Very useful information about the processes of rubber products undergoing continuous vulcanisation.

1. BY
PRYIABRATA GHOSH
IRI, KOLKATA
priyabrata01@yahoo.co.in

2. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINOUS VULCANISATION…………
1. STEAM CURING :
 METHOD OF OPEN STEAM CURING IS USED IN PRODUCTION OF
INSULATED CABLE AND WIRE. IT CONSISTS OF JACKETTED TUBES
WHICH OPERATE UNDER INTERNAL STEAM PRESSURE UPWARD
OF 200 P.S.I.
 IN OPERATING A CLOSED CHAMBER STEAM VULCANISER, THE
CURING CYCLE CONSISTS OF A RISE TO THE PREDETERMINED
PRESSURE, A DEFINITE PERIOD AT THAT PRESSURE AND A BLOW
DOWN TO ATMOSPHERIC PRESSURE. TOO SLOW RISE OR
PRESSURE FLUCTUATION MAY CAUSE POROSITY ETC.
 IN CONTINOUS VULCANISATION PROCESS, THE EXTRUDED WIRE
OR CABLE COVERING GOES DIRECTLY FROM THE TUBE INTO THE
STEAM AT CURING PRESSURE AND PASSES THROUGH A WATER OR
CONDENSATION SEAL PRIOR TO EMERGING FROM THE TUBE AT
ATMOSPHERIC PRESSURE.
P-1 …………CONTD……..

3. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINOUS VULCANISATION…………
2. FLUIDISED BED 1. STEEL TANK
2. POROUS CERAMIC
TILES
3. ELECTRICAL
HEATING
ELEMENT
4. GAS INLET PIPE
5. INLET FOR STEAM,
AIR & INERT GAS.
6. DRAIN VALVE
7. TANK FILLED
WITH GLASS
BEADS
8. EXTRUDER HEAD
9. HAUL-OFF
10. EXTRUDATE
11. EXHAUST SYSTEM.
P-2

4. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINOUS VULCANISATION…………
 THE SYSTEM CONSISTS OF A METAL TROUGH INSIDE WHICH
IS LAID A FLOOR OF POROUS CERAMIC TILES. JUST ABOVE
THE TILE HEATING ELEMENTS (e.g., 52 KW FOR A 42’ BED)
ARE MOUNTED WHICH IS COVERED TO A DEPTH OF ABOUT
4” BY SPHERICAL GLASS BEADS (0.1 – 0.2mm dia).
 AIR IS PASSED UPWARDS THROUGH THE MASS OF
REFLFECTIVE GLASS BEADS AT A VELOCITY IN ORDER TO
SUSPEND THE GLASS BEADS; - THUS GIVING A FLUIDISED
STATE.
 THE RUBBER EXTRUDATE IS THEN PASSED THROUGH THE
FLUIDISED BED BEING PRACTICALLY IMMERSED AND
CONTINUALLY SPRAYED BY THE BOILING ACTION OF THE
GLASS BEADS. THE LENGTH OF THE UNIT DEPENDS UPON
THE SPEED OF VULCANISATION, AREA AVAILABLE AND THE
TYPE OF PRODUCT TO BE CURED.
P-3 ………CONTD…………

5. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINOUS VULCANISATION…………
 EXAMPLE :-
AN UNIT OF 42’ LONG IS MOST COMMON FOR POLYCHLOROPRENE
CELLULAR EXTRUDATES. OUTPUT IS 42’/MIN., i.e., CURING TIME IS 1
MIN. AT A TEMPERATURE OF 2250C. (UPTO 2400C TEMPERATURE CAN BE
MAINTAINED).
 THIS PROCESS IS EXCEPTIONALLY GOOD FOR MANUFACTURE OF
CELLULAR PROFILES DUE TO ABSENCE OF DEFORMING STRESSES.
 CERTAIN POLYMERS OXIDISE AT THE HIGH CURING TEMPERATURE IN
CONJUNCTION WITH THE HOT AIR USED IN THE BED.
POROSITY :-
 DUE TO ABSENCE OF PRESSURE DURING CURE, THE ENTRAPPED AIR,
REMAINING IN THE EXTRUDATES, EXPANDS WHILE CURING AND
CAUSES CONSIDERABLE POROSITY AND DIMENTIONAL INSTABILITY,
LOW STRENGTH PROPERTIES AND POOR APPEARANCE.
 SO, FOR CONTINUOUS VULCANISATION, VENTED EXTRUDER WAS
DEVELOPED. THIS INVOLVES APPLYING A VACUUM TO THE HOT
(ABOVE 900C) COMPOUND TO ELIMINATE ENTRAPPED AIR, MOISTURE
AND VOLATILE MATTERS. VENTED OR VACUUM EXTRUDER CONSISTS
OF A WARMING-UP ZONE, A DEGASSING ZONE AND A COMPACTING
ZONE. GREAT CARE SHOULD BE TAKEN IN CONTROLLING THE
TEMPERATURES OF THE ZONES ALONG THE BARREL.
P-4 ………..CONTD………

6. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINOUS VULCANISATION…………
3. L.C.M. (LIQUID CURING MEDIUM) OR MOLTEN SALT BATH :
A STAINLESS STEEL OR MILD STEEL TANK (30 – 40’ LONG)
CONTAINS THE SALTS. IT IS HEATED UNIFORMLY BY MEANS
OF INFRA-RED HEATERS (72 KW FOR 40’ TANK) AND TEMP. IS
CONTROLLED ACCURATELY BETWEEN 200 – 3000C.
THE UNIT IS FITTED WITH ONE OR TWO STAINLESS STEEL
BELTS DEPENDING ON THE UNIT’S LENGTH WHICH
TRANSPORTS THE EXTRUDATE THROUGH THE MOTEN SALTS.
UNDER SUDDEN STOPPAGE (DUE TO POWER FAILURE OR
MECHANICAL FAULTS ETC.) THE BELTS AUTOMATICALLY LIFT
THE PRODUCT CLEAR OF THE SALTS.
THE COMPOSITION OF SALTS IS :-
POTTASIUM NITRATE – 53%
SODIUM NITRITE – 40%
SODIUM NITRATE – 7%
IT HAS MELTING POINT OF 1420C AND DENSITY OF 1.9 AT
2000C.
P-5 …………..CONTD…………

7. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINUOUS VULCANISATION…………
ON LEAVING THE BATH, THE EXTRUDATE IS COATED WITH A FINE
LAYEROF MOLTEN SALT. THIS IS REMOVED IN A HOT WASH. AT THE
END OF THE LINE, THERE IS A HAUL-OFF WHICH CONTROLS THE
SPEED OF THE EXTRUDATE.
THE OXIDISING NATURE OF THE SALTS REPRESENTS A SEREOUS
FIRE HAZARD. ANOTHER DISADVANTAGE IS THE UNNECESSARY
REMOVAL OF SALT BY WASHING IN HOT WATER AFTER CURING.
OPERATING HAZARDS :- EXTREMELY SOPHISTICATED FIRE
EXTINGUISHING APPARATUS HAS TO BE FITTED TO THE SALT BATH
UNIT TO ELIMINATE RELEASED ISOTHIOCYANATE FUMES BY
EXTRACTION PROCESS, WHICH CAUSE SEVERE IRRITATION TO THE
EYES OF THE OPERATORS. ALSO, EXTRUDATES WHICH HAVE BEEN
VULCANISED ARE PLACED IN A WELL VENTILATED ROOM FOR AT
LEAST 24 HRS.
P-6 ………..CONTD……….

8. PRESSURISED LIQUID CONTINUOUS VULCANISATION (PLCV)
THIS PROCESS IS SAME AS LCM , BUT THIS IS UNDER PRESSURE OF ABOUT
2 atmosphere. (1 atmosphere = 1 bar = 1.0133 x 105 Pascal = 0.103 MPa)
THIS METHOD IS ONLY USED FOR MANUFACTURING CABLES .
PRESSURISATION HELPS TO REMOVE / REDUCE THE POROSITY AND
UNIFORMITY OF THE PRODUCT.
P-7

9. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINUOUS VULCANISATION…………
4. MICROWAVE CURING :-
THIS IS BASED ON HIGH FREQUENCY HEATING OF RUBBER. THE
SYSTEM CONSISTS OF A VULCANISING TUNNEL THROUGH WHICH
A GLASS FIBRE BELT RUNS. THE MAGNETRONS ARE MOUNTED
WITH THE SUPPORT FRAME. A WEDGE GUIDE CENTERING
SYSTEM IS ALSO FIXED TO ENSURE THAT THE PROFILE TRAVELS
IN THE CENTRE OF THE ELECTRICAL FIELD. THE LENGTH OF
THE TUNNEL AND MAGNETRON SIZE CONTROL THE HEAT
INPUT.
DUE TO THE NATURE OF MICROWAVE HEATING WHICH HEATS
FROM THE INSIDE OUTWARDS, THE THINNER SECTIONS OF THE
CROSS-SECTION EXTRUDATES REMAINS UNDERCURE/UNCURE.
SO, IT IS NECESSARY TO USE SUPPLIMENTARY HEATING IN THE
FORM OF HOT AIR OR INFRA-RED AT THE END OF THE UNIT TO
ENSURE COMPLETE VULCANISATION.
THE PROCESS IS COMPLETELY CLEAN, NO PREHEATING IS
NEEDED AND THICKER SECTIONS CAN BE CURED FASTER. THE
SYSTEM IS BEST USED FOR THE POLAR MATERIALS. NON-POLAR
MATERIALS HAVE TO BE COMPOUNDED VERY CAREFULLY.
P-8 ………….CONTD…………

10. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINUOUS VULCANISATION…………
THEORY OF MICROWAVE HEATING :- THE RUBBER IS
HEATED BY PLACING IT IN A ELECTROMAGNETIC ALTERNATING FIELD.
POLAR MOLECULES IN THE COMPOUND ARE EXITED. THE RATE OF
HEATING IS DEPENDENT ON THE DIELECTRIC PROPERTIES OF THE
COMPOUND AND THE FREQUENCY OF THE ALTERNATING FIELD. IN
MICROWAVE HEATING, THE FREQUENCY IS 2450 Mc/SEC. USUALLY USED;
WHEREIN DIELECTRIC HEATING 300 – 1000 Mc/SEC. IS USED.
DIELECTRIC HEATING :- IN DIELECTRIC HEATING, ENERGY IS
APPLIED TO A PREFORMED BLANK OF RUBBER COMPOUND BY PLACING IT
IN BETWEEN TWO PLATES OF ELECTRODES. 13000 VOLTS ACROSS THE
ELECTRODES IS APPLIED. MORE POLAR THE COMPOUND, FASTER IS THE
HEATING.
POWER (KILOWATT) = 4.2 X K X W X T X SH
HEATING TIME IN SEC.
WHERE, K = CONSTANT, DEPENDS ON DIELECTRIC
PROPERTIES OF THE COMPOUND.
W = WEIGHT OF THE COMPOUND IN Kg.
T = TEMPERATURE RISE.
SH = SPECIFIC HEAT OF THE COMPOUND.
P-9

11. PRIYABRATA GHOSH, IRI, KOLKATA
CONTINUOUS VULCANISATION…………
COMPARISION OF MICROWAVE AND L.C.M. METHODS
MICROWAVE L.C.M.
COMPOUND MUST BE SUFFICIENTLY
POLAR.
CUSTOMERY MIXES WILL DO . BUT FASTER
CURING AGENTS SHOULD BE ADDED.
RELATIVELY THICKER SECTIONS ARE
CURED FASTER AND MORE UNIFORMLY,
BECAUSE HEAT IS GENERATED INSIDE THE
MATERIAL.
COMPARATIVELY SLOWER; NARROW
STRIPS AND THIN HOSES ARE CURED
QUICKER.
INTRICATED SHAPES ARE NOT DISTROTED
AS LONG AS ENERGY IS ABSORBED.
MAY DEFORM COMPLICATED SHAPED
PRODUCTS WITH FEED BELTS AND IN
CONTACT WITH FLUID (BUOYANCY
EFFECT)
CLEAN PROCEEDS AS WELLAS SURFACE OF
THE PRODUCT.
CONTAMINATION ON THE PRODUCT; SALTS
ARE WASHED OFF. INTRICATED SHAPES
MAY TRAP CONTAMINATION.
POWER EFFICIENCY IS BETTER. (ALL
ENERGY GOES IN THE PRODUCT)
HEAT LOSSES BY RADIATION;
VENTILATION SYSTEM IS REQUIRED.
NO FIRE PRECAUTIONS ARE NEEDED. THE OXIDISING NATURE OF SALTS
REPRESENTS A SERIOUS FIRE HAZARD.
NO LOSS EXCEPT ENERGY. REMOVAL OF EXCESS SALT BY WASHING IN
HOT WATER AFTER VULCANISATION.
END