2.3. Laminate Manufacturing

Herein, for the first time, a commercial lifting magnet was utilized to apply the compaction pressure on the vacuum bag lay-up during cure. First, the lay-up was prepared on a 6.45 mm-thick magnetic tool plate following the conventional wet lay-up/vacuum bag (WLVB) procedure, and then the lifting magnet was placed on the vacuum bag. Since the area under the magnet is smaller than the surface area of the laminate, the magnet was slid by hand along the length of the laminate to compact the entire surface. It is expected that sliding the magnet while applying compaction pressure on the lay-up would help to squeeze out the excess resin and remove mobile voids, both of which will significantly enhance the mechanical properties of composites. However, the improvements that can be achieved under compaction pressure are influenced by the reinforcement’s type. Basically, the random mat is thicker than the woven fabric and has a greater number of pores in its structure, so it is compacted more under pressure, resulting in higher improvement. In addition, as mentioned in previous studies [35,36,37], applying successive compaction cycles on the preform reorganizes the fiber network and may reduce the resin-rich areas and void content of the final part. Thus, sliding the magnet was performed in one pass, as well as in multiple passes over the entire laminate surface.

In this study, the laminates are fabricated under nine different scenarios listed in Table 2. Two identical laminates are fabricated for each scenario. These scenarios were selected to: (i) assess the feasibility of using a hand-held lifting magnet in WLVB processes; (ii) compare the quality of the random mat and plain weave WLVB laminates made by a lifting magnet with the ones fabricated by conventional WLVB laminates; and (iii) determine the effect of multiple passes of the magnet on the quality of WLVB laminates. In this regard, the baseline laminates, WLVB-RM-4-0 and WLVB-PW-6-0, were manufactured using a conventional WLVB method without applying external pressure. In the WLVB-RM-4-0 laminates, 4 plies of random mat, and in the WLVB-PW-6-0, 6 plies of plain weave fabric, were utilized. Then, for comparison, the random mat and plain weave laminates, WLVB-RM-4-1 and WLVB-PW-6-1, respectively, were fabricated using only one pass of the lifting magnet. Considering that additional resin is removed with each pass of lifting magnet, the number of passes is considered an important parameter affecting the quality of the final part. Thus, WLVB-RM-4-6, WLVB-RM-4-12, and WLVB-RM-4-18 laminates are fabricated by 6, 12, and 18 passes of the magnet on the saturated random mat preforms, respectively. However, for plain weave laminates, WLVB-PW-6-6 and WLVB-PW-6-12, only 6 and 12 passes of the magnet were utilized, respectively. The reason for this is that the lay-up consisting of 6 plies of plain weave is much thinner than the 4 plies of the random mat, so the applied magnetic pressure is much higher. Thus, for plain weave laminates, it is expected that fewer passes would be sufficient to capture the full benefits of magnetic compaction.

Designations for the laminates fabricated with different fabric types and different manufacturing processes.

In this study, all the laminates consist of either 4 plies of random mat or 6 plies of plain weave, 152.4 mm × 203.2 mm (6″ × 8″) E-glass fabrics. For the matrix material, epoxy and hardener were mixed for 5 min at 350 rpm using a mechanical mixer. The mixture was then degassed around 15 min to remove entrapped air until all visible bubbles disappear. The visual demonstration of the composite lay-up preparation using the WLVB process can be found in Ref. [33]. Briefly, the steps are as follows: the 38.1 × 25.4 cm² area of 400-series stainless steel tool plate was covered by a release film for easy removal of the laminate. Prior to laying the fabric, a coat of resin was applied on the tool plate and spread using a squeegee. The first ply was then placed on the resin, and a stainless-steel roller was used to enhance the impregnation and remove entrapped air. Once this is completed, more resin was poured onto the ply and uniformly spread over the fabric with a squeegee. This process was repeated until all the plies were placed. A 0.3 mm-thick, 152 mm × 203 mm, aluminum caul plate, taped to a piece of 216 mm × 267 mm perforated release film, was sprayed with PTFE release agent and placed on top of the saturated preform. The perforated release film was then taped down to the base plate to create a constraint on resin flow during vacuum and cure. For the random mat laminates, this constraint was 20 mm, and for the plain weave laminates, it was 25 mm away, circumscribing the entire lay-up. Bleeder material is placed on top of the release film to absorb the excess resin, and a thru-bag vacuum outlet connector is then placed on top of the bleeder, away from the preform. A vacuum bag was placed over the entire area and the edges of the bag were sealed to the tool plate. After 45 min from the start of fabric lay-up, the vacuum pump pulled a negative pressure of 95 kPa, and the tool plate was heated to 60 °C by the flexible silicone heat sheets secured to the bottom surface of the tool plate. Both vacuum and temperature were held constant for 8 h until the laminate was fully cured.