基于網(wǎng)頁的測試技術(shù)學習、實驗系統(tǒng)設(shè)計說明書

基于網(wǎng)頁的測試技術(shù)學習、實驗系統(tǒng)設(shè)計說明書,基于,網(wǎng)頁,測試,技術(shù),學習,實驗,試驗,系統(tǒng),設(shè)計,說明書,仿單 編號： 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