Double seam

A double seam is the closure between a can body and a can end (or lid).

Originally, the can end was soldered or welded onto the can body after the can was filled.^[1] However, this introduced a variety of issues, such as foreign contaminants (including lead and other harmful heavy metals). The double seam was later developed as a cheaper and safer alternative and quickly replaced the welded seam.

The double seam is made using a double seamer, which can have just one or a number of heads or seaming stations. The double seam is formed by mechanically interlocking five layers of material together: three layers of the can end and two layers of the can body. Each seaming head typically consists of two rolls, a first operation roll and second operation roll, and a chuck. Some seaming machines have two first operation rolls and two second operation rolls and a few machines use a method called "rail seaming" which requires no rolls. During the seaming operation, the can end is lowered on to the filled can body and held down by the chuck, which acts as an anvil to the seaming operation. The first operation roll then engages the can end against the can body thereby folding the end curl around the flange of the body. In some seaming machines, this is done as the can is turning at high speed. In other seaming machines, the can is stationary and the first operation roll (or rolls) spins around several times to ensure a complete first operation. After the first operation is complete, the first operation roll disengages from the can and the second operation roll then engages the can. The purpose of the second operation is to iron out the double seam into its final shape and remove the voids between the layers of can and end material. In practice, ironing out all of the can and end material in a double seam without leaving some voids is impossible without the use of a sealing compound.

Defects

The production of a high-quality double seam is dependent on several factors, including conformity to the set can and end specifications, the quality of the seamer tooling used and its compliance with the can and end being used, the condition of the seaming machine and the setup of the seaming rolls, lifter pressure and other components. When the machinery is set up correctly and the incoming materials (cans, ends, tooling, etc.) comply to the set specifications, the result should be^[2] ideal first and second operation seams.

A problem in any one of these factors and others can contribute to seam defects that have an adverse effect on the ability of the can to withstand foreign contamination and keep the product from leaking or reduce its shelf life. Below is a list that can be used as a reference.^[3]

Cover droop - A droop is a smooth projection of a double seam below the bottom of a normal seam. The droop may occur at any point of the double seam.
Cover Vee - "Vees" or "lips" are projections of the double seam below the bottom of a normal seam that resemble a "V" shape. There is usually no overlap of the cover hook with the body hook and these defects usually occur in small areas of the seam.
Cut Over - A "cut-over" is a seam defect where the top of the inside portion of the seam has become sharp enough to fracture the metal. As in the definition of "sharp seam", this condition usually occurs at the side seam juncture of a three piece container.
Dead Head - A deadhead or spinner (also referred to as slips or skids) is an incomplete seam caused by the chuck spinning in the countersink during the seaming operation.
False Seam - A "false seam" is a seam or portion of the seam which is completely unhooked, and in which the folded cover hook is compressed against the folded body hook. A false seam is not always detectable in an external examination.
Knocked Down Flange - A knocked down flange is a critical seam defect and it occurs when the cover and body hooks do not interlock dues to a bent can flange before double seaming.
Long Body Hook - A long body hook is a condition where the body hook approaches or exceeds the maximum recommended specification.
Long Cover Hook - A long cover hook is a condition where the cover hook length approaches or exceeds the recommended guidelines.
Loose First Operation Seam - Loose first operation seams may not allow sufficient tuck up of the cover curl to form a sufficient amount of cover hook and overlap in the finished seam.
Pleats, Puckers and Spurs - A pleat is a fold in the cover hook that extends from the cut edge downward toward the cover hook radius and sometimes below this radius in a sharp vee or spur shape. A pucker is a condition which is intermediate between a reverse wrinkle and a pleat, where the cover hook at the cut edge is locally distorted downward without actually folding. A spur is a localized irregularity characterized by a sharp protrusion at the bottom of the double seam. It is usually accompanied by a pleat or vee in the cover hook.
Seam bump - Seam bumps are a relatively short area of the double seam, where the seam thickness suddenly increases by 0.004" (0.1 mm) or more. They are predominantly found on welded and two-piece cans with long body hooks and are usually seen on the can filler's end when hot filled products exceed 185 degrees (Fahrenheit). The cover hook radius may be pulled away from the body wall.
Sharp seam - A sharp seam is a condition where the seam has a sharp edge and / or radius on the upper inside edge of the countersink wall. Sharpness is a conditional defect and must be evaluated by degree (slight vs sharp). A slight condition must be carefully watched and corrected as soon as possible. Left unchecked, this defect can cause cut overs.
Short Body Hook - A short body hook is a condition where the body hook does not meet the minimum required specification.
Short Cover Hook - A short cover hook is a condition where the cover hook does not meet the minimum recommended specification.
Sprung seam - A sprung seam is a condition where the seam is pulled away from the body wall. In extreme cases, the seam is pulled away from the body wall all the way around the can.
Tight First Operation Seam - Tight first operation seams can create flatness at the bottom of the first operation seam throughout its length. The cover hook may also be turned into the body hook. Overly tight first operation seams tend to create more reverse wrinkles in the cover hook.

Government regulations

In the United States, the production and quality of double seams is regulated by the US Department of Health and Human Services - Food and Drug Administration (FDA) and the Department of Agriculture (USDA). The plant's responsibility towards the government depends on the product being canned. The FDA regulates all components of canning low acid canned foods, including handling of empty containers, glass, metal and plastic containers.^[4] The USDA regulates canning of animal products.^[5]

US FDA regulations regarding low acid canned food

Before regulating inspection procedures and frequencies, the regulation pertaining to double seams establishes the terminology, explains how a seam is formed and lists the possible double seam defects.

Responsibility for the quality of the double seam

The regulation states that the can manufacturer is responsible for providing the can filler with guidelines and specifications for the can seam: "Can seam guidelines (specifications) are provided to the low acid canned food manufacturer by the supplier of the container and end being used. The guidelines detail the measurements, in thousands of an inch, of each attribute of the double seam for both the first and second seaming operations. They also provide a set-up aim or ideal starting dimensions for the set up of the seamer. The operating limits set the range for good practice." While specifying these guidelines is important, the plant is responsible for the double seam quality: "It is extremely important to understand that seam guidelines by themselves cannot be used for determining the quality of a double seam. The seam guidelines are to be used in setting up the double seams initially and maintaining seam integrity during production. Final acceptability of the double seam should be based on total evaluation of the seam by a qualified person and not on dimensions alone. Good seaming practice requires constant visual examination, frequently scheduled tear-down evaluation, machine maintenance, and immediate correction of unacceptable conditions." The regulation also states that if the double seams do not meet the guidelines, the can filler plant "must know what steps to take to correct the condition".

Critical seam parameters

Since there are several parameters that can be used to judge seam quality, the FDA regulation specifies the most critical parameters: "The most critical attributes to consider in judging the quality of the double seam are overlap and tightness (wrinkle). If one of the can seam measurements (i.e., body hook) is slightly beyond the specified guidelines but the rest of the seam is evaluated and the overlap and tightness (wrinkle) are within specified guidelines, then adjustments to the seamer can be made at the next scheduled shut-down. In this instance, the manufacturer should identify the out-of-guideline measurement and document they have evaluated the rest of the double seam, but did not find immediate corrective action necessary. However, if overlap measurement or tightness rating evaluation are below the minimum guidelines a resample from the questionable seaming station should be made. If the resample continues to show out-of-guideline measurements in overlap and/or wrinkle the machine should be stopped and adjusted."

Evaluation requirements - non-destructive testing

The FDA regulations specifies the frequency and requirements of non-destructive testing: "21 CFR Part 113.60(a) requires a visual examination of at least 1 can per seaming head by a qualified container closure inspector at intervals of sufficient frequency. The regulation requires that double seamed containers be visually inspected for gross closure defects such as sharp seams, cut-overs, deadheads, false seams, droops and broken chuck. The frequency of the visual examination should be made at intervals not to exceed 30 minutes (of operational time); and additional visual examinations must be performed immediately following a jam in a closing machine, after closing machine adjustment, or after startup of a machine following a prolonged shut-down. An example of a prolonged shut down may be when the plant ceases production at 6:00 PM and restarts production at 8:00 AM the next day."

Evaluation requirements - destructive testing

The FDA continues by detailing the requirements of double seam destructive testing: "The double seam teardown examination is a destructive test. Tools that are used to perform this test include a seam micrometer, countersink gauge can opener and nippers. Optional equipment for seam teardown examinations include a seam saw, seam projector and seam scope. Although it is not imperative the investigator carry this equipment to each LACF inspection, it is very important that they know how to operate this equipment and read measurements from the micrometer, seam projector or seam scope. It is also important that the investigator know how to determine the tightness or wrinkle rating of the cover hook. Knowledge of the procedures used to perform double seam teardown examination are essential to evaluating the firm's knowledge and ability to do this examination. Attachment 14 explains the procedure for using a seam projector for examining a cross section of the seam. Attachment 15 explains the can seam micrometer and procedure for use, and Attachment 16 explains the use of a seamscope for the same exam.

The requirements for double seam examinations are specified in 21 CFR Part 113.60(a)(1). The regulation states that teardown examinations shall be performed by a trained closure technician at intervals of sufficient frequency to ensure proper closure. The teardown examinations shall be made on the packer's end double seams on at least one can from each seaming head to ensure maintenance of seam integrity. Sufficient frequency is defined in the regulation as intervals not to exceed 4 hours (operational time).

The regulation allows for two different methods of double seam examination; the "micrometer" method or the "optical" method.

If the processor is using the micrometer method the regulation requires that three measurements are taken at points approximately 120° apart around the double seam. On three-piece cans the first measurement can be taken directly across from the side seam and the next two measurements are then taken 120° to either side of the first measurement. On three-piece cans the measurements must be taken at least one-half inch from the side seam juncture as the juncture may interfere with a true seam measurement.

Micrometer measurements are made and recorded in thousandths of an inch. The high and low measurements are recorded on the double seam teardown examination record. If the manufacturer is using the micrometer method the required measurements are:

Cover hook length
Body hook length
Width (also referred to as length or height)
Tightness (by observation for wrinkle)
Thickness

Optional measurements are:

Overlap (by calculation)
Countersink

The regulation specifies the formula used to calculate overlap when micrometer measurements are used:

CH + BH + T (.010in)* − W, where
CH = cover hook
BH = body hook
T = cover thickness *(general practice use .010 inches for tin plate thickness)
W = width

Measurements used to calculate the overlap should not be averaged. In fact, the lowest values should be used to determine the worst-case scenario. For example, to calculate the worst-case scenario you should use the lowest measurements for CH and BH and the highest measurements for W.

If a seam scope or seam projector is used (optical method) to make the seam measurements, the required measurements are:

Body hook length
Overlap
Tightness (observation for wrinkle)
Thickness (determined by micrometer measurement if the optical instrument cannot read this value)