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Making steel and aluminium aerosol cans

High speed precision engineering



Image: Thyssen-Krupp

Tin plated steel arrives in coils from the steelworks. Each one is about 4km long and a metre wide.
A colour printing press prints the can graphics directly onto the steel sheet before the sheet is slit and chopped to smaller sheets for bending into the can cylinders.



This sheet is slit and then chopped into rectangles for each can.

Welding the seam

To understand the process of using wheels as electrodes, lets look at something a bit bigger than an aerosol can. A lot bigger!





For aerosol cans a much faster machine is needed and the cans need to move in one direction. They are bent around the arm that carries the inner wheel just before they are welded.

Here is a slightly faster machine. Look out for the copper wire running over spindles leading to the welding electrode wheels.



If you want to see really fast can welding, look at the videos on this site. (Serious speed freaks only)

http://soudronic.com/3-piece-cans/canbody-welders

In the videos look out for the copper wire running over a set of reels. This is the way that a perfect new copper surface meets the can at every point along the seam.

If we look at a seam using a USB microscope or a low power binocular microscope we can see that the seam has a regular pattern. This is caused by pulsing of the welding current. The weld is not a continuous line, but a series of overlapping "nuggets". The timing of the pulses and the speed of the cans has to be precisely controlled. If the cans go too fast there will be gaps and the can will leak. If it goes too slowly, the heat will melt a hole in the can. The green strip is the edge of a plastic ruler marked in mm.




Image: Crown Holdings

In the pictures above and right try to identify:
  1. the slightly overlapping can edges
  2. the copper electrode wire around each wheel
  3. The forming rollers that bend the can into shape


The final process is to add the top (cone) and bottom (dome)






Questions and Activities about steel cans

1. Either from the microscope view above or using your own USB microscope and a mm scale work out the pulse frequency of the weld current if the can seam is is 20 cm long and moves through the welder in 0.5 s.

2. Why does the printing on the can stop few millimetres from the seam line?

3. Why is copper used for the electrode wheels and the wire in the welding machines?



  CLICK HERE FOR ANSWERS>
 

Making aluminium cans





About half the cans made in the UK are aluminium. They are lighter, they have no seam, and smaller cans can be made.

The manufacturing process involves forming a can from a single slug of aluminium using a punch and die at huge pressure. Aluminium is a soft metal. When it is subjected to the huge forces of the punch it heats up and flows like a liquid.

You can see an animation of the process here (opens in a new window).

VIEW ANIMATION>

 





Questions about aluminium aerosol cans


1. Why is the extrusion process used for aluminium cans not suitable for steel cans?

2, What is the difference between the printing process between aluminium cans and steel cans?



  CLICK HERE FOR ANSWERS>


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How do you turn a flat sheet of steel into a cylinder with a leak proof seam?

This is a close up of the seam.




The solution is to weld the sheet edges together using electric current, as shown the the diagram below.. Two electrodes press against the steel, one from outside and one from inside. The electrodes are connected to a power supply that drives a huge current of up to 5000 Amps through the metal. This melts the two overlapping layers together. This is called resistive welding. The current is pulsed at high frequency, one pulse for each spot or 'nugget' on the weld.





To make a long seam the electrodes need to be wheels to roll along the can.


Image: spotweldinc.com

Making the perfect electrode wheel

For a perfect leak-proof welded seam the electrode wheels must also be perfect. After only one turn a copper wheel would pick up blobs of molten metal and be useless. The solution is to run copper wire round the wheels that is never re-used. The copper is recycled and made into new wire so the cost is reduced. You can see the copper wire in the second video.



Image: mechanicalengineeringblog.com

The copper wire always presents  perfect new electrode surfaces as it is only used once. Getting the inner wheel into the inside of the can is not so easy as the can is moving in one direction. The trick is to curl the can into a cylinder just before it gets to the wheels.











Image: Crown Holdings

Close up of the weld. There is a "nugget" for each pulse of current. The nuggets must just overlap. The slightest gap would cause a leak. Engineers regularly inspect the welds with a microscope.











Image: Crown Holdings

A can approaches the wheels from the left. It has just been curved around the inner wire.

Hiding the seam

The seam on steel cans is a problem for can designers. The can is printed when it is flat sheet and then bent and welded. As ink is an insulator it must not cover the seam because it will stop the welding current from flowing though the sheet metal. This leaves an unprintable stripe on the can.


Image: Colep

Engineers have managed to reduce the width of the stripe from 5mm to 1.5mm. Now that you have seen how the seam is made you can see that this is a big precision engineering achievement.






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