21st Century Manufacturing Episode 2 Part 8 pptx

334 Plastic-Products Manufacturing and Final Assembly Chap. 8 8.3 PROCESSING OF PLASTICS I: THE INJECTION MOLDING METHOD 8.3.1 Overview Injection molding is a key production method for the casings of consumer products. It is cheap, reliable, and reduces the device's weight compared with metals. The ST Microelectronics' 'IouclrCblpt'" (the case study in Chapter 2) and the InfoPad cas- ings (the case study in Chapter 6) were made in this manner. A very wide array of consumer products ranging from toys to telephones to automobile parts is also made through this process. In Figure 8.4,some general features of the mold are shown. A simple bucket or cuplike component could be made in the gap between the core and the cavity shown at the bottom right. In this case the paning plane, shown on the bottom left, could be at the lip of the bucket. For other products the parting plane might be less conve- niently located. Thus, a ridge is often visible around plastic toys and simple appli- ances where the parting plane has been located Further finishing by hand might be desirable for such parts. Hand finishing might also be needed (a) for the injection's "gate marks" that maybe visible as tiny "pimples" on the surface of a part and (b) for the "ejector-pin" marks that for a relatively small object like a cellular phone casing are usually about 6 mm (about 0.25inch) in diameter. The reader might be interested in picking up any familiar plastic consumer product and searching for these inevitable markings. How- ever, these are often located in noncosmetic areas of the part, because of the cost involved in finishing operations. Not surprisingly, they are often found on the bottom or base of the object. Injection molding is shown in Figure 8.5.Pellets of the desired thermoplastic are loaded into the hopper on the right side and heated as they are pushed by a screw 1 Parting direction z x~Yx -y ·"2 Principal directions FJeure 8.4 &sic meld description, including slider for an 'Ullden:ui. Cavity direction Und:n::Ul Molding Parting plane Pull-out Cavity 8.3 Processing of Plastics I: The Injection Molding Method 335 (,) Hopper Mo,,", half M~ld Y r Clamp I I Injection Gate (bl (dl Molded part F1guJe 8.S Injection molding with reciprocating-screw machine. through heating zones. This melted and mixed material is forced through a nozzle into the mold by a hydraulic ram. The thermoplastic then cools, sets, and hardens in the mold. Once the two halves of the mold have been separated, the ejector pins facilitate "popping the part" out of the lower part of the mold. The process is now reviewed in more detail. Nozzle injection machine Sprue bush Filled ,Sprue half Impression Back-flow atou valve N?zzle r Hydraulic motor 'Sprue 'Sprue bush Gate land -Ccld slug "Runner Branch runner Sprue Gate . Parting· surface 33. Plastic-Products Manufacturing and Final Assembly Chap. 8 8.3.2 The Reciprocating-Screw Machine The reciprocating-screw machine is the most used machine in industry. It is shown in the open position in Figure 8.5a. Below the machine is a single-cavity mold and a multiple-cavity mold. Note that the machine is horizontally constructed and operated. Thus the mold is effectively laying on its side in comparison with Figure 8.4. In Figures 8.5b and 8.5e the cavity is on the right and the core is on the left. Molten plastic is shot into the mold from the nozzle of the reciprocating-screw machine and into the sprue of the mold. If there are multiple cavities (Figure 8.5d), the sprue feeds the runners and gates. The barrel of the machine is heated, and as the screw pushes the pellets tor- ward, there is additional heating from the mechanical pulverizing effect. In fact the machine is designed to operate in two distinct phases: • Step 1,plasticizing the thermoplastic: the combined action of the heaters and screw feed creates a metered volume of liquid polymer that arrives at area Y, just behind the nozzle. Since the nozzle is small in diameter, it is sealed shut at this point by a cold slug of plastic from the previous shot. • Step 2,injection into the mold: the screw action stops, and the whole stationary screw is now used as a ram to force the liquid out of the nozzle, through the sprue, and into the mold. Immediately before the ram action, the mold has been closed so that the liquid polymer fills the impressions shown as the dark areas in Figures 8.5b and 8.5L:. It should also be noted that a nonreturn valve just behind the Tamhead pre- vents the liquid from moving backward into the screw channels. Following this two- step process several things happen in unison. The part begins to cool down, and after a sufficient waiting period, the mold can be opened to eject the part. To reduce the cooling period, the mold isactively cooled by water lines. But during the cooldown, the screw can begin to turn again to collect its next shot of polymer pellets and move back to create space in position Y for the next shot. 8.3.3 Computer Aided Manufacturing McCrum, Buckley, and Bucknall (1997) describe the research into equipment and control design in recent years. The screw has the triple function of transporting the pellets, compressing and melting them with help from the heater zones, and then, during the ramming action, having enough strength to pump the melt through the nozzle into the die.The clearance between the barrel and the flat lands of the screw flights is only 10- 2 mm, demanding that the machine screw and the barrel be made from hardened steel with wear resistant coatings. Internal pressures of 100 Mpa are typical. CNC controllers are used to monitor various sensors in the system and to adjust the various parameters shown in Figure 8.6.The key features include: •Thermocouples for the temperatures of the barrel, nozzle, and mold • Pressure sensors on the screw/ram 8.3 Processing of Plastics I: The Injection Molding Method MoldprasureooDtrol Closed loop control of mold pressure during packing phase to ensure consistent part density. Measured mold pressure iscompared to desired value and hydraulic pressure is changed accordingly. CushionfshotJize eontrol Automatically maintains a constant volume of material in front of the screw after the mold is fiUed.This ensures that the material in the mold is subjected to the proper pressure during the packing phase and a1sothat there is sufficient material to properly pack the parts. 337 Velodty to pressure oontrol Transition from velocity program to ram pressure control phase is automatically activated using measured machine or plastic conditions. Transfer mode is selectable from mold pressure, hydraulic pressure, or ram position. RampressureooDb:oI Initiated after the velocity phase to provide dosed loop control of hydraulic pressure during pack hold and screw recovery phases. Maintains controlled pressure during these critical phases despite changes in oil temperature, valve leaking, or system Loading. Proxnunmed injection (velocity control) Closed loop control of ram speed as plastic is being injected into the mold. Separate velocity steps can be programmed to occur anywhere within the active ram stroke. F1gufe tl6 Control of injection molding (courtesy of Barber-Colman DYNA Products) •Pressure sensors monitoring the pressure in the mold • A screw position sensor (potentiometer or LVDT), which also measures velocity during the ramming step The controller is used to optimize the cycle and to pack and hold the polymer in the mold under the continuing pressure of the ram. Two methods for doing this are described in Section 8.3.5. 8.3.4 Behavior of the Polymer inside the Mold during the Filling Stage When a polymer cools in the die cavity, it shrinks dramatically and grips the sides of the core walls. During the milling operations (Chapter 7) to create the molds, these -Moldpressuresensor Position sensor Hydraulic sensor.?" 338 Plastic-Products Manufacturing and Final Assembly Chap, 8 walls must therefore be tapered to allow easier part ejection. The volume contrac- tions of a polymer between its liquid temperature and room temperature are of the order of 10% at normal atmospheric pressure. This presents another problem for the manufacturing of plastics because voids and sink marks would occur if the polymer were allowed to shrink this much without special controls. These voids can reduce the mechanical properties and cause cosmetic imperfections. Figure 8.7 from McCrum and colleagues (1997) shows how this volumetric shrinking isaddressed. Lines a, b, and c in the diagram represent isobars of increasing pressure, with line c being the highest. The details are explained inthe next paragraph. The cycle is as follows: • At the highest temperature and specific volume (V A) the liquid plastic is first pressurized, in the mold, to a pressure of approximately P = 10 3 atmospheres. This leads to a volume decrease of about 10% (i.e.,A taB between lines a and c). In Equation 8.2 below, K is the bulk modulus, which is 1 GPa, typical of most liquids: 6.V P 10 3 X 1.01 X 10 5 -y- = K = ~ = 0.101 (8.2) • In other words when the liquid polymer is rammed in at this pressure, and the mold is 100% filled, it will contain 10% more polymer than if it were filled at 1 atmosphere. • In the next phase, between Band C, the part in the mold cools under pressure (value, C), and the ram keeps additional liquid flowing into the mold to com- pensate for contractions. • At point C, the gate freezes over, sealing the mold and preventing further packing. • Between C and D, the plastic cools at constant volume (V D) under decreasing pressure until atmospheric pressure returns again at position D. v,· , , , I Glassy Room T g temperature Increasing pressure Viscous Temperature Figure 8.7 Liquid polymer follows the path A to B as a pressure of approximately io' atmospheres is applied. The liquid is "packed" between B and C.At point C, the gate freezes over. Between C and D, the plastic cools at constant volume (V D) (adapted from McCrum, Buckley, and BucknaU,l997). Liquid Packing 8.3 Processing of Plastics I: The Injection Molding Method 339 •From D to E the mold cools naturally and undergoes normal thermal contrac- tion. If the polymer had been allowed to cool naturally, the free thermal shrinkage would have been = (1 - V ElVA) = 10% to 15%. However, because of the pressurized cooling cycle,the contraction is = (1- VEIVD) = 1%. The maximum available clamping force is one of the key specifications in designing and costing equipment. Other main specifications include: (a) shot size, which is expressed as a mass equivalent volume of polystyrene, and (b) ejection stroke, which limits the maximum part depth that can be molded. 8.3.5 Controlling the Polymer Cool down There are two main methods to control the pressurized cooldown in Figure 8.7.These are summarized in the next two diagrams from the Barber-Colman Company. Actu- ally,in both cases, Step 1,the mold filling with the ram, is similar: • Step 1,for filling: a sequential velocity profile isprogrammed in, shown by the steps on the left of Figure 8.8. The velocity is high at first to accelerate the liquid polymer into the mold, but then it decreases as filling gets completed. ven Mold fiUingphase +~Ram pressure control phase Under control of programmed I I I injection } Pack_Hold +k-Back " , in I Pressure i : : IVel s TIm, Position 2 , Fill i Pack iHOld ~./"t'ss, :1 ~lire Screw rotate Position) Position 4 PositionS Flame U Injection molding cycles.The upper figure ill ram pressure controlled, and the lower figure is mold pressure controlled (courtesy of Barber-Colman DYNA Products). Velocity Vel 4 340 Plastic-Products Manufacturing and Final Assembly Chap. 8 • Step 2, alternative 1 to packing, for the packing and holding phase (along A to B to C in Figure 8.7): the ram pressure is used to control the process. The pres- sure sensor on the ram is used to monitor and control the highest pressure during packing the mold to its desired mass. The system then transfers to the lower holding pressure until the gate freezes. and then after a set time the screw/ram rotates and moves back, shown on the extreme right of the figure. •Step 2,alternative 2 to packing: amore direct method isto install a pressure sensor in the mold cavity.Figure 8.8 shows that this allows a more graceful release of the mold pressure and a direct measure of the processes inside the cooling mold. 8.3.6 Ejection and Resetting Stage In the ejection and resetting stage, the cavity and core are separated as the mold is opened, and the ejector pins "pop" the part away from the die surface. The schematic of a "two-plate mold" shown in Figure 8.9 is from Glanvill and Denton (1965). It indi- cates that real production molds are rather more substantial than might be suggested by the earlier figures showing general nomenclature. In fact, even Figure 8.9 is a more simplified view of a mold. Classical texts in this area, such as that by Pye (1983) and by Glanvill and Denton (1965), show that many extra alignment pins and backing plates are needed beyond those shown in the schematic. The extra bulk. sup- port blocks, ejector plates, and damping plates reflect the facts that molding pres- sures are high and production runs are long (e.g., 10,000 to 10,000,000 parts). Thus a substantial device is needed so that the ejector pins and moving parts do not wear out. The downside is that molds open relatively slowly with hydraulic or mechanical actuators, and it takes care to lift the parts out of the cavity after the ejector pins have pushed it off the core plate (center of Figure 8.9). As much ejection area as possible is desirable to minimize part distortions and to minimize cycle time. 8.3.7 Positioning the Gate Figure 8.10a shows some typical gate designs, and Figure 8.1De indicates the pre- ferred sprue/gate for a cup shape. The gate is a very small orifice, and as the liquid is forced through, additional shearing causes a temperature rise of about 20°C,which is beneficial in mold filling. At a later stage, the small orifice is beneficial again because the polymer first freezes at the gate, thereby separating the molded part from the material in the barrel. The correct gate design is also the key to efficient mold filling, as shown on the right of Figure 8.10c. Finally, the gate should be placed in a position that creates an acceptable part. Key design issues include: • Positioning the gate on a surface that is noncosmetic. For consumer electronics this usually means it will be on the inside of the casing. In other situations it should be inexpensive to remove or present no finishing problems. •Avoiding placing the gate near a stress concentration point of the product.' 8.3 Processing of Plastics I: The Injection Molding Method 341 Sprue bushing Clamping plate Cavity plate Core plate Support plate Spacer blocks Ejector retainer plate Ejector plate Rear damping plate F1pre 8.9 Two-plate mold based on G1anvill and Denton (1965) Schematic courtesy of Boothroyd et al.(1994) . •Positioning the gate where it will facilitate air expulsion as the injected liquid cools down. •Avoiding the formation weld lines at cosmetic or stress critical areas. The reader might be wondering how the air escapes as the ram fills the mold. In general, minute vents are installed on the parting plane, allowing the air to escape. Air can also be made to escape around the ejector pins.The position ofthe gate relative to these minute vents is important: there should be an easy flow of air from the gate-location side of the mold to the vent positions. If the air is not prop- erly vented, it will become superheated, because of the high pressures involved, and scorch the part. Locating ring- Ejector pins 342 Plastic-Products Manufacturing and Final Assembly Chap. 8 ~~~G Ring gate Fan gate Diaphragm gate Sprue gate :3= O::u a= ~~ ~~e ~~ ~~ (a) Weld line (b> (0) Figure 8.10 (a) Various gate designs in the top figure. The lower figure shows (b) an inefficient gating method and (c) the preferred method (on the right) to fill JI cup (lIdllpt"d fmm JrT Jlnd Pye) 8.3.8 Design Guides for the Part to Be Molded The design of the mold requires great experience to account for the shrinkage' of the thermoplastic during cooling, the draft angles that must be added to vertical walls so that the part can be ejected, and the location of the parting plane. Some other key design guides are shown in Figure 8.11. This figure is taken from Magrab (1997) and is based on the guides by Bralla (1998) and by Niebel, Draper, and Wysk (1989). The guides recommend: • The avoidance of undercuts for simplification of ejection • Uniform wall thickness for obtaining uniform shrinkage and avoiding warpage • Dual diameters for deep holes to stiffen core pins under pressure and for cooling "Iypical shrinkages between the CAD geometry and the final part are 1% to 2%. A typical draft angle of 1 to 5 degrees is needed on a CAD design for a part that will be injection molded. These values will depend on the plastic and the texture used. Rather than quote values that may date quickly, for exten- sjvelistings,thereaderisreferredto< __ ~>. Runner Sprue gate Gate Representative Design Guidelines for Injection Molding 8.3 Processing of Plastics I: The Injection Molding Method BetterGuideline 343 Maintain uniform wall thickness and provide for graduaJ changes in wall thickness For a deep blind hole, use a stepped diameter Maintain uniform wall thickness in thermoset parts. A bead at the parting line facilitates removal of mold flash, Use decorative designs to conceal shrinkage. Avoid undercuts and variation in wall thickness. Deliberately offset side walls to hide defects caused when mold halves do not line up properly. Minimum spacing for holes and sidewalls. Minimum distance between a hole and the edge of the part. AA q l IT1 B B8 o iiJ U E] E3 B Jm bit? B EJ Fieme 8.11 Design guides for plastic injection molds. Reprinted with permission from Integrated Product and Process Design and Development by E. B. Magrab. Copyright CRC Press, Boca Raton, Florida. •Minimum spacing distances from holes to holes and from holes to walls to ensure that the mold fills correctly and uniformly in the spaces between features • Cavity shapes that reduce "flash" at the parting line •The use of decorations to divert the eye from difficult-to-mold areas (One "trick," for example, is to scribe shallow circles around the gate position.After fum [...]... toendorseitsproducts to showthepowerofDFAlDFM n but 2 Plastic-Products 360 Manufacturing and Final Assembly Chap 8 1,4(){l 1,3{l{) Compaq's Decade of Gnrwtb 1 ,20 0 UOO UlOO 900 · ~80 0 !,700 ~ j; 600 Annualprofits 500 400 'I()O 20 0 100 0 1 986 1 987 1 988 1 989 1990 199\ 1m FIpre 1.16 in 19911Wd 1993 1994 1995 1996 1997 Compaq's application of DFA and JIT.It deliberately reduced profits 19 92 bUlled to rapid growth fur Compaq... the sheet as shown in Figure 8. 15 Equating 8. 3 and 8. 4 yields (8. 4) Plastic-Products Manufacturing 3 48 and Final Assembly Chap 8 (average thickness) (pressure) Clamp Clamp (radius of circular dome) Figu~ 8. lS n' Differentiating 10(e) = Dome section showing various parameters In(?) this expression n +In(R) (m' - 10(s) - + oln(t» ~_E = ~~ +~ P E _ ~ _ s R m'~ t oF, oR, and 85 are incremental changes with... margins from above 40% to 20 % to 25 %.ln the period from 19 92 to 1994 the cost of Compaq's basic 486 DX2/50 machine was reduced from over $3,000 to around $1 ,80 0 This "shook up" the other PC clone makers considerably and was a clear step in the direction of turning the PC into a commodity item The following list delineates Compaq's strategies in the early 1990s, while Figure 8. 16 illustrates its success:... redesign of its standard printer, reduced the "part count," and used snap fits as much as possible rather than screws This had a great impact on the part count, which was reduced from 1 52 to 32 Consequently the assembly time, also related greatly to the thermoplastic snap fits, was reduced from 30 to 3 minutes (see Dewhurst and Boothroyd, 1 987 ) Figure 8. 12 shows mold designs that create such snap-fit... unique orientation Plastic-Products 3 52 Manufacturing and Final Assembly Chap 8 • If asymmetrical, can they at least he oriented in a repeatable way? • Have screws been eliminated as much as possible? • Can lead-in chamfers be used as shown in Figure 8. 17a? •Will parts tangle, nest, or interlock and cause problems when people or machines try to pick them up? • WlIl any parts "shingle" down onto each other... e Therefore to obtain an even thickness 8R + - 01 ~ m'- R t distribution throughout the thermoformed dome, the strain at any two points must be the same, that is, (Be) e ,i- This can be achieved by 1 ~ -n-e [BP + -m' B'] BR P 8. 7 THE COMPUTER AS A COMMODITY: AND MANUFACTURING 8. 7.1 System Molded R a high value of Configuration Case n >0 DESIGN and Packaging (8. 5) t and a low value of m', FOR ASSEMBLY... Perforated breaker plate Screw Die Barrel Control thermocouples Meterlna sectiort Figure8.13 Flighl Compression section Extrusion with the die on the left F~d pocket Feed section Plastic-Products Manufacturing 346 and Final Assembly Chap .8 \ Extruder Die, Penson Mold ~ OM nozzle c A D Figure 8. 14 Blow molding of plastic bottles 8. 5 PROCESSING OF PLASTICS III: BLOW MOLDING The blow-molding process produces the... eliminated? • Are parts rigid enough to withstand buckling during push assembly as shown in Figures 8. 17c, 8. 17d, and 8. 17f? • Has weight been minimized? Third, the design for manufacturability of the subcomponents should be analyzed: • Have tooling requirements been minimized and standardized? • Can the present shop equipment cope with all the assembly needs? • Are subcontractors supplying parts according... distance, s, that subcomponents have to be moved and (b) to increase the "give" in the assembly Figure8. 18 Fitts's tapping task 8. 7 The Computer as a Commodity: Design for Assembly and Manufacturing 353 with as large a value of w as can be permitted without compromising the functioning of the device 8. 7.6 Design Checklist for Electromechanical Devices For the assembly of an electromechanical device,... features that simplify the assembly of one component with another (Figure 8. 17), and by not 'fighting gravity (Anyone who has unscrewed a sump plug to change his or her oil will appreciate thisl) 8. 7 The Computer as a Commodity: Design for Assembly and Manufacturing 351 (a) Push and twist Multiple peg hole Screw Force fit Flip part over Simple peg hole Provide temporary support Insert peg and retainer . present.However,thebookremainswiththeseweU-known,early 19905 eventsat Compaq,nottoendorseitsproductsbuttoshowthepowerofDFAlDFM. 360 Plastic-Products Manufacturing and Final Assembly Chap. 8 1,4(){l 1,3{l{) 1 ,20 0 UOO UlOO 900 · ~80 0 !,700 ~ 600 j; 500 400 'I()O 20 0 100 0 1 986 1 987 1 988 1 989 1990 199 1m 1993 1994. profit margins from above 40% to 20 % to 25 %.ln the period from 19 92 to 1994 the cost of Compaq's basic 486 DX2/50 machine was reduced from over $3,000 to around $1 ,80 0. This "shook up". land -Ccld slug "Runner Branch runner Sprue Gate . Parting· surface 33. Plastic-Products Manufacturing and Final Assembly Chap. 8 8.3 .2 The Reciprocating-Screw Machine The reciprocating-screw