Modern Plastics Handbook 2011 Part 7 ppsx

Instead, a melt stream shoots straight into the cavity, only stopping when the stream contacts the end of the cavity. The remaining melt then fills the cavity as fountain flow. As shown in Fig. 5.73, 166 this jetting produces as weld line within the cavity. The jetting is not aes- thetically pleasing, and the weld line is not as strong as the surrounding material. While jetting can be reduced or eliminated by reducing the velocity of the melt front, the effects of injection velocity are very complex and difficult to predict. 166 Typically, enlarging the gate and runner, reducing gate land length, and promoting mold-wall contact by locating the gate so that the flow is directed against a cav- 5.112 Chapter Five Melt Melt front Melted plastic material Frozen layer (a) (b) Figure 5.72 Fountain flow. (a) Top view; 166 (b) side view. 167 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.112 ity wall are more effective in eliminating jetting. In contrast, when cold melt is slowly injected into the cavity, the melt freezes as it flows into the cavity. While this cold flow produces poor surface and part properties, it can be eliminated by increasing the injection speed, melt temperature, and/or mold temperature. Rapid injection can also cause degradation of the polymer at the gate. Gate blush, a discoloration outside the gate, can be accompanied by a reduction in part properties, particularly impact strength. 168 Dieseling occurs when air trapped in the cavity burns. This produces discoloration or burning of parts, particularly at the end of the fill, and is often accompanied by short shots. Reducing injection speed, increasing melt temperature, and increasing gate size all tend to reduce or eliminate gate blush whereas remedies for dieseling include reducing the injection velocity, cleaning or enlarging mold vents, and redesigning venting in the mold. Since hydraulic pumps maintain a constant or controlled flow of hydraulic oil, the flow to a particular hydraulic cylinder is regulated by either a flow control valve, a proportioning valve, or a servo valve. A flow control valve merely restricts the flow at a particular opening size until the valve’s setting is changed. These values are typically used in “pressure-controlled’ machines where the injection speed is a percentage of the valve’s full open position. A proportional valve is a directional valve which adjusts the flow of oil in response to the position of an electric solenoid whereas a servo valve controls the flow of oil proportional to an electrical feedback signal. The latter system maintains a constant flow rate even with changes in the force against which the flow is working. Proportional valves or servo valves control Processing of Thermoplastics 5.113 Figure 5.73 Jetting. 166 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.113 fill for injection-velocity-controlled machines and allow profiling of injection velocity. With all-electric machines feedback to the servo motor also provides tight control of the injection velocity. When the injection velocity remains constant, the velocity of the melt front varies with the cross-sectional area of the mold cavity. The melt front speeds up when the cavity narrows or thins and slows down in wider or thicker sections of the cavity. Since the changing velocity of the melt front alters the orientation of the polymer chains in a cavity, extreme changes in melt front velocity can produce differential shrinkage in the part. By profiling injection velocity, the injection speed can be var- ied in response to changes in part geometry. The proportional or servo valve is given two or more velocity settings and (usually) a percentage of the total shot size for which each injection rate is valid. Although the maximum number of velocity settings in the profile varies with the machine manufacturer, velocity profiles are typically determined using flow analysis software. The injection pressure setting is the maximum pressure that can develop in the hydraulic lines that feed the injection cylinder(s). Thus, injection pressure is typically read as hydraulic pressure and is the setting on a pressure control valve. The valve opens when the line pressure reaches the set value, and the remaining hydraulic oil is returned to the reservoir. For pressure-controlled machines, the injection pressure is typically a percentage of the maximum hydraulic pressure whereas with injection-velocity-controlled machines, the injection pressure (or fill pressure high limit) is read as a hydraulic pressure. The injection pressure is read directly for all-electric machines. For all machines, the injection velocity is controlled by the settings for injection velocity and injection pressure. In pressure-controlled machines, the injection speed is set using a flow control valve, but the desired injection speed is not achieved unless pressure is high enough. Thus, pressure is incremented until the entire shot size is injected into the mold, and the actual injection speed is not usually known. With injection-velocity-controlled machines, the hydraulic pressure at transfer (line pressure) is typically displayed on the control system. If the injection pressure (fill pressure high limit) is greater than the hydraulic pressure at transfer, the injection speed is usually constant for the entire injection time (or travel). Since the injection pressure is simi- larly displayed in all-electric machines, exceeding the pressure limit during filling also causes the injection speed to rapidly decrease. The viscosity of the hydraulic oil can severely affect injection pressure. While hydraulic oils are fairly Newtonian, their viscosities will decrease with increasing temperature. Decreases in oil viscosity generally affect pressure settings; pressure develops more rapidly in the hydraulic lines, causing pressure control valves to open prematurely. 5.114 Chapter Five 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.114 Consequently, many injection molding machines monitor and/or control oil temperature. Typically, hydraulics are run prior to molding to heat the oil to an acceptable temperature, and an oil temperature setting or window is often an interlock on the molding cycle. Oil is also cooled by water that is forced through a cooling manifold. This pre- vents degradation of the oil. Oil is also filtered to prevent wear in the hydraulic cylinders and lines. Injection pressure is a primary factor affecting molding velocity and part quality. The pressure is influenced by part, gate, runner, and sprue dimensions, part surface area, melt temperature, mold temperature, injection velocity or injection time, and polymer viscosity. While pressure increases as part thickness and gate, runner, and sprue dimensions are decreased, part thickness and the pressure required to fill thin-wall parts is typically the most important factor in determining whether a part will fill. 169 Injection pressure also increases with part surface area since this increases the drag on the polymer melt. Although the pressure decreases with increased mold and melt temperatures, melt temperature has a greater effect than mold temperature. Finally, injection pressure is typically high at low injection times and high injection speeds because the polymer chains cannot orient in the direction of flow. At greater injection times or slower speeds, the required pressure decreases. However, if the injection time is too long or the velocity too slow, the polymer melt cools, thereby increasing melt viscosity and injection pressure. Injection time, the maximum time for which injection can occur, is the setting on a timer. When this time is set for the transfer technique (transfer from fill to pack), it determines the time in which the cavity fills. However, if other transfer techniques are used, the time setting is merely a safety or default value. Thus, if transfer does not occur by the other technique, the machine switches to pack or second stage at the set time. When time is set for the transfer technique, injection time is incremented until the entire shot size is injected into the mold and the plastication begins immediately. When other transfer techniques are used, injection time is set 1 to 2 seconds higher than the time required for injection. The switch from one part of the injection molding cycle to the next is called transfer. Although the transfer from fill to pack or from first stage to second stage, can occur using time, ram position, hydraulic (line) pressure, nozzle pressure, cavity pressure, tie bar force, and tie bar deflection techniques, the time is used for other stages of the molding cycle. Time is the oldest and easiest control for transfer in injection molding. The control device is merely an electrical timer. Thus, for timed transfer from fill to pack, the pressure and injection speed are maintained for a specified length of time. However, since the timer does not Processing of Thermoplastics 5.115 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.115 control the pressure/injection speed interaction during filling nor the viscosity of the resin, timed transfer is considered the least reproducible method for the transfer from fill to pack. Although not considered ideal, timed transfer still typically determines the pack/hold, hold/cooling, and cooling/mold open transitions. Ram position is commonly used for transfer from fill to pack in injection molding. For this, the machine can monitor the position of the ram using one or more transducers or a linear variable differential transformer (LVDT). When the set position is reached, the machine switches control. While such controls are typically used with injection velocity–controlled machines, position transfer can be attained when the shot size indicator (on the injection unit) trips an electrical microswitch. Unlike timed transfer, position transfer delivers a constant volume (shot) of melt to the mold. In conjunction with controlled ram velocity, such transfer permits uniform delivery of polymer melt to the mold cavities. Ram position has been suggested as a control for packing, but is not generally available on commercial machines. Hydraulic pressure is available on most injection molding machines, but is not as commonly used as position for the transfer from fill to pack. In hydraulic pressure transfer, the machine switches from fill to pack when the pressure in the hydraulic lines behind the injection cylinders reaches the set position. With well-maintained machines and properly set controls, hydraulic pressure transfer is more consistent than ram position transfer. While position transfer delivers a constant volume of material, polymers expand upon heating, and the constant volume of melt does not always produce consistent part weights. Hydraulic pressure can compensate somewhat for changes in viscosity and for expan- sion of the melt. However, hydraulic pressure transfer is not easily set up as position transfer. Although cavity pressure transfer is considered the most accurate method for transfer because it measures both material changes (such as viscosity) and machine behavior, it is not commonly used for transfer. As shown in Fig. 5.74, 170 both cavity and hydraulic line pressure enable fairly accurate transfer from fill to pack by monitoring the sharp increase in pressure that occurs when the cavity is completely filled. Since only the cavity pressure technique can measure peak packing pressure, it permits an accurate switchover from pack to hold. For either method, the transfer from hold to cooling occurs at the time when a consistent part weight is reached. Since cavity pressure must be mon- itored in the mold, the expense and positioning of the pressure transducers is the major problem. Cavity pressure measurement is also highly dependent on the position of the pressure transducer. Ideally, a pressure sensor can be placed about one-third of the way into the cavity. However, multicavity molds, particularly family molds or those with artificially balanced runner systems require multiple transducers. Additionally, the 5.116 Chapter Five 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.116 transducers can be mounted flush with a cavity wall or behind ejector pins. The former is more accurate, whereas the latter allows the transducers to be used in other molds without disassembling the mold. Nozzle pressure transfer is a method for reducing the cost, but maintaining some of the advantages of cavity pressure transfer. For this, a pressure transducer is installed in the nozzle of the injection molding machine. Since this transducer is farther from the mold cavity, it is not as accurate as cavity pressure measurements are. Nozzle pressure is also not always useful for family molds or those with artificially balanced runner systems. Consequently, nozzle pressure is also not commonly used as a transfer method. Tie bar deflection is relatively uncommon transfer technique. Typically a transducer or other device measures the strain in a tie bar. Since this strain changes with the cavity pressure, measuring tie bar deflection is a simpler (and less expensive) way to monitor the cavity pressure. This method has been shown to correlate with cavity pressure. 171,172 However, the relatively small changes in strain have produced practical problems in amplifying the signals from the transducers. With a single gate, a cavity with a uniform wall thickness fills in the manner shown in Fig. 5.72a. 166 When the cavity has more than one gate, the polymer flows out from each gate and joins at a weld line. Processing of Thermoplastics 5.117 Hydraulic line pressure Cavity pressure Gate freezes Mold opens Fill Pack Hold Cooling Time Pressure Figure 5.74 Typical hydraulic line and cavity pressure traces for a molding cycle. 170 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.117 Since the polymer chains in the flow front are oriented perpendicular to the direction of flow, the chains diffuse across the weld line. Thus, weld lines are weak areas in the part. If the flow fronts meet at wider angle (Ͼ135°, 173 the polymer chains can flow together, thereby produc- ing stronger weld lines called melt lines. As shown in Fig. 5.75, 173 weld and melt lines also occur when the melt must flow around an obstruc- tion, such as a hole in the part. Weld lines are also produced when the part thickness is not uniform. In this case, the melt flows first through the thicker sections and then across the thinner sections of the part in an effect called race tracking (Fig. 5.76 174 ). Since they are weak, weld lines are not typically located at critical areas in the part. They can be eliminated in multigated parts by the use of sequential valve gating, in which the next in a series of valve gates is opened when the flow front reaches the gate position. During filling, the polymer melt freezes at the cavity wall to form the frozen layer. Melt adjacent to this is dragged along the frozen layer and then frozen. Since the frozen layer insulates the cavity, the melt in the center cools more slowly. This produces the orientation shown in Fig. 5.77a. 175 The melt frozen at the wall and the melt dragged along the frozen layer is highly oriented whereas the melt in the center has relaxed. This effect also depends on mold and melt temperature and the pressure on the melt. As illustrated in Fig. 5.77b, 175 the orientation is greater near the gate and lowest at the end of the fill (where cavity pressure is lowest). Material cools as the hot melt enters the cooler mold, and as the polymer cools, it shrinks. Packing forces material into the mold to compensate for shrinkage whereas the holding stage pressurizes the gate to prevent melt from flowing back into the delivery system. The second stage (or the holding stage) incorporates both packing and holding. For packing the shot size is increased by 10 to 20 percent. This provides material for packing the cavity. The packing pressure is typically set at 50 to 60 percent of injection pressure or hydraulic pressure at transfer. High pressures tend to force the mold open, thereby causing flash, while low pressures are not sufficient to force material to the end of the cavity. The packing time is typically incremented until the part no longer exhibits sink marks and voids (Fig. 5.78 176 ) or until the part weight is constant. Since holding keeps pressure on the melt until the gate freezes, the pressure is usually set to less than 50 percent of the injection pressure. The time corresponds with gate freeze-off and is typically determined by measuring part weight as an indication of gate freeze-off. Since higher mold and melt temperatures allow greater relaxation and crystallization, they increase shrinkage. In general, slow injection produces greater shrinkage because the polymer is cooled as it is injected, providing greater orientation, and packing the cooled resin is difficult. 5.118 Chapter Five 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.118 However, fast injection causes shear heating of the melt, thereby requir- ing the longer cooling times that facilitate relaxation and crystallization. Increased packing of the mold will reduce shrinkage, but this is limited by the gate freeze-off time. Molds with unbalanced filling will also exhib- it over- and underpacking; this creates nonuniform shrinkage in the part. Once the mold is filled and packed and the gate freezes off, the injection molding machine switches to the cooling stage. The amount of cooling is determined by the cooling time. While the melt in the mold cools to solid, Processing of Thermoplastics 5.119 1. Melt fronts approach 2. Weld line forms 3. Meld line forms Meld line Weld line Melt Melt Melt Melt ⍜ Figure 5.75 Weld lines and meld lines. 173 Figure 5.76 Race tracking. 174 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.119 5.120 Chapter Five 0.04 0.03 0.02 0.01 Orientation Or 0 0.7 Wall thickness, mm 1.4 T W = 55°C V F = 290 mm/s polystyrene T M = 240°C T M = 280°C (a) Flow direction Flow direction Perpendicular Molecular orientation -1 0 Close to the gate Far from the gate 1-10 1-10 1-10 1 2h (b) Figure 5.77 Orientation developed during filling. (a) Across the cavity thickness; (b) along the length of the cavity. 175 0267146_Ch05_Harper 2/24/00 4:56 PM Page 5.120 [...]... Glanvill, A B., The Plastics Engineer’s Data Book, p 102 97 Berins, M L., Plastics Engineering Handbook, p 105 98 Hensen, F., Plastics Extrusion Technology, p 163 99 Petrothene® Polyolefins, p 75 100 Rauwendaal, Polymer Extrusion, p 443 101 Fisher, E G., Extrusion of Plastics, pp 222–223 102 Berins, M L., Plastics Engineering Handbook, p 1 07 103 Petrothene® Polyolefins, p 62 104 Ibid., p 67 105 Ibid., p... Cahners Books, Boston, 1 972 , p 222 114 Strong, A B., Plastics, p 295 115 Ibid., pp 296–2 97 116 Petrothene® Polyolefins, p 84 1 17 Berins, M L., Plastics Engineering Handbook, p 1 17 118 Wire and Cable Coaters’ Handbook, DuPont, Wilmington, Del., 1968, p 22 119 Petrothene® Polyolefins, p 85 120 Berins, M L., Plastics Engineering Handbook, p 116 121 Petrothene® Polyolefins, p 73 122 Finch, C., presentation... 171 – 172 53 Rosato, D V., and D V Rosato, Plastics Processing Data Handbook, Chapman & Hall, New York, 1989, p 93 54 Rauwendaal, C., Understanding Extrusion, pp 69 70 55 Rosato, D V., and Rosato, D V., Plastics Processing Data Handbook, p 105 56 Steward, E L., “Control of Melt Temperature on Single Screw Extruders,” 57th Annual Technical Conference of the Society of Plastics Engineers, 1999, p 195 57. .. 39 171 Ulik, J., “Using Tie Rod Bending to Monitor Cavity Filling Pressure,” ANTEC’ 97, 19 97, p 3659 172 Mueller, N., private correspondence, 1999 173 C-Mold Design Guide, p 144 174 Belofsky, H., Plastics: Product Design and Process Engineering, Hanser Publishers, New York, 1995, p 304 175 Potsch, G., and W Michaeli, Injection Molding: An Introduction, Hanser Publishers, New York, 1995, pp 116–1 17 176 ... 1995, p 12 81 Glanvill, A B., The Plastics Engineer’s Data Book, p 89 82 Bikales, N M., Extrusion and Other Plastics Operations, p 39 83 Hensen, F., Plastics Extrusion Technology, 2d ed., Hanser Publishers, New York, 19 97, p 106 84 Petrothene® Polyolefins, p 49 85 Hensen, F., Plastics Extrusion Technology, p 1 07 86 Fisher, E G., Extrusion of Plastics, pp 216–2 17 87 Rauwendaal, Polymer Extrusion, p... 76 Rauwendaal, C., Plastics World, vol 50, no 4, 1992, p 68 77 Rauwendaal, C., Plastics Formulating and Compounding, vol 2, no 1, 1996, p 22 78 Mielcarek, D F., “Twin-Screw Extrusion,” Chemical Engineering Progress, vol 83, no 6, 19 87, p 59 79 The MEGA Compounder: Productivity by Design, Werner and Pfleiderer, Ramsey, N.J., 19 97 80 Callari, J., “Mega Machine is Fast, Productive,” Plastics World, vol... area of the molded part or parts at the mold parting surfaces Overall cycles depend on molding material, part thickness, and mold temperature, and may be about 1 min for parts of 3-mm thickness to 5 or 6 min for parts of 8-cm thickness (Fig 6 .7) The process is generally used for high-volume production because the cost of a modern semi-automatic press of modest tonnage, say 50 02 671 46_Ch06_Harper 6.8... pp 3–4 70 Cincinnati Milicron, Cincinnati Milicron Austria, Vienna, 1998 71 White, J L., Twin-Screw Extrusion, p 229 72 Twin Screw Report, American Leistritz Extruder Corp., Somerville, N.J., November 1993 73 Baird, D G., and D I Collias, Polymer Processing, p 2 17 74 Leistritz Extrusionstechnik, Leistritz Aktiengesellschraft, Nurnberg, Germany, 1998 75 White, J L., Twin-Screw Extrusion, p 248 76 Rauwendaal,... Injection Molding Handbook, 2d ed., Chapman and Hall, New York, 1995, pp 222–223 155 Belofsky, H., Plastics, p 298 156 C-Mold Design Guide, AC Technology, Ithaca, N.Y., 1994, p 173 1 57 Injection Molding Reference Guide, p 17 158 C-Mold Design Guide, p 29 159 McCrum, N G., C P Buckley, and C B Bucknall, Principles of Polymer 02 671 46_Ch05_Harper 2/24/00 4:56 PM Page 5.125 Processing of Thermoplastics 5.125... 68 106 Vargas, E., Film Extrusion Manual, pp 338–339 1 07 Petrothene® Polyolefins, p 78 108 Vargas, E., Film Extrusion Manual, p 2 47 109 Petrothene® Polyolefins, p 65 110 Strong, A B., Plastics: Material and Processing, Prentice-Hall, Englewood Cliffs, N.J., 1996, p 258 111 Fisher, E G., Extrusion of Plastics, pp 2 27 228 112 Ibid., pp 238–241 02 671 46_Ch05_Harper 5.124 2/24/00 4:56 PM Page 5.124 Chapter . Books, Boston, 1 972 , p. 222. 114. Strong, A. B., Plastics, p. 295. 115. Ibid., pp. 296–2 97. 116. Petrothene® Polyolefins, p. 84. 1 17. Berins, M. L., Plastics Engineering Handbook, p. 1 17. 118. Wire. Twin-Screw Extrusion, p. 248. 76 . Rauwendaal, C., Plastics World, vol. 50, no. 4, 1992, p. 68. 77 . Rauwendaal, C., Plastics Formulating and Compounding, vol. 2, no. 1, 1996, p. 22. 78 . Mielcarek, D. F.,. Publishers, New York, 19 97, p. 106. 84. Petrothene® Polyolefins, p. 49. 85. Hensen, F., Plastics Extrusion Technology, p. 1 07. 86. Fisher, E. G., Extrusion of Plastics, pp. 216–2 17. 87. Rauwendaal, Polymer

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