Since the late 1980s, China’s can-making industry has experienced significant advancements in the production technology and processes of three-piece tinplate food cans. A landmark development was the abandonment of the century-old soldering tin can production techniques and the introduction of advanced foreign resistance welding equipment and scientific can-making processes. This revolutionized the entire metal can industry.

Basic Principles of Resistance Welding

The fundamental principle of all resistance welding methods lies in harnessing the thermal effects of electric current when it flows through a conductor, resulting in a temperature rise due to the material’s electrical resistance. Resistance welding machines utilize the heat generated by the flow of current through the welding circuit, applying simultaneous pressure to permanently fuse the metal together for welding.

1. Spot Welding

The principle of spot welding is illustrated in Figure 3-35, where B₁ and B₂ are welded together. The necessary current flows through electrodes E₁ and E₂, and simultaneous pressure is applied. According to Joule’s law, the heat generated between the electrodes is determined by the power W.
in the formula
W – power
I – effective value
(1J is equal to 0.239cal of heat, that is, 1cal=4.185J)
When welding current, welding time, and electrode pressure are properly coordinated, the welding material generates the required heat, with the majority concentrated at the welding joint. Some heat is lost through water-cooled electrodes, and additional heat is radiated from the cool working parts. With longer welding times, heat is also transferred to the surrounding air through thermal radiation.

The total resistance during welding comprises material resistance and contact resistance, as shown in Figure 3-36.

When two conductors make contact under pressure, the resistance at the contact point is termed contact resistance or transfer resistance. Due to the non-uniformity of the contact surfaces, smaller areas make initial contact, deforming higher points until the entire contact surface fuses into one, as depicted in Figure 3-37.

The actual contact surface (A_0 is slightly smaller than the so-called relative contact area (A) because achieving an ideal smooth surface is nearly impossible. The individual conductive surface is termed a face (a), while all local conductive surfaces together form (A).

As the electrode pressure increases, the actual contact surface also increases, and contact resistance decreases. Only when all contact surfaces (A) are equal, their contact points are equal, but such equal contact surfaces are rare. For each contact point, the heat generated by the welding current is not uniform, leading to the softening or melting of some contact points, eliminating resistance between the electrodes.

Plastic deformation of contact points and the formation of new contact points increase the contact area, continuing until the actual contact surface (A) equals the relative contact area \(A\). Contact resistance exists only for a certain period during the welding process, starting with current application and ending with material welding, achieved by completely contacting the thin top layer through melting. Figure 3-38 illustrates the heat distribution during spot welding.

2. Seam Roller Welding

Seam roller welding, also known as seam spot welding, replaces spot welding electrodes with welding wheels. Depending on the spacing of the weld points, seams can be spot-welded (with large spacing) or seam-welded (overlapping weld points), as shown in Figure 3-39. The shape of the weld seam is depicted in Figure 3-40.

To ensure good welding quality of the tin-plated surface, the welding electrodes must be kept clean. To achieve this, two slotted welding wheels are used, with a flat copper wire arranged on top. This ensures that any debris on the cylindrical surface collects on the copper wire and does not adhere to the welding wheels. By continuously removing tin debris from the welded part, the electrical contact surface remains clean at all times, as shown in Figure 3-42.

Strict adherence to the specifications for pressing the copper wire is necessary during welding to ensure reliable and good contact of the contact surface.

(3)The relationship between input voltage, frequency and welding current When roller seam welding is used, there is a welding point for each half-wave of the voltage applied to the welding wheel. For seam welded vessels, the welding speed of the welding wheel is limited by the voltage frequency, see Figure 3-43.

The calculation of solder point spacing is: welding speed/2f welding frequency. For example, if a welding machine v₂=50m/min, f=500Hz, the welding point spacing is:

In typical production processes, metal containers have different requirements for weld point spacing depending on the application:
1. For pressurized spray cans, the weld point spacing is generally controlled within 0.8–1 mm.
2. For beverage and food cans, the weld point spacing is typically controlled within 1–1.2 mm.
3. For containers with lower air tightness requirements, such as some types of dry powder and tea containers, the weld point spacing can be controlled at 1.2 mm or above.
4. Thermal Sections of Weld Points
The heat of the weld point can be divided into the following sections (see Figure 3-45):
I: Radiance from the previous weld point and the rising segment of the current waveform, as shown in the schematic.
II: Peak current segment generating the weld point.
III: Radiance from Zone II and the descending segment of the illustrated current waveform.
II provides the majority of the energy for welding. The energy amplitude of Zones I and III is determined by the weld point spacing.

Impact of Material on the Welding Process

The characteristics of the material and the selected standards have been detailed in previous sections. The influence of the material on the welding process is illustrated below.

In the market, due to the diverse types of tinplate used for can production, the impact on the welding process is determined not only by the material’s basic characteristics (primary cold-rolled CA, BA materials, secondary cold-rolled DR materials) but also by the tempering degree of tinplate and the passivation of the tinplate surface. However, the most significant impact on welding is the amount of tin coating on the tinplate.

As seen in Figure 3-46, when the welding pressure is at a constant value (50 dan), a lower tin coating amount (≤1 g/m²) on the tinplate leads to higher contact resistance, making welding more difficult.

Resistance Welding Machine

Since the introduction of the world’s first semi-automatic resistance welding machine in 1953, with continuous development in resistance welding theory and machine design over half a century, an increasing number of new and high-speed resistance welding machines have entered the market. Currently, the world’s fastest resistance welding machines can reach speeds of up to 1000 cans/min.

The mainstream resistance welding machines for the production of food cans are fully automatic resistance welding machines with speeds ranging from 150 to

400 cans/min. Semi-automatic resistance welding machines, due to their lower speed, are increasingly less used in the production of food cans. Below, we will use an example of a fully automatic medium-speed resistance welding machine (see Figure 3-47) to introduce the main components of a resistance welding machine and its basic functions (see Table 3-2).

The material feeding device B sucks tinplate one by one from the hopper A and then pushes it flat between the first pair of conveying rollers. The control device is responsible for timing. After monitoring by the tinplate double-sheet electronic sensor C (or alternatively, the line marking device E) and the flexible iron device F, the tinplate is introduced into the automatic rounding device G and shaped into a circle. Double-sheet tinplate is fed into the double-sheet collector by the tinplate double-sheet ejector D.

The tinplate, shaped into a simplified can body, is conveyed by the chain claw of conveying device 1 to conveying device 3J. These two conveying devices move synchronously but with a different sequence of actions. Conveying device 3J receives the can body M and pushes it without displacement between the upper and lower two welding wheels L. After passing through the two welding wheels L, the completely welded can body is transferred to the can-receiving belt N. It then passes through the output belt O or the bracket conveying device P (optional), threading the can body through the seam coating system (if installed) and delivering it to the next process on the production line. The welding current is in the on state throughout the entire operation, even during pauses in continuous welding operations. At the beginning and end of each can body welding, the welding current can be reduced or increased.

In addition to the basic functions of the resistance welding machine mentioned above, modern resistance welding machines have added many auxiliary functions, such as oxygen-free welding devices (also called nitrogen protection devices), online monitoring devices for welding quality (monitor), mercury-free welding devices, etc. These enhancements ensure that products produced by resistance welding machines are more aesthetically pleasing, safer, and more environmentally friendly.