The art of follow up in glass container Quality Control.
Human manual inspections are an important part of the Quality Control plan in a glass container plant.
In the Hot End that type of inspection is performed with some limitations since at that process stage the glass container is not completely formed. By the other hand, the advantage in performing it at that moment is that the operator who inspects can immediately act on the process and undertake the appropriate corrections.
On the contrary, at the Cold End the inspection can be complete and thorough. The glass container is finished and can be handled securely. But once detected, the operator is not able to correct the defect. Its task is to give feedback to the ones who can do it.
In fact, put it in very simple terms we can say that in the Cold End everything is all about Quality Control. The value added by the Cold End to the product is “adding” Quality.
The action of the Cold End operators does not limit itself simply to: “rejecting defective units”. Like every other control performed at this stage, there are two important objectives to meet.
For sure, by one side, prevent - using all the available tools - that defective units reach the pallet and consequently are sent to the glass fillers (The Final Customer!).
And, by the other side, give accurate and timely information to the Hot End operator (the line counterpart), regarding the quality of the inspected product. As said previously – with the exception perhaps of the Cold End Coating – the Cold End operator does not perform process adjustments/corrections. But its action is pivotal in providing inputs for process correction upstream.
Human manual inspections performed at the end of the annealing lehr are an important step in the different Quality Controls made at the Cold End area. The two goals to meet – reject defective units and provide feedback – have here their specific procedures as well.
In this post I will focus the discussion on the some of the actions necessary for providing valuable feedback and following up defects in the inspection done at the end of the annealing lehr in the Cold End area of a container glass plant.
One of the most important advantages of Lehr inspection is that the results will show exactly what the quality level is before automatic inspection.
As the bottles come out of the Lehr and they are still stacked in a specific pattern, it is easy to select a specific mould from this pattern for inspection. Moulds will be stacked according to a set pattern, according to the I.S. Machine Timing.
While bottles are on the Lehr in a set pattern we can do effective and proper follow up. When the containers leave the Lehr exit they become a mixed population. When mould numbers are mixed, it is not possible to select each individual mould when taking a sample.
This is very useful when inspecting and doing follow up on cavity related defects. It is important to know that most of common process defects are mould / cavity related.
Recognised to be an important tool for defect follow-up, it is responsibility of all involved – and now I am specifically thinking in the Hot End personnel – to assure that the best regular container pattern (the possible that makes the job easier for the Cold End operator) – is achieved in the lehr.
Once the bottles come out of the Lehr and there are at least 10 or more rows without a break, the Quality Inspector must take and note on the stacking pattern board, or in some other kind of visual aid, of the full set of cavities by number. By picking up any mould from the Lehr, the problem mould can be traced immediately by utilizing the board.
Making sure the board is kept up to date at ALL times.
Regularly checking correctness of board.
Check after any gap appears in the stacking pattern: IS machine cavity/section stopped. There should be a visual and dimensional verification to the concerned cavities before and after the interval. The cavities that are in the vicinity of the intervals should be visually inspected, looking especially for the detection of glass stuck inside the containers. Verification and follow-up should be maintained.
If a fault is informed by the Hot End the necessary follow-up must be done on the Lehr.
Immediately checking the cavities signalled with the defect and initiating the rejection if the defect is confirmed at the end of the annealing lehr. Reporting fault assessment to the Hot End with percentage of occurrence, begin, end of systematic rejection.
By the time the Lehr length is almost over, Hot End Operator should have checked another set and should have fixed the fault or changed the mould or blow it off at the Hot End.
If the defect it is not confirmed at the lehr, the inspection to the signalled cavities should be kept by a typical minimum period of time of the lehr time plus 30 minutes. Usually rechecked at five minute intervals. At least five bottles of each offending mould must be taken and inspected.
Identifying trends is a key aspect of the inspections:
If the same mould runs with the same defect for three or more checks (Defect trend);
If the same mould runs with various defects for three or more checks (Sectional trend).
If a mould is found defective with the same defect for three or more checks throughout the shift. (It is a trend primarily because the problem was not successfully rectified in the first place.)
All of this constitutes valuable feedback for Hot End Correction.