Quality Control (QC) activities in a glass container plant – Overview.
In the glass container manufacturing process everything starts with the reception and storing of the different raw materials.
As a first step, in order to make it possible to control the quality of the incoming raw materials, it is necessary that the glass manufacturer establishes a technical specification for each raw material that is used to produce glass. Cullet – recycled glass from external or internal origin – should be treated like any other raw material; therefore a technical specification should be developed as well for cullet.
Typically the specification defines optimum and normal reject limits for key parameters specific of each raw material: chemical composition and grain size.
Percentages for the concentration of main oxides, desired an undesired (contaminations) are established.
The goal it is not having a detailed chemical composition description of the raw material but rather focus on the oxides that are considered to be important. That is, those that can affect the outcome of the melting process, in the end the glass quality.
Grain size is another key parameter to control in a raw material. Economical and quality concerns are associated with it.
For cullet are established limits for contaminants: foreign materials (ceramic, organic) and colours (especially important if producing in flint glass colour). If the cullet used in the plant has an external source – bought from a supplier, opposed to an internal source, coming from internal glass rejections – it is of special importance the control of its quality. The uncertainty is greater.
Usually this is done by operating a formal compliance certificate system.
Ideally these technical specifications should be part of the commercial contract between glass manufacturer and the raw material supplier. As a minimum they should be of proven knowledge by the supplier.
Once established technical specifications for the raw materials, physical and chemical checking of incoming raw materials on receipt can be undertaken.
These controls are undertaken either in a local laboratory in the glass plant or subcontracted to an outside accredited lab. The latter has been of preference – as long as the response time is adequate – to decrease costs.
Nowadays these checking’s have largely been replaced by the provision of supplier certificates of conformance, by the access of the glass manufacturer to the raw material supplier’s process control information complemented by audits of the raw material supply locations. The purpose of such audits is to ensure familiarity with the process control methods and standards used by the supplier and to confirm their ability to adequately control the raw material within the glass manufacturer technical specification. These audits are part of the organization supplier audit plan.
Depending of the type of system that is in place to assure the quality of the raw materials received, the incoming check can vary from a confirmation of the nature of the material, the quantity being delivered and a validation of the certificate of conformance.
If a chemical and/or physical checking is required small samples can be taken and retained for laboratory examination. This may be done either on a random basis or regularly in accordance with an inspection plan.
A typical check that is done locally at the plant at the moment of raw materials receiving is the moisture content of the sand. It is necessary to control this to tight limits and if these are exceeded then corrections have to be made to the batch composition to compensate at the time of mixing. Also there are obvious economic implications in receiving a truck load of sand with excess of moisture.
Other materials that are used – or aid – in the container glass manufacturing process are also submitted to some kind of quality check. Among these we can refer: moulds, packaging material and coating materials.
In what concerns moulds, typically compliance certificates are required together with data from the checks carried out from the supplier. Alternatively, or sometimes additionally, the glass manufacturer samples the incoming mould items and carries out his measurements against a defined plan and the mould specification. Again, to decrease costs the latter option has becoming less used by the glass manufacturers.
The typical approach concerning other materials is the monitoring of the suppliers through certificates of conformance systems and supplier audits whenever appropriate and integrated in the supplier audit plan of the organization.
As in other cases the observed tendency is not to exert the control at plant reception using plant resources but rather to have tools that allow supplier monitoring continuously or at prescribed occasions.
The result of the batching, mixing and melting of the raw materials is glass. Independently of its end usage – that is, if it is going to be moulded into a bottle or jar – there are a couple of controls that should be performed to assess the quality of the molten glass.
The density test is a quick and easy way to indirectly detected unexpected changes in the glass composition due to mistakes in the raw materials batch and mixing process. The test involves the determination of the density of a glass specimen and comparison with previous results, monitoring significant deviations.
Homogeneity test indicates the presence of any inhomogeneous glass which shows up as coloured streaks under polarized light. The degree of inhomogeneity – permanent induced stress - can be quantified if necessary by examination under a petrological microscope. These inhomogeneities are known as cords.
Bubble and seed – gaseous inclusions in the glass, potentially stress concentrators – count, give an indication regarding the quality of the melting process. The result of the count is compared against the glass manufacturer established rejection limits.
Stones are solid un-molten material embedded in the glass that acts as stress concentrator - induces permanent stress in the glass surface. The total amount of stone contamination – stone count - is compared with established rejection limits. In parallel the origin of the stone is determined by visual comparison using reference literature.
If producing coloured glass it is fundamental to control the glass colour. Glass colour can be measured using a spectrophotometer to determine: dominant wavelength, purity and brightness. Again, the result achieved is compared against the established colour standard.
Manual QC starts at the Hot End. The Hot End comprises basically glass conditioning and container forming.
Immediately after the glass containers leave the forming machine. The Hot End operator should perform a visual and dimensional verification.
Samples from each individual mould cavity on the forming machine – a stratified sample -are taken off at regular predetermined intervals and allowed to cool for inspection in a light box. They are inspected visually and gauged either with fixed go-no go gauges or by a conventional S.P.C. measurement system where this method of control is preferred or more appropriate.
S.P.C. system is preferred since it is of preventive and not of reactive nature.
The glass weight is another parameter which is regularly monitored at the Hot End.
In fact – although sometimes undervalued – the detection of defects at this stage it is very important. Here it is possible to act immediately on the machine for correction. There is no significant delay involved between the moment of defect detection and the action for correction.
One of the idiosyncrasies of this manufacturing process is that – simply putted – the latter part of the process is almost entirely dedicated to QC activities. We call this area the Cold End.
In the Cold End, immediately after the containers leave the annealing lehr and after the application of the Cold End Coating, they are submitted to a second QC, now performed by a Cold End (Quality) operator. Again, we have an attributes inspection for stratified sampling. This inspection is performed at regular intervals in accordance with an inspection plan.
The controls are similar to those performed on the Hot End but with the advantage that at this stage the container is annealed and coated (hot end and cold end coated). That is, the container is finished.
The aim of this control is to detect and eliminate defects and give important information to the hot end for the correction of defects. In fact one of the key aspects for the success of a Quality Control System in a glass container plant is precisely the communication between Hot End and Cold End operators.
The results of these controls are recorded either electronically or on paper documents (each defect has a specific code which facilitates the communication and recording).
After this manual inspection the containers are placed in single lines and passed by various automatic inspection machines. This type of inspection should be the most effective. Every single container produced is inspected as opposed to the other inspections that are based on sampling plans.
Associated with the inspection machines we have a crucial tool used for defect elimination. That is the mould number code engraved in the containers as a dot or alphanumeric code. This code allows the automatic – and reliable – rejection of specific mould numbers by the inspection machines.
Important as well is to assess regularly the effectiveness of the inspection machines. The monitoring of such condition is done with the use of challenging samples. These are samples of containers with specific defects used to verify if the machines are performing as expected. However as simple as this may seems, rules and procedures must be followed in order to assure that only the best and most adequate samples are used. Otherwise it will be a wasteful and ineffective operation.
Inspection machines can be grouped – generically - in 3 big groups depending of the type of inspection they perform.
Sidewall inspection machines, inspect for defects – visual and some dimensional – in the body of the glass container.
Base and finish inspection machines, inspect – as it says – the base and finish areas of the containers.
These two groups of inspection machines use cameras for performing their inspection.
A third type of inspection machine - multi inspection machine - uses light reflection principles to detect and automatically reject defects. These machines mechanical in nature – several inspection stations, containers indexed by a star wheel – also check selected dimensions such as the bore of a bottle or the waviness of the sealing surface of a jar and glass thickness, amongst other possible detections.
These three groups of machines are installed sequentially in the production line and sometimes subdivided in equal parallel branches on the Cold End.
After the inspection machines in the cold end usually we have a human visual inspection as the containers pass in front of an on line light box (light screen).
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.
The objective of this inspection is often source of much misunderstanding.
Here we want to assess the visual quality of the containers and therefore check the effectiveness of the inspection machines regarding this particular aspect. 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.
Only in very particular situations – if the inspection machines cannot properly visually inspect the container due to its geometry or engravings (limitations to inspection) – this inspection can be used 100% of the time as a backup visual inspection but with obvious limitations.
Care should be taken to frequently rotate the operators that perform this inspection due to visual fatigue.
A final statistical check, or audit, is done as the containers are being, or when they have been, assembled on pallets for shrink-wrapping. Random samples are made accordingly with a sampling plan and the containers are visually inspected.
For the purpose of these checks, imperfections are often classified into three main groups: critical, major and minor defects. AQL levels are established in the organizations for each type of defect and the result of this inspection is compared against that standard.
This final inspection is used to formally approve the batch in production. We should be able to validate all the controls that are upstream.
Once the containers have been assembled onto pallets, each pallet is assigned a sequence coded label. The information in the label can be cross-referenced to all control inspection records. The batch sequence code number on the pallet label therefore provides traceability to the rest of the manufacturing chain and the other on-line control points.
Where traceability of individual containers is required this is achieved by marking each individual bottle at the time of production usually with an ink jet mark. Although very often a customer requirement, this marking is very useful for the producer as well. Especially when there is a need to investigate causes for specific failure or defect.
In parallel in the Quality Laboratory a series of parameters are tested in order to determine the container resistance and compliance with specifications.
Typical tests encompass: thermal shock, internal pressure, impact test, vertical load, annealing level, hot end coating level and cold end coating level.
Also, quantified dimensional measurements – in the production line the assessments are qualitative: go, no-go! – are performed and calculated simple averages and ranges. These results are handled statistically to detect trends and trigger corrective actions at the earliest possible moment.
The frequency of testing depends on several factors. These include the method of test, the magnitude of the control values, and how quickly a feature could go to out of control.
Resources are scarce and it is the task of the Quality Control Manager to determine were efforts must be made and where they can be relieved. The outcome of this management is documented in the Quality Control Plan for the specific container.
Samples from each mould are checked when making a routine control test. Apart from the annealing and cold end coating test, where the samples are selected based on their position in the annealing tunnel.
A failure during a routine process control test initiates an immediate retest of a larger sample (usually 3 to 6 containers from the mould cavity concerned) to establish whether or not there is a downward quality trend.
When sub-standard ware is detected, all the containers from the suspect mould cavities are rejected until there is a successful test. In addition, all the ware packed as good from the suspect mould cavities since the last successful test is regarded as suspect and reinvestigated so that all substandard ware can be rejected.
In the end – regardless of how cliché this might sound – people make the difference between an effective and an ineffective quality control system.
Key aspects encompass their attitude when performing the prescribed tasks and decision making. There is a significant connection between knowledge, skill and good decision making. Correct decisions are best made by the person performing the task.
The operator’s key role is to follow standard procedures relative to process control and document problems and suggestions.
Understanding customer needs and interacting with the customer via plant visits and cross functional teams is absolutely critical. Observe their filling lines and get direct feedback on the quality of the glass containers.
Every opportunity to interact with customers should be taken on a regular basis.