**6. Compression molding**

**Figure 16.** Ground PTFE powder being mixed with virgin PTFE powder.

182 Sintering Techniques of Materials

**Figure 17.** Mixture being cold pressed into pellet.

virgin PTFE

**Figure 18.** Fillers like graphite or pigments normally do not melt or agglomerate nor interact with PTFE during sinter‐ ing at atmospheric pressure but this is not the case with reprocessed PTFE when used alone or used as a filler with Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. Its advantage lies in its ability to mold large, fairly intricate parts.

Initially it was thought that the die and plunger could be heated during compression in the hydraulic press. A strip heater was wound around the die to heat the die with the help of a heating programmer and a thermocouple was inserted into a hole made specifically for this purpose in the die to measure the temperature during compression, The experimental arrangement is shown below.

**Figure 21.** Compression molding

The strip heater wound around the die, however, could not heat it to the desired temperature even after several hours of operation as the heat capacity of the die made out of hardened steel was much high and so this attempt was discontinued. Instead, after cold pressing in the press, the die and plunger itself with the PTFE disc inside it was kept inside the high capacity air over under mechanical pressure.

**Figure 22.** Cold pressed pellet along with the die and plunger being pressurized with the help of a C-clamp

**Figure 23.** Die under C-clamp kept inside the air over for sintering near 400°C

**Figure 21.** Compression molding

184 Sintering Techniques of Materials

over under mechanical pressure.

The strip heater wound around the die, however, could not heat it to the desired temperature even after several hours of operation as the heat capacity of the die made out of hardened steel was much high and so this attempt was discontinued. Instead, after cold pressing in the press, the die and plunger itself with the PTFE disc inside it was kept inside the high capacity air

**Figure 22.** Cold pressed pellet along with the die and plunger being pressurized with the help of a C-clamp

**Figure 24.** Repro filled PTFE Discs when sintered inside the die and plunger under C-clamp show discoloration which runs through its volume although good surface smoothness is seen and cracks disappeared totally. This was attributed to carbon production on reaction of volatiles with the die material. Even 100% virgin PTFE pellet show slight discolor‐ ation but only on its surface (both sides) when sintered under C clamp

**Figure 25.** 10% mixture, mixee mixed, 300 deg C, 2h pre-heated, 2500 psi, No pressure during sintering at 400 deg C in a furnace. Sample is white in color but surface is rough.

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 and sintering without pressure.

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

#### **6.1. Sintering treatment**

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 strength as it reduced the polymer strength.
