Page 39 - MetalForming June 2017

Fig. 3—Beverage-can tops undergo a four-stage process that avoids air cracks when the pull tab is attached.
hemisphere is not additive but interactive; add a stretch- form area to a simple cup draw and you create additional problems.
Beverage Containers Need Tops
State-of-the-art beverage containers feature anti-leak tops without any soldering or other hole-closing process required (Fig. 3). The four-step process to create the tops:
1) The flat-top blank is embossed in the middle of the top. This creates a round surface where objects can be hung. 2) The next step is a redraw that makes the embossment
longer and narrower.
3) A spacer is placed over the embossment, followed by the pull tab.
4) Finally, the tab is crushed by the embossment with no leaks.
Combining Stretch and Draw
Working with stretch and draw presents thoughts of fun and play. Unfortunately, such projects can be exceptionally frustrating (Fig. 4). First, the stretch and draw zones are separated by the shock line—rarely is this line at the joint where the stretch radius joins the forming cup. The unbal- ance of strain often depends on the current problem. Not all deep-drawn cups have flat bottoms; some have other configurations.
To illustrate the problem, Professor Donald Eary (General Motors Institute, 1985) studied the stretch/draw and shock- line problem. To define the needed corrective actions, he developed a chart containing some 30 forming variables. The chart recommended a corrective action based on the type of forming issue and the forming variable. For example, if you had a problem with binder force during a stretch operation where the sheetmetal was moving in the flange, the chart suggested the draw solution to prevent wrinkles
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Fig. 4—Placing a stretch component on the bottom of a deep- drawn can may create a major problem.
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