Special Mold Concept

Previous Page

conventional mold and have to be compensated by material savings. Thus, material
losses 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. headlamp
reflectors), production without cold-runner cassettes is often not conceivable [6.46].
Cold-runner technique for thermosets is also used in the so-called common-pocket
process (Figure 6.62). A combination with this process is the RIC technique (Runnerless
Injection Compression), which reduces scrap to a minimum in a simple way. At the same
time flashing is diminished. The plasticated material flows through a temperaturecontrolled runner into the slightly opened mold and is distributed there. The material is
pushed into the cavities and formed by the clamping motion of the mold. The material
distributor 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].

Figure 6.62 Cold runner mold
Bucher/Mueller system with tunnel gate
[6.47]

6.11

Special Mold

Concepts

6.11.1 S t a c k M o l d s
A special mold design has come into use, the stack mold, for molding shallow, small
parts in large quantities such as tape cassettes. Here, cavities are located in two or more
planes corresponding to two parting lines and are filled at the same time (Figure 6.63).
A molding machine with an exceptionally long opening stroke is needed. An increase in
productivity of 100% as one might expect from doubling the number of cavities cannot

be realized because of the time needed for the longer opening and closing strokes. The
increase in productivity is about 80% [6.49]. The clamping force should be 15% higher
than for a standard mold [6.49].
Hot manifolds are now employed exclusively. A stack mold with two parting lines has
three main components, a stationary and a movable mold half, and a middle section. It
contains the runner system (Figure 6.64).


Ground connection

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 Mold
plates; 11 Leader for mold plates; 12 Leader bushing; 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 extended
nozzle; 24 Sprue; 27 Stripper ring; 28 Ejector pins; 29-35 Ejector system; 39 Retainers;
42 Heated nozzle; 4 5 ^ 6 Interlocks


The mold section mounted on the movable platen and the center section are moved in the
direction of the machine axis during demolding. With this, the extension is removed
from the nozzle. The extension has to be sufficiently long that no leakage material can
drop 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 today
with telescopic extensions and without nozzle retraction. While the outer section on the
clamping side is mounted on the movable machine platen and moves positively with it
during mold opening and closing, special elements are necessary to guide and control the
movement of the center section. Because of the frequently large size of the molds
utilizing the whole platen area, center sections are attached to the tie bars or are guided
by 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 were employed which used separate hydraulic
cylinders 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 strokes
of different lengths. This allows the molding of parts with one height in one stack and
parts 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 normally
mounted at the level of the mold center and feeds the melt into the middle of the hot runner
manifold. From there, the melt is distributed uniformly to all cavities of both mold daylights.


Movement with hydraulic cylinder
Figure 6.65

Movement with toggle

Movement with racks

Methods of moving center section of stack molds [6.55]

Figure 6.66
Toggle mechanism
for stack molds
[6.55]

Movable

clamping platen

Stationary
clamping platen

Stripper ring

Ca
l mp ram
Clamping unit

Stripper plate

Rack or lever system

Figure 6.67 Ejector drive for stack molds [6.55]

adjusted within a wide range depending on pivotal point and toggle geometry. At the
same 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 Figure
6.67 is less rigid and permits a gentle start and build-up of demolding forces because of
springs in the pulling rods connected to the crank drive.
6.11.2 M o l d s for M u l t i c o m p o n e n t Injection M o l d i n g
There are a large number of multicomponent injection molding techniques, in terms of
processes and of names, which are explained in Table 6.6 [6.56, 6.57].


6.11.2.1 Combination Molds
Two-component combination injection molding in which two melts are introduced into
the 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 off
when 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

Definition

Multicomponent injection molding All injection molding methods in which two or more
materials are processed
Several melts are injected via several gate systems into
Composite injection molding
the cavity in succession
2-Color injection molding

As above, but using one material in different colors

Multicolor injection molding

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

2-Component sandwich injection
molding

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

Bi-injection

Two melts are injected simultaneously via two gating

systems into the cavity

This separation has allowed the development of two-component combination injectionmolded parts, such as housings with integrated seals.
The separation may be effected in either of two ways: by the rotating mold systems
shown in Figure 6.68 and by the non-rotating core-back technique shown in Figure 6.69
[6.57-6.59].
Molds with Rotating Mold Platen or Rotating Mold Half
A rotating mold has several gating stations and different cavities. For a two-colored part,
the first colored section is created by injection at the first mold position. After sufficient
time has elapsed for the melt to cool, the mold opens and the mold-part section turns
180° into the second position. The mold closes to form the second cavity into which the
second color or another material is injected via a second injection position. In the first
mold position, meanwhile, the first molded-part section is being created again. In a
similar fashion, three-colored parts can be made using three injection and mold positions
and rotations of 120°. The mold is rotated either by means of a standard rotary platform
that can be attached to the machine, irrespective of the mold, or by means of a rotary
device integrated into the mold that allows a rotary plate to operate. The advantage of
the standard rotary platform is its universal method of use, and in the smaller and less
expensive design of the molds used. Usually the mold platen on the ejector side is
designed to be the rotating side since rotation of the nozzle-side mold platen is more
complicated in terms of gating system and rotating system. These molds require high
precision mold making but are dependable in operation and do not require any elaborate
melt feed [6.57, 6.60]. Typical applications are car tail light covers [6.60], three-colored
keyboards [6.61], and the vent flaps of the Golf motorcar [6.62].


Non-rotary mold system
Core/back technq
i ue
Rotary mold systems

Rotary platform
Rotary mold
Spiders or cores
Transfer technique

Figure 6.68 Rotary mold systems for
composite injection molding

Figure 6.69 Core-back technique

Molds with Rotary Cores or Spiders
In this technique, only part of the ejector- or nozzle-side cavity with injected premolding is rotated (Figure 6.70). Both mold platens remain in position.
Molds with Transfer or Insert Technique
After the pre-molding is made in the first cavity, it is transferred by a handling device or
by hand into the second cavity and molded to produce the final part with a second
material. The term transfer technique is also used to describe using a different machine
for molding to produce the final part. Generally, these molds are preferred to rotary
molds for economic reasons because the complicated rotary device can be dispensed
with, and usually more cavities can be accommodated on the mold platen. Furthermore,
thermal separation of the pre-molding and final-molding positions is easier to
accomplish (particularly important for thermoplastic-thermoset laminates). Disadvantages are the need for precise centering of the pre-moldings [6.57].
Molds with Retractable Slides and Cores (Core-Back Molds)
With comparatively low mold costs, it is possible to produce multicolor or multicomponent injection molded parts in one mold without the need for opening the machine
in between and further transport of a molded part by means of the core-back technique.
The cavity spaces for the second material are first closed by movable inserts or cores and
are opened only after the first material has been injected. The components can be
arranged beside, above, or inside each other. This method does not suit material pairs that
will not join or bond to each other since it is not possible to produce effective undercuts
for interlocking with the injection partner. Furthermore, injection in these molds can only
be carried out sequentially and not in parallel as in other methods. This results in longer

cycle times [6.57]. Separate temperature control of the cores or inserts is beneficial since
the temperature of the impact surface onto which the second melt is injected can be
controlled more accurately [6.64].
In combination injection molding, the rotary mold systems often employ hot runners
for the pre-molding so as to yield a gateless pre-molding, since the gate interferes during

Rotary platform technique,
schematic
[Source: Nefctal, 1997]

Figure 6.70
[6.63]

Overmolding by the rotary technique. Here: toothbrush made of two components


rotation or transfer [6.64, 6.65] and would otherwise have to be removed prior to
transfer.
The choice of method for a particular molded part must be established individually
from technical and, economic aspects for every application. It must be remembered,
however, that rotary mold systems are generally more expensive because of the need for
two cavities and from the machine point of view, need a large distance between tie bars
in order to be rotatable. Rotary molds do, however, offer greater design freedom and the
possibility of thermal separation of the kind required for the manufacture of rubberthermoplastic combinations (e.g. PA/SLR).
6.11.2.2 T w o - C o m p o n e n t S a n d w i c h Injection M o l d s

In contrast to combination injection molding, sandwich molding theoretically does not
require a special mold technology and may be performed with standard injection molds.

Two melts are injected through a joint gating system into the cavity, to form a core and
an outer skin. The melts meet in an adapter between the nozzle peaks of the injection
units and the sprue bushing of the mold. It should be noted that all deviations from
rotationally symmetrical molded-part geometry with central gating cause non-uniform
core material distribution.
6.11.2.3 Bi-lnjection M o l d s

In this injection molding method, two different melt components are fed into the cavity
simultaneously through different gating systems. The weld line is affected by the positions of injection and wall thicknesses in the mold as well as by the injection parameters
of the two components [6.68].

References
[6.1]

KegelanguB, SchirmanguB, RinganguB, BandanguB. Technical Information, 4.2.1, BASF,
Ludwigshafen/Rh., 1969.
[6.2] SpritzgieBen von Thermoplasten. Publication, Farbwerke Hoechst AG, Frankfurt/M.,
1971.
[6.3] Sowa, H.: Wirtschaftlicher fertigen durch verbesserte AnguBsysteme. Plastverarbeiter, 29
(1978), 11, pp. 587-590.
[6.4] Kohlhepp, K. G.; Mohnberg, J.: SpritzgieBen von Formteilen hoher Prazision, dargestellt
am Beispiel von Polyacetal. Kunststoff-Berater, 10 (1974), pp. 577-584.
[6.5] Crastin-Sortiment, Eigenschaften, Verarbeitung. Publication, Ciba-Geigy, Basel, August
1977.
[6.6] Christoffers, K. E.: Formteilgestaltung, verarbeitungsgerecht. Das SpritzguBteil. VDIVerlag, Dusseldorf, 1980.
[6.7] TunnelanguB, AbreiB-Punkt-AnguB. Technical Information, 4.2.3, BASF, Ludwigshafen/Rh., 1969.
[6.8] Thonemann, O. E.: AnguB- und Anschnitt-Technik fur die wirtschaftliche Herstellung von
SpritzguBteilen aus Makrolon. Plastverarbeiter, 14 (1963), 9, pp. 509-524.
[6.9] Kunststoffverarbeitung im Gesprach, 1: SpritzgieBen. Publication, BASF, Ludwigshafen/Rh., 1979.
[6.10] SpritzguB-Hostalen PP. Handbook. Farbwerke Hoechst AG, Frankfurt/M., 1980.

[6.11] Vorkammer-PunktanguB-Isolierverteiler. Technical Information, 4.2.4, BASF, Ludwigshafen/Rh., 1969.


[6.12] Durethan BK. Tech. Ringbuch der Farbenfabriken, Bayer AG, Leverkusen, 1967.
[6.13] Goldbach, H.: HeiBkanal-Werkzeuge fur die Verarbeitung technischer Thermoplaste (ABS,
PA, PBT, PC). Plastverarbeiter, 29 (1978), pp. 677-682, and 30 (1979), pp. 591-598.
[6.14] Gauler, K.: HeiBkanalsysteme mit Ventilanschnitten. Kunsstoffe, 87 (1997), 3, pp. 338-340.
[6.15] Homes, W.: KaskadenspritzgieBen vermeidet Bindenahte. Kunststoffe, 86 (1996), 9,
pp. 1269-1272.
[6.16] Husky: Husky-HeiBkanalsysteme. Werkschrift der Husky Injection Moulding Systems
Ltd., Bolton, Canada, D-62507 Wiesbaden-Igstadt, Phone: +49 611 950 850.
[6.17] Leidig, K.; Poppe, E. A.; Schinner, K.: Technische Kunststoffe: Die Top Ten der SpritzgieBprobleme. Schwierigkeiten mit HeiBkanalen (8), DuPont de Nemours GmbH,
D-61343, Bad Homburg.
[6.18] Eiden, G.: Werkzeuge fiir die Herstellung von Prazisionsteilen aus technischen
Thermoplasten. Reprint of 8th Tooling Conference at Wiirzburg: Der SpritzgieBformenbau
im internationalen Wettbewerb. September 24-25, 1997.
[6.19] Braun, P.: Innenbeheizte HeiBkanalsysteme. Kunststoffe, 87 (1997), 9, pp. 1184-1886.
[6.20] HeiBe Seiten. Plastverarbeiter, 48 (1997), 10, pp. 96-98.
[6.21] Braun, P.: Universelles HeiBkanalsystem. Osterr. Kunststoffzeitschrift, 28 (1997), 5/6,
pp. 98-102.
[6.22] Sander, W.: Homogene Schmelze-Temperaturverteilung mit Standard-Elementen. Plastverarbeiter, 42 (1991), 12, pp. 55-59.
[6.23] RoBbach, R.: Schmelzefilter fur den SpritzgieBer. Kunststoffe, 85 (1995), 2, pp. 193-195.
[6.24] Zimmermann, W.; Hack, K.: Einfaches HeiBkanal Schmelzeleitsystem fiir SpritzgieBwerkzeuge. Plastverarbeiter, 44 (1993), 4, pp. 18-22.
[6.25] Konstruktionszeichnung. H. Weidmann AG, Raperswil/Schweiz.
[6.26] HeiBkanalsystem - Indirekt beheitzter Warmeleittorpedo C.2.1. Technische Kunststoffe,
Berechnen - Gestalten - Anwenden. Publication, Hoechst AG, Frankfurt/M., October
1979.
[6.27] Mit Microelektronik Temperaturen genau regeln. Plastverarbeiter, 41 (1990), 5, pp. 78-80.
[6.28] Der HeiBkanal. Publication, Plastic Service GmbH, Mannheim.
[6.29] Catalog. Firma Hotset, Ludenscheid.

[6.30] Publication. Tiirk und Hillinger, Zuttlingen.
[6.31] Warmeverluste an HeiBkanalblocken - Reflektorbleche schaffen Abhilfe. Plastverarbeiter,
37 (1986), 4, pp. 122-123.
[6.32] Schauf, D.: AnguBloses Entformen von SpritzguBteilen. In: Das SpritzgieBwerkzeug. VDIVerlag, Dusseldorf, 1980.
[6.33] Krehwinkel, T.; Schneider, Ch.: Kaltkanaltechnik-pro und contra. In: SpritzgieBen und
Extrudieren von Elastomeren, VDI-Gesellschaft Kunststofftechnik, VDI-Verlag, Dusseldorf, 1996.
[6.34] Cottancin, G.: Gummispritzformen mit dem Kaltkanalverfahren. Kautschuk Gummi
Kunststoffe, 33 (1980), 10, pp. 839-841.
[6.35] Kaltkanaltechnologie beim ElastomerspritzgieBen. Publication, Klockner Ferromatic
Desma, Werk Achim, 1983.
[6.36] Schneider, Ch.: Das Verarbeitungsverhalten von Elastomeren im SpritzgieBprozeB.
Dissertation, RWTH, Aachen, 1987.
[6.37] MaaB, R.: Die Anwendung der statistischen Versuchsmethodik zur Auslegung von
SpritzgieBwerkzeugen mit Kaltkanalsystem. Dissertation, RWTH, Aachen, 1995.
[6.38] Cottancin, G.: Gummispritzformen fiir das Kaltkanalverfahren. Gummi, Asbest, Kunststoffe, 33 (1980), 9, pp. 624-633.
[6.39] Bode, M.: SpritzgieBen von Gummiformteilen. In: SpritzgieBen von Gummiformteilen.
VDI-Verlag, Dusseldorf, 1988, pp. 1-33.
[6.40] Cottancin, G.: Gummispritzpressen mit dem Kaltkanalverfahren. Gummi, Asbest, Kunststoffe, 33 (1980), 9, pp. 624-633.
[6.41] Lommel, H.: Einfliisse der ProzeBgroBenverlaufe un der Werkzeuggestaltung auf die
Qualitat von Elastomerformteilen. Unpublished report, IKV, 1984.
[6.42] Holm, D.: Aufbau von Werkzeugen fiir SpritzgieBmaschinen. In: SpritzgieBen von
Elastomeren. VDI-Verlag, Dusseldorf, 1978.
[6.43] Benfer, W.: Rechnergestiitzte Auslegung von SpritzgieBwerkzeugen fiir Elastomere.
Dissertation, RWTH Aachen, 1985.


[6.44] Menges, G.; Barth, R: Erarbeitung systematischer Konstruktionshilfen zur Auslegung von
Kaltkanalwerkzeugen. AIF-AbschluBbericht, IKV, Aachen, 1987.
[6.45] Kloubert, T.: Reduzierung von ausgehartetem AnguBmaterial beim SpritzgieBen von
rieselfahigen 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 Verarbeitung von Duroplasten auf SpritzgieBmaschinen in
Kaltkanalwerkzeugen. Plastverarbeiter, 31 (1980), 8, pp. 467^69.
[6.48] Braun, U.; Danne, W.; Schonthaler, W.: AnguBloses Spritzpragen in der Duroplastverarbeitung. Kunststoffe, 77 (1987), 1, pp. 27-29.
[6.49] Hotz, A.: Mehr-Etagen-SpritzgieBwerkzeuge. Plastverarbeiter, 29 (1978), 4, pp. 185-188.
[6.50] Hartmann, W.: SpritzgieBwerkzeuge in Etagenbauweise mit "Thermoplay"HeiBkanaldiisen fur Verpackungs-Unterteile aus Polystyrol. Plastverarbeiter, 32 (1981), 5,
pp. 600-605.
[6.51] Moslo, E. R: Runnerless-Moulding. SPE-Journal, 11 (1955), pp. 26-36.
[6.521 Moslo, E. R: Runnerless-Moulding. New York, 1960.
[6.531 Lindner E.; Hartmann, W.: SpritzgieBwerkzeuge in Etagenbauweise. Plastverarbeiter, 28
(1977), 7, pp. 351-353.
[6.54] Schwaninger, W.: Etagenwerkzeuge insbesondere als Alternative zum Schnellaufer. Der
SpritzgieBprozeB, Ingenieurwissen, VDI-Verlag, Dusseldorf, 1979.
[6.55] Publication, 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-SpritzgieBteilen. Mehr Farben - Mehr Materialien - Mehr Komponenten - SpritzgieBtechnik, SKZ,
Wurzburg, May 1992, pp. 83-102.
[6.58] Eckardt, H.: Verarbeitung von TPE auf SpritzgieBmaschinen und Moglichkeiten der
Anwendung des Mehrkomponenten-SpritzgieBens. Thermoplastische Elastomere im
Aufwartstrend, SKZ, Wurzburg, October 1989, pp. 95-123.
[6.59] Kraft, H.: "Erweiterte Anwendungsbereiche fur das Mehrkomponenten-SpritzgieBen". In:
Kunststoffe, 83 (1993), 6, pp. 429-433.
[6.60] Bodeving, C : "Horizontalmaschine mit Drehtisch". In: Kunststoffe, 85 (1995), 9,
pp. 1244-1254.
[6.61] "Beweglich aus dem SpritzgieBwerkzeug". In: Plastverarbeiter, 36 (1985), 4.
[6.62] "Vollbewegliche Lufterklappen bei jedem Spritzzyklus". In: Plastverarbeiter, 44 (1993), 2.
[6.63] Netstal News. No. 31, Netstal-Maschinen AG, Nafels/Schweiz, April 1997.

[6.64] "HeiBkanaltechnik beim Mehrkomponenten-SpritzgieBen". In: Plastverarbeiter, 45 (1994),
10.
[6.65] Krauss, R.: SpritzgieBtechnik fur die Mehrfarbentechnik. In: Konstruieren von SpritzgieBwerkzeugen, VDI-Verlag, Dusseldorf, 1987, pp. 6.1-6.18.
[6.66] Mehr-Rohstoff-Technologie. Publication. Battenfeld, Meinerzhagen, September 1996.
[6.67] "Combimelt-Technologie fur Mehrfarben und MehrkomponentenspritzguB". In: KGK
Kautschuk, Gummi, Kunststoffe, 49 (1996), 7-8.
[6.68] Johannaber E; Konejung, K.: Mehrkomponenten-Technik beim SpritzgieBen. Mehr
Farben - Mehr Materialien - Mehr Komponenten - SpritzgieBtechnik, SKZ, Wurzburg,
May 1992, pp. 7-20.