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编号： INJECTION MOLDINGIn the injection molding process, thermoplastic resins are melted and the melt is forced (injected) into a mold. After this melt cools until the polymer solidifies, the parts are removed (ejected) from the mold. Injection molding permits mass production net shape manufacturing of high precision, three-dimensional of plastic parts. 1. One of the most common plastics manufacturing processes, injection molding can produce parts weighing as little as fraction of a gram or as much as 150 kg. 2The process currently consumes 30% of polymeric resins. 3 Of which 90% are thermoplastics capable of being remelted. Major advantages include capabilities to produce parts with: 1) virtually unlimited complexity, 2) fine details and good surface appearance, 3) controlled wall thickness and excellent dimensional stability, and 4) requiring limited or no finishing.The five important in the injection molding process are the 1. Injection molding machine 2. Mold3. Material 4 Method5. Man (i.e., operator). While there are many variations of each M, this discussion is limited to single-stage reciprocating screw injection machines and the flow of thermoplastic polymer melts in two-plate cold runner molds. Injection Molding MachinesAs illustrated in Figure 1, injection molding machines have three major components: the 1) injection unit, 2) clamping unit, and 3) controls. The injection unit plasticizes (melts) and injects the polymeric material into the mold. The clamping unit supports to the mold and provides the mechanisms for opening and closing of the mold and for ejection of molded parts.Figure1. Injection molding machine.4.During a thermoplastic molding cycle, the clamp of the injection molding machine closes, thereby closing the mold. Molten plastic located between the nozzle and screw in the barrel of the injection unit is forced (by the screw) into the mold. The controlled volume of melt injected into the mold typically fills the cavities to about 95 to 98% of their total volume. After injection is completed, the screw is pressured for a given period of time. In this packing stage, more melt is forced into the mold to compensate for shrinkage of the melt as it cools. The packing stage is followed by a holding stage, in which a controlled pressure is exerted on the screw for a specific length of time. Holding pressure prevents the melt from flowing back into the runners. When the gate freezes (solidifies), melt can no longer exit the cavity and the holding stage ends. While the melt cools immediately upon entering the cavity, a “formal” cooling stage follows the holding stage. During this period, the part cools until it is capable of withstanding ejection forces. The screw also rotates to melt more plastic and build up the molten plastic shot for the next molding cycle. At the end of the cooling stage, the mold is opened and the part is ejected.The times associated with a conventional molding cycle are shown in Figure 2. For 2 to 3-mm thick parts, filling occurs in less than 5 s, packing requires one-third of the fill time 5, the holding time depends on the gate size, and cooling is longest part of the cycle. Thin-walled parts (i.e., wall thickness is les than 1 mm), however, filling in less than 1 s, typically have no packing or holding stage, and cool rapidly.Figure2. Molding cycle diagram.6.1.1. Injection UnitThe injection unit must 1) melt the polymeric material and forms the shot, 2) transfer (inject) the melt into the mold, 2) build up packing and holding pressures, 3) bring the nozzle into contact with the sprue bushing of the mold, and 4) generate contact pressure between the nozzle and sprue bushing. While single stage ram plunger, dual stage ram plunger, and ram assisted-screw plunger machines are available 7 the most machines contain reciprocating screw injection units. As illustrated in Figure 3, these units contain a 1) hopper, 2) screw motor, 3) injection mechanism (hydraulic cylinders or motor-driven systems), 4) barrel and screw, and 5) nozzle.Figure3. Reciprocating screw injection unit.8.Polymer pellets are fed (by gravity) though the hopper, pass through a cooled feed throat, and drop onto the rotating screw. The feed throat is water cooled to prevent bridging (i.e., partial melting) of the resin. When the motor rotates the screw, friction from the rotating screw and conduction from the electric resistance heater bands surrounding the barrel melt the polymeric material. The rotating screw also conveys the polymer toward the nozzle. At the end of the screw, the molten polymer passes through a non-return valve, which prevents the melt from flowing back towards the hopper. Melt is trapped between the blocked nozzle and the non-return valve (Figure 4). This melt forces the screw backwards (i.e., towards the hopper). The screw stops rotating when sufficient melt has become trapped between the nozzle and non-return valve. While this melt is called the “shot size,” the measurement is axial travel of the screw or “stroke.”Figure4. Barrel, screw, non-return valve (check ring) and nozzle.9.The injection unit or sled also travels toward and away from the clamping unit. To facilitate purging, change nozzles, or adjust the travel distance, the unit is backed away from the stationary platen. During molding cycles, the injection unit is forward to that the nozzle and sprue bushing make intimate contact. The contact pressure (i.e., pressure exerted between the nozzle and the sprue bushing) ensures proper alignment of the nozzle and sprue bushing and also keeps the two items in contact during injection. The injection unit of a mold machine is specified by its:1. Shot size. The shot size is the maximum amount of plastic that can be injected in one molding cycle and is rated in ounces of general purpose polystyrene (GPPS) for U.S. machines10 and cm3 for European and Asian machines11 For best quality, parts must use about 60 to 70% of a machines rated shot size12. Smaller shot sizes produce greater irregularities and loss of precision, whereas larger shot sizes do not allow sufficient melt cushion for packing and for inefficiencies in plastication.You will be using a micro injection molding machine with a 3-g shot size, but the Department of Plastics Engineering has machines with shot sizes of 3-g to 8 oz.2. Plasticating capacity and recovery rate. The plasticating capacity is a measure of the amount of plastic that can be melted and homogenized per unit of time (lb/hr or kg/hr), with insufficient plasticating capacity (with respect to the shot size) producing unmelted plastic and too high plasticating capacity causing thermal degradation due to longer dwell time in the barrel. The recovery rate is a measure of the volumetric output of the injection molding machines (expressed in3/s). Both recovery rate and plasticating capacity are determined by running polystyrene at 50% of maximum capacity.3. Maximum injection velocity. The maximum injection velocity in a conventional injection molding machine ranges from 150 to 250 mm/s (6 to 10 in/s)13 and can be as high as 2000 mm/s for thin wall machines14.The micro injection molding machine that you will be using has a maximum injection velocity of 160 mm/s.4. Maximum available injection pressure. In all injection molding machines, injection velocity and injection pressure are linked so that the set injection velocity cannot be maintained without sufficient pressure. The maximum available injection pressure for a standard injection molding machine is 138 MPa (20,000 psi) and can be as high as 324 MPa for thin wall machines 15.1.2. Clamping UnitThe clamping unit supports the mold; opens and closes the mold; holds the mold closed during injection, packing and holding; and holds the ejection unit. The two major types of clamps are hydraulic systems and toggle clamps. A hydraulically-actuated toggle clamp is shown in Figure 5; (note: you will be using an electrically-actuated toggle, but I could not find a good picture of one). As with all clamping units, the toggle unit contain a stationary plane (1) and a moving platen (3) on which is mounted the mold (2). These platens and the tailstock platen (at the end of the machine) are usually supported by tie bars (7). The stationary plane has a hole, which facilitates mounting of the mold and allows the nozzle to contact the sprue bushing. The ejection system (4 and 5) is usually supported by the moving platen.In hydraulically-actuated toggle clamps, a toggle mechanism (6) provides a mechanical linkage between the moving and tailstock platens and the hydraulic cylinder (9). Forward actuation of the cylinder extends the toggle, thereby moving the moving platen and closing the mold. Full extension of the toggle provides the clamp force required to keep the platen closed during the molding cycle. Reverse actuation of the cylinder retracts the toggle and opens the mold. Clamp adjusting ring gear and a hydraulic motor (9 and 10) adjust the position of the moving platen relative to the tailstock platen. This movement is called the die height adjustment and allows different-sized molds to be fit into the clamping unit. Finally, if material is trapped between the two sides of closing mold, this produces a pressure in the hydraulic cylinder. Therefore, mold protection typically consists of reversing the clamp motion when mold cannot close, but the pressure has reached a preset level. The time of forward motion without closing the mold can also be limited to a preset value. Electrically-actuated toggle clamps are very similar hydraulically-actuated toggles, but electric motors (with various mechanisms) drive the toggle and the ejection mechanism.Figure5. Hydraulically-actuated toggle clamp unit 16.In hydraulic clamps (Figure 6), there are two hydraulic cylinders and no toggle. The small double acting traverse cylinder is actuated to open and close the mold, but the larger main cylinder helps provide the clamp force. To efficiently fill the latter cylinder, a prefill valve is opened before the traverse cylinder starts to close the clamp. Hydraulic oil is suctioned from the main or an auxiliary tank to the main clamp by the movement of the traverse cylinder. When the mold halves touch, the prefill valve is closed and further movement of traverse cylinder compresses the oil in the main cylinder, thereby producing the clamp force. This process is reversed when the mold is opened and a separate cylinder provides for part ejection. With hydromechanical clamps, the mold is open and closed using a toggle mechanism whereas the clamp force is produced by one or more hydraulic cylinders 17.Clamping units are specified by the clamp force. In addition, a number of parameters limit the size of the mold that can be mounted in the machine.The micro injection molding machine that you will be using has a clamp force of 3 tons, but the Department of Plastics Engineering has machines with clamp forces from 3 to 100 tons.Figure6. Hydraulic clamp unit18.During mold open, the part remains with the moving side of the mold, usually with the assistance of a sprue puller. An ejection system (Figure 7) usually detaches the part from the mold. To separate the part from the mold, a hydraulically or electrically-actuated ejection platen is forced forward (i.e., toward the stationary platen). Knockout rods, that connect this platen to the ejector platen in the mold, force the ejector plate forward. Thus, the ejector pins mounted to the ejector plate know the part out of the mold 19. Ejector return pins help return the ejector pins to the retracted position as the mold closes for the next cycle.Figure7. Ejection unit20.2. Injection MoldsA part is formed, cooled and injected in the injection mold that is mounted between the stationary and moving platen of the molding machine28. Therefore, the mold one or more hollow cavities shaped like the desired product. As shown in Figure 8, a typical mold is a series of plates. The cavity and core plates contain the geometry of the parts and runner system (if needed). The ejector pins and sprue puller are mounted between the ejector and ejector (or sprue) retainer plates. These plates are supported by top and bottom clamping plates, a support plate, and support pillars. The mold splits between the cavity and core plates to produce the two mold halves. The cavity or A side of the mold is mounted to the stationary platen while the core or B side is mounted to moving platen. These halves are aligned using four leader pins and bushings.Figure8. Cross-section of a typical two-plate mold21.Melt is delivered to the mold at the sprue bushing, which fits into the top retainer and cavity plates. A locating ring surrounding the sprue bushing aligns the mold with the stationary platen, thereby aligning the sprue bushing and nozzle. Melt flows from the sprue bushing into the runners, through the gates, and into the cavities. Figure 9 presents the layout of cavities in a multi-cavity mold. The sprue delivers melt from the nozzle to the runners, which split the melt stream for delivery to the four cavities. The core and cavity design control the shape, size and surface texture of the molded part. Cavities are located between the cavity and core plates, with placement and parting line locations dependent of part design. The term “mold half” does not mean that the two mold halves are of equal width. As illustrated in Figure 9, the sprue is tapered to facilitate the part release. Runner diameters typically have round (shown in Figure 9), trapezoidal, or modified trapezoidal cross-sections because these designs provide the best surface-to-volume characteristics 22,23,24. The dimensions of these runners depend on the material and size of the part 25. When melt flows from the runners to the cavities, the melt passes through a reduced cross-sectional area in the mold called a gate. Gates control the melt flow entering the cavity and ease separation of the molded part from the runner system 26. The design, sizing, and location of the gate influence the 1) shear experienced in the gate, 2) direction melt flow (i.e., orientation) and level of balanced cavity filling, 3) presence of flow instabilities, such as jetting, 4) location of vent and parting lines, 5) number and strength of weld and meld lines, 6) the amount of runner scrap, and 7) the need for secondary operations. Figure9. Layout of cavities in a multi-cavity mold27.In general, gate size is determined by the part wall thickness 28, overall part size, and material properties. Thicker parts require larger gates to facilitate packing, but a gate depth less than the part thickness allows for proper ejection without an ugly gate vestige (i.e., mark left on the part when the gate is removed). With thin-walled parts, the gate depth may be larger than the part thickness to decrease the fill pressure28. Parts with long flow lengths and large cavity surfaces need larger gates to reduce fill pressures and to prevent premature gate freeze off. Higher viscosity resins also require larger gates than easier flowing resins, larger gate cross-sections reduce the shear applied to the polymer melt, and short lands decrease the occurrence of jetting and other flow instabilities. Generous radii on the cavity side of the gate also create laminar flow and prevent jetting28. Some materials have wide processing windows while other materials can only be used with a narrow range of molding conditions. This behavior often occurs when small gates tend to cause thermal degradation of the material and excessive residual (molded-in) stresses in the part. Although too small a gate results in loss of strength of the steel in the land area and may cause the steel to break 29, long lands promote jetting. Therefore, the land length of the steel (i.e., gate) is usually 50% the gate depth. Injection molds have traditionally been machined from tool steel. Mold temperature is controlled from water lines that are drilled in the core and cavity plates of the mold. Water heated or cooled in a mold temperature controller and pumped into the mold. Since machining cannot produce smaller features, newer tooling has included electroformed nickel (used for digital versatile disks) and silicon inserts produced using conventional semiconductor fabrication (exposure and etching) processes.3. Polymer MaterialsPolymers are long chain molecules made up of simple repeating molecular units (i.e., mers). The net effects of having long chains are chain entanglement, a summation of intermolecular forces, and time scale of motion 33. Several factors, including the polymers molecular weight and molecular weight distribution, its thermoplastic or thermoset nature, its molecular configuration, and its structure affect the performance of polymers. Melt viscosity and degradation mechanism of plastics are also important when considering the flow of polymer melts through gates.3.1. Molecular Weight and Mechanical PropertiesDuring polymerization, chain length can be varied and not all chains will have the same length. Molecular weight is a measure of the average chain length while molecular weight distribution (MWD) is a measure of the range of chain lengths. Three molecular weights are typically reported for polymers. The number-average-molecular weight, , is mean chain length and provides an estimate of intermolecular attraction and the number of end groups in a resin. In contrast, the weight-average-molecular weight, , “counts” the longer chains, thereby producing an estimate of chain entanglement. Finally, the z-average-molecular weight, , favors very long polymer chains and has been correlated with melt strength in materials used for blown film extrusion and extrusion blow molding. While the polydispersity index, PI, (1)Does not exactly measure molecular weight distribution; PI is generally used to express the range of chain lengths.As illustrated in Figure 11, the effect of molecular weight on mechanical properties varies with the specific property. Increasing chain entanglement causes properties, such as melt viscosity and Izod impact resistance, to increase with molecular weight.Viscosity, h, has related to weight-average-molecular weight using Mark-Houwink equation 34: (2)Where K and a are empirical constants. For linear polymers, a is 1.0 until chain entanglement occurs (i.e., for oligomers) and a is 3.4 after the molecular weight has exceeded the critical molecular weight. Properties that depend on intermolecular attractions and the number of end groups initially increase with number-average-molecular weight, but remain constant after attaining a threshold molecular weight. These properties include the tensile strength, flexural modulus, and glass and melt transition temperatures.Figure11. Effect of molecular weight on the mechanical properties.35.Polymers typically used in injection molding have molecular weights greater than the critical or threshold molecular weights. In general, polymers prepared via addition polymerization (e.g., polyethylene, polypropylene, polystyrene, polymethylmethacrylate, and polyvinyl chloride) have higher molecular weights than those prepared using condensation polymerization (e.g., polycarbonate, polyacetal, polyamides or nylons). Improperly dried condensation polymers are also more susceptible to chain scission and the subsequent reduction in molecular weight. Consequently, condensation polymers are more likely to show the effects of molecular weight degradation in the measured tensile and flexural properties, but all polymers exhibit these effects as changes in melt viscosity and impact properties. The effects of heat history, however, depend on the specific characteristics of the polymer. In recent studies36,37 impact modified polystyrene (HIPS) and polycarbonate showed significant decreases in the melt flow and Izod impact properties with subsequent recycling histories, but much smaller changes in tensile and flexural properties.Molecular weight distribution (MWD) is usually a function of the polymerization technique and resins heat history. Metallocene and Zeigler-Natta provide relatively narrow molecular distributions (i.e., PI 2 to 4) whereas as free radical catalysis and chrome catalysts yield materials broader molecular weight distributions (i.e., PI 20 to 50)38. Broad molecular weight distribution improves the processability of many resins, but reduces properties like heat sealability in polyethylene blown films.3.2. Thermoplastic and Thermoset PolymersPolymers are classified into two classes: thermoplastics and thermosets. In a thermoplastic resin, the long chain molecules are held together by relatively-weak intermolecular attractions, such as van der Waals forces and hydrogen bonding. When the material is heated, the intermolecular forces weaken and polymer chains separate. Thus, the resin softens, eventually becoming a viscous melt, and the thermoplastic upon cooling. This behavior of the thermoplastic is repeatable and allows reprocessing of thermoplastics. In contrast, thermoset resins are initially individual polymer chains that can be dissolved and can flow. Upon heating, however, covalent bonds or cross-links form between the polymer chains. This irreversible cross-linking produces three-dimensional networks, thereby preventing or limiting reprocessing of these resins. Thermoplastic resins usually contain long-chain polymers. Thermoset resins, however, can be divided into two groups: thermoset rubber and thermoset oligomers. Thermoset rubbers are long chain polymers with reactive sites that facilitate cross-linking. This group includes polyisoprene, styrene-butadiene rubber (SBR), and nitrile rubber. The second group contains materials like phenol formaldehyde, melamine formaldehyde, unsaturated polyester, and epoxies. These thermoset resins are usually produced in two stage chemical reactions, with chemically reactive shorter chain molecules or oligomers formed in the first stage. When these oligomers are heated, they form reactive sites. For example, phthalic acid and glycerol are initially condensed to form a branched A-stage resin (Figure 12a) and these branched molecules cross-link to produce the three-dimensional structure shown in Figure 12b.a)b)Figure12. a) A-stage resin and. b) cross-linked three dimensional structure, where P is phthalic acid and G is glycerol39.3.3. Polymer ConfigurationsFigure 13 presents the effect of processing on the configurations (arrangements) of thermoplastic polymers. With the exception of thermotropic liquid crystalline polymers (LCPs), all polymers are amorphous in the melt state. In this state, the polymer chains form a random mass (i.e., random coil configuration), which is the most energetically favorable state. LCPs, however, are rod shaped in the melt because of their rigid polymer structures.Figure13. Effect of processing on the morphology of thermoplastic resins40.Upon cooling, polymers with irregular structures remain amorphous. This amorphous morphology gives the polymers broad softening ranges and low shrinkage because the randomly-ordered chains slowly expand and contract. The random chain ordering also 1) allows penetration of chemicals, thereby giving the polymers low chemical resistance, 2) cannot resistance fatigue and wear, and 3) provides only one index of refraction, allowing the polymers to be transparent. Amorphous plastics include polystyrene, polymethylmethacrylate, polycarbonate, and polysulfone.Highly ordered polymer structures allow other polymers to form a combination randomly-ordered chain segments and well-organized structures (i.e, crystallites) when the melt cools. This semi-crystalline configuration gives the polymers sharp softening points as the crystallites melt; high shrinkage associated with forming tightly ordered structures; good chemical, fatigue, and wear resistance offered by the crystallites, and two indices of refraction (i.e., one for each phase) to make the plastics opaque. Semi-crystalline plastics include polyethylene, polypropylene, aliphatic polyamides (e.g., polyamide-6,6), polyacetal, polyethylene terephthalate (PET), and polyphenylene sulfide.When the temperature exceeds a polymers glass transition temperature, Tg, the amorphous regions of the polymer soften, but any crystallites remain intact. As the temperature increases, the amorphous regions expand, thereby soften the plastic. With amorphous polymers, the material gradually becomes a fluid enough to melt process. The crystallites in semi-crystalline polymers, however, do not become disordered until the temperature reaches the polymers melting transition, Tm. Once the crystallites “break up,” the polymer rapidly soften enough to undergo melt processing. For polymers, like ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE), increasing temperature causes the covalent bonds in the main polymer chains to break before the polymer can flow. Thus, UHMWPE and PTFE can not be processed using convention melt processing methods.3.4. Polymer StructureThe structures of polymeric repeat units determine their stiffness, polarity and intermolecular bonding. Incorporation of stiff side groups or rigid aromatic rings into a polymer repeat increase the modulus (stiffness) of the molten and solid polymer as well as increasing its critical transition temperature. Thus, polyethylenes Tg is about -120C, while the Tg values for polystyrene and polycarbonate are 100 and 150C, respectively. More polar repeat units also allow for stronger intermolecular bonding. For example, polyethylene which has only van der Waals interactions has a Tm of 110C and a flexural modulus of 1110 MPa, whereas the hydrogen bonding in polyamide-6,6 provides a Tm of about 200C and a flexural modulus of 2800 MPa.Table 1 summarizes structure-property relationship for polymers. In general, higher molecular weight can improve the tensile strength, toughness, hardness, chemical resistance, and softening temperature of plastics, whereas broad molecular weight distributions reduce most of these properties. Greater levels of crystallinity and cross-linking enhance the tensile strength, modulus, hardness, chemical resistance, and softening temperature of polymeric materials. Finally, the strength of the primary and secondary bonds, the location of transition temperatures, and the morphology are determined by the chemical and steric structures 41.Table1. Structure-Property Relationships for Polymers42Polymer resins can be homopolymers, copolymers, or blends. Homopolymers contain one type of repeat units, while copolymers are polymerized using two or more monomers. Low density polyethylene (LDPE), polypropylene, and other homopolymers may actually contain a small amount of a second repeat unit, but typically have a well-defined processing window. In contrast, the processing conditions for copolymer depend on the ratio of components and their arrangement within the polymer chain. Random copolymers, like poly(ethylene-co-vinylacetate) and poly(styrene-co-acrylonitrile), consist of a random arrangement of the two mers and are generally single phase systems. Since block copolymers include long segments of each mer, they tend to phase separate (i.e., form separate regions containing each component). Block copolymers include thermoplastic polyurethane elastomers, polyether amides, and styrene block copolymers such as SEBS.Blends, such as poly(acrylonitrile-butadiene-styrene) (ABS), are mixtures of two or more polymers. Since the polymers do not mix well, most blends exist as phase separated systems. This behavior improves the impact resistance of polystyrene in impact modified polystyrene (HIPS), but can produce complex morphologies that are sensitive to part design and processing conditions. For example, polycarbonate/ABS, a three component blend in which the three phases are polycarbonate, polystyrene-co-acrylonitrile, and polybutadiene, shows significant differences in mechanical properties and surface morphology (which affects painting and plating) when the processing and design produce high residual stresses 43.Few commercial plastics resins are solely polymer. Additives are incorporated into the polymer to alter the bulk properties, either for a wide range of end uses (e.g., antioxidants) or for specific applications. The main categories of additives are the fillers, plasticizers and softeners, lubricants and flow promoters, anti-aging additives, flame-retardants, colorants, blowing agents, cross-linking agents, and UV stabilizers. Fillers, such as glass fibers, carbon black, and talc, enhance the mechanical properties. Plasticizers aid in the flowability of the polymer while the lubricants reduce the friction of the moldings. Flame retardants prevent or limit the burning of polymers. Blowing agents are used to produce gas-filled cells in the polymer matrix which enhance the properties and lower the weight of the polymer.3.5. Melt ViscosityMelt viscosity, the resistance of a polymer melt to flow, varies with processing conditions, particularly flow rate (i.e., shear rate), temperature, and pressure, and with the polymeric material, i.e., the structure, molecular weight, and additives. Viscosity increases for more rigid repeat units, with molecular weight, and when filler and fibers are incorporated into the polymer. As shown in Figure 14, low shear rates do not affect melt viscosity. When the shear rate reaches a critical value, however, the polymer chains begin to align in the direction of flow and the melt viscosity begins to decrease with increasing shear rate. The linear decrease (on a log-log curve) in viscosity with increasing shear rate is known as the power law region and can be modeled using: (3)Where k is the consistency index and n is the power law index. Not all polymers have the same sensitivity to shear rate, and so the power law index varies from about 0.15 for thermoset rubbers to 0.30 for polystyrene and polyvinyl chloride to 1.0 for Newtonian fluids like water. Melt viscosity decreases with increasing temperature. For narrow ranges of melt temperature, this decrease can modeled using an Arrhenius equation: (4)Where hR is a reference viscosity, Ea is the activation energy for flow, R is the universal gas constant, and T is the absolute temperature. Activation energies typically vary from about 7 kJ/mol for thermoplastic vulcanizates to 100 kJ/mol for materials like PMMA and polycarbonate. The WLF equation is employed when the temperature dependence of melt viscosity is modeled over larger temperature ranges. Higher pressures produce in an increase melt viscosity that can be expressed as: (5)Where a is an empirical constantFigure14. Effect of shear rate on melt viscosity44.3.6 DegradationThermal stability, the ability of a polymer to withstand processing and service (use) temperatures, is a function of the polymer structure and molecular weight. Aromatic rings in the polymer backbone tend to enhance stability whereas some functional groups like halides (e.g., chloride in PVC) often leave the chain easily. High molecular weight causes more entanglement, which limits the mobility needed for degradation, and reduces the number of unstable end groups.During processing, thermal, mechanical, chemical, and radiation-induced degradation occurs in polymers. Thermal degradation produces random chain scission, cross-linking, chain depolymerization, and unzipping of substituent groups. With random chain scission, covalent bonds are broken and the subsequent reduction in molecular weight decreases viscosity and selected mechanical properties (as discussed in Section 1.3.1). Cross-linking, which increases melt viscosity and material stiffness, occurs when unsaturation exists in polymers like the polybutadiene phase of HIPS and ABS. Chain depolymerization is the unzipping of polymer chains, which occurs with polyacetal, polystyrene, and polymethylmethacrylate, whereas unzipping of substituent groups is represented by the dehydrohalogenation of chloropolymers and fluoropolymers. Mechanical stress can break polymer chains, usually in the middle, but frictional heating more commonly produces thermal degradation. Chemical degradation happens at high temperatures when water, acids, bases, solvents, or other materials attack the polymer or when the polymer oxidizes. Finally, radiation treatments can produce chain scission or cross-linking in the polymer. Although thermal degradation often results from a combination of overly-high temperatures and/or too much residence time, the most common cause of degradation is the failure to properly dry the resin particles.4. Method (Process)The injection molding process consists of plasticating the resin (Figure 15a), injecting the molten polymer into the mold (Figure 15b), packing more melt into the mold to compensate for shrinkage, maintaining a holding pressure on the melt until the gate solidifies, and cooling the part in preparation for removal from the mold (Figure 15c). At the beginning of the injection molding cycle, clamp is open, the ejector platen is retracted, and the injection unit contains a plasticated shot. The nozzle and the sprue bushing must also contact each other before the start of melt injection 45. After the mold closes, the axial movement of the screw forces the melt into the mold through the nozzle. The end of mold filling is determined by the ram position, time, or pressure. Once the mold is filled, pressure behind the screw in the packing stage forces melt into the cavity to compensate for shrinkage. Subsequent holding pressure prevents melt from leaking out of the cavity until the gate freezes off. In some machines, the packing and the holding stages are combined into the second stage. Figure15. Injection molding process46. Cooling and plastication occur after the holding phase. Melt in the mold is cooled until the part can be ejected. During cooling, the screw rotates to build a shot for the next cycle. Back pressure controls the additional mixing of the melt. Once the shot is built, the screw is retracted axially to allow decompression or suck back to relieve the pressure on the melt and the part is ejected completing the injection molding cycle.4.1. Filling StageDuring the first or filling stage, the mold cavities are filled with molten resin. The critical parameters for molding filling are the 1. Shot size. Shot size is optimized by filling 95 to 98% of the cavity. For thin wall parts, the entire cavity is filled because the part solidifies before packing47. Too little material results in a short shot that usually affects packing and shrinkage in the molded part. Excess material squeezes out of cavity as flash which damages the mold at the parting line. 2. Injection velocity. Injection velocity is the rate at which the screw moves forward. During filling (injection), the screw travels at a constant velocity and develops either a pressure in the hydraulic lines controlling screw movement or a load on the servo motor in all-electric machines. The injection speed is usually optimized to produce defect-free parts. Very slow filling provides poor surface and part quality while rapid injection can causes jetting flow instabilities and degradation at the gate.3. Injection pressure. The injection pressure or fill pressure high limit setting permits development of enough pressure to sustain a constant injection velocity. The pressure required to fill the mold depends on the thickness and flow length of the part 48 and the viscosity of the polymer melt. As shown in Figure 16, the minimum pressure occurs at a moderate injection velocity. Figure16. Relationship between injection pressure and injection velocity. 4. Injection time. The molding machine requires a sufficient fill time limit to complete the filling stage. Otherwise, the machine switches prematurely to the packing stage. 5. Switchover technique. Switchover or transfer is when the machine changes from the velocity-controlled injection stage to the pressure-controlled packing stage. This switchover can occur by time, position, or pressure. With timed transfer, the screw is driven forward for a set period of time, whereas for position transfer, the screw travels a specified distance. Pressure transfer occurs when the pressure behind the screw, in the nozzle, or in the cavity reaches a preset value. Timed switchover is not commonly used because it produces poor repeatability. Most molders use position or pressure transfer. 4.2. Packing and Holding StagesSince the melt cools as it enters the mold, the melt contracts or shrinks. The second stage or pack stage is necessary to force more melt into the mold to compensate for this shrinkage. When no more material can be forced into the mold, melt can still leak back through gate. Therefore, a third stage or hold stage applies forces against the material in the cavity until the gate freezes. In some machines, pack and hold are combined into a single second or holding stage. Packing controls include packing pressure(s) and a packing time. The melt viscosity and the pressure on the ramdetermine the movement of the ram during packing and holding. The packing pressure is usually 80 to 120% of the fill pressure5. Although packing times vary considerably, Scales and Nunn5 suggested that the packing time should be about 30% of the fill time. Holding controls also include holding pressure(s) and a holding time. Typically, hold pressures are lower than pack pressures in order to reduce the level of retained stress at the gate. Holding times, however, are usually longer and depend on the gate thickness. 4.3. Cooling Phase After the part is filled and packed, the molten polymer is cooled sufficiently to withstand the ejection forces. To accomplish this, the part temperature at ejection must be below a distortion temperature, which is usually the Vicat softening temperature. This cooling processing is slow, and so, the cooling time consumes almost 70% of the total cycle time. The rate of cooling determines the physical appearance of the part. If the part is cooled too rapidly, it becomes brittle and lacks gloss. Cooling rapidly results in poor crystal growth and hence poor crystallinity. Poor crystallinity affects the part quality and the part becomes brittle. Slow cooling increases the level of crystallinity in semi-crystalline materials. Uniform cooling improves part quality by reducing residual stresses and maintaining dimensional accuracy and stability 49. 4.4. PlasticationDuring cooling, the screw rotates to melt and homogenize the polymer. The length of screw rotation is determined by the shot size. Plastication in injection molding machines has three stages: screw rotation, soak, and injection (screw forward). Since screw rotation primarily melts the polymer using friction, it is the most desirable mechanism. Therefore, the screw speed a setting on a flow control valve or servo motor is set so that plastication ends just before the cooling stage times out. The extruder delay timers can delay the screw rotation to facilitate this goal. A back pressure setting restricts the movement of the hydraulic fluid or servo motor. This back pressure facilitates mixing, but increases the plastication time. Once the desired shot size is attained, the screw rotation stops. Then the molten plastic is heated by conduction from barrel walls in a soak stop. Since this soak increases melt temperature, reduces melt viscosity, and may degrade polymer melt, it is usually limited. The melt is also sheared during injection pressure to increase melt temperature and reduce melt viscosity. Typically, the screw speed is set at about one-half the maximum value (e.g., 100 rpm) and the back pressure is set to about 10% of the maximum value. Then the extruder delay time is adjusted so that plastication ends just before the cooling stage times out. 4.5. Starting Up the Injection Molding MachineUpon starting up an injection molding machine, the power, water, and sometimes hydraulics are turned on. The heater bands surrounding the barrel will melt residual plastics materials in the barrel. This “soak” typically requires one-half to a full hour for the machines in the Plastics Departments laboratories. Failure to pre-heat the plasticating unit often results in a broken screw. The water cools the feed throat, any hydraulics, and the mold. The hydraulic motors may be turned on to pump the hydraulic oil and raise its temperature to a set level. The mold temperature controller is usually turned on somewhat later as its “soak” time is usually shorter.Barrel, nozzle, and mold temperatures are determined by the resin being molded. These can be obtained from suppliers literature or generic sources, such as ./. Since the feed throat of the cylinder (barrel) must be kept cool, a rising barrel temperature profile (from the rear to front of the barrel) is usually used. A temperature differential of 38C (100F)1 or more is not uncommon. The range is subject to the requirements of the cycle and the size of the shot as compared to the capacity of the machine. Fast cycles and large shot sizes may require a more even, or even a reversed, temperature profile.When the “soak” is completed, the injection molding machine is ready for operation. In the first operational step, the barrel is purged. Plastic is conveyed through the barrel by rotating the screw to build up a shot and then “injecting” the shot onto a purge plate. This process of rotating the screw and injection occurs until degradation-free polymer is flowing through the barrel. When purging is complete, a shot is collected for the first injection molding cycle. The mold is then closed, and the plasticating unit or sled is moved forward to contact the stationary platen. Finally, the mold is opened, and the injection molding machine is ready to cycle.Appendix A contains the procedure commonly used for optimizing processing parameters. 5. Man (Operator)The man is machine operator who plays an important role in maintaining the part quality during mass production irrespective of the process being either manual or automatic. Various methods are employed in industries to eliminate the errors caused due to manual labor. Examples of such methods are semi-automatic operation, continuous, lights out, and MPX. 5.1 Safety Considerations1. Safety glasses should be worn at all times during the experiment.2. The safety shield must be in position when melt is purged (air shots) from the barrel.3. Do not place hands or other foreign objects (sticks, tools) in the hopper area when the machine is operating.4. Do not tamper with any of the machines safety devices.5. Do not operate the machine in the “automatic” mode.6. Wear heat-resistant gloves when removing parts from the mold.7. Do not operate the machine if a member of the group is behind the machine.8. Examine rubber hoses from the mold circulating unit for cracks or any other potential cause of failure.注 塑在注射成型工艺中，热塑性树脂熔化、融化物被 (注射)到模成型。直到聚合物熔体冷却后,从模具取出。大规模生产注塑允许网络制造高精度的三维的形状的塑料配件。1. 注塑成型最常见的一种塑料的制造工艺，可产生部分重量高达150公斤。2. 现在注塑工艺中有30%的树脂过程中消耗。3. 其中90%塑料可以被再利用的。主要优点包括：1)几乎无限的复杂性,2)良好的外观,3) 很好的控制的壁厚和尺寸稳定性,4)需要投入很少。注塑工艺的五个重要的部分：射出成型机模型原料方法工作人员虽然理论是无限的,但讨论的是有限的,单螺杆注塑机、往复流动的热塑性聚合物熔体在分开两半的冷流道模具。射出成型机注塑机有三个主要组成部分（如图1）: 1)喷射装置,2)夹持装置,3)控制部分。压射系统把融化的高分子材料注入到模子。夹紧装置对模具支持和提供机制的开模和闭模、注塑件的排出的。图1 注塑机在一个塑料成型周期、螺杆合上模具。熔化塑胶螺杆之间、喷油嘴在喷射装置 (通过螺丝)进入模具。控制量的融化通常注入填充模具腔的总量大约95%到98%。注射后完成时,螺杆是压力维持一段特定的时间。在这个阶段,更多的融化料压入模具来补偿收缩胶料。持有压力防止融化回流，当融化物冷却到不能再退出腔阶段结束。这是，注塑成型进入一个冷却阶段，在这段时间里,直到它冷却能够承受弹射力量。这个阶段螺杆仍然旋转，让更多的塑料融化，做好下一阶段的准备。在最后的阶段,模具的冷却,部分被打开。常规成型周期如图2所示。2-3mm厚的部分,在不到5秒时三分之一,包装要求填写一次,保温时间取决于闸门大小,冷却时间最长的部分的周期。薄壁零件(例如,壁厚是1毫米),然而,填写小于1,通常没有进入包装阶段,并迅速冷却。图2 成型周期图1.1喷射装置压射系统必须1)溶化的高分子材料和形式的喷射,2)转移(注射)融入模具,2)建立包装和压力,3)把喷嘴接触物料套管的模具,以及(4)产生压力。往复式螺丝注射机。如图3 ，这些单位包含1)料斗,2)螺杆运动,3)喷射机制(液压缸或机动系统),4)桶和螺丝,和5)的喷嘴。图3往复式螺丝注射机。聚合物颗粒饲料(由重力)进入料斗,透过冷却饲料的喉咙,落到旋转螺杆。料斗喉咙是水冷，以防止树脂部分熔融。当电机旋转螺杆，摩擦从旋转螺钉和传导从电阻炉带围绕每桶融化的高分子材料。旋转螺杆还转达了聚合物对喷嘴。结束时的螺钉，熔融聚合物穿过止回阀，防止熔体从回流对漏斗。熔体是被困喷嘴之间的封锁和止回阀（图4 ）。这个融化部队向后螺杆(例如,向料斗)。当足够的螺丝停止转动被融化已成为喷嘴和逆止阀。这就是所谓的测量轴向旅游的螺丝或“划尾浆”。图4桶、螺杆、逆止阀(检查)、喷嘴。注射口也随着滑板转动滑行。便于清洗,改变喷嘴,或调整出行距离,单位为从静止式。为了便于清洗，更换喷嘴，或调整滑行的距离，放弃使用固定板。在注射成型周期中,注射机构向前运动,使直喷嘴也浇口直接接触。接触压力(即喷嘴和物料之间的压力)是喷嘴和浇口保持进门连接。注塑模具的机器单位是指：1.喷射的尺寸。喷口的大小是最大的塑料注射成型可在一个周期,被认为是在盎司的通用聚苯乙烯(GPPS)为美国机械10与cm3欧洲和亚洲的机器11,最好的品质,部分必须用大约60% 70%的额定大小12。小尺寸较大的不规则性和射击精度损失,而不允许足够喷口尺寸为包装、熔体垫塑化效率低下。你会用一个微型注塑设备有3射击的大小,但有机械工程塑料的大小的3至8盎司。2. 塑性和回收率。橡胶压片机压的能力是衡量的数额塑料可以融化、均质单位时间(磅/小时)或（公斤/人力资源),橡胶压片机能力(就开枪大小)生产塑料和橡胶压片机压过高的能力产生热降解由于不再停留的时间。恢复率是测量的体积输出的注塑机(表示,在3 /秒)。双方的回收率和橡胶压片机能力取决于运行聚苯乙烯在50%的最大值。3. 最大的注射速度。最大的注射速度在常规射出成型机的范围很广,从150 - 250毫米/秒(6 - 10 / s)13和高达2000毫米/秒的薄壁机器14。使用微射出成型机,你就可以具有最大注入速度为160毫米/秒。4.最大限度的压力。在所有的注塑机，注射速度和注射压力有联系，以便确定注射速度不能维持足够的压力。可用的最大值喷油压力为一个标准的注塑机是138兆帕（ 20,000磅） ，可高达324兆帕 15 。1.2夹紧装置夹紧装置支持模具；打开和关闭模具；在注射模具闭着,包装和保持并持有脱模装置。这两种主要夹液压系统及切换夹。一个液压钮钳如图5。(注:你将会用一个液压钳,但是我找不到一个好的照片)。跟所有的夹紧装置,由单位包含一个固定架(1),一个动人的滚筒(3),安装模具(2)。这些平台和架尾滚筒(末端的机器)通常是支持领带酒吧(7)。固定架滑板都有孔,方便安装的塑造和允许喷嘴接触物料套管。动滚筒(4和5) 通常是被系统支持。在液压钮夹、一个触发器的机制(6)提供了一个机械之间的联系,移动和尾架挡板、液压缸(9)。向前延伸的动作气缸的移动,移动的切换,合上了模具。滚筒伸展的触发器提供所需的夹具力保持滚筒关闭在成型周期。反向动作气缸收回曲轴和开模。环形齿轮和夹具调整液压马达(9和10)调整位置的移动滚筒相对的尾架滚筒。这个动作是所谓的高度调节,并允许的随机性,要融入了模具的夹紧装置。最后,如果被困两边的关闭,这产生了压模的液压缸。因此,模具保护典型地由逆转动作时,却无法接近模具的压力已经达到了一个预设级别。向前移动的时候没有合模也可以被局限于一个预设值。电动钮夹非常相似,但是液压切换各种电动马达驱动机制(曲轴和喷发的机制)。图5.液压钳夹具单元16。在液压钳(图6),还有两个液压缸,没有钮。双缸驱动来开启和关闭,但更大的模具主要圆筒帮助提供夹力。有效填充后者气瓶、50mn切边液压机充液阀打开之前圆筒开始关闭了横夹。液压油从主或辅助主钳运动的导线缸。当模具半场的触碰,50mn切边液压机充液阀关闭和进一步的运动的压缩油缸,主要生产夹力。这个过程是颠倒的模具是打开的时候,另一缸为产品顶出。用流体力学夹、模具的开启和关闭的使用一个触发器机制而产生的夹持力由一个或更多的液压缸(17)。指定的单位是夹夹力。此外,一个参数的数量限制大小的模具,可以安装在机器。微射出成型机,你将使用带有夹子力,但系3万吨塑料机械工程有力量从3夹100吨。图6.液压钳单位(18)在模具打开,部分仍然与移动的模具,通常用的流道牵引。一个弹射系统(图7)通常分离部分模具。分开的部分,液压模或电开关弹射滚筒向前推进(即走向静止式)。连接杆、敲推到这个从动盘在模具、力式的顶针板。因此,推板安装推的了解部分模具(19)。帮助弹射杆的位置为模具闭缩为下一个周期。图7 弹射单位 20 2. 注塑模具组成部分，冷却和注入的注塑模具的安装与固定和移动板的成型机构28 。因此，模具的一个或多个空心型腔形状像所期望的产品。所示，图8 ，一个典型的模具是一系列的盘子。腔和核心板包含的几何形状的零件和热流道系统（如果需要） 。注射器针和安装在浇道之间的弹射器和喷射器（或腹泻）聘板。这些板支持的顶部和底部夹紧板，支持板，并支持支柱。模具型腔之间的分裂和核心板，模具生产两半。腔或A一侧的模具安装在固定板的核心或B方是安装在移动式。这些半对齐使用四个领导人引脚和衬套。图8 一个典型的两分模具(21)熔模,送到注入套管,这为最高的护板、腔。一个定位环环绕的套管的模固定阶段,从而调整物料台板衬与喷嘴。融化流进赛跑的物料套管,穿过大门,进了蛀牙。图9洞室布局的礼物是加模具。物料从喷嘴送融化的选手,分裂融化流交付给四腔。核心和型腔设计控制的形状、大小、表面纹理的塑造的一部分。蛀牙,位于腔和核心板块之间,放置位置的依赖和切割线设计。术语“塑造半个”并不意味着这两个模具部分相等的宽度。如图9、物料的部分是圆锥促进释放。一般有圆直径者(如图9)、梯形、或修改,因为这些设计梯形截面特性提供最好的分型面(22),(23)、(24)。这些选手的尺寸大小取决于材料和部分(25)。当熔体流动的人从腔,化解经过截面积减少在模具称为一扇大门。控制熔体流动进入型腔和减轻分离的塑造,部份横浇道系统26。设计、浆纱、位置的大门的剪切经历影响;(2)在城门口、熔体流动方向(例如,定位)和水平的灌装,3)平衡腔内流动的不存在,如喷射,4)位置的排气和分离线,5)号码和力量的焊接及meld线,6)的转轮废铁,7)需要次要作业。图9 在一个多型腔布置模具27一般来说，门的大小是由部分壁厚 28 ，整体的一部分大小和材料特性。较厚部分需要更大盖，以促进包装，但门深度小于部分厚度允许适当弹射没有一个丑陋的门遗迹（即商标的一部分留在门口时被删除） 。随着薄壁零件，门深度可能大于部分的厚度，以减少填补pressure28 。长流部分的长度和大腔表面需要更大的大门，以减少填补压力，并防止过早门冻结了。高等粘度树脂还需要更大的盖比更容易流动树脂，大门口横截面减少剪切应用于聚合物熔体，减少和短期的土地发生的喷射和其他流量不稳定。慷慨的腔半径的大门也创造层流和防止溢出28。有些材料加工窗口时,有广泛的其它材料只能使用范围的成型条件。这种行为通常发生在小盖往往导致热降解材料和过多的残余应力(嵌入)的一部分。虽然太小的一门损失的强度的钢材的陆地面积,这可能会使钢打破(29岁),长期地促进喷射。因此,土地长度的钢(例如,门)通常是50%的大门的深度。传统上,注塑模具加工从工具钢。模具温度控制从水中钻的线,这是公司的核心和多腔模具。水加热或冷却在模具温度控制器和泵入模具。自从不能产生较小的特点、机械工具包括电铸镍(用于数字多功能光碟)和的生产使用传统半导体硅来制造衬垫(暴露与刻蚀)过程。3.高分子材料长链分子聚合物的分子(也就是简单的重复,河区域并)。网上有长链的影响,最后是链间纠缠的力量,时间尺度的运动33。几个因素,包括聚合物的相对分子质量和分子量分布,其性质,它还是一种热固性塑料的分子结构,其结构性能的影响。熔化粘度和降解机理的塑料也同样重要。当考虑到聚合物熔体流动通过大门。3.1分子力的影响在聚合，链长可以多种多样，不是所有的连锁店具有相同的长度。分子量是衡量的平均链长，而分子量分布（分布）是衡量各种链长。三分子量通常报告的聚合物。人数平均分子量，是指链长，并提供了一个估计间的吸引力和一些高端群体的树脂。相比之下，重均分子量的时间越长链，从而产生一种估计链缠结。最后，厂平均分子量， ，喜欢很长的聚合物链，并已与熔体强度的材料用于吹膜挤出和挤出吹塑。虽然多分散指数， (1)不准确测量相对分子质量分布,通常是被用来表达范围的链。正如在图11的效果分子量的力学性能随具体的财产。增加的原因链纠缠特性，如熔体粘度和冲击性，增加分子量。粘度、h、范围已涉及到使用方程重量34。 (2)在一个有经验的常量K。对于线性聚合物,一个是1.0 (如,齐聚物发生后,一个是3.4%)的分子量已超过临界分子量。属性依赖电子景点和数量的增加,最初末端重量,但依然不变实现了一个门槛分子量。这些性质包括抗拉强度、弯曲模量、玻璃、熔体温度。图11 效果的分子量的力学性能35通常用于注射成型聚合物分子量比有批评或阈值的分子量。一般来说,聚合物经加成聚合准备的(例如,聚乙烯、聚丙烯、聚苯乙烯、基于、聚氯乙烯)有较高的分子量比那些利用缩合反应(如聚碳酸酯,聚缩醛树脂、聚酰胺或尼龙)。不当乾缩合聚合链也更容易断裂及随后的分子量下降。因此,缩合聚合更易展示效果的分子量的退化、抗弯性能量张力,但是所有的聚合物展示这些影响熔融粘度变化和冲击性能。加热历史的影响,然而,依赖于特定的特征的聚合物。在最近的研究m.北京:36、37冲击改性聚苯乙烯(臀部)和聚碳酸酯呈显着减少对熔体流动指数和悬臂式冲击性能和后续循环记录,但较小的变化、抗弯抗拉伸性能。分子量分布(淤)通常是一种功能的聚合技术和树脂的热量的历史。茂金属和齐格勒提供相对狭小的分子分布(例如,PI 2 - 4),而作为自由基的催化和催化剂材料更广泛的分子量分布(例如,PI 20到50)m.北京:38。宽广的分子量分布的加工性能的改善,但减少许多树脂密封性能如热在聚乙烯吹膜。3.2热固性塑料和聚合物聚合物可分为两类:热塑性塑料,热固性塑料。在一个热塑性树脂、长链分子是由比较脆弱间点,如范德华力、氢键。当资料被加热,电子力量削弱和聚合物链分开。因此,该树脂软化,并最终成为粘性融化、热塑性塑料在冷却。这种行为是重复的,并允许再热塑性塑料的。相反,热固性树脂是最初的个人聚合物链,可以溶解和流通。然而,在加热时,债券或交联键:之间形成了聚合物链。这个不可逆的交联生产三维网络,从而防止或限制的处置这些树脂。热塑性树脂通常含有银杏聚合物。热固性树脂,然而,可以分为两类:热固性塑料橡胶及热固性齐聚物。长链聚合物热固性塑料橡胶是促进和活性位点进行交联。这个小组包括聚异戊二烯、国外橡胶(丁苯橡胶)、腈橡胶。第二组含酚类物质,如甲醛、苯甲醛、三聚氰胺的不饱和聚酯树脂、环氧树脂。这些热固性树脂通常产生于两个阶段的化学反应,与化学反应短链分子或齐聚在第一阶段。当这些齐聚物加热时,它们形成活性位点。例如,邻苯二甲酸和甘油是最初的浓缩形成一个分支A-stage树脂(图12a),这些分枝的分子结构进行生产如图12b。a)b)图12 a）树脂. b)交三维结构,P是邻苯二甲酸和G甘油m.北京:39。3.3聚合物配置图13介绍了影响加工的配置（安排）的热塑性聚合物。除热致液晶聚合物（ 发
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