Factors Affecting Extrusion
Shape is a determining factor in the part's cost and ease with
which it can be extruded. In extrusion a wide variety of shapes
can be extruded, but there are limiting factors to be considered.
These include size, shape, alloy, extrusion ratio, tongue ratio,
tolerance, finish, factor, and scrap ratio. If a part is beyond
the limits of these factors, it cannot be extruded successfully.
The size, shape, alloy, extrusion ratio, tongue ratio,
tolerance, finish, and scrap ratio are interrelated in the
extrusion process as are extrusion speed, temperature of the
billet, extrusion pressure and the alloy being extruded.
In general, extrusion speed varies directly with metal
temperature and pressure developed within the container.
Temperature and pressure are limited by the alloy used and the
shape being extruded. For example, lower extrusion temperatures
will usually produce shapes with better quality surfaces and more
accurate dimensions. Lower temperatures require higher pressures.
Sometimes, because of pressure limitations, a point is reached
where it is impossible to extrude a shape through a given press.
The preferred billet temperature is that which provides
acceptable surface and tolerance conditions and, at the same
time, allows the shortest possible cycle time. The ideal is
billet extrusion at the lowest temperature which the process will
permit. An exception to this is the so-called press-quench
alloys, most of which are in the 6000 series. With these alloys,
solution heat-treat temperatures within a range of 930°-980° F
must be attained at the die exit to develop optimum mechanical
At excessively high billet
temperatures and extrusion speeds, metal flow becomes more fluid.
The metal, seeking the path of least resistance, tends to fill
the larger voids in the die face, and resists entry into
constricted areas. Under those conditions, shape dimensions tend
to fall below allowable tolerances, particularly those of thin
projections or ribs.
Another result of
excessive extrusion temperatures and speeds is tearing of metal
at thin edges or sharp corners. This results from the metal's
decrease in tensile strength at excessively high-generated
temperatures. At such speeds and temperatures, contact between
the metal and the die bearing surfaces is likely to be incomplete
and uneven, and any tendency toward waves and twists in the shape
As a rule, an alloy's higher
mechanical properties means a lower extrusion rate. Greater
friction between the billet and the liner wall results in a
longer time required to start the billet extruding. The extrusion
ratio of a shape is a clear indication of the amount of
mechanical working that will occur as the shape is extruded.
Ratio = area of billet/area of shape.
the extrusion ratio of a section is low, portions of the shape
involving the largest mass of metal will have little mechanical
work performed on it. This is particularly true on approximately
the first ten feet of extruded metal. Its metallurgical structure
will approach the as-cast (coarse grain) condition. This
structure is mechanically weak and shapes with an extrusion ratio
of less than 10:1 may not be guaranteed as to mechanical
As might be expected, the
situation is opposite when the extrusion ratio is high. Greater
pressure is required to force metal through the smaller openings
in the die and extreme mechanical working will occur. Normally
acceptable extrusion ratios for hard alloys are limited to 35:1
and for soft alloys, it is 100:1. The normal extrusion ratio
range for hard alloys is from 10:1 to 35:1, and for soft alloys
is 10:1 to 100:1. These limits should not be considered absolute
since the actual shape of the extrusion can affect results. The
higher the extrusion ratio, the harder the part is to extrude
which is the result of the increased resistance to metal flow.
Hard alloys require maximum pressure for extrusion and are even
more difficult because of their poor surface characteristics
which demand the lowest possible billet temperature.
Difficulty factor is also used to determine a
part's extrusion performance.
Perimeter of Shape/ Weight per Foot.
per foot is of primary importance because of the consideration
for profitable press operation. As might seem obvious, a lighter
section normally requires a smaller press to extrude it. However,
other factors may demand a press of greater capacity such as a
large, thin wall hollow shape. Though it has low weight per foot
it may take more press tonnage to extrude it. The same reasoning
applies to the factor as with the extrusion ratio. A higher
factor makes the part more difficult to extrude consequently
affecting press production.
ratio also plays an important role in determining a part's
extrusion performance. The tongue ratio of an extrusion is
determined as follows: square the smallest opening to the void,
calculate the total area of the shape, and then divide the
opening squared by the area.. The higher the ratio, the more
difficult the part will be to extrude.
order to help us understand your needs and requirements and
service you better, the following is a check list of things to
consider when submitting items to an extruder for quoting or new
- Description or drawings of the part- talk to the
extruder early before the design is finalized.
- Specifications to be met; Federal specs, military, ASTM,
- Alloy and temper; if unknown, indicate requirements for
strength, corrosion resistance, machinability, finish,
weldability, to aid the extruder in making a recommendation.
- End use length and purchase length.
- Tolerances; commercial, per drawing, other.
- Surface Finish; mill, anodize, paint, exposed surfaces,
- Packaging; acceptable maximum and minimum weight per
package and shipping and handling requirements.
- Secondary fabrication requirements-mitering, punching,
bending, anodizing, drilling, etc.
- Product end-use.
- Quantity needed; this order and on an annual basis.
- Shipping date.
- Special quality considerations.