WO2005016632A1

WO2005016632A1 - Method for assembling sheet moulding compounds or stacks thereof, precisely in terms of weight, in order to produce smc parts

Info

Publication number: WO2005016632A1
Application number: PCT/EP2004/008374
Authority: WO
Inventors: Uwe Habisreitinger; Bernhard Nordmann
Original assignee: Daimlerchrysler Ag
Priority date: 2003-08-05
Filing date: 2004-07-27
Publication date: 2005-02-24
Also published as: DE10335730B3

Abstract

The invention relates to a method for producing SMC parts from fibrous, reactive sheet moulding compounds, whereby at least one blank (24, 25) is cut out of a sheet moulding compound strip (22) and is provided as a raw material. The position of said raw material is defined in a heated moulding tool (18) of a moulding press, is moulded by extrusion to form an SMC part, and is thermally hardened. The aim of the invention is to meet, in a highly precise manner, the nominal weight stipulated for the raw material to be inserted for each production cycle, despite a variable weight per unit area of the sheet moulding compound strip. To this end, blank parts equal in area are always respectively used to form the raw material, such that the nominal weight of the raw material is adjusted if need be for the maximum weight per unit area of the associated sheet moulding compound strip, but in other cases, however, there is first a mass deficit in relation to the nominal mass. Before the raw material is further treated, the mass deficit thereof in relation to the nominal mass is respectively determined by weighing, and said deficit is respectively made up from a dosing mass, a strand of the same type of a pasty resinous compound (43) and/or small cut particles (30) of the same type of a sheet moulding compound, being distributed on the raw material as a dosing mass in such a quantity that the nominal mass is individually reached.

Description

Process for the precise assembly of resin mats or resin mat stacks for the production of SMC parts

The invention relates to a method for producing SMC parts from fiber-containing, reactive resin mats according to the preamble of claim 1, as for example from the contribution by R. Brussels and U. Weber "SMC parts fully automatic" published in the magazine plastics , Volume 79 (1989), pages 1149-1154 - briefly cited below with [1] - emerge as known.

According to the literature reference [1], a certain amount of a mixture of reactive thermosetting synthetic resin and fibers is assumed in the production of SMC parts, which is weight-matched to the finished component. The coordinated amount of raw material is obtained by cutting out cut parts of a certain size and shape from a fiber mat web (prepreg web) delivered in roll form and by folding the cut parts together into a stack of mats. Such a mat stack is inserted in the correct position in an open mold of a press. The molding tool is heated to a temperature at which the reactive synthetic resin reacts chemically and sets. By initially slowly closing the molding tool in the press, the introduced raw material is initially only heated, as a result of which the synthetic resin becomes soft and flowable. The molding tool is then closed under controlled force and speed, whereby the softened raw material flows away to the side and thereby the cavity of the molding tool Fills the tool completely. After this engraving has been filled in, the mold is kept closed for a while with a defined force, so that the synthetic resin can fully react and harden. Only then can the mold be opened and the finished SMC part removed from it.

The article cited at the beginning [1] refers, among other things, to a fluctuating basis weight of the resin mats. Despite all efforts of the resin mat manufacturers, the resin mat sheets could not be manufactured with sufficient accuracy and / or consistency in the basis weight even today. For this reason, special measures must be taken to ensure that the mass of resin mats introduced into the mold is always of the same size, at least within a certain tolerance, in preparation for each production step of an SMC part. The higher the quality requirements for the finished product, the less the resin mass introduced can scatter around a target value. In [1] it is mentioned that the problem of the fluctuating basis weight of the resin mat web and the consequent problem of the exact dosing of the raw mass can be alleviated if the quality requirements for the finished SMC pressed part could be reduced. However, if the SMC components to be manufactured are thin-walled shell components with high quality requirements, the mass of the raw material to be filled should, if possible, be metered in with a significantly smaller fluctuation range upwards and downwards compared to a target value than the fluctuation range of the basis weight of the resin mat web , If the amount of raw material entered is too small, surface roughness as well as thin and weak points in the component arise, which in extreme cases can be perforated. If, on the other hand, too much raw material is dosed into the mold, the wall thickness becomes at least locally too large, which may lead to component distortion; in any case, overdosed parts are not true to size. Material also swells in the event of an overdose along the parting line of the molding tool, which not only leads to oversized burrs and corresponding additional work when deburring, but also to increased tool contamination and thus to an increase in the side work "tool cleaning", thus overall reducing productivity.

In the automated process for the production of SMC parts described in the literature reference [1], the blank parts stacked up as a raw material to form a stack of mats are all rectangular in shape and all have the same width, namely the width, in one direction transverse to the resin mat web the trimmed Harzmattenbahn itself. The cut parts are produced by cross cutting the resin mat web, using a pneumatically driven high-speed knife which is moved across the resin mat web, which is supported at the point of the cut by a narrow profile. The high-speed knife presumably emerges from the underside of the resin mat sheet to be cut and into a longitudinal slot of the supporting profile. To compensate for a change in the basis weight of the resin mat web, the rectangular dimension of the cut parts lying in the longitudinal direction of the resin mat web is used. When monitoring the target weight of the resin mat stack to be maintained, it is not the cut-off cut parts that are weighed, but the finished SMC part. Depending on the deviation of the finished weight of the SMC part from a target weight, the blank parts for the next SMC part are cut longer, shorter or consistently. A fundamental disadvantage of the control strategy known from [1] for maintaining the target weight of the raw mass to be introduced is that a control intervention to correct the actual controlled variable - raw mass for the nth part - from the target / actual deviation of a variable other than the measured variable - namely finished part mass of the n + lth part - is made dependent. The measurand "finished part mass of the n + lth part" does not have to be representative of the actual control variable "raw mass for the nth part". In [1] an attempt is made to detect possible differences between the two or to predict that the thickness of the resin mat web will be recorded continuously. Because of the fundamental differences between the measured variable on the one hand and the controlled variable on the other hand, a very large proportion of the SMC parts manufactured according to [1] will deviate from the target weight to be achieved; the control strategy known from [1] only works due to such a deviation. Apart from this, a further disadvantage of the method according to [1] is that the cut parts have to be rectangular with a width corresponding to the width of the resin mat web. However, this requirement can only be optimally approved for a limited range of components.

In the case of processes for the serial production of SMC parts, which are frequently carried out manually, the mass of resin mats introduced into the mold is individually weighed, which is also done manually and is very cumbersome for automation of the process. Rectangular blank parts are usually cut out of a quasi-endless fiber mat with a sharp knife on a steel base and weighed. If the target weight of a blank to be inserted into a mat stack is too large, an edge strip or a triangular piece is cut off on a long side of a blank, whereby the target weight is approximated but not exactly achieved. Above all, such a correction greatly changes the target shape of the blank, which has a disadvantageous effect on the molding process and the component quality. If, on the other hand, the target weight of a blank is too low, the next blank to be inserted into the raw mass of the current SMC part is cut slightly larger than the target shape or a small waste part from an earlier correction process is added. This type of correction has an adverse effect on the subsequent the molding process and the component quality. Incidentally, this manual weighing of the raw material quantity can only approximate the target weight with a very large spread, which is hardly less than the weight spread of the resin mats themselves. For this reason, a relatively large number of rejects and quality fluctuations occur in SMC parts production with manual raw material weighing.

EP 461 365 B1 - briefly cited below with [2] - shows a process for the production of molded plastic parts from thermoplastic, in which a weight-matched amount of heated and softened thermoplastic is placed in an open mold of a press by closing of the molding tool, the plastic mass is extruded into the cavity of the molding tool and then the workpiece still in the molding tool is cooled and finally removed from it. The special feature of the method described in [2] is the provision of the heated thermoplastic in a flat preform that is roughly matched to the shape of the cavity of the molding tool, with the molding compound distribution within the preform also being roughly adapted to the requirements of the cavity of the molding tool is. For this purpose, a thin, wide extrudate strand of hot plastic mass is placed on the heated and reversibly driven conveyor belt of a belt scale and weighed at the same time. The extrudate strand is placed on the conveyor belt in a meandering manner and with a variable mass distribution due to a slow oscillating movement of the conveyor belt in the conveying direction or counter to it and due to a specific belt speed that can deviate from the extrusion speed. In this method for the compression molding of thermoplastic, too, the plastic mass to be introduced into the cavity of the molding tool should correspond exactly to a target weight, on the one hand to ensure that the cavity is completely filled, and on the other hand that the molding tool is completely closed without being oversized To allow ridges to form. In the method shown in [2], this is achieved by continuously weighing the extrudate strand deposited on the belt scale. After a part of the extrudate strand still hangs on the extrusion nozzle towards the end of the formation of a preform and the entire plastic mass intended for the preform is not loaded on the balance, the extruded strand is already shortly before the target weight is reached, ie when a certain weight threshold is reached separated in a very short time. If the weight of the preform, which is now completely on the belt scale, lies within a predetermined tolerance range, it is transferred to a downstream conveyor, which in turn deposits it in the opened mold. If, on the other hand, the preform formed is too heavy or too light, it is discarded and its plastic mass is recycled. The weight threshold for triggering strand separation for the next preform is also corrected accordingly, ie if the preform is too heavy, the weight threshold is changed in the direction of a lower threshold weight and vice versa. However, this type of control of the weight of the plastic mass to be introduced into a molding tool cannot be transferred to the processing of fiber-reinforced thermosets, ie not to the portioning of resin mat blanks.

The object of the invention is to improve the generic method in such a way that, despite fluctuating basis weight of the resin mat web, the target weight prescribed for the resin mats to be inserted can be adhered to with great accuracy for each production cycle of an SMC part, without significantly increasing the shape of the individual cut parts change.

Based on the generic method, this object is achieved according to the invention by the characterizing features of claim 1. Then the raw mass is initially made available with a more or less large mass deficit compared to the target mass, using blank parts that always have the same area; at most with the maximum basis weight of the associated resin mat web, the target weight of the raw mass can be achieved in this first process step. This mass deficit is determined in each individual case by weighing prior to further processing of the raw mass in the molding tool, and then additional resin mass is individually metered in from a dosing mass, so that the desired target mass is achieved individually within a narrow tolerance range. In this case, a strand of pasty resin material of the same type and / or small particles cut from resin mats of the same type can be / are distributed as metering mass on the raw material provided.

Expedient embodiments of the invention can be found in the subclaims; otherwise the invention is explained below using various exemplary embodiments shown in the drawings; show:

1 is a schematic overall view of a process plant in a plan view,

2 shows the cutting table with the line tear on a resin mat web for cutting the parts of a five-part resin mat stack,

3 shows a scale for weighing the resin mass to be made available from a plurality of stacked blank parts for a new workpiece, the target weight being controlled by sprinkling small chips of a similar resin mat, 4 shows the course of the basis weight of a resin mat web to be processed over its longitudinal extent in diagram form, and

Fig. 5 shows another scale for weighing the resin composition consisting essentially of only a single blank for a new workpiece, the target weight being controlled by extruding a similar, pasty resin composition.

The method on which the invention is based for the serial production of SMC parts is explained on the basis of the method scheme according to FIGS. The SMC parts are produced from fiber-containing, reactive resin mass, which is provided as a preliminary product in the form of a quasi-endless resin mat web 22 wound up into a supply roll 1. In order to maintain the reactivity of the synthetic resin in the resin mat web 22, the latter is covered with a protective film 26 which is only removed shortly before the resin mat is processed and rolled up to form a separate roll 2. As can be seen more clearly in FIG. 2, the protective film is deflected towards the winding 2 against a direction of processing of the resin mat by means of a turning bar 12 located in the vicinity of the cutting table 3. The side edges of the resin mat web are unsuitable for further processing and must be cut off, which in the exemplary embodiment shown is carried out by a cutting tool 20 with a saw blade 21 driven in a rotationally oscillating manner. The side waste strips 28 cut off at the edge of the web are likewise deflected into waste containers 14 via turning bars 13.

The usable part of the resin mat web 22 is disassembled on the cutting table 3, which is provided with a very hard, seamless glass plate. Cut parts 24 and 25 (FIGS. 2 and 3) of a defined shape and size are cut out and these are stacked to form a multilayer resin mat stack 31 of a certain number of layers and arrangement. If there are waste parts that cannot be used immediately, they are discharged into a corresponding waste container 4 and collected therein. In the exemplary embodiment shown in FIG. 5, the raw material to be inserted - apart from the resin material metered in to achieve the target weight - consists in each case of only a single blank 23.

In the exemplary embodiment shown in the figures, the cutting parts are cut mechanically and automatically by means of a programmable cutting robot 5. In order to be able to cut and shape any desired shaped and / or arranged, in particular differently sized, cutting parts from the resin mat web 22 in an automated and trouble-free manner, a high-frequency rotary oscillating tool is used as the cutting tool Saw blade 21 is recommended, which - driven by an electric tool 20 with an integrated oscillation gear - performs small rotary strokes around a fixed central position. During the cutting process, the resin mat web 22 on the cutting table 3 is supported by a smooth, continuous and shock and joint-free glass plate which is harder than the cutting teeth of the saw blade.

On a separate weighing and stacking device 6 - see also FIG. 3 - the cut parts cut by the robot 5 on the table 3 are stacked to form a resin mat stack 31, the cut parts being handled and moved by a handling robot 7, which in turn is operated with a specially for this task and this substrate trained resin mat gripper 27 is equipped. After the resin mat stack 31 has been formed in the appropriate form for a new workpiece and the latter with respect to its solvent important has been balanced, this is inserted by the handling robot in a defined position in a heated mold 18 of the molding press 8.

The molding tool 18 is closed by the press until the shaping surface of the cavity comes into contact with the inserted resin mat stack and clamped in the closing direction with a defined, initially still small force. Through contact with the hot tool, the resin mass heats up and softens. Due to the closing force of the molding tool, the resin mass begins to flow and thereby completely fills the cavity of the more and more closing molding tool 18. The tool is then held in the closed state for a certain time with increased force, the resin composition thermally curing. After this curing time has expired, the press 8 opens the tool, the finished SMC part generally remaining in the lower, stationary mold half. With a removal robot 9 provided with a removal tool 29, the SMC part can be removed from the press and stored in a cooling station 11. While the cutting and handling robots 5 and 7 prepare a new resin mat stack, the opened molding tool 18 is cleaned by two cleaning robots 10, so that it is ready to receive a new resin mat stack.

Before going into the actual invention in more detail, the underlying problem should be explained in advance in connection with the diagram shown in FIG. FIG. 4 shows in diagram form the course of the basis weight f of a resin mat web, the average basis weight f _{g of} which is assumed to be 400 g / dm ² = 100%. The reason for the fluctuations in weight per unit area of the Harzmattenbahn is its density or density, which is not constant with sufficient accuracy, whereas the thickness of the Harzmattenbahn is quite uniform in the longitudinal and transverse directions. Nevertheless, the fluctuating basis weight of the resin mat web should continue to be spoken of, which is at least correct in the result. FIG. 4 is intended to clarify that the local basis weight fi can deviate significantly upwards and downwards from the average value f _q , it being assumed that the extreme values each deviate upwards or downwards from the average value by the same difference + Δf. In the embodiment shown in FIG. 4, which represents the basis weight of a narrow, longitudinally parallel longitudinal strip within the resin mat web, the spread of the basis weight fi of the resin mat web is assumed to be + 5%, which is quite realistic. This scatter value is too high for the serial production of SMC parts. In the case of qualitatively demanding SMC parts, the actual weight of the raw material inserted in the mold may differ by + 3 - 5 per mil compared to the target weight.

In order to be able to comply with the target weight prescribed for the resin mats to be inserted for each manufacturing cycle of an SMC part with high accuracy, despite the fluctuating basis weight of the resin mat web, the blank part (s) 23, 24, 25 always tailored according to shape and / or size so that the target weight of the raw mass is at most at maximum basis weight (f _q + Δf) of the associated resin mat web 22, whereas in most other cases a more or less large mass deficit compared to the target mass consists. In a first step of the process, an underweight of the raw mass is therefore not only accepted, but in most cases it is even sought. When using a resin mat web, whose local basis weight fi compared to an average basis weight f _{q of} the resin mat web gradually downwards the maximum fluctuation range + Δf fluctuates, the or the cut parts 23, 24, 25 are always cut with the same area with an area F resulting from the following relationship: F = G _so ιι / (fq + Δf). Here Gsoii means the weight of the target mass to be processed for the SMC component. The raw mass made available in the first process step in the form of the blank parts of equal area is thus on average smaller than the target mass to be aimed for by the percentage fluctuation range.

Before the raw material is inserted into the mold 18, the mass deficit of the raw material provided is determined in each individual case by weighing on a balance 15 or 15 '. The deficit determined is metered individually for each individual case from a dosing mass provided, which is individually distributed over the raw mass in such an amount that the desired mass to be aimed for is achieved within a narrow tolerance range. The dosing compound is metered in as long as the cut parts provided are still on the scale, with metering according to the scale display. Accordingly, dosing compound is supplied as long as the target weight has not yet been reached. In the embodiment shown in FIG. 3, small particles 30 cut from resin mats of the same type are used as the dosing composition, whereas in the embodiment according to FIG. 5 a strand 43 made of pasty resin composition of the same type is used. These exemplary embodiments will be discussed in more detail below.

The particles 30 used as dosing mass can advantageously be obtained from the edge strips 28 separated laterally from the resin mat web by cutting them into particles with an edge length of 2-6 cm. The usability of these edge strip particles can be improved by admixing a fiber-free resin. With an edge strip width of, for example, 4 cm, square particles can easily be obtained by cross-cutting the edge strip at a distance of 4 cm. If, due to the workpiece, waste parts between the waste parts cannot be avoided, in particular when multi-layer mat stacks 31 are used as raw material, these waste parts collected in the container 4 can also be processed into particles and can be used appropriately. If the particles used as a dosing mass are obtained from a continuous fiber-reinforced resin mat web, they should be cut out of it parallel and transverse to the fiber course of the fibers embedded in the resin mat web. As a result, the fibers remaining in the particles are least shortened and least damaged.

So that the reactivity of the particles 30 does not change, they are stored in a closed container 35, in which an atmosphere saturated with solvent vapor is maintained. This can e.g. this is done by providing a flat shell or channel separated from the particles and filled with solvent within the container. To meter particles 30 out of the container 35, they are conveyed out of the container 35 by means of a screw conveyor 36 through an opening which can be closed by means of a flap 37.

The weighing and stacking device 6 provided in the process diagram according to FIG. 1 is enlarged in FIG. 3 and shown in perspective. A weighing plate 16 is arranged inside the scale 15, on which a stepped stacking device 17 for the raw material to be provided in multiple layers is fastened. A large basic blank 24 is first deposited in stages on the device, whereupon two smaller blank parts 25 are deposited in each case on the individual steps and a five-part mat stack 31 is thus formed. In the procedure according to the invention, this next a mass deficit compared to the target weight to be maintained. As soon as the last cut part has been put down, which can be recognized for the balance by approximating the weight to the target weight, the deficit mentioned is displayed on the balance 15. At the same time, the dosing process for a feed of dosing compound to compensate for the deficit can be initiated automatically.

In the exemplary embodiment shown in FIG. 3, a horizontally movable container 35 is attached for the feeding of dosing mass, inside which the small particles 30 are stored, which are cut from a similar resin mat web. At the bottom of the container there is arranged a screw conveyor 36 which can be driven in rotation and which extends towards an end face of the container. There is an opening corresponding to the cross section of the screw conveyor, which can be closed by a flap 37. To add particles 30 to the resin mat stack 31, the flap 37 is opened and the screw conveyor is rotated so that the particles are conveyed out of the opening and remain on the uppermost layer of the resin mat stack. By slowly traversing the container horizontally during dosing according to the arrows indicated, the ejected particles are evenly distributed on the surface of the resin mat stack. Shortly before the target weight is reached, the flap 37 is closed and the screw conveyor is stopped; the particles still in the trap completely fill the target weight. The traversing device then returns the container 35 to the starting position, so that it is ready for a new dosing process.

In the exemplary embodiment illustrated in FIG. 5, the raw mass to be introduced into the molding tool is only provided in a single blank 23 on the weighing plate 16 'of the balance 15'. Here, too, a more or less large mass deficit is usually found. In the process 5, the mass deficit is compensated for by metering in a pasty mass, which is deposited in the form of an extruded strand 43 on the blank. When the extrudate strand is put down, the balance 15 'and with it the cut part are traversed at a speed corresponding to the exit speed of the extrudate strand - meandering horizontally - and thus distributed evenly on the top of the cut part. The extruded, fiber-reinforced plastic material is mixed and extruded from the two components of the plastic, which are stored separately, in separate containers 42 and 42 ', respectively, in a twin-screw extruder 40. Fibers are already mixed in at least one of the two components, but preferably in both, so that the extrudate strand is also fiber-reinforced. For the rapid closure of the extruder, the latter is provided with a closure head 41 which cuts off the extrudate strand at the exit point, for which purpose a closure needle projecting axially into the exit opening can be provided. In addition, the extruder must be able to be opened easily so that it can be cleaned of any plastic residues that have reacted within the extruder.

In the device according to FIG. 5, too, the dosing process is started automatically by the scale 15 ', namely as soon as the blank is placed on the weighing plate 16' and is also stopped automatically shortly before the target weight is reached. The lead at which metering is stopped is matched to the weight of the vertical part of the extruded strand that has not yet been placed on the weighing plate.

Thanks to the individual replenishment of plastic mass on the provided raw mass, its target weight can be adhered to precisely and reproducibly within a narrow tolerance range in each individual case. A cumbersome change in the surface area of the cut parts, which is dependent on the web weight, is not necessary for this. It can SMC parts are also produced to a high quality level. In addition, residual waste from the production process can be put to good use. In this way, not only primary material can be saved, but also disposal costs can be avoided.

Claims

claims

Process for the serial production of the same SMC parts from fiber-containing, reactive resin mats, in which at least one blank part is cut out of a quasi-endless resin mat web for a specific SMC part and determined as a single-layer raw material or - after stacking several blank parts into a resin mat stack Number of layers - a multi-layer raw mass is provided, which raw mass is shaped into the SMC part after extrusion of the heated raw mass in the closing mold after it has been inserted into a heated mold of a molding press, this is then thermally cured in the closed mold and the fully hardened SMC Part is taken from the opened mold, characterized by the common features of the following:

|> the blank (s) (23, 24, 25) forming the raw mass is or are always cut according to shape and / or size in such a way that at maximum surface weight (f _q + Δf) the associated resin mat web (22) sets the target weight of the raw mass, in most other cases, but initially there is a more or less large mass deficit compared to the target mass, t> before inserting the raw mass into the molding tool (18) in each individual case the mass deficit of the provided raw mass compared to the desired target mass determined by weighing (16, 16 '), D> the determined deficit of raw mass is metered individually for each individual case from a dosing mass, with a strand (43) of pasty resin mass of the same type and / or small as dosing mass , particles (30) cut from resin mats of the same type are distributed on the raw mass in such an amount that the target mass is reached individually.

2. The method according to claim 1, characterized in that when using a resin mat web (22) whose local basis weight (fi) compared to an average basis weight (f _q ) of the resin mat web (22) up and down by the maximum fluctuation range (± Δf ) fluctuates, that the cut part (s) (23, 24, 25) is or will be cut consistently with such a surface area (F) that results from the relationship F = G _so ιι / (f _q + Δf) , where G _means the weight of the target mass to be processed for the SMC component.

3. The method according to claim 1, characterized in that the dosing mass (30, 43) is metered in as long as the raw mass provided is still on the balance (15, 15 ') on which the mass deficit compared to the raw mass has already been determined, and that according to the display of this balance (15, 15 ') is metered in during the metering phase.

4. The method according to claim 1, characterized in that the particles used as a dosing mass, obtained from an endless fiber-reinforced resin mat web, are cut out of it parallel and transversely to the fiber course of the fibers embedded in the resin mat web. Method according to claim 1 or 4, characterized in that the particles (30) used as dosing mass are stored in a closed container (35) in which an atmosphere saturated with solvent vapor is maintained.

Method according to Claim 5, d a d u r c h g e k e n n e e c i n h e t s ds s for dosing particles (30) from the container (35) these are conveyed out of the container (35) by means of a screw conveyor (36) through an opening which can be closed by means of a flap (37).