PTFE Bellows can used at 3.5 kg / cm² at room temperature directly and upto 7.5 kg / cm² at room temperature with sleeves inside the bellows. Pressure sustaining capacity of PTFE bellows can be further increased by inserting spring steel ring the groove of bellows or further increase by S. S. brading to rate the bellows at 25 to 50 kg /cm² at room temperature. The increase in temperature considerably drops the rating. PTFE bellows can be fabricated for sustain more pressure, However its flexibility is inversely affected. PTFE Bellows can be used for vacuum applications with or without sleeves.

Continuous working temperature recommended for PTFE is 150-200°C. though it can be used as intermittently up to 250°C. After 250°C up to 350°C it does not decompose but it starts loosing its shape and does not remain at place. However the continuous working temperature drops down as the pressure increases.

Polyethylene is low cost similar material having good chemical resistance, better mechanical properties, slightly inferior electrical properties and poor thermal properties.

PTFE can be directly moulded to intricate shapes by process known as isostatic moulding. However the machinery required for processing is costly and high capital cost is involved. This process also demands high quantity to economically viable project. However most of processors use compression moulding , sintering, machining process to manufacture the product where intricate shapes are inforced by final machining. In this low scale quantity is possible to manufacture, however wastage in machining is very high.

PTFE bellows can have the flexibility of around maximum 25% in axial direction & lateral movement of 250 is attainable. The bellows can be design &fabricated for more flexibility , however their pressure sustaining capacity is adversely affected & there is the risk of rupture due to sudden increase in pressure , temperature , vibrations & shocks in the service & hence not recommended.

Very Very minuet circumferential lines at intervals of 5 to 10 mm are stroke lengths and not defects. However if deep these become weaknesses. The minute lines can be removed by centre less grinding for better appearance.

Shrinkage of PTFE Components during coarse of period is very common problem. It is result of release of thermal stresses and can be minimised by ultra slow sintering-annealing cycle in down mode. However total removal is not possible.

Taking the factor of safety – 2, it should be @ 1/2 of max compressive strength of that grade. E.g. 25 % Glass filled PTFE Compressive Strength @ 60 Kg/cm² Maximum Design Pressure @ 30 Kg/cm²

25 % Carbon filled PTFE Piston Rings for lubricated / dry running Air Compressors can be used up to 350 – 400 Kg/cm² pressure and piston velocity 300 m/min.

Filled PTFE Valve seats can be used for valves working up to 50 Kg/cm² @ ambient temperature.
V-Ring Packages in filled variety can be used even up to 1000 Kg/cm² @ ambient temperature.
PTFE Houses with helical S.S. Brading can be used for pressure as high as 200 Kg/cm² @ ambient temperature.
However this pressure sustaining capacity drops very fast as the working temperature increases.

The welding of conventional PTFE is possible, however the bond strength is weak & joint can not be guaranteed for leakage or service. However chemically modified PTFE can be welded into bond strength as good as base material. Please refer to page TFeM for more details.

For higher temperature (more than 200) for seats/seals of values, Pure graphite or metal seats can be used.

Hindustan Nylons does not manufacturer the parts of expanded PTFE. The expanded PTFE is low density PTFE having pores, sponge like flexibility. It is used as Gasketing / Sealing material. Due to it’s sponginess, it fills in rough surface of substrate and you get the leak proof joints even at low torque tightening of bolts.

PTFE thread seal tapes are available in different densities & different thickness. If the thickness is same, the due to different densities ranging from 0.5 to 1.5 the prices are varying.

As most of the fillers like Glass / Carbon / Bronze etc have decomposition temperature more than PTFE. Filler content can be measured by decomposition of pre weighed sample at temperature more than 450 & then weighing the residue i.e. filler.

Basically PTFE is nonstick material & adhesion of PTFE to other substrate materials is technically not possible. So when bonding is required as special chemical treatment (generally sodium itching) must be used to change the molecular structure of surface of PTFE. The chemical treatment generally used sodium itching, facilitates change in structural arrangement of carbon & Fluorine atoms on PTFE surface & removal of some fluorine atoms from the surface leaving a carbonaceous backbone responsible for adhesion. The surface of itched PTFE is light brown to dark brown. As this is only surface, treatment the inherent properties of PTFE remain unchanged.
The general purpose ARALDITE can be used for & as gluing agent for bonding.
The itched PTFE surface is light sensitive & undergo ageing & should be protected from light, or should be used as soon as possible after treatment. The UV/daylight exposure will fade away itching colour gradually by destroying carboneous layer & reduce bondality. Hence PTFE itched components to be stored for long time prior to bonding – be covered wrapped in black opaque plastics film & kept in dry plastics container wrapped in away from day light / sunlight / UV light.
The quality of chemically treated PTFE shapes can be verified by following methods :

• Visual :
  1. The surface color of itched PTFE should be light brown to dark brown.
  2. The color should be uniform & than should not be any white patches.
  3. There should not be any scratches/cuts on itched surface.

• Peel Off Test (Type Test) :
  1. Itched PTFE strip of around 100 mm w x 300 mm L x 1.5 mm thick is to be glued to stainless steel plate . The glue generally ARALDITE to be applied to both itched PTFE surface & polished S.S. plate in longitudinal & transverse direction & fixed. The max pressure & @ 10 kg/cm² is applied & curing temperature of 125°C is set.
  2. After curing & cooling upto Room temp, the edge of PTFE strip is removed by knife by armed 50 mm The pulling load / weight is applied from 2 kg. onwards – upto commencement of PEEL off from S.S. plate The value of load at peel off commencement should be minimum 2.5 kg/cm width.

• Breaking load test (Type Test) :
  1. The single lap joint of sodium itched PTFE strips of 25 x 25 mm² is prepared after using ARALDITE as gluing agent. After curing, the load is applied from 2 kg. onwards on this joint & the breakage load is measured when the joint is apart. The values should be min 20 kg/cm- width.

PTFE once polymerised during sintering, each not be reused / regenerated / repolymerised. In some countries, PTFE scrap / waste / cutting chips are ground to the regular @ 300 microns size & used in 10 to 15 % as filler in PTFE to reduce the cost. The properties of such compound are inferior to virgin PTFE.

Due to the release of Thermal & Mechanical stresses after peeling , the strips / sheets have warped / curved surfaces, If premium grade – fine powder 50 µ PTFE is used to manufacture this products – these defects can be prevented from occurring. However the cost can be around 25% extra.
Also strips / sheets are hot calandered – there can be somewhat improvement.
PTFE strips / sheet when opened made flat – you will always find them bent. To avoid this – slitting of edges on slitting machine – is only solution . However there is heavy cost increment due to labour cost as well as wastage.