The most common glass container defects and how a properly set up IS machine prevents them

This overview links common defects to the parts of the hot end worth checking. It is not a diagnostic guide or a list of acceptable tolerances; those depend on the container, process and production line.

Why defects are found at the end of the line but cannot be fixed there

The cold end allows cooled containers to be inspected, but the point at which a defect is detected does not establish where it originated. Uneven wall thickness may have arisen as the gob entered the blank mould, during glass distribution or in final forming. Rejecting a container does not, by itself, change the forming conditions.

Trace each defective container back to its production time, section and mould. Allow for its passage through the annealing lehr and any conveyor delays, so that you do not compare it with settings that were not yet in effect when it was formed.

Follow the glass from the gob through delivery, blank forming and final forming to transfer into the lehr and annealing. The melting furnace and batch can affect glass properties, but the focus here is on variation along the forming line. At each inspection point, ask a specific question: was the variation already present before the next process step?

Compare reject rates over equivalent periods, keeping start-up separate from steady production. Pack-to-melt — the weight of acceptable packed containers divided by the weight of glass melted — indicates overall performance, not the cause of a defect. Also track acceptable output against plan and customer complaints by batch.

How gob formation and delivery affect the container

A gob is a separated portion of molten glass; feeding is the process that produces it. As well as weight, check its shape, thermal condition and entry into the blank mould. Link gob-weighing records to the gob, section and sampling time.

1. Uneven glass distribution. This may originate in gob formation or delivery, or during blank forming. It shows up as differences in wall thickness around or along the container. Check gob weight and shape, and how it enters the blank mould; asymmetry alone does not prove a delivery fault.
2. Variation in container weight. This may originate in the feeder before the gob is cut. It shows up as differences in the weight of comparable containers over time. Compare those weights with gob-weighing records and feeder operating conditions, rather than relying only on a whole-shift average.
3. Surface folds. These may be associated with poor gob entry, glass folding over itself or its thermal condition during blank forming. They appear as localised ripples or folded areas on the surface. Their position can help narrow down where to investigate, but does not establish a single cause.

The timing of the feeder plunger, tube and shears affects gob formation and cutting. Assess the effect of any change with the glass temperature conditions and production speed recorded. When assessing a servo feeder for gob control, process repeatability matters more than simply having an adjustable parameter. Changing several settings at once makes causes harder to distinguish.

Assess the gob delivery system as a whole. It includes scoops, troughs and deflectors. For this overview, the key checks are smooth delivery and consistent gob arrival. Diagnosing individual components requires a separate procedure.

The blank-forming process also affects the result. Blow and Blow (BB) uses air, while Press and Blow (PB) uses a plunger. Narrow Neck Press and Blow (NNPB) uses pressing for narrow-neck containers. The same visible defect therefore cannot automatically be attributed to the same setting across these processes.

Which defects may be linked to the mould and its cooling

The mould affects both shape and heat removal. When quality is unstable, compare actual cooling and operating conditions, not just the saved recipe. Allow for the time needed to reach thermal stability after a product changeover.

4. Body ovality. This may arise during final forming or when a still-soft container deforms after the mould opens. It is identified by different dimensions measured in different directions across the same section of the container. Check its relationship to the mould, cooling and take-out timing; the measurement alone will not distinguish every mechanism.
5. Raised mould seam. This may form along the mould parting line during forming. It appears as a pronounced ridge where the mould halves meet. As well as mould closure, check timing and thermal conditions against the specific mould set.
6. Fine cracks around the finish. These may arise from local thermal or mechanical stress during forming and transfer. They appear as short cracks in a particular part of the finish. Repeated occurrences on the same section are a reason to investigate, not proof of inadequate cooling.

For an ISX machine, assess whether the settings and cycle repeatability suit the container. Any reduction in rejects must be demonstrated by production records under defined conditions; the machine type alone is not evidence.

Lightweighting bottles requires even glass distribution, stable cooling, and repeatable movements and timing. Reducing gob weight is not enough; testing must confirm that the container remains acceptable.

How annealing and loading affect container defects

Annealing relieves internal stress through controlled cooling. What matters is the temperature history of the container, not just the controller setting: air temperature is not the same as glass temperature. Monitor belt speed, lehr loading and container spacing.

7. Excessive residual stress. This may be associated with insufficient temperature equalisation or an unsuitable cooling profile. It is identified by a failure in stress testing under the specified method. Before drawing a conclusion, check the temperature profile, container layout and differences in wall thickness.
8. Cracked base. This may originate where the container experiences an excessive temperature difference or mechanical stress. It appears as a crack through the base area. Finding it after the lehr does not, on its own, establish annealing as the cause; the container’s condition before entry also matters.
9. Contact damage to the surface. This may occur during lehr loading, through contact between containers or with guides. It appears as localised marks, scuffs or damage at a recurring height. Check container position, spacing and the sequence of movements before lehr entry.

Record the temperature profile by zone alongside production loading. Compare setpoints with measured values and verify how measurements were taken. A lehr’s maximum design temperature is not an annealing recipe for a particular bottle. Without reference to its geometry and passage through the lehr, it cannot explain a stress-test result.

Assess the annealing lehr in relation to the actual flow of containers from forming. Uneven loading changes local loading and airflow around the containers. Spacing is therefore a quality-control check, not merely a conveyor issue. Detailed diagnosis of alignment is beyond the scope of this overview.

When comparing results, retain each container’s position across the belt and record interruptions to the flow and product changeovers. Otherwise, a local or temporary variation may prompt an unnecessary change to the entire temperature profile.

How to record defects so they can be traced to a cause

A useful record links the defect type to the container, the BB, PB or NNPB process, section and mould. Add the line, shift, detection time and estimated time of production. Note any connection with a product changeover and the most recent adjustment. This is a recommended recording structure, not a site-specific operating standard.

Track repeated defects by time and production position, not just by shift totals. Record the number of containers inspected as well as the number found defective. Where a container has several defects, set a counting rule so that the sum of defect categories is not mistaken for the overall reject rate. Keep customer complaints linked to batches and available production records.

For each adjustment being assessed, record who is responsible, which parameter changed and the conditions for evaluating the result. Where the process allows, avoid changing several independent settings at once. Check the result only on containers that actually passed through the process after the change. This reduces guesswork after a product changeover and avoids repeating adjustments whose effects have not been demonstrated.

If you cannot trace a defect to a particular stage, seek an independent hot-end audit. Prepare samples, a map of occurrences by mould, the settings history and records of passage through the annealing lehr. The outcome should be a prioritised sequence of checks and evidence for any subsequent feasibility study of a technical change. Set its timetable according to the data available and the scope of the assessment.