It is a process in which a hot mass of polymer
is fashioned into a continuous sheet by passage
through a system of heated rolls that is a calender.
The sheet is then cooled and wound up into rolls.
It may be embossed with patterns before winding
or it may be deposited on to a continuous web
e.g. fabric, paper to form an adherent coating.
Calendering process is used to manufacture unsupported
films/sheets either flexible or rigid and transparent
or coloured opaque. It is also widely used to
produce laminated products like leathercloth or
The machine comprises of an arrangement (stack)
of rolls mounted on bearings supported by side
frames and supported with roll drives and heating
arrangements. Commonly used calenders are four-roll
machines whereas five-roll versions are used for
special products such as rigid thin sheeting.
In this cases the additional nip helps in improving
the surface finish.
PVC is first compounded with appropriate additives
in blender's or high speed mixer. This PVC compound
is then fed either to a Banbury mixer or a Buss-Kneader
for proper homogenisation of the compound. After
this the compound is then passed through a two-roll
mill. The small compounded sheets emerging out
of the two-roll mill, are then feed to a metering
extruder or a stainer. The function of a stainer
is to continuously feed the calender at a controlled
rate and to filter out foreign particles.
The material is fed from the strainer to the
nip of the calender. The conveyer connecting strainer
and the nip of calendering machine is fitted with
metal detectors, to prevent entry of metal objects
into system and thus prevent damage to the calender
The material entering from the nip gets transformed
into a continuous sheet. This sheet is progressively
pulled through a series of rolls to resurface
each of the two sides of the sheet. The different
configurations of calendering rolls are as follows:
SELECTION OF CALENDERING CONFURIGATION
- Temperature range requirement
- Nip clearance
- Chemical characteristics of the polymer and
- Surface characteristics of sheet
The inverted " L" configuration is commonly used
in PVC calendering because
- it provides good visibility and accessibility
of all the rolls in the stack
- imparts good rigidity to the sheet because
of the vertical super imposition of the four
rolls for the following
- it provides an excellent surface contact to
both the side of sheets
- ease in carrying out thickness corrections
RANGE OF PRODUCTS
- overwraps, shrink films, blister films
- clear, translucent rigid sheets for bliste
- opague flexible film for liners and heavy
- clear film for blood bags
- printed and/or embossed flexible sheets for
curtain, table-cloths, place mats, mattresses
& bed covers
- printed flexible sheets for rainwear, baby
- flexible sheets as liners for canals, silos
and silo covers
- flexible sheets for covering machines and
In this process, the polymer is melted so that
it can be introduced under pressure into the cavity
of a mould. Unlike extrusion, injection is a discontinuous
process that produces articles in their final
shape (more complex than is possible with the
extrusion process). The articles produced may
be rigid or soft, compact or cellular. Blow molded
vinyl container range from wide mouth jars to
bottles with molded-in handles.
An injection cycle can be broken down into several
- In the first stage the correct quantity of
polymer is heated to the required temperature
to obtain the desired flow characteristics;
- In the injection stage the molten polymer
is injected into the mould;
- The polymer must be sufficiently fluid to
avoid problems at this stage.
- Finally the item is cooled and solidifies
to the shape of the mould.
|MARKET TRENDS AND POTENTIALS
||ELECTRICAL CONDUIT FITTINGS
||ELECTIRCAL & TELECOMMUNICATIONS
||CONSUMER HOME ENTERTAINMENT
INJECTION MOULDING MACHINE SPECIFICATIONS
- Suitable short weight capacity
- Suitable injection pressure capacity
- Clamping tonnage required should be adequate
- Enough daylight availability
INJECTION MOULDING CYCLE
- Mould gets closed
- Injection of molten polymer
- Hold on period to prevent volumetric shrinkage
- Screw suck back
- Refilling of next shot/cooling period
- Mould opens
- Article is ejected
PVC RESIN FOR INJECTION MOULDING
Generally low K-value is preferred for injection
moulding. K-values ranging from 50 to 60 exhibit
good processability due to higher flow ability
incontours of injection moulds. High shear viscosity
of such resins is very low which is suitable for
injection moulding process where very high shear
forces are generated.
Pre-hardened, nitrided or stainless steel tooling
are recommended for PVC> Moulds should have sufficient
draft (0.5 deg. c.) per cavity for easy release.
Mould shrinkage is very low usually 0.004 to
0.006 cm/cm for rigid components.
To prevent localised brining and short shots,
moulds should be vented to allow release of trapped
air and other gases.
Sprue- busing should have 2.5 deg. included angle,
entry to sprue should be larger than nozzle exit
diameter of scrue. Heated surue bush is not recommended
Runners should be full-round type and more than
6 mm in diameter to prevent pressure losses. Runner
layout should be balanced for preventing overpacking
of few cavities.
Cold slug well is important in PVC processing
since cold material from the tip of the barrel
flows in first. If it fills the cavity it may
give rise to surface defect. Hence, it is allowed
to flow into the well opposite the runners.
Gates should be polished smooth with all rough
edges and sharp corners removed. Land length should
be as short as possible.
A short weight of 60 to 75% of barrel capacity
rated in PVC is recommended. This will minimize
melt residence time in the barrel enabling processing
at higher temperatures.
Clamp capacity of about 2 to 3 tons/inch 2 projected
area is recommended.
Screws with a compression ratio of 1:5:1 to 2:1
and L/D ratio of 16:1 to 24:1 are recommended
The screw should be deep nitrided and plated
to prevent chemical attack.
A screw tip of the smear-head design is preferred
to minimize shear heating and hang-ups.
Nozzle should be as short as possible.
A separate heater is recommended with separate
control. A reverse taper nozzle is preferred to
break off sprue at back in nozzle.
Normally compounds are not pre-dried but if exposed
to high humidity can result into part blemishes,
inconsistent cycle, localized burning, and power
weld-line strength, silver streaking at the gate.
If required, pre-dry compound at 60-65 deg. c
for 1 to 2 hours.
Moulds should have good water circulation for
good productivity and optimum appearance. Water
temperature of 21 to 38 deg. c. are recommended.
Generally, temperatures around 196 deg. c. are
recommended. Beyond 216 deg. c, the compound may
degrade. Generally, a flat profile is set on heaters
across the barrel length.
A rotating speed of 40 to 50 rpm is satisfactory.
Very high rpm can lead to shear heating and degradation.
Slow to medium injection speeds should be utilized.
If excessive speeds are used, frictional heat
will burn the material.
INJECTION AND HOLDING PRESSURES
Injection pressure just sufficient to fill the
cavity should be used till screw fills the cavity.
Then it should be brought down to 1/3rd to 2/3rd
during holding phase. Sink marks opposite to gate
indicate need for more injection pressure and
sink mark near gate need for increase hold pressure.
Shut-off nozzles are not recommended since material
tends to hang up. Barrel is generally retracted
immediately after hold pressure phase and suck-back
is taken. Material is allowed to drool from open
nozzle during refilling stage.
|PVC polymer (K-value 55-60)
|Stabiliser: Tin mercaptide
||1.0 - 2.0 phr
||2.0 8.0-10.0 phr
|Processing aid-acrylic type
||3.0 1.5 -2.5 phr
|Internal/External: Calcium Stearate
| Fatty acid easter type
|External : Polyethylene wax
|Pigment: Titanium dioxide
|Colourant: Pthalocyanine type
TESTING OF PIPE FITTINGS
As per BIS, following standards have been recommended
for testing of pipe fittings:
IS 7834 Injection moulded PVC socket fittings
with solvent cement joints for water supplies
IS 10124 Specifications for fabricated PVC fittings
for potable water supplies
Main tests under these standards are:
STRESS RELIEF TEST
In this test, fittings are heated in air-circulating
over for one hour at 150 deg. c + or - 4 deg.
C. They are allowed to air-cool and examined subsequently
In this test, the amount of extractables from
water in contact with fittings for 48 hours have
Lead - 1.0 mg./ltr in first extraction (1 ppm)
- 0.3 mg/ltr in third extraction (.3 ppm)
Dialkyl tin C4 - 0.02 mg/ltr (0.02 ppm)
Other toxic substances - 0.01 mg/ltr (0.01 ppm)
SHORT TERM PRESSURE TEST
In this test, fittings are subjected to 4.2 times
nominal pressure for 1 hour and subsequently examined.
There should no failure.
COLOUR FOR FITTINGS
|Class2 (0.4 Mpa)
|Class3 (0.6 Mpa)
|Class4 (1.0 Mpa)
Extrusion is the process in which hot, soft plastic
is squeezed through a hole of the correct dimensions.
It hardens as it cools. Used for making continuous
lengths like plastic sheets or pipes.
Extrusion is a process in which the polymer resin
in the form of granules or powder is first converted
into a homogeneous melt, which is subsequently
passed through die to give products of desired
With extrusion, long lengths of product are made
by heating vinyl compound until it is softened
or melted, and forcing it through a uniquely shaped
opening or die. Extrusion permits high production
rates for large-volume products like house siding,
pipe, and wire and cable coverings. Growing markets
for extruded vinyl products include window frames,
vertical window blinds, medical tubing, and gutter
and downspout systems.
Extruder is made up of a cylindrical hallow barrel
which is heated by electrical heater's. The barrel
houses a screw having helical threads running
across the length of the barrel. Design of the
screw and barrel is based on the polymer being
processed and the performance requirements of
The screw is connected to a reduction-gear box
and is supported by thrust bearings, which are
designed to take high back pressure that is generated
in the barrel. The reduction-gear box is driven
by A.C or D.C electric motor.
The polymer resin is added to one end of the barrel
through a hopper. It is gradually conveyed across
the barrel by the rotational action of the screw.
As the polymer progresses along the screw it is
transformed from solid state to a semi-solid state
because of the external heat as well as the frictional
heat that is being generated due to the rotation
of the screw within the barrel.
The screw can be divided into three major
Feed: It picks up the cold polymer material from
the hopper and feeds it to the compression zone
Compression/Transition: Transition of material
from solid form to molten form takes place because
of heat and shearing action of melt in this section.
Metering: It acts like a metering pump and delivers
the melt to die, at a constant rate
It serves as a connection between the extruder
and the die. It houses the breaker plate and screen
Breaker plate: It breaks the helical flow
of polymer melt formed inside the barrel into
Screen pack: Filter out foreign particle
or contamination from the melt and also helps
in increasing the back pressure within the barrel
Shape and size of a product to be manufactured,
is determined by the shape of the die. The polymer
melt is passed through an annular (blown film)
or flat slit (cast film/sheet extrusion) opening,
the squeezing action on the melt orients the polymer
chains in one direction. This orientation enhances
the properties of the product .
BLOWN FILM CIRCULAR DIE:
FLAT DIE (CAST FILMS/ SHEET)
POST-EXTRUDER: DOWNSTREAM EQUIPMENT
Unlike molding process, extrusion is a continuous
process giving two-dimensional products. Hence
there is a need of downstream equipment's, like
calibration unit, cooling unit, take-up unit and
a winding/stacking unit.
The extrusion products can be broadly divided
into following area's of application, each having
its own distinct technology for manufacturing:
- Fiber & Filaments
- Wire & Cable
- Compounded pellets