38. 1. Introduction
All the processing stages between manufacture of the crude polymer and the final molding step are called the compounding processes. These include storage, transportation, metering, mixing, plastication and granulation. Sometimes additives may be added to the polymer mix to allow production on an industrial scale. Addition of plasticizers renders flexibility and ductility to rigid and brittle polymers. In a similar way, fillers can enable to enlarge the volume and also has influence upon the polymer properties.
38. 2. Processes
The various compounding processes are –
1. Mixing: Mixing allows obtaining a homogeneous mixture of polymer and additives. The additive is distributed uniformly throughout the crude polymer, without any unnecessary stressing .Mixing can occur under the action of gravitational forces in a tumble mixer ; or in a ribbon mixer , where materials are mixed in a horizontal barrel in a spiral manner. Such mixers are collectively known as cold mixers.
Another class of mixers are the hot mixers, wherein the polymers are heated by contact with heated metal components. Such mixers are also known as high – speed or turbo mixers. This method is widely used in the plastic industry , especially in the compounding of poly vinyl chloride (PVC).
2. Rolling and Kneading: Before processing , the plastics are compacted, melted, homogenized and modified with additives. These include rolling, in which the plastic is passed between two counterrotating , heated rolls wherein it is pressed into a thin layer. It is an open process in which additives can be added, and other scrap materials may be reintroduced. PVC is one plastic which extensively employs this process. Moreover, the material temperature can be controlled precisely and cleaning of the
machine is easy.
3. Pelletizing : It is the method of producing granules of equal size and shape. This enables the optimum feed behaviour on the processing equipment. Pelletizing can be done in hot or cold conditions.
Using a pelletizing die, strands, ribbons or sheets are formed ; and after solidification, they are chopped into the desired shapes. This is one procedure strictly followed in cold pelletizing. However, in hot pelletizing , the plasticated material is sent through a die , and a blade that rotates on the surface of the die plate chops the exiting strands. The melt is then cooled ; and subsequently the water is removed.
4. Shredding and Grinding : In thermoplastics processing, a variety of semifinished and finished products are generated. These products, inorder to be made suitable for processing, must be cut to produce a pellet form. A range of particle sizes is obtained in the process.
Regranulating is achieved through cutting mills. The material is added through a feed system wherein a rotor revolves with high momentum. The system is further equipped with multiple cutting heads that operate against stationary cutting edges. The rotating cutting heads are divided and displaced with respect to each other in some other machines. As soon as the desired particle size is obtained, the material is ejected through a sieve , the pores of which determine the size of the particles. The housing and rotor may be equipped with water cooling apparatus to remove the excessive heat generated.
5. Storage and Transportation :The storage and transportation of base polymers and intermediate products are achieved through automatic equipments. These equipments have an edge over the manually operated ones as they ensure improved working conditions , lower work place , lower contamination of materials and subsequent reduction losses. Moreover, accident hazards are also reduced. Large external silos are used for the storage of polymers, either in powder form , or in the pellet form. By incorporating vibrating frames, bunker cushions, slotted shelves or similar techniques as ventilation, the formation of granule bridges is avoided.
The materials are transported via road or rail. Reserve materials and additives are transported in ships in bulk containers, drums or sacks. Pneumatic equipments are generally preferred for conveying powders and pellets. The conveyors used may be high pressure conveyors, medium pressure conveyors or low pressure conveyors. Fill level sensors help in controlling the transport interval.
Inorder to meter liquids such as plasicizers or other additives for compounding, oscillating displacement pumps are used. Metering of soilds can be done using gravimetric metering equipments or volumetric metering equipments.
38. 3. TECHNIQUES FOR PRESSURELESS PROCESSING
The Pressureless Processing Techniques are used for starting materials such as monomers, plastic solutions and dispersions that can flow and that can be formed without application of external forces. The pressureless processing
techniques include –
1. Casting (Figure 38.1) : By polymerizing monomers in molds, thick – walled articles, embeddings, sheets and blocks can be produced. These are preferentially made up of polystyrene, polyamides and polymethylmetacrylate. Inorder to avoid thermal stresses, adequate temperature regulation is required. The mold material choice is dependent upon the use and surface quality of the moldings.
Film casting is mainly used in the photographic industry for the manufacture of cellulose acetate films. In this method of casting, polymer solution, melts or dispersions are cast directly into a metal drum or a rotating belt ; and from a slot die into precipitation bath.
Slush molding enables the production of hollow articles which are made from PVC pastes. These are made using two part metal molds, which are heated and filled with the paste. The paste gels on the outermost edges forming a skin ; and subsequently the excess paste is poured off. The remaining layer is gelled in an oven and after cooling, a hollow mold is obtained.
Slush molding can be further modified into Rotational casting for the production of hollow articles, made from PVC. The only difference is that in rotational casting, the amount of PVC paste is metered to the mold, which makes the removal of excess paste and post gelation techniques redundant. The mold is rotated about two perpendicular axes, while heating, and this enables uniform wetting of the cavity surface. Roatational casting has the advantage of low investment cost over slush ,molding.
In the manufacture of thick-walled , symmetrical structures such as pipes and sockets, centrifugal casting may be used. A partially filled mold is rotated rapidly about an central axis. Centrifugal force press the material outward in the mold, and in the process, the material gets compacted.
Figure 38.1 . Casting Machine a) Motor ; b)Drive Shaft; c)Molds
2. Dipping (Figure 38.2) : Sometimes, molds or articles are dipped into solutions , dispersions, pastes, melts or powders. Later on, the adhering material on the surface is solidified by heat treatment.
For manufacturing articles such as gloves, boots etc. in which one side remains open , appositivemold is immersed and subsequently the casting is gelled. A series of identical molds mounted on dipping frames are used for the mass production of such articles. During the dipping process, degassed PVC paste placed in a vessel is raised and lowered. Sometimes, the dipping frame is moved, while the vessel is stationary. The immersion and removal stages are carried out slowly to allow the material to flow homogeneously from the mold and to prevent the introduction of air into the paste.
The gelation is usually carried out in circulating air oven, and during the process the paste temporarily becomes fluid. The molds containing the gelled coating are immersed in water for cooling. It is cooled to around 50 C and at this temperature, the moldings can be easily removed from the molds. Cables and wires are extensively coated using this process.
Figure38.2 .Dipping Unit. a)Vessel; b)PVC paste; c)Dipping frame; d)Molds
3. Coating (Figure 38.3):This process is used in the production of selfsupporting films by coating sheet materials such as paper and textiles. The most important materials that are used forthe production of coatings are
The equipment for coating consists of an unwinding station, an expander, coating head, gelling channel, cooling system, and a rewind station. The disk brakes slows down the unwind station, and the rewind station enables the product to be held under tension. A smooth finish to the sheet is provided by the expanders. With the help of the coating head, the PVC paste is spread upon the moving sheet manually or mechanically. Thereafter, a doctor blade is employed to spread the sheet to required thickness. A revolving roller supports the sheet, due to which an exactly defined contact area is maintained. The rolls distribute the escaping paste and form a film. The rotation speeds of the rolls can be adjusted.
Hot air or infrared dryers supply the heat required for gelation of the paste. However, circulating air heating in combination with infrared heaters is
used in some cases.
The cooling effect must be high enough so that no adhesion between the individual layers occurs. The cooling is attained by two water – cooled , corrosion – resistant hollow drums.
Figure 38.3 . Coating Unit
a) Unwinding Station; b) Expander; c)Coating head; d)Gelling Tunnel; e)Cooling Roll; f)Rewind Station
4. Foaming: By adding physical or chemical blowing agents to a polymer, foams are generated. Physical blowing agents include gases and low-boiling compounds , whereas ; chemical blowing agents generally are gas – releasing compounds. Only a few materials are employed in the manufacture of foams , such as polystyrene , PVC, LDPE as well as phenolic and polyurethane resins.The foaming process must begin when the polymer is in a flowable form and allows bubbles to develop. The conditions are fixed, once the bubbles reach the optimum size.
38. 4.Polymer Processing under pressure
Sometimes the molding process is carried under high pressure in which the solid thermoplastic materials are melted, molded and cooled subsequently. Some of the commonly used techniques for processing under pressure are –
1. CompressionMolding: For the manufacture of thick – walled components and those that cannot be plasticized in extruders, compression molding is used. It plays an important role in the processing of elastomers and
A press consisting of fixed and moving platens is used. The plunger and cavity components are mounted on these plates, and the parting plane is kept horizontal. The cavity is filled with the cold plastic and is melted by heating the mold under low pressure. When the melting is complete, the mold is closed and pressure applied. The molding is removed by releasing the press once the ejection temperature is reached.
Compression molding is uneconomical due to the fact that the process has a long cycle time ; as the material in themold is heated and cooled by conduction via the mold surface.
2. Rolling and Calendering (Figure 38.4) : The rolling mills consist of two rolls that are used exclusively for the compounding of thermoplastics. The product that results from rolling is a sheet that can be granulated after cooling or fed in plasticated form directly into the processing equipment. PVC and rubber are processed using this process.
When the rolls are more than to in number, and the polymer is to be shaped using these rolls; then the machine is called a calendar. The number of rolls in a calendar can be uptoseven ; however, most commonly , four rolls are used. The rolls are made of polished cast iron, and are driven individually and heated. The heating medium is usually oil or water. The temperature and speed increases from one roll to the next. The roll mill must be able to withstand the extreme forces to which it is exposed, and hence, be made
Thecalenders are one of the most expensive types of processing equipments and are employed for the manufacture of a few special products like floor coverings and films made of plasticised PVC.
Figure 38.4 .Calendering line
a) Winder and Edge Cutter; b)Cooling Rolls; c)Four-roll calendar; d)Extruder; e)Mixing Roll Mill.
3. Extrusion (Figure 38.5):This method enables the attainment of awide range of film thickness, and large scaling processing. Moreover, there is no necessity for expensive solvents in the recovery systems.
A device known as extruder is used to melt the polymer, which is given the specific shape by a die. Later on, a cooling system is used to solidify it.
At the onset , the raw material is dried using heated air or vacuum, before it is melted in the extruder. The extruders may be single – screw extruders, or twin screw extruders, the single screw extruder being the most common. The melt is filtered depending upon the desired purity, and then the filtered raw materials are sent through the slot die. The width and thickness of the film are determined by the width of the die and the die gap respectively. The die gap can be further adjusted during production.
The melt film emerging from the die is cast on to a highly polished roll. A vacuum device prevents the entrapment of air between the melt and the roller surface. The final film that is obtained is wound onto a roll after trimming off the edges. In a separate process, the film is cut into the designate width.
Figure 38.5 . Extrusion Molding
a) Extruder; b) Filter; c)Slot Die; d)Air Knife; e)Cooling Roll; f) Device for measuring film thickness; g)Tension rolls; h) Corona Treatment; i)Edge Treatment; j) Winding
4. BlowMolding (Figure 38.6): This is the most common method for manufacturing hollow articles from thermoplastics . The most simple products and articles that are made by blow molding are bottles, ventilation ducts, suit cases and fuel tanks for automobiles. Two processes are generally followed for blow molding –
a. Extrusion Blow Molding : An extruded tube or parison is elongated to reach the required length . Theparison is then enclosed by themold , which is then cut by a blade. It is then conveyed to the blowing station. In the blowing station, a blow mandrel is inserted into the mold and the actual process is subsequently carried out. It is then cooled and the finished product is removed by opening the mold. The open mold is then transferred to theposition below the extrusion die inorder to receive a new parison. The
process then continues.
b. Stretch Blow Molding : This is a special form of blow molding in which the mechanical properties are considerably improved. A high degree of orientation is introduced in the plastics by drawing near the glass transition temperature or crystalline melting points. The molding material is drawn not only in the circumferential direction, but also in the machine direction. A mechanical ram is used for drawing in the longitudinal direction. The process is carried out under low temperature with the application of high
PVC, polypropylene and poly ethyl terephthalate are some of the materials for use with this process.
Figure 38.6 . Blow Molding Process
5. Injection Molding (Figure 38.7) :TheInjection Molding technique is significant because of its ability to manufacture complex molding geometries in a single stage operation. The process is often suitable for mass production operations because of the high degree of automation. The Injection molding technique is a discontinuous process . The plastic pellets or granules are melted and injected under pressure into the moldcavity , wherein it is thermally cross-linked or solidified by cooling.
Through a runner system the hot melt prepared in the injection cylinder is injected into the cavity of the mold. The pressure is increased towards the end of the filling stage because of the increasing length of the flow path. After the injection phase, the holding pressure stage follows. It reduces the volume losses due to cooling of the melt , and thus, avoids marks and voids in the molding. After this , the molding is cooled sufficiently so that it is dimensionally stable for ejection. When the holding pressure phase is finished, the plastic is prepared for the next cycle by the plastication unit by rotating the screw. The melt is transported to the area in the front of the screw by the helical backward movement of the screw in the plastication cylinder against a back pressure. After this the molding is cooled and is mechanically removed from the mold via a handling equipment. The entire process is fully automated and is coordinated by a control unit.
Figure 38.7 . Injection Molding Machine
a) Clamping unit; b) Mold; c)Plastication Unit; d) Control Unit; e)Temperature Control Unit
Q. 1. What are the typical additives used in polymer products ?
Ans. 1.Polymer additives include materials which make and modify polymers. These include surfactants, chain transfer agents, modifiers, plasticizers, stabilizers, initiators and cross – linking agents.
Q. 2. Explain Glass Transition Temperature
Ans.2. Glass Transition Temperature of a non – crystalline material is the temperature at which the material changes its behaviour from its glassy state to a rubbery state. Glassy state refers to the state at which a material is hard and brittle; whereas , rubbery state refers to the state at which the material is
flexible and elastic.
Q. 3. Apply your knowledge in Chemical Engineering and outline one problem one may encounter in polymerization processes.
Ans. 3. There are certain compounds like styrene, which are used in the polymerization process. Styrene is an olefin, and is unstable as a pure liquid. Such compounds may explode, even in the absence of oxygen.Many polymers can be recycled, which reduces the disposal problems. But inorder to do so, the polymers must be separated from each other at first, and this process may be very difficult and expensive.
Incineration is a process which is used in many polymerization processes. But this leads to release of high amount of heat energy , which could otherwise be used for various useful purposes. Moreover, carbon dioxide is produced during incineration , which leads to global warming.
Q. 4. Match the polymer processing techniques for making various products.
Group 1 Group
a. Rubber sheet 1. Rotational
b. HDPE water storage tanks 2. Compression
c. Bicycle paddle 3. Calendering d. Glass reinforced thermosetting plastics
Q.5 Mention some of the advantages of blow molding as compared to injection molding.
Ans.5 Some of the advantages of blow molding as compared to injection molding are-
Q.6 What are the different types of injection molding machines that may be used?
Q.7 What are the different types of screw zones in a extrusion machine ?
Q. 8 In molding process, the polymer material will try to stick to the wall of the mold (container). How can you prevent it?
Ans. 8 The polymeric material can be prevented from sticking to the walls of the mold by changing the viscosity of the polymer. This can be achieved by selecting the MOC of the container (mold) in such a way that it will create substantial viscosity gradient between the walls and the polymeric material. It can also be done by heating the walls of the mold externally.