Method and apparatus for blow molding curved hollow bodies made of plastics

During blow molding of curved plastic hollow bodies, the parison is first extruded as a whole in a substantially straight line and vertically from top to bottom. The parison is then severed from the extrusion die and the severed parison precurved. Finally, the precurved parison is inserted as a whole into the blow mold and blown. An apparatus for carrying out this method comprises releasable support means for accommodating and holding the parison, curving means for preliminary curving of the parison, tilting means for tilting the support means into the horizontal position and a blow mold, the precurved parison being placed as a whole in one mold half of this blow mold.

The invention relates to a method as defined in the preamble to patent 
claim 1. In addition, the invention relates to an apparatus as defined in 
the preamble to patent claim 2. 
In a known method and known apparatus of this type (JP-PA No. 53-121062), 
the extruded parison which is still connected to the extruder die is 
gripped at its lower, free end by a support pin, is curved or bent into a 
U shape by displacement of the pin and secured to the inner face of one 
section of the mold. In this way, it is possible to curve the parison only 
in one plane but not in three dimensions. 
In a further known method for blow molding curved hollow bodies made of 
plastics (DE-PS No. 29 27 098), the parison is, during its extrusion from 
the die, immediately inserted into a two-dimensionally or 
three-dimensionally curved mold cavity in a lower mold section. For this 
purpose, either the mold section or the die have to be moved accordingly. 
This has the disadvantage that large masses have to be displaced, i.e. in 
one case the lower mold section and in the other case the entire extruder 
which mounts the die. Moreover, the lower part of the parison which is 
inserted into the lower mold section is cooled to a greater extent than 
its free, upper section. This means that an unevenly heated parison has to 
be blown once the upper mold section has been mounted in place and this 
can lead to hollow bodies which are not completely blown and have 
differences in their wall thicknesses. 
The object of the invention is to remedy these shortcomings and propose a 
method of the type in question, with which the parison can be curved or 
bent three-dimensionally, no large masses have to be displaced and the 
parison does not experience any substantially varying degrees of cooling 
during the curving process. 
This object is accomplished in accordance with the invention by the 
characterizing features of patent claim 1. 
An apparatus for carrying out the inventive method is the subject matter of 
patent claim 2. Features of preferred embodiments of this apparatus are 
specified in subclaims 3 to 6.

FIG. 1 is a schematic illustration of an extruder 1, which is arranged on a 
machine frame 3 together with a blow mold 2. Plastic material, for example 
polyethylene or polypropylene, introduced into a supply funnel 4 is melted 
in the known manner in the extruder and conveyed to a die 5 comprising an 
interior mandrel via a transport means which is not illustrated. The 
plastics material exits vertically from the die from top to bottom in the 
form of a tubular parison 6 having a predetermined length. Once the 
parison 6 has the desired extrusion length it is separated from the die 5 
at its upper end. This state is illustrated in FIG. 1. 
The tubular parison is extruded from the die 5 along a support means 7, the 
front view of which is illustrated in FIG. 2 as seen in the direction of 
arrows 2 in FIG. 1. The support means 7 comprises, in the illustrated 
embodiment, a strip consisting of two articulatedly connected and 
substantially straight members 8, 9, the cross-sectional form of which is 
shown in FIG. 4. Both strip members 8, 9 have a cavity 11, which extends 
longitudinally to the axis of the strip and is closed at both its upper 
and lower ends. The cavity can be connected via nipples 12, 13 to a 
reduced pressure or vacuum source, which is not illustrated. Bores 14 lead 
from the cavity 11 to the front face of the strip members 8, 9. 
The connection between the two members 8, 9 is such that it is possible for 
one member to pivot relative to the other in the plane of drawing of FIG. 
2, as indicated by dash-dot lines. 
One strip member 8 is connected with the shaft 15 of an electric motor 16 
which is itself supported on a table 17 of the machine frame 3. With the 
aid of the motor 16 the strip-like support means 7 can be swiveled out of 
the vertical position into a horizontal position indicated in FIG. 1 by 
dash-dot lines and swiveled back again, in the directions indicated in 
FIG. 1 by the double-headed arrow A. 
The two strip members 8, 9 swivel relative to one another by means of 
known, for example pneumatic, means which are not illustrated. 
As illustrated, the parison 6 is extruded from the die 5 along the support 
means 7 in the immediate vicinity of the outer openings in the bores 14 in 
which a vacuum effect may be generated When the parison 6 is extruded to 
the desired length along the support means 7, a reduced pressure or vacuum 
is generated in the cavity 11 in the strip members 8, 9 so that the 
tubular parison 6 is sucked in and held on the support means 7. 
Subsequently, the upper end of the parison 6 is separated from the die 5 by 
cutting means 18 which are known per se and indicated in FIGS. 3 and 5. 
The parison 6 is then bent in the manner illustrated in FIG. 5 by swiveling 
the lower strip member 9. 
Subsequently, the motor 16 is actuated to swivel the support means 7, 
together with the parison 6 held thereon, through 90.degree. into the 
horizontal in the direction of arrow B (FIG. 6). 
As shown in FIGS. 1 and 6, the lower mold half 21 of a blow mold which 
comprises a mold recess 20 is arranged below the die 5 and the support 
means. The mold recess 20 is shaped to correspond to the form of the 
precurved parison 6 (FIG. 5). 
By switching off the vacuum in the support means 7, the precurved parison 6 
is allowed to drop into the lower mold half 21. Subsequently, cf. FIG. 8, 
the lower mold half 21 is brought together with an upper mold half 22 
which has a corresponding mold recess 20. Once the upper mold half 22 has 
been placed in position on the lower mold half 21, the parison 6 is blown 
in a manner known per se to the desired shape of the hollow body. 
As illustrated in FIG. 1, the upper mold half 22 is displaceable upwards 
and downwards by means of a piston-cylinder unit 23 mounted on the machine 
frame 3. The lower mold half 21, which is first of all located beneath the 
support means 7 (illustrated in dash-dot lines in FIG. 1), is then moved 
with the parison 6 inserted therein so that it is below the upper mold 
half 22. The upper mold half 22 is then lowered onto the lower mold half 
and the parison 6 blown. 
As illustrated and described, only the lower strip member 9 of the support 
means 7 has to be swiveled to precurve the parison 6 (FIG. 5). A small, 
e.g. pneumatic, low-powered drive motor would be adequate for this 
purpose. In contrast to the state of the art, it is not necessary to move 
any heavy masses using considerable power resources. Since, in addition, 
the areas of contact between the parison 6 and the support means 7 (cf. 
FIG. 4) may be designed so that they are relatively small, the parison 6 
loses, locally, very little heat to the support means 7 while it is held 
thereon and so a parison 6 which has an essentially uniform temperature is 
inserted into the blow mold 21, 22 and then blown with a uniform wall 
thickness. 
In the illustrated embodiment, the parison 6 is bent only once (in the 
plane of drawing of FIGS. 2, 3 and 5) by swiveling the strip member 9. To 
achieve a compound curvature the support means 7 must consist of a 
plurality of articulatedly connected strip members. It is then possible to 
bend the parison 6 not only in one plane, i.e. two-dimensionally, but also 
in three dimensions. For this purpose, the joint axis of the relevant 
strip member must extend at right angles to the joint axis between the 
strip members 8 and 9 which is illustrated in FIG. 2. This makes it 
possible to bend the parison 6 out of the plane of drawing of FIGS. 2, 3 
and 5. The parison 6 which is curved three-dimensionally is then inserted 
into the mold halves 21, 22, the mold recesses 20 of which are curved 
three-dimensionally in a corresponding manner. In this way, relatively 
complicated shapes can be produced for extrusion blow-molded hollow 
bodies. 
FIGS. 9, 10 and 11 show a different embodiment of a means for supporting 
and precurving freshly extruded, hot parisons. 
A support chain 27 is arranged beneath the die 5 and the cutting means 18 
so as to be parallel to the direction of extrusion, i.e. vertical. The 
members or links of this chain are formed by blocks 28 which are 
articulatedly connected with one another. As shown in FIG. 10, 10 blocks 
28 may, for example, be provided. Only 4 of these blocks are visible in 
the cut-off illustration of FIG. 9. Each block 28 has two jaws 31, 32 for 
clamping a parison 6 (FIG. 11). These jaws are pivotally mounted and 
actuatable by means of a power drive, which is not illustrated and can, 
for example, be electromagnetic or pneumatic. Pins 33 protrude from the 
blocks 28 and these pins bear guiding heads 34 on their free ends. In 
order to produce an articulated link connection, connecting bars 35 extend 
in two superposed layers between the guiding heads 34. These bars are 
articulatedly connected to one another via bolts 37 and to the heads 34 
via threaded pins 36. 
A guide means 38 in the form of a curved profiled tube 39 having a C-shaped 
cross section is associated with the support chain 27. The interior cross 
section of the profiled tube 39 is dimensioned such that the tube can 
accommodate and guide the heads 34 of the blocks 28 with their associated 
link connections, as illustrated in FIGS. 10 and 11, the blocks 28 with 
the jaws 31, 32 hereby being guided on the outer side of the tube 39 in 
accordance with its curvature. 
In addition, a power-actuated slide means 41 with a driver arm 42 is 
provided. The arm 42 may be moved upwards and downwards in the direction 
of arrow C (FIG. 9) or from side to side in the direction of arrow D. 
When moved to the right, the arm 42 is brought into engagement with the 
upper block 28 of the support chain 27. When the arm is displaced in the 
direction of arrow C, the support chain 27 bearing the freshly extruded 
parison 6 is moved into the guide means 38. The parison 6 carried by the 
jaws 31, 32 of the blocks 28 (FIG. 11) is then bent or curved to the shape 
of the guide means 38, for example in the shape of a "U". 
The guide tube 39 is connected via a shaft 43 with a drive motor, such as 
the electric motor 16 in FIG. 2. This means that the tube can be turned 
from the vertical position shown in FIG. 9 into the horizontal position 
shown in FIG. 11. 
Once the support chain 27 bearing the parison 6 has been fully inserted 
into the guide tube 39, the arm 42 is withdrawn (by displacement in the 
direction of arrow D). Subsequently, the guide tube 39, together with the 
support chain 27 and the curved parison 6 held thereby, is swiveled into 
the horizontal position (FIG. 11), whereupon the jaws 31, 32 are released 
and the parison 6 can drop into the lower mold half 21 which is ready to 
receive it. The production,process is then continued as described in 
conjunction with FIGS. 1-8. 
Finally, the guide means 38 is returned to the vertical position and the 
support chain 27 moved back by the arm 42 into the position illustrated in 
FIG. 9 so that another parison can be extruded and taken up. 
In the embodiment illustrated in FIGS. 9-11, the support chain 27 is 
pressed into the guide tube 39 by the arm 42. Instead, it would be 
possible to provide a drawing means, for example in the form of a wire 
extending through the tube 39, on the lowest block 28 of the chain 27. The 
support chain 27 could then be inserted into the guide means 38 by a 
pulling action. 
Relatively complicated parisons6 having compound curvatures can also be 
produced with the aid of a guide means 38 of the type described when the 
tube 39 is bent accordingly. In particular, three-dimensionally curved 
parisons can also be manufactured with the guide means 38 when the guide 
tube 39 is bent in three dimensions. In this case, care must merely be 
taken that the link connection between the individual blocks 28 of the 
support chain 27 is also movable in three dimensions. It is then possible, 
for example, to curve the parisons to the shape of a three-dimensional 
spiral. 
As shown in FIG. 11, the pivotable jaws 31, 32 grip the parison 6 over only 
a very small areaof contact and so the parison loses practically no heat 
during preliminary curving. 
Instead of holding the parison 6 on the blocks 28 with the jaws 31, 32 the 
blocks can also be designed in a similar manner to the strip members 8, 9 
of FIG. 4, i.e. they are connected to a suction means and hold the parison 
6 by means of a vacuum or negative pressure. The fundamental mode of 
operation is not altered in any way by this. On the other hand, the strip 
members 8, 9 could also have pivotable jaws 31, 32 for holding the parison 
6 instead of the bores 14. 
Although it has proven particularly favourable to insert the parison 6, 
which is in a horizontal position (FIGS. 6 and 11), from above into the 
lower mold half 21, it is also fundamentally possible to avoid swiveling 
the support means 7 and the guide means 38 into the horizontal position 
and, instead, to introduce the precurved parison in a more or less 
vertically aligned position into the mold halves of the blow mold, which 
must then be arranged such that they are also substantially vertical. 
In the illustrated embodiments of the method, the parison 6 is first 
precurved substantially in the vertical plane and the precurved parison is 
then tilted into the horizontal. In principle it is also possible, in 
particular in the embodiment of FIGS. 1 to 8, for the parison to be tilted 
first of all into the horizontal and then curved. A third possibility is 
for the parison to be curved and tilted into the horizontal at the same 
time. 
In FIG. 8, a funnel or the like may also be arranged at the inlet end to 
the tube 39 to facilitate insertion of the support chain 27. 
Alternatively, a number of blocks 28 of the chain 27 may already be 
located in the vertical branch of the guide tube 39 during extrusion of 
the parison 6. 
The guide means 38 can also be arranged at the upper end of the support 
chain 27 according to FIG. 9. In this case, the slide means 41 first 
withdraws the chain 27 supporting the parison 6 away from the vertical 
axis of the die 5 and then draws the blocks 28 of the chain 27 one after 
the other into and through the guide means so that the parison 6 is curved 
accordingly.