Plastic Product Material and Process Selection Handbook Part 7 doc

194 Plastic Product Material and Process Selection Handbook Figure 4.2 Three basic parts of an injection molding machine (courtesy of Plastics FALLO) The part taken from the mold is, in most cases, a finished product ready to be packed and shipped or ready to be used as a part of an assembled unit. In contrast to metal forming, there is very little if any wasted material in injection molding. For cold runner TP systems most runners and sprues are reground and reused. By using hot runner molds, the sprue and runner systems remain in a melted state in the mold and become part of the next finished part (Chapter 17). The hot runners can be thought of as an extension of the plasticizing chamber. IM lends itself readily to automation in varying degrees, depending upon the ingenuity of the machine and mold designers. The machine manufacturer can usually add components to the basic machine to implement any desired automatic arrangement. Molding cycles are relatively fast, and with new mold design developments constantly in progress, and centering on faster heat transfer within the mold, the molding cycle is continually being reduced. Modern methods of material handling, both of the raw plastic and the finished product, are becoming more generally employed, further reducing costs of the finished part (Chapter 18). The reduction of raw material inventory is seriously considered, and studies of production control methods are no longer uncommon, even by the small molder. Methods such as these are important in making molded parts less expensive to manufacture. 4.Injection molding 195 Plastic is usually purchased in pellet form and heated in the plasticizer and/or preheated prior to entering the plasticator until it reaches a viscous state in which it can be forced to flow into the mold cavities. Each plastic differs in its ability to flow under heat and pressure. For the best result, correct melting temperature, injection pressure, and mold filling speed must be determined from experience or by trial for the particular plastic and mold used. Some molding conditions require that both the speed of injection and injection pressure varies during the filling process. A heat-sensitive plastic may be degraded if too fast a fill rate is used. Forcing the plastic through orifices at too high a velocity may increase the shear and temperature enough to cause overheating and burning. Thin-walled parts require a fast fill rate to prevent chilling of the plastic before the cavity has properly filled. Some molded parts carry both thin and thick sections, plus such interrupted flow patterns as are required to move around cored holes. Demanding requirements such as these require considerable versatility in the design of the IMM injection unit. The programming of different injection speeds and pressures during the forward travel of the screw or plunger greatly aids in filling cavities properly. Programming or multi-stage injection is standard equipment on most machines. The clamp tonnage of a machine must have sufficient locldng force not to cause the parting of mold halves; it resists the force of melted plastic moving at high pressures into the mold halves. If the mating surfaces of the mold are forced apart, even a few thousands of an inch (depending on type plastic), fluid plastic will flow out and produce flash (Chapter 17). Molding system The IMM process can be identified by its most basic three popular methods of operation that are the hydraulic, electrical, and hybrid types. The two basic plasticizing systems used are the single-stage and the two-stage molding systems (Figures 4.3 and 4.4); there are also B-stage molding units, etc. The single-stage is also lmown as the reciprocating screw IMM. The two-stage has other names such as the piggyback IMM that can partially be related more to a continuous extruder (Chapters :3 and 5). Different IMMs meet specific parts such as different qualitative requirements to mold dry cycle, injection rate, injection pressure, 196 Plastic Product Material and Process Selection Handbook Figure 4,3 Schematics of single and two-stage plasticators Figure 4~ Simplified plastic flow through a single-stage IMM clamping force, platen size and daylight opening, maximum screw stroke, etc. 3 The feature of shot size or IMM capacity represents the maximum usable volume of melt that is injected into the mold. It is usually about 30 to 70% of the actual available volume in the plasticator. The difference basically rclates to thc plastic materials melt behavior, and provides a backup safety factor to meet different mold pacldng conditions. Shot size capacity may be given in terms of the maximum weight that can be injected into a mold cavity(s), usually quoted in ounces or grams of general purpose polystyrene (GPPS). Since plastics 4-Injection molding 197 have different densities, the better way to express shot size is in terms of the volume (in. 3 or cm 3) of melt that can be injected into a mold at a specific pressure. Rate of injecting the shot relates to IMM's speed and also the process control capability of cycling the melt to move fast-slow- fast, slow-fast, etc. into the mold cavity(s). Injection pressure in the barrel can range at least from 2,000 to some plastics up to 4:5,000 psi (14 to 310 MPa). The characteristic of the plastic being processed defines what pressure is required in the mold to obtain acceptable products. Based on what cavity pressure is required, the barrel pressure has to be high enough to meet pressure flow restrictions going from the plasticator into the mold cavity(s). The molding cycle is the complete repeating sequence of operations in the process. One cycle represents the time period, or elapsed time, between a certain point in one cycle and the same point in the next. Most of the time is the cooling phase that is usually at least 60%. To shorten cycle time lies principally in assessing all the capabilities of the IM process in addition to designing the part and the mold. Thus what is needed is a device for achieving optimum designs of part and mold. Program systems that provide for computer simulation of the IM process are used for this purpose. 3 The availability and performance of relevant software programs provide guidelines so that one can develop continuing experience. In support of this approach are software programs to reduce cycle time by evaluating the actual IM process operational settings. Oil hydraulic systems have been the major method used in operating IMMs. MI electrical machines as well as hybrid (hydraulic/electrical) are now also used. Electric and hybrid eliminate many variables from the hydraulically operating IMMs. 3, 17s Clamping Design Controllable actions of IMM clamps exist. Their operating mechanisms are identified as mechanical or toggle, hydraulic, electrical, and hybrid (hydromechanical) (Figure 4.5). Each has advantages and disadvantages. 3 Toggle clamps are more popular in smaller-tonnage machines because the mechanisms are inexpensive to manufacture and require less- complicated circuitry. Most electric IMMs use toggles. Hydraulic clamps are used extensively on machines in the medium- capacity range of about 150 through 1000 tons, with highest pre- dominance in the 250 to 700 ton range. They offer flexibility in machine setup and operation. Since tonnage can be developed at any point along the clamp stroke, just setting limit switches at the desired points along a calibrated scale does mold setup. Clamp slowdown, mold 198 Plastic Product Material and Process Selection Handbook Figure 4.5 Example of mold operation controls close, slow-mold breakaway, and fast-open functions are all adjustable. This versatility is particularly useful in complicated molding applications: molds with core pulls or unscrewing dies, multiple plate molds, or delicate parts requiring careful mold handling. 3 Electrical clean operating IMMs are available from many sources worldwide. In the past few decades the all-electrical IMMs have been producing all types, shapes, and size molded products. Different electrical designs are used. As an example servomotors are connected to the ballscrews through a heaw-duty timing belt and pulleys. Die height is set by a servo-driven, chain-and-sprocket arrangement. The plastic- ator is directly driven through a timing belt. Its design objective is high speed that meets the objective with sub-one-second dry-cycle times. 3 The hybrid is a combination of hydraulic and electrical. In turn these basic systems provide many different IMM designs to meet different product requirements. Each system provides advantages such as fast moving of platens, reducing size of hydraulic cylinders, and/or reduced operating costs. Examples of these hybrid operating systems meet the molders different molding requirements. A popular example that has been used for many decades is the electric screw drive system design in hydraulic operating IMMs. 3 Tiebar The clamping tiebars (rods) can be used to support the fixed and movable platens on which the mold is attached. They serve as equally loaded tension support members of the clamp when the mold is closed. The opcn distance between tic rods through which the mold must fit and eject molded parts sets up thc maximum outside dimensions of the mold that can be used. Diffcrcnt designs arc used to meet diffcrcnt 4.Injection molding 199 processing requirements such as permifing installing molds that would occupy the complete platen minus the tie rod circular areas. There are designs used to unlock one to all tiebars, those with one to four retractable tiebars, three tiebars, and the tiebarless where no tiebars are used. Tiebarless design is of a C-frame (also called U frame, open frame, etc.) construction targeted to provide clamping pressure and proper parallelism as well as operating platens. The fundamental purpose of these different actions is to provide faster automated mold changes (in and out of the IMMs). Each system provides its own advantages (and limitations) for specific operations required in the different operating IM plants. During clamping and when applying pressure on the molds, the tie rods stretch. If everything is in balance, the platens and mold stretch evenly. The distance the rods stretch is directly proportional to the applied load. Sensors, such as electrical strain gauges, can be used to detect the stretch or load applied and if an unbalance situation occurs, an indicator can alert the operator or the process control system. Bar sensing can also be used as a means of signaling the switch from pack to hold pressures, a potential alternate or support to pressure transducer use. In use are retractable ticbars. Different designs are used to unlock a tie bar. Principle reason is to permit installing molds that would occupy the complete platen minus the tie rod areas. Thus the mold literally has holes. Very popular are tiebarless systems which are also used. Without the tiebars, larger molds can reduce IMM cost, mount larger molds in a smaller IMM, permit quicker to easier mold mounting, no holes in molds, simpler part handling automation, etc. Machine Control Machine process controls coordinate individual functions of the clamp, injection unit, ejector mechanism, and mold systems and accessories such as core pulls and unscrewing dies for threaded parts. The more advanced controls employ a feedback system (closed loop) to provide much tighter control over actual parameters vs. setpoints. High-level controls are capable of communicating with auxiliary equipment such as chillers, hopper loaders, mold temperature controllers, robots, etc., and displaying all machine parameters and conditions (Chapter 3) These never ending advanced controls allow interfacing many machines to a common host computer that allows plant-wide monitoring of the overall production status. The many software developments are rapidly changing the character of the molding machine. 3 200 Plastic Product Material and Process Selection Handbook Machine startup/shutdown For IMM startup experience provides a guide to setting plasticizer heat profile as well as other settings. Otherwise start with the plastic manufacturer's recommendations. There arc different starting points for the various types of plastics that have to be interfaced with the different capabilities of IMMs to be used. The time and effort on startup make it possible to achieve maximum efficiency of performance vs. cost for the processed plastics. Information on process control settings developed can be stored and applied to future setups. Recognize that two identical IMMs usually require slight different settings to maximize their per- formance. Figure 4.5 provides examples of controls. The term process control has often been used when machine control is actually performed. As the knowledge base of the fundamentals of the molding process continued to grow, the control approach is moving away from principally press control and closer to real process control where material response is monitored and then moderated or even managed (Chapter 3). For startup mold setup is important. It includes: 1 determining plastic requirements based on type of mold to be used [cold runner (includes nozzle into cavity) or hot runner (only cavity)] ~83 2 locate proper KO bars with all having equal lengths 3 select eyebolt hole which yields a level hang/lift 4 level mold and clamp to fixed platen 5 line up locating ring 6 slowly close mold 7 open moving platen and install KO bars if KOs are acting as pullbacks 8 tighten bars malting certain they bottom out against the ejector plate 9 close platen 10 clamp mold to moving platen, remove safety straps, unhook hoist 11 open mold to desired daylight and set slowdown switches so that no high impact on the mold will occur 12 fine tune the final switch positions by repetitive small adjustments 13 connect all required power (electric, hydraulic, and/or pneumatic 14 check powered functions to ensure they are operating correctly such as electrical heaters just long enough to prove functionality avoiding excessive heat buildup before water is connected 4. Injection molding 201 15 conncct water lines 16 turn water on (electric heaters off) and examine for leaks. 433 Startup process control involves the machine operation and behavior of the plastic. Most important is the interaction between the machine operation and plastic behavior from the plasticator into the mold cavity(s). Principally the processing pressure and temperature vs. time determine the quality of the molded product. The design of the control system has to take into consideration the logical sequence of all these basic functions. They include injection speed (pressure dependent), clamping and opening the mold, opening and closing of actuating devices, barrel temperature profile, melt temperature, mold temper- ature, cavity pressure, 184 holding pressure, and mold cooling rate. These controls are essential to produce molded quality products and minimize cycle time. Quality features include mechanical properties, dimensional accuracy, absence of distortion, and surface quality. Molding a product (part) involves the three stages of fill, pack, and hold. The following guide provides a simplified example for IM plastics. Start with the plastic melt temperature at the mid-point of your supplier's recommended range. Know the actual melt temperature (not the barrel temperature set points). As with the melt, the mold temperature should be centered to the recommended range. Fill the mold as fast as you can and as far as you can. Separate speed from pressure. (Peak pressure during fill should never reach the injection pressure set point). Pacldng should be as slow as possible via separating speed from pressure. Priority for termination of pack is the same as fill. The ability to pack on velocity is dependent on the hydraulic and/or electric architecture of the machine. Few presses are able to do this, which requires the operator to pack on machine pressure. Unfort- unately, a constant pressure applied to a variable like plastic leads to a product that varies. Hold with enough pressure and time to prevent plastic melt discharge from the mold until the gate seals. Ideally, hold should be a zero velocity setting with whatever pressurc needed being available. The criteria for determining cooling time now occurs when the product can accept the force of ejection and does not distort (hold time is cooling time). 202 Plastic Product Material and Process Selection Handbook For the majority of plastic materials, you should always run with a cushion. Back pressure and screw rpm should be minimized. The goal should be to plasticate to the shot limit just before the cooling timer times out. Once the process has been optimized, plastic conditions should be recorded such as fill time, peak pressure at fill, cavity pressure, 184 melt temperature, mold temperature, melt flow rates, and gate seal time. Record all basic machines setpoints on the setup sheet such as the transfer time (fill time) and weight, overall cycle time, and total shot weight, part weight, % runner, etc. Start molding short shots and gradually increase the shot size as the injection speed while watching for flash or burning. Short shots that exhibit flash and/or burns indicate problems with tooling. Processing around a tool problem is a temporary resolution at best. Goal is to fill the mold as fast as possible. An ideal approach would provide a product 95% filled using 90% of the maximum injection rate of the press to operate with maximum efficiency. With the cavity approximately 95% filled leave the shot limit alone and start to lower the cut-off position. This will allow completing the fill portion of the cycle and using the inertia of the ram to pack out the product. With certain hydraulic IM machines that use servo valve technology for injection speed and pressure, it is not possible to completely separate fill from pack. This is best accomplish on machines that use a dual valve system. Lower the cut-off position until the product cavity is packed out making sure that a melt cushion exists. It may be necessary to increase the speed setting on the last step, but packing should be done as slow as possible. After completing the packing start adding hold pressure and time period. Pressure should be high enough to keep plastic from discharging and time adequate to allow the gate to freeze (melt solidifies). Gate seal time can be determined by looldng at cavity pressure at the gate or by weighing the product without the runner. After hold time is complete, delay the start of screw rotation (or decompress before starting the screw) to allow the pressure ahead of the screw to decay. Once the optimum cooling time is determined, screw rpm should be adjusted to minimize residence time (Figure 4.6). Profiled backpressures are not recommended. The slower the screw is allowed to turn the better the mixing action. Look at the peak pressure reached during fill and set the system pressure about 10% higher than this peak pressure. Minimizing the time used to open the mold, eject the product, and close the mold. 4. Injection molding 203 Figure 4~6 Plastic residence time Summarizing and providing additional details to what has been reviewed follows. If required, purge barrel free of degraded resin. Set machine for semi-auto and start cycle; observe screw. Set barrel temperature profile based on experience or start with resin supplier recommendation. Determine or estimate the shot size 386 and set machine for approximately 2/3 of the mold's full shot. Set decompression stroke. Set a position transfer point (if machine is so equipped) approximately an inch from bottom. Set first stage pressure at 50% for starters and ultimately set at 100%. Estimate and set second stage time with pressure at zero. Set melt injection velocity to maximum. Adjust velocity and/or pressure as needed; if the fill was fast and short, the pressure can be increased. The fill pressure should bc set high enough so the fill speed is not pressure limited, but controlled by velocity sctpoints. Estimate and set cooling time. Set backprcssurc at 50 psi and gradually increase if necessary. After observing each cycle, the shot size and transfer point will be adjusted frequently to set the process so that the first stage accomplishes 95 to 98% of the fill as measured by shot weight. Once the first stage shot size, transfer, velocity and pressure arc set, we can set 2nd stage packing pressure. Adjust pack pressure as needed, but do not [...]... two materials can bc the same or different and they can be molded to bond together or not bond together If materials arc not compatible, the materials will not bond so that a product such as a universal or balland-socket joint can be molded in one operation If they are compatible, controlling the processing temperature can eliminate bonding A 214 Plastic Product Material and Process Selection Handbook. .. they are easy and simple to operate Suited for economical production particularly of large, complex, 3-D parts Usual cavity pressure is 200 to 500 psi (1.4 to 3.5 MPa) An accumulator can be 222 Plastic Product Material and Process Selection Handbook used between the plasticator and mold providing a means of injecting a very large shot Since low pressures are used, the parts surface... simplified schematic of the extrusion process Information on dies (mono-layer and coextruded) used in extrusion is in Chapter 17 Figure 5.1 Simplified example of a single-screw extruder Extrusion is the single most popular process for forming TPs It processes over 36wt% of all plastics consumed worldwide into products 228 Plastic Product Material and Process Selection Handbook such as ranging from rather... 2 2 0 Plastic Product Material and Process Selection Handbook using several lasers into a box of gas At the point where the laser beams intersect, the energy would produce a reaction that causes a part to form without a mold is~ Monosandwich molding Two-component injection machines, using separate injection units for each of the two components perform the standard sandwich molding process This process. .. optimized set of process variables for the expected quality of a part (Figure 4.9) Figure 4.9 Quality surface as a function of process variables 208 Plastic Product Material and Process Selection Handbook Another example of a processing window study concerns thin-wall molding In this study the effects of IM conditions and critical design parameters on the filling, dimensional stability, and crystallization... to prevent the burning of plastic by the compressed, hot air at the bottom of the cavity Molding machines and tooling for small parts are not just smaller versions of their regular larger molding counterparts IM in micron 21 8 Plastic Product Material and Process Selection Handbook sizes can be performed in special designed IMMs and molds 38, is0 Some micromolders use screw -and- barrel molding machines... Figure 4 ~7 Molding area diagram processing window concept Figure 4,8 Molding volume diagram processing window concept 206 Plastic Product Material and Process Selection Handbook any process is not of poor product design but instead that the processes operated outside of their required operating window Many different processing window studies arc conducted As an example an injection molded radar application... shock and vibration requiring a much stronger machine then a plastic IMM Thixotropic molding The patented thixotropic technology is called Thixomolding Traditional die casting machines use a large pot of molten Mg that can 226 Plastic Product Material and Process Selection Handbook be hazardous to workers and creates sludge that is foundry waste By contrast thixotropic IMMs use chips of Mg similar to plastic. .. shot material is wasted Worse, the manufacturer loses control of the part- molding process What is occurring is molding the sprue and runner with the part becoming a ldnd of by -product The Sesame is designed so that molders can use a smaller runner and sprue, which gives them more control over the amount of plastic and pressure used to form the part itself A smaller runner and sprue also means less material. ..204 Plastic Product Material and Process Selection Handbook overpack Recheck cushion Some cushion should be maintained Set screw speed so that recovery is completed just prior to next cycle, but not limiting cycle time If flash occurs slow the velocity Maximizing Processing Window Control For startups a processing window is determined that sets controls to fabricate acceptable products It sets . the product can accept the force of ejection and does not distort (hold time is cooling time). 202 Plastic Product Material and Process Selection Handbook For the majority of plastic materials,. Molding volume diagram processing window concept 206 Plastic Product Material and Process Selection Handbook any process is not of poor product design but instead that the processes operated. 194 Plastic Product Material and Process Selection Handbook Figure 4.2 Three basic parts of an injection molding machine (courtesy of Plastics FALLO) The part taken from the