Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics

Transcription

1 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics David Trudel-Boucher 1, Bo Fisa 1, Johanne Denault 2 and Patrick Gagnon 2 1 Department of Mechanical Engineering, École Polytechnique de Montréal, C. P. 6079, Succ. Centre-Ville, Montreal (Quebec) Canada H3C 3A7 2 Industrial Materials Institute, National Research Council Canada, 75, de Mortagne, Boucherville (Quebec) Canada J4B 6Y4 Received: 4 October 2004 Accepted: 31 January 2005 SUMMARY Thermoforming of unconsolidated and consolidated polypropylene/glass fibre fabrics was studied for two mould geometries, i.e. a square mould and a mould with three studs. The principal objective of this study was to evaluate the feasibility of direct thermoforming of unconsolidated fabrics in complex moulds, since this process offers the potential of reducing the cost of the raw material by eliminating the separate consolidation step performed prior to moulding. In this work, the unconsolidated and consolidated fabrics plies were heated in a convection oven and then quickly transferred to a press to be simultaneously conformed and (re-)consolidated using a rubber punch and a metal cavity. Results show that similar product quality (evaluated by the void content) can be obtained with unconsolidated and consolidated fabrics once conformation to the mould shape is achieved. A minimal forming temperature of 215 C is required to achieve conformation to the mould shape, while minimal forming pressures of 2 and 3 MPa must be applied to achieve the same objective for the consolidated and unconsolidated fabric, respectively. Increasing the punch hardness, varying the fabric orientation and increasing the number of vents could reduce the pressure necessary to produce conformation to the mould shape. INTRODUCTION Over the last decade, continuous fibre thermoplastic composites have been progressively introduced to different fields of application and are now widely used in transportation as well as in the aerospace industries 1-3. However, to make composites more attractive for the industry it is necessary to reduce the production cost of manufactured components. One way of achieving this is to use faster and more cost-effective moulding processes, such as stamp forming and rubber-forming. Over the last decade, a lot of attention has been focused on the development of these processes to produce complex parts in short cycle times 4-6. These works also led to a better fundamental understanding of the deformation mechanisms as they occur during moulding since, unlike metallic sheets, a continuous fibre reinforced composite can be considered as essentially inextensible. For these materials, the dominant deformation mechanisms were found to be interlaminar shear and intralaminar shear. The former involves the fabric plies slipping on top of each other while in the latter the warp and weft rovings are rotated against each other, and this leads to a local reduction in the fabric surface 7. However, since the volume has to be conserved, the intralaminar shear results in a thickening of the fabric. Additional deformation mechanisms such as resin percolation across and along the fabric thickness have also been found to occur during moulding of complex part geometries. Thermoforming is generally accomplished using already consolidated plates 8 in which impregnation and wetting of the reinforcing fibre and void elimination are accomplished prior to moulding. Although the use of consolidated material simplifies the process, the cost of the raw material is obviously higher. Therefore, it would be interesting to adapt the moulding process in such a way that unconsolidated fabrics could be used instead of consolidated plates. The potential economy would then come from cost reduction related to the raw Polymers & Polymer Composites, Vol. 13, No. 6,

2 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon material, since the separate consolidation step (prior to moulding) would no longer be required. Until now, only a few investigations have been published on the thermoforming of unconsolidated fabric. Ostgathe et al. 9 compared the stamp forming of unconsolidated and consolidated fabric for the simple case of a flat plate. Their results showed that important reductions in the flexural modulus and strength are observed when unconsolidated fabrics are used. Therefore, they recommended the use of consolidated fabric to obtain better mechanical properties and to improve the surface finish. Thermoforming of unconsolidated fabric was also studied by Wakeman et al. 2 using a flat plate closed-mould. Their results showed that several plies of unconsolidated fabric could be successfully processed. In this study, the pre-heat temperature, tooling temperature and time at consolidation pressure were found to be the most important parameters, while the consolidation pressure was seen to have much less influence on the flexural properties and on the void content. It should be noted that the use of a flat closed-mould inhibited the deformation of the fabric and restrained the amount of possible transverse flow. Since as mentioned earlier, these deformation mechanisms play an important role in achieving good conformation to the mould shape in thermoforming of complex parts, significant differences can be expected between stamp forming and rubber-forming (where open moulds are generally employed) and the use of a flat closed-mould. To evaluate these differences, stamp forming and rubber-forming of unconsolidated fabrics were studied by Trudel-Boucher et al. for two relatively simple mould shapes 10,11. In these studies, the consolidation quality was characterized by the mechanical properties and by the void content, while the conformation to the mould shape was evaluated by comparing the moulded parts to the mould shape. For a flat plate 10, a forming pressure of 3 MPa had to be applied before the fabric temperature fell below 170 C to obtain optimal performance. In the case of a more complex bath-shaped cavity 11, a forming pressure of 3 MPa had to be applied before the temperature of the fabric fell below 190 C. The higher temperature required for the bath-shaped cavity was attributed to the greater deformation of the fabric and to the lubricating role played by the polymer, which was required to achieve the fabric deformation 12. In both cases 10,11 similar product quality was obtained with the consolidated and unconsolidated fabrics. It was therefore concluded that one-step thermoforming of unconsolidated fabrics is a promising forming process technology, and should be object of further studies. This paper presents an experimental investigation on rubber-forming of several unconsolidated and consolidated continuous-fibre composite fabrics. The aim of this work was to compare the (re-)consolidation and conformation to the mould shape of several unconsolidated and consolidated fabrics for two complex-part geometries with different levels of difficulty regarding the conformation, in order to evaluate the feasibility of thermoforming unconsolidated fabric. To achieve this objective, the influence of forming pressure, forming temperature, fabric orientation, punch material and number of vents were studied in the first part of this work for a single fabric in the consolidated and unconsolidated state. Optimal moulding conditions were determined for this fabric and then used to compare several additional consolidated and unconsolidated fabrics. In the second part of this work, conformation to the mould shape and the consolidation quality of unconsolidated fabrics were compared using the more complex of the two mould geometries available. The presence and the importance of moulding defects for each material were also characterized. Finally, the results of complementary experiments are reported, in which recycled material and unreinforced polypropylene were used to improve the surface finish. Materials EXPERIMENTAL Experiments were conducted on four different types of continuous fibre fabrics. The weaves of all the fabrics were made of commingled E-glass and polypropylene fibres, commercially available under the trade name of Twintex. A description of the four fabrics studied is given below: 1485 g/m 2 (44 oz/yd 2 ) balanced 2:2 twill fabric 1485 g/m 2 (44 oz/yd 2 ) unbalanced 2:1 twill fabric 743 g/m 2 (22 oz/yd 2 ) 2:2 twill fabric with larger tows 743 g/m 2 (22 oz/yd 2 ) 2:2 twill fabric with smaller tows For the 1485 g/m 2 unbalanced 2:1 twill fabric, the stacking sequence was chosen so that a symmetric lay-up with the same number of fibres oriented in each principal direction was obtained. All the fabrics had fibre weight fractions of 60%, which resulted in a nominal fibre volume fraction of 35% after consolidation. Four layers of 1485 g/m 2 and eight 544 Polymers & Polymer Composites, Vol. 13, No. 6, 2005

3 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics layers of 743 g/m 2 fabric were superposed to obtain an approximate thickness of 4 mm after processing. The consolidated plates necessary for the thermoforming of consolidated fabrics were produced by compression moulding using a press with heated plateaus. During the consolidation cycle, the fabrics were heated to a temperature of 200 C, held at this temperature for 5 minutes and then cooled to room temperature at 10 C/min. A constant pressure of 0.69 MPa was applied during the entire moulding cycle. These conditions have been shown to lead to composites with optimal mechanical properties 13. In some experiments designed to improve the surface finish, plates of recycled material and of unreinforced polypropylene were used to replace some unconsolidated fabric plies. Plates of recycled material were produced by shredding discarded pieces of consolidated materials and moulding them into 1 mm thick plates using the compression moulding cycle described above. These recycled fabric plates had a fibre weight content of 60% and an average fibre length estimated at 2 mm. The plates of unreinforced polypropylene were cut from extruded sheets. Moulds Two different mould geometries were used in this study so that the level of difficulty could be progressively increased. Both mould were positioned in the middle of a flat section delimited by a small inclined contour. Curvature radii were between 12.7 and 3 mm and the de-moulding angle was equal to 2. Further details of the two mould geometries are given below: Two polyurethane resins were used to fabricate the rubber punch. The first resin had a 70 Shore B hardness, while the second and harder resin had a 70 Shore D hardness. To build the punch, plaster moulds were fabricated by immersing a metal punch machined to perform match-die stamp forming in a liquid plaster solution. Once the plaster solution had solidified, the metal punch was removed and the plaster cavity was used as a mould to cast the polyurethane resins. The shape of these plaster cavities could be easily modified by adding or removing plaster. To fabricate a punch with two polyurethane resins, the harder resin was first cast in the bottom of a plaster cavity. Once this first polyurethane started to solidify, the softer polyurethane resin was cast on top. The bond obtained between the two rubbers was adequate to withstand the stresses associated with the moulding and de-moulding operations. Figure 1. Trimmed parts moulded from unconsolidated fabrics using (a) the square mould and (b) the mould with 3 studs (a) Square mould (Figure 1a): Dimensions of the square section: 9 x 9 x 4 cm Total depth (including inclined contour): 6 cm Area: 500 cm 2 (b) Mould with three studs (Figure 1b): Dimensions of the deep section: 37.5 cm in diameter and 5 cm in depth Total depth (including inclined contour): 7 cm Special features: One large circular 3 cm diameter stud and two smaller 2 cm diameter studs of 1, 0.5 and 1.5 cm in height, respectively Area: 575 cm 2 Polymers & Polymer Composites, Vol. 13, No. 6,

4 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon Processing Procedure In this study, the rubber-forming process was used to investigate the thermoforming of unconsolidated and consolidated complex parts. One of the advantage of this manufacturing process resides in the fact that the flexibility of the rubber punch can lead to a (quasi-)homogeneous pressure distribution on the composite 4, thus allowing the fabric to be successfully draped and (re-)consolidated during moulding. In this work, both the unconsolidated and the consolidated fabrics were first clamped in a blankholder consisting of a rectangular frame to which constant-force springs were fixed (Figure 2). The blankholder with the fabrics was then placed in a convection oven to be heated above the melting temperature of the polymer matrix. A natural convection oven (DESPATCH) was used to heat the fabric, since previous results have shown that a uniform temperature profile can be obtained through the thickness using this heating process 10. Once heated, the unconsolidated and consolidated fabrics were rapidly transferred from the oven to the press. Thermoforming was then performed using a metal bottom die and a urethane punch mounted on a 250 kn hydraulic press (MTS A-01) using a constant loading rate of 100 kn/s. Characterization Void Content In order to characterize the consolidation, the apparent void content (X v ) was determined by comparing the experimental and theoretical densities as follows: X v = ρ t ρ e ρ t (1) where ρ t is the theoretical density and ρ e is the density determined experimentally by measuring the mass of the specimen in air and in water. The theoretical density of a fully consolidated composite can be calculated by equation (2): ρ f ρ m ρ t = w f ρ m + w m ρ f (2) where ρ m is the matrix density, ρ f is the fibre density, and w m and w f are the matrix and fibre weight fractions, determined by pyrolysis of the samples for 4 hours at 500 C. The fibre weight fraction was also used to characterize the matrix flow in several locations. Figure 2. Blankholder used to hold the fabric layers Conformation to the Mould Shape The conformation to the mould shape was first evaluated visually to determine whether the fabric successfully draped the mould shape. For the cubic cavity, conformation to the mould shape was also evaluated by comparing the angle of the female die at the intersection of the bottom and vertical sides of the cavity with the angle of moulded parts in that location. To measure the angle of the moulded parts, 2 cm strips were cut in the middle of the parts and the angle was then measured using a digital protractor (MITUTOYO PRO 360). RESULTS Square Parts Punch Design The first task performed in order to achieve conformation to the mould shape consisted in determining the appropriate punch geometry. To achieve this objective, a polyurethane punch with a 70 Shore B hardness was initially used (this punch material was successfully used previously with the bath-shaped cavity 11 ). As suggested by Robroek 4, the punch was made slightly smaller than an exact match-die punch would have been, to reduce the pressure on the composite during the forming stage. Specifically, a clearance of 6 mm between the rubber tool and the metal cavity was used, which is 2 mm more than the nominal composite thickness. Because of the punch shape, pressure on the vertical walls was only due to the deformation of the punch as the pressure on the bottom of the mould started to grow. However, this punch geometry resulted in 546 Polymers & Polymer Composites, Vol. 13, No. 6, 2005

5 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics poor conformation of the corners delimiting the bottom and vertical walls of the moulded parts, even for high forming pressures and temperatures (Figure 3). This lack of conformation is probably associated with the barrelling of the rubber punch caused by the excessive tensile forces exerted by the composite 4. Consequently it was concluded that the rubber punch had to be slightly wider at the base to drape the bottom corner of the parts (Figure 4). Clearances between the rubber punch and the metal cavity of 6 and 5 mm were used in the side and corner of the mould, respectively. An analogous approach has previously been used by Robroek et al 4 to control the deformation sequence and to prevent excessive pressurization of the sliding composite during the forming phase. Three vents Figure 3. Poor conformation at the corner delimiting the bottom and the vertical walls of the moulded parts were also machined in each corner of the steel cavity, making a total of twelve. With this punch geometry the fabric conformation to the mould shape was achieved. This point will be expanded upon below in the following section dealing with the influence of the forming pressure and temperature. Effect of the Forming Pressure and Temperature The effects of the forming pressure and of the forming temperature were investigated because they are among the most critical forming parameters. If the applied pressure is too low, a satisfactory conformation of the fabric to the mould shape will not be achieved. Insufficient forming pressure can also lead to incomplete (re-)consolidation of the fabric, which can cause several defects such as delamination and a high void content 7. Re-consolidation is necessary because of the deconsolidation occurring by separation of the already consolidated plies during the heating stage 14. On the other hand, too high a forming pressure is known to lead to excessive resin percolation and to an important thickness variation 15. As for the forming temperature, it should be just high enough to allow the polymer matrix to wet the fibres completely while its viscosity remains relatively low 16. The moulding temperature should also be high enough to ease the deformation of the fabric. However, the temperature should not cause the matrix to degrade during heating 10. The following section describes the influence of the forming pressure and forming temperature for two stacking sequences. First, the influence of the forming pressure and forming temperature was determined for the consolidated and unconsolidated 1485 g/m 2 2:2 twill fabric having a (±45) 4 stacking sequence. Then, for comparison, the influence of the forming pressure was determined for the same fabric using a (0/90) 4 stacking sequence. Figure 4. Diagram of a rubber punch with a slightly wider base 6 cm 5 cm vents rubber punch metal mold (±45 ) 4 Stacking Sequence In this part of the study, the effect of the forming pressure and temperature was studied for the 1485 g/m 2 balanced 2:2 twill fabric. To perform this investigation, the rubber punch with a slightly wider base (described above) was used. On the basis of previous studies 10,11, the forming pressure was varied between 0.25 and 4.0 MPa, while the forming temperature was varied between 170 and 215 C. The influence of the forming pressure was determined using a constant forming temperature of 215 C, while the influence of forming temperature was studied using a constant pressure of 4 MPa. Polymers & Polymer Composites, Vol. 13, No. 6,

6 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon The fabrics were moulded using a (±45 ) 4 stacking sequence, meaning that the fibres were oriented along the diagonal of the blankholder. Figure 5. Variation of void content with forming pressure for the 1485 g/m 2 balanced 2:2 twill measured (a) on the vertical side and (b) on the bottom Results obtained for the consolidated and unconsolidated fabrics showed that complete conformation was not achieved below a forming pressure of 4.0 MPa. At lower pressures, incomplete conformation of the corner delimiting the bottom and vertical walls of the parts was observed, as previously shown in Figure 3. The same kind of defect was observed for forming temperatures lower than 215 C. To further characterize the conformation of the composite to the mould shape, the angles of the corner delimiting the vertical walls and the bottom of the parts were measured for mouldings made from consolidated and unconsolidated fabrics processed at 4 MPa using a forming temperature of 215 C. Essentially identical angles of 87.4 for the unconsolidated fabric and 87.6 for the consolidated fabric were obtained, which is only slightly below the 88 of the metal cavity. This can be considered a good conformation to the mould shape. Because similar angles are measured and because the same moulding pressure and temperature were used, it can also be concluded that neither the unconsolidated nor the consolidated fabric offer any significant advantage in terms of conformation to the mould shape in the present case. Visual inspection of the moulded parts revealed that a glossier surface finish could be observed on the outside surface of the parts. This feature is attributed in part to the different surface finish of the dies, the metal cavity having a mirror-like surface finish while the rubber punch has a mat surface finish. The difference in surface finish between the two surfaces of the moulded parts can also be associated with resin percolation through the thickness of the fabric. The percolation of resin creates a matrix-rich layer at the outside surface of the moulded parts, which gives a glossy appearance to the parts on that side. This is similar to what has been previously observed by Robroek et al. 4 and Trudel-Boucher et al. 11 for different part geometries. (a) (b) The void content was measured in the side and bottom of the parts for the entire range of forming parameters investigated, including those which gave poor conformation. Results for the forming pressure and temperature are presented in Figure 5 and Figure 6, respectively. From Figure 5 it can be seen that the void content measured on the side of the cavity remained almost constant over the entire range of forming pressure, while the reduction in the void content was gradual on the bottom of the parts. In the case of the unconsolidated fabric, this is very different from what was observed for a simple flat part 10 or for a bath-shaped cavity 11, for which a forming pressure of approximately 3 MPa was necessary to achieve similar values of the void content. The low void content measured at low forming pressures may be explained by the 548 Polymers & Polymer Composites, Vol. 13, No. 6, 2005

7 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics contributions of different stresses on the composite during moulding: Figure 6. Variation of void content with forming temperature for the 1485 g/m 2 balanced 2:2 twill measured (a) on the vertical side and (b) on the bottom High shear stresses act on the fabric along the vertical walls of the mould when it deforms to drape the cavity. Normal pressure (due to the deformation of the rubber punch) also acts on the fabric during moulding. The addition of these two different stresses resulted in stresses that were locally higher than the nominal value, calculated by devising the load applied by the press by the area of the mould, thus explaining the low void content measured at low forming pressures. The high shear stresses were attributed to the depth and to the small de-moulding angle of the moulded parts. Since void contents lower than 2.5% were found to correspond to the maximum mechanical performance 11, an excellent level of consolidation was achieved with both unconsolidated and consolidated fabrics. (a) The variation of the void content with the forming temperature is shown in Figure 6. The forming temperature does not have a strong influence on the void content, since the porosity decreases only slightly from 2.5 to 2% for the unconsolidated and consolidated fabric when the forming temperature is increased. However, considering the scatter in the data, this reduction in porosity is rather insignificant. It also differs from what has been previously observed with simpler moulds 10,11. This small variation of the void content with the forming temperature is probably related to the superposition of shear and normal stresses during forming, as mentioned previously. It can be concluded that similar void contents (and thus a similar quality of consolidation) can be obtained for consolidated and unconsolidated fabrics. Therefore, as for the conformation to the mould shape, there is no significant advantage related to the use of consolidated fabrics, as far as the consolidation is concerned. (b) (0/90) 4 Stacking Sequence Because the fabric must experience considerable deformation to successfully drape the mould shape, the importance of the fabric orientation should not be overlooked. In this section, the influence of the forming pressure was studied for the 1485 g/m 2 2:2 twill fabric and a (0/90) 4 stacking sequence, meaning that the fibres were oriented parallel to the sides of the blankholder. To compare the results with those presented above for a (±45) 4 stacking sequence, the forming pressure was investigated for the same range of forming pressures. Parts were then characterized in terms of their conformation to the mould shape and their void content. Results obtained for the (0/90) 4 stacking sequence showed that a forming pressure of 3 MPa was Polymers & Polymer Composites, Vol. 13, No. 6,

8 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon sufficient to achieve conformation to the mould shape for the consolidated and unconsolidated fabrics. This is 25% less than the forming pressure determined above for a (±45) 4 stacking sequence. Therefore, varying the fabric orientation is a good way of limiting the high pressure involved with the thermoforming process of a complex part. The lower pressure corresponding to the conformation to the mould shape is likely related to the amount of intralaminar shear occurring during moulding. For a (0/90) 4 stacking sequence, the fabric must experience intralaminar shear mainly in the corners delimiting two vertical walls 17. For the (±45) 4 stacking sequence intralaminar shear is higher, since the fibres have a natural tendency to re-orient themselves towards the stress direction. Lower intralaminar shear then leads to smaller thickness variations, which thus produces a lowers shear stress opposed to the downward movement of the fabric. This constitutes a possible explanation for the lower conformation pressure observed for the (0/90) 4 stacking sequence. The conformation to the mould shape was also characterized by measuring the angle of the corner delimiting the bottom and vertical walls of parts moulded using a forming pressure of 3 MPa and a forming temperature of 215 C. Values of 86.3 and 86.1 were measured for the unconsolidated and consolidated fabric, respectively. These angles are similar to those obtained for the (±45) 4 stacking sequence, and remain close to those of the cavity. Figure 7. Variation of void content with forming pressure for a (0/90) 4 stacking sequence (a) on the vertical side and (b) on the bottom (a) Results obtained for the variation of the void content are shown in Figure 7. On the vertical side of the unconsolidated fabric a slightly lower void content was measured at a higher pressure than at a lower pressure (Figure 7a). For the consolidated fabric, the porosity on the side of the moulded parts decreased up to a pressure of 1 MPa and then remained approximately constant at just under 2%. The variations in the void content differed from those ones obtained for the (±45) 4 stacking sequence, for which the void content remained almost constant for the entire range of forming pressures. For the bottom of the parts (Figure 7b), approximately constant values of the void content were measured for the unconsolidated fabric, while a plateau of approximately 2.0% was obtained for the consolidated fabric at a forming pressure of 1 MPa. The void content measured in the vertical sides and bottom of the parts are thus slightly lower for a consolidated fabric than for an unconsolidated fabric when a (0/90) 4 stacking sequence is used. This is different than what was observed for a (±45) 4 stacking sequence, where similar void content were obtained for the unconsolidated and consolidated fabric. The (b) difference in the variation of the void content in the side of the cavity obtained between both stacking sequences studied may be related to the different amount of intralaminar shear occurring during moulding. As discussed above, the intralaminar shear was lower when conforming a (0/90) 4 fabric. The lower intralaminar shear resulted in smaller thickness variations and, consequently, in a lower shear stress along the vertical walls. Since, for the 550 Polymers & Polymer Composites, Vol. 13, No. 6, 2005

9 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics unconsolidated fabric, a high pressure is required for the matrix to flow into the smallest interstices (fibre bundles), a reduction of the applied stress results in higher void content. For the consolidated fabric the de-consolidation occurring during the heating stage results mainly in interlaminar porosities 14 that can be more easily removed even for lower applied pressure. That explain the lower void content measured for the consolidated material for the (0/90) 4 stacking sequence. Effect of the Punch Material and of the Vent Number Experiments were performed using two different rubber materials to determine the influence of the punch hardness on the conformation to the mould shape and on the consolidation of the fabric. Also, to evaluate the influence of the number of vents on the conformation to the mould shape, parts were moulded using six or eighteen vents instead of twelve as used previously. Therefore, new vents were machined while others were closed. In this part of the study, the conformation to the mould shape was evaluated by determining the forming pressure at which the corner delimiting the vertical walls and the bottom of the parts can be successfully draped by the fabric. The void content was then determined for this pressure only. All experiments were conducted on the unconsolidated and consolidated 1485 g/m 2 balanced 2:2 twill fabric using the optimal moulding parameters determined above, i.e. a moulding temperature of 215 C and a (0/90) 4 stacking sequence. Three different punches of the exactly same shape and size were compared: The first punch used was the 70 Shore B urethane punch employed in the previous section to determine the influence of the moulding pressure, moulding temperature and fabric orientation. The second punch used was made of two different types of rubber. The upper part of the punch was fabricated using a 70 Shore B urethane, while the slightly wider base (Figure 4) was made from a harder urethane having a 70 Shore D hardness. The third punch was fabricated entirely from a 70 Shore D urethane. The comparison of the punch materials was performed using only six vents spaced at an average distance of 6 cm. Results are summarized in Table 1 (6 vent columns). For the softest punch (70 Shore B) and for the punch made of two different rubbers, conformation of the corner delimiting the bottom and vertical walls was achieved for a forming pressure of 4 MPa for both (unconsolidated and consolidated) fabrics. Conformation to the mould shape was obtained for a forming pressure of 3 MPa for both fabrics with the hardest punch. The lower conformation pressure determined using the hardest punch may be explained by a reduction in the barrelling of the punch, which eases the deformation of the fabric by reducing the normal pressure applied on the fabric during the moulding stage. The difference in results obtained between the hard punch and the punch made of two different rubbers may be explained by comparing the latter to two springs in series. As the system was loaded, the presence of a stiff component did not impede the component with the lower rigidity from experiencing important deformation. In the present case, that may be lead to a similar barrelling of the punch made of two rubbers and of the softest, with the same results regarding the restriction of the deformation. The de-moulding of parts processed with the harder punch was more difficult, the higher stiffness of the punch making it more difficult for the operator to slip the moulded parts off the wider base of the punch. Void contents measured in the vertical walls are indicated in parenthesis in Table 1 (6 vent columns). From these values, Table 1. Influence of the punch material and number of vents on the conformation pressure in MPa (P) and % void content (V) for the 1485 g/m 2 balanced fabric Unconsolidated Consolidated 6 Vents 18 Vents 6 Vents 18 Vents Punch material P V P V P V P V 70 Shore B Shore B + 70 Shore D Shore D Polymers & Polymer Composites, Vol. 13, No. 6,

10 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon we can conclude that the punch hardness did not influenced the consolidation quality, since void contents of about 2% are found in all cases. To determine the influence of the number of vents, twelve vents were added in the corner delimiting the vertical walls and the bottom of the mould, making a total of eighteen vents. Vents were spaced out at an average distance of 2 cm. The conformation pressure and the void content were obtained for all three punches described earlier, and the results are presented in Table 1 (18 vent columns). For the punch having a 70 Shore B hardness and the one made of two different rubbers, the conformation of the corner delimiting the bottom and vertical walls was achieved for a forming pressure of 3 MPa compared to 4 MPa when only six vents are used. However, it was not possible to reduce the conformation pressure for the unconsolidated fabric, which remained equal to 4 MPa. For the 70 Shore D urethane punch, conformation of the unconsolidated fabric was achieved for a forming pressure of 3 MPa. This is identical to what was obtained when fewer vents were used. For the consolidated fabric, the conformation pressure was decreased to 2 MPa by using more vents. This is 33% lower than the value obtained for an unconsolidated fabric. The reduction in the conformation pressure obtained for the 1485 g/m 2 consolidated fabric by increasing the number of vents indicates that the presence of entrapped air hinders the achievement of the conformation to the mould shape. However, the number of vents used was not sufficient to influence the results obtained for the unconsolidated fabric, probably as a result of the considerable amount of air that has to be evacuated from these materials. Finally, from Table 1 it can be seen that the number of vents did not influence the void content in the vertical walls. Thus, the number of vents did not influence the level of consolidation. Effect of the Fabric Material In this section, conformation to the mould shape and the consolidation quality of four unconsolidated and consolidated fabrics is compared. As in the previous section, conformation to the mould shape was evaluated by determining the pressure at which the fabric was successfully draped to the mould shape. To determine this pressure, parts were moulded at forming pressures between 1 and 4 MPa until moulding defects were eliminated. The consolidation quality was then evaluated by measuring the void content at this conformation pressure. Parts were moulded using a (0/90) 4 stacking sequence and 18 vents, since these conditions gave the bests results (see above). To avoid de-moulding difficulties, a punch with a 70 Shore B hardness was employed. Because preliminary results have indicated that conformation was difficult to achieve for some of the fabrics, the forming temperature was raised from 215 to 230 C. This was because it was shown previously in this study that conformation can be improved by using higher forming temperatures. Results for the different fabrics are summarized in Table 2. For the 1485 g/m 2 2:2 twill fabric, results obtained at 230 C were identical to those presented in the previous section for a forming temperature of 215 C. This means that conformation to the mould shape was achieved at 4 and 3 MPa for the unconsolidated and consolidated fabric, respectively. For the 743 g/m 2 fabric with the larger tow, conformation to the mould shape was also achieved at forming pressures of 4 and 3 MPa for the unconsolidated and consolidated fabric. It has to be mentioned that conformation of this fabric was not possible at 215 C. A possible reason explaining the higher temperature needed to achieve conformation to the mould shape may be the difficulty of evacuating the air from the unconsolidated layers since eight plies instead of four are needed to obtain a 4 mm thickness for this fabric. For the 743 g/m 2 fabric with the smaller tow, conformation was achieved at a pressure of 4 MPa for the consolidated fabric while conformation could not be obtained within the capacity of the press (4.5 MPa) for the unconsolidated fabric. The poor conformation of this material may be related to the difficulty of evacuating the air Table 2. Influence of the fabric architecture on the conformation pressure and void content Consolidation Pressure (MPa) 1485 g/m 2 balanced 743 g/m 2 with large tow 743 g/m 2 with small tow 1485 g/m 2 unbalanced Unc. Cons. Unc. Cons. Unc. Cons. Unc. Cons Void Content (%) Polymers & Polymer Composites, Vol. 13, No. 6, 2005

11 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics from the eight unconsolidated layers due to the closely woven architecture of this fabric. However, the higher pressure to conform this fabric in the consolidated state (Table 2) tends to indicate that the deformation of the fabric is also more difficult. For the 1485 g/m 2 unbalanced fabric, conformation to the mould shape could not be achieved. For this material, conformation was only possible on two of the four corners delimiting the vertical walls and the bottom of the parts. This result can be attributed to the unbalanced architecture of the fabric. The consolidation of the four studied fabrics was evaluated by measuring the void content at the pressure corresponding to the achievement of the conformation to the mould shape. However, when conformation to the mould shape could not be achieved, the consolidation was evaluated by determining the void content for a forming pressure of 4 MPa. Results presented in Table 2 show that similar void content of approximately 2% are measured for all fabrics. Therefore, although important differences were observed for the conformation to the mould shape, a good level of consolidation was obtained for all the fabrics. Mould with Three Studs In this section, the level of difficulty was increased by moulding deeper parts with three studs at their bottom (Figure 1b). As for the square mould, the first step consisted in designing the proper mould shape using the adequate punch material. Then, the formability of the four unconsolidated fabrics studied was compared. To achieve this objective, parts were moulded using a forming pressure of 4 MPa and a forming temperature of 230 C. These forming parameters were chosen using the results obtained for the square mould which have shown that high moulding pressure and temperature were preferable to mould unconsolidated fabrics. The formability of the fabric was characterized in terms of fabric deformation, void content and fibre weight fraction measurements. Finally, experiments were performed by replacing unconsolidated fabric plies by recycled material and unreinforced polypropylene in order to improve the surface finish. to the cubic mould, the clearance between the rubber punch and the metal cavity for the mould with three studs was of 6 and 4.5 mm in the mould sides and bottom, respectively. However, this punch was not stiff enough in certain locations to resist to the high forces exerted by the composite during moulding and therefore the punch has buckled in some locations. This caused a poor conformation of the fabric where the walls of the rubber punch are thin, i.e. near the deepest cavities (Figure 8). Therefore, a punch of the same geometry made of two different rubbers was fabricated. The bottom of the punch was made from a 70 Shore D urethane while the upper part was made from a 70 Shore B urethane. The higher hardness of the bottom of the punch was chosen to obtain good conformation near the cavities by avoiding the buckling of the punch. A punch made of two different rubbers was preferred to a punch entirely made of a hard rubber to avoid de-moulding difficulties (see above). Knowing that the presence of entrapped air was identified as a limiting factor for the achievement of conformation to the mould shape (see above), 45 vents were machined every 1.2 cm in the radius delimiting the bottom and vertical sides of the mould. Conformation to the Mould Shape In this section, the conformation to the mould shape is first characterized by optical observations in order to identify the main defect visible from the surface. Then, the deformation of the fabric is studied by Figure 8. Poor conformation near the deepest cavity obtained with a 70 Shore B urethane punch, due to the buckling of the rubber punch between the vertical wall and the cavity Punch Design According to the results obtained for the square parts, a 70 Shore B urethane punch slightly wider at the base than in the side was first fabricated to achieve conformation to the mould shape. Similarly Polymers & Polymer Composites, Vol. 13, No. 6,

12 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon performing pyrolysis in several locations. In general, parts moulded from the four unconsolidated fabrics exhibit a fairly good conformation to the mould shape. However, different types of defects have been observed on the moulded parts depending on the type of fabric used. At several locations in the vertical sides of the parts made from the 1485 g/m 2 unbalanced 2:1 twill fabric and from the 743 g/m 2 fabric with the smaller tow, areas of stretched fibres have been observed (Figure 9). From the visual inspection it seems that these areas of stretched fibres correspond to a kind of dry-spot, since the fibres can be easily peeled from the surface. This was confirmed by fibre weight content measurements of 66% performed in one of these locations for the 1485 g/m 2 unbalanced fabric. These areas of stretched fibre are probably caused by high shear stress along the vertical walls during moulding. For all fabrics, observations of the surface of the vertical walls showed that the weaving patterns were seriously altered (Figure 10). These defects are probably related to a high amount of intralaminar shear, which result in the exceeding of the locking angle. However, a good conformation to the mould shape was observed in these areas even if the fabric was subjected to an important deformation. Figure 9. Stretched fibre observed on the side of a moulded part, for a 1485 g/m 2 unbalanced fabric Figure 10. Deformation of the weaving pattern of the glass fibre, for the 743 g/m 2 fabric with the smaller tow For all fabrics, the conformation of the deepest cavities inside the parts was not perfectly achieved, as shown in Figure 11. However, by opposition to the defect mentioned above, this lack of conformation was not apparent from the outside of the parts. During moulding, high tensile forces are developed in the fabric to accommodate the complex mould geometry near the cavities. These tensile forces resulted in the thinning of the material. Therefore, to successfully achieve conformation to the mould shape, any reduction in the fabric thickness should be compensated by a certain amount of matrix flow. However, due to the high fibre content (60 wt%) of the fabrics, the amount of resin able to flow to fill the space between the metal cavity and the rubber punch was insufficient. This resulted in a poor conformation of the deepest cavity. To ease the matrix flow and thus improve the conformation to the mould shape, it would have seemed appropriate to reduce the matrix viscosity by increasing the matrix temperature. However, that was not possible because of matrix degradation at forming temperatures higher than 230 C. It would also have been tempting to reduce the flow stresses by reducing the forming rate, thus easing the matrix flow, but that was not possible either, due to the high cooling rates of the material 10,11 and thus to the short amount of time available to complete the forming operation. Finally, Figure 11. Poor conformation observed at the deepest cavity of the 743 g/m 2 fabric with the larger tow 554 Polymers & Polymer Composites, Vol. 13, No. 6, 2005

13 Thermoforming Complex Parts from Unconsolidated and Consolidated Polypropylene/Glass Fibre Fabrics as was observed for the cubic cavity, a glossier surface finish was apparent on the outside surface of the moulded parts. As for the square mould, this is probably due to the different surface finish of the metal cavity and rubber punch, and to matrix percolation across the thickness of the fabrics. As stated earlier, the fabrics were subjected to important tensile stresses during moulding. To verify whether these high stresses caused fibre breakage, pyrolysis of moulded material from the deepest cavity and the vertical wall located beside this cavity were performed. Plies were then examined individually to detect fibre damage. The observations performed in these areas showed no evidence of fibre breakage. Pyrolysis of material from the areas shown in Figure 8 and Figure 9 were also performed to characterize the deformation of the fabric. Results for material from the area of stretched fibres showed no sign of fibre breakage and the very regular fibre pattern observed suggests that the locking angle of the fabric was not exceeded. Pyrolysis of the area presented in Figure 10 showed a highly distorted fabric pattern, confirming that the locking angle was exceeded. As for material from the area of stretched fibre, no indication of fibre breakage could be identified in this location. Void Content and Fibre Weight Fraction To characterize the consolidation, the void content was measured on the side and bottom of the moulded parts. Results presented in Figure 12a show that similar void contents were found in both locations for all fabrics. For both 1485 g/m 2 fabrics and for the larger tow 743 g/m 2 one, void contents of approximately 2% were obtained, while for the 743 g/m 2 fabric with the smaller tow, the void content was slightly higher, close to 2.7%. This is probably associated to the difficulty of evacuating trapped air from the closely woven fabric. The fibre weight contents are shown in Figure 12b. In all cases, they were higher than the nominal value of 60%. This can be explained by matrix flow over a large distance, as was observed previously 11. With the exception of the 743 g/m 2 fabric with the larger tow, the fibre content was higher at the bottom of the parts than in the sides. As mentioned previously, due to the design of the rubber punch, the initial contact between the fabric and the dies occurred at the bottom of the cavity during moulding. It is only as the moulding pressure increased that the rubber punch experienced the deformation that allows the pressure to be partially transferred to the vertical side of the parts. Therefore, the bottom of the part Figure 12. (a) Void content and (b) fibre weight content, measured on the side and at the bottom of the moulded parts Void content (%) Fibre weight content (%) g/m 2 2:2 twill g/m 2 2:2 twill was initially subjected to a higher pressure, which may have forced more of the matrix to flow out from the bottom of the part. Surface Finish Side Bottom 1485 g/m 2 2:1 twill Side Bottom 1485 g/m 2 2:1 twill 743 g/m 2 2:2 twill (large) 743 g/m 2 2:2 twill (large) (a) 743 g/m 2 2:2 twill (small) (b) 743 g/m 2 2:2 twill (small) Although the fabrics could be successfully draped to the metal cavity, defects such as areas of stretched Polymers & Polymer Composites, Vol. 13, No. 6,

14 David Trudel-Boucher, Bo Fisa, Johanne Denault and Patrick Gagnon fibres on the outside (Figure 9) and incomplete conformation near the deepest cavity in the inside (Figure 11) were observed. These visible defects may not be acceptable for certain applications. Therefore, two different solutions were investigated to improve the appearance of the moulded parts. The first consisted of replacing several plies of unconsolidated fabric by a plate of recycled materials, produced by shredding consolidated discarded pieces and moulding them using the compression moulding cycle described above. The plates of recycled material were thus inserted at mid-thickness to replace 2 plies (1 mm) of the 743 g/m 2 unconsolidated fabric with the larger tow. Because the parts made of recycled materials were produced from the same fabric used in this study, the fibre weight content of the final parts was not significantly influenced. However, the use of shredded materials was expected to reduce the mechanical properties of the parts. During moulding, the recycled material was able to flow through the thickness of the fabric towards the surface of the parts. That resulted in a significant improvement in surface finish, since some of the roughness associated with stretched fibres could be eliminated. Complete conformation of the deepest cavity inside the parts was also achieved, as shown in Figure 13. Predictably, complete conformation was more easily achieved if the unconsolidated fabric was moulded together with a material that could flow to compensate for the thickness variations of the glass fibre weave. Figure 13. Good conformation observed at the deepest cavity of the 743 g/m 2 fabric with the larger tow, using recycled material The second way to improve the surface finish consisted of adding an 0.8 mm unreinforced polypropylene sheet on the top of a 1485 g/m 2 unconsolidated fabric during the heating stage. Parts were then moulded with the added polypropylene sheet facing the metal die. Because the polypropylene sheet was added during the heating stage, the melted polypropylene did not flow through the thickness across the unconsolidated plies, but rather remained at the surface due to its lower temperature. The addition of the polypropylene sheet did result in a more glossy surface finish of the parts. However, the roughness of the surface associated with areas of stretched fibres could not be completely eliminated. Some work should be done with different polypropylene sheet thicknesses to determine whether further improvements to the surface finish are possible. Because the polypropylene sheets remained on the surface, the addition of a matrix rich layer did not resulted in an improvement in the conformation of the deepest cavities inside the parts, as observed with the use of recycled material. Nevertheless, the addition of a polypropylene sheet allowed a significant improvement in the overall surface finish on the outside of the moulded parts. DISCUSSION Conformation to the Mould Shape In this paper, the approach used consisted of determining successively the influence of the most important forming parameters. This allowed us to progressively improve the forming process while developing a good knowledge of the influence of some of the forming parameters. The biggest difficulty encountered has been to drape the corner delimiting the bottom and vertical walls of the parts. To achieve conformation, a forming temperature between 215 and 230 C had to be used. The forming temperatures used in this work are thus very similar to those used in other thermoforming processes used with fibre reinforced polypropylene 18,19. To achieve the same objective, forming pressures between 2 and 4 MPa had to be applied to process consolidated and unconsolidated fabric. These pressures were higher than the 0.7 to 1.5 MPa used in previous studies to mould similar unconsolidated and consolidated fabrics 11,13,16. The high pressures determined in this study to achieve conformation to the mould shape are thought to be associated with high stresses acting opposite to the pressure applied by the rubber punch during moulding: 556 Polymers & Polymer Composites, Vol. 13, No. 6, 2005