Extrusion head

An extrusion head with an annular extrusion die gap or orifice for fabricating a hollow extrudate of thermoplastic material. The extrusion head has a central die core and outer die casing ring cooperating to define the die gap. The inner boundary of the die gap is formed by a wall of the die core, while the outer boundary of the die gap is formed by a wall of the casing ring. At least one boundary wall of the die gap comprises independently movable sections that move with respect to the other boundary wall to change the width of the die gap.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an extrusion head having a central die core and 
outer die casing ring cooperating to define an annular exit gap or orifice 
for fabricating hollow extrudates. The cross-sectional dimension of the 
gap is uniformly changeable by axial displacement of the die core and/or 
the die casing ring relative to each other; and one or both of the die 
members is segmented with each segment adjustable to vary the 
cross-sectional dimension of the gap at prescribed points 
circumferentially about the gap. 
2. Description of the Prior Art 
Extrusion heads have been developed where the die gap of the exit die is 
changed in the circumferential direction at prescribed points to attain a 
specified distribution of wall thickness in the hollow extrudate. This is 
desirable, for example, when blow molding a preform of hollow extrudate 
between the segments of a blow mold cavity. There, when the extrudate is 
clamped by the mold segments, the extrudate will traverse different 
stretch paths in the area of the mold clamping points than in the 
remaining portions of the mold. When an extrudate of uniform wall 
thickness, the walls of the finished blown product will have variable 
thickness in relation to the extent to which the extrudate stretches at 
different locations within the mold. 
By means of a specific distribution of wall thickness in the extruded 
preform, it has been possible to balance out weak or thin areas that would 
otherwise be created during the blow molding process. This has been 
accomplished by extruding the preform with a wall thickness of different 
dimensions in such a manner that the wall is thicker in areas of longer 
stretch paths and thinner in areas of shorter stretch paths. Thus, after 
the blow molding process has been completed, that is when the stretch of 
the material has terminated, the hollow body that is formed will have 
walls of equal thickness. 
In one prior method, a flexible die casing ring has been placed adjacent 
the exit of the annular die, as disclosed in DT-OS No. 26 54 001. In order 
the change the annular gap of the die at prescribed points, this ring is 
deformed in the radial direction within its elastic range through the 
action of tension and pressure screws. According to another method 
disclosed in DT-PS No. 1 161 412, the die casing itself is elastically 
deformable in the radial direction at prescribed points. In order to avoid 
joints between the deformed die ring and the recesses of the housing, in 
which the ring is guided, a ring lining is here inserted between the ring 
and the hollow extrudate as disclosed in DT-OS No. 26 10 668. 
Prior attempts at a solution require the application of considerable 
deformation forces to the casing die ring. The wall thickness of the ring 
must be sufficient to withstand the high mechanical and thermal die 
stresses created during the extrusion processes. Furthermore, such a 
deformable ring must be capable of springing back into its original 
circular shape when it is relieved from deformation forces. Consequently, 
for these reasons, the wall thickness of the die ring may not fall below a 
certain value. 
A further disadvantage of prior ring constructions relates to possible 
warping. When a closed ring is deformed, the circumferential length of the 
ring remains constant. Pressing in at one point of the ring can simply 
cause an undesirable bulging out at another point, so that there is merely 
a change of shape of the ring rather than a controllable change in 
diameter. 
The larger the volume of the container that is being fabricated by the blow 
molding process, the more material must be utilized. Consequently, there 
will be a larger aggregation of excess material at the critical points 
behind the mold separation seams formed by the clamped segments of the 
mold. For this reason, in order to constrict the die gap to form thin 
extrudate at the selected points, the associated ring areas must traverse 
comparatively large deformation movement at these same points. The heavier 
the design of the ring cross-section, the less is its deformation 
capability, so that the deformation that can finally be achieved is not 
sufficient to remove sufficient material at the desired extrudate points. 
Under extreme operating conditions, the ring can experience permanent 
material fatigue, which will cause the ring to break, or permanently 
deform, which renders the ring useless. With the prior art constructions, 
it is therefore only possible to provide for a few force application 
points for deforming the ring over its circumference. 
In the prior art techniques for changing the die gap in the circumferential 
direction at prescribed points, the mode of behavior of the plastic in the 
flowable state has been a decisive factor. The assumption has been that 
smooth continuous extruding surfaces of the die gap were needed to create 
smooth thickness changes in the wall thickness of the extruded preform 
material. Consequently, the prior techniques provided for smooth 
transitions in the local displacement regions of the gap. The most 
favorable smooth transitions were produced by radial deformation of the 
die casing ring in the elastic range of the ring material. Therefore, the 
above-described disadvantages of limited constriction of the die gap and a 
limited number of constriction points about the die gap periphery had to 
be accepted. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, smooth 
transitions in the extruded preform can be created without requiring a 
single deformable die casing ring or similar structure for providing the 
extruding surfaces of the die core or die ring with smooth transition 
surfaces. More particularly, at least one of the boundary walls of the die 
core and die casing ring that define the die gap comprises abutting 
movable sections disposed in side by side relation around the gap 
periphery. In one embodiment, the die core is solid while the die casing 
ring is made up of the movable sections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The die core 1 is formed by the mandrel of the extrusion head 1A. The 
mandrel moves in the axial direction. The casing ring 2 of the extrusion 
head also moves in the axial direction and is attached to the exit end 2A 
of the extrusion head housing 3. As is conventional, the die core 1 is 
shown as having a lower end with a tapered surface extending radially 
outwardly as measured in the axial direction along which material is to be 
extruded. Thus, relative axial displacement of the die core and casing 
ring will change the thickness of the extrudate being formed. 
In accordance with the teachings of the present invention, the die casing 
ring is constructed with separate sections 4 mounted about the inner 
periphery of the casing ring 2. Each section is L-shaped with one leg 
extending in an axial direction and having a wall surface 4A facing the 
inner die core 1. The other leg extends in a radially outward direction 
and defines the lower end of the die casing ring. The legs are joined at 
the edge 8 which forms the lower boundary wall of the casing ring 2. A 
guide ring 5 is fastened in the housing 3 of the extrusion head 1A and 
partially covers the axially extending legs of the sections 4. The guide 
ring 5 has a radially inwardly facing wall that contacts the extrusion 
material and provides a smooth inner surface for the die casing ring in 
the adjusted positions of the sections 4 as will be more fully evident 
from the following description. 
Each of the die ring sections 4 is axially adjustable by means of 
positioning screws 6 so that the lower outer boundary edge of the radially 
opposed portion of the die gap can be changed accordingly. Specifically, 
the die gap opening at each die section around the gap will depend on the 
height position to which the lower edge 8 of each section is set in 
relation to the opposed wall surface of the die core 1. The height 
position of the sections 4 are set individually or in groups. Accordingly, 
the extruded tube material, shown at 7, will have walls of variable 
thickness at prescribed points and in accordance with the adjustment of 
each section 4. The thickness of the extrudated tube material will thus 
increase or decrease as a result of changes of the tube's outer 
circumference, while the internal contour of the tube will remain the 
same. The inner circumference and the overall thickness of the tube is, on 
the other hand, changed symmetrically by changing the axial or height 
adjustment of the solid die core 1 relative to the casing ring 2. 
FIG. 1 shows various axial or height adjustments a, b, c, d, and e of the 
individual sections 4 and various height displacements of the casing ring 
2. The corresponding thicknesses of the tubular preform material 7 are 
also shown. The sections 4 are adjusted to set the circumferential 
thickness distribution of the tubular preform material 7 as required for 
each blow molding process. 
The guide ring 5 covers the upper ends of all of the sections 4 to form a 
smooth bridge between them and the axially adjacent wall of the die head. 
Accordingly, axial adjustment of the sections can be made without changing 
the wall surface along which the extrudate material moves except at the 
lower edges 8 of the sections where the die gap is to be controlled. The 
fact that only the lower end edges 8 of the sections 4 establish the die 
gap results in the shortest holding time for the thermoplastic material 
during the critical velocity range in the die gap. This is so because the 
paths over which the material experiences the highest flow speeds are kept 
short. 
Instead of deforming the material of one of the die sections, the device 
according to the invention moves individual sections or groups of sections 
in order to locally change the dimension of the die gap. As a result, 
comparatively lesser displacement forces are needed to move the sections 
compared to the high displacement forces necessary to deflect the prior 
art type of casing rings. Thus, the sections may be moved over greater 
displacement ranges. They can also be moved in an arbitrary fashion to 
produce corresponding arbitrary changes in the die gap width and frequency 
of change of gap width about the circumference of the gap. Although the 
transitions between the lower edges of the sections are discontinuous in 
accord with their height adjustment, no discontinuities occur on the 
surface of the hollow extrudate material. Instead, the transitions between 
the different thicknesses in the extrudate material are smooth and 
gradual. 
Hydraulic devices, (not shown), change the axial height adjustment of the 
casing ring 2, as preprogrammed during the tube extrusion process. This 
produces a corresponding change in the axial setting of the lower edges 8 
of all the sections 4 with respect to the outer surface of die core 1. 
In accordance with the teachings of the present invention, it is to be 
understood that instead of having the die casing ring segmented into 
sections, the outer surface of the die core can be formed of individual 
sections which are adjustable with respect to a solid casing ring. Also, 
the die core and the die casing ring can both be formed of adjustable 
individual sections.