The extrusion of PTFE granular powders

The extrusion of PTFE granular powders

Technical Service Note F2

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Page

Section 1. Types of Fluon® granular PTFE 5

powder for extrusion

Storage and handling precautions 5

Section 2. The Extrusion process 6

Powder feed 6Powder compaction 6Sintering 6Cooling 6

Section 3. Extrusion pressure 7

Surface finish of die tube and mandrel 7Length of cold zone above the heated 7section of the die tubeTemperature of PTFE powder in the 8cold zone of the die tubeLength of heated section of die tube 8Temperature of heated section of 8die tubeTemperature of die tube beneath 9heated sectionSpeed of ram in die tube 10Size of PTFE powder charge 10Brake pressure 13

Section 4. Extrusion equipment 14

Power cylinder 16Ram 16Die tubes 16Mandrel 16Die heating and temperature control 17Powder feed 18Brake 18

Page

Section 5. Die tube design 20

General 20Bursting pressure 20Fatigue strength 20Metals for the construction of die tubes 20Tube sizes 21

Section 6. Extrusion conditions 22

Selection of powder grade 22Extrusion pressure 22Temperature profile 23Charge length 23Rate of powder compaction 23Ram-down time 23Ram-up time 23Extrusion rate 23

Section 7. Operation of the extruder 24

Operating procedure 24Die cleaning 24Representative extrusion conditions 24

Section 8. Profiled extrusion 27

Section 9. Quality assessment 28

Qualitative tests 28Quantitative tests 28

Section 10. Diagnosis of faults in extrudates 29

Fluon® technical literature 34

Contents

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Although Fluon®* PTFE (polytetrafluoroethylene) is athermoplastic, in the temperature range between itscrystalline melting point (327 - 340°C; 621 - 644°F) and theonset of rapid thermal degradation ( >410°C; >770°F) it hasan extremely high viscosity and is liable to fracture whensubmitted to shear stresses. In consequence PTFE cannotbe injection moulded or melt extruded like otherthermoplastics and fundamentally different methods ofprocessing it have had to be developed. With granularPTFE powders these methods all involve compacting thepolymer powder at a relatively low temperature, exposingthe resultant preform to a temperature above thecrystalline melting point of the polymer and, finally,cooling it to ambient temperature.

Ram extrusion is a method of operating the process toproduce continuous lengths of extrudate and, for thisprocess to operate efficiently, PTFE powders must havegood flow characteristics so that they feed readily into theextruder die tube. This is achieved by using either of twotypes - pre-sintered, or agglomerated - both of which arefree-flowing.

Three grades of unfilled Fluon® used for extrusion areG201, G401 and G307. Fluon® G201 is a free-flowing pre-sintered grade whereas Fluon® G401 and G307 areagglomerated and free-flowing. Typical properties aregiven in the table below.

AG Fluoropolymers also offer the FC800 range of glassfibre filled free flowing grades of PTFE which are suitablefor ram extrusion.

STORAGE AND HANDLING PRECAUTIONS

Kegs of powder should be stored in cool dry conditions,preferably between 10 and 18°C (50 and 65°F). Excessivelywarm powder will have impaired powder flow andhandling properties, whilst atmospheric moisture maycondense on excessively cold powder if the keg is openedin a warm room: such condensation may cause poorquality extrudates. Kegs of powder which have beenexposed to extremes of temperature should be allowed tostand, unopened, until they have attained workshoptemperature.

Within its working temperature range PTFE is acompletely inert material, but when heated to its sinteringtemperature it gives rise to gaseous decompositionproducts or fumes which can produce unpleasant effectsif inhaled. Fumes can be produced during processing: forexample, when the material is heated to sinter it, or whenbrazed connections are being made to cable insulatedwith PTFE. The inhalation of these fumes is easilyprevented by applying local exhaust ventilation toatmosphere as near to their source as possible.

Smoking should be prohibited in workshops where PTFEis handled because tobacco contaminated with PTFE will,during burning, give rise to polymer fumes. It is therefore,important to avoid contamination of clothing, especiallythe pockets, with PTFE and to maintain a reasonablestandard of personal cleanliness by washing hands andremoving any PTFE particles lodged under the fingernails.

More complete guidance is given in the Association ofPlastics Manufacturers in Europe (APME) ‘Guide for thesafe handling of Fluoropolymers’.

Section 1. Types of Fluon® granular PTFEpowder for extrusion

Property Unit G201 G401 G307(pre-sintered) (agglomerated) (agglomerated)

Bulk powder density g/l 675 725 725Mean particle size µm 500 1500 675Ultimate tensile strength MPa 17 - 19.5 17 - 20 17 - 20

lbf/in2 2500 - 2800 2500 - 2900 2500 - 2900

Elongation at break % 300 - 375 300 - 350 300 - 350

The figures given above represent typical values and should not be used for specification purposes.

* Asahi Glass Company trade mark

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A quantity of polymer in powder form is fed into one endof a straight die tube of uniform bore where it iscompacted by a ram and forced along the tube whichincorporates a heated sintering zone. The ram is thenwithdrawn, the die tube re-charged with powder and thecycle repeated. In this way the compacted powder isforced step by step through the heated section of the dietube where it is sintered and then through a cooler sectionfrom which it emerges in a continuous length.

POWDER FEED

To obtain a product with uniform properties and free ofdistortion it is important that the powder is fed into the diein charges of uniform weight and it is essential that thepowder is evenly distributed throughout the die cavity.Changes in the quantity of powder fed to the die will affectboth the extrusion pressure and the extrusion rate. Thismay give a product with varying properties. In theextrusion of tubing, uniform distribution of powder in thedie cavity is of particular importance because unevendistribution can produce tubing which is eccentric andbent.

The flow of PTFE powder is adversely affected by a rise intemperature or by excessive working, i.e. exposure tosevere agitation, compaction or shearing. Consequently,to fill a die cavity satisfactorily it is necessary not only touse powders with inherently good flow properties butalso to control their environment. The temperatureadjacent to the top of the die tube and in the feed systemshould be kept in the range 21-30°C (70-85°F). It shouldnot be so low that moisture condenses into the powder.

POWDER COMPACTION

During the compaction stage the PTFE powder ispreformed to a substantially void-free condition and theextrudate moved through the die tube. The processmaintains pressure on the molten PTFE in the sinter zoneto coalesce the powder particles and the compactedpowder charges. To achieve good quality extrudate thepowder must be compacted reasonably slowly and thepressure held on the preform for long enough to allowany air that is mixed with the powder to escape. Anyentrapped air will produce voids in the preform. Inaddition the correct level of pressure must be developedin the die tube. Too low a pressure will produce a voidedpreform which will in turn, after sintering, produce aporous extrudate. Too high a pressure will result inadjacent charges which will not weld together properly,yielding an extrudate with distinct marks or even cracks ateach charge-join.

SINTERING

To be acceptable the extrudate must be sinteredthroughout its whole cross-section. This means that as itpasses through the heated zone of the die tube the wholeof the compacted powder must be brought to a

temperature above the melting point of the polymer andheld there under pressure long enough to form a coherentextrudate. The powder is heated by conduction, so thetime taken to bring it to the sinter temperature throughoutits cross section will depend on the size and shape of thecompacted preforms as well as their heat transferproperties. Those with a large cross-sectional area willtake longer to reach the sinter temperature than thosewith a small cross sectional area. The rate at which theindividual particles and adjacent charges coalesce willdepend on the temperature and the pressure to whichthey are exposed. The maximum sinter temperaturewhich can be used is limited by the need to avoid over-sintering the surface of the extrudate in the time taken tosinter its centre satisfactorily.

With large diameter rods where the centre will take muchlonger to reach the sinter temperature than the surface,the sinter temperature may have to be kept as low as370°C (700°F) to avoid surface degradation. With smalldiameter rods where the sinter temperature is quicklyattained throughout the whole cross-section, the sintertemperature can be higher and the dwell time kept low toavoid degradation. It follows, therefore, that each part ofthe extrudate must stay in the heated zone of the die tubefor a certain minimum time in order to get a satisfactorilysintered extrudate. This means that for each die tubeheated length and for the particular extrusion conditions,especially temperature profile, there is a maximumextrusion rate at which satisfactorily sintered extrudatecan be obtained. Extrudate of higher tensile strength maybe obtained by letting the extrudate remain in the heatedsection of the die tube for longer than the minimum timeneeded to produce satisfactory sintering. However, thedwell time of the polymer at the sinter temperature mustnot be so long that significant decomposition occurs.Decomposition will mar the appearance of the extrudate,reduce its mechanical properties and, eventually, causeinternal cracks.

COOLING

The rate at which the extrudate cools from the meltlargely determines the degree of crystallinity of theextrudate and therefore affects its dimensions andproperties. Very rapid cooling, especially of largediameter rod, can produce a highly stressed extrudatewhich must be annealed before it can be machinedsuccessfully to close tolerance components, and unevencooling usually distorts the extrudate. To produceextrudates, especially those with large section - say 25mm (1 inch) which are essentially stress-free and withaccurate dimensions, the cooling rate must be strictlycontrolled. If a reasonably long length of die tubeprotrudes beneath the heated zone the cooling rate of theextrudate can be controlled by controlling thetemperature of this portion of the die tube. If not, theextrudate as it emerges from the die tube, can beenclosed in a loosely fitting tube, the temperature ofwhich can be controlled.

Section 2. The Extrusion process

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To produce extrudate of good quality, it is necessary togenerate the optimum extrusion pressure for the particularpowder in use and its value can be calculated by dividingthe force applied to the extrudate when it begins to movedown the die tube by the cross-sectional area of the ram. Ifthe overall extrusion pressure is too low the extrudate willbe porous; if it is too high the charges of powder will beweakly joined together.

It has been found out that the optimum extrusion pressurefor agglomerated Fluon® powders lies in the range 7-10MPa (1000-1400 lbf/in2). If the extrusion pressure exceedsabout 15 MPa (2100 lbf/in2) the charge-joins becomenoticeable and eventually weak. The optimum extrusionpressure for pre-sintered Fluon® powders is in the range12-35 MPa (1800-5000 lbf/in2). Control of the extrusionpressure is of utmost importance in controlling the qualityof the extrudate produced from the various types of PTFEpowder. How this control can be achieved is bestunderstood by discussing the factors which influence theextrusion pressure.

SURFACE FINISH OF DIE TUBE AND MANDREL

The surface finish of the die tube and mandrel affect themovement of the extrudate; the rougher these surfacesthe more difficult it is to push the extrudate through thedie tube.

LENGTH OF COLD ZONE ABOVE THE HEATED SECTION

OF THE DIE TUBE

The extrusion pressure developed during extrusion isgreatly affected by changes in the area of contact betweendie surfaces and PTFE powder in the relatively cool regionof the die tube above the heated section. This is illustratedin Table 1. The surface area of the die components in thisunheated region can be altered by raising or lowering thealuminium blocks which carry the die heaters.

Table 1. Effect of the length of the unheated die tube (above the heated section) on the extrusion pressure

10mm diameter rod

Length of unheated section* Extrusion pressureof die tube above the heated (MPa)

90 mm 26105 mm 45120 mm 55150 mm 74

* The top 60 mm of the die tube was enclosed by the water-cooled platen, the remainder above the heated zones wasexposed to air and not deliberately cooled, heated or insulated

Die tube

Diameter 10.64 mmTotal length 1550 mmHeated length 660 mm

Heating arrangements Three separately controlled zones each comprising two1.5 kW heater bands

Temperature profile (top) Zone 1 380°CZone 2 400°C

(bottom) Zone 3 380°C

Extrusion conditions

Powder Pre-sintered Fluon® G201Penetration of ram tip into die tube 30 mmTotal cycle time 9 secondsRam dwell time at bottom of stroke 2 secondsPowder compaction rate 7.5 mm/sExtrusion rate 3 m/h

Section 3. Extrusion pressure

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TEMPERATURE OF PTFE POWDER IN THE COLD ZONE OF

THE DIE TUBE

The temperature of the PTFE powder in the cold zone ofthe die tube above the heated section also affects theextrusion pressure; the cooler the powder in this regionthe higher the extrusion pressure. The temperature at thetop of the die tube can be controlled by circulating wateraround the tube and by adjusting the temperature of thetop heated zone.

LENGTH OF HEATED SECTION OF DIE TUBE

The area of contact between the die wall and the polymergel in the heated section affects the extrusion pressuregenerated during extrusion. The surface area of the diecomponents in this region is controlled by the length of dietube maintained at a temperature above the melting pointof the polymer. The longer this heated length the higherthe extrusion pressure developed during extrusion, asillustrated in Table 2.

TEMPERATURE OF HEATED SECTION OF DIE TUBE

The temperature profile along the heated section of the dietube also affects extrusion pressure. Lowering thetemperature in the first heated zone raises the extrusionpressure by altering the length and the temperature of thecolumn of compacted powder above the polymer gel. Thisis illustrated in Table 3. Adjustment of the temperature inthe first heated zone affords a convenient way ofcontrolling the extrusion pressure during extension.

Raising the die temperature in the middle and lowerheated zones of the die tube may increase the extrusionpressure generated. However, adjustments to thetemperature in these regions have much less effect thanadjustments of similar magnitude to the temperature ofthe heated zone at the top of the heated section. Inaddition the extent of any change in temperature is limitedby the need to sinter the extrudate fully but to avoid anydecomposition at its surface.

Table 2. Effect of the heated length of the die tube on the extrusion pressure

10mm diameter rod

Heated length of the die tube Extrusion pressure (MPa)

440 mm 33660 mm 40900 mm 52

Die tube

Diameter 10.64 mmTotal length 1550 mmHeated length 440 mm, 660 mm, 900 mm (see above and below)Unheated feed length at top of die tube 90 mm

Heating arrangements 440 mm 660 mm 900 mmTwo separately Three separately Four separatelycontrolled zones controlled zones controlled zoneseach comprising each comprising each comprisingtwo 1.5 kW two 1.5 kW two 1.5 kWheater bands heater bands heater bands

Temperature profile (top) Zone 1 370°C 370°C 370°CZone 2 400°C 400°C 400°CZone 3 - 380°C 400°C

(bottom) Zone 4 - - 350°C


Powder Pre-sintered Fluon® G201Penetration of ram tip into die tube 40 mm 40 mm 40 mmTotal cycle time 18 seconds 12 seconds 9 secondsRam dwell time at bottom of stroke 12 seconds 6 seconds 3.5 secondsExtrusion rate 3.2 m/h 4.5 m/h 5.8 m/hDwell time in heated zone 7.5 minutes 8.8 minutes 9.3 minutes

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Table 3. Effect of the temperature of the first (top) heated zone on the extrusion pressure

10 mm diameter rod 25 mm/15 mm diameter tube

First zone Extrusion First zone Extrusiontemperature pressure (MPa) temperature pressure (MPa)

380°C* 18 380°C 12360°C 22 360°C 14340°C 27 340°C 17

* There was slight back extrusion with the ‘first zone’ temperature at this setting i.e. 380°C

Die tube

Diameter 10.64 mm 29.2 mmTotal length 1550 mm 1080 mmHeated length 660 mm 920 mmUnheated feed length of die tube 90 mm 130 mm

Mandrel

Diameter (in feed area) — 15.1 mmLength projecting beneath heated zone — 60 mmShape — Slight taper extending over

approximately 200 mm in that partof the mandrel in the sinter zone.Parallel before and after this part.

Heating arrangement Three separately controlled Four separately controlled zones each comprising zones each comprisingtwo 1.5 kW heater bands two 1.5 kW heater bands

Temperature profile (top) Zone 1 See top portion of table See top portion of tableZone 2 400°C 400°CZone 3 350°C 400°C

(bottom) Zone 4 — 350°C


Powder Pre-sintered Fluon® G201 Pre-sintered Fluon® G201Penetration of ram tip into die tube 30 mm 40 mmTotal cycle time 12 seconds 17.5 secondsRam dwell time at bottom of stroke 5 seconds 7 secondsPowder compaction rate 7.5mm/s 7.5mm/sExtrusion rate 3 m/h 2.6 m/h

TEMPERATURE OF DIE TUBE BENEATH HEATED

SECTION

Provided the polymer is allowed to freeze and no externalbrake is used, the temperature of the die tube below theheated section has no effect on the extrusion pressuredeveloped during extrusion of rod. This is because the rodshrinks away from the die walls as it freezes and cools,with the result that the walls of the die lying beneath theheated section do not restrict the movement of the rod.

During the extrusion of tube using a mandrel whichextends some way beyond the heated section of the dietube, the extrudate will shrink away from the die wall ontothe mandrel. This can severely restrict its movement andcause a very high extrusion pressure. In thesecircumstances the temperature of the die tube beneaththe heated section can affect the extrusion pressure bycontrolling the extent to which the extrudate shrinks ontothe mandrel. The length and shape of the mandrel alsoaffect the extrusion pressure generated.

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Table 4. Effect of the PTFE powder compaction rate on the extrusion pressure


Rate at which Extrusion Rate at which Extrusionthe ram pressure the ram pressurecompacts the (MPa) compacts the (MPa)powder powder

5 mm/s 23 5 mm/s 1210 mm/s 27 10 mm/s 1815 mm/s 33 15 mm/s 2220 mm/s 37 20 mm/s 2725 mm/s* 45

* At this closing speed there will be a danger of trapping air

Die tube

Diameter 10.64 mm 29.2 mmTotal length 1550 mm 1080 mmHeated length 660 mm 920 mmUnheated feed length at top of die tube 90 mm 130 mm

Mandrel

Diameter (in feed area) - 15.1 mmLength projecting beneath heated zone - 60 mmShape - Slight taper extending over

approximately 200 mm inthat part of the mandrel inthe sinter zone.Parallel before and after this part.

Heating arrangements Three separately controlled Four separately controlledzones each comprising zones each comprisingtwo 1.5 kW heater bands two 1.5 kW heater bands

Temperature profile (top) Zone 1 370°C 350°CZone 2 400°C 400°CZone 3 380°C 400°C

(bottom) Zone 4 - 350°C


Powder Pre-sintered Fluon® G201 Pre-sintered Fluon® G201Penetration of ram tip into die tube 30 mm 40 mmTotal cycle time 11 seconds 17.5 secondsRam dwell time at bottom of stroke 2, 4.5, 5.5, 6, 6.2 seconds 5.5, 9.5, 10.5, 11 seconds

(depending on the rate of ram movement-altered to keep the overallcycle time constant)

Extrusion rate 2.1 m/h 2.4 m/h

SPEED OF RAM IN DIE TUBE

Increasing the speed at which the extrudate is moved overthe die surfaces during the compaction stroke increasesthe extrusion pressure, as illustrated in Table 4. Thisspeed is controlled by the rate at which the ram movesinto the die tube and can readily be changed. Adjustingthe rate of movement of the extrudate in the die tubeduring the compression stroke of each cycle is therefore aconvenient way of controlling the extrusion pressure

during extrusion. In practice the rate at which the ram canbe moved is limited by the need to avoid trapping air withthe powder particles as they are compacted.

SIZE OF PTFE POWDER CHARGE

Increasing the ratio of powder charge length to crosssectional area of extrudate increases the extrusionpressure generated during extrusion.

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Thus changing the powder charge length by altering thepenetration of the ram tip into the die tube is a way ofadjusting the extrusion pressure developed, as illustratedin Table 5. The practical limits to the adjustment of thepowder charge size are, firstly, the need to maintainoutput and-secondly, to avoid trapping air in thecompacted powder and getting the weak and marked

charge joins which accompany the use of an excessivecharge size. Such a fault is illustrated in Figure 1

(see page 12).

When producing extrudate with a large cross-sectionalarea it is often impracticable to change the charge lengthenough to affect the extrusion pressure significantly.

Table 5. Effect of the PTFE powder charge size on the extrusion pressure


Length of Weight of Extrusion Length of Weight of Extrusionpowder powder pressure powder powder pressurecharge charge (MPa) charge charge (MPa)

15 mm 0.8 g 18 20 mm 5.9 g 930 mm 1.6 g 19 30 mm 8.8 g 1045 mm 2.4 g 21 40 mm 11.8 g 11

Die tube


Mandrel

Diameter (in feed area) - 15.1 mmLength projecting beneath heated zone - 60 mmShape - Slight taper extending over

approximately 200 mm in thatpart of the mandrel in thesinter zone.Parallel before and after this part.

Heating arrangements Three separately controlled Four separately controlledzones each comprising zones each comprisingtwo 1.5 kW heater bands two 1.5 kW heater bands

Temperature profileZone 1 370°C 350°CZone 2 400°C 400°CZone 3 350°C 400°CZone 4 - 350°C


Powder Pre-sintered Fluon® G201 Pre-sintered Fluon® G201Penetration of ram tip into die tube 15 mm 30 mm 45 mm 20 mm 30 mm 40 mm(length of powder charge)Weight of powder charge 0.8 g 1.6 g 2.4 g 5.9 g 8.8 g 11.8 gTotal cycle time 7.5 s 15.5 s 25 s 9.5 s 13.5 s 19 sPowder compaction rate 7.5 mm/s 7.5 mm/s 7.5 mm/s 10 mm/s 10 mm/s 10 mm/sRam dwell time at bottom of stroke 2 s 8 s 16 s 2 s 5.5 s 11 sExtrusion rate 2 m/h 2 m/h 2 m/h 2.2 m/h 2.2 m/h 2.2 m/h

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Figure 1. Weak and marked charge joins in PTFE extrudate as a result of an excessive charge size

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BRAKE PRESSURE

Applying a brake to the extrudate as it emerges from thedie tube is an effective way of controlling its movementand therefore of increasing the extrusion pressure asillustrated in Table 6. However, if the movement of theextrudate is too severely restrained, considerable

pressure is built up in the extrudate, particularly in thepolymer gel during the compressive stroke of the ram,and maintained whilst the ram is kept at the bottom of itsstroke. When the ram is moved up, the pressure isrelieved by the extrudate moving back up the die tube, aneffect known as ‘back’ extrusion. When this becomessevere the extrusion conditions cannot be controlled.

Table 6. Effect of brake pressure on the extrusion pressure


Pressure of Extrusion Pressure of Extrusionair line pressure air line pressureactuating actuatingbrake (MPa) (MPa) brake (MPa) (MPa)

0 14 0 130.2 18 0.3 170.7 26 0.6 190.8* 39 0.8 22

* With the brake set at this pressure the powder back-extruded, reducing the rate to 1.2 m/h

Die tube


Mandrel

Diameter (in feed area) — 15.1 mmLength projecting beneath heated zone — 60 mmShape — Slight taper extending over

approximately 200 mm inthat part of the mandrel inthe sinter zoneParallel before and after this part.

Heating arrangements Two separately controlled Four separately controlledzones each comprising zones each comprisingtwo 1.5 kW heater bands two 1.5 kW heater bands

Temperature profile (top) Zone 1 370°C 350°CZone 2 400°C 400°CZone 3 — 400°C

(bottom) Zone 4 — 350°C


Powder Pre-sintered Fluon® G201 Pre-sintered Fluon® G201Penetration of ram tip into die tube 40 mm 40 mmTotal cycle time 23.5 seconds 20 secondsRam dwell time at bottom of stroke 3.5 seconds 9 secondsPowder compaction rate 2.3 mm/s 10 mm/sExtrusion rate 2 m/h* 2.2 m/h

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Ram extruders used to process PTFE powders are built toextrude either vertically downwards or horizontally. Theformer type is the more common.

Vertical ram extruders are usually mounted on raisedplatforms, the die tubes passing through a hole in theplatform. Typically, the height of the platform is chosen sothat the distance between the floor and the bottom of thelongest die tube is 2-3 metres (6-10 feet). With some sizescontinuous lengths of extrudate can be obtained bybending the extrudate through an arc of large radius and

allowing it to move horizontally as the extrusionproceeds.This technique is applicable only to extrudate ofrelatively thin section and small diameter, e.g. 10 mm (3/8inch) diameter rod, or tube of up to about 25 mm (1 inch) diameter.

Figure 2 is a photograph of a vertical ram extruderinstalled in the Fluon® Technical Service Laboratories.Figure 3 illustrates the main components of a vertical ramextruder and these are discussed in detail.

Section 4. Extrusion equipment

Figure 2. General view of a vertical ram extruder

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Power Cylindercontrolled byadjustable micro-switch arrangementon ram

Pressure gauge

RamRam Tip

Rotating feed tableCooling water channel

Electrical Connection

Heating zone 1

Heating zone 2

Heating zone 3

Brake (see Fig.5)

VibratoryFeeder

Weighing Device

Heater band

Die Tube

Control thermocouplefor each heating zone

Aluminium heatconservation blocks

Regulated compressedair supply

Extrudate

Powder hopper

Figure 3. Main components of vertical ram extruder

Heating zone 4

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POWER CYLINDER

This drives the ram, which in turn compacts the PTFEpowder in the die tube. It may be actuated pneumaticallyor hydraulically. The stroke of the cylinder should beeasily adjustable up to a maximum of 150 -200 mm so thatthe length of ram movement can be controlled asnecessary.

The maximum thrust delivered by the power cylindermust be sufficient to apply the optimum pressure to theextrudate of the largest cross-sectional area required. Thepower cylinder should be fitted with a pressure gaugecovering high line pressures (up to 10 MPa; 1400 lbf/in2)and preferably with a second one covering low linepressures (up to 1.5 MPa; 215 lbf/in2). Both should befitted with a flow regulator (e.g. a needle valve) to preventdamage from very rapid changes in the line pressureduring extrusion. In addition, the low pressure gaugeshould be fitted with a limit valve to prevent damage frompressure surges.

It is an advantage to have a high degree of control overthe movement of the piston in the power cylinder becausethis in turn controls the extrusion conditions. Firstly, thetime taken for each complete cycle of operations shouldnot vary. The extrusion rate and therefore the time atsinter temperature will then be constant, helping toproduce a uniform product. Secondly, the total strokeshould be adjustable; otherwise if, for example, the ram isnot withdrawn far enough it can interfere with the powderfeed into the die cavity and if it can be inserted into the diecavity only for a very short distance, the charge size willbe very small and this could severely limit the extrusionrate. Thirdly, the rate at which the piston moves should beadjustable and it is particularly useful if it can be moved atdifferent rates in different parts of its stroke. When theram tip is out of the die tube the piston can be allowed tomove more quickly than when it is compacting thepowder. When extruding large diameter rod it may beadvantageous to move the ram away from the compactedpowder slowly to prevent the production of a partialvacuum and the possible sucking of the powder chargeback up the die tube. Fourthly, the time spent at the limitsof the ram stroke should be controlled. The time in thewithdrawn position should be just sufficient to allow thepowder charge to be distributed uniformly in the diecavity; any further time spent in this position is wasted. Itis beneficial to keep the ram in contact with the powderfor as long as possible after the down stroke has beencompleted. This gives the air mixed with the powder timeto escape and minimises back extrusion. Certainly, if thisdwell time is too short the extrudate will be porous andsome back extrusion may occur.

The degree of control outlined above can be obtained byusing a flow valve to control the rate of flow of fluid into

the power cylinder, together with a combination of timersactuated by microswitches.

RAM

The ram can be attached to the piston of the powercylinder either directly or via a crosshead. The latterarrangement, which is illustrated in Figure 4, simplifiesthe alignment of ram and die tube. The ram is usuallymade in two parts, i.e. a stock which is attached to thepiston or crosshead and a removable ram tip which isfitted to the stock, usually by a screw thread. The ram tipis machined to fit accurately into the die tube but can bereplaced easily when it wears or if it is damaged. Ram tipsshould be made of a softer material than the die tube tominimise wear on the latter. A wide variety of materialscan be used satisfactorily but phosphor bronze isparticularly suitable.

The clearance between the ram tip and the die wallsshould be sufficient to permit the ready escape of the airin the powder during the compaction stroke but not solarge as to cause polymer ‘flashing’, i.e. the escape ofpowder between the ram tip and die walls. A radialclearance of 0.05 - 0.1mm (0.002 - 0.004 inch) has beenfound suitable for small diameters (< 15 mm, < 0.6 inch)and 0.075 - 0.125 mm (0.003-0.005 inch) for largerdiameters.

DIE TUBES

Die tubes used in the ram extrusion of PTFE powder arepreferably constructed from seamless tubing made from ahighly corrosion resistant grade of steel or from mild steelwhich is subsequently hard chrome or nickel plated.

They should have a good surface finish. 0.2µm (8µ in) Rawhich can be achieved readily by honing, has provedsatisfactory. Smoother surfaces offer no significantadvantages and can make it difficult to achieve adequateextrusion pressures. Much rougher surfaces can causeexcessively high extrusion pressures and the build-up ofpolymer on the walls of the die in the heated zone(skinning). This subsequently degrades and can lead to acontaminated extrudate or one with a poor finish.

Multiple cavity die tubes are generally used to increaseproductivity in the extrusion of small to medium sizerods and tubes.

MANDREL

To extrude tubing a mandrel must be positioned in the dietube. It can be made of the same material and have thesame surface finish as the die tube. To allow for shrinkageits diameter should be slightly greater than the internaldiameter required in the final tube. The amount ofshrinkage will be very dependent on the extrusion

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conditions, particularly the temperature of theextrudate as it leaves the mandrel, but willgenerally be in the range 5-10%.

The major effect of a mandrel on processing is toincrease the extrusion pressure generated duringextrusion, and the design of the mandrel isusually an attempt to limit or control thisextrusion pressure. The most obvious design fora mandrel is a simple rod or tube of constantdiameter. However, if such a mandrel is longenough to extend much beyond the heated zoneof the die tube so that the extrudate shrinks ontoit firmly, then a very high extrusion pressure willbe developed. Ending the mandrel just beyondthe heated zone will help minimise the pressurebuild up but the mandrel must still extend farenough for the polymer to cool below its gelpoint and become reasonably dimensionallystable before it leaves the mandrel. If it does not,the tube will be distorted, usually convoluted.

One means of reducing the effect of the mandrelon extrusion pressure is to taper the mandrel.The position at which the taper commences andthe degree of the taper strongly influence theextrusion pressure. If it begins at some pointbeyond the heated zone it will reduce the effect ofthe extrudate shrinking onto the mandrel as it cools. If it ispositioned in the sinter zone where the polymer begins tomelt it will reduce the pressure developed as the polymermelts and its volume increases. The greater the degreeand extent of the taper the more effective it is in reducingthe extrusion pressure. The exact configuration adoptedfor the mandrel depends on the reduction in extrusionpressure which must be achieved.

An alternative method of reducing the extrusion pressureis to make the mandrel reciprocate. On the extrusionstroke the mandrel is moved down with the ram, eitherhydraulically or being pulled down by the polymer. At thebottom of the stroke the ram holds the extrudate in placewhilst the mandrel is hydraulically pulled back up to itsstarting position. Avoiding the extrudate sliding down themandrel and the inside of the die tube at the same timereduces the force needed to push the extrudate down thedie.

DIE HEATING AND TEMPERATURE CONTROL

Die tubes are most commonly heated by electric bandheaters which are usually mounted on aluminium blocksbored to fit the die tube closely and of appropriate outerdiameter to fit heaters. Each block should be long enoughto hold at least two band heaters and the total length ofthe heaters form the ‘heated’ length of the die tube. Thealuminium blocks help to conduct the heat over the dietube surface and to avoid local relatively hot or cold areas.

The heated length should be split into zones which can beseparately controlled.

Four such zones should be used with long die tubes (e.g.those with heated lengths greater than 700 mm; 27.5inches) and fewer zones used with shorter die tubes, asconvenient. Each zone should develop 2-4 kW of heatdepending on the size of the extrudate being made.

The temperature of each zone must be controllable towithin a few degrees at any temperature within a verywide range up to about 420°C (788°F). This is best done byusing a current-proportioning controller coupled to athermocouple placed in the centre of the zone andextending through the aluminium block to touch the outersurface of the die tube. It must be held firmly against thedie tube using a vibration proof fastener. The controllercan also be used to indicate the temperature of the dietube but it is better to use additional thermocouples andrecorders to do this because they can be used as a checkon the performance of the controller units.

Whichever method is used accurate control andmeasurement of the die tube temperature is essential toget a product of uniform quality and to get reasonablereproducibility.

Figure 4. Vertical ram extruder showing ram, ram tip,

vibratory feed tray and rotating distributor bowl

17

It is also essential to keep the top of the die tube cool,preferably to 21 - 30°C (70 - 85°F), to ensure good powderflow and to help generate extrusion pressure. This is doneby circulating cold water through holes bored into the mainplaten of the extruder. It is sometimes also necessary toextend the distance between the top of the die and the startof the heated zone and to lower the temperature of the topzone of the die tube in order to avoid overheating.

The maximum rate at which tube can be extruded can beincreased by heating the mandrel as well as the die tube.This can be done only when the mandrel is large enough forelectric heaters to be positioned inside it. Its temperaturecan be controlled as described for the die tube.

POWDER FEED

For consistent and trouble-free extrusion a reliable methodof feeding a predetermined quantity of PTFE powder to thedie tube and of distributing it uniformly within the die cavityis required. The powder must not be contaminated ordamaged in the process. For example, systems in which thepowders are heavily worked either by stirrers in the hopperor by a screw feed mechanism, although suitable for therelatively hard particles of pre-sintered powders, are notsuitable for the softer particles of agglomerated free-flowpowders, which tend to compact into large lumps.

Other systems suitable for both agglomerated and pre-sintered powders based either on weight or volumemetering of the powder have been developed. The powderis stored in a steep sided hopper from which it can flowunder gravity into the tray of a vibrating feeder. A butterflyvalve placed in the top of the discharge duct from thehopper prevents over compaction of the powder in this ductwhich could lead to inconsistent feeding. The gap betweenthe discharge duct and the vibrator tray is adjustable anddetermines the depth of powder in the tray. Both theduration and amplitude of vibration are also adjustable,hence the amount of powder delivered from the end of thetray into the extruder feed area can be controlled accurately.In practice the feed controls are adjusted so that the amountof powder fed into the feed area is just enough to fill the diecavity after each ram stroke. The vibrator is activated by theram closing a microswitch as it moves up out of the dietube.

An enhancement of this system is to have the vibrator fillinga weighing device which stops the vibrator when thespecified weight of polymer has been delivered (see Figure3). This more precise method allows the extruder to run forlong periods without under or over filling the feed bowl.

The powder can be distributed in the die cavity in a numberof ways. With the production of rod a rotating distributorblade or an oscillating shuttle is effective. The oscillatingshuttle cannot be used to make tube because of thepresence of the mandrel, but a rotating distributor bladewhich collects the powder and moves it into the cavityaround the mandrel is effective.

BRAKE

A brake is a means of increasing the extrusion pressure byapplying a controlled resistance to the movement of theextrudate as it emerges from the die tube. A widely usedtype of brake is illustrated in Figure 5. The movement of theextrudate is resisted by a three-jaw collet held against it bypneumatic pressure. The latter can be adjusted to apply justthe brake load needed to get the required extrusionpressure. Once set, this type of brake will maintain the samebrake load on the extrudate despite small variations in theextrudate size and in extrusion conditions. By usinginterchangeable sets of collets this type of brake can beused with die tubes of different diameter. Care is needed inusing this brake to ensure that the junctions of the colletjaws do not score the surface of the extrudate, particularlyif the latter is hot and somewhat soft or if a very high brakepressure is used.

Another type of brake is in the form of a tapered tube whichis fitted to the bottom of the die tube. It relies for itsoperation on a small difference between its internaldiameter and that of the extrudate. The brake itself is notadjustable so the brake load applied to the extrudate can bechanged only by adjusting the extrusion conditions to varythe size of the extrudate and the degree of interferencebetween it and the brake. For consistent brake load theextrusion conditions must be constant to ensure that thesize and hardness of the extrudate do not change. Thisbrake system has the disadvantage that unacceptably highextrusion pressures can occur if the extrudate quality (inparticular, dimensions) changes during processing. The air-operated brake mentioned in the last paragraph willcompensate automatically for any changes in the extrudatedimensions and therefore does not involve the generationof excessive pressures. However, an advantage of thetapered type of brake is that it can be used, to some extent,to ‘size’ the extrudate and provided its surface is smooth, itdoes not mar the surface of the extrudate.

It is essential that before the extrudate passes through anybrake it has cooled sufficiently to be reasonably hard andresistant to distortion.

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Die Tube

Extrudate

Extrudate

Collets in ‘open’ position

Regulatedcompressedair supply

Showing the three collets incontact with the extrudate

Pneumaticallyoperatedpiston (three)controllingpressure of thethree collets(shown in openposition)

The three colletsare made to suitthe diameter ofthe extrudateand areremovable andreplaceable byothers to suitindividualextrudate sizes.

O-ring seal

O-ring seal

Figure 5. Three jaw collet for applying pressure to Fluon® PTFE

extrudate as it leaves the die

19

GENERAL

As die tubes are subjected to pressures up to 100 MPathey need to be designed as thick-walled cylinders. Themetal tube chosen should be designed with an adequatefactor of safety on its bursting strength and should also beone that fails in a ductile manner. Metals that fail in abrittle manner and split axially should be avoided.

BURSTING PRESSURE

The bursting pressure Pu of a tube is given by the meandiameter formula:

Pu = 2fu K - 1K + 1

where fu = ultimate tensile stress

K = ratio of outside diameter to bore

Thus, for a tube made in mild steel, fu = 500 MPa (70,000lbf/in2), and allowing a safety factor of 2.5 on a maximumoperating pressure of 100 MPa (14,000 lbf/in2):

K - 1100 x 2.5 = 2 x 500 K + 1

which gives K = 1.67

Suggested wall thicknesses for mild steel tubes of varyinginternal diameters are given in Table 7.

Table 7. Die tube wall thickness

Material: Mild steel, ultimate tensile stress 500 MPa Maximum working pressure: 100 MPa.

Bore Wall

thickness

10 mm 4 mm20 mm 7 mm30 mm 10 mm40 mm 14 mm50 mm 17 mm60 mm 20 mm

If extruder tubes are required to withstand very highpressures it will either be necessary to increase the K ratioor to use steel with a higher ultimate tensile strength - see‘Metals for the construction of die tubes’ (next column).

FATIGUE STRENGTH

Die tubes subjected either to repeated pressures where thepressure pulsates between zero and a peak pressure, or toa pressure which pulsates at a smaller amplitude, about amean pressure, may fail owing to fatigue. This occurs if themaximum shear stress induced at the bore of a cylinder ishigh enough to initiate a fatigue crack, which thendevelops through the wall of the tube with repeatedapplications of the pressure.

The fatigue strength of a tube is the maximum shear stressthat can be induced at its bore, without leading to fatigue.Tubes with smooth bores have a higher fatigue strengththan those with rough corroded or pitted surfaces whichact as stress raisers.

METALS FOR THE CONSTRUCTION OF DIE TUBES

The wall thickness of the die tube is governed by thestrength of the material chosen for its construction. Theaim should be to achieve a wall thickness not greater thanthat needed for the pressure duty, otherwise the accuracyof temperature control in the heated zone could beimpaired.

Reliable results have been achieved using mild steel tubeswith honed bores that are subsequently hard chromed.This gives a surface finish of better than 0.2 µm Ra.

Cold drawn, seamless, stainless steel tube has a higherultimate tensile strength than mild steel tube but may beless resistant to decomposition products. The ultimatetensile strength of stainless steel does depend upon theamount of work hardening that takes place during theforming process. Typical values are ultimate tensilestrength 600 MPa (87,000 lbf/in2)and fatigue shear strength170 MPa (24,600 lbf/in2). The latter can be as high as 250MPa (36,000 lbf/in2).

High strength alloy steels with ultimate tensile strength850 -1000 MPa are alternatives to chromium plated mildsteel but they are not usually available in tubular form.These steels should be considered, however, wherespecial sizes or duties are involved that require theprocurement of a specially designed tube length. Alsohoned, plain bore tubes in these steels have fatigue shearstrengths of up to 360 MPa (52,200 lbf/in2).

Nitrideable steels should also be considered for abrasionresistance. A bore with a nitrided layer up to 0.75 mm(0.030 inch) thick has a high abrasion resistance which isdesirable when extruding glass-filled materials. A nitridedbore also achieves a higher fatigue strength but theinternal pressure applied to the tube must not be highenough to cause the bore to yield, otherwise the nitridedlayer will crack and might allow a fatigue crack to develop.

Section 5. Die tube design

20

TUBE SIZES

Useful engineering standards giving data on standardbores and wall thicknesses are:

British Standard BS EN10216-1:2002Seamless steel tubes for pressure purposes. Technical delivery conditions. Non-alloy steel tubeswith specified room temperature properties

British Standard BS EN10216-2:2002Seamless steel tubes for pressure purposes. Technical delivery conditions. Non-alloy and alloy steel tubes with specified elevated temperature properties

British Standard BS3605-1:1991Austenitic stainless steel pipes and tubes for pressure purposes. Specification for seamless tubes

American Petroleum Institute API SPEC 5L Line Pipe

The diameter of die tubes and mandrels used to extrudePTFE powders must be somewhat bigger than thedimensions required in the final extrudate to allow forshrinkage. The amount of shrinkage is very dependent onextrusion conditions, but for solid rod will be in the range10 - 15% and for tubing about 9 - 12% for the outerdiameter and 5 - 10% for the inner diameter. Shrinkage isslightly greater for agglomerated than for pre-sinteredpowders. Where particularly accurately sized extrudate isrequired, it is advisable to size the die tube and mandrelempirically. This can be done by making prototypes basedon the lowest and highest figures of shrinkage of OD andID given above so that the die tube diameter and mandreldiameter will tend to be slightly smaller and biggerrespectively than is actually required. Extrudate can thenbe made from this prototype equipment under production

conditions and the actual shrinkage measured. The dietube and mandrel can then be modified as necessary andfinally honed to the required surface finish and chromiumplated.

The length of the die tube must be a compromise betweenattaining a high output and exceeding the pressure limitof the PTFE powder being used. When making solid rodthe ratio of the length of the heated part of the die tube tothe internal diameter of the tube should be about 75:1 forpre-sintered powders and about 35:1 for agglomeratedpowders to effect the best compromise. The unheatedlength of die tube protruding above the heated portionshould be at least 150 mm (6 inches) and preferably asmuch as 250 mm (10 inches) for large diameter tubes toallow good control over extrusion pressure. The length:diameter ratio of this part of the die tube markedly affectsthe overall extrusion pressure generated duringextrusion. The unheated portion at the bottom of the dietube should also be 150 - 250 mm (6 - 10 inches) long,depending on the diameter, to allow some control overthe cooling conditions.

Using these general proportions, representative lengthsfor die tubes suitable for making rods of variousdiameters are given in Table 8. Where shorter die tubesare used, the extrusion rate must be reduced to allow thepowder to stay in the heated zone long enough to effectcomplete sintering and an external brake may have to beapplied to attain sufficient extrusion pressure to avoidporosity in the product. Where longer die tubes are usedthe extrusion pressure may exceed the maximum suitablefor the powder and may cause some degree of visiblecharge joins (shot marking).

Table 8. Representative die tube proportions for making rod

Required Approximate Length Length Length Total Powderextrudate die tube of upper of of lower length typediameter internal unheated heated unheated

diameter portion portion portionmm mm mm mm mm mm

10 11.4* 100 850 200 1150 Pre-sintered20 22.7* 150 1700 200 2050 Pre-sintered30 34.1* 150 2550 250 2950 Pre-sintered25 28.7† 150 1000 150 1300 Agglomerated35 40.3† 200 1400 200 1800 Agglomerated45 51.7† 250 1800 250 2300 Agglomerated55 63.2† 250 2200 300 2750 Agglomerated

* Assuming a shrinkage of 12% † Assuming a shrinkage of 13%

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SELECTION OF POWDER GRADE

The choice of PTFE powder type for making any particularextrudate is dictated by the extrusion pressure generatedduring extrusion. There is no definite pressure at whichthe change from one type of powder to another shouldtake place but as a guide pre-sintered powders should beused when the extrusion pressure is high, say >12 MPa (> 1800 lbf/in2) and agglomerated free flowing powdersshould be used when the extrusion is low, say <12 MPa (< 1800 lbf/in2).

In practice, high extrusion pressures are usuallyencountered when extruding small diameter rod or thinwalled tubing, and low extrusion pressures whenextruding large diameter rod or thick walled tubing. Thisis because the die tubes used to make small sectionextrudates usually have a high ratio of die wall surfacearea to extrudate diameter, with the result that thefrictional resistance to movement of extrudate is high.With die tubes capable of making large sectionextrudates, it is usually impractical or prohibitivelyexpensive to make the die tubes long enough to have ahigh ratio of die wall surface area to extrudate diameterand consequently the extrusion pressure generated whenextruding PTFE through them is low. Thus, in general, pre-sintered powders should be used to make small diameterrod (say <25 mm; 1 inch diameter) or thin walled tubing(say <10 mm; 3/8 inch wall), while agglomerated freeflowing powders should be used to make large diameterrod (say >25 mm; 1 inch diameter) or thick-walled tubing(say >10 mm; 3/8 inch wall).

Both pre-sintered and agglomerated Fluon® granularPTFE powders suitable for ram extrusion are available.Fluon® G201 is a pre-sintered powder recommended forextrusion where the extrusion pressure is high. Fluon®

G307 and G401 are agglomerated powders suitable forextrusion where the extrusion pressure is low. Fluon®

G307 and G401 can be extruded under identical

conditions to yield very similar products. Fluon® G307 isalso suitable for general purpose and automatic mouldingand is the choice where an ‘all-round’ powder is wanted.The coarser particle extrusion powder Fluon® G401 ispreferred, because of its particularly good powder flow,where there are difficulties in feeding powder into the dietube. Factors determining the choice of powder aresummarised in Table 9.

EXTRUSION PRESSURE

As described in ‘Extrusion Pressure’ (page 7), theextrusion pressure generated is largely determined by thedesign of the die tube, but the extrusion pressure can beadjusted within reasonable limits by changing theextrusion variables. Thus, in the extrusion of rod, theextrusion pressure can be raised by:

(1) Increasing the length of the feed zone of the die tube -that part of the die tube protruding above the heated zone. (2) Decreasing the temperature of the feed zone. (3) Increasing the length of the heated zone. (4) Decreasing the temperature of the first heated zone. (5) Increasing the rate at which the ram moves when incontact with the powder-this increases the rate at whichthe extrudate is moved over the die tube surface duringthe compressive stroke. (6) Applying an external brake. (7) Increasing the powder charge length by increasing thepenetration of the ram tip into the die tube.

The extrusion pressure can be lowered by reversing thesechanges.

With tubing, the extrusion pressure can be adjusted in thesame way as with rod, except that the effect of changingthe heated length or the temperature of the bottom heatedzone can be different. Thus, decreasing the heated lengthor lowering the temperature of the

Section 6. Extrusion conditions

Table 9. Selection of grade of Fluon® granular powder for extrusion

Extrusion pressure Typical extrudate RecommendedMPa lbf/in2 Fluon® grade

12 - 80 1800 - 11400 Small diameter rodbut preferably up to about 25 mm (1 inch)18 - 35 2500 - 5000 and G201

Thin section tubingup to about 10 mm (3/8 inch)wall thickness

3 - 12 400 - 1800 Large diameter rod G307 or G401but preferably > 25 mm (1 inch) (G401 is 7 - 10 1000 - 1400 and preferred where

Large diameter tubing there are any> 10 mm (3/8 inch) powder feedingwall thickness problems)

22

bottom heated zone, when making tube, can increase theextrusion pressure generated by enabling the extrudate tofreeze onto the mandrel.

To get a satisfactory extrudate the overall extrusionpressure must be appropriate for the type of powderbeing extruded and it must also be generated at thecorrect level in the feed zone and on the molten extrudatein the sinter zone. In practice this means balancing thefeed zone length and temperature against conditions inthe remainder of the die tube.

TEMPERATURE PROFILE

It is impossible to be specific about the temperature atwhich the controllers should be set because therelationship between the set temperature of thecontrollers and the actual temperature of the die tubevaries from one extruder to another and must beestablished empirically. However, as a general guide, thedie tube itself should be maintained at the followingtemperatures:

Top heated zone 350 - 370°C (660 - 700°F)Middle heated zone(s) 370 - 400°C (700 - 750°F)Bottom heated zone 350 - 370°C (660 - 700°F)

When extruding thin sections the temperatures can lie atthe top end of the ranges given, and in somecircumstances the middle zones can be kept high, withdegradation of the polymer being avoided by keepingshort the dwell time of the polymer in the sinter zone. Thisis only possible with thin sections where the heat spreadsacross the whole cross-section quite quickly.

When thick sections are being extruded the die tubetemperatures should be kept near the bottom of theranges to avoid degrading the polymer near the die wallsin the time taken for the powder in the centre of theextrudate to melt.

If the temperature of the top zone is too high there is adanger of overheating the feed area causing poor powderflow and back extrusion. In some circumstances the topzone temperature can be lower than 350°C (660°F) in orderto attain the extrusion pressure needed to get asatisfactory extrudate.

When the bottom end of the die tube or mandrel is verynear the bottom of the heated section of the die tube, thebottom zone temperature may have to be lower than therange indicated in order to let the polymer freeze beforeleaving the mandrel or die tube.

CHARGE LENGTH

This is governed by the amount of powder fed into the dietube during each cycle and therefore by the distance to

which the ram tip penetrates into the die tube at eachstroke. As a general rule it is preferable to use a small ramtip penetration and a short overall cycle rather than a longpenetration and long cycle to attain a given output.However, if the penetration is very small it may provedifficult to feed the correct amount of powder consistentlyand in practice a ram tip penetration of one to three timesthe ram tip diameter is usually satisfactory. Very long ramtip penetrations (say five to six times the ram diameter)should be avoided because of the danger of trapping airin the compacted powder.

RATE OF POWDER COMPACTION

This should be kept as low as possible, consistent withadhering to the required overall cycle time, in order tominimise the chance of trapping air in the compactedpowder. Generally, compaction rates of 5 to 20mm/second (0.2 - 0.8 inches/second) are satisfactory. Therate of compaction is the rate at which the ram moveswhen in contact with the powder.

RAM-DOWN TIME

This is the time the ram is kept at the bottom of its stroke.It should be as long as possible consistent with keeping tothe required cycle time and should never be less thanabout two seconds.

RAM-UP TIME

This should be kept to the minimum necessary to allowthe powder to be distributed uniformly in the die cavity.

EXTRUSION RATE

The extrusion rate is usually governed by the need to letthe extrudate remain in the heated zone of the die tubelong enough to sinter satisfactorily. In this situation,provided the extrusion pressure is within the workingrange for the type of powder being used, the maximumrate at which a satisfactory product can be produced canbe determined by gradually increasing the extrusion rateuntil thin sections cut across the extrudate just show signsof porosity. The maximum acceptable rate will then beslightly less than this.

Occasionally the output is controlled by other factors. Forexample, with very thin section extrudate the maximumextrusion rate is often controlled by the size of the powdercharge which can be used without entrapping air in thecompacted preform and the minimum cycle time of theextruder. With large section extrudates the need to allowthe extrudate to cool and harden enough to resistdistortion as it leaves the die tube or mandrel determinesthe maximum practicable extrusion rate.

23

OPERATING PROCEDURE

Before commencing extrusion the following checkprocedure should be followed:

(1) Ensure that each thermocouple is properly located andmaking good contact with the bottom of the hole in thealuminium block. Also that each thermocouple isconnected to the correct control unit. (2) Check the alignment of the ram tip in the die tube bymoving it down slowly into the die tube. Ideally it shouldhave a uniform clearance around its perimeter. Certainly itshould not rub against the die tube at any point. (3) Check the length of penetration of the ram tip into thedie tube and adjust as required. (4) Check that water is flowing through the main platen tocool the feed area.

If the die tube is empty, the initial powder charges can beprevented from falling through the die tube by pluggingthe latter with a wad of degreased, unsintered PTFE tapeor compacted Fluon® G163 powder. If polymer extrudateremains in the die tube from the previous run, then this ofcourse forms a plug.

The temperature controllers and all other instrumentsshould be set as required and the heaters switched on.To avoid oversintering any polymer remaining in the dietube from earlier runs, the feeding cycle should be startedas soon as the die tube temperature reaches 330°C. Thefeed system controls should be adjusted so that the cavityis consistently just filled with powder during each cycle.As soon as extrudate emerges from the die tube, the rateof extrusion should be checked with a stop-watch and ruleand the overall cycle time, or charge length, or both,adjusted as necessary to get the desired extrusion rate.

The extrusion pressure will probably not reach a steadyvalue representative of the extrusion conditions until a fulldie length of extrudate has emerged. Only if the extrusionpressure is obviously outside the working range of thetype of powder being extruded should adjustments bemade to alter the extrusion pressure before it has reacheda steady value.

The first extrudate to emerge from the die tube afterstarting extrusion will probably be undercompacted andundersintered and it will not be until a full die length ofextrudate has emerged that its quality will berepresentative of the extrusion conditions. It can,therefore, be misleading to check the quality of theextrudate very soon after starting extrusion. An idea of thequality of the extrudate can be obtained quite easily andquickly by examining thin sections cut from the extrudate,as described under ‘Quality Assessment’, page 28). Thesetests enable faults in the extrudate to be identified quickly

so that remedial action can be undertaken early with theminimum loss of material.

If changes are made in extrusion conditions a full dielength must be processed before the extrudate emergingfrom the die tube will be representative of the newextrusion conditions. Indeed, if the changes involve thedie tube temperature, it is probable that even moreextrudate will have to be made before new equilibriumconditions are established.

Once steady operating conditions have been establishedthe powder feed should be checked periodically to ensurethat the die cavity is being filled properly and the extrusionpressure should be monitored to get early warning of anytrouble, for instance the die tube becoming blocked by a‘skin’ of polymer. Also the appearance of the surface andof thin sections cut from the extrudate should be carefullyexamined periodically to ensure that the quality remainssatisfactory. Short lengths of extrudate should be retainedfor quality control tests such as longitudinal tensilestrength and elongation, relative density and dyepenetration.

At the end of the run the heat is switched off but extrusionis continued until the die tube temperature falls to about330°C (625°F) or until the extrusion pressure reaches themaximum permissible level. This minimises the exposureof the polymer in the die tube to very high temperaturesand so limits the occurrence of degradation and the build-up of ‘skin’ on the die walls.

DIE CLEANING

The appearance of a rough surface finish or a ‘black’carbonised ‘skin’ on the extrudate indicates that the dietube needs cleaning. These effects usually only appearafter many hours of running but the use of very high dietemperatures or stopping the extrusion process withoutfirst reducing the temperature to about 330°C (625°F),accelerates the rate at which the die tube becomes ‘dirty’.The die tube can usually be cleaned without stopping theextrusion by filling the die cavity with some clean,degreased bronze or brass turnings and extruding themthrough the die tube. They act as a mild abrasive andscrape the skin of decomposed polymer from the diewalls. If this does not eliminate the problem the extrusionmust be stopped, the extrudate withdrawn from the dietube and the latter scraped clean using a brass wire brushand a fine grade polishing paper.

REPRESENTATIVE EXTRUSION CONDITIONS

Extrusion conditions which have been found suitable formaking good quality rods and tubes of various sizes aregiven in Tables 10 and 11. Some properties of theextrudate are also listed.

Section 7. Operation of the Extruder

24

Table 10. Representative extrusion conditions for rods of various diameters

Approximate diameter of rod 10 mm 20 mm 25 mm 40 mm

Die tube

Diameter 10.6 mm 23 mm 29.2 mm 46 mmUnheated length at top of die tube 90 mm 150 mm 130 mm 145 mmHeated length 900 mm 1700 mm 920 mm 1260 mmUnheated length at bottom of die tube 400 mm 450 mm 30 mm 65 mmTotal length 1550 mm 2300 mm 1080 mm 1570 mmHeated length/diameter 85:1 74:1 32:1 27:1Water cooling Over top Over top Over top Over top

60 mm 60 mm 60 mm 60 mm

Heating arrangements Four separately Four separately Four separately Four separatelycontrolled controlled controlled controlledheated zones heated zones heated zones heated zoneseach with each with each with each withtwo 1.5 kW two 1.5 kW two 1.5 kW two 1.5 kWheater bands heater bands heater bands heater bands

Temperature profile (top) Zone 1 380°C 350°C 350°C 350°CZone 2 400°C 400°C 400°C 400°CZone 3 400°C 400°C 400°C 380°C

(bottom) Zone 4 350°C 350°C 350°C 320°C


Powder type Pre-sintered Pre-sintered Agglomerated AgglomeratedG201 G201 G307/G401 G307/G401

Penetration of ram tip into die tube 25 mm(1) 65 mm 30 mm 40 mmApproximate powder charge weight 1.5 g 15 g 15 g 35 gTotal cycle time 5 seconds 11.2 seconds 20 seconds 25 secondsPowder compaction rate 35 mm/s 22 mm/s 4.3 mm/s 4.2 mm/sRam dwell time at bottom of stroke 1.5 seconds 1.25 seconds 5 seconds 5 secondsBrake None None None NoneExtrusion pressure 82 MPa 32 MPa 8 MPa 9 MPaExtrusion rate 6.8 m/h 4.4 m/h 2.4 m/h 1.6 m/h(2)

1 kg/h 3.9 kg/h 2.7 kg/h 4.1 kg/h

Extrudate properties

Diameter 9.4 mm 20.2 mm 25.8 mm 39.1 mmShrinkage 11.7% 12.2% 11.7% 15%Texture Good Good Good GoodRelative density 2.16 2.16 2.15 2.16Tensile strength 22 MPa 20 MPa 18.5 MPa 18 MPa

(1) 10 mm diameter rod: The extrusion rate was limited by the minimum cycle time of the extruder and thepowder charge size used, not by sintering conditions. The output could be raised by increasing the chargesize but would eventually be limited by the problems of entrapping air and inducing faults at the charge joins.(2) 40 mm diameter rod: The distance between the bottom of the heated zone and the bottom of the die tubeis short. The extrusion rate was therefore limited by the need to allow the extrudate to cool and hardensufficiently to resist distortion as it emerged from the die tube. The bottom heated zone temperature wasalso kept low (320°C) to help promote cooling.

25

Table 11. Representative extrusion conditions for tubes of various sizes

Approximate tube size-outside/inside diameters 25/15 mm 40/30 mm 60/50 mm

Die tube

Diameter 29.2 mm 46 mm 69 mmUnheated length at top of die tube 130 mm 146 mm 155 mmHeated length 920 mm 1250 mm 1700 mmUnheated length at bottom of die tube 30 mm 114 mm 145 mmTotal length 1080 mm 1510 mm 2000 mm

Mandrel

Diameter(in feed area) 15.1 mm 36 mm 56.2 mmLength projecting beneath heated zone 60 mm Nil NilShape Slight taper extending over approximately 200 mm in that part of

the mandrel in the sinter zone

Heating arrangements Four separately Four separately Four separatelycontrolled controlled controlledheater zones heater zones heater zoneseach having each having each havingtwo 1.5 kW two 1.5 kW two 2 kWheater bands heater bands heater bands

Temperature profile (top) Zone 1 350°C 350°C 380°CZone 2 400°C 400°C 400 CZone 3 400°C 370°C 400°C

(bottom) Zone 4 350°C 325°C 325°C


Powder Pre-sintered Pre-sintered Pre-sinteredFluon® G201 Fluon® G201 Fluon® G201

Penetration of ram tip into die tube 40 mm 40 mm 40 mmApproximate powder charge weight 10 g 15 g 25 gTotal cycle time 10 seconds 14.5 seconds 12.5 secondsPowder compaction rate 12.5 mm/s 5 mm/s 6.7 mm/sRam dwell at bottom of stroke 3 seconds 2 seconds 2 secondsExtrusion pressure 43 MPa 45 MPa 53 MPaExtrusion rate 4.6 m/h 3.2 m/h(1) 3.5 m/h(1)

3.8 kg/h 3.6 kg/h 7.4 kg/h

Extrudate properties

Outside diameter 26.2 mm 41.6 mm 62.5 mmInside diameter 14.1 mm 32.7 mm 51.6 mmShrinkage, outside diameter 11.2% 9.6% 9.2%Shrinkage, inside diameter 6.2% 9.4% 8.2%Texture Good Good GoodRelative density 2.16 2.14 2.17Tensile strength(2) 23 MPa 20 MPa 27 MPa

(1) 40/30 and 60/50 tubes: The temperature of the bottom heated zone was kept low (325°C) to help the extrudate to cool andharden sufficiently to resist distortion as it emerged from the die tube. Even so, the usable extrusion rate was limited by theneed to avoid distortion of the extrudate as it emerged from the die tube. (2) Tensile strength: This was measured by machining a sleeve (2.5 mm thick) from the middle of the extrudate, cutting thisalong its length, opening it out and stamping a test specimen from the sheet obtained.

26

Section 8. Profiled extrusion

Granular PTFE is normally extruded as rod or tube ofcircular cross-section, but it is also possible to produceextrudates of non-circular section.

One method is to construct a die tube with matching ramto the required profile, but disadvantages of this methodare that the design and construction of such a die can becomplex and the cost high.

An alternative is to add a forming section to the end of aconventional die tube and use this to change the crosssection of the extrudate from circular to the desired shape.This technique is not expensive in terms of equipmentand, if the extrusion conditions are correctly chosen, it willgive extrudates of adequate dimensional and thermalstability. Limitations to this process are defined by thefollowing constraints:

(1) The maximum dimension of the profiled cross-section

should not exceed the diameter of the circular die tube.

(2) Cross-sectional area reduction from circular to profiledshould not exceed the ratio 2:1.

(3) Profiled cross-sections should be symmetrical.

Sections that have been successfully produced include:square, rectangular, hexagonal and cruciform.

The important controlling parameter in the process is thetemperature of the die tube at the transition from circularto profiled section. This temperature is normally in theregion of 330°C (625°F) but some variation may benecessary to suit individual cross-section areas andshapes.

Figures 6 and 7 illustrate the die configuration andprocessing conditions for two examples.

150mm

150 mm

110mm

920 mm560 mm

60 mm200 mm

100 mm

watercooling

watercooling

360oC380oC

380oC

380oC

350oC

380oC

380oC

310oC

Polymer Fluon® G307Die tube diameter 29.2 mmRam tip penetration 30 mmCycle time 24 secondsExtrusion pressure 13 MPaExtrusion rate 2.5 m/hourDimensions of insert 12 mm x 27 mm

Polymer Fluon® G201Die tube diameter 10 mmRam tip penetration 30 mmCycle time 9 secondsExtrusion pressure 40 MPaExtrusion rate 4.0 m/hour

27

Figure 6. 10.5 x 25.5 mm profiled extrusion Figure 7. Cruciform profiled extrusion

The quality of extrudate can be assessed both by simplequalitative tests such as examining its appearance,texture and porosity and by quantitative tests such as themeasurement of ultimate tensile strength and elongationand of the relative density. The qualitative tests aregenerally quick to perform and are very useful in helpingto optimise the extrusion conditions at the beginning of arun and in checking that the product remains satisfactorythroughout the run. The quantitative tests are useful as ameasure of product consistency.

QUALITATIVE TESTS

Surface appearance

The surface should be viewed in good light. It should besmooth, a uniform white colour and the charge joinsshould be barely visible. It is normal for extrudates fromagglomerated powders (G307, G401) to be glossier thanthose from pre-sintered powders (G201). The commonsurface defects, together with suggestions about theircause and eradication, are described in ‘Diagnosis offaults in extrudates’, (page 29).

Transverse sections

These indicate how well the individual charges of powderhave been compacted and sintered. Thin sections (0.1 - 0.2 mm; 0.004 - 0.008 inch) of substantially uniformthickness are cut from the full width of the extrudate,perpendicular to the axis of the extrudate, and thenviewed by transmitted light. Sections cut from extrudateproduced under correct processing conditions have auniform texture and appearance across their wholediameter. Sections cut from extrudate made from pre-sintered powders have a more grainy appearance thanthose from agglomerated powders which have a finetexture with no obvious particle boundaries.

Defects such as under-sintering, under-pressing or airentrapment will show in transverse sections as voids, orexcessive grain structure dispersed through the section orconcentrated in certain areas.

Longitudinal sections

These indicate how well the extrudate has been sinteredand how strongly the powder charges are joined together.Thin sections (0.1 - 0.2 mm; 0.004 - 0.008 inch) ofmaximum width are cut along a piece of extrudate severalcharge lengths long. A wood plane is useful for preparingthese sections, the extrudate being planed away until thecentre of the extrudate is reached and a section thencarefully cut away. The sections are evaluated by viewingthem by transmitted light and stretching them by hand.

Sections from extrudate which has been processedcorrectly have a uniform appearance; those from pre-sintered powders are somewhat grainy while those fromagglomerated powders have a fine texture with noobvious particle boundaries. When pulled they can bestretched a considerable amount before breaking. Porousareas with voids indicate that the extrudate has beenunder-sintered or under-pressed. Cracks running alongthe centre of rod indicate over-sintering. Cracks across thesections at charge-joins are caused by trapped air or over-pressing. If the sections snap when extended only slightly,the charge joins are weak, probably as a result of over-pressing.

Porosity

Cracks and porosity in extrudates can be identified byusing a penetrant dye such as ‘Ardrox’ 996P2*. Thesurface of a piece of extrudate several powder chargeslong is lightly abraded and the sample then immersed inpenetrant dye solution for at least two hours. It is thenremoved, excess dye washed off its surface using dye-removing solvent and running water and the sample cutin half along its length. The dye will have penetrated intoany cracks or porous regions, clearly identifying them.Good quality extrudate should not contain any such faultsand the dye should not penetrate into any part of it.

QUANTITATIVE TESTS

Relative density

The relative density of extrudates produced from granularPTFE powders will be dependent on the processingconditions, particularly the rate at which the extrudatecools. Generally, however, good quality extrudate willhave a relative density in the range 2.13 -2.19.

Tensile properties

The values obtained for tensile properties are dependenton the method of specimen preparation and on the testprocedure, so this test is really most useful as a measureof product consistency. There are, however, manyspecifications issued by national standards organisationsand other bodies which incorporate standardised testprocedures and limits for extrudates from granular PTFEpowders. Please contact AG Fluoropolymers for details.The most important international standards are ISO12086-1 and –2:1995 for raw materials and ISO 13000-1and –2:1997 for semi-finished products.

* Supplied by:- Chemetall plc, 65 Denbigh Road, Bletchley, Milton Keynes, MK1 1PB (UK)Tel. +44 (0) 1908 649333 Fax +44 (0) 1908 361872www.aerospace.chemetall.comin mid-Europe by Chemetall GmbH, Frankfurt a.M. Tel. +49 (0) 697165-0 and in the USA by Chemetall Oakite, 50 Valley Road, N.J. 07922, BerkeleyHeightsTel. +1 908 508 2214 Fax +1 908 464 7914 Toll-free 800 526 4473www.oakite.com

Section 9. Quality assessment

28

Section 10. Diagnosis of faults in extrudates

Table 12 lists some of the faults which may occur duringextrusion, with possible causes and suggested ways ofavoiding them.

Table 12. Common extrusion faults and their avoidance

Fault Possible cause Suggested corrective action

(1) Discoloured spots, black Oil contamination from the Check all hydraulic seals.or brown stains or streaks in extruder or parts of the Check for oil leakage in or around powder feedthe product powder feed system system and rectify any found. Clean the powder

feed system and extruder die. Check that air fromany compressor used is oil free.

orContamination from the Keep the area around the extruder clean. Keepworkshop area the drums of Powder closed when not in use.

orSmall fragments from plastic Re-align the ram tip in the die tube. Replace ramram tips, if used, which tip if damaged.decompose in the die tube

(2) Small white or grey Flakes of sheared polymer, Reduce ram clearances. Check that the ram tippatches or lumps in extrudate formed between ram tip and has sharp edges. Check that extrusion pressure

die walls, dropping on to the is not too high.extrudate Check that the top of the die tube is cool,

preferably 21 - 30°C (70 - 85°F).

(3) Rings of surface Contamination resulting from Stop the ram tip rubbing on die tube orcontamination at the junction abrasion between the ram tip mandrel byof each powder charge, and the die tube wall (and/or (a) re-aligning the ram tip in the die tube.possibly on one side of the mandrel in tubing extrusion) (b) increasing the clearance between ram tip andextrudate only. die walls or mandrel.(See Figure 8) (c) ensuring that the powder is uniformly

distributed in the die cavity. This is particularlyimportant when extruding tube because unevenpowder feed could force the mandrel to one side.

(4) Irregular black patches Die tube temperatures too high Check the set temperatures. If these are notor black stain adhering to the unusually high, check the performance of theextrudate controllers and the accuracy and location of the

thermocouples.

Prolonged extrusion without Clean the die tube by extruding one or twocleaning the die tube charges of brass turnings through it.

orCorrosion of the die tube by the As a final measure, shut down the extruder,extrudate empty the die tube and clean it as described in

the text (page 24).Check the die plating and the die surface for theeffects of corrosion, e.g. pitting.

(5) Rough, irregular surface Build-up of polymer on the As for (4) above.finish, possibly with patches of walls of the die tube as a resultpolymer skin adhering to it of prolonged extrusion without

cleaning the die tube. This ispromoted by the use of highdie tube temperatures

(continued)

29

Table 12. Common extrusion faults and their avoidance (continued)


(6) Score marks or cracks Too much brake pressure Reduce the brake pressure.running along the surface of orthe extrudate in the direction Extrudate too hot and soft on Reduce the temperature of the bottom heatedof extrusion entering the brake zone of the die tube. Reduce the extrusion rate.

orLocal skin or dirt build up Try cleaning the die tube by extruding one orin the die tube two charges of brass turnings through it. If this is

ineffective, clean as described on page 24.orBurr on the surface of the die Remove and check that these surfaces aretube or brake. smooth.

(7) Circumferential marks or Extrusion pressure too high Check the extrusion pressure and reduce it.cracks at the junction of See page 22 for methods of adjusting extrusionsuccessive powder charges, pressure. If these are not effective, clean diesometimes extending only tube to remove any deposits which may bepart way around the extrudate. restricting the movement of the extrudate,Weak bonds between the thereby causing a high extrusion pressure.powder charges or

Temperature too high at the Check that water is flowing through the channelstop of the die where the powder in the platen. Increase the rate of flow ifis compacted necessary.

Reduce the temperature of the topheated zone of the die tube, but take othermeasures to ensure that this does not causeexcessive extrusion pressure.

orToo much ram clearance, Use larger diameter ram tip to reduce clearancecausing sufficient flashing to between ram and die walls.interfere with the bonding ofthe successive powder charges

orCharge size too large. This is Reduce the charge size.usually confined to extrudate Reduce the overall cycle time to maintain outputwith a small cross sectional area. when charge size is reduced.

(8) Uneven charge lengths or Uneven powder feed and poor Check that the powder has not been overworkedwavy marks at the junction distribution of the powder in the or over-compacted in transit or in the feedof successive charges die cavity system. Keep the temperature of the powder in

the hopper and feed system in the range 21 - 30°C(70 - 85°F) Check the mechanism for deliveringpowder to the top of the die and for distributingit within the die cavity.

(9) Pitted surface, with waxy Degradation caused by over- Reduce the die temperature, particularly in thefeel; general darkening of whole sintering middle heated zones. Reduce the dwell time ofsurface the extrudate in the heated section of the die

tube by increasing the extrusion rate and/orreducing the heated length.

Check the accuracy of the temperature controlunits and thermocouples.

(10) Crack running along the Over-sintering As in (9).axis of extruded rod

30



(11) Porous regions in the Under-sintering Check that all heaters are working and that theextrudate concentrated near temperature control units and thermocouples arethe centre of rod and near accurate. Increase the die tube temperatures,the inner surface of tube particularly those in the middle heated zones.

Increase the dwell time of the extrudate in theheated section of the die tube by reducing theextrusion rate or extending the heated length.

(12) Porosity dispersed Extrusion pressure too low Increase the extrusion pressure as described inuniformly across rod or the text.tube section Use the extrusion pressure recommended for

the type of powder being used.orEntrapped air Lower the ram speed during compaction of the(See Figure 9) powder. Increase the ram clearance. Decrease the

charge size by reducing the distance to which theram tip penetrates the die tube.

(13) Internal voids or cracks, Entrapped air Lower the ram speed during compaction of theparticularly at the junction of (See Figure 9) powder. Increase the ram clearance. Decrease thepowder charges charge size by reducing the distance to which the

ram tip penetrates the die tube.

orMoisture in the powder Check that the powder is dry.

Check that the temperature of the powder in thefeed section is not below the dew point of the airin the vicinity of the extruder.

orCooling very thick Reduce the cooling rate as described on page 6.extrudates too rapidlyorSevere over-sintering As in (9).

(14) Tubing is convoluted Allowing the extrudate to leave Ensure that the extrudate is cool and thereforeor has a series of regularly the die tube at very near its gel firm enough to withstand buckling as it leavesspaced raised rings around point whilst using a mandrel the die tube. This can be done by lowering theits circumference usually which extends some way beyond temperature of the bottom heated zone, reducingcombined with cracks the die tube. The extrudate cools the extrusion rate or water-cooling that part of

rapidly as it leaves the die tube, the die tube beneath the heated zone.shrinks onto the mandrel and,therefore, has its movementrestrained.During the next compressivestroke the hot extrudate iscompacted against the materialstill on the mandrel.

(15) Curved or distorted Uneven cooling Shield the extrudate from draughtextrudate

orUneven distribution of the Keep the powder cool to maintain good flow.powder in the die cavity Check that the mechanism for distributing it in

the die cavity is functioning properly.

(continued)

31



(15) continued orMandrel off-centre in tube Re-position the mandrel and ensure that theextrusion powder is uniformly distributed around it to keep

it aligned.orObstruction, eg skin build-up, Clean the die tube as described on page 24.on one side of the die tube

(16) Eccentric tubing Uneven distribution of powder Keep the powder cool to help it to flow well.in the die cavity Ensure that the mechanism for distributing it in

the die cavity works efficiently.orMandrel off centre Re-align mandrel

(17) Back extrusion- Ram dwell time at maximum Increase the ram dwell time at the bottom of itsthe compacted powder charge penetration is too short strokemoves back up the die tubewhen the ram is withdrawn or

The top of the die tube where Increase the flow of water through the extruderthe powder is compacted is too platen. Reduce the temperature of the tophot heated zone.

Increase the distance between the top of the dietube and the start of the heated length.

orAir trapped with the powder Reduce the rate of ram movement whenduring compaction compacting the powder.

Increase ram clearances.Reduce charge size ie the extent to which theram tip enters the die tube.

orBrake restricts the movement Reduce the effectiveness of the brake.of the extrudate too severely

orBuild-up of skin etc. in the die Clean the die tube as described on page 24.tube which severely restricts thedownward movement of theextrudate.Usually occurs only afterprolonged use without cleaning

(18) Extrudate has a matt Surface finish of the die tube is Re-hone the die tube.surface poor

orThe extrudate leaves the die tube Cool the extrudate before it leaves the die tubein the gel state or very close to it by lowering the temperature of the bottom

heated zone, reducing the extrusion rate ordeliberately cooling the die tube beneath theheated zone.

orThe extrudate is not maintaining Apply a brake. It may be necessary after doing thiscontact with the die tube for to reduce the distance between the top of the dietotal length of gel zone tube and the beginning of the heated section or

reduce the rate of movement of the ram whilstcompacting the powder or raise the temperatureof the top heated zone to avoid getting anexcessive extrusion pressure.

32

Figure 8. Showing rings of surface

contamination at the junction of each

powder charge

Figure 9. Charge separation due to

entrapped air (on right)

33

The following is a comprehensive list of TechnicalService Notes on Fluon® PTFE. They are available fromthe AG Fluoropolymers sales office.

F1 The Moulding of PTFE granular powders

F2 The Extrusion of PTFE granular powders

F3/4/5 The Processing of PTFE coagulateddispersion powders

F6 Impregnation with PTFE aqueousdispersions

F8 Processing of filled PTFE powders

F9 Finishing processes forpolytetrafluoroethylene

F11 Colouring of polytetrafluoroethylene

F12/13 Physical properties of unfilled and filled polytetrafluoroethylene

F14 Isostatic compaction of PTFE powders

F15 Cast Film from Fluon® PTFE dispersion GP1

FTI500 Fluon® - A Guide to Applications, Properties& Processing

FTI800 Potential Material & Equipment Suppliers

Further Information

Information contained in this publication (and otherwisesupplied to users) is based on our general experience andis given in good faith, but we are unable to acceptresponsibility in respect of factors which are outside ourknowledge or control. All conditions, warranties andliabilities of any kind relating to such information,expressed or implied, whether arising under statute. tortor otherwise are excluded to the fullest extentpermissible in law. The user is reminded that his legalresponsibility may extend beyond compliance with theinformation provided. Freedom under patents, copyrightand registered designs cannot be assumed.

Fluon® grades are general industrial grades. It is theresponsibility of the purchaser to check that thespecification is appropriate for any individual application.Particular care is required for special applications such aspharmaceutical, medical devices or food. Not all gradesare suitable for making finished materials and articles foruse in contact with foodstuffs. It is advisable to contactthe AG Fluoropolymers sales office for the latest position.Users of Fluon® are advised to consult the relevantHealth and Safety literature which is available from theAG Fluoropolymers sales office.

Users of any other materials mentioned in thispublication are advised to obtain Health and Safetyinformation from the suppliers.

Note

The data on processing performance given in thispublication have been observed using the stated Fluon®

grades and machine conditions. These data arerepresentative but cannot cover all cases. Processors aretherefore advised to satisfy themselves of the suitabilityof any particular equipment or production methods forintended applications.

This edition ©AGFP September 2002

Fluon® TechnicalLiterature

34

35

UKFor fluoropolymer & AFLAS® enquiries from EMEA (Europe, Middle East & Africa): AGC CHEMICALS EUROPE, LTD. PO Box 4 Thornton Cleveleys Lancashire FY5 4QD UK Telephone: +44 (0) 1253 861951Fax: +44 (0) 1253 861950email: [email protected] web: www.agcce.com

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For fluoropolymer & AFLAS® enquiries from the Americas: AGC CHEMICALS AMERICAS INC. . Corporate Office 255 S. Bailey Road Downington, PA 19335 United States of America Telephone: +1 610-423-4300 Toll Free Telephone (US only) 800-424-7833 Fax: +1 610-423-4305 email: [email protected] web: www.agcchem.com

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Documents

The extrusion of PTFE granular powders