Sintering of PTFE billets for preventing cracks

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  process description    input data    results generated    uses    input screen    results screen

PTFE (poly tetrafluoroethylene) billets are made by compacting the PTFE powder (with or without fillers) under high pressure and then subjecting them to a controlled temperature cycle in a temperature-programmed oven. When PTFE is held above a critical sintering temperature for some time, the grain boundaries disappear due to molecular diffusion. This makes the billet homogeneous, mechanically strong and suitable for machining. The PTFE undergoes a phase transition at some temperature, which is accompanied by absorption of heat during heating. A similar transition takes place during cooling, at a slightly different temperature. In order to sinter the billet completely, it is necessary to ensure that even the center of the billet is held above the critical temperature for a specified time. This is done by programming the oven temperature appropriately.

Analytical solutions are available for heat conduction without a phase change for billets of any dimensions. Semi-analytical solutions are also available for heat conduction with a phase change for a wire (that is, a very long billet) and a flat plate (that is, a very short billet). This software package uses a numerical solution procedure applicable to the general case of a billet with comparable height and diameter, where phase transitions also take place. External heat transfer resistances are taken into account.
 

 

Process description

PTFE billets are made by compacting PTFE powder (mixed with fillers such as carbon, stainless steel, brass, asbestos, talcum etc. if desired) under high pressure. They are then heated under controlled conditions in a temperature programmed oven. When PTFE gets heated above the sintering temperature and is held above this temperature for some time, particle boundaries disappear due to diffusion. This makes the billet homogeneous, suitable for machining operations.

In order to ensure proper sintering, two factors are important :

(i) The center of the billet, which is the coldest part during heating, should get sintered completely.

(ii) The temperature gradient, that is, the difference between the wall temperature and the center temperature should be kept below a critical value. This is especially important during phase transitions. If the temperature gradient becomes too high, cracks appear in the billet due to the high thermal tresses, which make the billet unsuitable for machining.

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Input data

In order to use this software, the user has to supply the following information as input data :

(i) Billet size, that is, diameter and height

(ii) Billet physical propertries, that is, density, thermal conductivity and specific heat. These are determined by the type and amount of filler used.

(iii) Heat transfer coefficients for billet surfaces :  The heat transfer coefficients are usually high enough to be unimportant. The default values, that is, 500 kcal/hr.m2.C represent such high values.  When the heat transfer coefficients are high, the billet temperature  is controlled by the billet thermal conductivity, which is quite low for PTFE. The facility to change heat transfer coefficient values is provided for the sake of flexibility.

(iv) Oven temperature cycle : The oven temperature cycle consists of several periods of increasing temperature, constant temperature and decreasing temperature.

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Results generated

This software package generates the following results :

A.  Variation of the following parameters during the sintering cycle :

(i) The center temperature
(ii) The radial temperature profile
(iii) The billet wall temperature
(iv) Temperature gradient, that is, temperature difference between the center and the wall

B.  At the end of the cycle :

(i) Time taken by the center to reach the critical temperature for sintering
(ii) Time for which the center was held above the critical temperature
(iii) Time taken by the center to cool below the critical temperature

C.  Graphical results displayed during the sintering cycle :

(i) The center temperature
(ii) The radial temperature profile
(iii) Temperature variation throughout the billet, using two colors

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Utility of the software package

This software package can be used for :

(i) Calculating the center temperature and the temperature gradient throughout the cycle for a given sintering cycle. This information can be used to assess the suitability of the sintering cycle for sintering a billet of a given size.

(ii) Designing a suitable sintering cycle for a billet of given size, with specified type and amount of filler. This can be done by trying out several sintering cycles as in (i), or it can also be done directly by specifying the time for which the center should remain above the critical temperature, and the maximum temperature gradient. (This facility is not available in this demonstration version.)

(iii) Diagnosing problems that may exist in current operations

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Input screen


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Results screen


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