Solidification Process(2) - Polymer Processing (Chapter 8, 12) Seok-min Kim smkim@cau.ac.kr
Plastic Products Plastics can be shaped into a wide variety of products: Molded parts Extruded sections Films Sheets Insulation coatings on electrical wires Fibers for textiles Paints and varnishes Adhesives Various polymer matrix composites -2-
Trends in Polymer Processing Applications of plastics have increased at a much faster rate than either metals or ceramics during the last 50 years Many parts previously made of metals are now being made of plastics Plastic containers have been largely substituted for glass bottles and jars Total volume of polymers (plastics and rubbers) now exceeds that of metals -3-
Polymer A compound consisting of long-chain molecules, each molecule made up of repeating units connected together There may be thousands, even millions of units in a single polymer molecule The word polymer is derived from the Greek words poly, meaning many, and meros (reduced to mer), meaning part Most polymers are based on carbon and are therefore considered organic chemicals -4-
Synthesis of Polymers ( 폴리머합성 ) Nearly all polymers used in engineering are synthetic They are made by chemical processing Polymers are synthesized by joining many small molecules together into very large molecules, called macromolecules, that possess a chain-like structure The small units, called monomers, are generally simple unsaturated organic molecules such as ethylene C 2 H 4-5-
Linear, Branched, and Cross-linked Polymers Linear structure chain-like structure (Characteristic of thermoplastic polymers ) Branched structure chain-like but with side branches (Also found in thermoplastic polymers) Cross-linked structure - Loosely cross-linked, as in an elastomer - Tightly cross-linked, as in a thermoset -6-
Thermoplastic Polymers (TP) A thermoplastic polymer can be heated from a solid state to a viscous liquid state and then cooled back down to solid This heating and cooling cycle can be repeated multiple times without degrading the polymer The reason is that TP polymers consist of linear (and/or branched) macromolecules that do not cross-link upon heating By contrast, thermosets and elastomers change chemically when heated, which cross-links their molecules and permanently sets these polymers -7-
Mechanical Properties of Thermoplastics Low modulus of elasticity (stiffness) E is two or three orders of magnitude lower than metals and ceramics Low tensile strength TS is about 10% of the metal Much lower hardness than metals or ceramics Greater ductility on average Tremendous range of values, from 1% elongation for polystyrene to 500% or more for polypropylene -8-
Thermosetting Polymers (TS) TS polymers are distinguished by their highly cross-linked three-dimensional, covalently-bonded structure within the molecule Chemical reactions associated with cross-linking are called curing or setting In effect, the formed part (e.g., pot handle, electrical switch cover, etc.) becomes one large macromolecule Always amorphous and exhibits no glass transition temperature -9-
General Properties and Characteristics of Thermosets Rigid - modulus of elasticity is two to three times greater than TP Brittle, virtually no ductility Less soluble than TP in common solvents Capable of higher service temperatures than TP Cannot be remelted - instead they degrade or burn -10-
Elastomers Polymers capable of large elastic deformation when subjected to relatively low stresses Some can be extended 500% or more and still return to their original shape Two categories: - Natural rubber - derived from biological plants - Synthetic polymers - produced by polymerization processes similar to those used for thermoplastic and thermosetting polymers -11-
Classification of Shaping Processes Extruded products with constant cross-section Continuous sheets and films Continuous filaments (fibers) Molded parts that are mostly solid Hollow molded parts with relatively thin walls Discrete parts made of formed sheets and films Castings Foamed products -12-
Extrusion Compression process in which material is forced to flow through a die orifice to provide long continuous product whose cross-sectional shape is determined by the shape of the orifice Widely used for thermoplastics and elastomers to mass produce items such as tubing, pipes, hose, structural shapes, sheet and film, continuous filaments, and coated electrical wire Carried out as a continuous process; extrudate is then cut into desired lengths -13-
Extruder Screw Divided into sections to serve several functions: Feed section - feedstock is moved from hopper and preheated Compression section - polymer is transformed into fluid, air mixed with pellets is extracted from melt, and material is compressed Metering section - melt is homogenized and sufficient pressure developed to pump it through die opening -14-
Die End of Extruder Before reaching die, the melt passes through a screen pack - series of wire meshes supported by a stiff plate containing small axial holes Functions of screen pack: Filter out contaminants and hard lumps Build pressure in metering section Straighten flow of polymer melt and remove its "memory" of circular motion from screw -15-
Extrusion Die(1) Solid Cross Section Hollow Shapes -16-
Extrusion Die for Coating Wire Figure 13.11 Side view cross-section of die for coating of electrical wire by extrusion. -17-
Slit Die Extrusion Production of sheet and film by conventional extrusion, Slit may be up to 3 m wide and as narrow as around 0.4 mm A problem is uniformity of thickness throughout width of stock, due to drastic shape change of polymer melt as it flows through die Edges of film usually must be trimmed because of thickening at edges -18-
Calendering Feedstock is passed through a series of rolls to reduce thickness to desired gage Expensive equipment, high production rates Process is noted for good surface finish and high gage accuracy Typical materials: rubber or rubbery thermoplastics such as plasticized PVC Products: PVC floor covering, shower curtains, vinyl table cloths, pool liners, and inflatable boats and toys -19-
Injection Molding Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from cavity Produces discrete components almost always to net shape 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 -20-
Injection molding process Polymer 를 Hopper 로부터 Barrel 내로투입, Heater 와 screw 회전에의한마찰열로용융 Screw 회전으로용융수지를 screw 전방으로이동 Hydraulic unit force 로용융수지를 delivery system 을통해 cavity 내로충진 용융수지가충분히고화, 냉각될때까지냉각후취출 -21-
Injection molding process -22-
1. 가소화과정 (Plasticating Phase) -23-
2. 충진과정 (Filling Phase) -24-
3. 보압과정 (Packing Phase) -25-
4. 냉각과정 (Cooling Phase) -26-
Injection machine Cycle Mold is closed Melt is injected into cavity Screw is retracted -27- Mold opens
Injection Molded Parts Complex and intricate shapes are possible Shape limitations: Capability to fabricate a mold whose cavity is the same geometry as part Shape must allow for part removal from mold Injection molding is economical only for large production quantities due to high cost of mold -28-
Injection Molding Machine Two principal components: 1. Injection unit Melts and delivers polymer melt Operates much like an extruder 2. Clamping unit Opens and closes mold each injection cycle -29-
Injection Unit of Molding Machine Consists of barrel fed from one end by a hopper containing supply of plastic pellets Inside the barrel is a screw which: 1. Rotates for mixing and heating polymer 2. Acts as a ram (i.e., plunger) to inject molten plastic into mold Non-return valve near tip of screw prevents melt flowing backward along screw threads Later in molding cycle ram retracts to its former position -30-
Clamping Unit of Molding Machine Functions: 1. Holds two halves of mold in proper alignment with each other 2. Keeps mold closed during injection by applying a clamping force sufficient to resist injection force 3. Opens and closes mold at the appropriate times in molding cycle Types: 1. Toggle 2. Hydraulic -31-
The Mold The special tool in injection molding Custom-designed and fabricated for the part to be produced When production run is finished, the mold is replaced with a new mold for the next part Various types of mold for injection molding: Two-plate mold Three-plate mold Hot-runner mold -32-
유동시스템 1) 스푸루 (sprue) system 금형의입구에해당하는것으로용융된수지를러너에보내는역할 2) 러너 (Runner) 1러너의기능 : 러너의기능은성형기노즐에서밀려나온수지를캐비티까지안내하는길역할 2러너의설계시고려할사항 (a) 캐비티의개수및배열에따라배치방법을결정한다. (b) 수지의온도저하가최소가되도록한다. (c) 러너에서압력손실이최소가될수있는구조로한다. (d) 러너의체적은될수있는한작게되도록한다. 3콜드슬러그웰 : 고화된수지가캐비티안으로유입되는것을막음 -33-
3) 게이트 1 게이트의역할 - 충진되는용융수지의흐름방향과유량을제어러너측으로역류를방지한다. - 스프루, 러너를통과냉각된수지는좁은게이트를통과하는동안유동속도가빨라져마찰열이발생, - 러너가성형품에서용이하게절단되도록한다. 2 게이트의위치 - 게이트는성형품의가장두꺼운부분에설치하는것을원칙으로한다. - 성형품은외관상눈에띄지않는곳, 성형후게이트의끝손질이용이한부분에설치한다. - 웰드라인이생기지않는곳이나안보이는위치에설치한다. 3 게이트의크기 - 충진시간은게이트가클수록유리 - 잔류응력에의한변형, 휨에관해서는게이트가작은쪽이유리 -34-
Two-Plate Mold -35-
Two-Plate Mold Features Cavity geometry of part but slightly oversized to allow for shrinkage Created by machining of mating surfaces of two mold halves Distribution channel through which polymer melt flows from nozzle into mold cavity Sprue - leads from nozzle into mold Runners - lead from sprue to cavity (or cavities) Gates - constrict flow of plastic into cavity -36-
More Two-Plate Mold Features Ejection system to eject molded part from cavity at end of molding cycle Ejector pins built into moving half of mold usually accomplish this function Cooling system - consists of external pump connected to passageways in mold, through which water is circulated to remove heat from the hot plastic Air vents to permit evacuation of air from cavity as polymer melt rushes in -37-
Three-Plate Mold -38-
Hot-Runner Mold Eliminates solidification of sprue and runner by locating heaters around the corresponding runner channels While plastic in mold cavity solidifies, material in sprue and runner channels remains molten, ready to be injected into cavity in next cycle Advantage: Saves material that otherwise would be scrap in the unit operation -39-
Shrinkage Reduction in linear size during cooling from molding to room temperature Polymers have high thermal expansion coefficients, so significant shrinkage occurs during solidification and cooling in mold Typical shrinkage values: Plastic Shrinkage, mm/mm (in/in) Nylon-6,6 0.020 Polyethylene 0.025 Polystyrene 0.004 PVC 0.005-40-
Shrinkage Factors Fillers in the plastic tend to reduce shrinkage Injection pressure higher pressures force more material into mold cavity to reduce shrinkage Compaction time - similar effect longer time forces more material into cavity to reduce shrinkage Molding temperature - higher temperatures lower polymer melt viscosity, allowing more material to be packed into mold to reduce shrinkage -41-
Transfer Molding TS charge is loaded into a chamber immediately ahead of mold cavity, where it is heated; pressure is then applied to force soft polymer to flow into heated mold where it cures -42-
Bonding and Packing Schematic illustrations of IC packing Chip-on-board attachment -43-
Transfer-molding press schematic Runner design -44-
Blow Molding Molding process in which air pressure is used to inflate soft plastic into a mold cavity Important for making one-piece hollow plastic parts with thin walls, such as bottles Because these items are used for consumer beverages in mass markets, production is typically organized for very high quantities -45-
Thermoforming Flat thermoplastic sheet or film is heated and deformed into desired shape using a mold Heating usually accomplished by radiant electric heaters located on one or both sides of starting plastic sheet or film Widely used in packaging of products and to fabricate large items such as bathtubs, contoured skylights, and internal door liners for refrigerators -46-
Thermoforming -47-
Product Design Guidelines: Moldings Wall thickness Thick cross sections are wasteful of material, more likely to cause warping due to shrinkage, and take longer to harden Reinforcing ribs Achieves increased stiffness without excessive wall thickness Ribs should be made thinner than the walls they reinforce to minimize sink marks on outside wall -48-
Product Design Guidelines: Moldings Corner radii and fillets Sharp corners, both external and internal, are undesirable in molded parts They interrupt smooth flow of the melt, tend to create surface defects, and cause stress concentrations in the part Holes Holes are quite feasible in plastic moldings, but they complicate mold design and part removal -49-