DE10004146C2 - Arrangement for measuring the spread of a matrix material in electrically conductive reinforcement structures - Google Patents

Description

For the production of fiber-plastic composites (FKV) a Reinforcement structure impregnated with a matrix material (resin system). this happens For example, in processes that come under the generic term resin injection process liquid composite molding (LGM). This is done in a two-part tool a textile reinforcement material dry into the cavity filed. After the mold is closed, vacuum or pressure-assisted Injection of the matrix material, the filling time of the injection pressure, the Component size, the sprue system and that of the reinforcement material Flow resistance depends. After filling the mold, the matrix material hardens, the component can then be removed from the mold.

Glass, carbon and aramid are mainly used as fiber material. The mostly flat reinforcement structures are divided into knitted fabrics, knitted fabrics, scrims and fabrics. All structures have in common that fiber components are present in different directions and these are fixed to one another. A fabric is shown as an example in FIGS. 2 (top view) and 3 (section). Permeability is of particular importance for the characterization of the reinforcement structures. Fig. 4 shows the flow behavior to be examined on a sketch. The matrix material enters the cavity ( 13 ) via a central sprue ( 14 ) and spreads out in the form of an ellipse ( 15 ). Various methods are available for examining the propagation behavior. Above all, the detection of the flow front with the help of the image documentation is used here. However, this requires a shape with a transparent top tool. This leads to restrictions in terms of temperature control and pressure stability of the mold, but has the decisive advantage of being able to determine the flow front of the matrix material at different points in time at any time. This is not possible with systems such as pressure or temperature measurement, which only tap a signal at discrete points. Here the position of the flow font in a main axis must be interpolated in relation to a second one, so that the spread of the flow front can be described using an ellipse. To avoid this problem, a measuring principle based on a capacitor can be implemented.

Such a capacitor consists of two opposite conductive ones Surfaces and has three basic variables. This is the plate size that Plate distance and the dielectric constant of the medium, which is between the plates. In principle, there are three different types of Capacitance change possible with a measurement of the capacitors electrical capacity can be quantified. These variable sizes can also be effective as medium sizes, e.g. B. can the plate spacing or Dielectric constant across the capacitor may not be constant. The measurement of Capacity has long been possible using various methods.

This fact can be used as a secondary measurement principle for an original measuring size can be used. For this purpose, an arrangement is designed so that the size to be measured acts on a capacitor and its capacitance changes. Measuring systems of this type have been known for a long time. Examples are capacitive Distance measuring systems (change in distance), measurement of the overlap of Metal surfaces (change of surface), measurement of the portion of the liquid filled with liquid Capacitor (level sensor), measuring the progress of chemical Reactions by changing the dielectric constant (see e.g. Foldvari, Lion: Capacitive Transducers. Instruments & Control Systems, 11/1964).

From the document DE 28 45 269 A1 a method for determining the Water absorption of a fiber-reinforced plastic component with the help of a capacitive area sensor known. The function of this sensor is to measure the influence of Moisture towards the life expectancy of a fiber-reinforced plastic component determine. The sensor is laminated during component manufacture, or installed, but has no function during component production.

The impregnation of a non-conductive reinforcement structure, e.g. B. from Glass fibers with a matrix material can be measured with this method. For this, the Reinforcement structure placed or pressed between the two capacitor surfaces and by changing the ratio between air or vacuum and The resulting change in capacity is measured. From this one can refer to the Recalculate the proportion of the soaked reinforcement structure if calibration values available.

However, the reinforcement structure in such a capacitor is conductive, e.g. B. from Carbon, metal or conductive coated non-conductive fibers, can this conventional way the spread of the resin cannot be measured because the capacitor is short-circuited and also the conductive surface of the Reinforcement structure an electrical shield against an evaluation of the Represents processes inside the structure.  

The object of the invention is to overcome this difficulty bypass.

To do this, the surfaces of the capacitor plates must be electrically insulated ( Fig. 1). For this purpose, the insulating layers ( 3 ), ( 4 ) are applied or placed on the capacitor plates ( 1 ), ( 2 ) in a suitable manner. Then the reinforcement structure ( 5 ) lies in between at a potential between the two potentials applied to the capacitor.

This arrangement can be regarded as a series connection of four capacitors ( FIG. 5). The first capacitor on each side is formed by the insulating layers ( 3 ), ( 4 ). A further capacitor on each side are the cavities (gusset) ( 7 ), ( 8 ) between the insulating layer ( 3 ), ( 4 ) and the surface of the reinforcement structure ( 5 ). The conductive reinforcement structure ( 5 ) in the middle can also be used a connection ( 11 ) can be contacted.

The capacity of this arrangement depends on the thickness of the insulating layers ( 3 ), ( 4 ) and on the average distance between the surface of the reinforcing structure ( 5 ) and the surface of the insulating layers ( 3 ), ( 4 ). The penetration of the matrix material ( 6 ) into the interior of the reinforcement structure ( 5 ) does not change the electrical situation of the arrangement and the capacitance, since the conductive reinforcement structure acts like an electrical shielding of electrical fields. However, if the cavities (gusset) ( 7 ), ( 8 ) between the surface of the reinforcing structure ( 5 ) and the insulating layers ( 3 ), ( 4 ) are filled with the matrix material, the capacitance of the arrangement changes due to a change in the dielectric constant soaked part of the surface. Even in a highly compressed reinforcement structure ( 5 ), there are sufficient gussets for such a measurement. If a matrix layer is inserted between the directly adjacent part of the surface of the reinforcing structure ( 5 ) and the insulating layers ( 3 ), ( 4 ), the capacitance also changes due to a change in the average plate spacing. This applies to non-conductive matrices.

If the matrix material is conductive, the filled gussets act on the other Insulating layer adjacent surface of the matrix material as a second Capacitor plate. The capacity changes by changing the mean Plate spacing. Only that of the insulating layer is effective as the dielectric constant.  

The penetration of the spreading front ( 12 ) produces a change in the capacitance of the capacitor which depends on the proportion of the soaked surface in the total area and can be calculated back into the size of the soaked surface. Since an effect is used here that is only effective on the surface of the reinforcement structure, the three-dimensional shape of the capacitor plates is not important. Only the area and its length and width extension on the surface are decisive for the measurement.

When the matrix material spreads over the capacitor surface, the Gusset filled up discontinuously. This has pro depending on the gusset density An uneven increase in capacity with the progress of the surface Spreading front result. The effect is with capacitor plates, which are in the Ratios to gusset density are reduced by the large number of gussets. First capacitor sizes that cover only a few gussets are larger Unevenness in the measurement result to be expected.

There are several options for connecting such an arrangement and evaluating the capacities. One possibility is to connect the two capacitor plates ( 1 ), ( 2 ) to their connections ( 9 ) and ( 10 ) and to measure the capacitance ( FIG. 6). Since this is a series connection of capacitors, the total capacitance can be relatively small in smaller arrangements, so that a better and therefore more expensive capacitance measuring unit may be required. This problem can be solved by providing the reinforcing structure ( 5 ) with a connection ( 11 ) and connecting the connections ( 9 ) and ( 10 ) of the capacitor plates ( 1 ) and ( 2 ) together ( FIG. 7). This results in a parallel connection of the two capacitors and thus an increase in the capacitance. If the capacitor plates ( 1 ) and ( 2 ) are the same size, the capacitance increases by a factor of 4. If the measurement of the spread on one side of the reinforcement structure is sufficient, it is also possible to use a capacitor plate ( 9 ) and the connected reinforcement structure ( 11 ). be measured. The connection of the capacitor plate ( 10 ) can remain open ( Fig. 8) or be connected to the same potential as the reinforcing structure ( 11 ) ( Fig. 9). The capacitor between the connection ( 10 ) and the reinforcement structure ( 11 ) is therefore ineffective.

Compared to the measurement in series with the same capacitor plates, this represents a doubling of the capacitance and saves an insulated and connected capacitor plate in the event that the reinforcing structure and the connection ( 10 ) are connected to the same potential.

If it is necessary to separately measure the spread of the matrix material on both sides of the reinforcement structure ( 5 ), the capacitances at the connections ( 9 ) and ( 10 ) can be measured individually against the connection of the reinforcement structure ( 11 ) ( FIG. 10) . This last-mentioned form of measurement can be realized with any number of points of the reinforcement structure to be measured individually and the associated capacitor plates and capacitance measuring units. In this case there is also the possibility of interconnecting capacitor plates ( Fig. 11) in order to achieve further measuring effects (see below).

In order to characterize the spread of a matrix material in a reinforcement structure, it is generally customary to determine the shape and position of the spreading ellipse ( 15 ) starting from the injection point ( 14 ) ( FIG. 4). For this purpose, several capacitors for measuring the spread can be arranged in one tool. Curves 2 . Order (ellipse, hyperbola) are defined in the plane by at least five curve points. In the present zero point symmetry, however, three points are sufficient if they are not zero point symmetrical to one another. A particularly advantageous arrangement for determining these points is shown in FIG. 12, a measuring surface being used here for redundancy. The capacitor plates ( 16 ) to ( 21 ) are increasingly covered by the spreading ellipse ( 15 ), so that the measured increase in capacitance of the corresponding capacitor is proportional to the covered portion of the capacitor. This allows the distance covered by the propagation front over the capacitor to be determined. In the arrangement in FIG. 7, the two pairs of opposing capacitors are electrically connected in parallel, since a point-symmetrical spread of the front is generally expected. This leads to an increase in basic capacity and accuracy. The two mirror-symmetrically arranged capacitors serve to determine the rotation of the ellipse. This can be calculated by offsetting the signals from the two pairs of opposite capacitors. Only when the rotation is known can the complete shape and extent of the propagation ellipse be determined. As a material for the insulated capacitor plates with fixed insulation z. B. anodized aluminum or Teflon-coated steel. It is also possible to insert an insulating film, plate or fabric, scrim or fleece between the capacitor surface and the conductive reinforcement structure as insulation. As a material for an insulating interlayer z. B. Teflon, other plastics or glass, aramid or other non-conductive fibers. It is also conceivable that after impregnation this layer remains on the impregnated reinforcing structure, that is to say it is no longer detached.

When measuring a matrix material with an unknown dielectric constant or a reinforcement structure with an unknown shape of the gusset on the Surface or changed pressure on the reinforcement structure and thus Modified form of the gusset it is necessary to change the proportionality factor between measured increase in capacity and the portion swept by the flow front to determine the capacitor. This is after the complete sweep of the Capacitor areas through the flow front possible by the length of the Capacitor and the increase in capacity are related. For Regulation of the process parameters or for direct measurement of the spread of the Resin or liquid must be used at the beginning of the injection Information is available. One way to determine the proportionality factor at the beginning of the measurement is two capacitors electrically in parallel interconnect, the position of the capacitors is chosen so that the Beginning of sweeping through the industrious front did not occur at the same time. The two Capacitors can for example directly next to each other or to the gate of the resin or the liquid are point symmetrical and by a distance in Propagation direction to be shifted to each other. Then in the curve the a change in capacitance measured over time, at which the slope the curve changes. This change in slope takes place, albeit the second Capacitor is covered by the propagation front. From the well-known route between the beginning of the coverage of the two capacitors and the width of the capacitors, the proportionality factor for the curve sections before and  can be calculated after the kink. In the course of watering after the kink So the spread is calibrated with the proportionality factor and measured Regulation or immediate evaluation can be used. To one in the characteristic To get kink when the propagation front reaches a second capacitor, it is also possible to connect the two capacitors in series. The characteristic is however more difficult to interpret because the series connection makes them non-linear becomes. There are also one or more arranged in the direction of propagation short areas conceivable, those with a longer one in the direction of propagation Main capacitor plate are interconnected, and only while they are are overwritten by a change in slope in the characteristic curve, indicating that the corresponding point was reached by the spread front. Also lets out of this with the help of their surface and the wetted surface of the Main capacitor plate gain a calibration value for the proportionality factor.

There is also the possibility of one in the feed of the matrix material Install a capacitor with which a standardized dielectric constant is measured becomes. For this purpose, the geometry of the capacitor in the supply line is kept constant. A change results from the type of matrix material as a dielectric. This Measurement results are standardized to a system for which the Proportionality factor is known. This becomes a normalized dielectric constant certainly. The results of the measurements during soaking are standardized with the Dielectric constant weighted and deliver with the known Proportionality factor is the spread of the resin or liquid.

In addition, with the arrangement of capacitor plate and reinforcing structure described, it is possible to measure whether a reinforcing structure is inserted into the mold and to what extent the prescribed pressure is applied to the reinforcing structure, that is to say the desired fiber volume fraction has been set. A change in pressure causes a deformation of the gusset between the insulating layer and the surface of the reinforcing structure and thus also a change in the average distance between the surface of the insulating layer and the surface of the reinforcing structure. This changes the basic capacity and the proportionality factor. In a reinforcing structure, the capacitive effect in the arrangement at a certain, e.g. B. the prescribed pressure is known, it can be determined after insertion whether the measured capacity corresponds to the standard capacity for this pressure. It is also possible to calibrate the capacity via the pressure and thus to measure the pressure using this effect. In an arrangement in which the reinforcing structure ( 5 ) and thus its connection ( 11 ) is connected to the connection ( 9 ) or ( 10 ) of the plate ( 1 ) or ( 2 ) and against the corresponding other plate ( 2 ) or ( 1 ) is measured ( FIG. 5), a significantly lower capacity is measured if the reinforcing structure ( 5 ) is not inserted or is inserted in an insufficient amount, which corresponds to an insufficient fiber volume content. The same applies in the event that the fiber volume content is too high. In the event of wrinkling, which leads to non-uniform contact with the capacitor plates ( 1 ) or ( 2 ), a lower capacitance is measured compared to the standard capacitance. This can be used as part of quality assurance in a series production. B. the absence or the crease-free and correct insertion of the reinforcement structure and the application of the prescribed closing pressure are monitored.

The following representations Fig. 1 to Fig. 12 are attached:

Fig. 1: Measuring principle

Fig. 2: Typical structure of a reinforcement structure (top view)

Fig. 3: Typical construction of a reinforcing structure (average)

Fig. 4: Propagation behavior

Fig. 5: Equivalent circuit measurement setup

Fig. 6: Series connection

Fig. 7: parallel connection

Fig. 8: Single measurement with an open second capacitor

Fig. 9: Single measurement with short-circuited second capacitor

Fig. 10: Two individual measurements

Fig. 11: Several individual measurements

Fig. 12: Arrangement of the sensor surfaces

Claims (5)

1. Arrangement for measuring the spread of a matrix material in electrically conductive reinforcement structures and for monitoring the reinforcement structures for fiber volume content, characterized in that the part of the reinforcement structure to be measured is applied to an electrically insulated plate of a plate capacitor, and the change in capacitance between the plate and the Gain structure or between the plate and another electrically insulated plate of a plate capacitor applied to the reinforcement structure is measured with a capacitance measuring circuit. 2. Arrangement according to claim 1, characterized in that a matrix material is infiltrated into the reinforcement structure in such a way that it spreads along the plate of the plate capacitor and the measurement result of Capacitance measurement circuit for determining the spread of the matrix material over the plate of the plate capacitor is used. 3. Device according to claim 1, characterized in that several electrically insulated plates of plate capacitors are arranged so interconnected and the capacity as a measure of the progression of the propagation front with Capacitance measurement circuits that are measured with the measurement results a characterization of the spreading behavior of a matrix material in a electrically conductive reinforcement structure, especially the calculation of the shape and location of an ellipse of propagation.   4. Device according to claim 1 or 2, characterized in that two electrically connected in parallel or series connection Plates of plate capacitors, the position of which is selected so that it is separated from the Front of propagation of the matrix material offset by a known distance is reached be and from the course of the measured capacity, while only one of the Plates are swept from the spread front, calibration values for the further Measurement of the spread of the matrix material can be obtained. 5. Device according to claim 1, 2 or 3, characterized in that by measuring the capacity of one or more plates of Plate capacitors against a reinforcement structure or another plate of a plate capacitor the presence of the reinforcement structure that wrinkle-free occupancy and the fiber volume content or pressure between Reinforcement structure and plate is determined.