New Seven-Layer Rectangular Crimping Process: A Revolutionary Development in Steel Drum Manufacturing
Yang Wenliang
In recent years, advanced barrel manufacturing technologies from abroad have been increasingly introduced into China, driving continuous innovation and improvement in domestic steel drum production. Among these innovations, the emergence of the seven-layer rectangular crimping process has sparked a significant transformation in the industry. This new technique has not only challenged traditional methods but also redefined the concept of steel drum construction. Figure 1 shows a cross-sectional view of the first seven-layer rectangular crimping process developed in Germany, marking a major milestone in the evolution of steel drums.
Figure 1: Cross-section of a Seven-Layer Rectangular Crimping
The earliest steel drums in the world featured a five-layer rectangular crimp structure in the early 19th century. As their usage expanded, so did the demands placed on them. By the late 1980s, seven-layer round crimps had emerged globally and were gradually introduced to China. For the past three decades, this round crimp design dominated the market and became the standard structure for steel drums. However, with continuous improvements in quality and performance, the limitations of the round crimp began to surface. Especially in international transportation, where high durability is essential, there was a growing need for more robust and efficient crimping solutions. Recently, Germany pioneered the use of seven-layer rectangular crimps in critical applications, posing a significant challenge to traditional designs.
Figure 2 presents a cross-sectional view of a conventional seven-layer round crimp. Compared to the rectangular version, the round crimp has a larger area, a longer center hook, and better leakage resistance. From a mechanical strength perspective, the rectangular crimp offers a larger cross-sectional area, providing superior strength and impact resistance.
Figure 2: Cross-section of a Seven-Layer Round Crimp
During experiments, the seven-layer rectangular crimp demonstrated remarkable performance. It could withstand the most rigorous drop and pressure tests, significantly outperforming the traditional seven-layer round crimp. It is clear that this new technology will soon replace the conventional crimping structure and become the standard for steel drums in China in the near future.
So, how exactly is this innovative crimping structure achieved?
Dimensional Analysis of One- and Seven-Layer Rectangular Hemming Structures
The seven-layer rectangular crimping structure and its dimensions, as used in Germany, are shown in Figure 3. From the diagram, we can see that the barrel’s bottom depth is 15mm, the curl thickness is 9.8mm, the curl width is 13mm, and the outer corner radius of the curl is approximately 3mm. The steel sheet material used is 1.4mm thick, which is considered heavy-duty and suitable for critical applications. Currently, the average thickness of steel drums in China is around 1mm, with light-duty drums using 0.8–0.9mm and heavy-duty drums typically ranging from 1.2mm. The 1.4mm thickness is mainly used for dangerous goods such as yellow phosphorus packaging.
Figure 3: Seven-Layer Rectangular Crimping Structure and Dimensions
Based on the structure in Figure 3, the center head size of the seven-layer crimp is 6mm, meaning the minimum penetration at the center of the curl is 6mm. In contrast, the center of a conventional seven-layer round crimp is usually around 1mm. The body width of the barrel is 24.6mm, while the bottom width is 37.3mm—substantially larger than that of the round crimp. To achieve a seven-layer rectangular crimp, the process must be adjusted to increase the size compared to the traditional circular crimp. Otherwise, it would be impossible to achieve the desired structure.
Figure 4 shows the structural and dimensional layout of the barrel’s bottom cover after pre-rolling.
Figure 4: Structure and Dimensions of the Barrel Bottom Cover
Process Realization of Two or Seven-Layer Rectangular Crimping
To implement the seven-layer rectangular crimping, several process trials were conducted in a factory in Wujiang City, Jiangsu Province. It was found that achieving the seven-layer rectangular crimp using the existing seven-layer round crimping method was extremely challenging. Further analysis revealed that after pre-rolling the bottom cover, the groove shape of the first crimping roller should not be too large. If the groove is too big, it becomes difficult to wind the center of the crimp. When the second forming roller compresses the material, the size of the center hook cannot be achieved properly. On the other hand, if the first roller’s groove is too small, although the center can be wound, the overall material becomes too thick, making it impossible to install the rolling groove. After multiple trials, it was concluded that two rolling edges are insufficient for achieving the seven-layer rectangular crimp.
We then turned to the three-roller crimping machine produced by Xinhui Sansheng General Machinery Factory, which is a replica of German equipment. At that point, we realized that the German sealing machine was designed with three rollers for a specific reason. Figure 5 shows a physical image of the three-roller sealing machine manufactured by Xinhui Sansheng General Machinery Factory.
Figure 5: Physical View of the Three-Roller Sealing Machine
Figure 6 provides a partial view of the working portion of the three-roller sealing machine. Traditionally, the first roller was used for pre-rolling, the second for crimping, and the third for shaping. When used for seven-layer rectangular crimping, the first roller has a smaller groove to wind the center of the bead, the second roller has a larger groove to handle both the middle and outer parts of the crimp, and the third roller features a rectangular groove to complete the final rectangular shape.
Figure 6: Partial View of the Three-Roller Sealing Machine
The basic hemming process is as follows: Bucket body flanging → barrel bottom cover blanking → pre-rolling of the bottom cover → pressing the body and bottom cover together → first winding seal → second winding seal → third shaping. This process is more complex than the traditional seven-layer round crimping.
Through process adjustments and improvements, the structure and dimensions of the seven-layer rectangular crimp were finally achieved.
The exact groove sizes for each roller vary depending on the material thickness and the specific equipment used. Therefore, they should be adjusted according to the conditions of each enterprise. Generally, the first roller should have a smaller arc and groove width to effectively wind the center of the crimp. The second roller needs a larger arc and groove width to accommodate both the center and periphery of the crimp. The third roller must match the final rectangular shape of the crimp.
Through practical implementation, we have established the seven-layer rectangular crimping process and identified the special requirements for the sealing machine. We encourage domestic manufacturers to continue researching and practicing this innovative technique, aiming to develop even more advanced and simplified processes. We hope this article serves as a starting point for further development in this field!
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