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  1. Calendering

  2. Injection

  3. Extrusion

  4. Blowing



















Calendering

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 tablecloths.

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 rolls.

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
  • Width
  • Nip clearance
  • Chemical characteristics of the polymer and additives
  • 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

Packaging

  • overwraps, shrink films, blister films
  • clear, translucent rigid sheets for bliste packing
  • opague flexible film for liners and heavy duty sacks
  • clear film for blood bags

House Furnishings

  • printed and/or embossed flexible sheets for curtain, table-cloths, place mats, mattresses & bed covers

Clothing

  • printed flexible sheets for rainwear, baby diapers

Agriculture

  • flexible sheets as liners for canals, silos and silo covers

Industrial

  • flexible sheets for covering machines and office equipments

INJECTION MOULDING

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 stages :

  • 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
RIGIDS SEMIRIGIDS/FLEXIBLE
A PIPEFITTINGS A FOOTWEAR UPPERS
B ELECTRICAL CONDUIT FITTINGS B FOOTWEAR SOLES
C BUSINESS MACHINES C LUGGAGE DECORATIONS
D APPLIANCES D THERMOWARE/HOUSEHOLD DECORATIONS
E ELECTIRCAL & TELECOMMUNICATIONS    
F CONSUMER HOME ENTERTAINMENT    
G MEDICALS    
H SPECIALITY PLUMBING    
I FURNITURE    

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.

MOULDS

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 for PVC.

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.

EQUIPMENT

MACHINE SIZE

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.

SCREW DESIGN

Screws with a compression ratio of 1:5:1 to 2:1 and L/D ratio of 16:1 to 24:1 are recommended for PVC.

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

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.

PROCESSING

DRYING

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.

MOULD TEMPERATURE

Moulds should have good water circulation for good productivity and optimum appearance. Water temperature of 21 to 38 deg. c. are recommended.

BARREL TEMPERATURE

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.

SCREW RPM

A rotating speed of 40 to 50 rpm is satisfactory. Very high rpm can lead to shear heating and degradation.

INJECTION SPEED

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.

SCREW SUCK-BACK

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.

TYPICAL FORMULATIONS

PVC FITTING
PVC polymer (K-value 55-60) 100
Stabiliser: Tin mercaptide 1.0 - 2.0 phr
Impact Modifier-MBS/ABS 2.0 8.0-10.0 phr
Processing aid-acrylic type 3.0 1.5 -2.5 phr
Lubricants:
Internal/External: Calcium Stearate 0.8-1.0 phr
Fatty acid easter type 0.5-1.0 phr
External : Polyethylene wax 0.05-1.0 phr
Pigment: Titanium dioxide 3.0 phr
Colourant: Pthalocyanine type 0.02-0.03 phr

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 for defects.

EXTRACTION

In this test, the amount of extractables from water in contact with fittings for 48 hours have been restricted.

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) Blue
Class3 (0.6 Mpa) Green
Class4 (1.0 Mpa) Yellow

Extrusion

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 shape.

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

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 extruder.

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 zones:

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

ADAPTER

It serves as a connection between the extruder and the die. It houses the breaker plate and screen pack.

Breaker plate: It breaks the helical flow of polymer melt formed inside the barrel into linear flow

Screen pack: Filter out foreign particle or contamination from the melt and also helps in increasing the back pressure within the barrel

DIE

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.

EXTRUDER PRODUCTS:

The extrusion products can be broadly divided into following area's of application, each having its own distinct technology for manufacturing:

  • Films
  • Sheets
  • Pipes
  • Profiles
  • Coatings
  • Fiber & Filaments
  • Wire & Cable
  • Compounded pellets