PVC is converted in to final products by various processing techniques. A brief discussion of these processes follows:
EXTRUSION
Extrusion is defined as forcing a material through a die orifice.It is used to process plastic products with a continuous Cross section such as pipes , drinking straws, profiles ,sheets , tubes, rods and similar products. The plastic dry blend or compound is fed into the machine hopper (known as extruder). A rotating screw inside a heated barrel, getting softened by friction and heat, conveys this continuously. The softened plastic is then forced out through a die directly into cold water where the product is solidified.
From here it is conveyed onward into the take-off rollers. The shape of the die orifice determines the shape of the extruded article such as pipe, profile or a rod. The extrusion process is also the most used process for the production of PVC compound used as feed for some extrusion and injection moulding processes.
Blow moulding is a process for production of hollow articles. In general there are three types of blow moulding techniques :
1. EXTRUSION BLOW MOULDING :In this process, the PVC compound is melted and extruded into a hollow tube , commonly known as parison. The parison is captured by closing it into a cooled metal mould . Air is then blown into the parison inflating it into the shape of the desired hollow article. The article is ejected on cooling.
PRODUCTS PRODUCED : BOTTLES , ALL TYPES OF CONTAINERS.
2. INJECTION BLOW MOULDING :The technique is used to prepare single serve hollow plastic articles in bulk quantities. The polymer is injected into a core pin . The core pin is then rotated to a blow moulding station. to be inflated and cooled.
PRODUCTS PRODUCED : SMALL MEDICAL AND SINGLE SERVE BOTTLES
3. STRETCH BLOW MOULDING :The PVC compound is melted into a preform through an injection process. These performs are produced with necks and threads of the bottle on one end.The preforms are fed, later into an extrusion blow moulding machine where they are heated above their glass transition temperature with I.R heaters and then blown using high pressure into bottles using metal blow moulds.
PRODUCTS PRODUCED : BOTTLES , ALL TYPES OF CONAINERS
BLOWN FILM
The blown film production technique is used for manufacturing polymer films for various end-uses. The technique employs an extruder with a circular die, an air blowing mechanism attached with take-up rolls.
The plastic compound is fed in pellet form into the extruder hopper from where it is conveyed Forward by rotating screws under the impact of heat & friction and gets softened to a mouldable form. The softened plastic is then forced upwards through a circular die into the shape of a hollow tube. The tube Is continuously produced, expanded with air above the die in the shape of a bubble and collapsed by take-off Rolls.
The amount of air trapped in the bubble, the speed of the nip roller, and extruder out-put rate all Play a roll in determining the thickness and size of the film. The tube is continuously wound on the take off Rolls or fed directly into an on-line bag-making machine.
PRODUCTS PRODUCED: WRAPPING FILMS ,BAG LINERS,GROCRY BAGS, GARBAGE BAGS, SPL. OF GROCERY.
INJECTION MOULDING
This is one of the most common methods of shaping PVC and other polymers. The process of moulding is called injection moulding. The PVC compound in the form of dry blend or pellets is fed into the machine through the Hopper. The resin enters the injection barrel by gravity through feed throat.Upon entering into the barrel, the Resin is heated to the appropriate melt temperature. The molten polymer is injected into the mould by a reciprocating screw or a ram injector. The screw Injector is more suited for smaller parts. The mould receives the molten plastic and shapes it appropriately. The mould is cooled constantly to allow the mass to solidify and be cool to touch. Mould is opened and the moulded product is ejected. The injection moulding technique can be used for both rigid as well as plasticized PVC.
PRODUCTS PRODUCED :
RIGID PVC : PIPE FITTINGS AND SOME SPECIALITY ELECTRONIC COMPONENTS
FLEXIBLE PVC : FOOTWEARS , INSERTS
CALENDERING
Calender is a set of heated rolls to which pre compunded PVC stock (UPVC or PPVC) is fed to be rolled out Into a continuous sheet as it passes through a set of consecutive roll nips.it can be used to process rigid as well as flexible PVC .Calendering is considered to be the most sophisticated and versatile conversion technique among all vinyl processing techniques.
Calenders are generally four or five rolls arranged as L-type or inverted L-type shape.The later are more popular and most commonly used by the industry.
Sheets ranging from 2-50 thousandth part of an inch can Be produced on calender rolls. Sheets produced on calender rolls are much superior to those produced through Extrusion process.
In calendering process , first a dry blend is prepared in a mixer and then this dry blend is gelled either in the extruder or a two roll mill or both . Now , this gelled material is passed through the rolls of the calender to form a film or sheet.The thickness of the sheet is determined by the gap between the rolls. Temperature of the rolls is kept between 180-205 º c. Film /sheet thus formed is taken off by another roll and wound. The speed and temperature of rolls are optimised such that the desired quality of product is achieved.
PRODUCTS PRODUCED:
RIGID PVC: THEMOFORMED PACKS AND CONTAINERS (BLISTER PACKS,PACKS AND TREYS ) LINING & TRIMS FOR VEHICLES,DISPLAY SIGNS, VENETIAN BLINDS
A plastisol is liquid or a semiliquid suspension of emulsion grade PVC in plasticizers . Various additives like Stabilizers, Kickers, Colors, viscosity modifiers, blowing agents, fillers and others as required for processing and product properties are also present in the plastisol. The plastisol can be processed into end products through a number of techniques such as spread coating, dip coating and moulding.
1. KNIFE COATING : This technique is generally used for production of artificial leather, rexine and insoles etc. The process has three stages:
TOP COAT
MID COAT OR FOAM COAT
BASE COAT
Different plastisols are prepared for different coats. Coating is generally done by a specially designed knife.Top Coat is applied first followed by middle coat and base coat top coat is applied on release paper and passed through an oven. To carry out pre-gelaton of plastisol. The temperature of the oven is kept between 150- 160 º c. Foam coat is then applied and passed through a series of ovens here complete gelaton and foaming takes place. The temperature profile here is kept between 170-210 ºC. The temperature profile is direct or reverse. At the end of the process Leather cloth is separated from the release paper and the release paper is reused for next operation. In plastisol processing, choice of heat stabilizers and kickers is very critical as it determines and controls the Product quality and line speed.
2. DIP COATING : When the plastisol coating becomes a functional part of the mould itself, the process is called Dip coating, the metal substrate may or may not require a primer coat.
HOT DIP COATING : Hot dip coating is by far one of the most common plastisol processing technique. Tthe inherent heat gelling characteristics of the plastisol cause the coating to Gel on the hot object. The thickness of the coating depends on the mass of the part , the preheat Temperature and the time the hot object is allowed to remain in the dip tank. Plastisol viscosity does not play any roll in deciding the coating thickness.
The subsequent fusion of the coating is carried out to obtain maximum physical performance of the Coating.
PRODUCTS PRODUCED: COATED TOOL HANDLES AND SIMILAR PRODUCTS
COLD DIP COATING : Cold dipping does not require pre-heat of the metal part. The amount of plastisol pick-up by The metal part depends largely on the viscosity and thixotropic ratio of the plastisol. In cold dipping a subsequent fusion step is always required.
Products produced : coated tool handles and similar products
3. MOULDING :
SLUSH MOULDING: The moulding is used to produce hollow flexible items by filling a female mould with plastisol, Heating sufficiently to gel a layer of plastisol on the inner mould surface and then draining the excess plastisol. The gelled layer is then subsequently fused and stripped from the mould.
PRODUCTS PRODUCED : OPEN PARTS SUCH AS DOLL HEADS, INTERIOR AUTOMOTIVE PARTS.
ROTATIONAL MOULDING : Rotational moulding is a process whereby hollow flexible or rigid forms having complex shapes can be moulded efficiently. The process is invariably used for producing closed parts such as play balls. However, open parts can also be produced either by insulating select areas of the mould or by trimming fused material in selected areas of the finished part. Rotational moulding consists of two part mould filled with pre-determined weight of plastisol, inserted into a heated oven and rotated on the mould wall in two planes simultaneously.
PRODUCTS PRODUCED: CLOSED OBJECTS LIKE PLAY BALLS.
DIP MOULDING : Dip moulding refers to the process of dipping a male mould , or a mandrel, gelling a plastisol Layer, fusing the gelled plastisol and then stripping the hollow part .
PRODUCTS PRODUCED: GLOVES , BALLOONS ETC
OPEN MOULDING : Open moulding is a process of moulding directly in or onto a finished article such as Automotive air filters. After pouring a room temperature plastisol into a room temperature Mould, the part is fused.
PRODUCTS PRODUCED /APPLICATIONS : AUTOMOTIVE AIR FILTER, BODY UNDERCOATS AND SIMILAR APPLICATIONS.
