Method of extrusion blow molding flat bottles

A method of making flat bottles with flat or concave bottom by free extrusion of a plastic tube includes blowing of an intermediate form with circular cross section and shaping the latter essentially mechanically to the flat bottle at simultaneous final blowing for achieving clean radii. It is essential that the intermediate form is blown at a length corresponding to about the height of the flat bottle, with a circumference in the body area of -1 to about -10%, however, in the bottom near area of the side walls from 0 to about +15%, with regard to the corresponding circumference of the flat bottle. The radius of curvature at the junction from the body wall to the bottom should correspond to about the radius of curvature at the junction from the body wall of the broad side of the flat bottle to its bottom. The bottom part corresponding to the pinched end of the tube is pinched off during shaping.

BACKGROUND OF THE INVENTION 
For producing hollow plastic articles with rotationally symmetric cross 
section along their entire length and their entire height, respectively, 
e.g. round bottles, an extruded tube is generally used having a diameter 
which corresponds for example to the diameter of the neck of the hollow 
article to be produced and a wall thickness which exceeds the wall 
thickness of the finished hollow article. The tube is blown up in a 
blowing mold to the finished hollow article without experiencing an 
irregular wall thickness distribution. 
For hollow articles with a cross section deviating from the circular cross 
section like e.g. flat bottles, a production in this manner is not 
applicable as correctly reasoned in detail in the US-PS No. 3,892,829. 
Therefore, the mentioned US-PS provides a separate intermediate mold 
station in which an intermediate form is produced the length of which 
being considerably smaller than the height of the flat bottle and the 
surface of which being 25% smaller than the surface of the flat bottle to 
be produced. Only then can it be prevented during shaping of the 
intermediate form to the finished flat bottle in a final blowing mold that 
parts of the intermediate form are squeezed between both mold halves of 
the final blowing mold during their closing and that so-called dog-ears 
are obtained. The thus necessary biaxial stretching of the plastic 
material leads, however, to an irregular wall thickness distribution over 
the circumference of the flat bottle because the stretching occurs 
exclusively in the surface areas which do not yet bear against the cooled 
inner wall surface of the final blowing mold, thus preferably in the area 
of the narrow sides and the bottom of the flat bottle. In order to 
maintain a sufficient wall thickness, it is necessarily required to work 
with a surplus of material. The finished flat bottle has therefore an 
unnecessarily great wall thickness in the area of both its broad sides. 
Apart from the thus redundantly great material consumption such an 
excessive wall thickness results in an extended cycle period since 
evidently the cooling period is dependent on the area with the greatest 
material strength. This fact is in particular relevant when the 
intermediate form is not produced from a preform which is injected-molded 
around a hollow mandrel but is made from a freely extruded tube since the 
tube end sealed through pinching is arranged in the bottom of the 
intermediate form and the flat bottle, respectively, and causes there an 
especially high material accumulation. The method could not succeed in 
practice. Rather, the production of flat bottles with or without grip is 
usually accomplished through a tube blowing method in which an extruded 
tube is used with a circumference nearly equal to the greatest 
circumference of the flat bottle to be produced. The tube is continuously 
extruded in suspended position with a wall thickness desired for hollow 
articles and, after attaining a length corresponding to the height of the 
flat bottle, is transferred to a blowing mold in which the tube-like 
preform is blown up to the final flat bottle by means of a blow pin. The 
blow pin simultaneously calibrates neck and mouth of the bottle while the 
blowing mold during closing has already pinched off the considerable extra 
material portion especially in the neck area and shoulder area. 
Since depending on the shape of the flat bottle the pinched off material 
portion may amount from 10% up to 25% and more of the weight of the 
finished bottle, this type of production is relatively uneconomic since 
the need of machinery, especially, however, the power demand for drive, 
heating and cooling of the extrusion blow-molding apparatus is 
considerably higher than is necessary for the production of the actual 
flat bottle because the pinched-off material portion, too, must certainly 
be liquefied at first and then also cooled and a separate punching 
apparatus for its removal from the flat bottle as well as transport 
apparatus and regenerating apparatus for reprocessing the waste material 
are required. 
The DE-PS 27 20 448 discloses an apparatus for production of flat bottles 
which includes a blowing mold essentially provided with two mold halves 
wherein each mold half includes two mold parts movable perpendicular to 
the parting plane of the mold with one mold part forming the neck, the 
narrow sides and the bottom of the flat bottle, and the other mold part 
forming the broad sides of the flat bottle. A tube-extruded or 
injection-molded preform is used with a diameter approximately equal to 
the neck diameter of the flat bottle. The preform is blown up to the flat 
bottle during closing of the mold wherein the closing step proceeds in 
such a manner that initially the respective first or outer mold parts of 
both mold halves are closed. This material-saving process requires, 
however, a very precisely controlled blowing action. Further, the problem 
of undesired material accumulation in the bottom of the flat bottle still 
cannot be overcome when using an extruded preform. The method results in 
an essentially uniform wall thickness only in flat bottles whose radii of 
curvature is not too small because in these cases the material will still 
be stretched in the respective edge areas and corner areas upon already 
completely closed mold. It should be considered, however, that a relevant 
part of flat bottles used in the market and demanded from fillers have 
relatively sharp edges and thus small radii of curvature. In view of its 
costs, an blow-molding apparatus must basically, i.e. after installation 
of the respective blowing molds, be capable to produce all conventional 
flat bottles. 
SUMMARY OF THE INVENTION 
The invention is thus based on the object to create a method of producing 
flat bottles and an apparatus for carrying out the latter which allow the 
production of flat bottles in all conventional designs with nearly uniform 
wall thickness at least over the major part of its height, that is from 
bottom to shoulder, in a material-saving way and at short cycle periods. 
In accordance with the invention, this object is attained by the method 
stated in patent claim 1. In contrast to conventional wisdom, the solution 
is based on the teaching that the intermediate form which is known per se 
must have a greater circumference in the area of its junction in the 
bottom than the finished flat bottle in the same area in order to achieve 
the required wall thickness in the corners of the flat bottle, that is at 
a location where a respective broad side coincides with a narrow side and 
the bottom. The circumference of the body of the intermediate form changes 
continuously from a normally slight undersize relative to the respective 
circumference of the flat bottle in direction to the bottom to a usually 
slight oversize (&lt;15%) with the oversize being dependent in the individual 
case on how effectively the radii of curvature mentioned in the 
characterizing part of claim 1 approach each other, on the one hand, and 
on how long the pinching edge of the bottom part pinched off during 
forming to the flat bottle is, on the other hand. 
By further developing the method as stated in claim 2, the shaping of the 
bottom of the flat bottle can be optimized. 
With regard to the apparatus, the object upon which the invention is based 
is attained by the features stated in claim 5. In this connection, the 
invention is based on an extrusion blow-molding apparatus according to the 
US-PS 3,892,829, however, in an embodiment operable with a freely extruded 
tube. Still, the presently proposed method can be carried out only with a 
final blowing mold essentially known per se from the DE-PS 27 20 448.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show a longitudinal section and a cross section of an 
intermediate mold including two mold halves 1a and 1b movable 
perpendicular to their parting plane and defining a mold cavity in which 
an intermediate form 2 is blown by means of a blow pin 3 out of an 
extruded plastic tube 4 indicated in broken line. The plastic tube 4 has 
been freely extruded previously either continuously or by means of an 
extruder screw, closed at its lower end and for its stabilization slightly 
preblown preferably during the extrusion so as to receive in reality a 
slightly oval shape in longitudinal section before being transferred to 
the intermediate mold 1a, 1b and blown to the illustrated intermediate 
form 2. 
The intermediate mold 1a, 1b is provided above its cutting edge 5 in the 
bottom of the mold with a calotte-shaped depression for receiving the 
material accumulation necessarily occurring at the pinch-off edge of the 
plastic tube 4. The depression may, however, be cup-shaped in the event 
the pinched off material accumulation should be of larger volume. 
Subsequently arranged thereto toward the outside is an only slightly 
convex bottom area. In addition, the intermediate mold halves 1a and 1b 
define a mold cavity which produces an intermediate form 2 with a mouth 
and neck area completely or nearly completely corresponding to the 
respective parts of the finished flat bottle while the circumference of 
the intermediate form (and of the mold cavity, respectively) is 
dimensioned in the shoulder and body area up to about 10% smaller and in 
the bottom near area up to 15% greater than the respective circumference 
of the flat bottle. The diametrical variations are--as 
illustrated--continuously. In the event the shoulder area of the flat 
bottle to be produced has characteristic edges, the intermediate form must 
also be provided there with a circumference greater by few percent than 
the finished bottle has at the same level. 
Basically, the circumferential overdimension of the intermediate form is 
the greater the more characteristic the edges and corners of the finished 
bottle. Only then, thinning of the wall thickness and possibly even 
appearance-impairing swellings of these critical areas are prevented. 
Furthermore, the radius of curvature R at the junction of the body wall to 
the bottom of the intermediate form is about equal to the radius of 
curvature of the corresponding junction of the flat bottle to be produced. 
The length of the intermediate form (and of the form cavity of the 
intermediate mold, respectively) is almost equal to the height of the flat 
bottle to be produced, provided that the calotte-shaped center piece 2a is 
not considered upon determination of the length because it is pinched off 
due to the contained material accumulation so that material as well as 
cooling time is saved. 
This sizing of the intermediate mold 1a, 1b and of the intermediate form 2, 
respectively, allows the latter to be shaped to the finished flat bottle 
within a final blowing mold without experiencing any relevant material 
stretching and thus variations of the wall thickness. The initial position 
of this forming process is illustrated in FIGS. 3 and 4 while the 
intermediate position is shown in FIGS. 5 and 6. A precondition for this 
shaping is the application of a final blowing mold which consists of four 
parts and includes two mold halves A and B shiftable perpendicular to the 
parting plane of the mold and each of which including two mold halves 7a, 
7b and 8a, 8b. The mold halves 7a, 7b are movable toward each other 
independently from the mold halves 8a, 8b. The mold halves 8a, 8b are 
slidably guided in the mold halves 7a, 7b so that the latter define an 
outer frame whereby the parting plane extends preferably in the area of 
the narrow radii of the finished flat bottle. 
FIGS. 3 and 4 illustrate the transfer position in which the intermediate 
form of FIGS. 1 and 2 still suspending from the blow pin 3 is disposed 
between the open mold halves A and B. 
FIGS. 5 and 6 depict the state after closing the frame-like mold halves 7a, 
7b and the partial closing of the mold halves 8a, 8b. At this stage, the 
cutting edge 9 provided in the bottom parting plane of the frame-like mold 
halves 7a, 7b has already severed the calotte-shaped, thickened center 
piece of the intermediate form 2'. The diameter of this thickened center 
piece 2a (and, respectively, the calotte-shaped depression providing the 
latter and arranged in the mold bottom of the intermediate form) is 
dimensioned in such a manner that it is about half of the long axis of the 
bottom of the flat bottle. The inner mold parts 8a, 8b forming the later 
broad sides of the flat bottle, are just about in contact in the shown 
position with the intermediate form 2' which is kept at sufficient 
internal pressure by sealing off the blow air channel 3a of the blow pin 3 
by means of a not shown valve. Consequently, the bottom area of the 
intermediate form 2' begins already to bear against the contour of the 
frame-like mold parts 7a, 7b. Despite comparable appearance, this 
"pre-bottom" is thus not the thickened calotte-shaped center piece 2a of 
the bottom of the original intermediate form 2. Subsequently, the inner 
mold parts 8a, 8b are closed by the remaining displacement path as 
designated with "a". Thus, the intermediate form 2' nearly conforms to the 
contour of the finished flat bottle 11 which is illustrated in a sectional 
view in FIG. 7 along its short transverse axis and in a sectional view in 
FIG. 8 along its long transverse axis. For that, the final blowing mold is 
moved under a final blow pin and acted upon with full final blow pressure. 
The final blow pin is of same design as the blow pin 3 and thus not 
illustrated. After the cooling period, the final blowing mold is opened 
and the flat bottle 11 is ejected. 
In the above-described method, the final blowing mold receives the 
intermediate form from the intermediate blow station after opening and 
removing of the intermediate blowing mold 1a, 1b from the blow pin 3, is 
closed at least partially, preferably however completely and is moved into 
the final blow station beneath the final blow pin. 
A modification of this method avoids a too strong cooling of the 
intermediate form in this phase i.e. from the transfer to the final 
blowing mold beneath the blow pin 3 until being acted upon by pressure by 
means of the final blow pin. This modification recommended when 
temperature sensitive plastics are concerned is attained by completely 
closing the final blowing mold 7a, 7b, 8a, 8b in any event when being in 
the transfer position beneath the blow pin 3 and by allowing the full 
final blow pressure to act at least momentarily upon the intermediate form 
2 by means of the blow pin 3 so that the intermediate form 2 is already 
completely shaped in this intermediate blow station to the flat bottle 11. 
In this manner, surface irregularities are prevented in view of premature 
cooling. The final blowing mold is then transferred to the final blow 
station beneath the final blow pin which injects compressed air to act 
again upon the flat bottle 11 during the remaining cooling period. In this 
in a way two-step final blowing, the first step (by means of the blow pin 
3) should be kept as short as possible in order to prevent a lengthening 
of the cycle period in view of the time the final blowing mold is kept in 
the intermediate blow station. 
While the invention has been illustrated and described as embodied in a 
Method of and Extrusion Blow-Molding Apparatus for Producing Flat Bottles, 
it is not intended to be limited to the details shown since various 
modifications and structural changes may be made without departing in any 
way from the spirit of the present invention.