They see me rollin’……

The Basics of Extruding

Extrusion involves ramming metal through a die with extreme force.  This process is usually done with a heated billet, but it can be done cold if you enough force and believe in yourself.  Ever use one of those angel hair pasta makers on playdough?  It’s the same idea.  The playdough starts out as some shape, a ball or rectangle, or likely mashed together (the ‘billet’ if you will) and is pressed through the pasta maker, and you get nicely made strings of playdough.  We can do this with metal too!  We can make some pretty neat shapes via extrusion – some very complicated cross-sections that wouldn’t be very easy at all to make with any other process.  Actually, pasta is a great a well known example.  If you take a look in your cabinet, chances are you’ll have some type of pasta.  Maybe several types.  Penne.  Rigatoni.  Fusilli.  Ruote (look this one up).  They’re all shapes that have been extruded and then cut into the proper length.  They all have a constant cross section.  This is exactly what extrusion is good for. Making long pieces with a uniform (constant) and sometimes quite complicated (and even beautiful) cross section.  This long piece can then be cut into the necessary length.  Anytime you see something with a constant cross section, there’s a good chance that the piece was extruded.  

The billet is forced through the die, which creates the desired cross section, and the extruded piece moves continuously through the die.  The billet is forced by a ram.  Once the ram reaches its full stroke, there will be a small piece of billet left over called the butt end.  So here’s an important question: should we extrude a piece while it’s hot?  Or while it’s cold?  Let’s run through some benefits and drawbacks of each.  And ‘hot’ isn’t very precise.  How hot is hot?  Generally, it means above the recrystallization temperature (explanation here).  You’ll find some similar advantages when it comes to hot and cold extrusions as we did with forgings.  Generally speaking, if it’s hot, then it requires less force, and you avoid work hardening the part, so it remains more ductile.  Dimensional tolerances are slightly worse, and the surface finish isn’t quite as nice.  It also adds the process of heating up the piece first, which is a consideration.  With cold, more force is required and some materials might not be able to be extruded.  As well, sometimes complicated shapes can be more of a challenge.  But you’ll end with a nicer piece – close to the dimensions you specify, and with a nice surface finish.  

Some more temperature considerations with extrusions: high temperatures tend to be hard on the dies, and so they tend to wear out quickly.  Good lubrication helps with this.  Cold extrusion limits the oxidation that takes place.

What Materials Can We Use?

What materials can be extruded?  Many, but some are better than others.  As with other manufacturing methods, softer and more ductile metals are usually easier to extrude – they are easier to deform, and so require less force.  It’s difficult to build these rams in the first place, with the force required, so trying to extrude high carbon steel isn’t ideal.  Aluminum, on the other hand, is ideal; copper, magnesium, zinc and tin alloys will also work nicely.  So will some low carbon steels.  What’s also nice about the extrusion process is that you can usually extrude pretty brittle materials, because the process is mostly compressive and shear stresses – so you can avoid tensile forces which tend to tear a material.  Moreover, the parts have excellent dimensional tolerance and a very nice surface finish (those two seem to go hand-in-hand, don’t they?).  Additionally, extruding tends to create elongated grains, which can be a good thing (read up here on grain structure again).

Extrusion Ratio

An interesting measure to be aware of is the extrusion ratio.  It’s simply the area of the billet (the starting area before the piece has been forced through the die) divided by the area of the extruded piece.  And we’re talking cross-sectional area here.  For example, if you start out with a piece that has a cross sectional area of 10 cm x 10 cm (100 cm squared) and your extruded piece has a cross sectional area of 5 cm x 5 cm (25 cm squared…) then the extrusion ratio is 4.  Since the extrusion process is so effective at deforming the metal without tearing it to shreds, these ratios can become quite large.  Obviously, it will always be above 1, unless somehow you aren’t reducing the area of the piece but only changing the shape (which I don’t believe is possible with this method).  Obviously, the extrusion ratios possible will depend on things like the material, the shape you’re trying to extrude, how quickly you’re extruding it – but extrusion ratios typically range from 2 to 100, and maybe even more in certain cases.

Basics of Rolling

Let’s move onto rolling.  Rolling is a similar mechanism to forging – but instead of striking the piece with a press or hammer, the piece is rolled through, well, rolls.  Rolls are just cylinders that force the material to become a certain shape.  You know what baseball pitching machines look like?  It’s basically that.  A similar analogy would be rolling out dough with a rolling pin.  This process is used extensively and is especially good for reducing the thickness of a material, and producing a nice, constant thickness – for example, to make sheet metal, or aluminum foil.  Besides making flat pieces of metal, you can roll material to create various cross sections, such as an I-beam.  I-beams are used extensively for structural parts, and you can probably see them in many buildings, if any structural components are exposed.  You’ll also see them on bridges, and certain car parts can have an I-beam like shape.

Hot and Cold Rolling

Now as you may expect at this point, material can be hot or cold rolled, and again, you’ll see many of the same benefits and drawbacks of each.  When a material is hot rolled, it occurs above the recrystallization temperature (just like our forging and extrusion processes), meaning that you get a nice equiaxed microstructure.  Again, rolling something hot prevents work hardening.  Something that maybe hasn’t been mentioned with previous hot processes is that if you end up with non-uniform cooling due to part geometry (an I-beam is a good example), it can impart residual stresses on the material.  Again, oxidation becomes a more serious concern at high temperatures – the surface of the part becomes covered in something called ‘scale.’  If you watch a video of any hot manufacturing process, you’ll see this kind of nasty looking surface appear.  An advantage of hot rolling is that you can reduce the thickness of the material significantly with a single pass – considerably more so than cold rolling. With cold rolling you can avoid this oxidization and receive better tolerances.  Work hardening occurs, which might increase the strength of the material considerably.  

Shape of Rolls and Deflection

You can get pretty imaginative with the shape of the rolls, and how you apply them to the material.  The rolls certainly don’t have to be flat – of course, this is required for reducing the thickness of the material uniformly, i.e. making sheets (the name for this is flat rolling).  But you can have various shapes of rolls.  Roll forming is the process of create long parts with continuous cross section, similar to extrusions.  You can run a piece of material through different sets of rolls, each which change the shape slightly.  This is a relatively quick process, great for producing large quantities, and large parts.  I-beams are made like this; large I-beams would be difficult to extrude.

Here’s something interesting about rolling: because the forces are high, the rolls themselves might deflect a little bit.  SInce they are supported at the ends, this deflection will be the highest in the middle.  Since the shape of the rolls has been altered, so too will be the shape of the resulting material.  If you have straight rolls and they deflect in the middle, then you’ll end up with a curved piece that’s slightly thicker in the middle.  Of course, whether this happens or not depends on how much force is required to roll the material, and how stiff the rolls are, etc.  But if it does become an issue, it can mitigated by making the rolls slightly thicker in the middle to begin with – when they begin to deflect, they straighten out and you’re left with a flat piece.