The most commonly used materials for extruded stems are high-quality wrought AISI H-13 (1.2344) hot-work tool steel and forged tool steel. The stem is heated to austenitizing temperature, quenched, and tempered to a hardness in the range of 415-460 Brinell (Rockwell C44-48). Its strength depends on its sustained hardness at operating temperature and the absence of internal stresses.
The Role of the Extruded Stem
The extruded stem must operate repeatedly and continuously under very high compressive loads. Therefore, it is crucial that it remains perfectly aligned throughout its entire stroke length. Alignment should be checked weekly.
Due to the high pressures to which the extruded stem is subjected, the load must be applied evenly. Uneven loading will eventually lead to bending or fracture. This can be caused by a variety of reasons, such as improper installation, platen deformation, and press misalignment.
As mentioned above, these situations must be carefully avoided, as catastrophic fracture of the extruded stem is possible.
Extrusion rods typically use a bayonet coupling that quickly and securely locks the extrusion head to the rod. This makes replacing very hot blocks much easier and faster than with traditional screw- or rod-type couplings. Supplementary stabilizing studs are used to prevent lateral movement on newer presses where alignment is not an issue. Care must be taken when designing and machining bayonet couplings in extrusion rods to avoid potential stress risers that could cause breakage.
Extrusion Rod Maintenance
Extrusion rods should be stress-relieved periodically, depending on the size of the press and the number of thrusts.
Existing extrusion rods can be adapted to the bayonet system by attaching a spacer. The extrusion rod threads are cut and machined in the hole to attach the stud. It is important that the extrusion rod and spacer faces match exactly, with no gaps. Dowel pins must be installed across the diameter of the extrusion rod/spacer to prevent rotation. If gaps develop between the extrusion rod and spacer, the stud will bear the full force of the plunger, causing the threads to foul and becoming extremely difficult or impossible to remove.
Extrusion rods should be placed vertically in an oven and heated to 1000°F (540°C) at a rate not exceeding 1000°F (550°C) per hour, then maintained at this temperature for one hour per inch (25 mm) of extrusion rod diameter. Remove from the oven and allow the extrusion rod to cool in still air at room temperature.
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