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6 1 1 S p e c i a l M o l d C o n c e p t s

6 1 1 1 S t a c k M o l d s

A special mold design has come into use, the stack mold, for molding shallow, smallparts in large quantities such as tape cassettes. Here, cavities are located in two or moreplanes corresponding to two parting lines and are filled at the same time (Figure 6.63).A molding m achine with an exceptionally long opening stroke is needed. An increase inproductivity of 100% as one might expect from doubling the number of cavities cannotbe realized because of the time needed for the longer opening and closing strokes. Theincrease in productivity is about 80% [6.49]. The clamping force should be 15% higherthan for a standard mold [6.49].

Hot man ifolds are now em ployed exclusively. A stack mold with two parting lines hasthree main components, a stationary and a movable mold half and a middle section. Itcontains the runner system (Figure 6.64).

Figure 6.62 Cold runner moldBucher/Mueller system with tunnel gate

[6.47]

conventional mold and have to be compensated by material savings. Thus, materiallosses in a corresponding eight-cavity mold could be reduced from 12 to 3% [6.47].

In cases where multiple gating is needed for certain moldings (e.g. headlampreflectors), production without cold-runner cassettes is often not conceivable [6.46].

Cold-runner technique for thermosets is also used in the so-called common-pocketprocess (Figure 6.62). A combination w ith this process is the RIC technique (RunnerlessInjection Com pression), which reduces scrap to a minim um in a simple way. At the sam etime flashing is diminished. The plasticated material flows through a temperature-controlled runner into the slightly opened mold and is distributed there. The material ispushed into the cavities and formed by the clamping motion of the mold. The materialdistributor penetrates the tapered sprue bushing and closes it against the parting line.Temperature control keeps the material in the runner fluid and ready for the next shot[6.48].

Previous Page

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Figure 6.63 Stack mold with hot manifold [6.47]A, B Parting lines; 1-3 Leader for center plate; 4a-4b Mold plates; 7-8 Cores; 9-10 Moldplates; 11 Leader for mold plates; 12 Leader bu shing; 15 Heater for sprue; 16 Hot manifold;17 Sprue to molded part; 18 Mold plate, as per 9; 19-23 Central sprue to machine as extendednozzle; 24 Sprue; 27 Stripper ring; 28 Ejector pins; 29-35 Ejector system; 39 Retainers;42 Heated nozzle; 4 5 ^ 6 Interlocks

Ground connection

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The mold section mounted on the movable platen and the center section are moved in thedirection of the machine axis during demolding. With this, the extension is removedfrom the nozzle. The extension has to be sufficiently long that no leakage material candrop onto the leader pins and stick there during mold opening. This would impede their

proper functioning [6.53, 6.54]. For this reason many stack molds are operated todaywith telescopic extensions and without nozzle retraction. While the outer section on theclamping side is mounted on the movable machine platen and moves positively with itduring mold opening and closing, special elements are necessary to guide and control themovement of the center section. Because of the frequently large size of the moldsutilizing the whole platen area, center sections are attached to the tie bars or are guidedby means of guide bars with guide shoes [6.53, 6.54] (see Figure 6.64).

Today the motion is primarily produced by toggles or sometimes by racks(Figure 6.65). Previously systems w ere employ ed w hich used separate hydra uliccylinders for moving the center section.

Toggle and rack control open at both parting lines smoothly and simultaneously.Toggle controls also offers the option of using, within a certain range, opening strokesof different lengths. This allows the molding of parts with one height in one stack andparts with a different height in the other one. The curves of the opening path can be

Sprue

Figure 6.64 Stack mold [6.16]Hot runner system for stack mold manifold and sprues and gates molds. The sprue is norm allymounted at the level of the mold center and feeds the melt into the middle of the hot runnermanifold. From there, the melt is distributed uniformly to all cavities of both mold daylights.

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Figure 6.67 Ejector drive for stack molds [6.55]

adjusted within a wide range depending on pivotal point and toggle geometry. At thesame time ejection is actuated by the same elements that move the center section.Various kinds of toggle control are shown with Figure 6.66. The rack control in Figure6.67 is less rigid and permits a gentle start and build-up of demolding forces because ofsprings in the pulling rods connected to the crank drive.

6 1 1 2 M o l d s f o r M u l t i c o m p o n e n t I n je c t io n M o l d i n g

There are a large number of multicomponent injection molding techniques, in terms ofprocesses and of names, which are explained in Table 6.6 [6.56, 6.57].

Rack or lever systemtrpper plateCamp ram

Campng unit

Movabecampng paten Strpper ring

Stationarycampng paten

Figure 6.66

Toggle mechanismfor stack molds[6.55]

Figure 6.65 Method s of moving center section of stack molds [6.55]Movemen with racksovemen with toggeovemen with hydrauc cynder

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This separation has allowed the development of two-component combination injection-molded parts, such as housings with integrated seals.

The separation may be effected in either of two ways: by the rotating mold systemsshown in Figure 6.68 and by the non-rotating core-back technique shown in Figure 6.69[6.57-6.59].

M olds with Rotating Mold Platen or Rotating M old HalfA rotating m old has several gating stations and different cavities. For a two-co lored part,the first colored section is created by injection at the first mo ld position. After sufficienttime has elapsed for the melt to cool, the mold opens and the mold-part section turns180° into the second position. The mold closes to form the second cavity into which thesecond color or another material is injected via a second injection position. In the firstmold position, meanwhile, the first molded-part section is being created again. In a

similar fashion, three-colored p arts can be made using three injection and mold positionsand rotations of 120°. The mold is rotated either by means of a standard rotary platformthat can be attached to the machine, irrespective of the mold, or by means of a rotarydevice integrated into the mold that allows a rotary plate to operate. The advantage ofthe standard rotary platform is its universal method of use, and in the smaller and lessexpensive design of the molds used. Usually the mold platen on the ejector side isdesigned to be the rotating side since rotation of the nozzle-side mold platen is morecomplicated in terms of gating system and rotating system. These molds require highprecision m old making but are dependable in operation and do not require any elaboratemelt feed [6.57, 6.60]. Typical applications are car tail light covers [6.60], three-coloredkeyboards [6.61], and the vent flaps of the Golf motorcar [6.62].

6 11 2 1 Com bination Mo lds

Two-component combination injection molding in which two melts are introduced intothe cavity in succession via separate gating systems requires special mold techniques

since those areas of the mold that become filled by the second melt must be blocked offwhen the first material is injected, in order that it does not penetrate into those areas.

Table 6.6 Definition of several multicomponent injection molding processes

Process name

Multicomponent injection molding

Composite injection molding

2-Color injection molding

Multicolor injection molding

2-Component sandwich injectionmolding

Bi-injection

Definition

All injection molding methods in which two or morematerials are processed

Several melts are injected via several gate systems intothe cavity in succession

As ab ove, but using one m aterial in different colors

Same as 2-color injection molding, but using more than2 colors

Two melts are injected in succession through a gatesystem, to form a core and outer layer

Two melts are injected simultaneously via two gatingsystems into the cavity

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Molds with Rotary Cores or SpidersIn this technique, only part of the ejector- or nozzle-side cavity with injected pre-molding is rotated (Figure 6.70). Both mold platens remain in position.

Molds with Transfer or Insert TechniqueAfter the pre-molding is made in the first cavity, it is transferred by a handling device orby hand into the second cavity and molded to produce the final part with a secondmaterial. The term transfer technique is also used to describe using a different machinefor molding to produce the final part. Generally, these molds are preferred to rotarymolds for economic reasons because the complicated rotary device can be dispensedwith, and usually m ore cavities can be accomm odated on the mold platen. Furtherm ore,thermal separation of the pre-molding and final-molding positions is easier toaccomplish (particularly important for thermoplastic-thermoset laminates). Dis-advantages are the need for precise centering of the pre-moldings [6.57].

M olds with Retractable Slides and Cores Core-Back M olds)With comparatively low mold costs, it is possible to produce multicolor or multi-component injection molded parts in one mold without the need for opening the machinein between and further transport of a molded part by means of the core-back technique.The cavity spaces for the second m aterial are first closed by movab le inserts or cores andare opened only after the first material has been injected. The components can bearranged beside, abov e, or inside each other. This m ethod does not suit material pairs thatwill not join or bond to each other since it is not possib le to produce effective undercutsfor interlocking with the injection partner. Furtherm ore, injection in these m olds can onlybe carried out sequentially and not in parallel as in other methods. This results in longer

Figure 6.68 Rotary mold systems forcomposite injection molding

Figure 6.69 Core-back technique

Rotary mold systemsRotary platformRotary moldSpiders or coresTransfer technique

Non-rotary mold system

Core back technque

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rotation or transfer [6.64, 6.65] and would otherwise have to be removed prior totransfer.

The choice of method for a particular molded part must be established individuallyfrom technical and, economic aspects for every application. It must be remembered,

however, that rotary mold systems are generally more expensive because of the need fortwo cavities and from the machine point of view, need a large distance between tie barsin order to be rotatable. Rotary molds do, however, offer greater design freedom and thepossibility of thermal separation of the kind required for the manufacture of rubber-thermoplastic combinations (e.g. PA/SLR).

6 .11 .2 .2 Tw o - C o m p o n e n t S a n d w i c h I n j e c t i o n M o l d s

In contrast to combination injection molding, sandwich molding theoretically does notrequire a special mold technology and may be performed with standard injection molds.Two melts are injected through a join t gating system into the cavity, to form a core andan outer skin. The melts meet in an adapter between the nozzle peaks of the injectionunits and the sprue bushing of the mold. It should be noted that all deviations fromrotationally symmetrical molded-part geometry with central gating cause non-uniformcore material distribution.

6 . 11 . 2 . 3 B i - l n j e c t i o n M o l d s

In this injection molding method, two different melt components are fed into the cavitysimultaneously through different gating systems. The weld line is affected by the posi-tions of injection and wall thicknesses in the mo ld as well as by the injection param etersof the two components [6.68].

R e f e r e n c e s

[6.1] KegelanguB, SchirmanguB, RinganguB, BandanguB. Technical Information, 4.2.1, BASF,Ludwigshafen/Rh., 1969.

[6.2] SpritzgieBen von Therm oplasten. Publication, Farbw erke Hoech st AG , Frankfurt/M.,1971.

[6.3] Sow a, H.: Wirtschaftlicher fertigen durch verbesserte AnguBsysteme. Plastverarbeiter, 29(1978), 11, pp. 587-590.

[6.4] Koh lhepp, K. G.; Mo hnberg, J.: SpritzgieBen von Formteilen hoher Prazision, dargestelltam Beispiel von Polyacetal. Kunststoff-Berater, 10 (1974), pp. 57 7-5 84 .

[6.5] Crastin-Sortiment, Eigenschaften, Verarbeitung. Publication, Ciba-Geigy, Basel, Aug ust

1977.[6.6] Christoffers, K. E.: Formteilg estaltung, verarbeitungsgerecht. Das SpritzguBteil. VDI-Verlag, Dusseldorf, 1980.

[6.7] TunnelanguB , AbreiB-Punkt-An guB. Techn ical Inform ation, 4.2.3, BASF, Ludwigs-hafen/Rh., 1969.

[6.8] Thonem ann, O. E.: AnguB- und Anschn itt-Technik fur die wirtschaftliche H erstellung vonSpritzguBteilen aus Makrolon. Plastverarbeiter, 14 (1963), 9, pp. 5 09 -52 4.

[6.9] Kunststoffverarbeitung im Ge sprach , 1: SpritzgieBen. Pub lication, BAS F, Lu dw igs-hafen/Rh., 1979.

[6.10] SpritzguB-Hostalen PP. Hand book. Farbwerke H oechst AG , Frankfurt/M., 1980.

[6.11] Vorkamm er-PunktanguB-Isolierverteiler. Technical Information, 4.2.4, BASF, Ludw igs-hafen/Rh., 1969.

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[6.44] Menges, G.; Barth, R: Erarbeitung system atischer Konstruktionshilfen zur Auslegun g vonKaltkanalwerkzeugen. AIF-AbschluBbericht, IKV, Aachen, 1987.

[6.45] Kloubert, T.: Reduzierung von ausgehartetem AnguBmaterial beim SpritzgieBen vonrieselfahigen Duroplasten - Kaltkanaleinsatz und Partikelrecycling. Dissertation, RWTH,Aachen, 1996.

[6.46] Niemann, K.: Kaltkanaltechnik-Stand und Einsatzmoglichkeiten bei Duroplasten.AnguBminimiertes SpritzgieBen, SKZ, Wiirzburg, June 23-24, 1987.[6.47] Gluckau, K.: Wirtschaftliche V erarbeitung von Duroplasten auf SpritzgieBmasch inen in

Kaltkanalwerkzeugen. Plastverarbeiter, 31 (1980), 8, pp. 467^69.[6.48] Braun, U.; Danne, W.; Schonthaler, W.: AnguBloses Spritzpragen in der Duro-

plastverarbeitung. Kunststoffe, 77 (1987), 1, pp. 27-29.[6.49] Hotz, A.: Mehr-Etagen-SpritzgieBwerkzeuge. Plastverarbeiter, 29 (1978), 4, pp . 185 -18 8.[6.50] Hartmann, W.: SpritzgieBwerkzeuge in Etagenbau weise mit Therm oplay -

HeiBkanaldiisen fur Verpackungs-Unterteile aus Polystyrol. Plastverarbeiter, 32 (1981), 5,pp. 600-605.

[6.51] Mo slo, E. R: Runn erless-Moulding. SP E-Journal, 11 (1955), pp. 26 -3 6.[6.521 Mo slo, E. R: Runnerless-Mou lding. New York, 1960.[6.531 Lindner E.; Hartmann, W.: SpritzgieBwerkzeuge in Etagenbauweise. Plastverarbeiter, 28

(1977), 7, pp. 351 -353 .[6.54] Schwaninger, W.: Etagenw erkzeuge insbesondere als Alternative zum Schnellaufer. Der

SpritzgieBprozeB, Ingenieurwissen, VDI-Verlag, Dusseldorf 1979.[6.55] Pub lication, Husky .[6.56] Eckardt, H.: MehrkomponentenspritzgieBen - Neue Werkstoffe und Verfahren beim

SpritzgieBen. VDI-Verlag, Dusseldorf 1990, pp. 149-194.[6.57] Langenfeld, M.: Werkzeugtechnik zur Herstellung von Mehrkomponenten-SpritzgieB-

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[6.59] Kraft, H.: Erweiterte Anw endun gsbereiche fur das Meh rkompon enten-SpritzgieBen . In:Kunststoffe, 83 (1993), 6, pp. 429 -4 33 .

[6.60] Bod eving, C : Horizon talmaschine mit Dreh tisch . In: Kunststoffe, 85 (1995), 9,pp. 1244-1254.

[6.61] Bew eglich aus dem SpritzgieBwerkzeug . In: Plastverarbeiter, 36 (1985), 4.[6.62] Vollbewegliche Lufterklappen b ei jede m Spritzzyklus . In: Plastverarbeiter, 44 (1993), 2.[6.63] Netstal News. No . 3 1, Netstal-Maschinen A G, Nafels/Schweiz, April 1997.[6.64] HeiBkanaltechnik beim Meh rkompon enten-SpritzgieBen . In: Plastverarbeiter, 45 (1994 )

10 .[6.65] Krauss, R.: SpritzgieBtechnik fur die Mehrfarben technik. In: Ko nstruieren von SpritzgieB-

werkzeugen, VDI-Verlag, Dusseldorf 1987, pp. 6 .1-6.18.[6.66] Mehr-Rohstoff-Technologie. Publication. Battenfeld, Me inerzhagen, September 1996.[6.67] Com bimelt-Technologie fur Mehrfarben und Meh rkomp onentenspritzguB . In: KG K

Kautschuk, Gummi, Kunststoffe, 49 (1996), 7-8.[6.68] Johannaber E; Konejung, K.: Mehrkomponenten-Technik beim SpritzgieBen. Mehr

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