resin transfer moulding for aerospace structures || overview of fibre preforming

Overview of fibre preforming

Vivek Rohatgi, L. James Lee and Adrian Me/ton

6.1 FIBRE PREFORMING - WHY IS IT NEEDED?

6

Unlike the traditional process of autoclave curing of prepregs, in which the fibres and the resin are pre-mixed before lay-up and moulding, the fibre reinforcement in the resin transfer moulding (RTM) process is initially dry and is often assembled outside the mould in the shape of the finished part (Figure 6.1). The fibre assembly, called a preform, is then placed in the mould and impregnated with the liquid resin which polymerises to yield a rigid composite. The preforming process thus converts the fibre reinforcement into the part geometry.

The manner in which the fibre reinforcements are held together to form a fibre mat determines its architecture and is dictated by the size and performance requirements of the end application. During fibre assembly many single-fibre filaments are first brought together in the form of a bundle, also called a roving or a tow. The fibre tows are then held together and assembled by different preforming methods. Predominant fibre materials are glass, graphite and Kevlar®. Typical fibre reinforcements for aerospace applications include bidirectional wovens, unidirectional or bidirectional stitched mats and various other combinations of stitching, weaving, knitting, braiding and filament winding. Apart from the mechanical strength, a fibre reinforcement is also characterised by additional attributes such as the bulk factor, which is the ratio of the volume of the given mass of 'loose' reinforcement to the volume of the same mass after forming, and by drapeability, the ability of a fabric to conform to the contours of the mould cavity. Currently, one of the

Resin Transfer Moulding for Aerospace Structures. Edited by T. Kruckenberg and R. Paton. Published in 1998 by Chapman & Hall, London. ISBN 0412731509.

Fibre preforming - why is it needed? 149

Figure 6.1 Carbon fabric stiffener preforms. Reproduced courtesy of the Cooperative Research Centre for Advanced Composite Structures Ltd, Fishermens Bend, Vic., Australia.

bottlenecks in RIM process is the preforming stage. Usually, individual layers of the reinforcement must be cut and shaped to conform to the various curvatures of the tool surfaces, a process which can be very time consuming and therefore very cost-ineffective. In a way, the mould represents a 'prison cell' and the fibre reinforcement may not go quietly into it (Figure 6.2). Fasteners such as tape and staples are often needed to hold the fibre reinforcement in shape until the mould is closed, greatly slowing the turnaround time. Another challenge is to hold the fibres in place within the closed mould, as the pressure of the injected resin can shift the preform into unpredictable contours [1]. Moreover, parts with complicated geometries make accurate placement of the fibre reinforcement

150 Overview of fibre preforming

Figure 6.2 Schematic of preform placement in the mould.

quite cumbersome and may induce a number of defects. Stretching causes fibre thinning and reorientation. Bending results in thickness reduction and springback. In-plane compression causes wrinkles through buckling. Wrinkle formation and fibre thinning are undesirable from the point of view of the mechanical strength of the preform, and thickness reduction may lead to gaps between the preform and the mould surface. This together with fibre reorientation can cause channeling or 'racetracking' of the resin in regions of high permeability, resulting in the formation of voids and/or 'dry spots'. Incomplete fibre wet-out is detrimental to both the surface quality and the mechanical strength of moulded composite. Dimensional changes due to preform springback are also undesirable as they lead to additional machining costs to achieve the required tolerances. Obtaining net-shape preforms is therefore perceived as a prerequisite for cost-effective production of parts from the RTM process.

Although there are several commercial preform fabrication methods, the technology is not as well developed as other well-established netshape manufacturing techniques such as sheet metal stamping. The major hurdle is the lack of a sufficient knowledge base and reliable technology to produce preforms consistently with accurate dimensions and the desired fibre orientations. Nonetheless, owing to the sustained interest in recent years among researchers both in industry and in academia, preforming methods are gradually changing from art-based to engineering-based techniques. An understanding of the preform fabrication techniques and mechanisms governing the formability is important for optimal preform design of the wide range of reinforcements used

Use of binders and tackifiers 151

in the RTM process. This chapter covers the various aspects of forming the fibre reinforcement into net-shape assemblies that can be moulded into the final product. Extensive details of each preforming process are beyond the scope of this chapter, but can be found in the references cited.

6.2 USE OF BINDERS AND TACKIFIERS FOR FIBRE PREFORMING

Binder and tackifier materials form an important part of the preforming process. Their use not only facilitates stabilisation of the fibre mats but also allows for part consolidation (attachment of ribs, inserts and foam cores, etc.) to yield more integrated structural components.

Binders and tackifiers are thermoplastic or thermoset resins that are usually solid at room temperature but melt easily upon heating. On cooling, the resin resolidifies, bonding the fibre tows or plies together. The glass transition temperature (Tg) of the binder or tackifier should be low enough so that the preform can be shaped without overheating or overcuring the binder or tackifier. At the same time the Tg should be high enough so that the preform can be handled and stored at room temperature [2,3]. In high-performance aerospace composites a partially reacted matrix resin, commonly termed a reactive tackifier, is often used to achieve effective consolidation of fibre preforms. Another reason for using the tackifier is to obtain sufficient tack to reduce slippage between the layers and to minimise springback. The latter occurs when the elastic stresses stored in the fibres during deformation is greater than the adhesion forces due to tack [4].

The choice of a binder or tackifier is governed by several factors, such as compatibility with the matrix resin to be injected during the mouldfilling stage, operational effectiveness, process environment control, final product performance and the preforming technique. Typical thermoset binders are based on unsaturated polyester, polyurethane and epoxy resins. However, thermoplastics are the preferred binder materials as it is easier to control their rheological properties. The trade-off is that the mechanical properties of the moulded composite may be reduced because of incompatibility with the thermoset matrix. A commonly used thermoplastic for glass fibres is nylon; for carbon fibres, polyetheretherketone (PEEK) and polyphenylene sulphide (PPS) are often used. Binder concentration is typically limited to the range of c.4-8 wt% of the preform to provide sufficient rigidity to the preform without adversely affecting the mechanical properties of the finished part [5].

Tackifiers are mostly thermoset resins. The reactive tackifier can be either an uncatalysed or a catalysed thermoset resin, which is generally a partially reacted matrix resin. Typical examples are epoxy, polyester / vinyl ester (with suitable initiators and promoters), phenolic, polyimide


and bismaleimide resins [6]. Particularly suitable epoxy resins include polyglycidyl ethers of polyhydric phenols (compounds having an average of more than one phenolic group per molecule) such as diglycidyl ethers of bisphenol A having an epoxide equivalent weight from c.650-750. Vinyl ester resins include acrylates or methacrylates of polyglycidyl ethers of compounds having an average of more than one phenolic hydroxyl group per molecule. Reaction products (with an average molecular weight of 800-1400) of diglycidyl ether of bisphenol A and acrylic or methacrylic acid have been found to perform quite satisfactorily [6]. Consideration of the molecular weight of the tackifier is important for controlling the rheological properties of the resin during the heating and cooling stages of the preforming process. The effectiveness of the tackifier in minimising fibre springback strongly depends on the rheological properties of the polymer melt [7]. It is also desired that the tackifier dissolves in and reacts with the resin injected into the mould cavity during mould filling. The concentration of the tackifier ranges from 4-7 wt% of the preform [8]. As the density of the fibres is typically nearly twice that of the resin, the tackifier constitutes about 8%-14% of the total resin in the moulded part. Therefore, to obtain a homogeneous matrix and to avoid any deterioration in the mechanical properties, it becomes important that the chemistry of the tackifier chosen is compatible with the matrix resin [6,9]. It is, however, not necessary that the tackifier chemistry be identical to the matrix resin as long as the tackifier can react with the matrix. As an example, an epoxy resin tackifier can be used with a cyanate ester matrix, and vice versa [2,3].

6.3 FIBRE PREFORMING TECHNIQUES

Fibre preforming techniques can generally be classified into two broad categories, depending on whether the formation and the assembly of the fibre tows (mat forming) and conversion (shaping) into the final geometry of the part is carried out simultaneously or sequentially. Both the mat forming stage and the shaping stage, however, may involve several substages or processing steps. Another difference in the preforming techniques arises from whether or not the shaping process utilises a thermoplastic or a thermoset binder or tackifier.

6.3.1 SIMULTANEOUS PREFORMING PROCESSES

As shown in Figure 6.3, filament winding and overbraiding can be classified as simultaneous processes. To make preforms by filament winding, continuous fibre tows are wound tightly over a collapsible mandrel having the desired shape of the part. This approach is good for achieving high fibre content but is usually limited to axisymmetric

Continuous Fibre

~ Filament Overbraiding Winding

Fibre preforming techniques 153

r-------L---r-- - --- ---,

Mat Fonning

+ Textile Approach

t Stitching, Weaving Knitting, Braiding

Pack fonnation and Blanking

Shaping

+ Draping and Debulking

• Assembly

• Textile

t Cut and Sew

Sleeve-on

Figure 6.3 Classification of fibre preforming techniques for aerospace applications.

shapes. It is also difficult to 'lock' the fibre tows in place, which results in comparatively low interlaminar shear strength [10]. Overbraiding circumvents this limitation, and like filament winding produces preforms via helical wrapping of fibres onto a mandrel which duplicates the shape of the final part [11]. The overbraiding process produces an interlaced system of yarns which conforms to the shape of the mandrel, eliminating the need for subsequent cutting and shaping. The lay-up parameters for the fibre tows or yarns can be automated for precise placement and orientation. Three-dimensional braiding offers more flexibility in fibre placement [12] and improved through-the-thickness properties over twodimensional braided preforms.

6.3.2 SEQUENTIAL PREFORMING PROCESSES

Mat Forming Techniques

Most of the continuous fibre preforms are made by sequential processes. In these processes, the final fibre assembly or the preform is obtained by first forming the fibre mats, which are then shaped and assembled by one of the following methods.

Preform fabrication techniques for aerospace applications include methods based on traditional textile operations such as stitching, weaving, knitting and braiding (hence the name textile preforms). Textile processes, however, need to be carefully controlled to avoid any detrimental effect on the fibres (Figure 6.4). A convenient and economical way for holding the fibre tows together and maintaining their orientation is


Figure 6.4 Schematic of forces inflicted on the fibres during the weaving process.

by using a continuous stitch. The crossover or tricot, and the standard or chain stitch, are two types of stitches used [13]. Benefits of stitching include better interlaminar shear properties, damage tolerance and fibre alignment [14]. However, the use of stitches is not without disadvantages. Stitches can create high-density areas that may interfere with the flow and result in the formation of voids [15, 16]. Also, as a result of the presence of stitches anisotropic microcracking in composite parts has been observed [15]. To avoid these drawbacks other ways have been devised to hold the fibre tows. These include weaving, knitting and braiding the fibre tows in different patterns to obtain a self-supporting structure.

In weaving, yarns in one set, which runs along the length of the fabric, interlace by passing under and over the yarns in another set, which cross the fabric. Woven materials have good drapeability depending on the weave pattern and the tow size. Three basic weave patterns are plain, twill and satin. Depending upon the weave pattern, satin weaves can be further subdivided into different harness types (e.g. 3HS, 4HS, 8HS, etc.). Harness weaves are more common in aerospace preforms. Mock Leno weaves or other tight styles, though very tough, are not good in compressive modes. The main characteristic of woven fibres is the formation of crimp caused by the under-and-over weave pattern at the intersection of the warp (0°) and weft (90°) tows or yarns. Triaxially woven fabrics are made of three yarns interlaced at 60° angles. Woven fabrics can also be produced with through-the-thickness reinforcement. These include layer-to-layer interlock, through-the-thickness angular interlock and orthogonal and net-shape weave patterns [17, 18].

The knitting process produces little crimp and results in a less bulky reinforcement. In knitting, the interlacing is done by loops formed between neighbouring tows in one set (introduced sequentially in weft knitting, and in parallel in warp knitting). Knitted fabrics are ideal re-


inforcements for highly curved shapes because of their extremely high deformability [19]. Multiaxial warp-knit (MWK) fabrics also provide good conformability to complex shapes. The MWK fabric consists of warp, weft and bias (±8°) yarns held together by a chain or tricot stitch through the thickness of the fabric [20].

Like filament winding, braiding is used primarily when the desired fibre orientation is symmetrical about an axis [21]. The braiding process can be divided into three major categories: flat braiding [22], tubular braiding [23] and through-the-thickness, three-dimensional, braiding [12, 24]. Moreover, depending on the orientation of yarns in the braids, tubular or flat braids can be further classified into biaxial and triaxial braids. In biaxial braiding, two groups of yarns within the same plane intersect each other at a certain angle, ±8°, to the longitudinal axis of the braid. Triaxial braid is formed by inserting unidirectional yarns parallel to the longitudinal braid axis in the same plane as the biaxial yarns [25]. Other preforming techniques involve forming a hybrid structure consisting of different combinations of textile and chopped or continuous strand random fibre mats.

Binder and tackifier application techniques

The methodology of binder or tackifier application for stabilising layers of fibre mats into net-shape or near-net-shape preforms is quite similar to the use of thermoplastic powders in prepregs. Some of the common methods of binder and tackifier application utilise veils, liquid/solvent spray and powder. Veils can be placed between adjacent layers of fibre mat and then the ply stacks fused with heat and pressure to form the preform [9]. Alternatively, spraying of liquid with dissolved binder or tackifier onto each layer can be used, but this is not a very practical approach as it tends to increase the scrap factor and slows the production because of the need for the liquid/solvent to evaporate during bonding. Also, conventional liquid binders include stabilisers that present an environmental concern [1]. Application of the binder or tackifier in the form of small granules or thin fibres (c.l00-400 /-tm in diameter) is a better approach. This method offers the additional benefit of being environmentally friendly. The conventional method is to disperse the binder or tackifier between the layers of the fibre mat. Shields and Colton [26] have proposed a different method which involves application of the powdered resin to the fibre tows instead, with subsequent assembly of the tows to form the fibre mat. However, this approach limits the use of such tows to a specific application and thus is not very practical for large-scale commercial production of fibre preforms.

The most effective method of applying the powder binder or tackifier to the fabric is to use a coating machine. The two most commonly used


coating machines are of the overhead hopper type in which the amount of powder is metered onto the fabric by a fine bristle-covered roller [Figure 6.5(a)] and of the dip tank type [Figure 6.5(b)]. In the latter the fabric is passed through powder in a tank. Heat is applied to partially melt the particles thereby causing them to adhere to the fabric. The main components of both designs are an 'off-let reel', a 'wind-on reel', a 'heater bank' with 'shield plate', and an applicator hopper or tank. The fabric is wound from the off-let reel, under or through the hopper, between the heater bank and shield plate and then rewound on the wind-up reel. The optimum energy setting is gauged when the powder particles are well attached to the fibres but not dissipated into the fabric (Figure 6.6). If the heat relative to the feed speed is too high the particles will dissipate into the fibre tows. This will result in reduced bonding between the layers. Conversely, insufficient heat will fail to bond the particles and the powder will fall off the fabric. The best results are obtained when the tackifier particle is bound to the fabric as shown in Figure 6.6. The overall

MATERIAl HOPPER

APPLICATION ROLLER

WIND·UP REEL SHIELD PLATE~ OFF·LET REEL

MACHINE BED

(a) HEATER BANK

POWER ROLLERS DISTRIBUTION ROLLERS

HEATER BANK FABRIC

o 10} ~DI .~. Figure 6.5 Powder coating machines: (a) overhead type; (b) dip tank type.


BINDER LEVEL NO GREATER THAN 6 % TO FABRIC WEIGHT

WICKED IN TO FABRIC

CORRECT DUSTY

Figure 6.6 Schematic of the ideal distribution of binder or tackifier on the fibre preform.

appearance should be dull or dusty on the surface of the fabric. If after application of the tackifier the appearance has returned to the original shiny lustre (especially so with carbon fabric), the binder or tackifier will be ineffective.

Assembly and shaping techniques

For applications in which the final preform consists of several components with complex geometry it becomes very difficult to obtain a consolidated net-shape preform by means of a single method to replicate the shape of the entire part. It then becomes advantageous to break down the preform in several modules, fabricate the preforms separately for each module and then assemble the different preform structures as a single unit (Figure 6.7). In some cases, the subassemblies can be stitched, stapled or pinned together to improve the through-the-thickness properties. Shaping techniques which use binders and tackifiers are blanking and draping or debulking. Other methods are cut-and-sew and sleeve-on.

Blanking

Older methods of stabilising textile preforms with binder or tackifier involved selective application and removal of a hot iron. Since most reinforcing fabrics are poor heat conductors, hot iron tacking was done one ply at a time, which made the process both time and labour intensive. An improvement of this method involves laying up the individual plies over a diaphragm press, as shown in Figure 6.8, and directing a


PREFORMED 2-PLY OVERWRAP~

FLAT 8-PLY BLANKED

METALLIC INSERT/

THREE-DIMENSIONAL WOVEN TEE SECTION

FLAT 8-PLY BLANKED PROFILE

Figure 6.7 Assembly of various preforms and a metallic bush (all preassembled before being placed into the mould).

stream of heated air through all the layers to melt the resin. This results in faster heating as a result of convective heat transfer. Application of vacuum pressure behind the perforations serves three important functions. First, in the fabric lay-up process the vacuum can facilitate holding the individual plies in place until hot air is applied. Second, the vacuum can be used to promote hot air penetration through the laminate. Last, subsequent to the melting stage, the vacuum can be used to draw colder

Rubber Diaphragm

Vacuum Bed

Controls

Figure 6.8 Heated diaphragm mat consolidation press.


ambient air through the plies to cause the melt to solidify rapidly [27]. This not only enhances the production rate but also causes the resin modulus to increase quickly. Increase in resin modulus results in increased resistance to the relaxation of deformed elastic fibres, thereby minimising preform springback. Low-inertia infra-red heaters offer even faster heating rates as a result of heat transfer by radiation. For most epoxy-based tackifiers the typical heat setting temperature is 1000e for 1-5 min followed by cooling under pressure for the same amount of time. This process produces a dry workable sheet or board-like material which can be cut into blanks with multiple blade cutting tools or a roller press (Figure 6.9). It is usually much more economical to cut the dry fabric to net-shape than to machine a cured laminate with diamond cutters.

Draping and debulking

The term draping refers to the forced conformance of the fibre reinforcement over the tool surface. In contrast to blanking, in which the edges of the fibre reinforcement are clamped, draping involves deformation of a free boundary. Thus the dominant deformation mode is interfibre shear rather than stretching or compression as in blanking. Owing to the interfibre shear during draping, the initial angle between the weft and warp fibres may decrease [Figure 6.10] until the 'locking angle' is reached [28].

Draped preforms are often debulked to the desired fibre volume fraction and thickness with the aid of vacuum or a hydraulic press. Vacuum debulking is similar to vacuum bagging of prepregs. It involves placing the draped preform in a vacuum bag assembly. A typical as-

Figure 6.9 Blank form cutting roller press.


Warp Weft

Figure 6.10 Schematic depicting the change in fibre orientation during draping of woven fibre mat.

sembly consists of an envelope (made of high-temperature plastic film sealed to the perimeter of the mandrel). A runner (connected to a vacuum source) serves to pull the vacuum from the bag. Often, debulking is carried out in a heated oven to melt the tackifier resin dispersed between the adjacent fibre layers. After applying the vacuum for a specified time period the assembly is allowed to cool to room temperature. The solidified or cured tackifier bonds the fibre layers together, thereby imparting dimensional rigidity to the stacked laminate.

Cut-and-sew

Some low-volume applications utilise the cut-and-sew preforming technique. In this method laminates are precut and assembled into the shape of the part being produced, including the positioning of inserts or cores. The plies are held together by sewing in the thickness (z) direction. Common sewing threads are polyester, glass and Kevlar™, although carbon has also been used [29]. Although the process is slow, the structural efficiency is high because of the exact reinforcement placement and orientation [30].

Sleeve-on

This method is employed for shaping biaxial and triaxial braided fibre mats which are in the form of a 'sock'. The sock is then 'sleeved-on' to the

Net-shape preforming of woven fibre mats 161

........ ........

........ ........

'~~~--::Tl

/ ~

./ / / / ...::....-----::>..£-_----""'''''''----~j

/

Figure 6.11 Schematic depicting the change in fibre orientation during sleeve-on of a braided sock. e = intertow angle; / = interfibre distance; d = tool diameter.

mandrel having the shape and geometry of the part to be moulded. As in draping, the sleeve-on method also results in a change in fibre orientation from the initial configuration. The extent to which the intertow angle, 8, changes is a function of the radius of curvature and diameter, d, of the tool (Figure 6.11).

6.4 NET-SHAPE PREFORMING OF WOVEN FIBRE MATS BY MEANS OF TACKIFIERS

Although some patents describing the methodology for obtaining stabilised preforms for liquid composite moulding (LeM) processes exist [2, 3, 5, 6, 27], few studies have investigated in depth the relevant mechanisms for better design of preforming conditions to obtain near-net and net-shape preforms. Kittelson and Hackett [9] studied the influence of chemical compatibility of epoxy tackifier (PT 500) with an epoxy matrix resin (PR 500) on mechanical properties of moulded composites. They also did experiments to observe the effect of tackifier concentration and


debulking conditions on fibre springback in C-Spar preforms [Figure 6.12(a)] made of plain weave graphite fabric. However, the mechanisms causing the springback phenomenon were not addressed. A systematic investigation of the issues involved in preforming of tackified fibre mats has been conducted [7]. A reactive bismaleimide (BMI) and a nonreactive polymethylmethacrylate (PMMA) tackifier (c. 100 mm, powder) were used with woven (6k, 4HS) graphite fibre mats. Preforming experiments were carried out with the use of two different modes of fibre deformation, namely, V-shape bending and lateral compression [Figures 6.12(b) and (c)). The basic approach used and a summary of the findings obtained from that study is presented here. The specifics of the experimental set-up and other details are reported elsewhere [31].

The V-shape bending and lateral compression experiments both showed that at a specified concentration level increasing the degree of

• \~~

After springback

(b)

After springback

• Initial .~,'

(a)

Consolidated thickness

(e)

Thickness after springback

Figure 6.12 Schematic of spring back (sb) phenomena for three types of preform geometry deformation: (a) C-spar (sb = eO); (b) U-shape (sb = eO); (c) lateral compression (sb = L\H = H2 - H1). Source, part (a): Kittelson, J. L. and Hachett, S. C., Tackifier/resin compatibility is essential for aerospace grade resin transfer moulding, Proceedings of the 39th International SAMPE symposium, Society for the Advancement of Materials and Process Engineering, 1161 Parkview Drive, Covina, CA 91724-3748, pp. 83-96, 1994. Source, parts (b), (c): Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite moulding, Journal of Composite Materials, 31, 720-44, 1997.


cure of BMI tackifier reduced the amount of springback Also, for the same tackifier conversion, an increase in the concentration resulted in lower springback These results concur with the general observations of Kittelson and Hackett [9] for the C-spar type geometry. An interesting phenomenon, observed from U-shape bending experiments, was that when the same amount of tackifier was applied dissolved in acetone solvent, instead of in the powder form, fibre springback reduced from 5.3° to 2.2° (for eight layers, the fibre volume fraction, Vf is 0.55). The tack or interply adhesion, however, was greater in the case of the powder technique compared with when the tackifier was applied by the solvent method. The reason for this behaviour can be explained from the scanning electron micrographs [Figures 6.13(a) and (b)] which show the distribution of the tackifier in the two cases. When applied as a powder most of the tackifier remained on the fibre mat surface as coagulated chunks. In contrast, when applied from a solvent, very little remained on the surface and most of it formed a thin coating on the filaments.

In order to elucidate the mechanism for the observed preform springback, fibre layers were consolidated in a mould to different extents. The resulting microstructure was 'locked' by impregnating the fibres with an unsaturated polyester and subsequently curing the resin. Cross-sections of the moulded samples were then examined by means of scanning electron microscopy. Figure 6.14(a) is a micrograph of the crosssection of the sample with a fibre volume fraction, Vf of 0.32. The figure shows a gap between the adjacent layers. However, with an increase in the fibre volume fraction [Figure 6.14(b), V f = 0.45), the interlayer gap disappears, and fibre tows of adjacent layers start touching each other. Compaction beyond Vf = 0.45 causes further consolidation of the fibre tows as the smaller gaps between the filaments of the fibre tows start to get compressed [Figure 6.14(c), Vf = 0.67). These photomicrographs indicate two levels of consolidation and consequently suggest that there should also be two levels of springback - one arising from the deconsolidation of the small gaps within the fibre tows and the other arising from deconsolidation between the fibre layers.

Based on the experimental results, it was hypothesised that springback occurs when the force exerted by the elastic fibre preform is greater than the 'holding force' provided by the tackifier. The ratio of forces driving and opposing fibre springback can be expressed as in equation (6.1). The rationale is that the driving force exerted by the fibres depends on the relaxation characteristics of the preform, It, whereas the holding force is the product of the modulus, ETI which is a function of degree of cure for the reactive tackifier, and the wetted surface area coverage, Aw:

driving force (Ff) holding force = f ErAw

(6.1)


Figure 6.13 Photomicrographs of debulked fibre preforms with bismaleimide tackifier applied (a) as a powder; (b) dissolved in acetone. Source: Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite moulding, Journal of Composite Materials, 31, 720-44, 1997.

It was proposed in the study that the magnitude of the force exerted during springback for any number of fibre layers could be estimated from a model describing the compaction behavior of fibres, such as the finitely extendable non-linear elastic (FENE) spring model [equation 6.2] [32]:


Ca)

(b)

(e)

fibre layer

fibre layer

Figure 6.14 Photomicrographs (magnification x 200) showing the consolidation of interlayer and intralayer gaps with increasing fibre volume fraction, V( (a) VI = 0.32; (b) VI = 0.45; (c) VI = 0.67. Source: Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite moulding, Journal of Composite Materials, 31, 720-44, 1997.


where

Pa is the applied pressure; k is the spring constant of the fibres; VfO is the initial fibre volume fraction; VI, is the fibre volume fraction at any instant i; Vf'ax is the maximum fibre volume fraction achievable; n is a constant.

The underlying assumption of the proposed approach is that the force required to compress the fibres to a certain volume fraction should be similar (for minimal hysteresis) to that exerted by the fibres at the same volume fraction during the relaxation process. Thus information from compressibility curves, as shown in Figures 6.15(a) and 6.15(b) can be used to calculate the pressure required for achieving compaction to a desired fibre volume fraction and also to estimate the amount of fibre springback, which can be used for effective mould design. Figure 6.15(b) illustrates the applicability of the FENE model to describe the consolidation behaviour of eight layers of 6k, 4HS graphite fabrics (assuming Vf'ax = 0.901).

Analysis of the holding force for a reactive tackifier is complicated because of the combined effects of the increase in the modulus from chemical reaction and the increase in the area coverage by the melted tackifier. Thus, in order to decouple the effects of increase in modulus and surface area coverage, lateral compression experiments were conducted with PMMA tackifier. Different temperatures beyond the melting point of the tackifier were chosen to cover a broad range of melt viscosity. The premise was that for a thermoplastic tackifier the modulus would be the same for all cases upon cooling to room temperature, unlike the case for BMI tackifier. Any observed differences in the springback values should therefore come from differences in the surface area coverage by the tackifier rather than from differences in modulus. Since quantification of the actual surface area coverage is difficult because of the complex nature of spreading of a viscoelastic melt over a porous substrate, changes in the wetted area coverage were studied qualitatively. Figures 6.16(a)-(c) show the photomicrographs of the preforms after debulking for three of the runs, at 220°C, 250°C and 287°C, respectively. Although there is more spreading for debulking at 250°C as compared with the 220°C run, the melt remains on the surface and there is little intralayer coverage. At 287°C, however, the polymer melt migrates from the surface to within the fibre tows [Figure 6.16(c)] and forms a coating on the filaments. The reason for this can be explained by analysis of the forces favouring and resisting impregnation of the tackifier into the fibre tows. The capillary pressure, Pc, provides the main driving force favouring bundle impregnation, whereas resistance to deformation comes from the viscoelastic modulus of the polymer melt.


100~------------------------~--~--~~

.~ 80 S:: i :::l gJ 60 ~ a. c:

.~ 40

a. E o ()

(a)

r--.

20

50

45

'<Zj 40 0..,

'--'

~ 35 ~ 30 C) .... ~25 o ''820 ro S15 o UIO

(b)

5

o

___ 814dry

--- 814dry, stitched

---- 814 resin impregnated

Volume fraction

I .. experimental I

- model ! IJ T

J J ~

/ ./'

V

-o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Fiber

Figure 6.15 Compaction pressure versus fibre volume fraction: (a) 3k plain weave graphite fabrics (Brochier 814); (b) 6k, 4HS woven graphite fabrics. Source, part (b): Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite moulding, Journal of composite Materials, 31, 720-44, 1997; part (a) reproduced courtesy of the Cooperative Research Centre for Advanced Composite Structures Ltd, Fishermens Bend, Vic., Australia.


Figure 6.16 Photomicrographs of surface of polymethylmethacrylate tackified preforms debulked at: (a) 220°C; (b) 250°C; (c) 28JOC; at (i) low magnification, (ii) high magnification. Source: Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite moulding, Journal of Composite Materials, 31, 720-44, 1997.

Consequently, when the driving force equals or exceeds the resisting force it results in 'wicking' of the tackifier resin inside the fibre tows.

Figure 6.17 depicts the springback values as a function of debulking temperature for 3 wt% and 8 wt% of PMMA tackifier. Springback remains the same up to 235°C, then decreases gradually with increasing temperature, and levels off after 287°C. As the modulus should be the


(e)

Figure 6.16 c(i) & (ii)

same in all cases, initial decrease in springback can be attributed to the increase in the interlayer wetted area coverage arising from a decrease in the melt viscosity. Further decrease in springback is a result of the transition of the tackifier area coverage from interlayer to intralayer. Since for a given tackifier concentration tow impregnation implies complete intralayer spreading, the springback levels off beyond the transition region.


0.40

0.35-

I 10.30 - I .......

! 1 ..I<i • I g 0.25- I ,&J • bO • = • •

! ! '§' A rn 0.20-

0.15-

0.10 I I I I I I

200 220 235 250 270 287 305 310 Temperature (0C)

Figure 6.17 Spring back as a function of debulking temperature in fibre preforms with polymethylmethacrylate tackifier. - = 3 wt%, - - - = 8 wt%. Source: Rohatgi, V. and Lee, L. J., Moldability of tackified fibre preforms in liquid composite molding, Journal of Composite Materials, 31, 720-44, 1997.

The general conclusions drawn from this study are that the processing variables affecting fibre springback include tackifier concentration, preform debulking temperature and the location of tackifier (i.e. between the layers compared with within the fibre tows). The study revealed two levels of fibre consolidation: inter layer consolidation or compression of gaps between the layers, followed by intra layer consolidation or compression of gaps within the fibre tows. It was therefore reasoned that there should also be two levels of springback, one arising from the deconsolidation of the small gaps within the fibre tows and the other arising from between the fibre layers. Thus, for optimal interply adhesion and reduced springback, the preforming conditions should be controlled such that the tackifier is present at both the inter layer and intralayer levels.

6.5 DESIGN OF PREFORM TOOLS

The principal section of a preform tool is the plug form. The heated fabric blank is forced over and around the plug by a ring or matching cavity. Typically, tools are made from hard wood, plastics or of light metaL When the shape is simple the fabric can be formed over a plug-type tool with an elastomeric diaphragm. Where sharp corners exist in the plug

Design of preforming equipment 171

1ST MOVING BLOCKS 2ND STAGE MOVING BLOCK

Figure 6.18 Section through spring staged preform tooling.

form an intensifier should be used under the diaphragm to assist in forming such detail. Since the fibres are inextensible, special forming techniques are required for complex shapes, in which the fibre must be fed progressively into the shape, pulling material in from the sides. Spring-loaded tool sections can achieve this, as shown in Figure 6.18.

6.6 DESIGN OF PREFORMING EQUIPMENT

There are three simple stages in the preforming process:

1. locating the fabric; 2. heating the fabric to soften the binder or tackifier; 3. pressing the fabric to form.

The order of these steps can be changed to locating, pressing, heating and then cooling. A light metal frame can be used to locate the fabric and hold it in a controlled fashion. This must be carefully designed as the fabric will have the tendency to sag as the tackifier softens. The other concern with regard to location is that the frame should be indexed over the press tool in a consistent fashion to make the process repeatable.

The principal types of preforming equipment are linear [Figure 6.19(a)] and rotary [Figure 6.19(b)]. The first fixes the process stages in a consecutive line; the second arranges them around a column. The linear system (also known as the shuttle system) is often the first choice as it takes progressive steps to a goal and is then repeated. It is, however, the less efficient of the two in that the return of the fabric holding tray for the


UNLOAD STATlON

PREFORM PRESS

PACK LOCATlON

TOOL CAROUSEL

I (b) PACK LOCATION J L UNLOAD STATION

Figure 6.19 Typical preforming machines: (a) shuttle type; (b) carousel type.

next part constitutes lost time. Rotary or carousel-type rigs are more efficient in that the process is a continuous cycle, lending itself more easily to automation. The fibre blank is placed on to the table, rotated into the heater and is finally rotated into the press and formed. As each preform is retrieved from the press the vacant stage is filled with another fibre blank. This system can produce preforms at a rate of one every 45 s (based on a rib channel section 12" (308 mm) long, 3" (77 mm) wide and 1.5" (38 mm) deep). Refinements to the rig such as an automatic tool changer and simple pick-and-place, load and unload devices can automate the whole process.

6.7 PREFORM STORAGE

As a general rule, storage of preforms for prolonged periods of time should be avoided whenever possible. However, when storage becomes a necessity it is important to support the shape to prevent gradual relaxation. The shape of the preform does not need to be perfect as the mould will realign it to shape. However, the developed trim length must be maintained. A support block or a formed kit tray provides the best storage medium. The temperature and humidity of the storage space should be carefully controlled to avoid any post-forming problems with stored preforms.

Conclusions 173

6.8 CONCLUSIONS

This chapter has provided an overview of the different methods available for manufacturing fibre preforms for the RTM process. Characteristics of different preforming methods were discussed. The choice of one preforming method over the other is determined by several factors, such as the process ability (drapeability, bulk factor, wetting and permeability characteristics of the preform), the feasibility of obtaining net-shapes, the desired mechanical properties of the moulded part and, of course, the cost of production. Although all these factors are important, net-shape preforming is a prerequisite for RTM to reach the desired commercial potential. This necessitates a broader experimental database and simple though realistic models to characterise the mechanisms governing the changes in the fibre orientation during preforming. Moreover, in order to optimise the existing liquid moulding technology, it becomes imperative to integrate preforming analysis with mould-filling characteristics and mechanical properties of the moulded composite parts.

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