March 15, 2008
Extrusion is the process in which a continuous radial stretch flange is created by expanding a hole. Of all of the factors controlling the success of an extruding operation, material type and its mechanical properties are the most influential. Materials exhibiting good stretch capability and stretch distribution characteristics are best suited for extruding.
Editor's Note: This is Part I of a two-part series discussing critical variables controlling the success of an extruding operation. Part II discusses the effects of the die design and the use of ironing and preforms.
Extruding a hole sounds like a reasonably easy process. Just pierce a hole into a metal blank and bend the metal down to create a continuous radial profile. What could go wrong? A lot.
Splitting, incorrect dimensions, and the inability to achieve the correct height are just a few problems you can experience. Creating a successful extrusion is very dependent on a number of factors.
The term extrusion in metal forming has more than one definition.
The first definition is the process in which metal is pushed through an orifice with a specific profile. Think of a Play-Doh® Fuzzy Pumper®—you fill the hopper with the Play-Doh, push down on the handle, and out comes long strands of make-believe hair that you can use to decorate your character. Aluminum bars, tubes, and profiles are made using a similar method. Although this type of extrusion is sometimes used in stamping, such as fineblanking, it is less common than the type of extrusion described next.
The second definition of extrusion is the process in which a continuous radial stretch flange is created by expanding a hole in metal (see Figure 1). This process will serve as the focus for this discussion.
Of all of the factors controlling the success of an extruding operation, material type and its mechanical properties are the most influential. Materials exhibiting good stretch capability and stretch distribution characteristics are best-suited for extruding.
A ferrous metal's (carbon steel's) stretching characteristic, or the ability for it to work-harden and stretch uniformly, is best expressed by evaluating its n value. N value is defined as the material's work-hardening exponent. It is a numeric value roughly ranging from 0.100 to 0.300. The higher the number, the more evenly and uniformly the material stretches.
As odd as it may seem, steel needs to work-harden to exhibit good stretch capabilities. One of steel's unique properties is its ability to increase its hardness and strength as it plastically deforms. If steel work-hardens quickly during plastic deformation, it allows more surface area to deform, and good strain distribution results. In other words, more steel will stretch.
The metal's stretch distribution characteristics and its thickness are two of the most important factors controlling the success of a stretching operation (see Figure 2). Metals with poor stretch distribution characteristics stretch in a very confined, isolated area rather than over a large surface area. If only a small area of the extrusion stretches, most likely it will fail (seeFigure 3).
How the metal is cut before it is extruded can affect the results. Remember that the normal piercing process involves exceeding the metal's shear strength, which causes the metal to fracture or break free. However, before the metal fractures, the punch must cut or extrude the metal (Play-Doh extrusion). A portion of the resulting cut metal edge is the shear zone and a portion is the fracture zone. This shear/fracture relationship (often referred to as the metal's cut band) can be seen easily on the edge of any conventionally pierced part (seeFigure 4).
Stampers often find that they can prevent splitting by filing or sanding the edge of the pierced hole before extruding it. There are two reasons for this. First, the smooth portion of the cut has more edge-stretching capability than the fractured portion of the cut. Smoothing out this fracture zone of the metal results in more edge stretchability. Second, during the metal cutting process, the metal often is deformed in compression around the punch or cutting section. This metal deformation can work-harden the material at its cut edge. This cold-worked material has less stretchability than unworked material. Grinding the edge of the blank often removes the cold-worked material.
Many stampers resort to shaving the cut edge in an effort to smooth it and remove the work-hardened edge. Shaving is a method in which the edge of the prepierced hole is recut using a single-point cutting tool, like using the shaper of a lathe bit to remove a small portion of the perforated edge. Unfortunately, the small shavings often adhere to the tool. If these shavings remain on the tool, product failures and die damage can result.
One remedy is to use a vacuum unit, or bazooka vacuums, to pull the debris or shavings from the tool. You also can use compressed air to assist in removing the shavings.
Until next time … Best of luck!S