Mouldless VARTM

The manufacturing method chosen for the rudder was a mouldless VARTM pro-cess. This decision was made despite the drawbacks mentioned in section 2.2.2.

The economical and time advantages of not having to make a mould for a pro-totype was the main reason why the method was chosen. A relatively stiff foam combined with a rigid spoke construction was used to combat the warping dur-ing curdur-ing the mouldless method is prone to. The diminished surface quality of the mouldless method was not weighed heavily as it was a prototype and surface quality was not expected to have a great effect on the performance.

5.2.1 Material Selection

The resin system used for the Mouldless VARTM was the same as for the regular VARTM, described in section 5.1.1. The fibers used was the Easycomposites 200g 2x2 Twill and Saertex X-E-302g/m²from table 5.1.

As a core materialMyCell 80kgclosed cell PVC foam from Maricell was used.

This is a core material commonly used in the marine industry as it has a very low water absorption, and a good compressive strength of 1.22M Pa. The material technical data sheet of the foam can be found in the appendix B.

5.2.2 Test infusion

As the author had never done a mouldless VARTM before, a test infusion was done. This test infusion used a solid piece of theMyCell 80kgfoam cut in a similar shape to the rudder and the same fiber lay-up as in the final rudder.

The process started by wrapping the foam core with the glass fiber sheets. A spray adhesive,FusionFix GP Spray Adhesive, was used to bond the cloth to one end of the trailing edge. The cloth was then pulled tight over the foam core and adhered to the same side of the trailing edge using the same adhesive. The same process was done for both the glass fiber cloths.

The fiber clad part was then wrapped in peel ply and flow mesh, before the whole assembly was put into a prepared vacuum bag, figure 5.3. The inlet was placed on the leading edge of the rudder mock up and the outlet was placed ap-proximately 50mm away from the trailing edge, to ensure full infusion of the part before the resin reached the outlet. The inlet had a T-joint at the end and was wrapped in felt to ensure resin flow into the part.

The normal VARTM process was then followed with a pressure test, infusion, and curing under vacuum pressure. The result of the test infusion can be seen in figure 5.4.

52 A. Brandal: Development of a Modular Hull and Rudder System for an ASV

Figure 5.3:Test infusion during va-cuum bagging.

Figure 5.4: Fully cured mouldless VARTM test infusion.

The test infusion was insightful and several practical lessons were learned during the process. The process parameters that could be improved were:

• Oversize the vacuum bag compare to the geometry. Creation of folds allows the bag to better compact around the core.

• Place the inlet a distance away from the part. When the inlet in placed at the part surface the vacuum bag stretches around the inlet and does not properly compact the fibers to the core, resulting in air bobbles between the fibers and the core.

• Have a greater overlap of sheets at the trailing edge of the rudder. This ensures a stronger bond and makes the fiber lay-up much easier.

• Use the spray adhesion liberally, on both the trailing and leading edge.

Avoiding folds and creases is much easier when the fabric is secured in sev-eral places.

5.2.3 Final Transfusion

The final transfusion was done by wrapping the rudder core in the same way as the test transfusion described in section 5.2.2. Learning from the test transfusion the glass cloth was wrapped around the trailing edge with a minimum of 5cm clearance, pictured in figure 5.5.

The other process improvements from the test infusion were also followed and an overview of the rudder mid infusion can be seen in figure 5.6. The rudder shaft had to be sealed during infusion to be able to pull a vacuum. A double seal was implemented, one inside the vacuum bag and one in the other end of the rudder stock.

The process parameters used is presented in table 5.3. The resin mass was calculated as described in section 5.1.2.

The results from the mouldless VARTM were of high quality. There were no dry spots of air bobbles visible. The shell had been compacted into the core structure and no cavities or bridging could be found. Wrinkles were not found on the surface

Chapter 5: Manufacturing 53

Fiber Size [mm]

540 x 780 Lay-up

thickness[mm] 0.45 Resin mass

[k g] 0.4

# inlets 1

# outlets 1

Table 5.3:Mouldless-VARTM process parameters.

Figure 5.5:A close up of the cloth overlap at the trailing edge of the rudder.

of the part, but there was some fringing outside of the core edge as figure 5.7 shows. This fringing was easily cleaned up in the post processing.

Figure 5.7:Overview of mouldless VARTM mid transfusion.

54 A. Brandal: Development of a Modular Hull and Rudder System for an ASV

Figure 5.6:Overview of mouldless VARTM mid transfusion.