Polymer_Processing.ppt
- 1. POLYMER PROCESSING PRESENTED BY Dr. Hussin Al-Shafey 1
- 2. • Polymer processing :The operation by which solid or liquid polymers are converted to finished products. • Polymer processing consists of several steps: 1) Preshaping operations involving all or some of the following individual operations: Handling of particulate solids (particle packing, agglomeration, gravitational flow, compaction, and others); Melting or heat softening; Pressurization and pumping of the polymer melt; Mixing for melt homogenization or dispersion of additives; Devolatilization and stripping of residual monomers, solvents, contaminants, and moisture • The common goal of the above operations is to deliver thermoplastics or crosslinkable thermosets in a deformable fluid state that will allow them to be shaped by a die or mold; thereafter, they can be solidified by cooling below Tg or Tm (thermoplastics) or by a chemical reaction (thermosets). Polymer processing
- 3. 2) Shaping operations during which structuring occurs (morphology development and molecular orientation that modify and improve physical and mechanical properties). Principal shaping methods include die forming, molding, casting, calendering, and coating. 3) Postshaping operations, such as decorating, bonding, sealing, welding, dyeing, printing, and metallizing. • For thermoplastics, extrusion is the most popular with approximately 50% of all the commodity thermoplastics being used in extrusion process equipment to produce profiles, pipe and tubing, films, sheets, wires, and cables. • Injection molding follows as the next most popular processing method, accounting for about 15% of all the commodity thermoplastics processed. Other common methods include blow molding, Compression molding, thermoforming, and calendering. Polymer processing
- 4. Important plan before polymer processing • The primary (start materials) consideration of choosing the fabrication method is whether the material is thermoplastic or thermosetting. • Other (conditions) considerations are softening temperature, stability, • and of course the size and shape of the end product. • The methods of polymer processing, including: Plastics technology Fiber technology Elastomer technology Polymer processing
- 5. POLYMER PROCESSING Elastomer Technology Extrusion Molding Spinning Fiber Technology Plastics Technology Compression molding Injection molding Blow molding Thermoset molding Blown film process Cast film process Calendering Wire coating die Thermoforming by application of vacuum Wet Spinning Dry Spinning Melt Spinning Polymer Additive Batch mixing Reinforcement Vulcanization Solvents for coating resins Reinforced plastics Plasticization of polymers
- 6. The most important polymer processing are extrusion and molding processes which involve the following sequence of steps: Heating and melting the polymer Pumping the polymer to the shaping unit Forming the melt into the required shape and dimensions Cooling and solidification Other processing methods include calendering, blow molding, thermoforming, compression molding and rotational molding. 1- Plastic technology
- 7. Molding • The molding process starts by the forcing of divided plastic by the application of heat and pressure to flow, fill, and conform to the shape of a cavity (mold). • Thermoplastic materials have to be cooled below Tm or T g before being removed from the mold. • Thermoset materials and rubbers have to be heated long enough for some chemical reaction to occur that brings about a stable, network structure. • Although some thermosets are cooled before ejection from the mold, most can be ejected hot.
- 8. In compression molding, the preform (pre-weighed) amount of polymer is put between stationary (heated mold cavity ) and movable members of mold and closing the mold. Heat and pressure cause the material to liquefy and flow into the voids in the mold where it chemically reacts and hardens into the final shape. For a typical compression molding, the temperature are near 150 ̊C and pressure of 1000-3000 psi. The material is then allowed to cure and ejected from the mold. Parts made in this way would have spruces and runners which must be trimmed. Compression Molding
- 9. Platen Mold Plunger Guide Pins Mold Cavity Platen Hydraulic Plunger Heat and Cooling Heat and Cooling Hydraulic Pressure Compound to be molded Compression Molding
- 10. Injection molding • Polymer is heated to a highly plastic state (above Tg) and forced to flow under high pressure into a mold cavity where it allowed to cool for solidification and the molding is then removed from cavity. • This process is well suited for producing true three-dimensional shapes such as bottles, toys etc, which require fine details like holes, snaps and surface details. • Typical cycle time 10 to 30 sec, but cycles of one minute or more are not uncommon • Mold may contain multiple cavities, so multiple moldings are produced each cycle
- 11. Design for gating and feed system for the die is crucial to ensure complete die fill. It is important to design the molding so that solidification does not prevent complete mold filling. The design and location of the gates for entry of polymer is a crucial design details. For large part more than one gate may be required for proper flow of material. Mold must be designed in such a way that the solid part can be ejected without distortion. By considering proper orientation at the beginning, in may be possible to avoid expensive mold cost. If possible, design the part so that it can be ejected in the direction of mold closure. To minimize the shrinkage fillers like glass fiber, wood flour, are added during molding. With some part geometries, post mold shrinkage can lead to generation of high residual stress. Injection molding
- 13. Blow molding 1. Using this manufacturing process hollow parts are made like bottle or sphere etc. 2. Air is blown into a thin walled plastic cylinder called the parison. The parison is formed by melted plastic material being pushed through an extruder. 3. When the parison reaches a certain length, the two halves of the mould close around the parison sealing it at the bottom. 4. Compressed air is then used to inflate the parison to form the shape of the cavity inside the mould.
- 14. In blow molding, a hot tube of plastic material is placed between two halves of the mold. The mold is then closed and air or inert gas is blown at a pressure of 20 – 40 MPa which expands the hot tube outward to fill the mold cavity. The part cools, hardened and is ejected from mold. The process produces a part that is dimensionally defined on its external dimension. The process does not lend itself to incorporate design details such as holes, corners, narrow ribs, etc. The blow molding process is very fast and can produce part very economically. The process can be accomplished manually or by semi-automatic or automatic machines. Blow molding
- 15. • The polymer are heated just enough to make them fluid but not to initiate cure. • Molds are usually heated, and this plus the heating generated as the resin is injected into the mold raise its temperature enough to initiate cure. • The part may be removed from the mold when it is form stable but before the cure is completed. • Stored heat in the part allows the cure to be completed in a minute, before the part can cool. Thermoset molding
- 16. Extrusion Extrusion is one of the few continuous plastic processes . Extrusion is similar to injection moulding can be defined as the act of shaping a material by forcing it through a die. It is a Polymer conversion operation. Principle: • It comprises of forcing of a plastic material through an die by means of pressure. • The chief concern with extrusion of polymers is the die swell and the orientation. • In die swell, the extrudate swells to a size greater than the die from which it just exited. Thus the design must compensate for the swell. • Polymer molecules become highly oriented in one or two directions as a result of the strongly oriented flow inherent in the extrusion process. • Control of orientation can improve the property of the material.
- 17. Applications For compounding plastics Production of tubes ,pipes ,sheets ,film , wire coating and other continuous profiles. It is generally used in case of thermoplastic polymers and some thermosets too. Commonly used polymers are PE, PP, PS, PVC, polyamide, polyester. Extrusion Extrusion is divided into several sections
- 18. Extrusion
- 19. • Extruders are basically helical screw pumps that convert solid polymer material into melt , which is then delivered to the die. TYPES: Single screw extruder Twin screw extruder Extrusion
- 20. Single screw extruder Major components: Barrel, Screw Die - not an extruder component (Special tool that must be fabricated for particular profile to be produced ) Raw materials in the form of thermoplastic pellets, granules, or powder, placed into a hopper and fed into extruder barrel. The barrel is equipped with a screw that blends the pellets and conveys them down the barrel Heaters around the extruder’s barrels heats the pellets and liquefies them
- 21. Feed zone • In this 1st zone, the polymer pellets are conveyed to the subsequent zone. • Main function is to preheat the polymer. • Here the screw depth is constant. Compression zone • Here the screw depth increases. • It repels the air gap between the granules. • Heat transfer from the barrel wall is improved. • Density change is accommodated. Metering zone • Here screw depth is constant. • It homogenizes the melt and supply it to the die. Die zone • The final zone to shape the polymer melt. • It usually comprises of the perforated steel plate called breaker plate and a sieve pack. • It sieves out the extraneous materials(foreign particles). • It removes the turning memory from the polymer melt. • It allows head pressure to develop by providing a resistance for pumping action of metering zone.
- 22. There are three major classes of twin screw extruders: a) Co-rotating intermeshing, b) Counter-rotating intermeshing and c) Counter-rotating non-intermeshing. The screws are made up of conveying, kneading block and mixing sections. The screw design is frequently modular, which allows for a nearly unlimited number of possible screw configurations. Twin screw extruder
- 23. Blow film process (Film extrusion) • Blown film extrusion is the most important process for the production of thin plastic films . The Film is generally defined as a product thinner than 0.1 mm, while sheet is thicker than this. • The molten polymer is extruded through an annular die. In most cases the blown film bubble is formed vertically upwards. The maximum bubble diameter is usually 1.2 – 4 times larger than the die diameter. • The solidified film passes through a frame which pinches the top of the bubble and is taken up by rollers. Coextruded films with 3 – 8 layers (sometimes up to 11) are also produced by this process.
- 24. Cast film Process • Cast film and sheet extrusion involves extruding a polymer through a • flat die with a wide opening (up to 10 m), onto a chilled steel roller or rollers which quench and solidify the molten material. • The cast film process is used for very tight tolerances of thin film, or for low viscosity resins. Most • dies are of T slot design, which contain a manifold to spread the •flowing polymer across the width of the die, followed downstream by alternating narrow and open slits to create the desired • flow distribution and pressure drop. • Most cast film lines manufactured today are coextrusion lines, combining layers from as many as seven extruders into the product through multimanifold dies, or single manifold dies with the aid of feed blocks.
- 26. Calendering • The calendering of thermoplastics is an operation used for the production of continuous sheet or film of uniform thickness, by squeezing the molten material between a pair of heated driven rolls. • The molten material is fed to the calender rolls from a mixer and two roll mill system, or from a large extruder. • The major plastic material to be calendered is PVC. Products range from wall covering and upholstery fabrics to reservoir linings and agricultural mulching materials.
- 27. Wire coating die • In wire and cable coating processes, individual wires or wire assemblies are pulled at very high speed through a crosshead die, at right angles to the extruder axis. • In high pressure extrusion, the polymer melt meets the wire or cable before the die exit, for example insulating of individual wires. In low pressure extrusion, the melt meets the cable after the die exit, for example jacketing of assemblies of insulated cables.
- 28. Thermoforming by application of vacuum • Thermoforming techniques involve the softening of thermoplastic sheets by heat, followed by forming by the application of vacuum, pressure, or a moving plug. • Geometries of thermoformed products are usually simple (boxes, food trays, various containers, refrigerator liners, computer cases). Thermoforming competes with blow moulding and injection moulding. • The main advantages of this process are the relatively low cost of thermoforming machines and the very low cost of the moulds, and the ease of forming large area, thin section parts. • Disadvantages are the limited product shapes possible, difficulties in obtaining the required thickness distribution, difficulties in controlling molecular orientation and limitations in service temperature which may induce strain recovery or shrinkage.
- 29. Thermoforming refers to heating a sheet of plastic material until it becomes soft and flexible and then forming it either by vacuum, by air pressure or between matching mold halves. Following are the typical sequences used in thermoforming of polymers. A sheet of thermoplastic material is placed over a die and heated until it becomes soft. A vacuum is then created inside the die cavity which draws down the heated plastic sheet into the shape of the die. The material is then cooled, the vacuum is released and the final product is taken out Thermoforming
- 30. 2- Fiber technology Spinning Spinning is the process in which a polymer melt is forced through a small orifice called spinner jet which shapes it into a fiber form. Spinning types: a) Melt spinning b) Dry spinning c) Wet spinning All three types involve the formation of continuous filament strands by forcing a viscous fluid or solution of the polymer through the small orifices of a spinneret plate and immediately solidifying or precipitating the resulting filaments. In dry and wet spinning, the solvent is removed to form the solid filament. In dry spinning the solvent evaporates into a gas and in wet spinning the solvent is leached into a liquid path.
- 31. Melt spinning Melt spinning uses heat to melt the polymer to a viscosity suitable for extrusion. This type of spinning is used for polymers that are not decomposed or degraded by the temperatures necessary for extrusion. Melt spinning process use heat to melt the fiber to a viscosity suitable for extrusion through a spinneret (die) with numerous holes (one to thousands) The molten polymer is extruded at high pressure and constant rate through a spinneret into a relatively cooler air stream that solidifies the filaments. Lubricants and finishing oils are applied to the fibers in the spin cell. Lubricants are used to reduce fibers friction against themselves and against elements of the processing machinery. Once formed, the filament yarn either is immediately wound onto bobbins or is further treated for certain desired characteristics or end use. Polymers such as poly(ethylene terephthalate) and nylon 6,6 are melt spin in high volumes
- 33. Dry spinning The dry spinning process begins by dissolving the polymer in an organic solvent. This solution is blended with additives and is filtered to produce a viscous polymer solution, referred to as "dope", for spinning. The polymer solution is then extruded through a spinneret as filaments into a zone of heated gas or vapor. The solvent evaporates into the gas stream and leaves solidified filaments, which are further treated using one or more methods. This type of spinning is used for easily dissolved polymers such as cellulose acetate, acrylics, and modacrylics. Dry spinning is the fiber formation process potentially emitting the largest amounts of volatile organic compounds. Air pollutant emissions include volatilized residual monomer, organic solvents, additives, and other organic compounds used in fiber processing. The largest amounts of un-recovered solvent are emitted from the fiber spinning step and drying the fiber.
- 34. Dry Spinning
- 35. Wet spinning • Wet spinning is required when polymer’s solvents are not sufficiently volatile to permit rapid evaporation. Wet spinning also uses solvent to dissolve the polymer to prepare the spinning dope. • The process begins by dissolving polymer chips in a suitable organic solvent, such as dimethyl formamide (DMF), or acetone. The spinning solution is extruded through spinnerets into a precipitation bath that contains a coagulant (or precipitant) such as aqueous dimethyl acetamide (DMAc) or water. • The polymer is not soluble in coagulant but the coagulant is miscible with spinning dope solvent. • The essential feature in wet spinning is the transfer of the mass of the solvent from the polymer to the coagulating bath. When the solvent is solubilized out, the polymer structure precipitates into solid form. • Air pollution emissions in the wet spinning organic solvent are similar to those of dry spinning. Emissions occur as solvent evaporates from the spinning bath and from the fiber in post-spinning operations.
- 36. Wet Spinning
- 37. 3- Elastomer technology It is the process by which a network of crosslinks is introduced into an elstomer. Vulcanization effects are: It transforms an elastomer from a weak thermoplastic mass without useful mechanical properties into a strong elastic, tough rubber. Vulcanization takes place by heat in the presence of sulfur alone and this process is relatively low. It can be speeded by the addition of small amounts of organic or inorganic compounds known as accelerators. Vulcanization with sulfur takes place when 0.5-5 parts (by weight) of sulfur is combined with 100 parts of rubber. Sulfur chloride can vulcanize rubber without heat. Non sulfur compounds that can vulcanize rubber fall into three groups: 1. Oxidizing agents such as organic peroxide and nitro compounds 2. Generators of free radicals such as organic peroxides and azo compounds. 3. Phenolic resins . Vulcanization
- 38. Reinforcement • Even the vulcanized rubber don not exhibit good mechanical properties such as strength, stiffness, abrasion resistance and tear resistance required for many uses. • These properties can be enhanced by the addition of fillers to rubbers before vulcanization. Fillers are divided by two types: inert and reinforcing fillers. • Inert fellers such as clay and barites, which make the rubber mixture easier to handle before vulcanization, have little effect on its physical properties. Reinforcing fillers is responsible for the improvement of the above mentioned mechanical properties of the vulcanized rubber. Reinforcing fillers examples - Carbon black is the famous and the only important reinforcing filler for most elastomers, although silica and silicates fillers can be used in some cases as with the silicone elastomers. - The reinforcing action of fillers depends on its nature and the type of elastomer and the amount of filler.
- 39. Batch mixing • Batch mixing is generally carried out using two specially designed blades inside temperature controlled chamber. • It is most commonly used for compounding and mixing of rubber formulations. • Its most important commercial use in the incorporation of carbon black and other additives into rubber for the manufacture of automobile tires. • Laboratory size batch intensive mixers are extensively used for characterization of materials and processes.
- 40. Polymer additives • The primary reasons for using additives are: Property modification or enhancement Overall cost reduction Improving and controlling of processing characteristics •Important types of modified polymer include: polymer composites, polymer-polymer blends, and polymeric foams. Plasticization of polymers Plasticizer is a material that enhances the processibility or flexibility of the polymer with which it is mixed. The plasticizer my be a liquid or solid or another polymer. The rigid poly vinyl chloride is a hard solid material (used for example to make pipe and credit cards) but when mixed with 50- 100 parts by weight of phthalate ester plasticizers converts the polymer into leathery products useful for the manufacture of electrical coatings.
- 41. Plasticizers is used in surface coatings to enhance the flow and leveling properties of the material during applications and reduce the brittleness of the dried film. Rubbers are plasticized with petroleum oils, before vulcanization, to improve processibility and adhesion of rubber layers to each other and to reduce the cost and softness of the final product. Large quantities of these “oil-extended” rubber are used in tire compounds and related products. The oil content is about 50% of the SBR. Saturated hydrocarbons have limited compatibility with most rubbers and may seat out. Aromatic oils are more compatible and unsaturated straight chain and cyclic compounds are intermediate in solvent power. Plasticization of polymers
- 42. Reinforced plastics • Fiber reinforcement is used to increase rigidity, strength, and usage temperatures. Fiber reinforced materials are attractive construction materials because they are stiff, strong and light. • Glass fiber are the most widely reinforcing agents, although other fibrous materials, like aromatic polyamides confer advantages in special applications. • The improved mechanical properties of reinforced plastics require that the fiber length exceeded a certain minimum value. The aspect ratio (length/diameter) of the fiber should be at least about 100 for the full benefits of reinforcement. This is why carbon black are reinforcement only for hydrocarbon elastomer, but not for all plastic generally.
- 43. Solvents for coating resins • The surface coating must be in the form of soluble formulations which induce the use of solvents. Single solvent are rarely used because the requirements for evaporation rates, safety, solvency, and so on generally mean that a solvent blend is more effective and less costly. • The cheapest organic solvents are hydrocarbons, whereas most solvents for film-forming polymers are moderately hydrogen bonded. • The procedure can be used to blend solvents to nonsolvents (to reduce the cost) or even to make a solvent mixture from nonsolvents. The latter procedure must be used with caution for surface coatings. If the residual liquids after evaporation are non solvents, the final coalesced polymer film may have poor clarity and adhesion to the substrate. • The slowest evaporating component of the solvent blend should be a good solvent for the polymer, since the last solvent to leave the film has a strong influence on the quality of the film.