METAL EXTRUSION - IN BRIEF NOTES

Metal extrusion

Process 

Extrusion is a compression process in which work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The process is like squeezing toothpaste out of a toothpaste tube. Lubricant is provided to ease the passage of the metal through the die. Extrusion process is usually classified based on the physical configuration and working temperature.   

       Based on physical configuration it is classified as: direct extrusion and indirect extrusion. Direct extrusion is also called forward extrusion. In the direct extrusion process the metal billet is loaded into the container and the ram compresses the metal billet. 

The flow of material occurs in the direction of application of force through the opposite end as shown in the Figure M4.1.1. Hollow sections are possible to create by this process setup as shown in Figure M4.1.2. Indirect extrusion is also called backward extrusion process. In the backward extrusion process, the die is mounted on the ram. As the ram penetrates into the work, the metal is forced to flow through the die in the opposite direction of motion of the ram as shown in the Figure M4.1.3.

Figure M4.1.1: Direct Extrusion  

                                    Figure M4.1.2: Direct Extrusion to produce hollow cross-section 

                                                       Figure M4.1.3: Indirect Extrusion 

         Based on working temperature, classification of extrusion is as: hot extrusion and cold extrusion. Hot extrusion involves prior heating of billet to a temperature above its crystallization temperature. Cold extrusion is usually used to produce parts at room temperature.

Typical parts and applications

Any part with constant cross-section can be produced by this method. Cross-section that can’t be produced by normal machining process are often more economical by the extrusion process. A standard method of measuring capacity is used called circumscribing–circle-diameter (CCD). This is the size of circle into which the cross section will fit. For aluminum, the minimum CCD is 6.3mm and maximum CCD is 1.02m. For steel the diameter is small. Maximum CCD for steel is 150mm. In fact, extrusion over 30m in length and 1 Ton in weight can be made. Wall thickness for aluminum ranges from 1mm upward. For carbon steel, the minimum is 3.2 mm and for stainless alloys 4.8mm.

Suitable material for extrusion

Two factors affect the ease with which the metal can be extruded namely, required temperature and the temperature range. If the required temperature for extrusion is low and available temperature range is wide the extrusion will be better. The most common metals used for the extrusion are the following (listed in order of extrudability): 

1. Aluminum and aluminum alloys

2. Copper and copper alloys

3.Magnesium

4.Low-carbon and medium-carbon steels

5.Modified-carbon steels

6.Low-alloy steels

7.Stainless steels

General design recommendation

Though complicated shapes are possible to create, it is advisable to use standard cross-section whenever possible as shown in Figure M4.1.4.

 Figure M4.1.4: Standard extruded shapes available.

The problem with complicated shapers are: metal flows less readily into narrow and irregular die section, distortion and other quality problems, price of the customized dies are higher.

Detail design recommendation

The various design recommendations are as follows:

            Sharp corners are avoided for both internal and external corner of extruded part. If sharp corners are used various problems encountered are: less smooth flow of material through the die, increase tool wear, increased possibility of tool breakage, less strength in the part due to stress concentration. Recommended minimum radii for various metals and alloys are summarized in Table M4.1.1 for guidance. Figure M4.1.5 shows the good and bad practice in the design of cross-section of component to be extruded.

Figure M4.1.5: Good and bad practice in the design of cross-sections to be extruded

Table M4.1.1: Recommended minimum corner and fillet radii. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

• Section walls should be balanced as much as the design function permits as shown in Figure M4.1.5. 

•  Ribs are added in order to avoid the variation of flatness of a long thin section those having critical flatness requirement as shown in Figure M4.1.6. 

Figure M4.1.6: Ribs are added to the sections for long sections.

• Knife like edge part is avoided because it affects smooth flow of material through die. Holes in nonsymmetrical shapes should be avoided in less extrudable material as shown in Figure M4.1.7.

Figure M4.1.7: Knife edge should be avoided. 

• Abrupt changes in section thickness are avoided for less extrudable materials like steel as shown in Figure M4.1.8. 

Figure M4.1.8: Avoid abrupt changes in section thickness for less extrudable materials

• Recommendations for depth of indentation has been shown in the Figure M4.1.9.

Can’t be extruded in steel Can be extruded in steel

Figure M4.1.9: Design rules for indentations.

• The ratio of length to thickness of any segment should not exceed 14:1. For magnesium it is 20:1 as shown in Figure M4.1.10. 

Figure M4.1.10: The length-to-thickness ratio of any section of an extrusion of steel or other difficult-to-extrude material should not exceed14.

• Symmetrical cross sections are preferable to non-symmetrical designs to avoid unbalanced stresses and warpage. Design recommendation has been shown in the Figure M4.1.11. 

Figure M4.1.11: Nonsymmetrical shape by extruding asymmetrical section and dividing it in two.

Dimensional factor

Extrusion is a hot process and temperature and cooling rate variation affect the final dimension of the extruded parts. Hence, extruded parts are more inherent to piece-to-piece and drawing-topiece dimensional variation than parts made with other processes.         

Recommended tolerances

Table M4.1.2 summarizes the recommended tolerances for extruded parts for ferrous metal.

Table M4.1.2: Recommended Dimensional Tolerances for Ferrous-Metal Extrusions. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

Table M4.1.3: Recommended Dimensional Tolerances for Ferrous-Metal Extrusions. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)