DE20080290U1 - Device for introducing sensitive additives into thermoplastic material - Google Patents

Description

Device for introducing sensitive additives into thermoplastic

Plastic material

The invention relates to a device for introducing sensitive additives, in particular those sensitive to shearing or kneading, into thermoplastic plastic material, with a plasticizing zone in which a plasticizing screw mounted in a housing zone is present for converting the plastic material into the plasticized state, which plasticizing screw conveys the plasticized plastic material to a screw section of a relaxation zone in which at least one additive is fed to the plasticized plastic material via at least one line and mixed with the plasticized plastic material by means of a screw.

The usual procedures for introducing additives into plasticized thermoplastic material have been to add the additives to the plasticized material in the initial zone of an extruder or to mix the additive with the plastic material in a funnel using a mixing screw. It is also known to introduce glass fibers, which are a particularly delicate additive, into the plasticized plastic material in a relaxation zone of an extruder and to mix them with this material using a screw.

It has been shown that additives that are sensitive to shearing and kneading in particular suffer considerably when subjected to these known processes. This applies in particular to glass fibers, which are subject to a high risk of breakage. The more the glass fibers are broken when they are introduced into the plastic material, the more the elastic modulus of the glass fiber reinforced plastic material is reduced. Studies have shown, for example, that glass fibers that were 4 to 5 mm long before being introduced into the plastic material were reduced in length to around V ₁₀ to V ₁₅ when subjected to the known processes. It has also been shown that the wetting of numerous additives, particularly glass fibers, by the plasticized material was unsatisfactory, so that the coating of the additives, particularly glass fibers, by the plastic was not perfect.

The invention aims to at least reduce, if not avoid, these disadvantages and to improve a device of the type described at the outset so that the additives introduced into the plastic material, in particular those that are sensitive to shearing or kneading, e.g. glass fibers, are less or not damaged, but are better coated by the plastic material. The invention solves this problem by providing a dam body in a dam zone between the plasticizing zone and the relaxation zone, which dam body rotates with the plasticizing screw and shears the plastic material and which directs the flow

of the plasticized plastic material flowing over it and that, viewed in the direction of transport of the plastic material, at least one channel for plasticized plastic material branches off in front of this dam, which bypasses the dam and, after the outlet point of the line for the additive, opens into the screw section of the relaxation zone or into a housing section behind it. In this way, the total throughput of the plasticizing screw is divided. A portion is guided over the dam and, by the shearing caused by it and, if necessary, external additional heating, e.g. resistance heaters around the housing, is brought to a higher temperature than the material supplied by the plasticizing screw. The additive or additives are introduced into this material portion, which is brought to a higher temperature and therefore has a correspondingly low viscosity, which results in better wetting of the additives, in particular the fibers, e.g. glass fibers, and faster impregnation of the fiber material. The remainder of the total throughput of the plasticizing screw is then brought together and mixed with this premix. The pretreatment mentioned above makes it easier to mix the highly viscous melt with the additive-containing premix. The thermal damage to the plastic material caused by the influence of the bluff body or the additional heating must be kept within such limits that the advantage of protecting the additives, in particular the better wetting and fiber length retention and the associated increase in strength of the fiber-reinforced plastic material, outweighs this.

It is known (US 5 190 706 A) to divide the flow of molten plastic material into two paths in a device for extruding thermoplastic foam material, with a slide valve arranged behind the branching point of the second path (seen in the flow direction of the material), which is formed by two perforated plates that can be moved relative to one another, by means of which the plastic flow in the first flow path can be changed. Behind this valve, seen in the flow direction, additives are introduced into the plastic material, mixed with it and finally recombined with the portion of material fed via the second path. Such a device is not suitable for the introduction of sensitive additives.

According to a preferred embodiment of the invention, the damming body is a damming screw whose flight depth and/or flight pitch is smaller than that of the plasticizing screw. This damming screw brings the plastic material to the desired higher temperature in a simple manner due to its flat geometry, which is higher than that of the plastic material fed via the bypass channel. According to the invention, the damming screw expediently has the same direction of its flights as the plasticizing screw, i.e. it has no return thread. As a result, the plastic material subjected to high shear by this damming screw is fed in the direction of the

A relaxation zone is promoted so that the shearing effect is only short-term and damage to the plastic material is kept within acceptable limits.

According to a further development of the invention, the flight depth of at least part of the screw section of the relaxation zone is greater than the flight depth of the plasticizing screw in order to give the low-viscosity mixture containing the additive sufficient opportunity to mix intimately with the high-viscosity plastic mass fed via the channels forming bypasses.

Within the scope of the invention, it is sensible to arrange a screw section with multiple flights in the area of the inlet of the line for the additive compared to the subsequent relaxation zone in order to achieve a better initial distribution of the additive substances. Furthermore, within the scope of the invention, it is expedient to have the relaxation zone followed by a screw section whose flight depth is smaller than that in the relaxation zone. In this way, the mixture of the two partial flows is again subjected to a higher shear in a similar way to that in an extruder, which is, however, less than that caused by the bluff body, but the material mixture emerging from the device, which is provided with the additives, is sufficiently plasticized so that the resistance of the downstream tool is overcome.

For economic reasons and for reasons of a clear design, it is advisable to arrange the plasticizing screw, the dam body, the screw section of the relaxation zone and preferably also the screw section following the relaxation zone coaxially and to provide them with a common drive. According to a preferred embodiment, the arrangement is such that the previously mentioned components have a common housing which has a constant inner diameter.

For the supply of the additives, a stuffing and dosing element provided in the line supplying the additive is recommended within the scope of the invention, e.g. a screw, in particular a co-rotating twin screw, which presses the additive into the relaxation zone in order to achieve a reliable supply of the additive.

When operating the device according to the invention for introducing at least one sensitive additive, particularly one sensitive to shearing or kneading, into thermoplastic plastic material, in which the plastic material is brought into the plasticized state and then allowed to relax, whereby the additive is introduced into the relaxing plastic material and mixed with it, the procedure is such that the plasticized plastic material is divided into two paths, whereby in the first path the plastic material is brought to a higher temperature than during its previous plasticization by the action of a preferably short-term shear and into this more strongly heated plastic material

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the additive is introduced, and that the plasticized plastic material guided on the second path is combined and mixed with the plastic material part containing the additive after the additive has been introduced. While a single housing section guided over the bluff body is recommended for the first path, the second path can be formed by one or more channels through which the higher-viscosity plastic material is guided.

It is advisable to keep the amount of plastic material fed along the first path lower than the amount of plastic material fed along the second path. This keeps the influence of thermal damage to the plastic material to a minimum. It is also advisable to achieve the higher temperature of the plastic material fed along the first path by heating it from the outside if necessary, in addition to the effect of strong shearing.

In the drawing, embodiments of the device according to the invention are shown schematically.

Fig. 1 shows a first embodiment of the device according to the invention in horizontal section. Fig. 2 shows a variant embodiment in a section similar to Fig. 1. Fig. 3 shows a further variant embodiment for a detail in vertical section.

In the embodiment according to Fig. 1, the thermoplastic material to be processed, e.g. in the form of bags, film residues, bottles, etc., is filled into a shredder 2 from above via a conveyor belt 1. The shredder 2 has a pot-shaped receiving container 3, in the bottom area of which tools 4 rotate around the vertical container axis. These tools 4 chop and/or mix and/or heat the introduced plastic material and press it into an opening 5 arranged in the side wall of the receiving container 3. This pressing of the processed plastic material into the opening 5 is caused on the one hand by the centrifugal force applied to the material, but it is also supported by the fact that the working edges of the tools 4, which are formed in particular by knives, are set back against the direction of rotation (arrows 6), so that a spatula-like pressing of the plastic material into the opening 5 results. The opening 5 also forms the intake opening of a plasticizing zone 48, in which a plasticizing screw 8 is rotatably mounted in a housing 7. The core 9 of this screw is driven by a drive 11, in particular a motor with a gear, via a shaft 10. The plastic material introduced into the plasticizing zone 48 is carried along by the screw 8 in the direction of the arrow 12 and gradually converted into a plasticized state. This is facilitated by the fact that the core diameter of the screw 8 increases in the transport direction, so that the pressure or shear exerted on the plastic material gradually increases. The plasticized material thus converted into a highly viscous state is divided into two parts at point 13. The first, smaller part in terms of volume flows over a screw-shaped

Baffle body 14, which is rotatably mounted in a housing zone 15 and is rotationally connected to the plasticizing screw 8. The other, larger part of the plasticized material supplied by the plasticizing screw 8 enters two channels 16 connected to the housing zone 7, in which the plasticized material flows in the direction of the arrows 17. The channels 16 bypass the baffle body 14 and only after this (seen in the transport direction of the plastic material) do the plastic material rejoin the part of the material guided over the baffle body 14 at point 18. At this point, the channels 16 are connected to a housing zone 19, in which a screw 20, which is rotationally connected to the baffle body 14, is rotatably mounted, the core 21 of which gradually reduces in the transport direction (arrow 22) of the plastic material, so that the housing zone 19 forms a relaxation zone 49 for the plastic material.

As can be seen from Fig. 1, the screw forming the bluff body 14 has a flight pitch that is smaller than that of the plasticizing screw 8. Alternatively or additionally, the flight depth of the screw forming the bluff body 14 can be smaller than the smallest flight depth of the plasticizing screw 8. This ensures, on the one hand, that only a desired fraction of the total throughput of the plasticizing screw 8 can pass through the bluff body 14. On the other hand, the described flat geometry of the screw forming the bluff body 14 means that the plastic material in the housing zone 15 is brought to a desired higher temperature than this material has in the other zones of the device. This higher temperature is achieved by the high shear and, if necessary, by additional heating 51, e.g. with resistance heaters, to which this material is subjected in the housing zone 15. However, the direction of the pitch of the flights of the accumulation screw 14 is directed in the same way as the pitch of the plasticizing screw 8, so that the accumulation screw 14 also transports the material in the direction of the arrow 22. This ensures that the volume flow in the accumulation zone 50 is kept constant. The thermal damage to the plastic material, which occurs due to the increased temperature in the housing zone 15, therefore remains low and can be kept within predetermined limits with sensible operation.

The plastic material brought to the higher temperature by the blocking screw 14 and, if applicable, the additional heating 51 is thus more liquid or has a lower viscosity than the material supplied by the plasticizing screw 8. This low-viscosity material passes from the housing zone 15 into the relaxation zone 49 and is there combined with the additive or additives, which are introduced into the relaxation zone 49 via a line 24. The line 24 is connected laterally to the housing zone 19 of the relaxation zone 49 and forms a housing in which a stuffing and dosing element, in particular a co-rotating double screw 23 (two screws arranged parallel to one another, which rotate in the same

direction). This stuffing and dosing element is driven by a motor 25 with gear 26. The additives to be introduced can be dosed by means of speed control. The additive material to be introduced by the double screw 23 is introduced into the line 24 by a feed device 27, e.g. a funnel. The double screw 23 transports this material in the direction of arrow 28 and presses or stuffs it into the housing zone 19 at the outlet point 29 of the line 24. This outlet point 29 is located, as Fig. 1 shows, in the direction of arrow 22 (transport direction of the plastic material), in front of the point 18 at which the branched material flow guided via the channels 16 forming bypasses is fed back again. At this impression point 29, the screw 20 with its core 21 which decreases there already forms a relaxation zone, which promotes the intimate mixing of the additive supplied with the low-viscosity plastic material coming from the housing zone 15. This liquefied material wets the introduced additives intimately and results in rapid saturation of fiber material. The initial distribution of the additives forming the additives can be improved by the screw 20 of the relaxation zone 49 being multi-threaded in the area of the impression point 29. This multi-threaded section of the screw 20 extends to approximately the point 18 at which the bypass channels 16 open into the housing zone 19. From there, the screw 20 of the relaxation zone 49 has approximately the same pitch as the plasticizing screw 8, but has a reduced diameter of the core 21 to relax the plastic material it carries. In the housing section following point 18 in the direction of arrow 22, the low-viscosity plastic quantity to which the additives (reinforcing fibers and/or fillers) are added therefore has sufficient time and space to mix with the high-viscosity plastic flow supplied by the bypass channels 16. The aforementioned pretreatment facilitates the mixing of the main flow (high-viscosity melt) carried by the channels 16 with the additive-containing premix.

It is expedient to provide vent openings 30 in the relaxation zone 49 formed by the housing zone 19, through which a degassing of the material flowing in the housing zone 19 can be achieved.

Following the housing zone 19, the plastic material containing the additives passes into a further housing zone 31, in which a screw section 32 is mounted, the core diameter of which increases again to a value which is smaller than the largest core diameter of the plasticizing screw 8. In this way, the plastic material in the housing zone 31 is again placed under higher pressure and finally ejected through a discharge opening 33 in the direction of the arrow 34. Any device for further processing the plastic material containing the additives can be connected to this opening 33, e.g. a

Granulating device or a device for introducing the plastic material into molds, etc.

The bluff body 14 does not have to be formed by a bluff neck.

For example, it is possible to form the bluff body 14 from one or more bluff disks. In the case of a bluff screw, this does not have to be a single piece; it is possible, for example, to form the bluff body 14 from several screw sections connected in series, the flight geometries of which can be different from one another.

As can be seen from Fig. 1, the housing zones 7, 15, 19 and 31 have the same outer and inner diameter. This makes the manufacture of the entire housing much easier. Since the screws 8, 14, 20 and 32 are all connected to one another in a rotationally locked manner, only a single drive 11 is required. However, to simplify manufacture, it is recommended that the individual screw sections be manufactured separately and connected to one another in a rotationally locked manner in a known manner.

The embodiment according to Fig. 2 differs from that according to Fig. 1 in that the plastic material plasticized by the plasticizing screw 8 is subjected to filtering before it is degassed and then fed to the bluff body 14 or the bypass channels 16. For this purpose, a sealing screw 35 is connected to the plasticizing screw 8 in a rotationally locked manner, the screw flights of which have a pitch direction opposite to that of the plasticizing screw 8 and thus form a return thread. As a result, the material supplied by the plasticizing screw 8 in the direction of arrow 12 is forced to flow into a channel 36 in the direction of arrow 37, which channel 36 leads to a filter device 38, from which the filtered material flows back into an additional housing section 41 via a channel 39 in the direction of arrow 40. The filter device 38 is expediently designed as a tandem filter, with two filters connected in parallel and, during normal operation, being simultaneously flown against by the plastic material to be cleaned. As soon as contamination appears, the material flow to one filter can be shut off and this filter can be brought into a cleaning position or backwashed, while the other filter continues to operate normally.

In the housing section 41, a further relaxation zone 52 is formed by a reduced diameter of the core 42 of the screw 43 mounted in it, which is also designed as a degassing zone, so that the vent openings 30 are arranged here. If desired, however, degassing openings can also be provided in the housing section 19 following the bluff body 14.

However, the screw 43 forming the relaxation zone is followed by a screw section 44 which, in comparison to the screw 43 of the relaxation zone 52, has an enlarged core diameter in order to ensure the necessary melt pressure to overcome the channels 16.

As Fig. 3 shows, when processing free-flowing plastic material

A funnel 45 may also be provided for feeding this material in the direction of arrow 46 from above into the opening 5 of the housing zone 7. This can be assisted by having an indentation screw or another indentation tool in the funnel 45.

Claims (10)

1. Device for introducing sensitive additives, in particular those sensitive to shearing or kneading, into thermoplastic plastic material, with a plasticizing zone ( 48 ) in which a plasticizing screw ( 8 ) mounted in a housing ( 7 ) is present for converting the plastic material into the plasticized state, which plasticizing screw ( 8 ) conveys the plasticized plastic material to a screw section ( 20 ) of a relaxation zone ( 49 ) in which at least one additive is fed to the plasticized plastic material via at least one line ( 24 ) and is mixed with the plasticized plastic material by means of a screw ( 20 ), characterized in that between the plasticizing zone ( 48 ) and the relaxation zone ( 49 ) a bluff body ( 14 ) rotating with the plasticizing screw ( 8 ) and shearing the plastic material is arranged in a A damming zone ( 50 ) is present which limits the flow of the plasticized plastic material flowing over it, and that, viewed in the transport direction of the plastic material, at least one channel ( 16 ) for plasticized plastic material branches off in front of this damming body ( 14 ), which bypasses the damming body ( 14 ) and, after the opening point ( 29 ) of the line ( 24 ) for the additive, opens into the screw section ( 20 ) of the relaxation zone ( 49 ) or into a zone ( 31 ) located behind it. 2. Device according to claim 1, characterized in that the damming body ( 14 ) is a damming screw whose flight depth and/or flight pitch is less than that of the plasticizing screw ( 8 ). 3. Device according to claim 2, characterized in that the damming screw ( 14 ) has the same direction of its flights as the plasticizing screw ( 8 ). 4. Device according to one of claims 1, 2 or 3, characterized in that the flight depth of at least a part of the screw section ( 20 ) of the relaxation zone ( 49 ) is greater than the flight depth of the plasticizing screw ( 8 ). 5. Device according to one of claims 1 to 4, characterized in that in the region of the outlet point ( 29 ) of the line ( 24 ) for the additive there is a screw section ( 20 ) with multiple flights compared to the subsequent relaxation zone ( 49 ). 6. Device according to one of claims 1 to 5, characterized in that the relaxation zone ( 49 ) is followed by a screw section ( 32 ) whose flight depth is smaller than that in the relaxation zone ( 49 ). 7. Device according to one of claims 1 to 6, characterized in that the plasticizing screw ( 8 ), the dam body ( 14 ), the screw section ( 20 ) of the relaxation zone ( 49 ) and preferably also the screw section ( 32 ) following the relaxation zone ( 49 ) are arranged coaxially and have a common drive ( 11 ). 8. Device according to claim 7, characterized in that a common housing of constant inner diameter is provided for the plasticizing screw ( 8 ), the bluff body ( 14 ), the screw section ( 20 ) of the relaxation zone ( 49 ) and preferably also for the screw section ( 32 ) following the relaxation zone ( 49 ). 9. Device according to one of claims 1 to 8, characterized in that a stuffing and dosing element, e.g. a screw, in particular a co-rotating twin screw ( 23 ), is provided in the line feeding the additive ( 24 ), which presses the additive into the relaxation zone ( 49 ). 10. Device according to one of claims 1 to 9, characterized in that the relaxation zone ( 49 ) is longer than the accumulation zone ( 50 ).