Alignment and Changing of Molds

13 A l i g n m e n t a n d C h a n g i n g 13.1 Function of A l i g n m e n t of M o l d s Injection molds are mounted onto the platens of the clamping unit of the injectionmolding machine The clamping unit opens and closes them during the course of the molding cycle The molds have to be guided in such a way that all inserts are accurately aligned and the mold halves are tightly closed Without proper insert alignment, molded parts would exhibit deviations in wall thickness; they would not have the required dimensions Because guiding molds with the clamping unit alone is generally not sufficient, injection molds also need a so-called internal alignment It aligns both mold halves with the necessary precision and prevents their convolute joining 13.2 A l i g n m e n t w i t h t h e Axis of t h e Plasticating Unit Precise alignment is mandatory here Otherwise the nozzle could not be sealed by the sprue bushing, and undercuts occurring at the sprue would interrupt operation Therefore, alignments concentric with the sprue bushing are used almost exclusively For this purpose the mold is equipped with a flange-like locating ring, which keeps the sprue bushing in the mold (Figure 13.1) and matches the corresponding opening in the machine platen, the diameter of which depends on the size of the machine platen [13.1] Locating rings are readily available from producers of mold standards (see Chapter 17) and are machined from case-hardening steel or water-quenched unalloyed tool steel The locating ring has a tapered inner bore of appropriated dimensions to allow the nozzle tip to pass through A mold has usually only one locating ring If both mold halves are equipped with locating rings, then a loose fit is needed on the movable side to better align both sides This is only a help for setting up the mold Machn i e platen Locating ring Machine platen Heat insulating sheet Mold clamping plate Figure 13.1 Locating ring Figure 13.2 Split locating ring [13.5] The locating ring is slightly press-fitted into the mold plate and fits the machine platen with a close sliding fit Figure 13.2 shows a locating ring combined with a thermal insulation plate This design is particularly useful for adding an insulating sheet to the mold This is done for processing thermosets or thermoplastics if a high mold temperature is needed for molding precision parts 13.3 Internal Alignment a n d Interlocking The mold halves themselves have to be guided internally, by the tie bars of the machine, to obtain the needed accuracy In small molds this is done with leader pins; pins which protrude from one mold half of the opened mold and slide into precisely fitting bushings in the other mold half during mold closing This ensures a constant and accurate alignment of both surfaces of flat molds without shifting during injection and the production of moldings In molds with deep cavities, especially those with long and slender cores, a shifting of the core can occur during injection in spite of exact alignment with leader pins This has already been discussed in Chapter 11, "Shifting of Cores" Design examples for such molds were presented with Figure 11.21 Figure 13.3 shows an example for positioning and mounting a leader pin and the appropriate bushing Four leader "units" (pin and bushing) are usually required for proper alignment To facilitate the assembly and to make sure that the mold is always correctly put together, one leader pin is offset or made in a different dimension [13.2-13.5] The latter method may cause fewer difficulties, especially when standard mold parts are used If two leader pins, one diagonally opposite the other, are made Leader pin Leader bushing Locating sleeve Figure 13.3 Leader-pin assembly [13.5] longer, it is easier to slide the two halves together while placing the mold into the machine or during assembly The leader pins are positioned as close to the edge as possible to gain space for the cavity and an adequate number of cooling lines Effective alignment is possible only if close tolerances are kept between leader pins and corresponding holes This, however, causes considerable wear Therefore it is not prudent to let the pins slide directly into the respective holes of the mold plates As a matter of principle, leader bushings should be used to counteract wear and to enable worn-out parts to be exchanged easily Leader bushings, like leader pins, are made of case-hardened steel with a hardness of 60 to 62 Rc They are commercially available in various sizes and shapes Wear can furthermore be reduced by lubricating the pins with molybdenum disulfide For this purpose pins or bushings have oil grooves Leader pins without lubrication (Figure 13.7) should only be used for rather small molds or special applications in slides or with ball bushings (Figure 13.9) Low-maintenance operation is achieved through the use of leader bushings with solid lubricant depots [13.2-13.4] Leader pins and bushings are commercially available in various designs (Figure 13.5) Attention should be paid to the recommended fits (Figure 13.4) The length of leader pins depends on the depth of the cavity Leading has to begin before the mold halves are engaged Therefore, a sufficient length must be selected Shoulder leader pins (Figure 13.5) can, at the same time, pin mold plates together Commercial availability is treated in Chapter 17 Figure 13.4 Tolerances for leader pins Figure 13.5 Common design of leader pins The length of leader bushings depends on their diameter It should be 1.5- to 3-times the inside diameter (Figure 13.6) The corresponding holes in the mold plate have to be drilled according to instructions Figures 13.7 and 13.8 demonstrate the assembly of leader pins and bushings [13.4] Figure 13.7 shows the system for the design of simple molds and for leading slides A jig drill is needed for proper alignment of the holes in the various plates Figure 13.8 represents a system with shoulder leader pins Bores for pin and bushing can be drilled in a single operation (equal diameters) The system of Figure 13.9 is not used very often It is carefree and ensures a precise and low-friction movement, but at additional expenses With leader sleeves, mold plates can be aligned and connected with one another which would otherwise not be engaged by leader pin or bushing (Figure 13.10) The diameter of the sleeve is kept the same as that of the bushing or the shoulder of a shoulder leader pin Therefore, all holes can be machined in one pass The internal diameter is adequately large to permit unrestricted entering of pins The removal of the locating sleeves can be readily effected through the tapped hole at the end by means of an extractor, e.g., leader pin in Figure 13.11 Figure 13.7 Leader pin without oil grooves [13.4] Vent Figure 13.6 Shoulder bushings with common tolerances Figure 13.8 [13.4] Shoulder leader pin Among the multitude of leader-pin systems, those shown in Figures 13.11 and 13.12 should also be mentioned Both are based on components already described: leader pin, bushing and sleeve The system of Figure 13.11 has pins and bushings with threaded holes and tightening screws, which are propped on the opposite side by head supports Plates not engaged by pin and bushing are lead by sleeves This design is more expensive, however, than the one shown in Figure 13.3 but has some decisive advantages The assembly of the individual plates is not done with screws, and additional drilling of holes is unnecessary The plates are kept together with the tightening screws At the same time, the mold plate can be utilized better for accommodating cavity and cooling lines [13.3, 13.5] The system in Figure 13.12 shows a very different design of leader bushings and sleeves and their assembly in the mold The bushings consist of three parts: the bushing proper, a retainer ring, and a ring nut There are two locations for the retainer ring The bushings can be mounted flush with the plate surface or protruding by mm and Figure 13.9 Leader pin assembly with ball bushing [13.4] Figure 13.10 Locating sleeve [13.4] Figure 13.11 Leader pin assembly [13.5] a Leader pin with tapped hole for retainer screw, b Shoulder bushing with retainer screw, c Tubular dowel Figure 13.12 Leader pin assembly with several straight bushings [13.6] tightened with the nut In the protruding position any number of plates can be connected to one another This kind of bushing can take over the job of bushings with or without retainer ring and can also be used as a leader sleeve In this case, no ring nut is used The nut connects the bushings reliably to the mold plates This could save additional plates, which are needed in other systems for supporting the bushings The height of the mold is lower This may compensate the costs for the more elaborate design [13.32] To ensure proper operation, no lateral forces should act upon the leader systems If there is no lateral load, the required cross-section of the leader pins does not have to be computed For oblique pins, especially those acting on slides, the necessary cross-section should be calculated The same equations can be used as presented in Section 12.9.2.1 for slide molds 13.4 A l i g n m e n t of L a r g e Molds 0.02 Clearance Occasionally, no leader pins are used in large and deep molds such as those for buckets and boxes Guiding is left to the tie bars of the molding machine during opening and closing until short of complete engagement Since this accuracy is insufficient for proper alignment, special arrangements become necessary They are all characterized by the fact that the alignment does not begin sooner than shortly before the mold is closed Both mold halves brace one another when closed Of special advantage is the "pot" design (Figure 13.13) and its variations (Figures 13.14 and 13.15) because it also reacts Figure 13.13 Interlock machined into solid material [13.4] Figure 13.14 Interlock with attached gib [13.4] Figure 13.15 [13.4] Interlock with inserted gib against forces from cavity expansion The inserted ledges in the variations are easily replaceable after they are worn out More variations are presented with Figure 13.16 Frequently bolts are used as aligning interlocks fitted into both mold halves (Figure 13.17) Their center line is not in the plane of the parting line Thus, both mold halves are braced against one another after clamping Figure 13.18 shows such a mold Instead of cylindrical alignment bolts, rectangular interlocks made of shock-resistant tool steel can be employed Alignment of molds by such interlocks calls for high accuracy of machining, because later corrections are not practical Frequently, tapered interlocks according to Figure 13.19 are finally used The long design of the locating bolts and bushings, in contrast to the round design, allows different thermal expansion on the nozzle and clamping sides to be compensated If precise alignment and precision locating of the mold halves are necessary, the use of flat leaders with solid lubricant depots may be expedient (Figure 13.20) These permit Figure 13.16 Modified interlocks [13.7] Figure 13.17 Cylindrical interlock [13.4] Figure 13.18 Alignment with cylindrical interlock [13.8] precise centering even before the mold is completely closed, as well as compensation of differential thermal expansion of fixed and moving mold half Flat leaders may also be combined with circular (conventional) leader elements (Figure 13.21) Figure 13.19 Tapered interlocks [13.8] Male, Female Figure 13.20 Flat leaders with solid lubricant depots [13.2] 13.5 Changing Figure 13.21 Combinations of flat leaders with conventional leader elements [13.2] Molds 13.5.1 S y s t e m s f o r a Q u i c k C h a n g e o f M o l d s for Thermoplastics Injection molds are usually mounted to the machine platens by mechanical clamping devices (conventional mold clamps with bolts) and connected to power- and watersupply lines To this the mold is either horizontally or vertically brought into the machine by a lifting device Depending on size and weight of the mold and the number of connections, this leads to shutdown times, which may last from an hour to several days (Figure 13.22) [13.9] Such secondary times affect the productivity considerably, especially in the case of small batch sizes and thus frequent mold changes The development towards automation, the demand for more flexibility and better economics, lead by necessity to systems for a quick mold change [13.10-13.12] In spite of this, such systems have not prevailed so far There are two reasons for this One is the lack of compatibility among the various systems on the market today [13.13, 13.14, 13.16, 13.17] The second one is the need for a change of almost all molds used in a machine and the associated high costs Automatic change Weight of mold (tons) Clamping force (IcN) Manual change Tm i e for mold change (hours) Tm i e for mold change (minutes) Figure 13.22 Cutback in down time with rapidclamp systems for mold changes in injection molding machines [13.9] A quick-change system consists of several components which allows changing injection molds either fully automatically or semi-automatically, controlled by an operator Such components serve the function of - detaching and fastening the mold at the machine platens, - disconnecting and connecting the supply lines, - bringing the mold into the clamping unit or taking it out From this follows the need for these means of a quick-change system: - quick-clamping devices, - quick-connection couplings, - changing equipment Besides this, some more components are required for automation of mold changing, which have to be combined to one system (Figure 13.23) Only the combined action of all components permits flexible and automated injection molding [13.13] Mold design is mostly affected by quick-connection couplings Two solutions for quick-clamping devices have prevailed on the market One can distinguish between adaptive and integrated clamping systems, which are usually actuated hydraulically They can easily be inserted into a concept of flexible automation The adaptive clamping system has hydraulically actuated locking cylinders or ledges with integrated collets [13.9] mounted to the clamping platens of the machine, into which the precisely machined clamping plate of the mold is inserted They are mostly chamfered or provided with a groove (Figures 13.24 and 13.25) During clamping, the piston or ledge, which is also chamfered, is moved against a corresponding counter chamfer of the mold (Figures 13.24 and 13.25) The counter chamfer is about 5° This angle causes self-locking (as long as no oil has dripped on it) For reasons of safety, clamping elements are therefore equipped with a proximity switch as a standard [13.9, 13.16-13.19, 13.21] The integrated clamping system has a hydraulic locking device integrated into the clamping platens It clamps the mold either directly via bolts mounted on base plates of the mold [13.22, 13.23] or via its own bolts, which press flat against the edges of the mold edges (platens) [13.23] (Figure 13.26) Main computer Mold storage Mold-handling system Figure 13.23 Pre-heating location Rapid-clamp system Quc i k couplings Exchange equipment Machine control Components of automatic mold change [13.13] Figure 13.24 Adaptive rapid clamp system Hydraulic jack (left), Clamping ledge with integrated lugs, mold plate chamfered (right) [13.9] Figure 13.25 Adaptive rapid-clamp system A-E Variable dimensions of the individual design, top: Cylinder with sloped piston assuring a self-locking clamp with correspondingly sloped clamping face, bottom: Tilted cylinder causes selflocking clamp with level clamping face The equipment to automate the mold change is so multifarious today that not everything can be presented here All systems presented so far operate basically with the same locking mechanisms As soon as they are mounted to the clamping platen of the machine, they have to be looked at as a rigid system, which determines the size of the mold plate independent of the mold size Thus, the base plates of small molds may become disproportionately large, and for large molds one eventually has to switch to a machine with greater clamping force The solutions presented so far assume a rectangular clamping plate of the mold Figure 13.27 presents a design with hydraulic clamping slides acting on locating rings and integrated into the clamping platens of the machine This system is suited for rectangular as well as for circular molds [13.24, 13.25] Mechanical systems are possible for smaller molds [13.3, 13.19] A manually operated quick-clamping device is on the market as a supplement composed of standards [13.2] A totally different concept is the use of magnetic clamping plates The surface of the mold is kept uniformly on the clamping plate by a magnetic holding force generated by a permanent magnet No additional clamping devices are necessary The system is designed such that the magnetic force is maintained even during a power failure, enabling the mold to be held securely It only requires a short burst of current for switching the plate on and off The advantage of the system is that only the machine needs to be modified No changes have to be made to the molds A prerequisite, however, is the use of magnetizable materials Furthermore, when the mold is being designed, it must be borne in mind that any requisite thermal insulation plates have to be integrated before the mold Figure 13.26 Integrated hydraulic rapid-clamping system [13.23] clamping plate since the magnetic force only acts over a distance of roughly 12 mm in the plates most commonly employed in injection molding machines [13.26] Proximity switches ensure that the mold is securely mounted on the machine If, for instance, the mold moves by more than 0.2 mm from the magnetic plate during the opening process, the machine is stopped (Figure 13.28) According to the manufacturer, about 1,000 injection molding machines have been fitted out with magnetic plates Aside from the mold, there must be an automatic connection with the ejector system if spring-loaded ejectors are not used For this purpose, hydraulic clamping systems and mechanical couplings are resorted to [13.2, 13.17, 13.27] The use of quick-clamping systems for changing molds already results in a considerable shortening of the setup time, but it does not yet ensure a fully automatic change of molds This is only made possible by employing quick-coupling systems for energy supply and for sensors The following connections are required for - heat-exchange medium (oil, water), hydraulic oil for slides, core pullers etc., electric connections for heating (hot runners), connections for thermocouples and pressure sensors Design and installation on both the mold and the machine are set out in [13.27] Coupling systems are designed as modular systems and consist, depending on size, of individual couplings for energy supply and sensor connections, guide pins, lock and docking cylinders (Figures 13.29 and 13.30) Thus, the systems can be assembled as demanded by the application They are supplied as standards for manual and automatic operation Accurate assembly of these systems is mandatory for their trouble-free function A small inexactness during assembly can lead to inadmissible displacement, which results in leakage and premature wear Therefore, coupling elements are floating in the carrier plates in order to eventually allow an adjustment Thermal expansions can also be captured this way especially in large molds [13.9, 13.13, 13,18] Arnold-change equipment affects mold design, if at all, only insignificantly They are accessories, which change molds and, depending on makeup, can accept the function of a mold-conveying system The complete mold is always exchanged Depending on the standard of the equipment, molds are immediately ready Clamping slide Locating ring Stationary platen Clamping slide Figure 13.27 Rapid moldclamping system Mold clamped to locating ring [13.24] Magnetic circuit built into platen Platen Tie rod Proximity switch Figure 13.28 Magnetic mounting platen [13.26] Figure 13.29 Mold with rapidcoupling device [13.9] As soon as the mold is laterally placed into the machine and securely fastened the movable coupling half is inserted into the coupling half mounted to the mold by pressure cylinders Guide pins align both halves When they are fully connected, they are interlocked by locking cylinders Figure 13.30 Multiple coupling with interlock and insertion equipment [13.9] The model presented here with 32 water, six hydraulic and two compressed-air connections is only one eighth of the complete equipment The multiplecoupling system is part of the clamping ledge and is equipped with a central insertion cylinder and a locking cylinder on each side The locking cylinders have hollow pistons which carry the guide pins of the system As soon as the coupling halves are connected the pins become self-locking by means of a hydraulically actuated chuck for operation and are already brought up to working temperature during the change procedure The effectiveness of the quick-clamping systems is being continuously improved by the use of mold carriers The mold, located on the carriers which are attached to the side of the injection molding machine, can be prepared for production before being inserted into the clamping unit [13.23] (Figure 13.31) The system shown in Figure 13.31 has been developed for the production of small runs, prototypes, and test specimens It consists of a changing frame and various slots that contain the mold cavities [13.28, 13.29] The inserts can be locked by the quickclamping device mentioned at the outset and attached with the quick-fit couplings to the power supply A similar concept for multicavity molds is presented in [13.30] Figure 13.31 Mold carrier-enhances the effectiveness of rapid mounting systems [13.33] This system cannot be transferred to all types of molds, but offers significant cost advantages for small lots and with simple molds 13.5.2 M o l d E x c h a n g e r for Elastomer M o l d s A quick-change system for elastomers was designed (Figure 13.33) in accordance with the same concept as was illustrated in Figure 13.32 for thermoplastics For a "mold Mold-changing system Mold insertEjector Figure 13.32 Mold-changing system for small production runs [13.29] Figure 13.33 Modular mold design [13.31] Locating ring, Insulating sheets, Clamping plates, Cold manifold, Nozzle, Heated plate, Cavity retainer plates, Spacing blocks, Guide ledges Insert positioning Mold chassis change" only the forming mold plates are exchanged with this system The changing plates are designed in such a way that they not contain sensitive mold components, especially no heater elements or sensors Thus, they can be carried to a cleaning bath without much effort immediately after the exchange For the exchange of the mold plates an exchange system (Figure 13.34) was built, which operates fully automatically It takes the plates from a preheating station and carries them to the mold base To ensure a fully automatic operation, a cold-runner manifold was used, which was largely separated from the remaining mold and kept at about the temperature of the injection unit With this, the otherwise considerable material loss from the runner system could be circumvented The manifold was designed so that it could be exchanged effortlessly Gripper Gripper cylinder Exchange plate Locating plate Support Feeding plate Feeding cylinder Figure 13.34 top: System for exchanging retainer plates bottom: Components of the exchange table [13.31] References [13.1] [13.2] [13.3] Euromap Injection moulding machines Mould fixing and connection dimensions Europaisches Komitee der Hersteller von Kunststoff- und Gummimaschinen, VDMA, Frankfurt, 1995 Standards, Hasco, 1997, Liidenscheid Standards, EOC, Liidenscheid, 1996 [13.4] [13.5] [13.6] [13.7] [13.8] [13.9] [13.10] [13.11] [13.12] [13.13] [13.14] [13.15] [13.16] [13.17] [13.18] [13.19] [13.20] [13.21] [13.22] [13.23] [13.24] [13.25] [13.26] [13.27] [13.28] [13.29] [13.30] [13.31] [13.32] [13.33] Mohrwald, K.: Einblick in die Konstruktion von SpritzgieBwerkzeugen Garrels, Hamburg, 1965 Handbook of Standards, Sustan, Frankfurt, 1966 Standards, Prospectus, Zimmermann, Lahr/Schwarzwald Zawistowski, H.; Frenkler, D.: Konstrukcja form wtryskowych tworzyw termoplastycznych (Design of injection molds for thermoplastics) Wydawnictwo NaukowoTechniczne, Warszawa, 1984 Lindner, E.: SpritzgieBwerkzeuge fiir groBe Teile, Information from the Laboratory for Tech Application of Plastics, BASF, Ludwigshafen/Rh Handbuch fiir den Einsatz von Werkzeugschnellwechselsystemen in SpritzgieBbetrieben Prospectus, Enerpac, Diisseldorf, 1987 CIM im SpritzgieBbetrieb Carl Hanser Verlag, Miinchen, 1992 Fernengel, R.: Konzept einer modernen SpritzgieBfertigung Kunststoffe, 84 (1994), 10, pp 1361-1362 and pp 1364-1367 Rationeller fertigen mit Werkzeugwechselsystemen Plastverarbeiter, 45 (1994), 2, pp 87-89 Benfer, W.: Werkzeugwechselsysteme an SpritzgieBmaschinen Kunststoffe, 77 (1987), 2, pp 139-149 Heuel, 0.: Losen Schnellwechsel-Systeme fiir SpritzgieBformen alle Probleme? Kunststoffberater, 32 (1987), 11, pp 22-25 Spannen der Werkzeuge vereinheitlicht Plastverarbeiter, 38 (1987), 2, pp 82-84 Bourdon, K.: Zufiihrungs- und Entnahmetechniken an der SpritzgieBmaschine SKZConference, Wurzburg, June 19-20, 1991, pp 83-101 Schultheis, M.: Der SpritzgieBbetrieb und seine Wettbewerbsfahigkeit VDI-K, Dusseldorf(1992), pp 113-131 Handbuch fiir den Einsatz von Werkzeugwechselsystemen in SpritzgieBbetrieben Prospectus, Enerpac, Veenendaal, NL Prospectus Staubli AG, SeestraBe 250, CH-8810, Horgen/Ziirich Thoma, H.: Rechnereinsatz und flexible Maschinenkonzepte Kunststoffe, 75 (1985), 9, pp 568-572 Schneller Werkzeugwechsel moglich Plastverarbeiter, 37 (1986), 8, pp 56-57 Prospectus Engel, Schwertberg, Osterreich, 1995 Prospectus Arburg, LoBburg, 1996 DE PS 2938665 C2, Krauss-Maffei, 1984 Prospectus Netstal AG, Nafels, Schweiz, 1996 Prospectus Giihring Spanntechnik, Komwestheim, 1997 Euromap 11 Automatic mould changing on injection moulding machines Europaisches Komitee der Hersteller von Kunststoff- und Gummimaschinen, VDMA, Frankfurt, 1993 Backhoff, W.; Lemmen, E.: Stammwerkzeug mit wechselbaren Einsatzen Information for Tech Applications, 259/79, Bayer AG, Leverkusen, 1979 Spanner, P et al.: Flexibles Fertigungszentrum fiir das SpritzgieBen kleiner Serien Kunststoffe, 74 (1984), 9, pp 489-490 Vollaustauschbares Werkzeug-Wechselsystem Plastverarbeiter, 45 (1994), 2, pp 42-43 Weyer, G.: Automatische Herstellung von Elastomerartikeln im SpritzgieBverfahren Dissertation, Tech University, Aachen, 1987 Formenwechsel leichtgemacht Kunstst Plast., 32 (1985), 11, pp 12-13 Halbwerkzeugverarbeitung Technical Information Farbwerke Hoechst AG, Frankfurt, 1975