**7. Ram extrusion**

Ram extrusion enables continuous processing of PTFE [9,10]. The PTFE powder (virgin or reprocessed) is fed into a cylindrical extrusion pipe hydraulically while at the same time compressing it by means of a ram and transported through the pipe, which is heated up to sintering temperature in the range 370 to 400°C (Fig.26). The ram is then withdrawn, the die tube re-charged with powder and the cycle repeated. This way the powder is continuously fed into the heating section of the die tube where it is sintered and then it passes through a cooler section from which the finished products (rods, tubes etc) flow out continuously which are cut into desired lengths. Apart from PTFE materials like ultra high molecular weight polyethylene as well as their compounds can be ram extruded. Uniform distribution of powder into the cavity is essential. The powder should exhibit good flow properties. The extrusion pressure is in the range of 26 to 74 MPa depending on the length of the cold zone above the heated section of the die tube and the extrusion rate is 3 m/h. The heated length of the tube can vary from 44 to 90 cm.

The 50% recycled PTFE ram extruded PTFE rod made in China is shown in Fig.27. It is clear that the discoloration in ram extruded rods (top) made by us was due to the production of carbon (Fig.28). This conclusion was confirmed when the discoloration reduced on subsequent anneal at 400° C for 1h in air oven which led to the escape of carbon into atmosphere in the form of carbon di oxide gas. As a result the extruded rod (bottom) exhibited biscuit color all thorough its volume without any black patches. This is a significant result and confirms that the discoloration is caused by carbon production. The fluorine being a gas produced on disintegration of fluorocarbon perhaps has already escaped during the sintering while carbon in the absence of oxygen has deposited within the rod. Acid treatment before sintering has been found to improve the color further.

**Figure 26.** A typical ram extruder with 100% virgin extruded PTFE rod.

The reason for the discoloration could be due to carbon generation on reaction of volatiles with the stainless steel die under pressure. This can be avoided if a pathway can be provided for the escape of volatile gases during sintering under pressure. This will need fabrication of a new die and plunger with a series of holes. While high pressure (2000 to 3000 psi) is required during cold pressing powder into pellet, a relatively lower pressure (500 to 1000 psi) should suffice during sintering. Further efforts were made by reducing the die pressure to 2500 psi

This showed that mild pressure during sintering is a must. High pressures with clamp not only discolor the pellets but also fuse them with the die. So a compromise in pressure during

Sintering temperatures were varied from 350 to 450 deg C and duration from 15 min to 1h. From the points of view of polish, smoothness and strength, the best treatment was found to be 380-400 deg C, 1h which is the same used for sintering virgin PTFE. Lower temperatures resulted in poor strength due to under-sintering while higher temperatures resulted in poor

Ram extrusion enables continuous processing of PTFE [9,10]. The PTFE powder (virgin or reprocessed) is fed into a cylindrical extrusion pipe hydraulically while at the same time compressing it by means of a ram and transported through the pipe, which is heated up to sintering temperature in the range 370 to 400°C (Fig.26). The ram is then withdrawn, the die tube re-charged with powder and the cycle repeated. This way the powder is continuously fed into the heating section of the die tube where it is sintered and then it passes through a cooler section from which the finished products (rods, tubes etc) flow out continuously which are cut into desired lengths. Apart from PTFE materials like ultra high molecular weight polyethylene as well as their compounds can be ram extruded. Uniform distribution of powder into the cavity is essential. The powder should exhibit good flow properties. The extrusion pressure is in the range of 26 to 74 MPa depending on the length of the cold zone above the heated section of the die tube and the extrusion rate is 3 m/h. The heated length of the tube can vary from 44

The 50% recycled PTFE ram extruded PTFE rod made in China is shown in Fig.27. It is clear that the discoloration in ram extruded rods (top) made by us was due to the production of carbon (Fig.28). This conclusion was confirmed when the discoloration reduced on subsequent anneal at 400° C for 1h in air oven which led to the escape of carbon into atmosphere in the form of carbon di oxide gas. As a result the extruded rod (bottom) exhibited biscuit color all thorough its volume without any black patches. This is a significant result and confirms that the discoloration is caused by carbon production. The fluorine being a gas produced on disintegration of fluorocarbon perhaps has already escaped during the sintering while carbon

cold pressing as well as provision to let out the volatile gases are necessary

and sintering without pressure.

strength as it reduced the polymer strength.

**6.1. Sintering treatment**

186 Sintering Techniques of Materials

**7. Ram extrusion**

to 90 cm.

**Figure 27.** 50% recycled PTFE rod from Deqing VRT Plastic Industrial Co., Ltd.

**Figure 28.** A 40% repro + 60% virgin PTFE mixture was ram extruded into long rods of diameter 17 mm with the help of a local industry in India. The rods came out fine from the points of polish and strength but was dark in color with patches all through the length and volume due to carbon deposition.
