The Complete Guide to
Can Seam Defects

The integrity of a canned product depends almost entirely on the quality of its seam. A compromised seam is not merely a cosmetic defect or a quality inconvenience; it is a potential pathway for Clostridium botulinum, the anaerobic bacterium responsible for botulism, one of the most lethal foodborne illnesses known. Even minor seam deviations, ones that may not be visible to the naked eye, can allow microbial contamination over time or cause premature spoilage, putting consumers at risk and exposing producers to liability.

Regulatory agencies worldwide treat can-seam integrity accordingly. For low-acid canned foods, FDA regulations under 21 CFR 113.60(a) require regular closure observations, visual examination of at least one can from each seaming head at intervals not to exceed 30 minutes of operational time, and records of observations and corrective actions.

For acidified foods, analogous closure requirements are outlined in 21 CFR 114. 
The Can Manufacturers Institute (CMI) publishes detailed standards for seam
dimensions and defect classification. GFSI-benchmarked certification schemes, including SQF Edition 9 (FSC 15: Canning, UHT, and Aseptic Operations), BRCGS Global Standard for Food Safety Issue 9 (Section 6: Process Control), and IFS Food Version 8, include container closure integrity requirements within their thermal processing and canning modules. Consult the current edition of each standard for specific clause references applicable to your operation.

Seam defects remain a persistent risk to closure integrity, leading to spoilage, HFIs (Hermetic Failure Incidents), customer complaints, and, in severe cases, recalls. The reason is not a lack of awareness; it is a fundamental gap in how seam inspection has historically been performed.

Until recently, no system existed that could inspect every can at full line speed across the full 360 degrees of the seam without contacting or disrupting the production line. That gap is now closing, but understanding why it mattered requires starting from the basics.

The “double seam” is the industry-standard closure method for metal cans. Understanding how it is formed is essential context for understanding how and why it fails. The process described here applies to both two-piece (drawn) and three-piece (welded) cans; the primary difference is that three-piece cans have a side seam, which adds complexity at the crossover junction where the side seam meets the double seam.

A double seam joins two distinct metal components:

• The can body: The cylindrical body of the can, whose top edge is flanged outward during manufacturing
• The end (or lid): A stamped metal disc with a curved profile around its perimeter, called the curl

Double seaming is performed on a machine called a seamer or seaming machine, which operates through a two-operation sequence. Before seaming begins, the can body and end are brought together at the make-up point, where the end is placed on the flanged body, and the assembly is lifted onto the seaming chuck:

The double seaming process requires extremely precise mechanical control. The seaming rolls must apply exactly the right profile and pressure to produce the correct hook dimensions and compression. Variables that affect seam quality include:

Modern canning lines operate at speeds of 600 to 1,200+ cans per minute across multiple seamer heads. At those speeds, even minor mechanical degradation can produce hundreds or thousands of defective cans before it is detected through conventional inspection methods.

Seam Thickness (T) The total compressed thickness of the double seam, measured at its widest point. Excessive thickness may indicate insufficient roll pressure; insufficient thickness may indicate over-compression or metal fracture risk.

Seam Width (W) / Seam Length (SL) The vertical dimension of the seam from top to bottom. Seam width and seam length refer to the same measurement; terminology varies by region and canmaker convention. This measurement is critical because it directly relates to hook length; a seam that is too narrow may have insufficient hooks to maintain hermeticity.

Seam Thickness (T) The total compressed thickness of the double seam, measured
at its widest point. Excessive thickness may indicate insufficient roll pressure;
insufficient thickness may indicate over-compression or metal fracture risk.

Seam Width (W) / Seam Length (SL) The vertical dimension of the seam from top to bottom. Seam width and seam length refer to the same measurement; terminology varies by region and canmaker convention. This measurement is critical because it directly relates to hook length; a seam that is too narrow may have insufficient hooks to maintain hermeticity.

Body Hook (BH) The length of the can body flange that has been turned inward and formed into a hook during seaming. Adequate body hook length is essential for seam integrity. Short body hooks are a common and serious defect type.

Cover Hook (CH) The corresponding hook formed from the end curl. Cover hooks must meet minimum length specifications; short cover hooks are similarly indicative of a compromised seam.

The following section provides a detailed reference guide to the most commonly encountered can seam defect types, including their causes, visual and dimensional characteristics, and quality implications.

Consequences: 
Because the body hook is formed from the flange, a knocked-down flange directly results in a short or absent body hook; this, in turn, reduces overlap to critical or zero levels, with the same hermetic consequences as a false seam. Knocked-down flanges are a critical defect.

Consequences: 
Short hooks directly reduce overlap. Since overlap is the primary determinant of seam integrity, hooks at or below the minimum specification are treated as critical defects. The combination of a short body hook and a short cover hook, even if each is only slightly below specification, can produce overlap at or near zero.

Detection Challenge:

Sprung seams may appear normal externally; seam thickness measurement or X-ray is needed. Seam bumps are typically detectable by external visual or optical inspection.

Detection Challenge:

Cutovers at the crossover zone may not be visible externally and are easily missed by both visual and optical inspection. Teardown must be targeted at the crossover to detect them.

Detection Challenge:

Deadheads are typically detectable by visual inspection due to the obviously malformed seam profile. However, partial deadheads affecting only a portion of the circumference can be subtler.

Detection Challenge:

Peaked seams may appear acceptable externally. Cross-section or X-ray inspection is required to evaluate the internal seam profile.

An important insight for quality engineers is that seam defects are rarely random. They are almost always traceable to a specific seamer head, and often follow predictable patterns as that head degrades.

Modern canning seamers have multiple heads, commonly 4, 6, 8, or more, that rotate and each seam a can in sequence. When one head develops a mechanical issue (roll wear, chuck wear, timing drift, lubrication failure), the defects it produces appear periodically in the production stream, every 4th, 6th, or 8th can, depending on the
head count.

A crucial point for sampling-based inspection is that it may completely miss a consistently defective seamer head if the sampling frequency is not aligned with the number of seamer heads.

For example, a 6-head seamer producing one defective can out of every 6 represents a 16.7% defect rate from that head. Still, if you are pulling one teardown sample every 30 minutes from a line running 800 CPM, your sample is statistically unlikely to catch the defect consistently. It may go unnoticed until a customer complaint or a routine audit.

Seamer head tracking, the ability to associate each can inspection result with the specific seamer head that produced it, is therefore an essential capability for meaningful seam quality management. It transforms defect data from an aggregate metric into an actionable, head-specific maintenance signal.

The industry standard for seam quality verification is manual teardown, in which a trained operator removes a sample can from the line, uses a seam saw or strip can tool to cut and strip the seam, and then measures the hook dimensions with a micrometer at three or four points around the circumference.

Strengths: Teardown analysis directly measures seam geometry and is the basis for regulatory compliance documentation. It provides accurate hook, thickness, and width measurements.

Limitations:

• Destructive and slow: Each teardown takes several minutes and produces a rejected can. Typical sampling rates are 1 can per seamer head every 30–60 minutes.
• Very low statistical coverage: On a 1,000 CPM line, a 30-minute teardown interval means one sample per ~30,000 cans. Even pulling one teardown per head on a 6-head seamer at that interval covers only 6 of 30,000 cans, a coverage rate of 0.02%.
• Measures only sampled points: Even within a single can, teardown measures only 3–4 cross-sections. Localized defects (droops, false seams, spurs) affecting a small arc may not be captured.
• Lags mechanical degradation: By the time a teardown sample catches a deteriorating seamer head, thousands of defective cans may already be downstream.
• Operator-dependent: Measurement accuracy and technique vary by operator skill and attention.

The limitation is not statistical process control (SPC) itself. SPC remains essential in
every serious quality program. The limitation is the data behind it. When the data
comes from samples, even the best SPC system is still interpreting the process through snapshots. More checks and better gauges improve the inference, but they do not change its nature: you are still making decisions about cans you never directly examined.

There is also a less obvious limitation in how samples are collected. A can pulled during a deliberate line slowdown does not always represent seam formation under normal running conditions. Speed-related effects, including thermal expansion, vibration, and roll dynamics at full CPM, change how seams form. Defects that appear at production speed can disappear in a slowed sample and reappear at full output. Unless sampling is performed at true production speed from the heads that need attention, this blind spot persists.

Some operations use optical inspection systems, typically cameras, to detect visible seam defects. These systems are faster than teardown and can inspect every can.

Strengths: Non-destructive, 100% inspection of visible defects, relatively high throughput.

Limitations:

• Limited to surface defects: Optical systems can only detect what is visible on the exterior of the seam. Internal defects (false seams, short hooks, insufficient overlap) are entirely invisible to optical inspection.
• Angular coverage limitations: Most optical systems inspect from a limited number of fixed angles, potentially missing defects that are not in the field of view.
• Non-dimensional measurement: Optical systems detect shape anomalies but cannot measure seam dimensions, the data that actually determines seam integrity.

Visual inspection of seams by line operators, looking for visible droops, spurs, or anomalies, is a supplementary measure in many facilities. It is inherently low-throughput, unreliable for subtle or internal defects, and subject to fatigue and attention variability. It cannot be considered a quality control measure for regulatory compliance purposes.

X-ray inspection meets all four requirements. X-ray imaging enables visualization of internal seam condition and deviation signatures within the multilayer seam in real time. Unlike optical inspection, X-ray is not limited to surface features. It is also worth distinguishing inline seam inspection from contact-based seamer monitoring (proximity sensors, accelerometers, or other machine-mounted instrumentation). Machine signals provide useful information about the seaming process, but they are not the same as inspecting the finished seam itself. The finished seam on the can is the arbiter that matters, and that is what X-ray seam inspection evaluates directly. Importantly, inline X-ray inspection does not replace manual teardown; it transforms teardown from a discovery tool into a directed confirmation tool, enabling operators to verify specific heads exactly when the data indicates a concern.

The Peco InspX SeamTrac Legato is currently the only system that delivers on all four requirements simultaneously in an inline production environment. It is designed to mount over the existing single-file seamer discharge conveyor with no internal conveyor and no product contact, while using multiple overlapping X-ray beams to provide full-circumference coverage at speeds up to 1,200 CPM in as little as 7.5 feet of linear line space. All Peco InspX X-ray systems are designed to meet applicable radiation safety standards; contact Peco InspX for details on FDA CDRH compliance and state licensing requirements.

Detecting defective cans is necessary but not sufficient. The more powerful capability is the ability to attribute each defect to the specific seamer head that produced it, enabling maintenance teams to intervene on the specific head showing performance degradation before it produces a large volume of defective product.

The shift transforms seam inspection from a sampling method, which only catches defects based on periodic checks, to an architecture providing direct visibility into the process. The result is a reduction in the need for seamer adjustments and minimized production downtime, as the system addresses the root cause of defects rather than just filtering out faulty products.

The SeamTrac Legato’s Advanced Seamer Tracking capability assigns every inspection result to its originating seamer head in real time, generating the Peco InspX SeamScore. This per-head performance metric provides operators with actionable, targeted maintenance intelligence rather than aggregate line-level quality data. Combined with AI-directed predictive maintenance (PdM), the system identifies which heads are trending toward out-of-specification performance before they begin producing defects at scale. An On-Demand QA Sampling feature allows operators to trigger a targeted teardown sample from any specific seamer head without stopping the line, enabling verification exactly when and where the data indicates it is needed.

Fill level is directly related to seam integrity in one important respect: under-filled cans have excess headspace, which affects the counter-pressure dynamics during seaming and can contribute to seam formation problems. Overfilled cans create excess internal pressure that can stress the seam post-seaming. Integrated headspace monitoring, available in advanced inline inspection systems, allows these fill-level issues to be identified simultaneously with seam inspection, closing a second quality loop in a single pass.

The SeamTrac Legato includes integrated headspace monitoring as a standard capability, identifying under-fills and over-fills in the same pass that inspects the seam, with no additional station, no additional footprint, and no additional operator overhead.

360-Degree Coverage Confirm that the system’s inspection geometry provides overlapping coverage of the full seam circumference. Systems that inspect from only one or two angles have inherent blind spots for localized defects. The SeamTrac Legato uses multiple high-contrast X-ray beams with overlapping coverage to image the complete seam circumference on every can.

True Inline Operation at Full Line Speed The system must be capable of operating at your line’s full production speed, not a reduced inspection speed, without creating a bottleneck. Verify the system’s rated throughput in cans per minute and confirm it matches or exceeds your line speed. The SeamTrac Legato inspects at speeds up to 1,200 CPM, matching the output of modern high-speed canning lines.

Non-Contact Design The inspection system should mount over your existing conveyor without requiring can transfer into and out of the machine. Transfer points are a
leading source of line jams and contribute to Can damage. A non-contact, over-conveyor design eliminates both risks. The SeamTrac Legato has no internal conveyor. It mounts directly over the existing single-file seamer discharge conveyor with no transfer points and no product contact.

Seamer Head Attribution Every can inspection result should be attributable to the specific seamer head that produced it. This capability is what enables proactive maintenance rather than reactive defect removal. The SeamTrac Legato’s Advanced Seamer Tracking monitors, scores, and attributes every inspection result to its originating seamer head in real time via the Peco Seam Score.

AI-Assisted Predictive Maintenance Leading systems incorporate AI-driven analytics that track seamer head performance trends over time, alerting operators to heads that are degrading before they begin producing out-of-specification seams; this moves quality management upstream. The SeamTrac Legato incorporates AI-directed predictive maintenance (PdM) by seamer head, providing early warning of degrading heads at full production speed.

On-Demand Sampling Integration The system should support triggered teardown sampling, allowing operators to call a sample from a specific seamer head on demand, without stopping the line or disrupting production; this enables targeted quality verification when analytics indicate a specific head may be developing an issue. The SeamTrac Legato’s On-Demand QA Sampling allows operators to request a sample from any specific seamer head without a production interruption.

Headspace / Fill Level Monitoring Integration of headspace monitoring within the same inline system allows fill level and seam quality to be managed together, eliminating the need for a separate fill level monitoring station. The SeamTrac Legato includes integrated headspace monitoring as standard, identifying under-fills and over-fills in the same inspection pass.

Automated High-Speed Rejection When a critical defect is detected, the system must be capable of automatically ejecting the defective can from the line, reliably, at full line speed, without operator intervention and without disrupting surrounding cans. The SeamTrac Legato supports a full range of high-speed ejection and diversion options, automatically removing critical defects from the line without stopping production.

SCADA and Cloud Integration Modern production environments require seam inspection data to flow into plant-level SCADA systems and, where appropriate, cloud analytics platforms for fleet-wide monitoring and reporting. Verify that the inspection system supports your specific data architecture and connectivity requirements. The SeamTrac Legato supports OPC-UA connectivity and selected cloud data integration for fleet-wide monitoring and reporting.

Footprint and Installation Space on a canning line is finite. Systems that mount over the existing conveyor with minimal linear footprint enable installation in facilities where floor space is constrained and avoid the need for line reconfiguration. The SeamTrac Legato requires just 7.5 feet of linear line space and mounts over the existing conveyor with minimal line reconfiguration.

Does inline X-ray inspection replace teardown?

No. Destructive teardown remains essential for direct dimensional measurement, regulatory documentation, and seamer setup verification. What changes is the role teardown plays. Instead of functioning as the primary method for discovering problems, teardown becomes a directed confirmation tool. Continuous inline inspection identifies where risk is emerging; teardown verifies the specific condition on the specific head that the data points to. QA stops searching and starts confirming.

What about vision systems?

Optical and vision systems have value in the right application, but they can only detect what is visible on the exterior surface of the seam. Internal conditions (false seams, short hooks, insufficient overlap) are invisible to cameras. The environment immediately after the seamer is also challenging for optical systems: wet cans, steam, washdown spray, and variable lighting. X-ray penetrates the product and the multilayer seam structure to evaluate the internal seam condition directly, regardless of surface conditions.

We already have seamer monitoring sensors. Why do we need seam inspection?

Contact-based monitoring (proximity sensors, accelerometers, or other machine-mounted instrumentation) provides useful information about the seaming process. Still, it monitors elements of the machine rather than the result on the can. The finished seam is the arbiter that matters. Machine signals can indicate that something changed in the process; seam inspection confirms what that change actually produced on the can. The two are complementary, not interchangeable.

Is X-ray inspection safe for food products and plant personnel?

Yes. X-ray inspection systems used in food manufacturing operate at energy levels far below those that could affect food safety, nutritional value, or taste. All Peco InspX systems are designed to meet applicable radiation safety standards, including FDA CDRH (Center for Devices and Radiological Health) requirements and state licensing where applicable. The SeamTrac Legato is rated Safety Category 4, Performance Level E, and radiation exposure outside the cabinet is effectively zero during normal operation.

How does this affect our existing SPC program?

It strengthens it. SPC depends on the quality of the data behind it. Moving from periodic sample data to continuous 100% inspection data does not replace SPC; it gives SPC a fundamentally better data foundation. Trends that were previously invisible between samples become visible in real time. Head-level attribution means control charts can be maintained per seamer head rather than as line-level aggregates, making process signals more specific and actionable.

Fill level is directly related to seam integrity in one important respect: under-filled cans have excess headspace, which affects the counter-pressure dynamics during seaming and can contribute to seam formation problems. Overfilled cans create excess internal pressure that can stress the seam post-seaming. Integrated headspace monitoring, available in advanced inline inspection systems, allows these fill-level issues to be identified simultaneously with seam inspection, closing a second quality loop in a single pass.

The SeamTrac Legato includes integrated headspace monitoring as a standard capability, identifying under-fills and over-fills in the same pass that inspects the seam, with no additional station, no additional footprint, and no additional operator overhead.

Can seam integrity is not a secondary quality metric. It is the primary barrier between your product and the consumer; the single structural element on which the entire promise of shelf-stable, safe canned food depends.

The gap between what traditional inspection methods can detect and what is actually occurring across every seam on every can at full production speed is real, measurable, and consequential. Recalls, consumer complaints, and food safety incidents traced to seam defects are not failures of the canning process per se; they are failures of the inspection and monitoring systems that were supposed to catch those process failures before they reached the market.

Consider the contrast. Under traditional inspection, mechanical drift in a seamer head accumulates silently. A teardown sample may or may not catch it; if it does, the line holds while QA investigates. By the time the root cause is confirmed, thousands of suspect cans are already downstream, potentially in the cooker, in the warehouse, or on a truck. The response is forensic: which heads, which cans, how far back, and how much product is at risk.

With inline 360-degree inspection and head-level attribution, the sequence changes entirely. The system flags the head trending out of specification in real time. The operator triggers an on-demand QA sample from that specific head, confirms the condition, and initiates corrective action before defects reach scale. The seamer dashboard shows the before-and-after effect of the adjustment. No forensic investigation, no downstream holds, no recall risk accumulation. The problem is identified, verified, and resolved within the same production window it began.

Over time, this becomes more than an inspection system. A continuous production record across every head, every pocket, every shift, and every incoming material lot creates a foundation for better trend analysis, better maintenance timing, better supplier correlation, and better control of seam performance across lines and plants. Root cause analysis stops being a forensic exercise based on incomplete evidence between the last good sample and the first bad one. The process record is already there.

The Peco InspX SeamTrac Legato is the only system built specifically to close that gap. It is designed to inspect every can at line speed, attribute results to individual seamer heads, support headspace monitoring, and automatically reject critical defective containers, while mounting over the existing single-file seamer discharge conveyor and minimizing disruption to the production line.

ROI analyses developed for specific customer applications indicate payback periods that can be measured in months rather than years, driven by reduced downtime, increased OEE, fewer HFIs, and measurably improved seamer performance. One major food manufacturer has already committed to the SeamTrac Legato as its standard for seam process control and is executing a structured plant-by-plant rollout.

The question for canning operations is no longer whether this level of inspection is technically possible. It exists, it runs at your line speed, and it fits in 7.5 feet of your line. The question is whether the cost of not having it, in recall risk, downtime, wasted product, and consumer harm, is a cost worth continuing to accept.

To see the SeamTrac Legato in action or request technical details for your specific line configuration, contact Peco InspX at www.peco-inspx.com/products/legato or call +1 800 732 6285.

Abbreviations: AO (Actual Overlap), BH (Body Hook), BHB (Body Hook Butting), BRCGS (British Retail Consortium Global Standards), CDRH (Center for Devices and Radiological Health), CFR (Code of
Federal Regulations), CH (Cover Hook / End Hook), CMI (Can Manufacturers Institute), CPM (Cans Per Minute), EH (End Hook), FDA (Food and Drug Administration), FSC (Food Sector Category), GFSI (Global Food Safety Initiative), HFI (Hermetic Failure Incident), IFS (International Featured Standards), LACF (Low-Acid Canned Foods), OEE (Overall Equipment Effectiveness), OL (Overlap), PdM (Predictive Maintenance), SCADA (Supervisory Control and Data Acquisition), SL (Seam Length), SQF (Safe Quality Food), TB (Body Thickness), TE (End Thickness), TR (Tightness Rating), W (Seam Width), OPC-UA (Open Platform Communications Unified Architecture).

This appendix consolidates the regulatory and product-positioning references cited in the guide. Verify that all regulatory citations and product claims reflect the current editions and approved language at the time of publication.
• U.S. FDA; 21 CFR 113.60(a), Containers. For low-acid canned foods, the regulation requires regular observations for gross closure defects, visual examination of at least one can from each seaming head by a qualified closure inspector at intervals not to exceed 30 minutes of operational time, and records of observations and corrective actions. Official regulatory text is available via the eCFR; interpretive inspection guidance appears in the FDA’s “Guide to Inspections of Low-Acid Canned Food Manufacturers: Part 3”.
• FDA inspection guidance; Guide to Inspections of Low-Acid Canned Food Manufacturers: Part 3, Container/Closures. Useful reference for visual seam examination frequency, examples of gross closure defects, and teardown context for double-seam evaluation.
• Peco InspX approved public positioning reference; SeamTrac Legato brochure and current public product language. Approved positioning: the only inline, 360-degree, non-contact seam inspection system; multiple X-ray beams with overlapping coverage; mounts on the existing single-file seamer discharge conveyor; up to 1,200 CPM; 7.5 feet of linear line space.
• GFSI-code references. SQF Edition 9 (FSC 15), BRCGS Global Standard for Food Safety Issue 9 (Section 6), and IFS Food Version 8 each include container closure integrity requirements. Consult the current edition of each standard for specific clause references.