Seam formation and structure

Introduction

Seaming is the operation that allows the assembly of the lid to the body by attaching the flange and hem. The element that is formed by this operation is called "seam." The machine used to perform this operation is called "can seamer" (Figure 7).

In the case of three-piece cans, the body and the bottom of the box are assembled by seaming. However, this is done at factories making boxes and not at the canneries.

Formation of the seam

The seam is composed of three folds cover and two folds of the body, a seal suitable trapped in these multiple folds (Figure 8). It thus forms a rigid and hermetic sealing.

During seaming, the rim of the body and the lid (hem) overlap to form a strong mechanical structure (seam). The formation of the seam generally takes place in three steps (Figure 9):

1. The compression;
2. the first pass and
3. the second pass.

Compressing

On the box with the lid crimp in which the chuck came embedded, we apply a compressive force directed vertically from bottom to top, through a compression plate which presses the two elements of against each other. This force is developed by a spring more or less compressed (Figure 9).

The main role of compression is to maintain the cover throughout the operation of seaming. Excessive or insufficient compression can result in defects that can not seam to be corrected in subsequent phases.

First pass (rolling)

The first pass operation is also called "rolling operation" because it makes rolling the hem of the lid around the rim of the body. To do this, use a wheel with a profile (throat) Special (Figure 9).

The rolled obtained after the first pass is a determining factor in obtaining a proper crimp. It should be neither too loose nor too tight (Figure 10). Defects due to faulty first pass can not be corrected during the next phase. The first pass will be correct if the following conditions are met:

1. Good height of the seaming head (distance between the upper surface of the plate and the bottom of the mandrel).
2. Force of compression.
3. Proper alignment of seaming rollers with the chuck.
4. Proper tightening of the wheel first pass.

At the end of the operation of the first pass, the wheel of first pass shrinks and loses contact with the lid of the box a second time, then begins.

Second pass (relief)

The second pass is made using a suitable wheel characterized by a groove flatter than the first pass and used to crush the roll of first pass against the lip of the chuck (Figure 9). In other words, it helps tighten the seam. A good clamping depends on the setting of the wheel a second time, its profile and status.

Another role of the operation of the second pass is to spread the seal that fills the interstices within the seam, thereby improving the sealing of the sealing.

Figure 11 shows a section of seam after the second pass in the normal case.

Structures of the seam

The different structures of a seam are shown in Figure 12. The measurement and evaluation of these structures is essential for setting the first and second passes, but also to judge the tightness of the closure (see §: 6.2. Consideration dimensional).

The specifications of the seam (seam Standards) are provided by the manufacturer boxes and lids. For each characteristic measured are provided, generally, three sets of dimensions: the optimal value, the minimum and maximum value. The objective (optimal value) is an ideal size for adjusting the seamer. Tolerance or acceptance limits (minimum and maximum values) determine the range of values ​​for good seam. When these limits are exceeded, it is necessary to take corrective action.