Preform made of thermoplastic material, and device and method for producing a preform

Abstract

The preform consists of a thermoplastic material and is used to produce a blow-molded container. The preform has a tubular central region, a closed bottom, and an inlet section. The inlet section is opposite the bottom in the direction of the longitudinal axis and delimits the interior space. The wall thickness in the region of the bottom is at least partially smaller than the wall thickness in the central region. The base has a projection in the region of the inner or outer surface and optionally has a tab. The axial webs extend from the center of the base in the direction of the central region. The vertical tab can connect the at least two axial tabs to each other or can be implemented separately. After the material is supplied via the gate, the projection and optionally the tab are formed by the melting space and the flow channel.

Description

Technical Field

[0001] The invention relates to a method and a device for producing a preform for forming an advantageous preform dome geometry for a subsequent blow-molding process.

[0002] The invention ultimately leads to a considerable saving of raw materials and also achieves a quality improvement in the finished product.

[0003] Furthermore, the invention relates to a preform or preform having an improved bottom geometry. Background Art

[0004] A preform is an injection-molded blank made of at least one thermoplastic material, which is used in a blow-molding machine to produce a stretch-blow-molded plastic container.

[0005] In order to produce the preforms in the manner described according to the invention, the plastic raw material is plasticized and subsequently pressed under high pressure into a single-cavity or multi-cavity molding tool.

[0006] According to the existing technology, Figure 1 A preform is produced which consists essentially of a neck region, a handle region and a bottom dome in terms of geometry and which is hollow on the inside due to the use of a core in the forming tool. The neck region is formed in such a way that it can be configured, for example, to be reclosable with a screw cap. However, during the blow-molding process, the neck region does not undergo any further changes. In contrast, at an increased temperature, the handle region and the bottom dome are blown into a hollow body, whereby the plastic is stretched and significantly solidified. The region of the preform to be deformed thus ensures the quality of the bottle that is subsequently produced in terms of geometry in a manner that cooperates with the core geometry.

[0007] The forming tool is usually the highest investment in a production system. It is therefore of great value that the forming tool operates efficiently. In this way, a preform whose skin is in direct contact with the intensively cooled forming steel and therefore solidifies rapidly there can be demoulded without damage and without mechanical deformation, so that the forming tool is ready for the next production cycle without loss of time.

[0008] It should be noted that during the injection molding process, a holding pressure is always maintained throughout the preform via the gate in order to compensate for this defect during the solidification process of the preform (which is accompanied by shrinkage and thus leads to a lack of material) and thus avoid undesirable collapsed areas on the injection molded part.

[0009] In the usual rapid production cycles, considerable residual heat remains in the interior of the preform wall, which leads to reheating, whereby the preform can soften again and crystallize out, which renders it unusable.

[0010] It is therefore very advantageous to cool the preform further intensively during several production cycles after demolding in a simpler mold part, a so-called cooling jacket.

[0011] like Figure 1 The preform shown corresponds to the current state of the art, wherein, as already mentioned, it is inevitable that the wall thickness of the preform, in particular in the region of the preform dome and the shank, has a similar wall thickness. If the material freezes prematurely due to the thinner wall thickness in the base region or in the neck region, it is inevitable that during the cooling phase the entire preform, including the neck region, will shrink due to the holding pressure of the melt, which ultimately leads to undesirable collapse points in critical regions of the preform.

[0012] Therefore, according to the preform geometry of the present invention (the preform geometry is as follows Figure 2 The preform geometry shown in the figure and the advantages of which are explained below) cannot be produced in the known injection molding method or can only be produced by taking into account corresponding measures for maintaining the required holding pressure, because through the present invention, a wall thickness that is significantly thinner is achieved in the preform arch than in the subsequent handle area in order to exclude premature freezing in these thin areas and to avoid collapse points.

[0013] Another criterion for the subsequent blow molding process is that the temperature profile between the preform dome and the stem must make a sudden temperature jump from about 50-80° C. for the best results, which, however, is currently hardly achievable according to the prior art. This results in that, due to the gradual temperature change, the material in the base area cannot be pulled out into the bottle body in an optimal manner during the stretch blow molding process, which leads to unnecessary material losses. This can be greatly optimized by thinner wall thicknesses in the preform dome, but, due to the resulting rapid freezing in the thin area during the injection molding process, the holding pressure in the stem or neck cannot be maintained, which then leads to the collapse point and the neck no longer forms a sealed system in harmony with the closure. Summary of the invention

[0014] The central object of the present invention is to describe a method and a device with which preforms with a significantly more favorable wall cross section in the preform dome can be produced. The advantage is that the infrared heating of the subsequent blow molding machine can introduce heat energy more efficiently via the surface that is enlarged while the wall thickness is reduced, so as to bring the plastic in this area to a temperature that allows stretching more quickly. As a result, during the stretch blow molding process, the material can be pulled directly from the preform dome in an optimal manner that is favorable to the bottle base or the bottle body, which can achieve a significant saving of raw material.

[0015] The preform according to the invention can be cooled more efficiently in the forming tool because there is a smaller wall thickness between the melting space and the flow channel and the flow channel represents a larger surface. The exception to the more efficient cooling is the region of the melting space, because here, due to the reduced cooling in the forming tool, the residual heat acquired by the melt is higher than that acquired by the remaining preform region. The melt is injected directly into the larger melting space through the gate, where it is collected like in a basin and then bypassed in a controlled manner into the remaining preform body. The melting space thus ensures that the preform is constantly filled with melt and, during the holding pressure phase, supports the uniform distribution of the melt into the subsequent thinner wall thickness regions through its design. Through the melting space, the melt is uniformly guided to the corresponding flow channels, just like through an umbrella, or the melt is uniformly guided directly into the thinner wall thickness region of the preform dome. In the case of an optimized design of the melting space, in coordination with the corresponding bottle base geometry, the flow channel can even be omitted. In the case of supplementary cooling of the robot, a higher cooling efficiency is also achieved, since in the cooling jacket of the removal robot, a larger cooling surface can be used for the preform dome while the wall thickness is reduced, which counteracts resoftening and the associated loss of quality. The cooling jacket of the robot can better receive the preform during the transfer from the forming tool to the robot due to the projections and corresponding optional tabs, which are formed by the melting space and the optional flow channels in the outer preform dome. The preform dome has a larger contact surface with the cooling jacket, and this ensures more efficient cooling of the preform.

[0016] In addition, with the help of a special preform dome design, corresponding container bottom geometries can be accommodated. During the blow molding process, the crossbar hits the preform dome in the gate area during contact, cools the gate area in an uncontrolled manner, and thus prevents optimal stretching of the bottom area. According to the invention, due to the protrusions and tabs around the gate area, which are produced by the inner melting space in the tool and the optional vertical and / or horizontal flow channels, an optimized stretching of the preform dome can be achieved in a manner coordinated with the structural bottle bottom embodiment with the help of an adapted crossbar geometry. If a correspondingly larger melting space located inside the tool is used, the corresponding internal flow channel can also be omitted. The melting space is located in the future preform body directly after the gate, that is, the melt is collected here and then evenly distributed to the remaining preforms. In this case, without a flow channel, the melt is directly guided to the thinner wall thickness area via the melting space. This results in better shaping of the blow-molded bottle around the gate area or the base area, thereby avoiding sudden differences in wall thickness and material accumulations which, in turn, can lead to so-called stress fractures at the base, particularly in the case of carbonated beverages.

[0017] With the aid of the projections and the optionally introduced webs, the surface of the inner and / or outer preform dome can be enlarged in the entire base region. This has the advantage that the subsequent infrared heating of the blow molding machine can introduce heat more efficiently via the enlarged surface and can pull the material out of the preform dome better. The crossbar of the blow molding machine can better influence the wall thickness of the bottle base by better guidance or centering and by a precisely defined design of the inner projections and webs, so that the preform can thus be precisely extended in the axial direction.

[0018] To produce preforms of this type, three solutions are proposed below, which are applied in the bottom plate and / or in the core of the forming tool.

[0019] In a first variant, in the region of the bottom plate of the forming tool, it is possible, for example, to configure the forming of the outer preform dome in such a way that the largest part in the transition of the gate to the preform bottom is actually thin-walled, but at least one melt space, optionally two or more flow channels, are designed axially and / or vertically in such a way that they cannot freeze prematurely and thus maintain a holding pressure to the preform shank. The melt space surrounding the gate region has the purpose of guiding the melt in a better way into the corresponding flow channels or in the thin-walled wall thickness region of the preform dome in order to ensure a uniform distribution of the melt.

[0020] The size of this melting space depends on the preform size, weight, geometry and quality requirements of the corresponding blow-molded bottle. The preform dome formed by the melting space and the flow channel can be externally manifested as a projection or a tab on the preform, which has no negative effect on the subsequent blow-molding process if these tabs are distributed as symmetrically as possible on the circumference. During the stretch blow-molding process and later on the bottle, the tabs can even have a stabilizing effect. In addition, the projections and optional tabs are visible from the outside on the finished product in the first and third embodiments and represent a saving in terms of visual appearance.

[0021] Flow channels with the goal of influencing the bottle geometry are known from the prior art, for example from WO 2010 / 069042 A1 and US Pat. No. 5,455,088. In both documents, flow channels are described that influence the bottle body and the bottle base. The flow channel begins directly at the preform gate and ends in the prestressing handle. The advantage of both embodiments described above is that according to the invention, the melt space is first filled and then the melt is subsequently distributed into the thinner preform dome or into the corresponding flow channels, so as to ensure uniform filling of the preform.

[0022] In addition, the axial flow channels can be connected to one another vertically in order to support the dimensional stability of the preform during the holding pressure phase. Furthermore, the flow channels all end in the region of the preform dome without weakening the wall thickness of the future foot. This allows for uniform axial expansion and better temperature distribution in the preform dome without creating large wall thickness jumps between the preform gate and the outflow to the cylindrical preform body.

[0023] An alternative second variant of the invention is described here, in which it is possible, for example, in the core of the forming tool to configure the shaping of the preform interior design in such a way that the largest part of the inner preform dome is actually thin-walled, but at least one inner melt space, optionally two or more flow channels, are designed axially and / or vertically in such a way that they do not freeze prematurely and can therefore maintain a holding pressure up to the preform shank. By means of an adapted or reduced core cooling on the preform dome, the melt space and the flow channels can support the preform shank with the melt for a longer time during the holding pressure phase, in order to avoid collapse points in this way. When designing the flow channels, it is important to avoid undercuts, in order not to endanger the demolding of the preform, as in WO 2016 / 059135 A1, since the preform cannot be demolded here due to the undercuts. These flow channels are formed internally on the finished preform dome as tabs which, if distributed as symmetrically as possible on the circumference, do not have a negative effect on the subsequent blow-molding process, but even have a stabilizing effect during the stretch blow-molding process and later on the container. In addition, the inner projections and the tabs formed by the melt space and flow channels of the tool are not visible from the outside on the finished product, which is a major difference from the first and third solutions. If the design of the bottle base allows, the flow channels can be omitted and only the inner melt space in the tool can be used.

[0024] A third variant for optimizing the preform according to the invention in the base region is to implement a combination of a melt space and outer and inner flow channels in the preform dome. To this end, the base plate and the core are adapted together in the forming tool. Thus, inner / outer melt spaces, axial and / or vertical flow channels can be introduced simultaneously. In particular in CSD (Carbonated Soft Drinks) applications, where high internal pressures act, a more stable bottle base with a lower weight can be achieved by a targeted design of the preform dome or by introducing a combination of a melt space and axial / vertical outer flow channels and / or corresponding inner flow channels. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be described in more detail below based on embodiments with reference to the accompanying drawings.

[0026] Figure 1 A preform, such as is usually produced according to the prior art, is shown in cross section.

[0027] Figure 2The preform outer contour is shown in cross section, wherein the preform dome is configured in the forming tool during the injection molding process in such a way that it has at least one external projection, two axial and / or one vertical web, which are visible on the finished container.

[0028] Figure 3 The inner contour of the preform is shown in cross section, wherein the bottom area is configured in the forming tool during the injection molding process in such a way that it has at least one inner projection, two axial and one vertical web, and the at least one outer projection, two axial and one vertical web are not visible from the outside on the finished container.

[0029] Figure 4 The combination of the outer and inner contours of the preform is shown in cross section, wherein the base region is formed in the molding tool during the injection molding process such that it has at least one projection, two axial outer webs and at least one axial / vertical inner web.

[0030] Figure 5 Schematic diagram showing the flow channels on the preform dome from the outside and from the side.

[0031] Figure 6 A top view of an exemplary production assembly for a preform having an external projection and an external tab is shown.

[0032] Figure 7 An interior view of an exemplary production assembly for a preform having an inner projection and an inner tab is shown.

[0033] Figure 8 The cross section with the inserted transverse strut shows the inner contour of the preform with the recesses for the inner projections and the vertical webs.

[0034] Fig. 9 A bottom plate with cutouts for the melting space and the axial / vertical flow channels is shown.

[0035] Fig.10 A core with cutouts for the melt space and axial / vertical flow channels is shown.

[0036] Fig.11 A forming tool cavity for an injection molding machine with a melt space is shown.

[0037] Fig.12 Different inner / outer projection geometries are shown. DETAILED DESCRIPTION

[0038] In the following, the attached figures shall support the explanation of the manufacturing process of the prefabricated dome region.

[0039] According to the invention, all structural details and method details explained below can be realized individually or in any combination with one another. All device features can also be used within the framework of the method, and all method features can be realized in terms of the device.

[0040] Figure 1 A preform (1) produced according to the prior art is shown. Here, the wall thickness (10) in the preform crown (6) below the pouring gate (7) is similar to the wall thickness (10) in the shank region (5). Figure 2 A preform optimized for the blow molding process and having a reduced wall thickness (9) in the bottom region (6) can only be realized to a limited extent by injection molding technology due to the risk of the melt freezing, since the holding pressure that counteracts the shrinkage of the preform (2) during the cooling process can no longer act in the decisive region.

[0041] The present invention describes the following three solution variants: Figure 2 The invention relates to a preform (2) in the embodiment of the present invention. It should be mentioned here that all three methods produce at least one inner / outer melt space, an optional outer axial / vertical flow channel and / or an optional inner axial / vertical flow channel on the circumference of the described preform dome (6), but during the blow molding of the preform (2), the at least one inner / outer melt space, the optional outer axial / vertical flow channel and / or the optional inner axial / vertical flow channel have no adverse effect on the desired result. In contrast, in variants one and three, the formed protrusions and the optional tabs on the preform are visible from the outside and induce increased strength despite the material saving. In addition, in the case of an adapted bottle bottom geometry, the outer / inner flow channel can be completely omitted in the tool and can only be processed with the aid of the outer or inner melt space. From the gate, the melt enters directly into a melting space which is not in the material supply of the tool, but is always arranged after the gate region in a projection in the future preform dome as a material distribution center, the projection having a diameter of at least 2 mm to 30 mm ( Fig.12 ). In this case, the hot melt flow is not cooled immediately, but is firstly uniformly diverted into the following thin-walled area similar to an umbrella and cooled in a controlled manner by an adapted tool cooling section until the end of the holding pressure process.

[0042] In order to still be able to produce, for example, Figure 2 The preform (2) shown is configured as follows Fig.11The forming tool cavity shown has a melting space and an optional flow channel, so that at least one projection (8), two (preferably five) outer axial webs (11) and / or one outer vertical web (12) are formed via a thin-walled section (9) in the preform dome (6). Figure 2 The structural design shown during the injection molding process supports the maintenance of the holding pressure in the bottom region (6) of the preform (2). The surface contour of the preform (2) is continued in the region of the web, wherein the contour falls on the inside or outside in the region of the depression and thus a smaller wall thickness (9) is generated in the region of the depression.

[0043] The preform dome (6) has a uniform wall thickness in the gate region, has a projection (8) formed by the melting space (3) in the tool and ensures a continuous distribution of the melt, which is then guided into the preform shank (5) via a flow channel in the forming tool cavity through a recess, the flow channel being in the form of a web (11, 12, 13, 14) on the preform.

[0044] Ideally, the axial flow channel opens directly into the shank of the preform or can terminate in a freely definable manner, depending on the design of the base of the container to be produced, between the gate (7) and the shank region (5). This again compensates for the advance of the melt and avoids undesirable weld seams. The vertical flow channel visible on the preform as a web (12, 14) connects at least two axial flow channels visible on the preform as webs (11, 13) to one another in order to guide the melt simultaneously via the flow channels into the preform shank (5) during the holding pressure phase. In addition, the vertical flow channel supports the stability of the bottle base during the subsequent blow molding process in order to compensate for uncontrolled stretching caused by different wall thicknesses between the melt space and the respective flow channel. It is also possible to use only one vertical flow channel visible on the preform as a web (12, 14) with the goal of obtaining more material on a specific section of the bottle base in order to support the shape stability of the bottle base.

[0045] It is important here that these surfaces do not need to have an excessive length in the axial direction, but rather that the radial cross section in the region of the thin spots ensures the desired abrupt heat transfer for the blow molding process over a short length. This has the advantage that the thin spots can usually be implemented completely in a separate molded part of the preform dome. In addition, due to this implementation, the flow channels visible on the preform as webs (12, 14) are relatively short, which simplifies the thermal and rheological design of these flow channels: as the length decreases, premature freezing in the flow channels and the formation of weld seams are reduced.

[0046] However, in order to design the axial flow channel to be narrower, at least one additional vertical flow channel can also be integrated into the forming tool cavity as a connecting element between the axial flow channels, such as Figure 2 , 3 As shown in FIG. 4 by the webs (11, 12, 13, 14), the webs subsequently support the shaping of the preform (2) during the holding pressure phase. However, it is also possible to introduce only one outer vertical flow channel and / or only one inner vertical flow channel at the same time.

[0047] Figure 3 The preform (2) in the figure shows a second solution variant, in which webs are introduced into the inner contour of the preform (2) so that they are not visible from the outside on the blow-molded bottle, and at least two (preferably six) axial inner webs (13) and at least one vertical inner web (14) are integrated into the preform dome (6) in order to ensure pressure retention in the preform bottom area via the melt space (3) during the injection molding process.

[0048] and Figure 4 The combination of outer and inner webs is shown. The webs (11, 13 and 14) continue the original surface contour of the preform (1). The projection (8) on the preform is produced by the melting space (3) in the tool and ensures an optimal supply of melt to the preform (2) by the simultaneous and parallel introduction of inner / outer flow channels visible as webs (11, 13), wherein during the forming process the outer and inner flow channel contours, for example width and length, are identical and adapted to the surface contour of the preform (2). This prevents collapse areas in the shank (5) and the neck region (4) during the holding pressure phase.

[0049] exist Figure 5 The flow channel (15) marked by arrows in the figure shows how the melt flows via the gate into the melting space (3) visible here as a projection (8) and from there via five flow channels (visible here as webs) of sufficient width and length into the preform shank and thus maintains the holding pressure. The melt first accumulates in the subsequent melting space (3), which, due to its larger diameter ( Fig.12 ) and keeps the melt at a constant temperature, as in a box, in order to prevent: premature cooling of the melt before the injection molding process or the end of the holding pressure, resulting in collapse areas in the neck area (4) or even in the handle area (5).

[0050] exist Figure 6 and Figure 71 and 2 show webs with different inner and outer contours using different possible embodiments. The different geometries of the projection (8), the axial webs (11, 13) and the vertical webs (12, 14) can be clearly seen.

[0051] Figure 8 The crossbar (16) is shown in the locked position during the stretching process. When inserted into the preform (2), the crossbar (16) strikes the inner projection (8) and the vertical webs (14) of the inner preform dome (6). The inner contour of the axial / vertical webs can be consistent with the outer contour of the crossbar geometry in order to ensure that the preform (2) can be extended precisely in the axial direction. This has the advantage that the preform gate of the container is always accurately centered and that differences in wall thickness in the container bottom due to a displaced gate (7), so-called eccentricity (the preform gate is displaced away from the axial container center), are avoided.

[0052] In the injection molding mold, inner / outer melting space, axial / vertical inner or outer flow channel can be introduced according to Fig. 9 The bottom plate or according to Fig.10 in the core. Fig.11 The complete mold cavity with inner and outer melting space (3) is shown. This can be produced by a corresponding depression or by increasing the future wall thickness of the preform (2), by adapting in the bottom plate (17) and / or in the core (18).

[0053] exist Fig.12 In the embodiment of the present invention, two different forms of the projection (8) are implemented without inner / outer webs. The outer diameter of the outer / inner projection (20, 21) is a minimum of 2 mm to a maximum of 30 mm before the transition to the thin-walled wall thickness region (9) of the preform dome begins and ends in the transition to the preform shank (5). In this case, the thin-walled wall thickness region is designed to have a constant wall thickness a between the projection and the transition to the preform shank. Depending on the size of the inner / outer projection, the wall thickness can be implemented constantly until the transition to the preform shank, or it can be implemented with a narrowed or increased wall thickness a. The inner projection (20) in this example merges directly into the thinner wall thickness region (9) of the preform dome.

[0054] Reference numerals list

[0055] 1 Prefabricated parts according to the prior art

[0056] 2Preform with optimized thin-walled bottom area

[0057] 3 Melting Space

[0058] 4 Neck area

[0059] 5. Handle area

[0060] 6 Prefabricated vault

[0061] 7. Gate

[0062] 8 protrusion

[0063] 9 Reduced wall thickness in precast vaults

[0064] 10Normal wall thickness for injection molding process

[0065] 11 Axial flow channel on the outer contour of the preform

[0066] 12Vertical flow channels on the outer contour of the preform

[0067] 13 Axial flow channel on the inner contour of the preform

[0068] 14Vertical flow channels on the inner contour of the preform

[0069] 15 Runner

[0070] 16 Crossbar

[0071] 17 Bottom plate

[0072] 18 cores

[0073] 19 Prefabricated interior space

[0074] 20 inner protrusion

[0075] 21 External protrusion

Claims

1. A preform made of thermoplastic material for producing a blow-molded container, the preform having a tubular central region (5), a closed bottom (6) and a neck region (4), the neck region being opposite the bottom in the direction of the longitudinal axis of the preform and delimiting an inner space (19), and in the preform, the wall thickness in the region of the bottom (6) is at least partially smaller than the wall thickness in the central region (5), characterized in that The bottom (10) has at least one inner / outer projection in the area of ​​the surface and optionally has outer or inner webs (11, 12, 13, 14) with a constant wall thickness, which extend from the center of the bottom (6) in the direction of the central area (5).

2. The preform according to claim 1, characterized in that The projection and the optional webs (13, 14) are arranged on the inside relative to an interior space (19) of the preform (2).

3. The preform according to claim 1, characterized in that The projection and the optional webs (11, 12) are arranged on the outside relative to an interior space (19) of the preform (2).

4. The preform according to claim 1, characterized in that The webs (11, 12, 13, 14) are arranged on the inside and outside relative to an interior space (19) of the preform (2).

5. The preform according to any one of claims 1 to 4, characterized in that The material of the preform is injection molded.

6. The preform according to any one of claims 1 to 5, characterized in that The gate (7) is arranged in the region of the outer center of the bottom (6).

7. The preform according to any one of claims 1 to 6, characterized in that The wall thickness in the region of the base (6) is, at least in some regions, approximately 20% to 70% smaller than the wall thickness in the central section (5).

8. The preform according to any one of claims 1 to 7, characterized in that The projections and the optional inner and outer webs are designed as additional reinforcements for the bottle base without reducing the wall thickness in the region of the base (6).

9. A device for producing preforms (2) for producing blow-molded containers from thermoplastic material by injection molding technology, the device having an outer mold and a core arranged in a cavity of the outer mold, and in which the outer mold has a bottom and a neck area, the neck area being opposite the bottom in the direction of the longitudinal axis, characterized in that The distance between the core and the inner side of the outer mold in the region of the bottom is at least partially smaller than the distance between the core and the central section of the outer mold, and at least one recess and optionally a groove-shaped flow channel for the melt space extend from the center. The recess is introduced into the tool after the material supply and directly after the gate and is larger in diameter than the gate. The recess is produced by the core or the bottom plate and thus has an inner or outer melt space in the mold cavity, which can be seen as an inner or outer projection on the formed preform.

10. The device according to claim 9, characterized in that A recess for the melting chamber and an optional flow channel extend on the core starting from the center of the dome.

11. The device according to claim 9, characterized in that A recess for the melt chamber and an optional flow channel extend from the inner center on the outer mold.

12. The device according to any one of claims 9 to 11, characterized in that The spacing in the region of the base is approximately 20% to 70% smaller than the spacing in the central section.

13. The device according to any one of claims 9 to 12, characterized in that A melting chamber is arranged in the region of the bottom.

14. A method for producing a preform for producing a blow-molded container from a thermoplastic material, the preform having a tubular central region, a closed bottom and a neck region, the neck region being opposite the bottom in the direction of the longitudinal axis and delimiting an interior space, and in which the wall thickness in the region of the bottom is at least regionally smaller than the wall thickness in the central region, wherein: The bottom has at least one melting chamber in the region of the surface and optionally a flow channel which extends from the center of the bottom in the direction of the central region.

15. The method according to claim 15, characterized in that A holding pressure is generated in the final stage of the injection molding process.

16. The method according to claims 15 to 16, characterized in that The melt space is filled with plasticized plastic.

17. The method according to claims 15 to 17, characterized in that During the injection molding process, after the material has been supplied via the gate, first the melt space which is larger in diameter is filled with thermoplastic and then the spacing region between the core and the outer mold.

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