Preforming pressure

Eree-sintered articles do not have the same dimensions as the mold cavity because they shrink at right angles to the direction of the preform pressure and grow in the direction of the appHed pressure. 

Shrinkage of about 5-10% occurs at right angles to the direction of the preforming force. The amount of shrinkage is mainly dependent on the rate of cooling, but also to a minor extent, on the preforming pressure. 

PTFE resins exhibit a first-order transition at 19°C (66°F) due to a change of crystalline structure from triclinic to hexagonal unit cell (see Chapter 3, Section 3.2.1.3). A volume change of approximately 1% is associated with this transition (Figure 4.3). Another consequence is that the resin has a better powder flow below 19°C but responds more poorly to preform pressure. Billets prepared below this transition are weaker and tend to crack during sintering. For this reason, the resin should... 

The trapped air in the preform is under high pressure, theoretically equal to the preforming pressure. The air requires time to leave the preform because it is mostly contained in the void areas surrounded by enmeshed particles. Immediate sintering would lead to a rise in the already high pressure of the air and catastrophic cracking of the part as the PTFE melts and the mechanical strength declines. The preform should be allowed to degas which equalizes the internal air pressure to atmospheric pressure. 

Figure 5.7 Effect of preform pressure on mold shrinkage. (Courtesy DuPont.)...

Figure 5.10 shows the various steps of the sintering process. At first the preform completes its elastic recovery and begins to thermally expand past the PTFE melting point. The expansion can reach up to 25%-30% by volume depending on the type of resin, powder, preforming pressure, and temperature. Above 342°C, PTFE is a transparent gel due to the absence of a crystalline phase. 

Mold construction Inserting the preform Pressure injection... 

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