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New thermoplastic composite preforms based on split-film warp-knitting
PII: S1359-835X(98)00086-4
Composites Part A 29A (1998) 1511–1523 1359-835X/98/$ - see front matter 䉷 1998 Published by Elsevier Science Ltd. All rights reserved.
New thermoplastic composite preforms based on split-film warp-knitting H. Stumpf a,*, E. Ma¨der b, S. Baeten c, T. Pisanikovski d, W. Za¨h e, K. Eng f, C.-H. Andersson d, I. Verpoest c and K. Schulte a a
TU Hamburg-Harburg, Denickestrasse 15, D-21073 Hamburg, Germany Institut fu¨r Polymerforschung, Dresden, Germany c Katholieke Universiteit Leuven, Department MTM, Leuven, Belgium d Pro Eng Co AB/Lund Institut of Technology, Lund, Sweden e Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen, Germany f Engtex AB, Mulsjo¨, Sweden (Received 2 January 1998; accepted 6 May 1998) b
A newly developed type of dry thermoplastic textile preform incorporating non-crimp glass fibre reinforcements and matrix material in the form of split-film is presented. Weft-inserted warp knitting has been chosen as a textile production technique for its low cost. A specialized glass fibre/polypropylene matrix system has been proven to perform favourably in melt impregnation and to provide good composite properties. Some of the processing techniques to be applied to the new textile preform are presented, one of which is the QUIKTEMP concept for fast heating and cooling of tools for thermoplastic moulding. Composite plates produced from preliminary splitwarpknit structures reveal a good potential for cost-saving while reasonable mechanical properties can be maintained. 䉷 1998 Published by Elsevier Science Ltd. All rights reserved. (Keywords: thermoplastic composites; textile preform; split-warpknit; fast heating/cooling)
It is often stated that continuous fibre reinforced composites with the currently available thermoset matrices will never become a competitive alternative in mass applications to more conventional materials such as steel and aluminium. A major reason for this finding lies in the long cycle times associated with thermoset composite production. Consequently, a vast number of activities and projects currently pursued address the development and market introduction of cost-efficient thermoplastic composites. The latter bear quite some potential to achieve total cycle times in the order of 2 min or less, where cycle time is defined as the period of time for which one specific part stays in the process. When considering cost, it seems essential to have any kind of textile preform since it will hardly be economical in mass production to put in reinforcement rovings one by one, at least as long as continuous fibres are considered for reinforcement. Several developments now focus on the inclusion of both reinforcement fibres and matrix material in one textile preform structure. Clearly, the merit of such a structure is its great drapability combined with a
comparatively low cost, especially when evaluating the process as a whole. These textiles offer the chance to make the eventual composite part directly from the preform in one step. Hence, no intermediate processing stages (preimpregnation, pre-consolidation in a double-belt press or similar) are needed. Figure 1 displays the substantial advantage of this new method to make a composite over the more conventional methods with various intermediate stages, like thermoforming of organosheets. Special moulding techniques that allow the heating of the dry textile in the mould can ease the production of parts from dry textile preforms, although they are not mandatory. The materials are usually also processable in the classical GMT-like manner (heating outside the press, then compression in a cold mould). This issue will be addressed below. Commingled rovings have now become quite popular 2–4 for making many kinds of textiles, such as woven, knitted and braided preforms. In most applications, they replace pure reinforcement rovings. Due to their inherently even mixture of reinforcement fibres and matrix material, they usually yield a homogeneous distribution of fibres in the composite. They do, however, suffer from two problems:
* Corresponding author now at: Lufthansa Technit AG, Department FRA WS 3, Lufthansa Base, D-60546, Frankfurt/Main, Germany. Tel.: +49 69 696 94086, fax: +49 69 696 3103, e-mail: [email protected].
• relatively high cost of production, depending on the type of commingling applied; • inherent tendency to yield micro-waviness of the
INTRODUCTION
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Split–film warp–knitted composite preforms: H. Stumpf et al. reinforcement fibres, in case continuous matrix yarns are used. The latter is due to the fact that highly prestretched continuous thermoplastic yarns tend to contract when they are heated (‘memory effect’). The reinforcement yarns, on the other hand, contract little or not at all when the preform is heated. This leads to kind of a thermal misfit, possibly resulting in some waviness of the longitudinally stiff reinforcement fibres. This problem can be reduced by selection of an adequate matrix recipe and a low stretch ratio for the matrix yarns. However, a certain extent of stretch is necessary to ensure a sufficiently high tenacity of the yarn and thus its processability in a textile machine. It was the objective of the work currently presented to investigate alternatives for making a dry textile thermoplastic composite preform. Split-films, hence ribbons of a rather small cross-section, were chosen as a thermoplastic constituent to bring the matrix into a textile preform. Warp knitting was identified as an inexpensive and efficient method to produce preforms carrying uni- and multiaxial continuous reinforcement fibres. Pure warp knits from split-films (without reinforcement fibres) are actually widely used in the consumer goods industry. They serve, for instance, as a material to pack fruits and vegetables to be sold in grocery stores—a real low cost application. Weft-insertion warp knitting (WIWK) has been known for a long time in the composites field as a suitable technique for making ‘multiaxial fabrics’ to be used along with thermoset resins. The aim was now to introduce splitfilm in the respective structures, and thus to combine • • • •
low cost and high productivity of warp knitting; low cost of split-film as a thermoplastic constituent; great drapability of a knitted structure; highly oriented non-crimp reinforcement obtained with the weft-insertion technique.
The aim of this paper is to explore the principal feasibility of the technique just outlined, to develop suitable processing technologies and to investigate the composite mechanical properties achievable with such a preform. Since the focus of the project presently reported is in the mass application sector, special effort is undertaken to achieve a favourable
Figure 1
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mechanical performance to cost ratio rather than maximum mechanical properties.
MATERIAL DEVELOPMENTS Until now, the authors—for the sake of cost competitiveness—were mainly concerned with split-film co-knitted preforms for glass fibre/polypropylene (PP) matrix composites. This fibre/matrix combination appears to be quite difficult to handle in comparison with others. While the task has been completed to a considerable degree, the development efforts are now extended to the field of polyethyleneterephtalate (PET)/glass fibre composites. Due to the short cycle time, the processing route presented above needs materials with a low melt viscosity. In fact, as long it is low enough, the melt viscosity as such is not even the point of primary importance. The focus rather has to be on the wetting behaviour of a certain matrix material when being combined with a given fibre. It is, therefore, the combination of molten matrix material and fibre sizing that has to be considered. Glass fibres with various sizings were examined. The sizing basically has to fulfil three tasks: • protection of the fibres during textile production (a very demanding task when high production rates are considered); • adequate surface properties to allow easy wetting in combination with the selected matrix material; • good bonding to the selected matrix material. The fibres were manufactured and coated with a custommade amino silane sizing 1 by Glasseiden Oschatz GmbH. Glass fibres of type P319 by Vetrotex International and type EC 15 854 M28 by PPG Industries Fibre Glass bv served as a commercial comparison. Also, TWINTEX commingled roving by Vetrotex International was warp-knitted and employed for comparisons. The PP matrix to be chosen has to fulfil several requirements: • easy wetting of the glass fibres; • originally non-polar PP has to bond well to fibre sizing ( → adequate modification);
Comparison of processing methods (new concept for co-knitted, co-woven and mingled textile preforms)
Split–film warp–knitted composite preforms: H. Stumpf et al. • PP has to be suited for film extrusion; • adequate thermal stability. As a matrix material, four different grades with varying melt viscosities (3, 13, 25, 36 MFI 230/2.16) were blended with modifiers such as maleic anhydride grafted PP and acrylic acid/styrene grafted PP during film extrusion. All polymers were stabilized with a commercial additive. Before even making a textile structure, the fibre/matrix combinations were tested by means of a single-fibre pull-out (SFPO) apparatus. Detailed results are published elsewhere 1,5 . An optimized recipe for the sizing (termed PPKM) and the matrix modification (termed M1), respectively, was selected. Figure 2 gives an overview of typical interfacial shear strengths obtained, and compares the custom-made sizing PPKM to commercial ones. The sizing PPKM is more sensitive to changes in the matrix melt viscosity than the commercial fibre sizings, but the IFSS values are of similar magnitude for all three sizings. The matrix material (13 MFI w/o mod.) without any modification, however, shows a significantly lower IFSS value. This underlines the importance of the modification for good interaction between the fibre and the non-polar matrix PP. The frictional shear strength during pull-out (determined from the frictional sliding part of the force–displacement curve during SFPO) did not vary significantly for any of the tested specimens and was always around 4.5 MPa. The debonding work as a fraction of the total work recorded during SFPO is given in Figure 3. From this diagram, it can be concluded that the (relative) debonding work is significantly higher for all modified PP matrices due to the chemical bonding between fibre and matrix. The PP material with the best melt flow (36 MFI 230/ 2.16) was selected. As far as the sizings are concerned, it can be said that the PPKM sizing generally is equally well suited in terms of fibre–matrix interaction when compared the commercial sizings. Textile production trials, however, revealed that only the PPKM sizing could be used for warpknitting. The Vetrotex roving had to be excluded because of its high linear density, whereas the PPG glass fibres turned out to be easily damaged in the textile machine. Therefore,
Figure 2 Interfacial shear strengths (IFSS) obtained from single-fibre pull-out tests for various PP matrices and fibre sizings
glass rovings from Glasseiden Oschatz GmbH sized with PPKM were used for all further development work. In preliminary impregnation trials with single rovings and matrix films, the chosen fibre/matrix combination proved to allow for virtually complete infiltration of any fibre array. Hence, basically all the spaces between the fibres were filled with matrix, even though the impregnation times were on the order of 1 min or even lower. The development of an adequate PET matrix/glass fibre system is still in progress and will soon be reported.
TEXTILE DEVELOPMENTS A short introduction to warp knitting A principal set-up of the knitting elements in a warp knitting machine is shown in Figure 4. The yarn comes from a beam and runs through the guide bar (5), which makes a lapping movement through which the textile construction is determined. Each guide bar can act separately. The respective guide units place the yarn around the needle, which moves up and down and actually forms the loops. All needles are mounted on a needle bar (2) and act simultaneously in conjunction with tongue bar (4). The sinker units (3) hold the newly produced loops down on the trick plate (1) while a new row of loops is formed. A typical, simple warp knitted structure (tricot) is shown in Figure 5 on the right-hand side. In contrast to weft knitting, which in principle requires only one spool of yarn, warp knitting needs as many yarn ends as loops are to be formed in one step/one row. Therefore, the yarns are usually put on a so-called ‘beam’ before the actual knitting step. The result is a long drum that contains many parallel (say 1000, for instance) yarns spooled onto it, so that a family of parallel yarns can be obtained when pulling them off the drum. This makes warp knitting a technique particularly aimed at mass production. As far as the number of yarns is concerned, there is, however, one exception. For the so-called weft insertion, a
Figure 3 Debonding work, W d, as a fraction of the total work, W, obtained from single-fibre pull-out tests for various PP matrices and fibre sizings
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Split–film warp–knitted composite preforms: H. Stumpf et al.
Figure 4
Warp knitting machine, principal set-up of knitting elements (Mayer RS3 MSU-V, with magazine weft insertion)
minimum of only one yarn end is needed. In this case, according to the Karl Mayer magazine weft insertion technique, the weft inserted yarn is placed orthogonally to the machine (‘warp’, see Figure 5) direction, all across the needle row. It is then pushed forward by unit (4) to be laid into the row of loops during the loop formation process. Very much depending on the linear density of the weft inserted yarn, the volume fraction of these straight yarns in the ready textile can be quite high. In an extreme case, there are only a small number of knitted loops holding a large number of straight insertion yarns in place that actually take over the technical function of the fabric. Figure 6 shows the incorporation of the weft insertion in a production machine, where a family of parallel weft yarns is fed into the machine in order to decrease the yarn running speed. Unlike in the weft insertion case, it is also possible to make inlays in the warp direction. The straight inlay yarns are then placed between the needles by a guide unit like (5) in Figure 4. When combined with weft insertion, the result can be a biaxially reinforced fabric. With special machine set-ups even inserts inclined with respect to the principal machine directions (warp, weft) can be obtained and, hence, a multiaxially reinforced fabric. In conclusion, three distinct types of straight inlays and combinations can be produced: • • • •
Figure 5 Simple knitted structures
weft inserted inlays; inlays in the warp direction; angular reinforcement inlays; virtually all combinations of the above (‘multiaxial’ fabrics).
Split-film knitting Since all the structures contain split-film and reinforcement rovings of rather high linear density, special guide elements had to be used in order to prevent frequent yarn breakage during production. A typical tube guide finger is shown in Figure 7. This unit can guide reinforcement roving, split-film or a combination of the two. At present the split-film is cut from the extruded polymer film by a series of many parallel knifes and is then beamed. The beams are used for the warp-knitting operation. As the process progresses into industrial mass production, the
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Figure 6 Schematic diagram of magazine weft insertion (Mayer ‘MSU’)
beaming operation can be skipped, since the polymer film can be cut on-line and then directly be fed into the knitting machine. It should be kept in mind that already now the split-film technique can be run at a very low cost, especially when being compared to the incorporation of thermoplastic matrix fibres, that are usually melt-spun.
Split–film warp–knitted composite preforms: H. Stumpf et al. Trials are undertaken to fibrillate the film and vary its profile in order to enhance the melt distribution during the impregnation process. Textile structures All the structures listed above are examined in the project presently reported about. A variety of uni- and biaxial structures has already been manufactured on Karl Mayer machines (Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen) while the multiaxial structures are still in the process of being produced on Malimo machines (Malimo Maschinenbau GmbH, Chemnitz). All split-film co-knitted structures are termed splitwarpknits. Two basic types can be distinguished: (1) (a) split-warpknits containing split-film in the loops (and possibly other loops from some binding yarn); (2) (b) split-warpknits containing split-film only in the straight inserts, along with reinforcement rovings (in this case, the loops are exclusively formed from some binding yarn). Initially, only the first type of fabric was produced. The corresponding composite properties obtained so far are given below. For the sake of a more even fibre distribution
already in the textile preform, the second type is now being looked upon. In this case, it is possible to bring the inserts closer together, as the binding yarn can be of considerably lower linear density than the split film and the volume of yarn between the rovings can be kept low. Also, this type allows for the reinforcement yarn to be placed even more straight. In any event, however, the reinforcement yarn undulation in the currently considered types of non-crimp fabric is far less than that in a woven fabric. Figures 8 and 9 show uni- and biaxial versions of a fabric of type (a). In this sample, the loops consist exclusively of split-film and melt away during composite production. As already pointed out, the spacing between the rovings is rather large. However, this must not necessarily yield a matrix-rich region of equal size in the composite. By suitable stacking of several fabric layers, for instance, the rovings of one layer can fill the gaps in the adjacent layer. Also, there are other ways to spread the rovings during composite production by certain processing techniques. Figures 10 and 11 show the show uni- and biaxial versions of a fabric of type (b). The fibres are held together by a polyester binding yarn. The replacement with a polypropylene yarn, which would melt away during production, will also be investigated in the near future. In this structure, the split-film is exclusively placed parallel to the rovings, as shown in Figure 12. The reinforcement rovings are very straight and little gaps can be found in between them. The structure is very flat, which can, however, not be seen in the photographs. It was already mentioned that warp knits tend to have a very high drapability. Figure 13 gives an example in this regard. This structure was one of the very first to be produced within the project cited.
PROCESSING TECHNOLOGIES APPLIED IN THE PROJECT Figure 7 Guide finger used for split-film and reinforcement roving, respectively (in the figure, threaded with split-film)
Figure 8
In
all
material
developments
focusing
on
mass
Uniaxial split-warpknit type (a) with PP split-film in the loops (vertical is warp direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. applications, the processability under industrial conditions is crucial. When considering thermoplastic composites the minimum cycle time needed for proper consolidation of the preform or prepreg is of primary importance. However, certain material characteristics may yield substantial restrictions. Two alternative processing methods are considered within the project reported. On the one hand, a classical GMT-like method is applied, where a stack of dry textile material is heated externally, optionally precompacted, and then transferred to a cold press in order to make a composite by moulding. This method employs only well established technologies. On the other hand, a new processing concept termed QUIKTEMP is incorporated into moulding equipment. Since the considerations used apply to many dry textile preforms other than those actually investigated at present, an attempt was made to keep the presentation as general as possible.
production, except for the fact that there is no considerable flow of material in the mould, as the project is concerned with continuous fibre reinforced structures. Heating by conduction takes place between two hot plates, which can give some precompaction. Alternatively, a convective or radiative heating could be employed, which possibly would eliminate the necessity to use Teflon carrier foils (or similar release film). Either option (convective or radiative) would take larger heating times, however. The process as described in Figure 14 is easy to handle and does not need highly specific equipment (except for the mould). It suffers, however, from the fact that the handling of the molten fabric can be troublesome, despite the use of carrier films. Also, the films introduce additional cost and may be hard to separate from the composite surface after the processing when they are not wanted on the ready part. On top of that (especially for processing options without a flexible carrier and mechanical support during heating, respectively), some preforms may contain constituents that
Adapted GMT process Figure 14 shows a schematic sketch describing the process as applied at present. It is very similar to a GMT
Figure 11 Biaxial split-warpknit type (b) with PP split-film parallel to glass reinforcement; binding loops made from polyester yarn (vertical is warp direction) Figure 9 Biaxial split-warpknit type (a) with PP split-film in the loops (vertical is warp direction)
Figure 10 Uniaxial split-warpknit type (b) with PP split-film parallel to glass reinforcement; binding loops made from polyester yarn (vertical is warp direction)
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Figure 12 Uniaxial split-warpknit type (b) with binding loops made from polyester yarn. Reinforcement and split-film were inserted simultaneously by MSU magazine weft insertion
Split–film warp–knitted composite preforms: H. Stumpf et al. contract during heating and lead to shrinkage and curling of the textile (see earlier discussion). While this is not true for the optimized structures presently reported, it can be a real drawback with many thermoplastic textile preforms. Moreover, some thermoplastic matrices tend to exhibit an increased void content in the composite when the molten material is not continuously kept under pressure. This was found by Leterrier and G’Sell 6 and certainly applies to the transfer of the molten stack from the heating station to the press. The adapted GMT process may also require special precautions to prevent extensive oxidation and thus premature degradation of the thermoplastic textile constituents during preheating, since the heating station is essentially open to the ambient air. Other problems are the sinking of molten matrix material down to the bottom of the textile stack during prolonged preheating due to gravity. Generally, it can be stated that this process reduces the freedom of choice in certain process parameters (such as impregnation time) and may be hard to control in some respects. Nevertheless, it can be well worthwhile considering for its simplicity where the limitations are of no concern. QUIKTEMP concept A processing concept that does not exhibit many of the shortcomings mentioned in the last section was presented elsewhere 5, but shall now be described briefly. General description. It was pointed out earlier that the processing of many materials is greatly enhanced when they are continuously held under pressure in order to avoid shrinkage, excessive void formation 6 and the like. When highly oriented thermoplastic polymer products (fibres, films, ribbons etc.) belong to the constituents of a textile composite preform, it is often necessary to support the material mechanically throughout the heating phase in order to prevent contraction and/or curling due to the so-called ‘memory effect’.
Note that a classical GMT-like process does not fulfil this requirement. We are seeking an alternative processing method where the material is in a mould or other mechanical support device and no transfer of the material is needed between the heating/impregnation and cooling phases. The immediate implication is that the mould—or at least its surface—goes through the same thermal history as the part. This bears a severe heat transfer problem, when considering the desired cycle times in mass production, which are usually lower than 2 min. Most conventional moulds are made of steel or similar material and it is virtually impossible to heat them up and cool them down in a sufficiently short time. Some quick calculations yield that, even with highly conductive mould materials and with a drastically lowered thermally active mass, it is still very difficult to reach sufficiently high heating rates (i.e. of the order of 100 K min ¹1 or more). This results from the fact that only a certain number of heating elements can be placed in a given volume. Note that with the mould materials such as aluminium or copper alloys (which exhibit a low heat capacity and high heat conductivity), cooling with water does not present any problem in regard to the cycle time. From the thermal point of view, cooling rates of more than 250 K min ¹1 can easily be reached with an aluminium mould or similar material. It is thus logical to concentrate on the reduction of the heating time.
QUIKTEMP—a new concept for cycle time reduction In the first place, the QUIKTEMP concept aims at keeping the composite in one mould for the whole process due to the technical facts mentioned above. The newly developed concept is based on three observations. • The surface temperature of the mould on the cavity side is the quantity of primary interest to the part.
Figure 13 The split-warpknit is highly drapeable
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Split–film warp–knitted composite preforms: H. Stumpf et al. • A large temperature difference between two media (or two domains of one mould) can be employed to drive the heat transfer. • When optimizing the cycle time, the duration of the heating turns out to be the limiting factor. Cooling (with water) turns out to be much more effective than heating. The design shown in Figure 15 incorporates the finding that domains of the mould volume other than the cavity
Figure 14 Schematic diagram of the adapted GMT process
Figure 15 Newly developed mould design for fast heating and cooling
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surface region can be of different temperature. Hence, the mould consists of a large volume made of a material with high thermal inertia (e.g. steel) and a smaller volume of very low thermal inertia. While the first region acts as a heat storage and is held at a very high temperature level (e.g. 300 or 400⬚C), the second region—termed the ‘forming region’–is in contact with the part. It comprises heating and cooling systems which adjust
Split–film warp–knitted composite preforms: H. Stumpf et al. the temperature of the cavity surface to the desired value. During the heating phase, there will be an additional heat flux from the heat storage into the forming region as a significant temperature gradient exists in this direction. This boosts the heating. During cooling, the heat storage tends to decelerate the temperature change. However, as the cooling is by far more effective, this is not really of concern when considering the total cycle time. If required, one can highly increase the cooling rate by raising the coolant volume flux. On the other hand, the cooling systems ‘shield’ the cavity surface from a possible heat flux from the heat storage. Nevertheless, it should be noted that holding times at low temperature (part ejection and insertion of new material) should be minimized in order to save energy. This, however, will be done anyway in mass production.
The QUIKTEMP concept works in principle, which was shown by tests with a laboratory-scale mould 5. A significant increase of the heating rate could be obtained even with a not yet optimized version, as can be seen from the experimental data plotted in Figure 16. An industrial test tool for the manufacture of an automotive upper washer spring seat is currently being manufactured and will be operated in an industrial environment in the near future. This tool incorporates the QUIKTEMP concept and will be applied to the splitwarpknit material. As an outlook, Figure 17 gives a calculated spatial temperature distribution in the male half of the tool core at the end of the cooling phase. This figure was obtained from a finite element simulation (which has been shown to be a predictive tool of reasonable quality 5), the results of which will be compared to experimental data. Note that the heat storage temperature is somewhat arbitrary and, for demonstration, set to a quite high value of around 400⬚C. At this point it also should be noted that the QUIKTEMP concept is the subject of several international patent applications.
RESULTS OF PROCESSING AND MECHANICAL TEST DATA Mechanical properties obtained from the split-warpknits with an ideal process
Figure 16 Comparison of the new concept (with heat storage) and conventional heating (‘forming region’ without heat storage)
Before going into the various processing options, it was decided to examine the material properties obtained from a processing cycle optimized with respect to the composite mechanical performance (regardless of the cycle times,
Figure 17 Mesh and the spatial temperature distribution at the end of the cooling phase (one-eighth of male half of the tool core)
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Split–film warp–knitted composite preforms: H. Stumpf et al. which may be prohibitive in some applications). A comparison to materials made from commingled yarns (the results obtained with TWINTEX are considered here) was conducted. As optimal processing conditions, the following cycle was chosen: warming up to a processing temperature of 220⬚C, pressing at a lateral pressure of 25 bar for 5 min, then cooling down (all steps in the same mould). So far, for the split-warpknits, only a rather weak dependency on the applied pressure and the holding time at maximum temperature was found. The quality of the plates produced apparently was not much worse when choosing a pressure of 10 bar and a holding time around 1 min. Until now, mechanical test data with a sufficient statistical basis was only obtained from the first batches of textile material produced, i.e. the preforms of type (a) from above where the split-film is exclusively put in the loops. Note that the fabrics used for composite production were not precisely those shown in Figures 8–13—but they were certainly of the first type (a) and similar in all parameters. Results from the second type (b) of material will soon be published. Table 1 gives the results of tensile and bending tests conducted according to ISO 527 and ISO 178, respectively. The split-warpknits considered here were made from PP split-film (550 dtex) and PPKM-sized glass roving (300 tex). For the uniaxial version, the rovings were inlayed warpwise, where the eventual density was about 8 cm ¹1. In the biaxial case, the density was around seven warp yarns per cm and four weft yarns per cm. The uniaxial TWINTEX material was obtained by warp knitting of the RPP 75630
roving (glass/polypropylene commingled), where the roving was weft-inserted (5 wefts cm ¹1) and a polyester yarn (80 dtex f24) was used for the binding loops. For proper interpretation of the data, the fibre volume fraction has to be taken into account. For biaxial and woven structures, the fibre volume fraction values in parentheses give the fibre content oriented in the load direction. Figures 18 and 19 show the results of Table 1 normalized with respect to the fibre content in the load direction, where all values were scaled to a hypothetical reference composite having a fibre content of 25 vol.% in the load direction. It is evident from Figures 18 and 19 that the uni- and biaxial split-warpknits of type (a) compare well to the TWINTEX structures in all respects considered, except for the bending failure stress of the biaxial structure. At this point, it has to be mentioned that the biaxial bending specimens always failed very early by macroscopically visible buckling of the fibre bundles on the compression side. Hence, at a very early stage of the test, the stress profile was very much different from that in a proper bending test and the further force–displacement curve appeared as that in the case of plastic bending deformation (oscillating around the first failure load level and eventually rising considerably towards its end, which corresponds to the specimen being drawn into the supports). Consequently, the formulae for calculation of the longitudinal stress value corresponding to bending failure cannot really be applied any longer. The value, which is nevertheless given in Table 1, does actually not display the ‘bending strength’ usually cited, but rather has some arbitrary character.
Table 1 Properties of mono- and biaxial split-warpknits and monoaxial warp-knits and woven structures, respectively, from commercial commingled yarns Characteristics
PP36/M1/PPKM monoaxial warp knit
TWINTEX RPP 75630 mono- PP36/M1/PPKM biaxial warp TWINTEX TPP 70800 woven axial warp knit knit
Fibre vol. fraction (%) Tensile modulus (GPa) Tensile strength (MPa) Bending modulus (GPa) Bending failure stress (MPa)
23 15.2 348.4 13.4 229.7
46 38.1 729 26.4 434
39 (25) 17.7 447.1 14.5 154.5
37 (19) 12.7 297.6 10.5 245.5
Figure 18 Mechanical test results for the elastic modulus (upscaled for a hypothetical reference composite with V f ¼ 25% in the load direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. This argumentation is not to hide that the biaxial material in the form firstly considered (type (a)) cannot take as high transverse loads as the uniaxial split-warpknit and the TWINTEX materials, respectively. The reason for this, as pointed out, is early buckling of the fibre layers on the compression side (underneath the punch in three-point bending). After having conducted thorough studies of the bending behaviour, the authors identify the presence of large matrix-rich regions to be responsible for the effect in question. Matrix shear failure at these locations and facilitated crack propagation causes the early bucklingout of the fibres on the compression side. The presence of matrix-rich regions can be reduced by the application of certain processing techniques, but certainly the initial fibre placement in the textile has a significant influence as the single fibres do not move over long distances during short processing times. Two actions were taken to overcome this problem. Firstly, it was decided to have an optimized preform of type (a) which does not introduce as large spacings between the reinforcement rovings and thus matrix-rich regions. It is obvious from Figure 9 that there is some limitation in this regard, because the split-film loops will always cause a certain spacing between the rovings, especially in the biaxial case. Secondly, it was decided to extend the research to structures of type (b) as introduced above. From Figures 10 and 11, respectively, it is clear that the fibre distribution in the composite should be far better than in the case considered at present, since already in the preform structure the fibres are quite evenly distributed. The experimental investigations carried out with the newly produced structures (type (b)) are still in progress. Adapted GMT process The main goal of this study was to investigate whether it is possible to use a GMT-based production process for the split-warpknit composites. Furthermore, an optimization of the production parameters was carried out.
The influence of various parameters was investigated: • • • •
preheating temperature mould temperature pressure of consolidation holding time at pressure
For the investigations, biaxial split-warpknits of type (a) were chosen as for the optimized cycle from earlier and stacked to obtain a cross-ply material, [0,90] 3s. As already pointed out, from the present knowledge of the authors this does not appear to be the optimum split-warpknit structure. Test programs with the latest split-warpknit generation are currently prepared and will be reported. However, the results obtained so far are good for a principal evaluation of the feasibility of the adapted GMT process in conjunction with the split-warpknits. Conduction heating (using Teflon carrier/release films) turned out to be the most favourable heating method, particularly in terms of the heat-up time. The only parameter of major importance for the mechanical behaviour of the composite plates was the preheating temperature, hence the temperature to which the material is externally heated in an oven before it is transferred to the press to be formed in a cold mould. This temperature greatly influences the viscosity of the thermoplastic matrix and thus the ease of matrix wetting out and matrix flow through the densely packed fibre array, respectively. All other parameters examined only had a minor influence, which is especially remarkable as far as the pressure of consolidation and the mould temperature are concerned. The optimal processing parameter set for the adapted GMT process turned out to be: • • • •
preheating temperature, 280⬚C mould temperature, 125⬚C pressure of consolidation, 48 bar holding time at pressure, 300 syielding the properties:
• tensile modulus, 16 GPa • tensile strength, 306 MPa
Figure 19 Mechanical test results for the failure stress (upscaled for a hypothetical reference composite with V f ¼ 25% in the load direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. • • • •
shear modulus, 0.7 GPa ⫾ 45⬚ tensile modulus, 2.2 GPa Poisson coefficient, 0.58 fibre volume fraction, 47% (about 28% in the tensile test direction) • void fraction, 5%
mechanical properties obtained with the adapted GMT process. However, this might be completely different with preforms carrying the reinforcement yarns more evenly distributed already in the textile structure. Future investigations will focus on this aspect.
A downscaling of the tensile test values to a reference fibre volume fraction of 25% gives a modulus of 14 GPa and a strength of 273 MPa, which compares not too favourably to the values displayed in Figures 18 and 19 for plates obtained from the ‘classical’ production technique (as presented in the previous section). Microscopy yielded the presence of an increased number of matrix-rich regions and a less even fibre distribution when compared to the composites from the previous section. This fact and the void fraction of 5% can be claimed responsible for the lower
Preliminary prototype trials Some existing laboratory-scale and industrial moulds were used for test trials with the split-warpknits. Figure 20 displays a dome-type part which was obtained with a laboratory-scale production method derived from the QUIKTEMP concept. Figure 21 shows a shoe sole produced with the adapted GMT method, using an existing industrial tool. While the analysis and optimization of the fundamental material properties is still in progress as discussed, the material could already be shown to be processable under realistic conditions. The surface quality looks reasonable; more detailed evaluations (fibre content, fibre distribution, mechanical performance) will be conducted with the next batches of split-warpknits to be produced.
SUMMARY
Figure 20 Sample part (dome)
Figure 21 Sample part (shoe sole, made with an existing industrial mould)
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The material selection task has been very successfully completed to a considerable degree. A PP matrix/glass fibre system well suited to both split-warpknitting and composite production with short impregnation times has been developed. This system could also be transferred to other preforming techniques where easy melt impregnation in short times is needed. Warp knitting of the split-film to make so-called splitwarpknits does not present any principal problem. Uni- and biaxial preforms have been produced in large amounts. Especially, uniaxial structures can be manufactured with a
Split–film warp–knitted composite preforms: H. Stumpf et al. very high productivity. In a perspective industrialized process, the cost added to the pure material cost by the introduction of the matrix material in the form of split-films will be low. An adapted GMT process and QUIKTEMP, a new concept for fast heating and cooling of moulds in thermoplastics processing, has been presented. While the second concept is feasible with split-warpknits, it yields somewhat lower composite mechanical properties with the structures examined so far. It, nevertheless, can be very worthwhile to be considered in low-cost applications. The QUIKTEMP concept, on the other hand, is being incorporated in an industrial test tool to be operated in the near future. From pilot trials with laboratory-scale equipment, this processing technique appears very advantageous. The mechanical properties achieved with the first batch of split-warpknits produced turn out very promising. The uniaxial version compared favourably to the respective TWINTEX material. In the biaxial textiles, the spacing of the reinforcement fibres was rather large, which eventually led to a somewhat reduced maximum bending load of the biaxial composite because of matrix-rich regions. This problem will supposedly be overcome with an altered textile architecture currently being investigated. However, already in the current state of development, the split-warpknit structures—when being industrialized— might be
competitive in terms of the mechanical performance to cost ratio.
ACKNOWLEDGEMENTS This research is partially supported by the European Community Research Programme Brite-EuRam, Project No. BE7256-93, Contract Number BRE2-CT94-0552. The authors gratefully acknowledge the financial contribution by the commission.
REFERENCES 1. 2. 3. 4. 5. 6.
Ma¨der, E., Grundke, K., Jacobasch, H.-J., Panzer, U., Proceedings of the 31st International Man-made Fiber Congress, Dornbirn, Austria, 23–25 September 1992. Kaldenhoff, R., Wulfhorst, B., Franzke, G., Diestel, O., Offermann, P., Ma¨der, E., Proceedings of the 39th International SAMPE Symposium, Anaheim, USA, 11–14 April 1994, pp. 3036–3050. Ma¨der, E., Bunzel, U. and Mally, A., Technische Textilien/ Technical Textiles, 1995, 38, 205–208. Skop-Cardarella, K., Ma¨der, E., Proceedings of the 2nd National Symposium SAMPE Deutschland e.V., Dresden, Germany, 22–23 February 1996, pp. 28–40. Stumpf, H., Ma¨der, E., Schulte, K., Za¨h, W., Proceedings of the European SAMPE Symposium 1996 Basel, Switzerland, 28–30 May 1996, pp. 327–339. Leterrier, Y. and G’Sell, C., Polymer Composites, 1994, 15, 101– 105.
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Composites Part A 29A (1998) 1511–1523 1359-835X/98/$ - see front matter 䉷 1998 Published by Elsevier Science Ltd. All rights reserved.
New thermoplastic composite preforms based on split-film warp-knitting H. Stumpf a,*, E. Ma¨der b, S. Baeten c, T. Pisanikovski d, W. Za¨h e, K. Eng f, C.-H. Andersson d, I. Verpoest c and K. Schulte a a
TU Hamburg-Harburg, Denickestrasse 15, D-21073 Hamburg, Germany Institut fu¨r Polymerforschung, Dresden, Germany c Katholieke Universiteit Leuven, Department MTM, Leuven, Belgium d Pro Eng Co AB/Lund Institut of Technology, Lund, Sweden e Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen, Germany f Engtex AB, Mulsjo¨, Sweden (Received 2 January 1998; accepted 6 May 1998) b
A newly developed type of dry thermoplastic textile preform incorporating non-crimp glass fibre reinforcements and matrix material in the form of split-film is presented. Weft-inserted warp knitting has been chosen as a textile production technique for its low cost. A specialized glass fibre/polypropylene matrix system has been proven to perform favourably in melt impregnation and to provide good composite properties. Some of the processing techniques to be applied to the new textile preform are presented, one of which is the QUIKTEMP concept for fast heating and cooling of tools for thermoplastic moulding. Composite plates produced from preliminary splitwarpknit structures reveal a good potential for cost-saving while reasonable mechanical properties can be maintained. 䉷 1998 Published by Elsevier Science Ltd. All rights reserved. (Keywords: thermoplastic composites; textile preform; split-warpknit; fast heating/cooling)
It is often stated that continuous fibre reinforced composites with the currently available thermoset matrices will never become a competitive alternative in mass applications to more conventional materials such as steel and aluminium. A major reason for this finding lies in the long cycle times associated with thermoset composite production. Consequently, a vast number of activities and projects currently pursued address the development and market introduction of cost-efficient thermoplastic composites. The latter bear quite some potential to achieve total cycle times in the order of 2 min or less, where cycle time is defined as the period of time for which one specific part stays in the process. When considering cost, it seems essential to have any kind of textile preform since it will hardly be economical in mass production to put in reinforcement rovings one by one, at least as long as continuous fibres are considered for reinforcement. Several developments now focus on the inclusion of both reinforcement fibres and matrix material in one textile preform structure. Clearly, the merit of such a structure is its great drapability combined with a
comparatively low cost, especially when evaluating the process as a whole. These textiles offer the chance to make the eventual composite part directly from the preform in one step. Hence, no intermediate processing stages (preimpregnation, pre-consolidation in a double-belt press or similar) are needed. Figure 1 displays the substantial advantage of this new method to make a composite over the more conventional methods with various intermediate stages, like thermoforming of organosheets. Special moulding techniques that allow the heating of the dry textile in the mould can ease the production of parts from dry textile preforms, although they are not mandatory. The materials are usually also processable in the classical GMT-like manner (heating outside the press, then compression in a cold mould). This issue will be addressed below. Commingled rovings have now become quite popular 2–4 for making many kinds of textiles, such as woven, knitted and braided preforms. In most applications, they replace pure reinforcement rovings. Due to their inherently even mixture of reinforcement fibres and matrix material, they usually yield a homogeneous distribution of fibres in the composite. They do, however, suffer from two problems:
* Corresponding author now at: Lufthansa Technit AG, Department FRA WS 3, Lufthansa Base, D-60546, Frankfurt/Main, Germany. Tel.: +49 69 696 94086, fax: +49 69 696 3103, e-mail: [email protected].
• relatively high cost of production, depending on the type of commingling applied; • inherent tendency to yield micro-waviness of the
INTRODUCTION
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Split–film warp–knitted composite preforms: H. Stumpf et al. reinforcement fibres, in case continuous matrix yarns are used. The latter is due to the fact that highly prestretched continuous thermoplastic yarns tend to contract when they are heated (‘memory effect’). The reinforcement yarns, on the other hand, contract little or not at all when the preform is heated. This leads to kind of a thermal misfit, possibly resulting in some waviness of the longitudinally stiff reinforcement fibres. This problem can be reduced by selection of an adequate matrix recipe and a low stretch ratio for the matrix yarns. However, a certain extent of stretch is necessary to ensure a sufficiently high tenacity of the yarn and thus its processability in a textile machine. It was the objective of the work currently presented to investigate alternatives for making a dry textile thermoplastic composite preform. Split-films, hence ribbons of a rather small cross-section, were chosen as a thermoplastic constituent to bring the matrix into a textile preform. Warp knitting was identified as an inexpensive and efficient method to produce preforms carrying uni- and multiaxial continuous reinforcement fibres. Pure warp knits from split-films (without reinforcement fibres) are actually widely used in the consumer goods industry. They serve, for instance, as a material to pack fruits and vegetables to be sold in grocery stores—a real low cost application. Weft-insertion warp knitting (WIWK) has been known for a long time in the composites field as a suitable technique for making ‘multiaxial fabrics’ to be used along with thermoset resins. The aim was now to introduce splitfilm in the respective structures, and thus to combine • • • •
low cost and high productivity of warp knitting; low cost of split-film as a thermoplastic constituent; great drapability of a knitted structure; highly oriented non-crimp reinforcement obtained with the weft-insertion technique.
The aim of this paper is to explore the principal feasibility of the technique just outlined, to develop suitable processing technologies and to investigate the composite mechanical properties achievable with such a preform. Since the focus of the project presently reported is in the mass application sector, special effort is undertaken to achieve a favourable
Figure 1
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mechanical performance to cost ratio rather than maximum mechanical properties.
MATERIAL DEVELOPMENTS Until now, the authors—for the sake of cost competitiveness—were mainly concerned with split-film co-knitted preforms for glass fibre/polypropylene (PP) matrix composites. This fibre/matrix combination appears to be quite difficult to handle in comparison with others. While the task has been completed to a considerable degree, the development efforts are now extended to the field of polyethyleneterephtalate (PET)/glass fibre composites. Due to the short cycle time, the processing route presented above needs materials with a low melt viscosity. In fact, as long it is low enough, the melt viscosity as such is not even the point of primary importance. The focus rather has to be on the wetting behaviour of a certain matrix material when being combined with a given fibre. It is, therefore, the combination of molten matrix material and fibre sizing that has to be considered. Glass fibres with various sizings were examined. The sizing basically has to fulfil three tasks: • protection of the fibres during textile production (a very demanding task when high production rates are considered); • adequate surface properties to allow easy wetting in combination with the selected matrix material; • good bonding to the selected matrix material. The fibres were manufactured and coated with a custommade amino silane sizing 1 by Glasseiden Oschatz GmbH. Glass fibres of type P319 by Vetrotex International and type EC 15 854 M28 by PPG Industries Fibre Glass bv served as a commercial comparison. Also, TWINTEX commingled roving by Vetrotex International was warp-knitted and employed for comparisons. The PP matrix to be chosen has to fulfil several requirements: • easy wetting of the glass fibres; • originally non-polar PP has to bond well to fibre sizing ( → adequate modification);
Comparison of processing methods (new concept for co-knitted, co-woven and mingled textile preforms)
Split–film warp–knitted composite preforms: H. Stumpf et al. • PP has to be suited for film extrusion; • adequate thermal stability. As a matrix material, four different grades with varying melt viscosities (3, 13, 25, 36 MFI 230/2.16) were blended with modifiers such as maleic anhydride grafted PP and acrylic acid/styrene grafted PP during film extrusion. All polymers were stabilized with a commercial additive. Before even making a textile structure, the fibre/matrix combinations were tested by means of a single-fibre pull-out (SFPO) apparatus. Detailed results are published elsewhere 1,5 . An optimized recipe for the sizing (termed PPKM) and the matrix modification (termed M1), respectively, was selected. Figure 2 gives an overview of typical interfacial shear strengths obtained, and compares the custom-made sizing PPKM to commercial ones. The sizing PPKM is more sensitive to changes in the matrix melt viscosity than the commercial fibre sizings, but the IFSS values are of similar magnitude for all three sizings. The matrix material (13 MFI w/o mod.) without any modification, however, shows a significantly lower IFSS value. This underlines the importance of the modification for good interaction between the fibre and the non-polar matrix PP. The frictional shear strength during pull-out (determined from the frictional sliding part of the force–displacement curve during SFPO) did not vary significantly for any of the tested specimens and was always around 4.5 MPa. The debonding work as a fraction of the total work recorded during SFPO is given in Figure 3. From this diagram, it can be concluded that the (relative) debonding work is significantly higher for all modified PP matrices due to the chemical bonding between fibre and matrix. The PP material with the best melt flow (36 MFI 230/ 2.16) was selected. As far as the sizings are concerned, it can be said that the PPKM sizing generally is equally well suited in terms of fibre–matrix interaction when compared the commercial sizings. Textile production trials, however, revealed that only the PPKM sizing could be used for warpknitting. The Vetrotex roving had to be excluded because of its high linear density, whereas the PPG glass fibres turned out to be easily damaged in the textile machine. Therefore,
Figure 2 Interfacial shear strengths (IFSS) obtained from single-fibre pull-out tests for various PP matrices and fibre sizings
glass rovings from Glasseiden Oschatz GmbH sized with PPKM were used for all further development work. In preliminary impregnation trials with single rovings and matrix films, the chosen fibre/matrix combination proved to allow for virtually complete infiltration of any fibre array. Hence, basically all the spaces between the fibres were filled with matrix, even though the impregnation times were on the order of 1 min or even lower. The development of an adequate PET matrix/glass fibre system is still in progress and will soon be reported.
TEXTILE DEVELOPMENTS A short introduction to warp knitting A principal set-up of the knitting elements in a warp knitting machine is shown in Figure 4. The yarn comes from a beam and runs through the guide bar (5), which makes a lapping movement through which the textile construction is determined. Each guide bar can act separately. The respective guide units place the yarn around the needle, which moves up and down and actually forms the loops. All needles are mounted on a needle bar (2) and act simultaneously in conjunction with tongue bar (4). The sinker units (3) hold the newly produced loops down on the trick plate (1) while a new row of loops is formed. A typical, simple warp knitted structure (tricot) is shown in Figure 5 on the right-hand side. In contrast to weft knitting, which in principle requires only one spool of yarn, warp knitting needs as many yarn ends as loops are to be formed in one step/one row. Therefore, the yarns are usually put on a so-called ‘beam’ before the actual knitting step. The result is a long drum that contains many parallel (say 1000, for instance) yarns spooled onto it, so that a family of parallel yarns can be obtained when pulling them off the drum. This makes warp knitting a technique particularly aimed at mass production. As far as the number of yarns is concerned, there is, however, one exception. For the so-called weft insertion, a
Figure 3 Debonding work, W d, as a fraction of the total work, W, obtained from single-fibre pull-out tests for various PP matrices and fibre sizings
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Split–film warp–knitted composite preforms: H. Stumpf et al.
Figure 4
Warp knitting machine, principal set-up of knitting elements (Mayer RS3 MSU-V, with magazine weft insertion)
minimum of only one yarn end is needed. In this case, according to the Karl Mayer magazine weft insertion technique, the weft inserted yarn is placed orthogonally to the machine (‘warp’, see Figure 5) direction, all across the needle row. It is then pushed forward by unit (4) to be laid into the row of loops during the loop formation process. Very much depending on the linear density of the weft inserted yarn, the volume fraction of these straight yarns in the ready textile can be quite high. In an extreme case, there are only a small number of knitted loops holding a large number of straight insertion yarns in place that actually take over the technical function of the fabric. Figure 6 shows the incorporation of the weft insertion in a production machine, where a family of parallel weft yarns is fed into the machine in order to decrease the yarn running speed. Unlike in the weft insertion case, it is also possible to make inlays in the warp direction. The straight inlay yarns are then placed between the needles by a guide unit like (5) in Figure 4. When combined with weft insertion, the result can be a biaxially reinforced fabric. With special machine set-ups even inserts inclined with respect to the principal machine directions (warp, weft) can be obtained and, hence, a multiaxially reinforced fabric. In conclusion, three distinct types of straight inlays and combinations can be produced: • • • •
Figure 5 Simple knitted structures
weft inserted inlays; inlays in the warp direction; angular reinforcement inlays; virtually all combinations of the above (‘multiaxial’ fabrics).
Split-film knitting Since all the structures contain split-film and reinforcement rovings of rather high linear density, special guide elements had to be used in order to prevent frequent yarn breakage during production. A typical tube guide finger is shown in Figure 7. This unit can guide reinforcement roving, split-film or a combination of the two. At present the split-film is cut from the extruded polymer film by a series of many parallel knifes and is then beamed. The beams are used for the warp-knitting operation. As the process progresses into industrial mass production, the
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Figure 6 Schematic diagram of magazine weft insertion (Mayer ‘MSU’)
beaming operation can be skipped, since the polymer film can be cut on-line and then directly be fed into the knitting machine. It should be kept in mind that already now the split-film technique can be run at a very low cost, especially when being compared to the incorporation of thermoplastic matrix fibres, that are usually melt-spun.
Split–film warp–knitted composite preforms: H. Stumpf et al. Trials are undertaken to fibrillate the film and vary its profile in order to enhance the melt distribution during the impregnation process. Textile structures All the structures listed above are examined in the project presently reported about. A variety of uni- and biaxial structures has already been manufactured on Karl Mayer machines (Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen) while the multiaxial structures are still in the process of being produced on Malimo machines (Malimo Maschinenbau GmbH, Chemnitz). All split-film co-knitted structures are termed splitwarpknits. Two basic types can be distinguished: (1) (a) split-warpknits containing split-film in the loops (and possibly other loops from some binding yarn); (2) (b) split-warpknits containing split-film only in the straight inserts, along with reinforcement rovings (in this case, the loops are exclusively formed from some binding yarn). Initially, only the first type of fabric was produced. The corresponding composite properties obtained so far are given below. For the sake of a more even fibre distribution
already in the textile preform, the second type is now being looked upon. In this case, it is possible to bring the inserts closer together, as the binding yarn can be of considerably lower linear density than the split film and the volume of yarn between the rovings can be kept low. Also, this type allows for the reinforcement yarn to be placed even more straight. In any event, however, the reinforcement yarn undulation in the currently considered types of non-crimp fabric is far less than that in a woven fabric. Figures 8 and 9 show uni- and biaxial versions of a fabric of type (a). In this sample, the loops consist exclusively of split-film and melt away during composite production. As already pointed out, the spacing between the rovings is rather large. However, this must not necessarily yield a matrix-rich region of equal size in the composite. By suitable stacking of several fabric layers, for instance, the rovings of one layer can fill the gaps in the adjacent layer. Also, there are other ways to spread the rovings during composite production by certain processing techniques. Figures 10 and 11 show the show uni- and biaxial versions of a fabric of type (b). The fibres are held together by a polyester binding yarn. The replacement with a polypropylene yarn, which would melt away during production, will also be investigated in the near future. In this structure, the split-film is exclusively placed parallel to the rovings, as shown in Figure 12. The reinforcement rovings are very straight and little gaps can be found in between them. The structure is very flat, which can, however, not be seen in the photographs. It was already mentioned that warp knits tend to have a very high drapability. Figure 13 gives an example in this regard. This structure was one of the very first to be produced within the project cited.
PROCESSING TECHNOLOGIES APPLIED IN THE PROJECT Figure 7 Guide finger used for split-film and reinforcement roving, respectively (in the figure, threaded with split-film)
Figure 8
In
all
material
developments
focusing
on
mass
Uniaxial split-warpknit type (a) with PP split-film in the loops (vertical is warp direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. applications, the processability under industrial conditions is crucial. When considering thermoplastic composites the minimum cycle time needed for proper consolidation of the preform or prepreg is of primary importance. However, certain material characteristics may yield substantial restrictions. Two alternative processing methods are considered within the project reported. On the one hand, a classical GMT-like method is applied, where a stack of dry textile material is heated externally, optionally precompacted, and then transferred to a cold press in order to make a composite by moulding. This method employs only well established technologies. On the other hand, a new processing concept termed QUIKTEMP is incorporated into moulding equipment. Since the considerations used apply to many dry textile preforms other than those actually investigated at present, an attempt was made to keep the presentation as general as possible.
production, except for the fact that there is no considerable flow of material in the mould, as the project is concerned with continuous fibre reinforced structures. Heating by conduction takes place between two hot plates, which can give some precompaction. Alternatively, a convective or radiative heating could be employed, which possibly would eliminate the necessity to use Teflon carrier foils (or similar release film). Either option (convective or radiative) would take larger heating times, however. The process as described in Figure 14 is easy to handle and does not need highly specific equipment (except for the mould). It suffers, however, from the fact that the handling of the molten fabric can be troublesome, despite the use of carrier films. Also, the films introduce additional cost and may be hard to separate from the composite surface after the processing when they are not wanted on the ready part. On top of that (especially for processing options without a flexible carrier and mechanical support during heating, respectively), some preforms may contain constituents that
Adapted GMT process Figure 14 shows a schematic sketch describing the process as applied at present. It is very similar to a GMT
Figure 11 Biaxial split-warpknit type (b) with PP split-film parallel to glass reinforcement; binding loops made from polyester yarn (vertical is warp direction) Figure 9 Biaxial split-warpknit type (a) with PP split-film in the loops (vertical is warp direction)
Figure 10 Uniaxial split-warpknit type (b) with PP split-film parallel to glass reinforcement; binding loops made from polyester yarn (vertical is warp direction)
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Figure 12 Uniaxial split-warpknit type (b) with binding loops made from polyester yarn. Reinforcement and split-film were inserted simultaneously by MSU magazine weft insertion
Split–film warp–knitted composite preforms: H. Stumpf et al. contract during heating and lead to shrinkage and curling of the textile (see earlier discussion). While this is not true for the optimized structures presently reported, it can be a real drawback with many thermoplastic textile preforms. Moreover, some thermoplastic matrices tend to exhibit an increased void content in the composite when the molten material is not continuously kept under pressure. This was found by Leterrier and G’Sell 6 and certainly applies to the transfer of the molten stack from the heating station to the press. The adapted GMT process may also require special precautions to prevent extensive oxidation and thus premature degradation of the thermoplastic textile constituents during preheating, since the heating station is essentially open to the ambient air. Other problems are the sinking of molten matrix material down to the bottom of the textile stack during prolonged preheating due to gravity. Generally, it can be stated that this process reduces the freedom of choice in certain process parameters (such as impregnation time) and may be hard to control in some respects. Nevertheless, it can be well worthwhile considering for its simplicity where the limitations are of no concern. QUIKTEMP concept A processing concept that does not exhibit many of the shortcomings mentioned in the last section was presented elsewhere 5, but shall now be described briefly. General description. It was pointed out earlier that the processing of many materials is greatly enhanced when they are continuously held under pressure in order to avoid shrinkage, excessive void formation 6 and the like. When highly oriented thermoplastic polymer products (fibres, films, ribbons etc.) belong to the constituents of a textile composite preform, it is often necessary to support the material mechanically throughout the heating phase in order to prevent contraction and/or curling due to the so-called ‘memory effect’.
Note that a classical GMT-like process does not fulfil this requirement. We are seeking an alternative processing method where the material is in a mould or other mechanical support device and no transfer of the material is needed between the heating/impregnation and cooling phases. The immediate implication is that the mould—or at least its surface—goes through the same thermal history as the part. This bears a severe heat transfer problem, when considering the desired cycle times in mass production, which are usually lower than 2 min. Most conventional moulds are made of steel or similar material and it is virtually impossible to heat them up and cool them down in a sufficiently short time. Some quick calculations yield that, even with highly conductive mould materials and with a drastically lowered thermally active mass, it is still very difficult to reach sufficiently high heating rates (i.e. of the order of 100 K min ¹1 or more). This results from the fact that only a certain number of heating elements can be placed in a given volume. Note that with the mould materials such as aluminium or copper alloys (which exhibit a low heat capacity and high heat conductivity), cooling with water does not present any problem in regard to the cycle time. From the thermal point of view, cooling rates of more than 250 K min ¹1 can easily be reached with an aluminium mould or similar material. It is thus logical to concentrate on the reduction of the heating time.
QUIKTEMP—a new concept for cycle time reduction In the first place, the QUIKTEMP concept aims at keeping the composite in one mould for the whole process due to the technical facts mentioned above. The newly developed concept is based on three observations. • The surface temperature of the mould on the cavity side is the quantity of primary interest to the part.
Figure 13 The split-warpknit is highly drapeable
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Split–film warp–knitted composite preforms: H. Stumpf et al. • A large temperature difference between two media (or two domains of one mould) can be employed to drive the heat transfer. • When optimizing the cycle time, the duration of the heating turns out to be the limiting factor. Cooling (with water) turns out to be much more effective than heating. The design shown in Figure 15 incorporates the finding that domains of the mould volume other than the cavity
Figure 14 Schematic diagram of the adapted GMT process
Figure 15 Newly developed mould design for fast heating and cooling
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surface region can be of different temperature. Hence, the mould consists of a large volume made of a material with high thermal inertia (e.g. steel) and a smaller volume of very low thermal inertia. While the first region acts as a heat storage and is held at a very high temperature level (e.g. 300 or 400⬚C), the second region—termed the ‘forming region’–is in contact with the part. It comprises heating and cooling systems which adjust
Split–film warp–knitted composite preforms: H. Stumpf et al. the temperature of the cavity surface to the desired value. During the heating phase, there will be an additional heat flux from the heat storage into the forming region as a significant temperature gradient exists in this direction. This boosts the heating. During cooling, the heat storage tends to decelerate the temperature change. However, as the cooling is by far more effective, this is not really of concern when considering the total cycle time. If required, one can highly increase the cooling rate by raising the coolant volume flux. On the other hand, the cooling systems ‘shield’ the cavity surface from a possible heat flux from the heat storage. Nevertheless, it should be noted that holding times at low temperature (part ejection and insertion of new material) should be minimized in order to save energy. This, however, will be done anyway in mass production.
The QUIKTEMP concept works in principle, which was shown by tests with a laboratory-scale mould 5. A significant increase of the heating rate could be obtained even with a not yet optimized version, as can be seen from the experimental data plotted in Figure 16. An industrial test tool for the manufacture of an automotive upper washer spring seat is currently being manufactured and will be operated in an industrial environment in the near future. This tool incorporates the QUIKTEMP concept and will be applied to the splitwarpknit material. As an outlook, Figure 17 gives a calculated spatial temperature distribution in the male half of the tool core at the end of the cooling phase. This figure was obtained from a finite element simulation (which has been shown to be a predictive tool of reasonable quality 5), the results of which will be compared to experimental data. Note that the heat storage temperature is somewhat arbitrary and, for demonstration, set to a quite high value of around 400⬚C. At this point it also should be noted that the QUIKTEMP concept is the subject of several international patent applications.
RESULTS OF PROCESSING AND MECHANICAL TEST DATA Mechanical properties obtained from the split-warpknits with an ideal process
Figure 16 Comparison of the new concept (with heat storage) and conventional heating (‘forming region’ without heat storage)
Before going into the various processing options, it was decided to examine the material properties obtained from a processing cycle optimized with respect to the composite mechanical performance (regardless of the cycle times,
Figure 17 Mesh and the spatial temperature distribution at the end of the cooling phase (one-eighth of male half of the tool core)
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Split–film warp–knitted composite preforms: H. Stumpf et al. which may be prohibitive in some applications). A comparison to materials made from commingled yarns (the results obtained with TWINTEX are considered here) was conducted. As optimal processing conditions, the following cycle was chosen: warming up to a processing temperature of 220⬚C, pressing at a lateral pressure of 25 bar for 5 min, then cooling down (all steps in the same mould). So far, for the split-warpknits, only a rather weak dependency on the applied pressure and the holding time at maximum temperature was found. The quality of the plates produced apparently was not much worse when choosing a pressure of 10 bar and a holding time around 1 min. Until now, mechanical test data with a sufficient statistical basis was only obtained from the first batches of textile material produced, i.e. the preforms of type (a) from above where the split-film is exclusively put in the loops. Note that the fabrics used for composite production were not precisely those shown in Figures 8–13—but they were certainly of the first type (a) and similar in all parameters. Results from the second type (b) of material will soon be published. Table 1 gives the results of tensile and bending tests conducted according to ISO 527 and ISO 178, respectively. The split-warpknits considered here were made from PP split-film (550 dtex) and PPKM-sized glass roving (300 tex). For the uniaxial version, the rovings were inlayed warpwise, where the eventual density was about 8 cm ¹1. In the biaxial case, the density was around seven warp yarns per cm and four weft yarns per cm. The uniaxial TWINTEX material was obtained by warp knitting of the RPP 75630
roving (glass/polypropylene commingled), where the roving was weft-inserted (5 wefts cm ¹1) and a polyester yarn (80 dtex f24) was used for the binding loops. For proper interpretation of the data, the fibre volume fraction has to be taken into account. For biaxial and woven structures, the fibre volume fraction values in parentheses give the fibre content oriented in the load direction. Figures 18 and 19 show the results of Table 1 normalized with respect to the fibre content in the load direction, where all values were scaled to a hypothetical reference composite having a fibre content of 25 vol.% in the load direction. It is evident from Figures 18 and 19 that the uni- and biaxial split-warpknits of type (a) compare well to the TWINTEX structures in all respects considered, except for the bending failure stress of the biaxial structure. At this point, it has to be mentioned that the biaxial bending specimens always failed very early by macroscopically visible buckling of the fibre bundles on the compression side. Hence, at a very early stage of the test, the stress profile was very much different from that in a proper bending test and the further force–displacement curve appeared as that in the case of plastic bending deformation (oscillating around the first failure load level and eventually rising considerably towards its end, which corresponds to the specimen being drawn into the supports). Consequently, the formulae for calculation of the longitudinal stress value corresponding to bending failure cannot really be applied any longer. The value, which is nevertheless given in Table 1, does actually not display the ‘bending strength’ usually cited, but rather has some arbitrary character.
Table 1 Properties of mono- and biaxial split-warpknits and monoaxial warp-knits and woven structures, respectively, from commercial commingled yarns Characteristics
PP36/M1/PPKM monoaxial warp knit
TWINTEX RPP 75630 mono- PP36/M1/PPKM biaxial warp TWINTEX TPP 70800 woven axial warp knit knit
Fibre vol. fraction (%) Tensile modulus (GPa) Tensile strength (MPa) Bending modulus (GPa) Bending failure stress (MPa)
23 15.2 348.4 13.4 229.7
46 38.1 729 26.4 434
39 (25) 17.7 447.1 14.5 154.5
37 (19) 12.7 297.6 10.5 245.5
Figure 18 Mechanical test results for the elastic modulus (upscaled for a hypothetical reference composite with V f ¼ 25% in the load direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. This argumentation is not to hide that the biaxial material in the form firstly considered (type (a)) cannot take as high transverse loads as the uniaxial split-warpknit and the TWINTEX materials, respectively. The reason for this, as pointed out, is early buckling of the fibre layers on the compression side (underneath the punch in three-point bending). After having conducted thorough studies of the bending behaviour, the authors identify the presence of large matrix-rich regions to be responsible for the effect in question. Matrix shear failure at these locations and facilitated crack propagation causes the early bucklingout of the fibres on the compression side. The presence of matrix-rich regions can be reduced by the application of certain processing techniques, but certainly the initial fibre placement in the textile has a significant influence as the single fibres do not move over long distances during short processing times. Two actions were taken to overcome this problem. Firstly, it was decided to have an optimized preform of type (a) which does not introduce as large spacings between the reinforcement rovings and thus matrix-rich regions. It is obvious from Figure 9 that there is some limitation in this regard, because the split-film loops will always cause a certain spacing between the rovings, especially in the biaxial case. Secondly, it was decided to extend the research to structures of type (b) as introduced above. From Figures 10 and 11, respectively, it is clear that the fibre distribution in the composite should be far better than in the case considered at present, since already in the preform structure the fibres are quite evenly distributed. The experimental investigations carried out with the newly produced structures (type (b)) are still in progress. Adapted GMT process The main goal of this study was to investigate whether it is possible to use a GMT-based production process for the split-warpknit composites. Furthermore, an optimization of the production parameters was carried out.
The influence of various parameters was investigated: • • • •
preheating temperature mould temperature pressure of consolidation holding time at pressure
For the investigations, biaxial split-warpknits of type (a) were chosen as for the optimized cycle from earlier and stacked to obtain a cross-ply material, [0,90] 3s. As already pointed out, from the present knowledge of the authors this does not appear to be the optimum split-warpknit structure. Test programs with the latest split-warpknit generation are currently prepared and will be reported. However, the results obtained so far are good for a principal evaluation of the feasibility of the adapted GMT process in conjunction with the split-warpknits. Conduction heating (using Teflon carrier/release films) turned out to be the most favourable heating method, particularly in terms of the heat-up time. The only parameter of major importance for the mechanical behaviour of the composite plates was the preheating temperature, hence the temperature to which the material is externally heated in an oven before it is transferred to the press to be formed in a cold mould. This temperature greatly influences the viscosity of the thermoplastic matrix and thus the ease of matrix wetting out and matrix flow through the densely packed fibre array, respectively. All other parameters examined only had a minor influence, which is especially remarkable as far as the pressure of consolidation and the mould temperature are concerned. The optimal processing parameter set for the adapted GMT process turned out to be: • • • •
preheating temperature, 280⬚C mould temperature, 125⬚C pressure of consolidation, 48 bar holding time at pressure, 300 syielding the properties:
• tensile modulus, 16 GPa • tensile strength, 306 MPa
Figure 19 Mechanical test results for the failure stress (upscaled for a hypothetical reference composite with V f ¼ 25% in the load direction)
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Split–film warp–knitted composite preforms: H. Stumpf et al. • • • •
shear modulus, 0.7 GPa ⫾ 45⬚ tensile modulus, 2.2 GPa Poisson coefficient, 0.58 fibre volume fraction, 47% (about 28% in the tensile test direction) • void fraction, 5%
mechanical properties obtained with the adapted GMT process. However, this might be completely different with preforms carrying the reinforcement yarns more evenly distributed already in the textile structure. Future investigations will focus on this aspect.
A downscaling of the tensile test values to a reference fibre volume fraction of 25% gives a modulus of 14 GPa and a strength of 273 MPa, which compares not too favourably to the values displayed in Figures 18 and 19 for plates obtained from the ‘classical’ production technique (as presented in the previous section). Microscopy yielded the presence of an increased number of matrix-rich regions and a less even fibre distribution when compared to the composites from the previous section. This fact and the void fraction of 5% can be claimed responsible for the lower
Preliminary prototype trials Some existing laboratory-scale and industrial moulds were used for test trials with the split-warpknits. Figure 20 displays a dome-type part which was obtained with a laboratory-scale production method derived from the QUIKTEMP concept. Figure 21 shows a shoe sole produced with the adapted GMT method, using an existing industrial tool. While the analysis and optimization of the fundamental material properties is still in progress as discussed, the material could already be shown to be processable under realistic conditions. The surface quality looks reasonable; more detailed evaluations (fibre content, fibre distribution, mechanical performance) will be conducted with the next batches of split-warpknits to be produced.
SUMMARY
Figure 20 Sample part (dome)
Figure 21 Sample part (shoe sole, made with an existing industrial mould)
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The material selection task has been very successfully completed to a considerable degree. A PP matrix/glass fibre system well suited to both split-warpknitting and composite production with short impregnation times has been developed. This system could also be transferred to other preforming techniques where easy melt impregnation in short times is needed. Warp knitting of the split-film to make so-called splitwarpknits does not present any principal problem. Uni- and biaxial preforms have been produced in large amounts. Especially, uniaxial structures can be manufactured with a
Split–film warp–knitted composite preforms: H. Stumpf et al. very high productivity. In a perspective industrialized process, the cost added to the pure material cost by the introduction of the matrix material in the form of split-films will be low. An adapted GMT process and QUIKTEMP, a new concept for fast heating and cooling of moulds in thermoplastics processing, has been presented. While the second concept is feasible with split-warpknits, it yields somewhat lower composite mechanical properties with the structures examined so far. It, nevertheless, can be very worthwhile to be considered in low-cost applications. The QUIKTEMP concept, on the other hand, is being incorporated in an industrial test tool to be operated in the near future. From pilot trials with laboratory-scale equipment, this processing technique appears very advantageous. The mechanical properties achieved with the first batch of split-warpknits produced turn out very promising. The uniaxial version compared favourably to the respective TWINTEX material. In the biaxial textiles, the spacing of the reinforcement fibres was rather large, which eventually led to a somewhat reduced maximum bending load of the biaxial composite because of matrix-rich regions. This problem will supposedly be overcome with an altered textile architecture currently being investigated. However, already in the current state of development, the split-warpknit structures—when being industrialized— might be
competitive in terms of the mechanical performance to cost ratio.
ACKNOWLEDGEMENTS This research is partially supported by the European Community Research Programme Brite-EuRam, Project No. BE7256-93, Contract Number BRE2-CT94-0552. The authors gratefully acknowledge the financial contribution by the commission.
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Ma¨der, E., Grundke, K., Jacobasch, H.-J., Panzer, U., Proceedings of the 31st International Man-made Fiber Congress, Dornbirn, Austria, 23–25 September 1992. Kaldenhoff, R., Wulfhorst, B., Franzke, G., Diestel, O., Offermann, P., Ma¨der, E., Proceedings of the 39th International SAMPE Symposium, Anaheim, USA, 11–14 April 1994, pp. 3036–3050. Ma¨der, E., Bunzel, U. and Mally, A., Technische Textilien/ Technical Textiles, 1995, 38, 205–208. Skop-Cardarella, K., Ma¨der, E., Proceedings of the 2nd National Symposium SAMPE Deutschland e.V., Dresden, Germany, 22–23 February 1996, pp. 28–40. Stumpf, H., Ma¨der, E., Schulte, K., Za¨h, W., Proceedings of the European SAMPE Symposium 1996 Basel, Switzerland, 28–30 May 1996, pp. 327–339. Leterrier, Y. and G’Sell, C., Polymer Composites, 1994, 15, 101– 105.
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