TWIN SCREW EXTRUDERS

FEATURES OF SINGLE SCREW EXTRUDER

LOWER OUTPUT – MORE COST OF PRODUCTION

FIXED PRODUCTION – DUE TO OPTIMUM RPM

OPERATES THROUGH FRICTION – MORE DEGRADATION

LIMITATIONS OF L / D RATIO – 24 – 32

MIXING THROUGH BACK PRESSURE & FRICTION - NON HOMOGENITY AND LIMITATIONS IN QUALITY

HIGHER RESIDENCE TIME – MORE DEGRADATION & MORE DOSES OF STABILIZER

TWIN SCREW EXTRUDERSTHE DISADVANTAGES OF SINGLE SCREW EXTRUDERS

ARE RESPONSIBLEFOR THE DEVELOPMENT OF TWIN SCREW EXTRUDERS (T. S. E.).

THEY HAVE TWO INTERMESHING SCREWS ENCASED IN A MATCHING BARREL. THEY WORK LIKE A POSITIVE DISPLACEMENT PUMP, INDEPENDENT OF FRICTIONAL CRITERION.

TWIN SCREW EXTRUDERS ARE ESSENTIALLY GEAR PUMPS.

WHATEVER GOES IN – COMES OUT FROM THE DIE.

MORE THE BULK DENSITY OF DRY BLEND ENTERING IN THE HOPPER, MORE IS THE OUTPUT. WHETHER THE SCREWS ARE STARVED OR FILLED.

TYPE OF TWIN SCREW EXTRUDERS

       TWIN SCREWS CAN BE –

CO-ROTATING OR

CONTRA ROTATING.

       THE BARREL AND SCREW CAN BE –

PARALLEL, STEPPED, CONICAL OR SUPER CONICAL.

       OVER ALL OUTPUT OF TWIN SCREW EXTRUDERS IS MUCH MORE THAN SINGLE SCREW EXTRUDERS.

CO ROTATING VS. CONTRA ROTATING

     IN THE CO-ROTATING SCREWS, THE SHEAR ON THE POLYMER IS QUITE HIGH IN THE OVERLAP REGION, BECAUSE THE FLIGHTS ARE MOVING IN OPPOSITE DIRECTION.

       THIS GIVES BETTER MIXING. BUT DEGRADES PVC.

     CONTRA ROTATING SCREWS GENERATE LESS SHEAR IN THE OVERLAPPING REGION, SINCE THE

FLIGHTS TRAVEL IN THE SAME DIRECTION.

      THIS IS WHY CONTRA ROTATING SCREWS ACCOUNT FOR BETTER POPULARITY.

CONTRA ROTATING SCREWS

       IN CONTRA ROTATING SCREWS, AGAIN THERE ARE TWO POSSIBILITIES

WHEN VIEWED FROM THE TOP – THE SCREWS CAN ROTATE TOWARDS THE CENTER OR

TOWARDS THE SIDES.

       AS FAR AS MIXING OR HEAT TRANSFER IS CONCERNED, EITHER DIRECTION OF ROTATION WILL PROCESS THE POLYMER IN THE SAME MANNER.

       INWARD DIRECTION OF ROTATION MAY PULL THE MATERIAL FROM THE HOPPER INTO THE BITE OF THE SCREWS MORE EFFECTIVELY.

         HOWEVER THERE IS ALWAYS A DANGER THAT A

FOREIGN OBJECT IS PULLED INTO THE SCREWS.

ADVANTAGES OF TWIN SCREW EXTRUDERS

     IN CASE OF SINGLE SCREW EXTRUDERS, THE MIXING ACTION DEPENDS MAINLY ON BACK PRESSURE, RESULTING IN BACK FLOW IN THE METERING SECTION.

     IN TWIN SCREW EXTRUDER, ADDITIONAL MIXING TAKES PLACE AT THE POINT OF SCREW INTERMESHING.

      NARROW RESIDENCE TIME ENABLES LESSER DOSES OF STABILIZERS IN TWIN SCREW EXTRUDERS

       PROCESSING TEMP. RANGES FROM 180 –190 °C IN T. S. E. AS AGAINST 205 – 225 °C IN S. S. E.

ADVANTAGES OF TWIN SCREW EXTRUDERS

       LESS THERMAL HISTORY PROVIDES LARGE MARGIN AGAINST THERMAL DEGRADATION.

       L / D RATIO IS 12 TO 16 IN TWIN SCREW EXTRUDER. AGAINST 24 TO 32 IN SINGLE SCREW EXTRUDERS.

       MORE VISCOUS & CONSISTENT MELT AND MORE UNIFORM PUMPING ACTION GIVES SURGE FREE MELT DISCHARGE WITH CONSISTENT PRODUCT IN CASE OF TWIN SCREW EXTRUDER.

       VENTING OPERATION AT EASE.

       SELF CLEANING ACTION.

       RELATIVELY LOWER R. P. M. IN T.S.E. ( 15 – 30 ) AS AGAINST ( 40 - 60 ) R. P. M. IN S. S. E.

MIXING & SHEARING

     IN TWIN SCREW EXTRUDERS THE SCREW R.P.M. IS LOW AND HENCE THERE IS LOW SHEAR AT THE BARREL WALL RESULTING IN LOW MIXING.

     HOWEVER GOOD MIXING TAKES PLACE THROUGH

1. THE CLEARANCE BETWEEN FLANKS OF OPPOSING SCREWS, AS WELL AS

2. THE FLIGHT LAND OF ONE SCREW AND

THE BOTTOM CHANNEL OF THE OPPOSING SCREW PROMOTE INTERCHANGE OF MATERIAL BETWEEN SCREWS RESULTING IN GOOD MIXING.

ENERGY UTILIZATION

      MECHANICAL ENERGY FROM THE MOTOR IS USED FOR –

   MIXING AND IN TURN IN RAISING THE TEMPERATURE OF THE POLYMER.

THIS IS DESIRABLE, AS DUE TO POOR THERMAL CONDUCTIVITY OF THE POLYMER, HEAT TRANSFER FROM THE BARREL IS NOT SUFFICIENT.

  IF HIGH OUTPUT IS ACHIEVED WITH INSUFFICIENT MECHANICAL ENERGY, QUALITY OF THE PRODUCT MAY BE POOR DUE TO INSUFFICIENT FUSION.

COMPRESSION

THE BULK DENSITY OF THE DRY BLEND

ENTERING THE HOPPER IS ABOUT 0.6 g / cc. THE MELT DENSITY IS ABOUT 1.45 g / cc.

A CONSIDERABLE AMOUNT OF COMPRESSION MUST TAKE PLACE WITHIN THE EXTRUDER.

      THE SCREWS / BARREL MUST PROVIDE

COMPRESSION THROUGH THE DESIGN OF BARREL AS WELL AS DESIGN OF FLIGHTS.

THE EXTRUDER HAS FEED ZONE, COMRESSION ZONE AND METERING ZONE.

WAYS OF ACHIEVING COMPRESSION

1.    DECREASE THE PITCH OF THE SCREW FROM FEED END TO THE DISCHARGE. THE SCREW AT THE FEED END PUMPS THE MATERIAL AT A FASTER RATE THAN AT THE DISCHARGE, SO THAT MATERIAL IN BETWEEN IS COMPRESSED.

2.   THE FLIGHT DIRECTION CAN BE REVERSED OVER A SMALL LENGTH OF THE SCREW. THIS TECHNIQUE IS USED IN COMPOUNDING EXTRUDERS.

3.   INCREASING FLIGHT THICKNESS FROM FEED END TO DISCHARGE END.

4.   REDUCE THE DIAMETER TOWARDS THE DISCHARGE END BY USING TAPERED SCREWS AND BARREL.

5.   REDUCE BARREL AND SCREW DIAMETER STEP WISE.

PROCESSING IN TWIN SCREW EXTRUDERS

      THE FOCUS IN TWIN SCREW EXTRUSION IS DIVIDED IN FOUR PARTS -

1.   THE DRY BLEND MIXING OPERATION.  

2.   BETWEEN FEED ZONE AND VACUUM PORT. 

3.   BETWEEN VACUUM PORT AND DIE.

4.   WITHIN THE DIE.

       IN PVC PROCESSING WHAT MATTERS IS THETEMPERATURE OF PVC AND NOT THE

TEMPERATURE OF ANY PART OF THE EXTRUDER.

APPROPRIATE PRODUCT QUALITY

CONTROLCORRECT CORRECTINPUTS OUTPUTS

PLASTIC PROCESSING -1. PREPARING THE MATERIAL-

COLOURING, MIXING, DRYING ETC.2. MELTING & HOMOGENIZING3. SHAPING THROUGH DIE,MOULD,

FORMING4. COOLING THE SHAPED PRODUCT5. FINISHING-PRINTING, ASSEMBLY

ETC.

PROCESS

HOT MIXING

      UNIFORM OPERATION IN THE MIXING BATCHES IS THE KEY TO SUCCESSFUL EXTRUSION.

       MIX ALL THE BATCHES AT SAME

TEMPERATURE, TORQUE AND TIME AND FOLLOW THE SAME SEQUENCE OF ADDITION OF ADDITIVES.

      MORE THE DROP TEMPERATURE, MORE THE BULK DENSITY OF THE DRY BLEND.

     OPTIMUM DROP TEMPERATURE IS 120 °C

COLD MIXING

     COOL THE BATCH TO SAME TEMPERATURE.

     VARIATION IN THE FINAL TEMPERATURE OF THE DRY BLEND AFFECTS THE LOCATION OF

FUSION IN THE EXTRUDER. THE OPTIMUM COMPOUND TEMPERATURE FROM COLD MIXER IS 40 °C.

       TOO MUCH COOLING WILL RESULT IN

MOISTURE CONTENT IN THE DRY BLEND DUE TO CONDENSATION.

    CARBON BLACK HAS A VERY HIGH SURFACE AREA PER UNIT WEIGHT AND HAS AFFINITY FOR GASES AND MOISTURE, WHICH IT ABSORBS LIKE A SPONGE.

BETWEEN FEED ZONE & VACUUM PORT

      THE SCREWS SHOULD BE FILLED COMPLETELY FOR MAXIMUM OUTPUT.

     FAIRLY AGGRESSIVE HEATING IS REQUIRED IN THE FIRST TWO ZONES TO FACILITATE

EXTRACTION OF

1. VOLATILES,

2. MOISTURE

3. HCl GAS GENERATED DUE TO DEGRADATION.

DURING VACUUM DEGASSING.

AT VACUUM PORT      LOOK AT THE VACUUM PORT WITH A FLASH

LIGHT.

     NO DRY POWDER SHOULD BE VISIBLE AT ANY TIME FROM THE VACUUM PORT.

     THE DRY BLEND SHOULD LOOK POWDERY BUT STICKY LIKE WET SAND.

     THE GRAINS MUST STICK TOGETHER BUT SHOULD NOT FUSE. [ GELLATION ]

THE TEMPERATURE OF MATERIALAT VACUUM PORT SHOULD BE AROUND 150 °C.

EFFECT OF DRY BLEND TEMPERATURE

       DRY BLEND TEMP. PLAYS A VERY VITAL ROLE. IT SHOULD BE AROUND 40 °C.

       SUPPOSE YOUR DRY BLEND TEMP IS 20 °C MORE. THE TEMPERATURE AT VACUUM PORT WILL RISE TO

AROUND 170 °C, INSTEAD OF 150 °C.        THE TORQUE WILL RISE.

       THE MATERIAL WILL BE FUSED BEFORE VACUUM PORT.          THE PRODUCT WILL HAVE VOIDS DUE TO TRAPPED AIR

AND MOISTURE RESULTING IN POORER IMPACT AND MECHANICAL STRENGTH.

        DUE TO MORE HEAT AND TORQUE IT MAY DEGRADE,

FURTHER RESULTING IN BAD QUALITY.

BETWEEN VACUUM PORT & ADAPTER

     THIS IS THE AREA WHERE PVC SHOULD FUSE.OPTIMUM FUSION SHOULD BE 60 TO 65 %

      IF THE CONSISTENCY AT VACUUM PORT

LOOKS GOOD BUT THE TORQUE IS TOO HIGH, TRY TO INCREASE THE

ADAPTER TEMPERATURE BY FEW DEGREES.

      THE ADAPTER IS LIKE A VALVE.

-HIGHER TEMPERATURE OPENS IT UP.

-LOWER TEMPERATURE CLOSES IT.

LOCATION OF FUSION

     DEPENDING THE SIZE OF THE DIE, WE SHOULD BE ABLE TO CHOOSE THE PLACE OF OPTIMUM FUSION. SOME TIMES THE FUSION CAN TAKE PLACE EVEN IN DIE.

     IF YOU HAVE POOR FUSION AND THE

METERING ZONE TEMPERATURES ARE AT MAXIMUM LIMIT, TRY TO REDUCE ADAPTER TEMP LITTLE BIT.

     THE MELT WILL GET HOTTER AND THE TORQUE WILL RISE AND THEN FALL OFF AGAIN TO A

LEVEL JUST ABOVE THE INITIAL LEVEL.

CHANGES DURING EXTRUSION        ANY CHANGE YOU MAKE, DO IT GENTLY.

       TEMP. INCREASE SHOULD BE AT THE MOST 5 °C. AT A TIME.

       WAIT FOR 20 MINUTES BEFORE YOU MAKE ANOTHER CHANGE.

       ANY CHANGE IN ONE PART OF AN EXTRUDER AFFECTS ALL PARTS OF IT.

         NOTE ALL YOUR CHANGES IN A LOG BOOK.         MAKE SURE THAT THE SAMPLE CHECKED FOR PHYSICAL

PROPERTIES MATCHES CORRECTLY WITH THE CORRESPONDING EXTRUSION CONDITIONS, TO AVOID WRONG CONCLUSIONS.

         IF YOUR INPUTS ARE CONSISTENT, YOU MAY HAVE TO

MAKE MINOR CHANGES, BASED ON THE CONTROL PLAN DATA ON PARAMETERS AND THE TEST RESULTS AVAILABLE.

WHAT HAPPENS INSIDE THE DIE

      MELT FROM THE EXTRUDER ENTERS THE DIE EITHER THROUGH AN ORIFICE OR BREAKER PLATE AND SCREEN

PACK.

      EXTRUSION DIE IS ESSENTIALLY A STREAM LINED CHANNEL WHICH GIVES APPROPRIATE SHAPE TO THE POLYMER MELT.

      THE MELT COMING OUT OF THE DIE MUST LOOK SHINY WITH NO LUMPS, STREAKS OR ROUGHNESS.

      THE MELT TEMPERATURE SHOULD BE BETWEEN 185 TO 190 °C.

      THE TORQUE SHOULD BE 60 % OR ABOVE.

BASIC REQUIREMENTS

1. THE MELT VISCOSITY MUST BE MAINTAINED BY CONTROLLING THE TEMPERATURE.

2. DEGRADATION DUE TO OVER HEATING SHOULD BE AVOIDED.

  3. FLOW SHOULD BE SMOOTH. NO STAGNANT AREAS SHOULD BE PRESENT IN THE DIE.

4. CHANGE IN CROSS SECTION SHOULD BE GRADUAL.

 5. SUFFICIENT PRESSURE MUST BE MAINTAINED BY GRADUALLY REDUCING THE CROSS-SECTIONAL AREA OF THE DIE CHANNEL.

BASIC REQUIREMENTS

6.   THE MELT PATH RESTRICTION SHOULD BE ADJUSTABLE.

7.   THE DIE CONSTRUCTION SHOULD BE RIGID TO WITHSTAND INTERNAL PRESSURE.

8.   THE DIE SHOULD FILL COMPLETELY. [ INT. LUB]

9.   USUALLY TOWARDS THE EXIT END OF THE DIE REDUCTION IN CROSS SECTION IS AVOIDED. INSTEAD A PARALLELITY IS MAINTAINED.

THIS PARALLEL PORTION IS CALLED “ DIE LAND”.

TWIN SCREW DESIGN DETAILS

      INDIVIDUAL SCREWS ARE USUALLY CUT FROM ONE PIECE OF METAL.

        SOME MANUFACTURERS USE SMALL SCREW

SECTIONS THAT SLIDES ONTO SPLINED OR KEYED SHAFTS.

        THE TWIN SCREWS ARE NITRIDED FOR WEAR

RESISTANCE.

      CHROME PLATING IS AVOIDED AS ITS LOW COEFFICIENT OF FRICTION DEPRIVES THE MECHANICAL WORKING OF THE POLYMER.

TWIN SCREW DESIGN DETAILS

       HIGH PERFORMANCE SCREWS ARE TEMPERATURE CONTROLLED. THIS IS DONE BY PUMPING THE TEMPERATURE CONTROLLED FLUID THROUGH THE BORE OF THE SCREWS.

       IN CASE THE BULK DENSITY OF THE DRY BLEND VARIES FOR ANY REASON, GRAVITY FED HOPPERS CAN NOT BE USED.

       INSTEAD, A VOLUMETRIC SCREW FEEDER IS USED TO FEED THE MATERIAL WITH VARIABLE SPEED ARRANGEMENT.

BARREL DESIGNING DETAILS

       BARRELS ARE NITRIDED.

       THEY ARE SUBJECTED TO APPRECIABLE WEAR AND TEAR, DUE TO SPREADING FORCES BETWEEN THE SCREWS.

      CHROME PLATING IS AVOIDED AS THE DIFFERENCE IN

THE THERMAL EXPANSION BETWEEN THE CHROME PLATING AND THE STEEL BARREL INDUCE STRESSES THAT CAUSE CHROME PLATE TO PEAL OFF.

        A TWIN SCREW EXTRUDER CAN FUNCTION WITH MUCH MORE BARREL WEAR THAN SINGLE SCREW EXTRUDER.

         IT IS BETTER TO TOLERATE THE WEAR AND THEN

REPLACE THE BARREL.          A GROWING PRACTICE IS TO USE HARD ALLOY LINERS.

CONICAL VS. PARALLEL SCREW

       OVER A PERIOD OF TIME, DUE TO WEAR AND TEAR, THE OUTPUT IN PARALLEL SCREW DECREASES GRADUALLY.

       WHERE AS IN CASE OF CONICAL SCREWS THE OUTPUT CAN BE MAINTAINED BY

PERIODICALLY FORWARDING THE SCREWS.

       UNBALANCED FORCES ACTING ON THE SCREWS DO PRESENT A PROBLEM OF DEFLECTION AS THE SCREWS GET LONGER.

CONICAL VS. PARALLEL SCREW        CONICAL TWIN SCREWS HAS A DISTINCT

ADVANTAGE AS A CONICAL SCREW IS A MORE EFFECTIVE CANTILEVER BEAM AND RESIST BENDING BETTER.

       PARALLEL SCREWS LIE SO CLOSE THAT PROVIDING ADEQUATE THRUST BEARING IS A PROBLEM. OBVIOUSLY ONE SCREW SHAFT HAS TO

EXTEND BEYOND THE OTHER TO ACCOMMODATE APPROPRIATE THRUST BEARING.

         IN CASE OF CONICAL SCREWS THIS PROBLEM

IS SOLVED DUE TO MORE DIAMETER AT THE FEED END.

ADAPTER & DIE DESIGN       TO MINIMIZE THRUST LOADS, THE DIE SYSTEMS FOR TWIN

SCREW EXTRUDERS ARE DESIGNED WITH WIDE INTERNAL OPENINGS TO REDUCE THE BACK PRESSURE.

       THE PRESSURE VARIES INVERSELY WITH THE THIRD POWER OF THE THICKNESS OF THE OPENING. SO IT IS POSSIBLE TO REDUCE BACK PRESSURE BY USING WIDE CLEARANCE UP TO THE FINAL DIE OPENING.

       THE END OF THE TWIN SCREWS ARE NOT SYMMETRICAL. THEAMOUNT OF MATERIAL BETWEEN THE SCREW TIPS AND THE

DIE IS LARGE.

       IT IS IMPORTANT TO KEEP THIS TO A MINIMUM. AN EFFECTIVEDESIGN FOR A TWIN SCREW EXTRUDER ADAPTER HAS THE CAVITY TO MATCH THE CONICAL TIPS OF THE SCREWS.

       THESE CONICAL CAVITIES ARE CONVERGED TO A COMMON SMALL DIAMETER HOLE.

        ADAPTERS AND DIES ARE HARD CHROME PLATED FROM TIME

TO TIME.

FLOW RATE IN THE PROFILE DIE

      IF THE FLOW IS CHANGED BY CHANGING THE THICKNESS OF THE DIE OPENING, THE

VOLUME OF FLOW VARIES INVERSELY WITH THE CUBE OF THICKNESS OF THE OPENING.

       SINCE THE FLOW IN THE REST OF THE DIE MUST CHANGE IN OPPOSITE DIRECTION, THERE IS ACTUALLY A MULTIPLYING EFFECT.

       IN CASE OF PROFILE DIES, CHANGING DIE LAND LENGTH TO BALANCE THE DIE IS PREFERABLE BECAUSE THE EFFECT IS LINEAR.

        PLATE TYPE DIES OFFER DEGRADATION

PROBLEM AFTER 12 – 18 HOURS. THESE DIES CAN NOT BE USED FOR HIGH RATE EXTRUSION.