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

Abstract

The invention relates to a preform which consists of a thermoplastic material and is used to produce blow-moulded containers. The preform has a tube-like centre region, a closed base and a mouth section. The mouth section lies opposite the base in the direction of a longitudinal axis and defines an interior space. The wall thickness in the base region is at least partially less than in the centre region. The base has at least one protrusion and optionally a web in the region of an inner or outer surface. The axial web extends from the centre of the base towards the centre region. The vertical web can connect at least two axial webs to one another or be designed as a single web. The protrusion and the optional webs are moulded by a melting chamber and flow channels following the material feed via the sprue.

Description

[0001] Preform made of a thermoplastic material and device and method for producing a preform

[0002] The present invention relates to a method and a device for producing preforms for forming an advantageous preform dome geometry for the subsequent blow molding process.

[0003] Ultimately, the invention leads to a significant saving of raw materials and also achieves an improvement in the quality of the finished product.

[0004] The invention further relates to a preform with improved bottom geometry.

[0005] Preforms are injection-molded blanks made of at least one thermoplastic material that are used in blow molding machines for the production of stretch-blown plastic containers.

[0006] For the production of preforms described in this invention, raw plastic material is plasticized and then pressed into a single- or multi-cavity mold at high pressure. According to the prior art, preforms are produced as shown in Figure 1, which geometrically consist essentially of a neck and skirt region and a base, and which are hollow inside due to the use of a core in the mold. The neck region is shaped in such a way that it can be resealable, for example with a screw cap. The neck region, however, does not undergo any further changes during the blow molding process. The skirt region and the base, on the other hand, are blown into hollow bodies at elevated temperatures, whereby the plastic is stretched and solidifies considerably. Therefore, the preform regions to be deformed, in interaction with the core geometry, are geometrically responsible for the subsequent bottle quality.

[0007] Typically, the mold represents the largest investment in a production system. Therefore, great importance is placed on its efficient operation. The preform, whose outer skin is in direct contact with the intensively cooled mold steel and consequently solidifies quickly, is demolded without damage and without mechanical deformation, so that the mold is ready for the next production cycle without any delay.

[0008] It should be noted that during the injection molding process, a holding pressure is maintained throughout the entire preform via the sprue in order to compensate for this deficiency during the solidification process of the preform, which is accompanied by shrinkage and thus leads to missing material, in order to avoid unwanted sink marks on the molded part.

[0009] During typical rapid production cycles, considerable residual heat remains inside the preform wall, leading to reheating, which can cause the preform to resoften and crystallize, rendering it unusable.

[0010] It is therefore very advantageous to continue cooling the preform intensively after demoulding in simpler mould parts, in so-called cooling sleeves, during several production cycles.

[0011] The preform, as shown in Figure 1, corresponds to the current state of the art, where, as described, it is unavoidable that the wall thicknesses of the preform have similar thicknesses, especially in the area of ​​the preform crown and the shaft. If the material freezes prematurely due to thinner wall thicknesses in the base or neck area, shrinkage during the cooling phase cannot be avoided due to the subsequent pressure of the melt, which affects the entire preform, including the neck area. This consequently leads to undesirable sink marks in critical areas of the preform.

[0012] The preform geometry according to the invention, as shown in Figure 2 and the advantages of which are explained below, cannot therefore be produced in the known injection molding process or can only be produced by taking into account corresponding measures which maintain the required holding pressure, since this invention achieves a significantly thinner wall thickness in the preform tip than in the subsequent shaft area in order to exclude premature freezing in these thin areas and to avoid sink marks.

[0013] A further criterion for the subsequent blow molding process is that the temperature profile between the preform tip and the skirt would have to undergo an abrupt temperature jump of approximately 50 to 80°C for optimal results. However, this is hardly feasible given the current state of technology. In most cases, this leads to a gradual temperature transition that prevents the material in the base area from being optimally drawn into the bottle body during the stretch blow molding process, resulting in unnecessary material consumption. This could be greatly optimized by thinner wall thicknesses in the preform tip, but the resulting rapid freezing in the thin area during the injection molding process would prevent the holding pressure in the skirt or neck from being maintained. This would then lead to the aforementioned sink marks and the neck would no longer form a tight system, as would be the case with closures.

[0014] The central objective of the present invention is to describe a method and apparatus for producing preforms with significantly more favorable wall cross-sections in the preform crown. The advantage is that the infrared heaters of the downstream blow molding machines can more efficiently apply heat energy via this now enlarged surface area with a simultaneously reduced wall thickness, thus bringing the plastic in this area to a stretchable temperature more quickly. This allows the material to be optimally extracted directly from the preform crown during the stretch blow molding process, favoring the bottle base or body, thus enabling significant savings in raw materials.

[0015] Preforms according to this invention can be cooled more efficiently in the mold because the wall thickness between the melt chamber and the flow channels is thinner, and the flow channels have a larger surface area. One exception to this more efficient cooling is the melt chamber area, because the reduced cooling in the mold means the melt retains more residual heat than the rest of the preform. The melt is injected through the sprue directly into the larger melt chamber, where it is collected in a basin, and then diverted into the rest of the preform body in a controlled manner. The melt chamber thus ensures a constant filling of the preform with melt, and its design supports the even distribution of the melt into the subsequent, thinner-walled area during the holding pressure phase.The melt is evenly directed through the melt chamber, like a screen, either to the corresponding flow channels or directly into the thinner wall thickness area of ​​the preform tip. With optimal design of the melt chamber, the flow channels can even be omitted in coordination with the respective bottle base geometry. Higher cooling efficiency is also achieved in the post-cooling of the robotics, as a larger cooling surface can be used for the preform tip in the cooling sleeve of the removal robotics with a simultaneously reduced wall thickness, which counteracts resoftening and the associated reduction in quality. Due to the projection and the respective optional webs, which are formed by the melt chamber and optional flow channels in the outer preform tip, the cooling sleeve of the robotics can better accommodate the preform during transfer from the mold to the robotics.The preform dome has a larger contact area with the cooling sleeve and this ensures more efficient preform cooling.

[0016] In addition, a special preform dome design can be adapted to the respective container base geometry. During the blow molding process, the stretch rod hits the preform dome in the inner sprue area, cooling it uncontrollably and thus preventing optimal stretching of the base area. Due to the projection and the webs around the inner sprue area, which are created by the inner melt chamber and the optional vertical and / or horizontal flow channels in the mold, an optimized stretching of the preform dome can be achieved according to the invention with an adapted stretch rod geometry, in coordination with the constructive bottle base design. The respective inner flow channels can also be dispensed with if a correspondingly larger internal melt chamber is used in the mold. The melt chamber is located directly after the sprue in the future preform body, i.e.The melt is collected here and then evenly distributed throughout the rest of the preform. In this case, the melt is channeled directly through the melt chamber into the thinner wall thickness areas without any flow channels. This results in better shaping of the blown bottle around the sprue or bottle base area, thus avoiding abrupt changes in wall thickness and material accumulation, which can lead to stress fractures at the bottle base, especially in gas-containing beverages.

[0017] With the projection and the optionally incorporated ribs, the surface area of ​​the inner and / or outer preform dome can be enlarged across the entire base area. This has the advantage that the infrared heaters of the downstream blow molding machines can apply heat energy more efficiently via the enlarged surface, and the material can be drawn more effectively from the preform dome. The stretching rod of the blow molding machine can better influence the wall thickness of the bottle base through better guidance and centering, as well as the precisely defined internal design of the inner projection and the ribs, thus enabling the preform to be precisely lengthened axially.

[0018] For the production of such preforms, three solutions are proposed below, which are used either in the base plate and / or core of the mold.

[0019] In a first variant, for example, it is possible to design the outer preform crown in the area of ​​the base plate of the mold such that the majority of the transition from the sprue to the preform base is actually thin-walled, but at least one melt chamber, optionally two or more flow channels, are designed either axially and / or vertically so that they do not freeze prematurely and can thus maintain the holding pressure to the preform shaft. The melt chamber around the sprue area serves the purpose of directing the melt more optimally into the respective flow channels or the thin-walled area of ​​the preform crown to ensure even melt distribution.

[0020] The size of this melt chamber depends on the preform size, weight, geometry, and the quality requirements of the blown bottles. The preform dome formed by the melt chamber and the flow channels can appear on the outside of the preform as protrusions or ridges. These do not negatively impact the subsequent blow molding process, provided they are distributed as symmetrically as possible around the circumference. In the stretch blow molding process and later on the bottle, they can even have a stabilizing effect. Furthermore, the protrusion and the optional ridges in versions one and three are visible from the outside of the finished product, visually demonstrating the savings.

[0021] Flow channels designed to influence bottle geometry are known from the prior art, for example, in WO 2010 / 06 9042 A1 and US5,455,088. Both documents describe flow channels that influence the bottle body and bottle base. The flow channels begin directly at the preform gate and end within the preform shaft. The advantage over the two aforementioned designs is that, according to the present invention, a melt chamber is filled first, and only then is the melt distributed into the thinner preform dome area or into the respective flow channels, thus ensuring uniform filling of the preform.

[0022] Additionally, the axial flow channels can be vertically interconnected to support the preform's dimensional stability during the holding pressure phase. Furthermore, the flow channels all end in the area of ​​the preform crown without weakening the wall thickness of the future bottle base. This enables uniform axial stretching and better temperature distribution in the preform crown without large wall thickness changes between the preform gate and the outlet to the cylindrical preform body.

[0023] An alternative second variant, which the invention describes here, is possible in the core of the mold, for example, to design the shape of the preform's interior design in such a way that the largest part of the inner preform tip is actually thin-walled, but at least one inner melt space and optionally two or more flow channels, either axially and / or vertically, are designed in such a way that they do not freeze up prematurely and can therefore maintain the holding pressure to the preform shaft. By adapting or reducing core cooling at the preform tip, the melt space and the flow channels can support the preform shaft with melt for longer during the holding pressure phase in order to avoid sink marks. When designing the flow channels, it is important to avoid undercuts in order not to endanger the demolding of the preform, as for example in WO 2016 / 059135 A1, since the preform cannot be demolded here due to an undercut.These flow channels appear as ridges on the inside of the finished preform crown. These ridges do not negatively impact the subsequent blow molding process, provided they are distributed as symmetrically as possible around the circumference. Rather, they have a stabilizing effect during the stretch blow molding process and later on the container. Furthermore, the inner projection and the ridges, which are formed by the melt chamber and the flow channels of the mold, are not visible from the outside of the finished product, which is a significant difference from the first and third approaches. If the bottle base design allows it, flow channels can be omitted and only an inner melt chamber can be used in the mold.

[0024] A third variant for optimizing the base area of ​​the preform according to this invention is a combination of a melt chamber and outer and inner flow channels in the preform dome. For this purpose, the base plate and the core are adapted together in the mold. This allows for the simultaneous introduction of an inner / outer melt chamber and axial and / or vertical flow channels. Especially in CSD (carbonated soft drinks) applications, where high internal pressures are present, a more stable and lightweight bottle base can be achieved through targeted preform dome designs or the introduction of a melt chamber and a combination of axial / vertical outer flow channels and / or corresponding inner flow channels.

[0025] The invention is explained in more detail below and with reference to exemplary embodiments in the accompanying drawings. The drawings show, in Fig. 1, a preform in cross-section as it is typically produced according to the prior art.

[0026] Fig. 2 Preform outer contour in cross-section in which the preform dome was designed during the injection molding process in the mold as an example so that it has at least one outer projection, two axial and / or one vertical web, which are visible on the finished container.

[0027] Fig. 3 Preform inner contour in cross-section in which the bottom area was designed during the injection molding process in the mold so that it has at least one inner projection, two axial and one vertical web, which are not visible from the outside of the finished container.

[0028] Fig. 4 Combination of preform outer and inner contour in cross-section in which the bottom area was designed during the injection molding process in the mold so that it has at least one projection, two axial outer webs and at least one axial / vertical inner web.

[0029] Fig. 5 Schematic representation of the flow paths at the preform tip from the outside and from the side.

[0030] Fig. 6 Top view of an exemplary production arrangement for preforms with an outer projection and outer webs.

[0031] Fig. 7 Internal view of an exemplary production arrangement for preforms with an inner projection and inner webs.

[0032] Fig. 8 Preform inner contour with a recess for an inner projection and vertical web in cross section with retracting stretch rod.

[0033] Fig. 9 Base plate with a recess for a melt chamber and axial / vertical flow channels.

[0034] Fig. 10 Core with a recess for a melt chamber and axial / vertical flow channels. Fig. 11 Mold cavity for an injection molding machine with a melt chamber.

[0035] Fig. 12 Different inner / outer projection geometries

[0036] The following drawings are intended to support the explanation of the manufacturing process of the preform dome area.

[0037] All design details and process details explained below can be implemented individually or in any combination according to the invention. All device features can also be used within the process, and all process features can be implemented in the device.

[0038] Figure 1 shows a preform (1) produced according to the prior art. The wall thickness (10) in a preform tip (6) below a sprue (7) has a similar wall thickness (10) to that in a skirt region (5). Preforms optimized for the blow molding process according to Figure 2 with reduced wall thicknesses (9) in a base region (6) can only be produced by injection molding with restrictions due to the risk of the melt freezing, since the holding pressure, which counteracts the shrinkage of the preform (2) during the cooling process, can then no longer act in the crucial areas.

[0039] This invention shows three solution variants for how the preform (2) in Figure 2 can be produced. It should be noted that all three processes create at least one inner / outer melt chamber, 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 these have no adverse effects on the desired blow molding result when blow molding the preform (2). Quite the opposite: in variants one and three, the molded projections and optional webs on the preform are visible from the outside and suggest increased strength, even though material has been saved. Furthermore, with an adapted bottle base geometry in the mold, outer / inner flow channels can be completely dispensed with and only one outer or inner melt chamber can be used.The melt flows from the sprue directly into the melt chamber, which is not located in the mold's material feed, but always after the sprue area, in a projection of at least 2 mm to 30 mm in diameter (Figure 12) in the future preform dome, serving as a material distribution center. The hot melt stream is not cooled immediately here, but is first evenly redirected into the adjacent thin-walled area, comparable to a shield, and cooled in a controlled manner by an adapted mold cooling system until the post-press process is completed.

[0040] In order to be able to produce a preform (2) as shown in Figure 2 using conventional injection molding technology, a mold cavity as in Figure 11 with a melt chamber and optional flow channels is designed in such a way that at least one projection (8), two, preferably five, outer axial webs (11) and / or one outer vertical web (12) are formed over the thin wall (9) in the preform tip (6). The structural design during the injection molding process, as shown in Figure 2, supports the maintenance of the holding pressure in the base area (6) of the preform (2). The surface contour of the preform (2) is continued in the area of ​​the webs, whereby in the area of ​​the recess the contour falls on the inside or outside and thus generates a thinner wall thickness (9) in the area of ​​a recess.

[0041] The preform tip (6) has a uniform wall thickness in the sprue area, has a projection (8) which is formed by a melt space (3) in the tool and ensures a continuous distribution of the melt, which is then guided via the flow channels, which appear as webs (11, 12, 13, 14) on the preform, in the mold cavity through recesses in the preform shaft (5).

[0042] The axial flow channels ideally open directly into the skirt of the preform or, depending on the base design of a container to be produced, can end in a freely definable manner between the sprue (7) and the skirt area (5). This compensates for any overflow of the melt and avoids unwanted weld lines. The vertical flow channels, visible on the preform as webs (12, 14), connect at least two axial flow channels, visible on the preform as webs (11, 13), in order to simultaneously guide the melt via the flow channels into the preform skirt (5) during the holding pressure phase. In addition, the vertical flow channels support the bottle base stability during the subsequent blow molding process in order to compensate for uncontrolled stretching caused by the different wall thicknesses between the melt chamber and the respective flow channels.It is also possible to use only one vertical flow channel, visible on the preform as webs (12, 14), with the aim of obtaining more material at a specific bottle bottom section in order to support the shape stability of the bottle bottom.

[0043] It is important that these surfaces do not have to be excessively long in the axial direction, but that the radial cross-sectional area in the area of ​​the thin spots already ensures the desired abrupt heat transfer over a short length for the blow molding process. This has the advantage that the thin spots can usually be created entirely in the split forming parts of the preform dome. In addition, the flow channels, visible on the preform as webs (11, 12, 13, 14), are relatively short due to this design, which simplifies the thermal and rheological design of these flow channels in that premature freezing within the flow channels and the formation of weld lines decreases with decreasing length.

[0044] To make the axial flow channels narrower, at least one additional vertical flow channel can be integrated as a connecting element between the axial flow channels in the mold cavity, as shown in Figures 2, 3, and 4 by the webs (11, 12, 13, 14), which then support the shaping of the preform (2) during the holding pressure phase. However, only one outer vertical and / or only one inner vertical flow channel can be introduced simultaneously.

[0045] The preform (2) in Figure 3 shows the second solution variant, in which the webs are introduced into the inner contour of the preform (2) so that they are not visible from the outside on the blown bottle and at least two, better six axial inner webs (13) and at least one vertical inner web (14) are integrated into the preform dome (6) in order to secure the holding pressure during the injection process via the melt space (3) in the preform base area.

[0046] Figure 4, on the other hand, shows a combination of outer and inner webs. The webs (11, 13, and 14) continue the original surface contour of the preform (1). The projection (8) on the preform is created by a melt chamber (3) in the mold and, with the simultaneous, parallel introduction of the inner / outer flow channels, visible as webs (11, 13), ensures the optimal supply of the preform (2) with melt. The outer and inner flow channel contours match in terms of shape, such as width and length, and adapt to the surface contour of the preform (2). This prevents sink marks in the shaft (5) and neck area (4) during the holding pressure phase.

[0047] The flow paths (15) in Figure 5, marked by arrows, show how the melt flows via the sprue into the melt chamber (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 shaft, thus maintaining the holding pressure. The melt first collects in the subsequent melt chamber (3), which, due to its larger diameter (Figure 12) than the sprue (7), keeps the melt at a constant temperature like in a tank, to prevent premature cooling of the melt from causing sink marks in the neck area (4) or even in the shaft area (5) before the injection molding process or holding pressure is completed.

[0048] Figures 6 and 7 show webs with different inner and outer contours and different design options. The different geometries of the projection (8), axial (11, 13), and vertical (12, 14) webs are clearly visible.

[0049] Figure 8 shows a stretch rod (16) in the retracted position during a stretching process. As it moves into the preform (2), the stretch rod (16) encounters the inner projection (8) and a vertical web (14) of the inner preform tip (6). The inner contour of the axial / vertical web can match the outer contour of the stretch rod geometry to ensure that the preform (2) can be stretched precisely axially. This has the advantage that the preform gate of the container is always exactly centered and wall thickness differences in the container base due to offset gates (7), so-called off-centers (preform gate is offset eccentrically from the axial container center), are avoided.

[0050] In the injection mold, the inner / outer melt space, the axial / vertical, inner or outer flow channels can be incorporated into a base plate as shown in Figure 9 or in the core as shown in Figure 10. Figure 11 shows a complete mold cavity with an inner and outer melt space (3). This can be achieved either by respective recesses or enlargements, the future wall thickness of the preform (2), or by adjustments in the base plate (17) and / or the core (18).

[0051] Figure 12 shows two different shapes of projections (8) 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 into the thin-walled wall thickness region (9) of the preform tip starts and ends in the transition to the preform shaft (5). In this case, the thin-walled wall thickness region is designed with a constant wall thickness a between the projection and the transition to the preform shaft. Depending on the size of the inner / outer projection, it can be either constant or with a tapered or increasing wall thickness a up to the transition of the preform shaft. The inner projection (20) in this example flows directly into the thinner wall thickness region (9) of the preform tip.

[0052] List of reference symbols

[0053] State-of-the-art preform

[0054] Preform with optimized, thin-walled bottom area

[0055] Melting room

[0056] neck area

[0057] Shaft area

[0058] Preform dome

[0059] sprue

[0060] Projection reduced wall thickness in the preform dome normal wall thickness for the injection molding process axial flow channel on the preform outer contour vertical flow channel on the preform outer contour axial flow channel on the preform inner contour vertical flow channel on the preform inner contour

[0061] Flow path

[0062] horizontal bar

[0063] base plate

[0064] core

[0065] Interior of the preform inner projection outer projection

[0066] 17

Claims

A preform made of a thermoplastic material for producing bias-molded containers, said preform having a tubular central region (5), a closed base (6), and a neck region (4) which, in the direction of a longitudinal axis of the preform, is opposite the base and delimits an interior space (19), and in which a wall thickness in the region of the base (6) is at least partially smaller than in the central region (5), characterized in that the base (10), in the region of a surface, has at least one inner / outer projection and optionally one outer or inner web (11, 12, 13, 14) with a constant wall thickness, which extends from a center of the base (6) toward the central region (5). Preform according to claim 1, characterized in that the projection and the optional web (13, 14) are arranged on the inside with respect to an interior space (19) of the preform (2).Preform according to claim 1, characterized in that the projection and the optional web (11, 12) are arranged on the outside with respect to an interior space (19) of the preform (2). Preform according to claim 1, characterized in that the webs (11, 12, 13, 14) are arranged on the inside and outside with respect to an interior space (19) of the preform (2). Preform according to one of claims 1 to 4, characterized in that the material of the preform is injection-molded. Preform according to one of claims 1 to 5, characterized in that a sprue (7) is arranged in the region of an outer center of the base (6). Preform according to one of claims 1 to 6, characterized in that a wall thickness in the region of the base (6) is at least partially approximately 20 to 70% less than in the central section (5). Preform according to one of claims 1 to 7, characterized in that the projection, the optional, inside and outside webs are designed as additional reinforcement for the bottle base, without reducing the wall thickness in the region of the base (6).Device for the injection-moulding production of a preform (2) from a thermoplastic material for producing bias-moulded containers, which preform has an outer mould and a core arranged in a cavity of the outer mould, and in which the outer mould has a base and a neck region opposite the base in the direction of a longitudinal axis, characterized in that a distance between the core and an inner side of the outer mould in the region of the base is at least partially smaller than that between the core and a central section of the outer mould, and in that, starting from a centre, at least one recess for a melt chamber and optionally a groove-like flow channel extend. This recess is introduced into the mould after the material has been fed in, directly after the sprue, and is larger in diameter than the sprue.It is created either by a core or base plate and therefore has an inner or outer melt space in the mold cavity, which is recognizable on the molded preform as an inner or outer projection. Device according to claim 9, characterized in that the recess for the melt chamber and the optional flow channel on the core extend from the center of a dome. Device according to claim 9, characterized in that the recess for the melt chamber and the optional flow channel on the outer mold extend from an inner center. Device according to one of claims 9 to 11, characterized in that the distance in the region of the base is approximately 20% to 70% smaller than in the region of the central section. Device according to one of claims 9 to 12, characterized in that a melt chamber is arranged in the region of the base.A method for producing a preform from a thermoplastic material for producing bias-molded containers, which preform has a tubular central region, a closed base, and a neck region which lies opposite the base in the direction of a longitudinal axis and delimits an interior space, and in which a wall thickness in the base region is at least partially smaller than in the central region, wherein the base has at least one melt space in the region of a surface and optionally a flow channel web which extends from a center of the base towards the central region. Method according to claim 15, characterized in that a holding pressure is generated in a final phase of the injection molding process. Method according to claims 15 to 16, characterized in that the melt space is filled with plasticized plastic.Method according to claims 15 to 17, characterized in that during the execution of the injection molding process, after the material has been fed in via the sprue, first the melt space with a larger diameter and then a space between the core and the outer mold is filled with thermoplastic material. 16

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