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:
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.