What is an hollow die?
Types of hollow dies
A steel extrusion tool which forms extruded closed profiles containing one or more voids such as rectangular tubing. The tool generally consists of :
- a die cap that generates the outer surface of the profile and
- the mandrel or core which generates the inside contour.
Hollow or semi-hollow profiles are produced usually with foloving type dies:
- spider (taper seal)
Extruded sections produced on such dies have seams or longitudinal weld lines, due to the metal flow around the web supports (bridges) that hold the mandrel. The latter determines the inside contour of the profile being extruded. After flowing around the supports, the metal is fused in a weld chamber before passing through the die (die cap) proper .
The best definition of a hollow extrusion die is that it is made for the purpose of a double extrusion. This fact makes it a more complex to correct .
Hollow dies can be considerd as double-extruded due to the fact that:
- First the billet is divided into two, three, or more sections as it hits the face of the die
- After the billet has been sectioned on the face of the die, the sections will travel across and over the surfaces of the mandrel until they come up to the opening on the cap which will extrude the final form of the actual extruded shape (Figures 1 to 3).
Figure 1 – Hollow dies:
The billet is divided into two, three, or more sections as it hits the face of the die.
After the billet has been sectioned on the face of the die,
the sections will travel across and over the surfaces of the mandrel
until they come up to the opening on the cap
which will extrude the final form of the extruded shape 
The correction of hollow dies
The advantage of correcting hollow dies
The advantage of correcting hollow dies is that there are various places to apply corrections before you have to work the bearing itself:
- A well designed hollow die can be corrected by just grinding or working the flow in the ports
- In many cases an hollow die is corrected without working the bearings at all.
To correct the flow on a hollow die you should:
- Examine the ports, to make sure that the volume of aluminum to be carried by one port is balanced with the volume carried by the rest of the ports.
- This means you need to check the area of the side of the extrusion shape each port should feed (Figure 4).
Figure 5 shows a typical design showing the distribution of port areas to balance the flow through the die.
Heavy and light walls
Taking this into consideration, we then know that by opening or restricting the flow through the ports we speed up or slow down the flow of aluminum in any section of the shape being extruded. A section of a hollow extrusion will hold back also if that section is extruding light wall thickness. That is why you should look for other problems before opening up the ports on a hollow die.
The reasons a hollow open exrusion might not come out with light walls from a hollow die are as follows :
- Mandrel shifting in relation to the cap
- Poor feed on core
- Walls filed lightly.
In cases like the one illustrated in Figure 6 you should not make any correction until you have taken care of the shifting on the die.
- The ports of a hollow die around the mandrels act as a long bearing.
- The obstacles on the mandrel to facilitate the flow of aluminum while it travels along the surface of the mandrel.
- Correction is done by milling or grinding of obstacles (Figures 7A and 7B) .
As can be seen by Figure 6A, the angle being steep will create two obstacles on the side of the mandrel which will slow down the flow of aluminum. On Figure 6B, you see the mandrel with both obstacles removed, either by milling machine or by grinding. In this case the angle is less, with no obstruction, and more volume of aluminum goes through. Therefore this will speed up these two sides of the extrusion .
The die makers will often build these obstructions on hollow die manurels intentionally. Whenever a die designer feels he should slow down the flow to feed certain sections. he will do so and in many cases it is only a safety factor not to use a very heavy bearing. If it is not needed it can be easily removed by grinding.
- Luis Bello, Die Corrections for Changing Flow Characteristics, Aluminum Extrusion Technology Seminar, Chicago, 1977
- Influence of Al Microstructure on Hard Anodising Quality – Profile Material / Tom Hauge, Hydro Aluminium, Norway
IHAA Symposium, 25th of September 2014, New York
- Aluminum Technology Extrusion / P. Saha