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High-speed stamping quenches thirst for beverage cans

What you can learn from stamping at the speed of light

All of the components of a beverage can are stamped in a high-speed press. Would soda beverages be as popular if they could not be packaged in a portable, singular, recyclable can made inexpensively by a high-speed stamping process?

Metal food and beverage cans are widely recognized as low-cost, recyclable, and tamper-proof packages with long shelf lives. It may not be as well-known that metal stamping presses play an important role in their history and high-speed precision production.

Can Origins: All’s Fair in Lunch and War?

Like many inventions, the metal can developed as a result of wartime needs. In 1795 Napoleon offered 12,000 francs to anyone who could develop a method of preserving food for the French army. At the time, scurvy and hunger had disabled more of Napoleon’s soldiers than combat had.

The reward offer prompted Frenchman Nicolas Appert to develop the first metal can. Cans were hand-produced at the rate of 10 per day.

Canned food use grew and became a staple around the world, but the demand for metal containers increased dramatically in 1935 when the first flat-topped beer can was introduced in the U.S. It was met with huge market success.

The next invention in the evolution of the can occurred during a picnic. One big limitation of the early-generation beverage can was that an opener, or “church key,” was needed to open it. At a picnic one day, Ermal Fraze, a toolmaker from Dayton, Ohio, forgot his church key. Inventively he used a car bumper to open drinks for himself and his guests. That experience prompted Fraze’s invention of the pull-top can, which allows the user to merely pull a removable tab to access the drink.

raze worked closely with press manufacturer Nidec Minster in 1962 to perfect a production system for his new invention—an easy-open can-end press (see Figure 1). The pull-top can proved to be a popular improvement in beverage packaging, and can sales increased dramatically.

In the mid-1960s, a new process for making cans out of two metal parts—the body and the end (top)—was developed, significantly reducing the can’s cost.

Innovations and continual improvements have increased the thirst for beverage cans. To meet that demand, today’s can line production speeds approach 5,000 cans per minute.

Can Process Can Do Fast

The process of producing two-piece beverage cans starts with the blanking and forming of a cup from a large aluminum coil. Upon exiting the press, the cup is transferred to downstream equipment for additional forming, coating, decorating, cleaning, and so forth.

Most presses used for the cupping application are double-action presses, designed with an inner and outer slide (see Figure 2).

Figure 1
In 1962 Minster introduced this 45-ton cabinet crown press as the world’s first press designed to produce easy-open can lids. Early lid designs of this beverage can lid were called “tear top.”

Typically, the outer slide leads the inner slide by a phase angle which can vary depending on the application. In the cupping application, an outer slide and punch holder blank and grip the material. Then an inner slide and punch holder draw the cup. The cups produced must be of extremely high quality to eliminate problems such as tearing of the material during additional forming processes.

Today’s most advanced cupping presses can run a 14-out die up to 350 strokes per minute (SPM), producing up to 5,000 cans per minute.

The can’s end is commonly referred to as a shell (see Figure 3). A shell is blanked and formed in either a single- or double-action press from either steel or aluminum sheet or coil. The shell, like the cup, must be of extremely high quality to provide adequate strength and proper attachment to the filled can body.

It becomes even more critical that this process be precise when cans and ends are produced from thin material. Today up to 10,800 shells can be stamped per minute with 24-out dies running at up to 450 SPM.

Shells are transferred to high-speed precision presses, or conversion presses, to complete assembly with the easy-open end (see Figure 4). The conversion press forms the tab and attaches it to the shell, which undergoes additional forming operations.

The most precise operation is the score, or cut, in the material that allows the consumer to open the can by pulling on the tab. If the score is too deep, a “leaker” develops. If a score is not deep enough, the end cannot be opened properly. The score must be held within ±0.0001 in.—1⁄40 the thickness of a human hair. These forming operations require very precise presses equipped with thermal control systems to maintain extreme accuracy (see Figure 5).

Today easy-open ends for beverage cans are produced in a four-out die at 750 SPM to produce 3,000 ends per minute.

Optimizing Features

High-speed presses used to produce cups, shells, and easy-open ends are uniquely designed for each application. Some of the press features and requirements for these applications are:

High Operating Speeds. Press designs continually push the limit for higher operating speeds. Stroke lengths and reciprocating mass of the slide and upper punch holder must be minimized to keep dynamic forces within acceptable safety factors.

Dynamic Balance. A dynamic balancing system is necessary to compensate for increased inertia forces at higher operating speeds. This feature also results in minimal unbalanced dynamic forces being transferred into the plant floor and to other equipment.

Figure 2
Today’s most advanced cupping presses can run a 14-out die up to 350 strokes per minute (SPM), producing up to nearly 5,000 cans per minute. In a double-action press, the two slides operate independent of each other, allowing a slight offset to assist the drawing and forming of cups at high speed

Frame Construction. Major frame components are made of cast iron to optimize vibration absorption characteristics at higher speeds.

Quick-lift Slide. Stroke lengths have been reduced to enhance efforts to increase operating speed. However, shorter stroke lengths reduce the open space for die inspection, maintenance, tool replacement, clearance of jams, threading material, etc. The quick-lift slide feature allows a quick and easy method to raise the slide and allow these activities to take place in a less confined space.

Fast Starting/Stopping. It is important to be able to start and stop very quickly to make sure that zero or minimal scrap parts are produced during these conditions. To accomplish this, a high-performance clutch/brake system is necessary. Quick stopping also enhances die sensing efforts to minimize any damage that might occur in the case of a die jam or wreck. A hydraulic clutch/brake system can typically start and stop much faster than a comparable air unit. Also, today’s hydraulic clutch/brake systems require no adjustment and allow high single stroke rates during setup without causing an overheated condition. The zero-backlash design removes all free clearance in the unit, minimizing wear.

Bearing Design. Bearing design is crucial to the performance and precision of higher-speed presses. Worn or failed bearings can be very expensive and time-consuming to replace. Bearing systems typically are one of two types: an oil film or mechanical roller. Both types of bearing systems are optimal for certain applications. However, an oil film bearing system is considered a triple-action type using a combination of hydrostatic, hydrodynamic, and squeeze-film principles.

In all performance criteria such as static and dynamic stiffness, dynamic load capacity, deflection under load, and bearing life, an oil film bearing system matches or beats a roller bearing system, especially in the critical area of bearing life. With proper maintenance, an oil film bearing system is void of contacting surfaces, thus providing the opportunity for infinite life in those areas.

Bottom Dead Center (BDC) Repeatability. In a conversion press application, the precision and control of the slide BDC repeatability are critical. The accuracy of the slide BDC repeatability is a direct correlation to the press structure, bearing system, and slide guiding system. These press design parameters must work together to optimize BDC repeatability, resulting in product stability.

Thermal Shut Height Control. Especially in a conversion press application, a thermal shut height control system is required to maintain a consistent shut height. The best method to maintain shut height stability is to control the oil temperature of the lubricating oil being circulated throughout the press structure. This involves the capability to both heat and cool the oil based on the operating condition and plant ambient temperature.

Vibration Absorption/Monitoring. In conversion press applications, dies use stop blocks to help control and maintain the very critical tolerances of the score and other forming processes. The foundation of a well-designed conversion press includes massive cast-iron components. A cast-iron frame dissipates and absorbs vibration up to three times more than a steel-welded equivalent. A robust frame design also provides the additional benefit of being able to better withstand an unfortunate die wreck with minimal damage.

A press designed with substantial mass contributes to its ability to run at higher tonnages and speeds while maintaining low vibration levels. This is crucial for long-term reliability and optimal production efficiency. Presses now can also be equipped with vibration sensing and monitoring equipment that allows plant personnel to track vibration levels on a real-time basis, providing a visual indication of the vibration severity of a given press and die operation.

Off-center Loading, Die Tipping. Cupping and shell applications have virtually no off-center loading because the die pockets are perfectly symmetrical in both the front-to-back and right-to-left directions of the press centerline.

Figure 3
Precision is critical for end stamping from extremely thin material at SPMs up to 450.

However, the easy-open-end application is not symmetrical. In the early generations of conversion presses, high off-center loads created premature wear on both the press and die. In addition, they inhibited the press’s and die’s ability to reliably produce parts to the tight tolerances required.

Now conversion dies are positioned within the press structure in a way that minimizes off-center loading. A well-balanced load also minimizes the amount of stop block overstrike in the die, thus reducing overall press and die loading and vibration.

Reliability. Can plants run in a 24/7/365 continuous production schedule. In most plants, the equipment is operated at or near maximum rated speeds. Periodic maintenance is planned to keep the equipment in good running condition. The combination of continuous production schedules at maximum operating speeds requires that the equipment be available to run at all times. In a continuous production environment, there is no time to make up for lost production due to unplanned equipment downtime.

Durability. The continuous production in can plants running at extremely high speeds results in elevated cycle rates in regards to wear and tear on the equipment. For example, a conversion press running continuously at 750 SPM may run nearly 1 million cycles per day. Presses and other equipment in the line must be designed to withstand these cycle rates.

Parallels and Takeaways

The global market for beverage cans is estimated at approximately 325 billion per year. The stamping press technology’s high-volume capability has facilitated the growth and expansion of the beverage industry because it has provided it with a low-cost, mobile, safe, and sustainable package.

The requirements for precision, performance, reliability, and durability of the equipment and processes in can plants are very challenging, resulting in a high barrier to entry into this market. Companies with the proper equipment and supporting services can meet these challenges head-on.

Even those stamping manufacturers that don’t enter the can stamping market may find takeaways from its high-speed production methods to apply to their own stamping operations. Astute manufacturers are likely to recognize innovations that originated from high-speed processes.

Dynamic balancing, quick-lift slides, high-performance clutch/braking, triple-action bearing systems, thermal shut height control, and press structures that minimize off-center loading originated to meet the extreme demands of high-speed, high-production stamping, but can have applications for other stamping operations as well.

Greg Stueve is general manager, packaging group, Nidec Minster Corp., greg.stueve@minster.com, 419-628-2331, www.minster.com.