TONGUE RATIO CALCULATION IN EXTRUSION DIE DESIGN

This calculation will supply a value in the form of a ratio between the opening (or neck) of

a tongue detail to the total surface area of the tongue itself. The calculation will supply an

indication as to the strength of any such tongue and the likelihood of high deflection

occurring or even breakage during extrusion. It can also give advice to a die designer on the

construction of the die.

Extrusion ratio will also give some indication as to the strength of a die. For example a die

with a high extrusion ratio coupled with an area of the profile which has a high tongue

ratio, this is the ideal recipe for disaster.

Generally a tongue ratio of between 4:1 and 5:1 is the cut-off point at which the

majority of extruders would feel that the die would have sufficient intrinsic strength to

with-stand normal aluminium extrusion parameters without employing any special design

features for the die and the naturally higher cost for the die.

To calculate :-

Surface area of the total tongue

Smallest neck opening (SQUARED)

If tongue ratios exceed the standard figures then the extruder or die maker may advise on

one or more of the design features be incorporated in the die design :- Possibility of

consulting the customer requiring the extrusion with a view to adapting the design to make

the extrusion die less likely to suffer from damage.

Special heat-treatment for the die (e.g. triple tempering)

Special steel for the manufacture of the die (e.g. QRO 90, H10A)

Different die design characteristics.

Maximising the support strength (e.g. back-sparked tongue area)

Sinking or recessing the tongue or part of the tongue in the die face.

Suggested die design parameters

Tongue ratio 3:1 or below. (Good support required in the die)

Tongue ratio 4:1 - 5:1 (Give back sparked support and recess tongue face in die)

Tongue ratio 4:1- 5:1 (with high extrusion ratio) use special steel also.

Tongue ratio 6:1- 7:1 (with low extrusion ratio) use all of the above.

Also ensure that there is clearance for the extrusion to exit the die. If the extrusion twists in

the back of the die and gets stuck then the likelihood of the tool breaking is extremely high,

no matter what safety systems were introduced. Obviously the tongue area needs good

support but the part of the profile that is adjacent to the tongue does not require the same

amount of support and would facilitate the exit of the extrusion if it were to twist. Tongue

ratios above those already discussed (a different die design e.g. Step-over bridge)

The balancing of the flow and friction forces either side of the tongue are important. This is

generally not too much of a problem when wall sizes are equal, and therefore bearings

equal in length. Great care has to be taken with walls of unequal thickness because of the

die design with unequal bearing lengths. The lateral forces will tend to move the tongue

from the larger wall thickness towards the smaller wall thickness.