternate procedure within the glass plant. All of the challenge samples must be identified with a safeguard mark as well.

A safeguard mark is a mark placed in the challenge sample with the objective of assuring its rejection in case the sample it is not rejected in the tested machine. The mark assures that the rejection takes place in another one of the inspection machines downstream in the line. The mark facilitates its visual localization in the line if the above does not work or it is not possible to ensure (e.g. if tested the last inspection machine in the line).

Sample collection, selection and validation should comply with some specific rules.

In first productions and whenever it is necessary, defective samples are collected from the running production.

The types of defective samples to select are based in historical data of similar containers and past experiences, searching always for the most frequent and potentially dangerous defects.

Whenever possible and in case that that there are no sought after defects in the running production, simulated samples of defects should be used as challenge samples.

The validated challenge samples are identified and placed in the production line in specific boxes near the automatic inspection machines or near the end of the annealing lehr. All samples must be kept in good cleaning and conservation conditions.

The remaining samples that were selected but not validated should be kept and preserved for a future possible utilization in case of necessity.

For “non-first productions”, with the appropriate antecedence, the samples that were selected and validated in previous productions, should be gathered and placed in specific boxes to be sent to the Cold End of the respective production line. The samples must be in a clean and undamaged condition.

During the course of a production at the event of any loss or deterioration of the validated standard samples, the replacement of the sample shall be done as soon as possible.

First, it should be checked if there are - already selected, but not validated – equivalent samples for validation which can replace the lost or damaged samples, without any loss of quality in the inspection.

In the case that there are no compatible samples, they should be obtained from the running production, or simulated, and submitted for validation.

In the end of the production the samples must be withdrawn from the production line and stored assuring good preservation conditions.

Upon storing those samples should be checked for number and type. This is to assure that they can be used in future productions.

The criteria’s for validation of the challenge samples are based on:

Product specification;

• The degree of the challenge that the sample presents to the inspection machine (again, the defect should be as small as possible and detected by only one detector);

• The inspection capacity of the inspection machines installed in each production line, relatively to the defects in examination;

• Customer requirements (some glass fillers establish requirements for specific challenge samples – specific defects of concern – and requirements for run records and effectiveness rates).

For dimensional defects, acceptance and rejection samples may be created. These samples are important to assess if the rejection and acceptance limits of the inspection machine is correct. The acceptance limit is established by product specification. The selection of these samples is made through dimensional confirmation.

To run the challenge samples in the inspection machines some procedures must be followed.

Challenge samples are to be run at a minimum frequency of every four hours. Good practices advise to run the samples each two hours of production.

In the beginning of the shift all the challenge samples for the production line shall be checked for presence and condition.

If a sample is missing or cannot be used, that sample must be replaced as soon as possible. This occurrence should be recorded. In the case that there are no available replacing samples, they should be obtained from current running production and submitted for validation.

Each one of the challenge samples shall be run in the appropriate inspection machine. The test must also be conducted after every re-adjustment of the inspection machine.

The challenge samples should be run individually in the inspection machine. Only if the production line is running at higher speeds, a maximum of two samples may be run at the same time.

In each run, it shall be checked if each sample is rejected. The check is complete only after the sample is physically rejected by the machine air ejector. The check comprises that the ejector has sufficient strength to reject the container and that is synchronized.

At each run the “rejection safety” of each inspection machine (if present) should be tested once. Only if testing this feature, the standard sample shall be removed before it reaches the air ejector. Working correctly the machine should stop.

If the machine does not stop, it should be checked the cause of failure (e.g.: displaced ejector, deformed sample) and notified the inspection machine technician for adjustments.

It is expected that the run intervals will be missed only in cases of true emergencies, and that such cases will be relatively rare.

Each challenge sample shall be run through the inspection equipment (at least) three times at each check.

In order for the inspection equipment set-up to be considered satisfactory, the inspection equipment must reject the challenge sample all three times. Failure to reject three out of three indicates that the inspection equipment set-up is unsatisfactory.

This will require that immediate corrective action be taken on the inspection equipment, and may also necessitate a hold on existing ware. Upon completion of the corrective action, challenge samples must again be run, and must reject three out of three. This recheck must be documented.

It is a good practice to define Minimum Automatic Inspection Requirements for the organization.

These requirements establish the organization minimum requirements regarding Automatic Inspection. The aim is to standardize criteria and procedures regarding challenge samples.

These requirements for inspection equipment types are established by industry (market), defect or concern.

Due to shape and decoration considerations, container glass plants may be unable to comply with the minimum inspection requirements for certain containers or even to use the inspection equipment’s.

Every attempt should be made to use coarse inspection on the line. If there is an exemption and no automatic inspection is used, it must be provided for an alternate inspection method, such as light screen. Inspection frequency must also be increased.

It´s the communication, stupid!

By nature glass is a material which is peculiar. It is very strong if undamaged and very weak if severely scratched. Its strength it is highly dependable of the surface condition and the presence of stress concentrators.

Glass container production is a high speed mass production process.

Some defects may be present for a short time and then disappear, other might pop-up unexpectedly.

Glass container production will never result in “zero defective”. Glass containers are primary packaging and thus the allowable number of defective bottles is low.

Any glass production line therefore needs to have a variety of container inspections (machines, human) to detect and remove non-conforming bottles and jars (refer to post “Quality Control (QC) activities in a glass container plant – Overview.”).

All faults (defects, the stress concentrators) must be eliminated before they reach the palletizer. That is, so to speak, the primary objective of the quality control activities in a container glass plant.

Equally important is the collection and spread of all the data and feedback that outcomes from and concerns to all controls performed along the process. This is crucial for fault correction.

Often the fault is detected downstream (at Cold End) requiring a correction upstream (at Hot End).

The quicker the information reaches the agent that can act on the process – for correction – less defective units will be produced, rejected, scraped …

In this industry, it is easy to understand that for the effectiveness and efficiency of the Quality System is key one communication process: the communication between Hot End and Cold End.

It helps both areas. Improves and makes everyone’s task easier in the plant. Improves efficiency which means that improves Quality.

In a glass plant: high efficiency means low defect rate, which translates in production stability.

Communication must be bidirectional: from the Hot End to the Cold End and vice-versa. In the end communication is all about trust.

If the defects are timely informed, in an accurate and dependable way:

• Increases the trust placed by each one on its counterpart;

• This trust is returned in the form of better information;

• Trust is gained with rigor and accuracy in the information provided;

Accurate and detailed information must be exchanged, reporting:

• Mould number with problem;

• Inside or outside cavity;

• Area of the container with defect: finish, body, bottom, mould seam,…,

• Occurrence frequency;

• Rejection percentage of inspection machines (if detected at the Cold End);

• Samples of defective containers must be delivered to the Hot End (if detected at the Cold End);

When reporting defects to the Hot End – from the Cold End - one key aspect of the communication process is to prioritize what to communicate.

Prioritization, involves:

• Analyzing the different defects of the container;

• Order the defects by importance;

• Informing immediately the Hot End colleague of the most severe defects detected;

• Follow the less severe defects detected (prevent that they became more severe, follow-up trends);

• Inform less severe defects when there are no other priorities.

Nowadays is frequent to see glass plants operating some kind of computer based system that manages the data collection, spread and display. Records are performed electronically and messages are exchanged between areas.

Records and messaging comprise:

• All production losses;

• Information of moulds rejection by mould number reader and inspection machine;

• Warning messages (both ways);

• Information also when there are no rejections (allow to know production trends, are inputs to decision making);

• Mould change (verification and validation at Cold End);

• Hot End rejections: for later verification at Cold End (after lehr time);

Of the most importance is to record all occurrences: an inspection which was not recorded does not exist!

Two more aspects of communication in a glass plant should be mentioned:

• Communication between (within) shifts;

• Communication between shifts and day-shift team;

Communication between (within) shifts prevents surprises and facilitates the anticipation / prevention of problems. It facilitates the work of all and it must be oral and written (for the review of previous shift records).

It is done each with its counterpart. Major shift events must be reported. Also, the most frequent defects detected and mould numbers with more problems. Physically, the samples of the major defects detected must be reviewed together.

Communication between shifts and day-shift team it is done through Supervision and it is an important input for the Production Daily Meeting.

It is very important to leave - for review of the day-shift team – samples of defects whose rejection depends of the shift criteria (more frequently cosmetic defects).

Thus, visual standards of acceptance / rejection can be established and a criterion is standardized. Critical periods can be limited and decisions can be taken regarding the packed production (eg. colour variations, seeds and bubbles,…,).

It is crucial to convey to the Supervision the shift concerns and difficulties. These help to set the work priorities of the day-shift teams.