Plasticizing unit for an injection molding machine

A plasticizing unit for an injection molding machine includes a plasticizing cylinder; a plurality of separately energizable resistance heaters arranged on the plasticizing cylinder; clamping sleeves surrounding the resistance heaters and oriented generally coaxially with the plasticizing cylinder; and clamping elements tightening the clamping sleeves for pressing the resistance heaters against the outer face of the plasticizing cylinder. There is provided a protective casing surrounding the plasticizing cylinder and being attached thereto. Each resistance heater comprises a flat-tube heater body including consecutive first and second length portions. The first length portion contains heating wires and has a helical course about the outer face of the plasticizing cylinder. The second length portion forms an end of the flat-tube heater body and is free from heating wires and contains the lead wires. The second length portion is bent out of the helical course of the first length portion and is accommodate in a space defined between the outer face of said plasticizing cylinder and the protective casing. A terminal panel is secured to a rear terminus of the protective casing and is oriented generally perpendicularly to the longitudinal axis of the plasticizing cylinder. Supply cables within the protective casing are connected to the end portions of the heater bodies and to terminal plugs which are supported in the terminal panel for displacements of limited extent in a direction parallel to the longitudinal axis of the plasticizing cylinder.

BACKGROUND OF THE INVENTION 
This invention relates to an injection molding machine for processing 
synthetic materials and is of the type which includes an injection molding 
unit equipped with a changing device for replacing a plasticizing unit 
forming part of the injection molding unit. 
The plasticizing unit--to which the invention is more particularly 
directed--includes a plasticizing cylinder which has a plurality of 
independently controllable resistance heaters whose heating wires are 
supported by ceramic insulating bodies. The resistance heaters are clamped 
on the outer cylindrical face of the plasticizing cylinder by means of 
axially split tightening sleeves pressed together by tangentially clamping 
tightening elements. The plasticizing unit further has handle components 
for being grasped by conveying means which move the plasticizing unit for 
effecting replacement. There is further provided a stand on which the 
plasticizing unit may be set in a stable equilibrium after being separated 
from the injection molding unit. 
A known, automatically replaceable plasticizing unit of the above-outlined 
type equipped with the usual ceramic heater bodies includes a coupling 
block which is fixedly connected with the plasticizing cylinder and which 
includes the plastic material supply channel. The coupling block serves, 
among others, as the stand for the uncoupled plasticizing unit, as 
disclosed in European Patent No. 69,221 and German Offenlegungsschrift 
(non-examined published application) No. 3,229,223. The ceramic heater 
bodies are of the type disclosed in related brochures by the firms Erge 
Elektrowarme-technik Franz Messer, D-8563 Schnaltach, Hersbruckerstrasse 
31 and Ihne & Tesch, D-5880 Ludenscheid, Am Drostenstuck 18, both of the 
Federal Republic of Germany. 
Further, flat-tube heating bodies are known which find application mostly 
in industrial electric heating and apparatus construction for heating air 
and liquid and which are also adapted to perform contact heating. The 
initially linear flat-tube heater bodies are formed of a cross-sectionally 
flat tube of chromium nickel steel and heating wires which are embedded 
within the flat tube in a highly densified magnesium oxide or magnesium 
silicate material. Such flat-tube heater bodies which are flexible to 
allow easy bending thereof for adapting them to the specific 
configurational requirements, may be of the type disclosed in related 
brochures by the firms Eltra GmbH and Co. KG Leicht & Trambauer 
Elektrowarmetechnik, P.O.B. 1120, D-6102 Pfungstadt as well as Turk & 
Hillinger GmbH Elektrowarme, D-7200 Tuttlingen, Friedrichstrasse 21, both 
of the Federal Republic of Germany. Although such flat-tube heaters have 
been known and widely used in a great number of environments for decades, 
including the heating of injection nozzles in injection molding machines 
(as disclosed in German Utility Model No. 7,345,326), they could not be 
used in plasticizing cylinders of injection molding machines, one reason 
having been the problem of protecting the heater wire-free end portions of 
the heater coils and the connected supply conduits of the plasticizing 
cylinder against mechanical shocks and impacts externally of the heated 
zone. Such problems are not encountered in the heating of the less exposed 
injection nozzles, since their diameter is less than that of the 
associated plasticizing cylinders. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved plasticizing unit 
of the above-outlined type in which the handling of the plasticizing unit 
during replacement is facilitated while maintaining a superior energy 
efficiency and effecting a rapid temperature regulation. 
This object and others to become apparent as the specification progresses, 
are accomplished by the invention, according to which, briefly stated, as 
resistance heaters for the plasticizing cylinder flat-tube resistance 
heater bodies--for example, of the type identified earlier--are used which 
are bent to form heater coils and which are exposed to the centripetal 
pressure of the clamping sleeves surrounding the plasticizing cylinder. 
Further, end portions of the flat-tube heater bodies which are free from 
heating wires, but which include electric lead wires, are bent out of the 
helical course of the heater coils and are shrouded by a protective member 
spaced from the clamping sleeves. The protective member which, according 
to a preferred embodiment, is a cross-sectionally U-shaped casing, also 
serves as a stand for supporting the plasticizing cylinder when removed 
from the injection molding unit. A terminal panel is secured to a rear 
terminus of the protective casing and is oriented generally 
perpendicularly to the longitudinal axis of the plasticizing cylinder. 
Supply cables within the protective casing are connected to the end 
portions of the heater bodies and to terminal plugs which are supported in 
the terminal panel for displacements of limited extent in a direction 
parallel to the longitudinal axis of the plasticizing cylinder. 
For several reasons, a plasticizing unit structured according to the 
invention as outlined above may be handled easier during the exchange 
operation, while using a type of rapidly regulating resistance heater 
which could not be used in plasticizing cylinders heretofore and which is 
able to generate very high plasticizing temperatures. Thus, one reason 
being that the directly connectable plasticizing cylinder is freed from 
the coupling block which results in a significant weight reduction of the 
replaceable unit. Further, the protective member for the supply terminals 
may simultaneously serve, by virtue of its particular configuration, as a 
stand for the plasticizing cylinder after its release from the 
plasticizing unit, and also function as a holding arrangement engageable 
by the conveying device which executes the exchange of the plasticizing 
unit. 
The protective member thus protects a space where the bent-out end portions 
of the heater coils free-floatingly hold the cable carrying the lead wires 
for the heating wires of the heater coils. It is an advantage of this 
arrangement that the cables may, with the flexible lead wires, follow the 
temperature-caused expansions and contractions of the heater coils without 
resistance. 
The use of flat-tube heater bodies favors a rational mass production 
because the flat-tube heater bodies may be wound into coils of different 
inner diameters without appreciable additional technological input to 
adapt them to plasticizing cylinders of different diameters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to FIG. 1, the plasticizing unit shown therein is designed for 
an injection molding machine which has an injection molding unit equipped 
with an exchanging device for replacing the plasticizing unit. 
The plasticizing cylinder 10 of the plasticizing unit is provided with a 
plurality of independently controllable resistance heaters. The resistance 
heaters are flat-tube heater bodies bent to form heater coils 11 which are 
held on the outer face 10a of the plasticizing cylinder 10 by clamping 
sleeves exerting a centripetal pressure thereon. Each clamping sleeve is 
axially split and is thus constituted by a pair of identical shells 11c 
which are tightened to the cylindrical surface 10a of the plasticizing 
cylinder 10 by diametrically oppositely arranged and tangentially acting 
clamping bolts 26. For this purpose, as particularly well seen in FIGS. 5, 
each shell 11c has opposite longitudinal edges 11c' which, as viewed in 
cross section, are curved in a direction which is opposite to the 
direction of shell curvature. The edges 11c' belonging to different shells 
11c thus form pair-wise back-to-back oriented, coextensive trough-like 
structures. Each trough-like longitudinal edge 11c' receives a cylindrical 
support pin 25 of corresponding radius. The pins 25 are traversed by and 
are threadedly in engagement with the clamping bolts 26. Heater wire-free 
end portions 11e of the heater coils 11 are, together with supply cables 
15 containing lead wires 12 bent out of the curved course of the heater 
coil 11 and are screened by a U-shaped casing 13 which extends along a 
substantial length portion of the plasticizing cylinder 10 and which is 
spaced from the clamping shells 11c. The underside 13a of the casing 13 
serves as a setting face on which the plasticizing unit may rest in a 
stable equilibrium after it has been disconnected from the injection 
molding unit. 
With particular reference to FIGS. 1 and 2, the plasticizing cylinder 10 is 
connected to the casing 13 by means of horizontal webs 14 which are held 
in the vertical walls of the casing 13. U-shaped clamping yokes 14a cradle 
the plasticizing cylinder 10 and are held in the horizontal webs 14 by 
nuts 14b threaded on the ends of the yoke legs. 
With particular reference to FIGS. 2, 3, 8 and 9, the end portions 11e of 
the heater bodies (coils) 11 have a vertical orientation and are connected 
to the coiled part of the respective body 11 by a curved connecting 
portion 11f which has a course--from a horizontal plane b--b containing 
the cylinder axis a--a--that is directed oppositely to the course of the 
heater coils 11. The lead wires 12 are held in the cable 15 which is 
supported at the end of the terminal portion 11e in a freely floating 
manner. As a result, each end portion 11e and the cable 15 are capable of 
following without resistance the heat-caused expansions or contractions of 
the heating coils 11, while being situated in a space protected by the 
casing 13. As seen in FIGS. 1 and 3, the protected space is closed at the 
front by a vertical panel 22 secured to the casing 13. 
The U-shaped casing 13 is, by means of a top plate 13c, complemented into 
an axially extending tube of rectangular cross section which projects 
laterally to both sides of the center of gravity of the plasticizing unit. 
The supply cables 15 of the heater coils 11 extend within the rectangular 
tube to a vertical terminal panel 16 held at the rearward end of the 
plasticizing cylinder 10. Also referring to FIG. 1a, the plug terminals 17 
of the supply cables 15 are axially displaceably supported for limited 
shifting motions on horizontal support pins 18 of the terminal panel 16 by 
means of centering shoulders 17'. The pins 18, fastened on the terminal 
panel 16, penetrate the centering shoulders 17'. A coil spring 19 
surrounds each pin 18 and abuts on the one side on the shoulder 17' and on 
the other side on a head of the surrounded pin 18. Therefore centering 
shoulder 17' of the plug terminals 17 are pressed against the terminal 
panel 16. This arrangement ensures that jars and shocks, particularly the 
rhythmic axial shocks imparted on the plasticizing unit in the course of 
the periodic injection of the plastic material into the injection mold 
assembly have, even in the long run, no harmful effect on the contact 
terminals of the supply cables 15. 
To the vertical walls of the casing 13 radially symmetrical pins 13d are 
secured which serve as holding members for engagement by the conveying 
mechanism for the plasticizing unit. 
As seen particularly in FIG. 1, in each separate heating coil 11 the 
respective electric lead wires 12 are arranged only at one axial end 
thereof and extend in the heater wire-free end portion 11e of the 
respective heater coil 11. The heater coils 11 are arranged pairwise on 
the plasticizing cylinder 10 in a mirror image to one another in such a 
manner that the end portions 11e of adjacent heater coils 11 forming one 
pair are oriented towards one another. By virtue of this arrangement the 
terminal portions 11e of adjoining heater coils 11 may be bent out from 
their respective heater coils 11 at a common axial length portion of the 
plasticizing cylinder 10 which is not occupied by the clamping shells 11c. 
In this manner, the end portions 11e are arranged in pairs for attachment 
to the associated supply cable 15. 
With particular reference to FIGS. 5-8, the flat outer tube forming each 
heater coil 11 is of a chromium-nickel steel and is filled with a powdery 
ceramic mass, preferably magnesium oxide or magnesium silicate in which 
the spirally wound heater wire 11a is embedded. As may be observed 
particularly in FIGS. 1, 4 and 5, the inner diameter of the heater coil 11 
corresponds to the outer diameter of the plasticizing cylinder 10. The 
wall of the bilaterally symmetrical flat tube 11d of each heater coil 11 
is deformed by planar pressing such that its side oriented towards the 
axis a--a of the plasticizing cylinder 10 constitutes a contacting wide 
side 11d' which constitutes approximately one-third of the entire outer 
surface of the flat tube. The contacting wide side 11d' is pressed against 
the outer face 10a of the plasticizing cylinder 10 by the centripetal 
pressure exerted by the clamping shells 11c. The inner cylindrical face of 
the clamping shells 11c lies against the outer flattened side 11d" of the 
flat tube 11d which extends parallel to the contacting wide side 11d'. As 
seen in FIG. 4, the peripheral end portions 11c" of the longitudinal ends 
of the clamping shells 11c are radially inwardly bent toward the surface 
of the plasticizing cylinder 10 and extend, with their terminal edges 
11c'", at a uniform distance therefrom. As it may be particularly well 
seen in FIGS. 1 and 8, each heater coil has, in its mid portion, a greater 
pitch than at the flanking ends. In the axial section of the winding of 
greater pitch the plasticizing cylinder 10 has a radial bore 23 for 
receiving a heat sensor 29 whose conductor 30 joins the respective cables 
15. 
By virtue of the relatively large contact faces of the heater coils 11, 
formed by the wide sides 11d', there is achieved an improved heat 
conduction and, accordingly, a rapid heat regulation by the heater coils 
11 can be achieved with relatively small energy losses. 
The present disclosure relates to subject matter contained in Federal 
Republic of Germany Patent Application No. P 35 46 415.1 (filed Dec. 31, 
1985) which is incorporated herein by reference. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meanings and range of 
equivalents of the appended claims.