G. W. Dion Company
 
 
 
 

 

 

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Extrusion Tool Reengineering Pays Dividends

A North American extruder of sections for heat exchangers, together with the G. W. Dion Company LLC in Leonard Michigan, USA has improved the die arrangement in a way that achieved lasting die reliability.

Aluminum extrusions are ideally suited for heat exchangers. Freedom of cross-section design combined with the metal's good thermal conductivity enable hollow sections to be produced, which have a cross-section that assists flow together with very large heat transfer surfaces and thin walls. In automobiles the compact structure of the radiator and aluminum's low density lead to considerable savings of weight. The use of extruded sections in heat exchangers began many years ago, but began to rapidly expand in the 1990’s. They have since become so widespread is due to their ability to transfer large amounts of heat in a very restricted space. For this, designers utilize the property typical of aluminum that sections can be extruded with almost any desired cross-section. For these specific heat exchangers the shape chosen is a flat, thin-walled hollow section with internal webs and partition walls (Fig.1) optimized for heat transfer and hydraulics and a 6mm diameter tube with internal enhancements or serrations (Fig.2). However, the production of these delicate sections sets new challenges for extrusion processes and die manufacturing technology. For a North American producer, Gary W. Dion developed a die design whose aims were:
Fig 1: Fig 2:

- to increase die reliability,
- to reduce die set-up time and costs,
- to increase product quality and tolerances.

These sections are extruded in 1800 and 2250 ton extrusion presses.  The billet diameters on 6“ and 7“ respectively, with extrusion reduction ratios over 400:1.  Four strand dies were employed on these sections.

These goals were achieved by focusing on two areas:
 - Designing a tool assembly that would allow for minimum tool deflection.
- Reducing the tolerances on the mandrel inserts.


The Old Tool Design

 The dies were of a four-hole design, using replaceable die and mandrel inserts.  The die inserts were shrunk fit into separate die holders that were adjusted once placed in the housing to create the proper offset, allowing for tool deflection.  Because of the tool deflection during the extrusion process, the end product would many times extrude out of tolerance and the dies had to be pulled prematurely, many times without extruding any product at all.  Due to the instability of the tool assembly, many dies needed to be prepared and kept heated to the proper extrusion temperature to assure there was always dies ready in case of die failures.  Many times 3 or 4 dies would fail, producing very little if any product before one would to run the proper tolerances for any length of time.

The dies required anywhere from 3 to 5 hours to prepare them for production. This preparation consisted of:
- disassembling the die
- removing the mandrel and die inserts
- replacing the mandrel and die inserts
- reassembling the die
- adjusting the wall openings to create the proper offsets, allowing for tool deflection, (This was done by shimming or grinding the die holders to suit)


The New Tool Design

The new tool consisted of a 4-hole die ring or holder, four self-contained mini-dies and a backing plate. The mini-dies were a complete individual set containing die and mandrel inserts. This new design eliminated the out of tolerance condition because of the individual mini-insert component. As the tool assembly deflected under the extrusion pressure the mini-inserts moved as individual units keeping the die and mandrel inserts concentric. With this new design the dies were many times more stable and reliable. The mini-inserts required no special adjustments to set them up. This reduced the die assembly time substantially. Tool reliability increased substantially as did the final extruded product.


On average the old style dies would produce 20-50 charges or billets, while the new style dies produce 250 charges per set-up.  This increased productivity and reduced tooling costs substantially.


Development of Carbide Mandrel Inserts for 6 mm Enhanced Tubing

The mandrel inserts are manufactured from carbide. The original supplier was manufacturing the inserts to a tolerance of +/- .0005". Due to the high reduction ratio and the very thin wall thickness this tolerance range allowed for uneven run-out lengths and wall thickness variations. When extruding this type of profile it is extremely important to maintain the tightest possible tolerance allowing for the closest area per each of the four holes in the die. If one insert is on the higher side of the tolerance band and another is on the lower side, there will be considerable difference in performance. Many times this lead to tool failure, resulting in a premature die change. Also with this wide tolerance band the die life varied greatly.


With new machining processes, fixturing and materials the carbide mandrel inserts were able to be produced within tolerances of +.0002/-.0000".  This would allow for increased tool wear because the inserts were machined to high tolerance.  This would also allow for very uniform wall thicknesses making the volume of each hole very close to the other.  These close tolerances increased tool reliability and repeatability many fold.

Additionally the original inserts were supplied with an EDM finish on the bearing surface.  This surface caused much drag and tearing of the internal enhancements in the extruded tube causing tool failure, which caused a die change.  Using special fixturing in an Abrasive Flow Polishing Machine solved this problem.  This process accomplished two functions.  Not only could the carbide mandrel inserts be polished to a high finish, but also the final sizing could be brought closer to tolerance.


Conclusion

As described above these tooling improvements increased die reliability by over 8 times.  This briefly describes what can be accomplished with focusing on extrusion tool development.  Continual development and improvement is a must for any manufacturing company.  From design to manufacturing processes many things affect the quality of extruded products.  These issues also affect the cost per pound of these products.  Die reliability has a substantial impact on the bottom line.