DE102020111297A1 - Process for the production of a reinforcing fiber wadding for the production of fiber reinforced composite materials and fiber wadding device - Google Patents

Abstract

Method for producing a reinforcing fiber wadding for the production of fiber-reinforced composite materials, with at least the following steps: a) Feeding a quantity of loose reinforcing fibers and / or chopped off roving sections of reinforcing fibers into a funnel (10) which has a funnel opening (11) on its underside. b) formation of a reinforcing fiber wadding by: - separation of the loose reinforcing fibers and / or the roving sections emerging from the funnel opening (11); - and / or connection of reinforcing fibers by swirling by means of a knife element (22) and / or a rotating swirling element, which rotates below the funnel opening (10) in a wadding space (26).

Description

The invention relates to a method for producing a reinforcing fiber wadding for the production of fiber-reinforced composite materials with the features of the preamble of claim 1 and a fiber wadding device therefor.

In a fiber-reinforced composite material, tensile stresses are absorbed by suitable reinforcing fibers in order to relieve the matrix material in which the reinforcing fibers are embedded. As a result, significantly greater mechanical strengths can be achieved than with unreinforced materials of the same type. The reinforcing fibers, which usually consist of glass or carbon, are mixed into a fluid that is either the melted thermoplastic matrix material or at least one of several reaction components that can be hardened by chemical reaction.

For an effective improvement in the mechanical strength, a homogeneous distribution of the reinforcing fibers is necessary, and the reinforcing fibers must be made as long as possible. Both inhomogeneities with insufficient fiber content and agglomerates of fibers must be avoided. While long fiber bundles, fabrics or mats can be laid out with reactive resins before the matrix is formed, this is not possible with thermoplastic materials. With these, there are great difficulties embedding the reinforcing fibers in the thermoplastic matrix because, on the one hand, homogeneous distribution is only possible through pressure and / or vigorous mixing and kneading of the melt, which on the other hand results in high shear forces that considerably shorten the mixed-in reinforcing fibers and possibly also damage the matrix material. Thus there is a classic conflict of objectives between homogenization and material protection.

The object of the invention is thus to prepare reinforcing fibers in such a way that a simpler production of fiber composite materials is possible and / or the strength of the fiber composite materials is increased by a greater length of the embedded fibers.

This object is achieved by a method for producing a reinforcing fiber wadding for the production of fiber-reinforced composite materials with the features of claim 1.

Essential to the invention is the approach of not providing certain reinforcing fibers for mixing into a matrix material in a loose bed, but rather to form a cotton-wool-like, spin-shaped configuration therefrom.

The reinforcing fibers are severely separated by the method according to the invention when they are fed to the method as a bundle, and / or they are felted together when they have entered the process loosely, so that at the end they assume a loose, cotton-wool-like configuration which has a has a very low bulk density. The fibers have a slight connection to one another, so that the cotton-wool-like configuration is retained during further processing. On the other hand, the bond between each other is not so strong that great forces have to be applied in order to divide the wadding flakes or to separate several flakes from one another.

Reinforcing fibers, such as in particular glass fibers, are fed to the method according to the invention as bundles. These are shortened to a certain length either beforehand or online at the beginning of the procedure. In particular, the reinforcing fibers are in bundles, in the form of so-called rovings. The rovings are preferably already shortened before the beginning of the wadding, so that chopped off pieces of roving, so-called chopped stands, are used to carry out the process.

• - Die Schüttdichte ist um den Faktor 5 bis 20 erhöht;
• - Die volumenspezifische Oberfläche ist um den Faktor 5 bis 50 erhöht;
• - Die Faserlänge am Ende des Wattierungsprozesses ist lediglich um 0,5 bis 1,0 mm gegenüber der Faserlänge bei Aufgabe in den Prozess reduziert.

The following exemplary properties are characteristic of the reinforcing fiber wadding produced by the method according to the invention; these can be seen in comparison with the fiber bundles preferably fed into the process, in particular in the form of the chopped stands:

• - The bulk density is increased by a factor of 5 to 20;
• - The volume-specific surface is increased by a factor of 5 to 50;
• - The fiber length at the end of the wadding process is only reduced by 0.5 to 1.0 mm compared to the fiber length when fed into the process.

The cotton-wool-like configuration that can be achieved with the invention is also characterized, as can be seen from the feature of enlarging the volume-specific surface, that the openings between the reinforcing fibers are so large that they cannot only be penetrated by low-viscosity fluids such as water. Rather, they can also be penetrated without pressure by medium to high viscosity fluids such as a thermoplastic plastic melt or a gel-like reaction resin, possibly by capillary action.

• - Eine sehr große Faserlänge von mehr als 5 Millimeter, wäre für die Festigkeit eines daraus hergestellten faserverstärkten Verbundwerkstoffs zwar vorteilhaft. Andererseits müssen die Verstärkungsfasern, die über die Messerelemente oder andere rotierende Vereinzelungselemente vereinzelt werden, gut auflockerbar sein. Auch dürfen die internen Bindungen in der entstehenden Faserwatte nicht zu stark werden, damit eine Teilentnahme von Faserbauschen aus dem Pufferspeicher noch möglich ist.
• - Zu kurze Verstärkungsfasern hingegen lassen sich nur schlecht zu einem watteartigen Gebilde verfilzen und bringen bei der Herstellung eines faserverstärkten Verbundwerkstoffs unter Nutzung der nach der Erfindung hergestellten Faserwatte nicht die gewünschte Festigkeitssteigerung.

The fiber length when fed into the wadding process should be selected from the following points of view:

• - A very large fiber length of more than 5 millimeters would be advantageous for the strength of a fiber-reinforced composite material produced from it. On the other hand, the reinforcing fibers, which are separated by the knife elements or other rotating separating elements, must be easy to loosen. The internal bonds in the resulting fiber wadding must also not become too strong, so that partial removal of fiber tufts from the buffer store is still possible.
• Reinforcing fibers that are too short, on the other hand, are difficult to felt into a cotton wool-like structure and do not bring about the desired increase in strength when producing a fiber-reinforced composite material using the fiber cotton wool produced according to the invention.

From these points of view, an average fiber length of the reinforcing fibers present in the wadding form of 3 millimeters to 5 millimeters has proven to be particularly suitable.

According to the invention, an extremely loosened fiber configuration is obtained. The padded fiber bundles which are produced according to the invention have the advantage of being flowable and in particular being able to be fed continuously or discontinuously into a fluid, in particular into a plastic melt.

A fiber wadding device suitable for carrying out the method is specified in claim 8.

A “funnel element” within the meaning of the present invention is any feed container that has at least one opening on the underside and is designed in terms of its shape so that the reinforcing fibers placed therein can slide or fall into this opening. In addition to conical funnel shapes with a round cross section, pyramid or wedge shapes are also conceivable.

According to the invention, a “knife element” is a rotating element which has flat elements protruding radially from a drive shaft. These elements only need sharpened blades if the fibers are to be shortened when the reinforcing fibers pass from the funnel element into the milling unit. If the added reinforcing fibers or bundles thereof have already been pre-cut, the main purpose of the knife element is the separation. which does not necessarily require sharp knife blades.

The air flow provided according to an advantageous embodiment exerts a force on the fiber wadding present in the buffer store and enables it to be completely removed from the buffer store. In particular, the air flow makes it possible to divide the cotton wool-like fiber bundle into partial portions. The gate valve, which closes the buffer storage unit downwards, only needs to be partially opened in order to release part of the fiber wadding from the buffer storage unit by means of the air flow. Another portion remains in the buffer store so that all reinforcement fibers or fiber bundles newly released from the mill unit can dock directly to it. The wadding and felting process, once initiated, can therefore be continued continuously, and there is no need to wait for another wadding in the buffer store after removal.

It is particularly advantageous if a discharge channel is arranged below a discharge opening of the buffer store, through which a sucking air flow acts on the discharge opening of the buffer store.

The fiber wadding can be discharged from the discharge unit via a stuffing piston, provided that intermediate storage and / or packaging is provided.

For continuous further processing, it is advantageous to provide a discharge unit with at least one screw shaft, in particular a discharge unit with a pair of tamping screw shafts rotating in the same direction. As a result, the fiber bundles produced are drawn in length and the output is evened out.

At least one stirring element rotating within the buffer store is preferably provided. This distributes the reinforcing fibers on the intake gap between the tamping screw shafts rotating in the same direction and thus draws them in better. Furthermore, after the initial production of the wadding, by means of a significantly reduced shearing entry compared to the knife, for example by means of an additional stirrer element, more extensive wadding of the fibers can be achieved.

The device according to the invention also has the advantage that continuous and discontinuous partial processes can be coordinated with one another in such a way that a continuous discharge of fiber bundles is possible. The discontinuous sub-process, which includes the separation and accumulation of a sufficient amount of reinforcing fibers in the buffer storage, is coupled with the discharge via the motor-controlled gate valve so that a continuous discharge of individual padded fiber bundles is possible. In particular, the rotational speed of the cutting blade and the rotational speed of the stuffing screw shafts as well as the stroke frequency of the stuffing piston as a function of the loss of mass of added reinforcing fibers measured at the funnel. For this purpose, the mass flow of the fibers initially fed in as well as the padded fibers conveyed away by the screw shaft is measured gravimetrically. For example, the conventional "loss-in-weight" technology is used. The coupling of these two mass flows through the buffer is particularly advantageous for the overall process. Its filling level is determined by the opening amplitude and frequency. The filling level can be kept constant with the help of optical systems or with the help of a third scale by adjusting the opening amplitude and frequency accordingly during calibration.

Direct further processing is made possible, for example, in the form of inline compounding of a fiber-reinforced thermoplastic material for plastic injection molding processing. For this purpose, the discharge unit of the fiber wadding device can be connected directly to the side of an extruder in which a thermoplastic such as polyamide is first melted and homogenized. After homogenization, the pressure in the melt is almost completely reduced in a decompression zone. The application unit ends in the extruder exactly where the decompression zone is located, so that the padded fiber bundles can be mixed directly into the plastic melt that is present at this point without pressure or almost without pressure. Due to the large cavities and openings between the reinforcing fibers already described, the melt can flow into the padded fiber bundles without pressure. Clearly speaking, the fiber bundles have a certain absorbency, which enables problem-free wetting by the liquid plastic, so that in the inline compounding using the wadding method according to the invention, a fiber-reinforced plastic is obtained in which the fiber lengths are still present as they are in the fiber wadding device were manufactured. By feeding the reinforcing fibers into the plastic melt without pressure, damage in the form of fiber shortening is avoided. In addition, drive power in the discharge unit is avoided. The discharge unit of the fiber wadding device, which is directly connected to the extruder, is used in the manner of a stuffing screw, but there is no need for a stuffing in the sense of pushing it with force.

If the reinforcement fiber wadding is formed in the course of inline compounding, it is advantageous to heat the reinforcement fibers and / or the reinforcement fiber wadding produced. By feeding in fibers heated to melt temperature, local freezing of the melt on contact with the reinforcing fiber wadding is avoided. This ensures that the matrix material flows completely into the reinforcing fiber wadding and that it is not enveloped as a fiber bundle with air pockets.

A heatable funnel, for example, can be used for preheating.

• 1 eine Faserwattierungsvorrichtung nach der Erfindung im Schnitt;
• 2 eine Ausschnittsvergrößerung aus 1 und
• 3 die Massenströme in der Faserwattierungsvorrichtung in schematischer Ansicht.

The invention is explained in more detail below with reference to the exemplary embodiment shown in the drawings. The figures show in detail:

• 1 a fiber wadding device according to the invention in section;
• 2 an enlarged section 1 and
• 3 the mass flows in the fiber wadding device in a schematic view.

• - einem Trichter 10, der sich zu einer unten gelegenen Trichteröffnung 11 verjüngt;
• - einer Mühleneinheit 20;
• - einer Puffereinheit 30 und
• - einer Austragseinheit 40.

1 Fig. 3 shows the construction of a fiber wadding device 100 on average. This essentially consists of:

• - a funnel 10 leading to a funnel opening below 11 rejuvenates;
• - a mill unit 20th ;
• - a buffer unit 30th and
• - a discharge unit 40 .

The mill unit 20th is located directly under the funnel opening 11 . It has a housing in which an annular gap 21 is trained. In its center is a warehouse 24 for one drive shaft 23 intended. Immediately below the annular gap 21 is located in a wadding room 26th a rotating knife element 22nd that is about the drive shaft 23 is driven.

Below the mill unit 20th is a buffer unit 30th with a buffer tank 32 provided, which is formed in the illustrated embodiment as a cylindrical space which has approximately the same diameter as the funnel opening 11 and the wadding room 26th the mill unit 20th about that. The wadding room 26th is down through a gate valve element 31 compared to the storage tank below 32 lockable. The gate valve element 31 is movable via a further servomotor, not shown here. Via the opened locking slide element 31 can fiber bundles from the wadding room 26th down into the buffer tank 32 be delivered before the wadding room 26th is then closed again at the bottom in order to produce fiber wadding again.

In the buffer tank 32 is a stirrer element 33 arranged. This is essentially used to store those in the buffer memory 32 amount of fiber wadding present over the cross section of a feed opening 44 the discharge unit 40 to distribute evenly and to continue padding them gently. The stirrer element 33 is through the drive axle 23 also driven, with a gearbox in between for speed decoupling 34 is provided.

The discharge unit 40 is in the illustrated embodiment as a twin screw pair with two screw shafts 42 formed by a motor and gear unit 43 are driven in the same direction. The one in the discharge channel 41 arranged worm shafts 42 fill the cross-section of the discharge channel 41 essentially without touching the wall. The air gap between the outer edges of the screw flights of the screw shafts 42 and the inner wall of the discharge channel 41 should be at least 0.05 millimeters.

The preferred embodiment of the fiber wadding device 100 , in the 1 also sees a suction unit 46 before, which via a vent 45 with the discharge channel 41 communicates. The vent 45 is located below the feed opening 44 so that over the suction unit 46 a sucking air flow can be generated which acts on the buffer storage 32, in which the stirrer element 33 is arranged. During the gate valve 31 is open, the airflow also acts directly on the contents of the wadding space 26th .

• In den Trichter 10 werden trockene Verstärkungsfasern wie beispielsweise Glasfasern in Bündelform eingelegt. Diese rutschen nach unten in die Trichteröffnung 11 und von dort in den konisch zulaufenden Ringspalt 21 der Mühleneinheit 20. Die Verteilung der Verstärkungsfasern auf den Ringspalt ergibt sich im oberen Teil bereits durch die rotierende Antriebswelle 23, welche sich durch das Zentrum des Trichters 10 erstreckt. Beim Eintritt in die Mühleneinheit 20 wird durch deren Trichteröffnung 25 an der Außenseite wie durch eine Kegelform einer Lagereinheit 24 für die Antriebswelle 23 im Zentrum bewirkt, dass die Verstärkungsfasern gerichtet und vereinzelt in den Ringspalt 21 gelangen. Die in den Trichter 10 eingefügten Faserbündel bewegen sich allein aufgrund ihrer Masse, also ohne äußere Zwangskräfte, zur Mühleneinheit und werden dort kraftschlüssig in den Spalt eingezogen. Die Variierung des Spaltes zwischen Mühleneinheit und Zylinderwand ermöglicht die Adaptierung für verschiedene Faserdurchmesser.

The function of the fiber wadding device 100 According to the invention, with reference to the enlarged detail view in 2 explained:

• In the funnel 10 dry reinforcing fibers such as glass fibers are inserted in bundles. These slide down into the funnel opening 11 and from there into the conical annular gap 21 the mill unit 20th . The distribution of the reinforcing fibers over the annular gap results in the upper part from the rotating drive shaft 23 which extends through the center of the funnel 10 extends. When entering the mill unit 20th is through their funnel opening 25th on the outside like a cone shape of a bearing unit 24 for the drive shaft 23 in the center causes the reinforcing fibers to be directed and isolated in the annular gap 21 reach. The one in the funnel 10 Inserted fiber bundles move solely due to their mass, i.e. without external constraining forces, to the mill unit and are drawn into the gap there with a force fit. The variation of the gap between the mill unit and the cylinder wall enables adaptation for different fiber diameters.

Below the annular gap 21 rotates the knife element 22nd . The one from the annular gap 21 protruding fiber ends are from the knife element 22nd and fall into the wadding area below 26th . Already because of the rotation of the knife element 22nd and the air flow generated thereby causes turbulence in the reinforcing fibers, which lie against one another and create a spin-like or cotton-wool-like structure made of reinforcing fibers.

The gate valve 31 , which is the bottom of the buffer tank 32 forms, remains closed for a certain period of time, during which the separation from the bundle as well as the accumulation and felting of separated reinforcing fibers is made possible, which results in the formation of the reinforcing fiber wadding. After the preset time has elapsed, the locking slide opens 31 , but preferably not opening completely and not the entire cross-section of the buffer store 32 releases towards the bottom, but only from about 30% to 50% of the cross-sectional area per stroke. This makes it possible for parts of the fiber bundle produced to fall out, but at the same time for parts of the wadding space 26th remain so that the wadding process can continue.

So that a part of the produced fiber wadding from the buffer storage into the discharge unit 40 can be transferred, it must be deducted. The bonds between the individual reinforcing fibers are not particularly strong in themselves, but in total lead to the fact that the weight alone is insufficient for this from the buffer storage 32 to remove. For this purpose, the sucking air flow is used, which is generated by the blower unit 46 out through the openings one above the other 45 and 44 in the housing of the discharge unit 40 right up to the gate valve 31 acts up.

The one below the locking slide 31 arranged stirrer element 33 causes in addition to the air flow that from the half-open gate valve 31 emerging fiber bundles are conveyed away directly to the side. The main task of the stirrer element 33 however, consists in placing the fiber bundle in the feed opening 44 to distribute evenly and thus an indentation in the gap between the two worm shafts 42 to force. The drawn in fiber bundles are then over the screw shafts 42 conveyed away to the left and fed to another device for direct further processing or for packaging.

3 Fig. 3 shows the three in the fiber wadding device 100 separately controllable mass flows ṁ ₁ , ṁ ₂ , ṁ ₃ .

An input mass flow ṁ ₁ is determined by the speed of the drive shaft 23 in the mill unit 20th influences, which at the same time also affects the speed of the knife elements 22nd in the wadding room 26th is determined. The input mass flow ṁ ₁ can be determined gravimetrically, e.g. B. by load cells on the funnel element 10 or at the transition between the units 10 , 20th and 30th the fiber wadding device 100 and the discharge unit 40 .

An output mass flow ṁ ₃ is given with a given geometry of the screw shafts 42 determined solely by their speed. It can be through one of the fiber wadding devices 100 downstream gravimetric device such as a further dosing funnel can be measured.

The input mass flow ṁ ₁ and the output mass flow ṁ ₃ must be synchronized. This makes the draining like clogging the wadding space 26th avoided.

By having one over the locking slide 31 lockable buffer space 32 is provided, an intermediate mass _{flow ṁ 2} can be controlled, through the periodic opening and closing of the gate valve 31 . The intermediate _{mass flow ṁ 2} results as a function of the stroke frequency and stroke width of the gate valve 31 at the discharge opening. These parameters can be regulated within certain time limits solely on the dependence of a locally observed degree of filling in the wadding chamber, the superordinate synchronization with the input mass flow ṁ ₁ and / or the output mass flow ṁ _{3 being} required.

Claims (13)

Process for the production of a reinforcing fiber wadding for the production of fiber-reinforced composite materials, with at least the following steps: a) feeding a quantity of loose reinforcing fibers and / or chopped off roving sections of reinforcing fibers into a funnel (10) which has a funnel opening (11) on its underside; b) Forming a reinforcement fiber wadding by: - Separation of the loose reinforcing fibers and / or the roving sections emerging from the funnel opening (11); and / or connection of reinforcing fibers by swirling by means of a knife element (22) and / or a rotating swirling element which rotates below the funnel opening (10) in a wadding space (26). Procedure according to Claim 1 , characterized by the following further steps: c) Periodic transfer of the reinforcing fiber wadding from the wadding space (26) into a buffer store (32) by periodically opening a locking slide (31) arranged between them and d) discharging the reinforcing fiber wadding from the buffer store (32) ) by means of an air stream. Procedure according to Claim 2 , characterized in that a discharge channel (41) is arranged below a discharge opening of the buffer store (32) through which an air flow acts on the discharge opening of the buffer store (32). Procedure according to Claim 3 , characterized in that the reinforcing fiber wadding is discharged via a pair of coring screw shafts (42) rotating in the same direction, which are arranged in the discharge channel (41), the reinforcing fiber wadding being conveyed into a feed gap between the screw shafts (42) by means of the air flow . Procedure according to Claim 4 , characterized in that the reinforcing fiber wadding is distributed on the feed gap by means of a stirring element (33) rotating within the buffer store (32). Procedure according to Claim 3 , characterized in that the reinforcing fiber wadding is discharged by means of a stuffing piston arranged in the discharge channel. Method according to one of the Claims 4 until 6th , characterized in that - the speed of the knife element (22), - the stroke frequency of the locking slide (31) and / or the opening cross-section which is variable over the stroke and - the speed of the stuffing screw shafts (42) or the stroke and the stroke frequency of the stuffing piston are regulated in this way are that an input mass flow ṁ ₁ at the funnel (10 _{) corresponds to an output mass flow ṁ 3} at the end of the output channel (41). Fiber wadding device (100) for carrying out the method according to one of the preceding claims, at least comprising: - A funnel (10) which has a funnel opening (11) on its underside, - A mill unit (20) with at least one knife element (22) rotating below the hopper opening (11); - A buffer store (32) arranged below the knife element (22), a motor-driven locking slide (31) being arranged below the buffer store (32); - An air blower (46) with a pressure line and / or suction line acting on the buffer store (32). Fiber wadding device (100) according to Claim 8 , characterized in that a rotatable stirring element (33) is arranged between a discharge opening of the buffer store (32) and the feed opening (44) on the discharge channel (41). Fiber wadding device (100) according to Claim 9 , characterized in that the knife element (22) and / or the swirling element as well as the stirring element are driven via a common drive shaft (23) and are speed-decoupled from one another via a gear (34) arranged between them. Fiber wadding device (100) according to one of the Claims 8 until 10 , characterized in that in the funnel opening (11) and / or in the mill unit (20) an annular gap (21) tapering in the direction of fall is formed, in the center of which a bearing element (24) for a knife element (22) and / or the swirl element connected drive shaft (23) is arranged. Fiber wadding device (100) according to one of the Claims 8 until 11 , characterized : - that below a discharge opening of the buffer store (32) a discharge channel (41) aligned transversely to the direction of fall of the reinforcing fiber wadding is arranged with a feed opening (44) and - that a suction opening (45) of the air blower (46) is on the the feed opening (44) of the buffer store (32) is arranged opposite the side of the discharge channel (41). Fiber wadding device (100) according to Claim 11 , characterized in that a pair of counter-rotating stuffing screw shafts (42) is arranged in the discharge channel (41).

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