Device and method for reducing impurities

Abstract

The invention relates to a device (100) and a method for reducing impurities (V) in a plastic melt (K), in particular a plastic melt of thermoplastic material. The device comprises a plurality of openings (20) with bridges (21) therebetween. Cutting edges (22) are formed on the bridges (21).

Description

[0001] Device and method for reducing contamination

[0002] The present invention relates to a device and a method for reducing impurities in a plastic melt, in particular in a plastic melt made of thermoplastic material.

[0003] A wide variety of plastic containers are known from the state of the art. Various processes are known for the production of these plastic containers, particularly plastic bottles, the application of which depends not least on the plastics used. Plastic containers are usually manufactured using a blow molding process, in which plastic containers, such as plastic bottles, are inflated into their final shape using excess pressure in a blow mold.

[0004] Blow molding involves a distinction between different process technologies, the most notable of which are extrusion blow molding, injection blow molding, and injection stretch blow molding. In extrusion blow molding, a single- or multi-layer plastic tube is hot-extruded, inserted into a blow mold, and blown into a plastic container via a blow mandrel inserted into a mold cavity.

[0005] The injection blow molding process is a combination of injection molding and blow molding. It involves first producing a preform in an injection mold using an injection mold. The preform is then removed from the injection mold, conditioned if necessary, and placed into the mold cavity of a blow mold, where it is finally inflated under excess pressure to the shape specified by the mold cavity.

[0006] In the injection stretch blow molding process, the preform, which is inserted into the mold cavity, is additionally stretched with a mandrel during the blow molding process. The preform can be inflated immediately after its injection molding production.

[0007] In alternative manufacturing processes, the further processing of the preforms can also take place spatially and / or temporally separate from the preform production itself. Finally, it should also be mentioned that the preforms can also be manufactured using an impact molding process or an extrusion blow molding process.

[0008] Typically, the raw material is provided as granules, heated and melted in an extruder, and then conveyed to a blow mold or injection mold. In this mold, the melt is further processed, either by extruding a preform in the form of a tube or by injection molding to form a preform.

[0009] Global raw material resources are becoming increasingly scarce with an ever-growing population. At the same time, the increased demand is leading to ever-increasing environmental pollution. Therefore, efforts are being made to reuse used packaging. One method of reuse is recycling.

[0010] Many plastics can be reused multiple times. However, this requires that they be of high quality during recycling or at least exhibit a high degree of purity before reuse. Therefore, efforts are being made to install recycling plants directly upstream of production facilities so that, for example, recontamination during the transport of recycled granulate, also known as reclaimed material, can be prevented. However, such solutions are very expensive and require a high degree of process reliability.

[0011] During the recycling process, collected used containers are typically separated according to their composition and then shredded. A washing process typically also takes place to clean the materials and, if possible, separate them from foreign matter. The shredded plastics are then melted, and the melt is filtered. The filtered melt is subsequently cooled and further processed into granules. These granules are then typically filled into bags for transport.

[0012] It can also happen that different grades of recycled plastics result in inhomogeneous granules, which can have correspondingly different properties.

[0013] All of these factors lead to defects or contamination, which are detrimental to the subsequent blow-molding process. Defects can, in particular, cause blow-molded plastic containers to leak or burst, either during the blow-molding process itself or later in the filling process. This leads to machine downtime and, potentially, to extensive cleaning effort.

[0014] It is therefore an object of the invention to remedy at least one of the disadvantages of the prior art. In particular, a method and / or a device are to be provided that enable the production of high-quality plastic containers and reduce the rate of defective plastic containers. In particular, the reliability of the blow-molding or filling process is to be increased.

[0015] This object is achieved by the devices and methods defined in the independent patent claims. Further embodiments emerge from the dependent patent claims.

[0016] In particular, the claimed invention can reduce defects in containers, which in turn leads to machine downtimes during the production of plastic containers being reduced or at least minimized due to plastic containers becoming leaky or bursting at defects.

[0017] A device according to the invention for reducing impurities in a plastic melt, particularly in a plastic melt made of thermoplastic material, has a plurality of openings with intermediate webs. Cutting edges are formed on the webs. The cutting edges are arranged opposite to the flow direction of the plastic melt. Contaminants in the plastic melt therefore strike these cutting edges as they flow through the device.

[0018] In this case, the thermoplastic material can be a conventional or bio-based polyester or a polyolefin, in particular PET, copolymers of PET, PEE, HDPE, or PP, with or without coloring, whereby the material is recycled material or a mixture of virgin and recycled material. The thermoplastic material can also be entirely virgin.

[0019] In this context, contamination essentially refers to three categories of problems or defects that lead to the bursting of blow-molded containers and / or to leaks in blow-molded containers.

[0020] Bursting containers can lead to extended downtime in production facilities, as they require more or less intensive cleaning. Leaking bottles pose the risk that the product stored inside will slowly leak out or spoil, or, what may be considerably more devastating, that the contents of the container will become contaminated. This is particularly dangerous for products that must be sterile, for example, and / or products containing active medicinal ingredients. Aseptic filling lines, in particular, must be meticulously clean to prevent germs from growing on leaked contents and to prevent containers from becoming contaminated by germs.

[0021] Particularly during the melt formation of recycled material, so-called gel formation can occur. This type of gel formation refers to local areas in the material where the material is not a freely flowing thermoplastic polymer, but where the material has been cross-linked through the formation of bridges between the molecules. At the cross-linked areas, the material behaves differently during the blow molding process, and local thick spots form. The material accumulated in the thick spots is missing from other parts of the plastic container, leading to thin spots, containers bursting during blow molding, or even holes forming, thus causing containers to leak.

[0022] Defects in plastic containers caused by gel formation are a major problem. It is known that gels can pass through the filters during recycling or that such gels can also form after a filter in a recycling plant. Gels are often unavoidable in the injection molding of regenerated materials. In technical terms, people often talk about a PET gel, fisheye or cross-linked PET. In general, it can be said that gel formation can occur at local points in the material where the material is not a freely flowing thermoplastic polymer, but where the material has been cross-linked due to bridges forming between the molecules. At the cross-linked points, the material behaves differently in the blow molding process, and local thick spots form.

[0023] A second category of defects is caused by unmelted PET. Similar to the previously discussed thick material areas, this also involves an accumulation of unmelted PET, which leads to failure at or near this defect. Such unmelted PET is usually still clearly visible as white particles or granules in the transparent PET. In technical terms, this is often referred to as unmelt or recrystallization.

[0024] Such unmelts can also occur downstream of a recycling plant's filter, for example, because the PET doesn't crystallize evenly, doesn't build up uniformly, or differs chemically due to the type or concentration of comonomers. It can also happen that the material enters the extruder too cold, or that it absorbs too little shear heat due to sliding effects and is therefore not heated sufficiently.

[0025] A third category involves genuine contamination by foreign substances. These can include, for example, glass particles, metal, wood, paper, rubber, or other contaminants. In technical terms, such defects are often referred to as black dots, as these contaminations by foreign substances are typically visible and very often black.

[0026] The general term “impurities” refers to all defects as described here.

[0027] The origin of the contaminants varies widely. On the one hand, melt filtration of recycled granules is highly susceptible to failure, and contaminants are not filtered out. On the other hand, they can also arise later, for example, during remelting in the plastic container manufacturing process. Contaminants can also enter the system during filter changes.

[0028] The granules are often transported in bags, and PET, in particular, develops very high static charges. Such charges cause dirt particles to enter the granules. It has been found that these contaminants only cause problems above a certain size.

[0029] The device for reducing impurities enables the impurities to be reduced by the cutting edges on the webs so that they reach a size that usually no longer leads to defects in the blow-molded container.

[0030] A cutting edge is an element that allows a body to be mechanically split or separated. Cutting edges are formed by the meeting of two flat or curved surfaces at an acute angle. Their hypothetical intersection line has a radius of 0. This is not possible in practice; instead, the surfaces merge into one another at a radius. This radius is less than 0.2 mm, in particular less than 0.1 mm, and preferably less than 0.05 mm.

[0031] The webs preferably have a length in the direction of flow and a thickness transverse thereto. The length is at least three times the thickness, preferably five times the thickness.

[0032] In this case, the reduction of impurities is therefore also understood to mean the modification of the impurities, in particular cutting or slicing, since once they fall below a certain size they have little to no influence on the quality and accordingly no longer appear as impurities in the process.

[0033] This device can, for example, already be used in the recycling process; however, it is also suitable for use in the injection molding or blow molding process, specifically after the extruder, so that the melt passes through the device as the second-to-last step and is either injected into the mold or extruded, for example, as a tube in the final step. The openings of the device preferably have a clear diameter of less than 0.8 mm, in particular less than 0.6 mm, and preferably less than 0.3 mm.

[0034] These sizes ensure that all particles larger than 0.8 mm, 0.6 mm, or 0.3 mm either remain trapped in the device or are crushed by the cutting edges. If the contaminants are filtered out, they can no longer cause defects. However, if they are crushed instead of filtered out, they are usually so small that they also no longer cause defects. Crushing is particularly advantageous because the device, or rather its openings, are not clogged.

[0035] The cutting edges preferably have a cutting angle of less than 45°. In particular, the cutting angle is less than 35° and preferably less than 25°. However, the cutting angle is preferably at least 15°.

[0036] If the angle is less than 15°, there is a risk that the cutting edges will frequently break and thus become blunt relatively quickly. There is also a risk that the openings will clog relatively quickly at such shallow angles. If the angle is greater than 45°, most structures will no longer be cut properly and / or a relatively high resistance to the melt will occur. An angle of 35° achieves relatively good results.

[0037] The device is preferably made of a metallic material. Metals with a melting point of more than 400°C have proven particularly advantageous. The melting point is preferably greater than 800°C. Such metals can be processed using a three-dimensional metal printing process.

[0038] A melting point of more than 800 ° makes it possible to clean devices that are clogged or have contaminants by means of heat, i.e. to burn them out.

[0039] The device is typically tubular and has a first open end and a second closed end. This allows the melt to be introduced into the first open end and forced through the closed end and / or the side wall, where it can be filtered accordingly. The term "filtering" refers here to both the retention and cutting of impurities.

[0040] Preferably, the device is designed such that it tapers conically from the first open end to the second closed end. This allows for uniform filtering and, in particular, easier cleaning of the device.

[0041] The openings, and in particular the cutting edges, are preferably arranged along the entire device. This increases the flow cross-section, or rather the throughflow cross-section.

[0042] The device is preferably arranged in a housing. Such an arrangement allows the device to be easily attached and, if necessary, replaced.

[0043] A further aspect relates to an extruder head comprising a device for reducing impurities in a plastic melt. The extruder head comprises, in particular, a device as described here. The device has a plurality of webs with cutting edges. The device is arranged on the extruder head such that the plastic melt flows through the device. This means a flow through during intended use.

[0044] An extruder head with such a device with cutting edges makes it possible to filter the melt after extrusion and before introduction into a hot runner system, or to crush impurities contained therein in such a way that these have no negative effects or at least only acceptable negative effects on the container subsequently blown.

[0045] The device is preferably monitored by one or more pressure sensors that monitor the pressure profile as the fluid flows through the device. In particular, when used in an injection molding process, the injection pressure is monitored, and if the pressure rises above a defined threshold, for example, 10%, the user is instructed to clean the device.

[0046] The device is preferably arranged in a housing, wherein this housing is preferably arranged so as to be rotatable together with the device transversely to a flow direction of the plastic melt. This makes it possible to rotate the device so that it is or can be flowed through in the opposite direction to the original flow direction. This makes it possible to pressurise and rinse the device from its other side, and thus in the opposite direction to the usual flow direction. Contaminants on the device which could not be cut off can thus be removed from the device in a simple manner.

[0047] Additionally or alternatively, the device can be pivotally mounted parallel to a longitudinal axis of the extruder head. Such an arrangement makes it possible to pivot the device out of the processing path and clean and / or replace it accordingly. The longitudinal axis is typically defined by the flow direction at the end of the extruder head.

[0048] In particular, it may also be provided to provide multiple fixtures so that they can be pivoted into the machining path like a revolver drum, allowing them to be continuously exchanged. This enables uninterrupted machining or production.

[0049] A further aspect relates to an injection molding machine comprising a device as described herein or an extruder head as described herein. This enables the provision of an injection molding machine with coordinated components.

[0050] A further aspect relates to a blow molding machine comprising a device as described herein or an extruder head as described herein. This enables the provision of a blow molding machine with specifically coordinated components.

[0051] A further aspect relates to a method for reducing impurities in a plastic melt, in particular carried out using a device as described here. In the method, the impurities in the plastic melt are comminuted so that their size is smaller than 0.8 mm, in particular smaller than 0.6 mm, and preferably smaller than 0.3 mm.

[0052] Such a process results in the provision of a plastic melt which only contains impurities which essentially no longer have a negative influence on the finished blow-molded container.

[0053] In this process, the impurities are preferably separated during an extrusion process by webs with cutting edges arranged in the flow of the plastic melt. This enables the preparation of a high-quality melt with few impurities immediately before further processing.

[0054] A device and method according to the invention are explained using schematic figures. It shows:

[0055] Figure 1 : A device for reducing contamination;

[0056] Figure 2: a detailed view of Figure 1;

[0057] Figures 3A to 3D: Sectional views of different cross-sections of webs;

[0058] Figure 4: the crushing process;

[0059] Figure 5: the backwash process;

[0060] Figure 6: an extruder head;

[0061] Figure 7: the extruder head of Figure 6 during backflushing.

[0062] Figure 1 shows a device 100. The device 100 has a first open end 101 and a second closed end 102. From the first open end 101 to the second open end 102, the device 100 is essentially tapered. The device 100 has a plurality of openings 20, which are formed by intermediate webs 21. This results in a net-like appearance of the device 100.

[0063] Figure 2 shows a detailed view of Figure 1 in an enlarged representation. A plurality of openings 20 with intermediate webs 21 are visible. These openings 20 are essentially diamond-shaped. The clear diameter of these openings 20 is determined by inscribing a circle within these openings 20 and determining its diameter. In the present case, the openings 20 shown here have a clear diameter of 0.3 mm. In other words, a circle with a diameter of 0.3 mm can be inscribed within the diamond-shaped opening.

[0064] Figures 3A to 3D show a cross section through a web

[0065] 21 along the line BB as shown in Figure 2. Figure 3A shows a diamond-shaped web 21 which has a cutting edge 22 at one end. This cutting edge

[0066] 22 is arranged opposite to the flow direction of the plastic melt K, so that impurities V from the plastic melt K impinge on this cutting edge 22. This process will be explained in detail later in Figure 4. In this case, the cutting edge 22 has a cutting angle of approximately 33°.

[0067] Figure 3B shows a cross-section through an alternative embodiment of a web 21. The web 21 is lens-shaped in this case and also has a cutting edge 22 at at least one end. The tangentials to this cutting edge 22 form an angle of approximately 36°.

[0068] Figure 3C shows a cross-section through another alternative embodiment of a web 21. In this case, the web 21 is triangular and also has a cutting edge 22 at least at one end. The cutting edge 22 has an angle of approximately 11°. Although this angle of 11° is below the minimum angle of 15°, it may still be the right choice for certain plastics.

[0069] Figure 3D shows a cross-section through another alternative embodiment of a web 21. In this case, the web 21 is designed as a half lens and also has a cutting edge 22 at least at one end. The cutting edge 22 has an angle of approximately 40°. For the stability of the web 21, it is particularly advantageous if it has a length in the flow direction that is at least twice the dimension transverse to the flow direction.

[0070] Figure 4 shows the comminution process of a contaminant V within a plastic melt K. As can be seen, several webs 21 are arranged next to one another. The webs 21 each have a cutting edge 22. For the sake of clarity, only one web is provided with a reference symbol.

[0071] The proportions are also shown on one of the webs. As can be seen, it has a length L and a thickness D. In this example, the length L corresponds to four times the thickness D.

[0072] A contaminant V within the plastic melt K strikes the cutting edges 22 of the webs 21 in the direction of flow, which is shown by the arrows. The contaminant V is accordingly pressed onto the cutting edges 22 by the flowing plastic melt K and is thus pressed through the openings 20 located between the webs 21 and cut at the same time. In the direction of flow after the webs 21 there are now only contaminants V' which are significantly smaller than before the webs 21. These contaminants V' have a size which no longer has a negative influence on later processing steps such as injection molding or blow molding.

[0073] Figure 5 shows the backflushing process in a schematic representation. Shown is an injection mold 41, which is connected to a corresponding extruder head 40. Inside this extruder head 40 there is arranged a device 100 for reducing impurities in a plastic melt. In the top of Figure 5, the device 100 is arranged in the direction of flow so that it has a corresponding filtering effect. In the bottom of Figure 5, the extruder head 40 is spaced from the injection mold 41 and the device 100 is rotated by 180° so that it can be flushed by subsequent plastic melt in the extruder head 40. Impurities that were not cut in the device 100, but merely retained, can thus be flushed out of the device 100, as shown by the arrow.

[0074] Figure 6 shows an extruder head 40, within which a device 100 is arranged. The device 100 is also arranged in a housing 30. The housing 30 is rotatably mounted within the extruder head 40. In Figure 6, the flow direction of the plastic melt is from left to right. The plastic melt thus enters a first open end 101 (see Figure 1) and is pushed through the device 100 and filtered in the process. A large number of impurities are reduced in size as described in relation to Figure 4, but some of the impurities remain trapped within the device 100 and gradually clog it.

[0075] Figure 7 now shows the extruder head 40 when the device 100 is being cleaned. The housing 30 within the extruder head 40, together with the device 100, is arranged counter to the flow direction so that a plastic melt in the extruder head can act on the device 100 counter to the previous flow direction. This allows contaminants that have become trapped in the device 100 to be flushed out of the device 100. This enables simple and quick cleaning of the device 100.

Claims

Patent claims 1. Device (100) for reducing impurities (V) in a plastic melt (K), in particular in a plastic melt made of thermoplastic material, characterized in that it has a plurality of openings (20) with webs (21) located therebetween, cutting edges (22) being formed on the webs (21), the cutting edges (22) being arranged opposite to a flow direction of the plastic melt (K), so that the impurities (V) from the plastic melt (K) impinge on these cutting edges (22) when flowing through the device.

2. Device (100) according to claim 1, characterized in that the thermoplastic material is a polyester or polyolefin, in particular PET, copolymers of PET, PEF, HDPE, PP with or without coloring, wherein the material is virgin material or recycled material or mixtures of virgin material and recycled material.

3. Device (100) according to claim 1 or 2, characterized in that the openings (20) have a clear diameter which is less than 0.8 mm, in particular less than 0.6 mm and preferably less than 0.3 mm.

4. Device (100) according to one of claims 1 to 3, characterized in that the cutting edges (22) have a cutting angle of less than 45°, in particular less than 35° and preferably less than 25°, wherein the cutting angle is preferably at least 15°.

5. Device (100) according to one of claims 1 to 4, characterized in that the device is made of metal.

6. Device (100) according to one of claims 1 to 5, characterized in that the device (100) is tubular and has a first open end (101) and a second closed end (102).

7. Device (100) according to claim 6, characterized in that it is tapered from the first open end (101) to the second closed end (102).

8. Device (100) according to one of claims 6 or 7, characterized in that the openings (20) are arranged along the entire device (100).

9. Device (100) according to one of claims 1 to 8, characterized in that it is arranged in a housing (30).

10. Extruder head (40) comprising a device (100) for reducing impurities (V) in a plastic melt (K), in particular a device according to one of claims 1 to 9, with a plurality of webs (21) with cutting edges (22), characterized in that the device (100) is arranged on the extruder head in such a way that the plastic melt (K) flows through the device (100).

11. Extruder head (40) according to claim 10, characterized in that the device (100) is arranged in a housing (30), wherein the housing (30) is arranged rotatably together with the device (100) transversely to a flow direction of the plastic melt (K).

12. Extruder head (40) according to claim 10 or 11, characterized in that the device (100) is arranged pivotably parallel to a longitudinal axis of the extruder head (40).

13. Injection molding machine comprising a device according to one of claims 1 to 9 or an extruder head according to one of claims 10 to 12.

14. Blow molding machine comprising a device according to one of claims 1 to 9 or an extruder head according to one of claims 10 to 12.

15. A method for reducing impurities (V) in a plastic melt (K), in particular carried out with a device according to one of claims 1 to 9, characterized in that the impurities (V) in the plastic melt (K) are comminuted so that their size is smaller than 0.8 mm, in particular smaller than 0.6 mm and preferably smaller than 0.3 mm.

16. The method according to claim 15, characterized in that the impurities are divided during an extrusion process by webs (21) with cutting edges (22) which are arranged in the flow of the plastic melt (K).

Images