Processing of Mechanical Seal PTFE Bellows

PTFE bellows (referred to as PTFE bellows) has been widely used in various types of corrosion-resistant mechanical seals. The manufacture of bellows is divided into two steps, first making the blank and then turning it. Teflon blank tube stock. The whole barrel is composed of three sections, the middle section is pure PTFE, and the two ends are filled PTFE.

1. Raw material specifications
Pure tetrafluoroethylene raw material is mixed with SFX suspension powder and filler.
Fillers mainly include the following:
Graphite powder: chemical reagent grade.
Glass fiber: Use “three-no” glass fiber filaments without alkali, wax and surface treatment agent, crushed by ball mill and pass through a 150-mesh sieve.
Silica: Pass through a 270 mesh screen.

2. Mold design

The shrinkage rate of pure PTFE plastic is 5%, and the shrinkage rate of filled PTFE plastic is 2%~2.5%. The outer diameter d2 of the design die (see Figure 9-25) is calculated according to the shrinkage rate of 5% plus processing excess. Considering the amount of 3-4mm, the inner diameter d1 of the die is considered as the shrinkage rate is zero, that is, the actual size of the inner diameter of the part is subtracted from the machining allowance of 3-4mm. The height of the mold is four times the height h of the blank. The press mold, bottom mold and inner mold and outer mold all adopt dynamic fit.
The specific calculation method is as follows: set the inner diameter of the part as dn and the outer diameter as dw, then:
d2=dw+5%dw+(3-4)
d1=dn-(3-4)
D=d2+40
H=4h
H2=H+10
H1=10
The mold material is 45 steel.

3. Raw material ratio of bellows blank
1. Front section (moving ring section)
Formula 1: 80%-85% of PTFE suspended fine powder; 15%-20% of glass fiber powder
Formula 2: 80%-85% of PTFE suspended fine powder; 15%-20% of graphite powder

The mixture of the above formula can be fully mixed and stirred for 1-2 minutes in a 2950r/min small high-speed mixer (the diameter of the stirring paddle is 200-250mm). If it is found that it is still not fully mixed, stir again.
2. The middle corrugated pipe section adopts pure PTFE coarse powder.
3. Back section (fixed section)
90% PTFE suspended fine powder; 10% glass powder or glass fiber powder. The two ingredients are also mixed in a high-speed mixer.

4, blank pressing
Calculate the mass of the powder according to the density of 2.7g/cm³, weigh the materials separately, and add them to the mold in order. In order to make the stratification of the blank clear, each time you add a material, you can put the die into the mold and press it slightly, and then add the second material and the third material. After all the materials are added to the mold, press on the press with a pressure of 30-50MPa; the pressure holding time is 5-10MIN to fully exhaust (depending on the height of the parts), and the mold can be demolded after holding the pressure.

The height of the mold is sometimes too high, exceeding the maximum working height of the press, causing processing difficulties. At this time, the mold height H can be appropriately reduced to make H ≥ 2.5h (blank height), so that after each material is added, the initial pressure must be given on the press, and the pressure is 2-5MPa. Before adding the second material , the surface of the initially pressed material must be scratched with a small rod, so as to ensure the quality of the pressed blank.

5. Sintering of blanks
The pressed blank is sintered in a DL-151 high-temperature sintering furnace, and starts to be heated at a heating rate of 1-2 °C/min. When the temperature in the furnace reaches 320 °C, the turntable is turned on for 1 hour. Then continue to heat to 375–3800C at a heating rate of 1°C/min, and keep warm at this temperature. The holding time is calculated as 10min per millimeter of wall thickness. After heat preservation, cool down with the furnace (the furnace door cannot be opened), and naturally cool down to room temperature.

Due to the different raw materials, pressing and sintering processes of the bellows blank, the corrugated section will be brittle and cracked. Therefore, the experience should be summed up in time in the production, and the reasonable process should be determined according to the different raw materials. If the quenching process is used, the toughness of the bellows can be greatly improved.

There are two methods of turning PTFE bellows, single-tool or multi-tool turning. Due to the simplicity of a single-knife tool. It is also conducive to the adjustment of the wave distance, so more single-tool turning systems are used. Ordinary lathes are used, and tools can be made of tool steel or high carbon steel. Taking the 152 type mechanical seal bellows as an example, the car manufacturing process is briefly described as follows.

When turning, use a three-jaw chuck to directly clamp the PTFE blank, which will leave prestress on the softer blank. The parts that have been turned will be deformed, so the mandrel is used to support the inner hole to install the truck. it is good. The structure of the tool is shown in Figure 9-26. The tool holder is fixed on the tool holder. The shape of the cutter head is determined according to the requirements of the drawing, and the transverse distance δ between the two cutter heads is the axial wall thickness of the corrugation.

First process the end face of the moving ring and the shape of the bellows, and then use the bellows tool to turn the bellows. First, the inner wave, pay attention to control the depth of the corrugation, and then the outer wave. Axial feed makes the third wave. Then the inner and outer waves are in turn. Finally, turn around, support the inner hole with the mandrel, and finish the rest of the car.