This document discusses different molding techniques for rubber compounds: - Compression molding involves placing rubber in a heated mold cavity and applying pressure. Flash and semi-positive molds are discussed. - Transfer molding uses pre-heated rubber that is forced through holes into mold cavities. It allows for more complex shapes than compression molding. - Injection molding automatically feeds, heats, and injects rubber into a closed, heated mold. It produces consistent, high-quality parts at high speeds but requires specialized rubber compounds.

1. BY
PRIYABRATA GHOSH
IRI, KOLKATA

2. PRIYABRATA GHOSH, IRI, KOLKATA
MOULDING TECHNIQUES
 THREE BASIC TYPES OF MOULDING :-
a) COMPRESSION MOULDING :
THE RUBBER BLANK IS PLACED DIRECTLY INTO THE CAVITY OF
MOULD WHERE IT IS HEATED BY CONDUCTION AND FORCED TO
FLOW BY APPLICATION OF PRESSURE.
b) TRANSFER MOULDING :
IT USES PRE-WARMED RUBBER WHICH IS HEATED DURING TRANSFER
AND FORCED THROUGH HOLES INTO MOULD CAVITIES IN A THREE
PART MOULD.
c) INJECTION MOULDING :
IN THIS PROCESS, RUBBER COMPOUND IS PUSHED UNDER PRESSURE
FROM AN INJECTION HEAD WHERE IT HAS BEEN HEATED AND
PLASTICISED. THEN IT IS INJECTED INTO A CLOSED HEATED
MOULD WHERE CURE IS COMPLETED.
P-1

3. PRIYABRATA GHOSH, IRI, KOLKATA
THREE BASIC TYPES OF COMPRESSION MOULD. i) FLASH TYPE; ii)
SEMI-POSITIVE AND iii) POSITIVE MOULDS.
FLASH TYPE MOULDS :
 SIMPLEST MOULD IS THE STRAIGHT COMPRESSION OR FLASH
TYPE.
 IT IS LOADED BY PLACING EXCESS OF UNCURED COMPOUND IN
THE CAVITY. THEN THE LID IS CLOSED UNDER PRESSURE. EXCESS
MATERIAL IS FORCED INTO OVERFLOW CHANNELS.
 LEAST EXPENSIVE TO MANUFACTURE AND HAS THE ADVANTAGE
TO MANUFACTURE AND HAS THE ADVANTAGE OF BETTER HEAT
TRANSFER THAN OTHER TYPES.
 SUCH MOULDS ARE USED FOR PRODUCTION OF SHEETS, SLABS,
BELTING, TILING ETC.
P-2 ………CONTD………..

4. PRIYABRATA GHOSH, IRI, KOLKATA
COMPRESSION MOULDING……………
SEMI – POSITIVE MOULDS :
 CONSISTS OF TWO PARTS – A PLUNGER AND BASE.
 PLUNGER FITS LOOSELY INTO THE CAVITY AS WITH FLASH
MOULDING. PRESSURE ON THE COMPOUND IS GREATER THAN IN
STRAIGHT COMPRESSION MOULDING BECAUSE OF THE RESTRICTED
FLOW PAST THE PLUNGER TO THE OVERFLOW CHANNELS.
POSITIVE MOULDS :
VERY LITTLE USE NOW-A-DAYS.
THE PLUNGER FITS TIGHTLY INTO THE BASE. FULL LOAD IS APPLIED
DIRECTLY ON COMPOUND.
BLANK WEIGHT MUST BE CONTROLLED, OTHERWISE ARTICLE
THICKNESS WILL VARY.
AIR MAY ALSO BE TRAPPED WITH THE CAVITY.
DUE TO GREAT PRESSURE WHICH DEVELOPS ON THE RUBBER, GIVES
ADVANTAGE IN MOULDING BOTH EXTREMELY SOFT AND HARD
COMPOUNDS.
DISADVANTAGE IS THE LESSER HEAT TRANSFER, BECAUSE THERE IS A
GAP AT THE TOP PLATE FROM THE MOULD EXCEPT FOR THE AREA IN
CONTACT WITH THE PLUNGER.
P-3 ………..CONTD…………

5. PRIYABRATA GHOSH, IRI, KOLKATA
COMPRESSION MOULDING…… P-4

6. INVOLVES TRANSFER OF THE UNCURED COMPOUND FROM ONE
PLACE TO ANOTHER WITHIN THE MOULD.
MOULD IS CLOSED WITH THE CAVITIES EMPTY AND COMPOUND
IS PLACED IN A TRANSFER CAVITY ON THE TOP OF THE MOULD.
THE POT IS FITTED WITH A RAM OR PISTON WHICH IS INSERTED
OVER THE COMPOUND. AS FORCE OF THE PRESS IS APPLIED,
THE COMPOUND FLOWS THROUGH SPRUES/CHANNELS TO THE
CAVITIES WHERE IT IS VULCANISED.
TRANSFER MOULDING IS A COMPROMISE BETWEEN
COMPRESSION MOULDING AND INJECTION MOULDING. THIS
METHOD IS GOOD FOR MOULDING OF COMPLICATED SHAPES OR
BONDING WITH INSERTS (METALS) IN MANY WAYS.
IT ALSO PERMITS SHORTER CURE TIME BY PUTTING HIGHER
TEMPERATURES.
TRANSFER MOULDING CREATES “POT FLASH” (~ 20 – 25%) AND
THIS ADD TO MATERIAL AND LABOUR COST.
P-5

7. PRIYABRATA GHOSH, IRI, KOLKATA
TRANSFER MOULDING.............. P-6

8. PRIYABRATA GHOSH, IRI, KOLKATA
INJECTION MOULDING CAN BE DEFINED AS AUTOMATIC FEEDING,
HEATING AND PLASTICIZATION OF RUBBER COMPOUND AND
INJECTION IN MEASURED QUANTITY THROUGH A NARROW ORIFICE
INTO A TIGHTLY CLOSED HOT MOULD IN WHICH VULCANISATION
TAKES PLACE.
THE SPECIAL FEATURES OF THIS PROCESS ARE :
1) HIGH SPEED, HIGHLY AUTOMATED MOULDING TECHNOLOGY.
2) FINISHING OPERATION MINIMISED/ELIMINATED.
3) UNIFORM CURING OF VARIABLE THICKNESS COMPONENTS CAN BE
ACHIEVED WITH REDUCED CURE TIME.
4) GENERALLY MOST EFFICIENT USING SPECIALITY RUBBER
COMPOUNDS. PRODUCTS ARE CONSISTENTLY OF GOOD QUALITY
WITH HIGH DIMENSIONAL ACCURACY. THE ENTIRE OPERATION IS
EASY, CLEAN AND QUICK.
5) RUBBER COMPOUNDS MEANT FOR INJECTION MOULDING SHOULD
HAVE GOOD FLOW CHARACTERISTICS, GOOD HOT TEAR RESISTANCE
AND SHOULD HAVE NO REVERSION (MOULD CONTAMINATION) INSIDE
THE CLOSED MOULD.
6) COMPOUNDS SHOULD CURE AS RAPIDLY AS POSSIBLE.
P-7 ………….CONTD……….

9. PRIYABRATA GHOSH, IRI, KOLKATA
INJECTION MOULDING…………….
INJECTION MOULDING IS A CYCLE OF 4 STEPS :-
SOFTENING (PLASTICATION) INJECTION VULCANISATION
UNLOADING (DEMOULDING).
AUTOMATIC FEEDING OF RUBBER STRIP TO THE SCREW WHICH
ROTATES AND DRAWS RUBBER INTO THE FLIGHTS.
THE PLASTICIZED RUBBER COMPOUND PASSES THROUGH A NON-
RETURN VALVE AND IS ACCUMULATED IN THE BARREL BY
PUSHING THE PLUNGER IN THE BACKWARD DIRECTION.
THE MOULD OPENS AUTOMATICALLY AT THE END OF
VULCANISATION PERIOD AND THE PRODUCTS ARE EJECTED
AUTOMATICALLY OR REMOVED MANNUALLY.
THE MOULD IS THEN CLOSED AND THE MACHINE IS READY FOR
THE NEXT CYCLE.
P-8

10. PRIYABRATA GHOSH, IRI, KOLKATA
INJECTION MOULDING…… P-9
P-9

11. REACTION INJECTION MOULDING (RIM)
PRIYABRATA GHOSH IRI KOLKATA
 R I M WAS DEVELOPED FOR MIXING AND MOULDING LIQUID MATERIALS.
OPEN CASTING OF MIXED LIQUIDS IS COMMONLY USED BY THIS PROCESS.
 SUCH LIQUID MATERIALS REACT EXTREMELY FAST WITH EACH OTHER. SUCH
HIGH REACTION RATE REQUIRES VERY RAPID TRANSFER OF LIQUID FROM THE
MIXING CHAMBER TO THE MOULD.
 THE POLYOL BLEND AND ISOCYANATE STREAMS ARE PUT UNDER HIGH
PRESSURE (ABOUT 140 Kg / cm2).
 THE TWO STREAMS ENTER THE MIXING CHAMBER WHERE THEY STRIKE ONE
ANOTHER AT VERY HIGH VELOCITY CAUSING TURBULENCE. THIS TURBULENCE
RESULTS IN THROUGH MIXING OF THE STREAMS.
 THE MIXTURE THEN FLOWS INTO THE TUNNEL AT LOW PRESSURE. FROM
TUNNEL , THE MIXTURE FLOWS THROUGH THE GATE INTO THE MOULD CAVITY
TO FROM THE MOULDED PARTS.
 LOW FORCES ARE REQUIRED TO KEEP THE MOULD CLOSED, BECAUSE
MOULDING PRESSURE IS TYPICALLY ONLY ABOUT 3.8 Kg / cm2 OR LESS.
 THIS LOW MOULDING PRESSURE PERMITS USE OF LIGHT CLAMPS FOR R I M . IT
ALSO PERMITS MOULDING OF R I M MATERIALS IN NON – METALLIC MOULDS OR
P-10

12. PRIYABRATA GHOSH, IRI, KOLKATA
COMPRESSION TRANSFER INJECTION
1) SIMPLE MOULDING COMPLICATED MOULDING INTRICATE MOULDING
2) BLANK , WRONGSIZE CAN
CAUSE FLOW LINES, BLISTERS
AND UNFILLED PARTS.
EXCESSIVE SIZE IS WASTEFUL
AND CAUSE HEAVY FLASH
WHICH IS DIFFICULT TO
REMOVE AND MAY CAUSE
DIMENSIONAL PROBLEMS.
EXCESSIVE SIZE RESULTS IN
POT FLASH, NOT IN THE
MOULD CAVITY.
REDUCED STOCK
PREPARATION, AUTOMATIC
FEEDING.
3) MINIMAL FLOW. AMOUNT
OF FLOW INCREASED BY
APPLYING AND RELEASING
PRESSURE (BUMPING)
COMPOUND MUST FLOW
FREELY TO FILL MOULD
CAVITY PROPERLY.
VISCOSITY OF COMPOUND
SHOULD BE REDUCED TO THE
LOWEST LEVEL TO IMPROVE
FLOW.
4) COSTLY RAW STOCK
PREPARATION. NEEDS SHAPED
PREFORMS.
RAW STOCK NEEDS
PREPARATION, BUT NO
SHAPING IS REQUIRED.
SCREW UNITS MASTICATE AS
WELL AS HEATS.
5) LONG CURING CYCLE MODERATE CURING CYCLE VERY FAST CURING CYCLE.
6) MORE LABOUR TO LOAD
AND UNLOAD THE MOULD (IF
NOT FIXED). EACH CAVITY
SHOULD BE LOADED
INDIVIDUALLY.
MOULDS ARE HEAVY AND
NEEDS OPERATORS TO LOAD
AND UNLOAD.
P-11
OPERATOR ONLY UNLOADS
THE PARTS.
……..CONTD….

13. PRIYABRATA GHOSH, IRI, KOLKATA
COMPARISON OF COMPRESSION, TRANSFER AND INJECTION MOULDING.............
COMPRESSION TRANSFER INJECTION
7) FIT FOR LOW PRODUCTION MEDIUM SCALE-
PRODUCTION
LARGE VOLUME PRODUCTION
8) LEAST COST OF
PRODUCTION
COSTLIER THAN
COMPRESSION MOULDING
COST DEPENDS ON VOLUME OF
PRODUCTION
9) MOULD WEAR IS LESS MOULD WEAR IS HIGH MOULD WEAR IS HIGH
10) CAN USE INEXPENSIVE
STEEL FOR MOULD
CONSTRUCTION
MOULD COST HIGHER THAN
COMPRESSION MOULD
MOULD COST IS HIGH, IF
DAMAGED EXPENSIVE TO
REPAIR.
11) WORST FLASH CONDITION,
EXPENSIVE FINISHING
OPERATION
POT FLASH UPTO 25%,
COSTLY FINISHING
OPERATION.
NEGLIGIBLE FLASH, FINISHING
COST NEGLIGIBLE.
12) HEAT TRANSFER FROM
THE METAL OF THE MOULD TO
THE ARTICLE.
COMPOUND GETS HEATED AS
IT FLOWS FROM THE POT TO
THE CAVITY.
SCREW AND INJECTION HEAT
THE COMPOUND TO CURING
TEMPERATURE.
13) MOULDING TEMPERATURE
RANGE (1200 – 1600C)
MOULDING TEMPERATURE
RANGE (1300 – 1700C)
MOULDING TEMPERATURE
RANGE (1800 – 2400C)
14) OFTEN USED FOR HEAVY
MOULDINGS. INSERTS IN A
MOULD CAVITY TEND TO BE
DISPLACED AS THEYARE
POSITIONED IN AN OPEN
MOULD.
OFTEN USED FOR RUBBER TO
METAL INSERTS. BETTER
ADHESION OF RUBBER TO
INSERTS IS ATTAINED.
END
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