Large format composite pellets 3D printing firm Thermwood recently printed several sections from a 51-foot long yacht hull mold to demonstrate how a single hull mold may be sufficient to manufacture even larger vessels, such as yachts. The printed sections of the test mold are made of carbon fiber reinforced ABS from Techmer PM. ABS was chosen because of its physical properties and relatively low cost compared to other reinforced thermoplastics.
Thermwood has already 3D printed a full-size pleasure boat master pattern which has been used to produce multiple boat hull molds. While this demonstrated the value of additive manufacturing for small boat tooling, much larger vessels, such as yachts, require a different approach. In these instances, since only a single mold is needed, it is desirable to print the mold itself rather than print a plug or pattern from which multiple production molds can be made.
To demonstrate how this might work, Thermwood printed a 10-foot section from a 51-foot long yacht hull mold. This rather unique mold design was specifically developed for additive manufacturing. It is printed in sections, each about five feet tall. These printed sections are then bound together both chemically and mechanically using high strength polymer cables into two mold halves. The two mold halves then bolt together to form a complete female mold for the yacht hull.
There are several interesting aspects to this design. First, each mold section has a molded-in rocker. When the mold is fully assembled, it rests on the floor on these rockers. At this point, the mold can be rolled over to tilt about 45 degrees to either side, kind of like a giant rocking chair. This allows for easier access during the layup process. The molded wedges are then clamped to the rockers to hold the mold in the desired position. Once the hull has been laid up and fully cured, the mold is rolled to level and the printed wedges are clamped to both sides, holding them level. The two mold sides can be un-bolted and slid apart to release the finished boat hull.
The test pieces that Thermwood printed show that this approach will work in practice. ABS was chosen because of its physical properties and relatively low cost compared to other reinforced thermoplastics. Certain thermosets will work directly on the ABS molded surface using just traditional mold release practices, however, other thermoset materials are based on solvents that can chemically attack the ABS polymer. To prevent this, Thermwood has experimented with several protective coatings including traditional mold gel coats.
While virtually all coatings tested worked at room temperature, large thermoset molds often experience some heat as a result of the exothermic reaction that occurs during the thermoset curing process. Thermwood tested each of the coatings at 200°F and found that some worked at both room and elevated temperature, while others that worked at room temperature did not work well at elevated temperatures. While it appears that this approach will work today for certain thermosets, the ideal would be to develop a low-cost polymer that is chemically resistant to the other thermoset solvents and eliminate the need for a protective coating.
These demonstration pieces were printed and trimmed on Thermwood’s 10×10 foot LSAM MT, the smallest and lowest cost additive manufacturing system currently available from Thermwood. The entire mold section, made of four printed pieces, weighs 4,012 pounds and required 65.5 hours to print. This demonstration shows that a really large machine is not strictly necessary to produce a really large part. With a little imagination and some creative engineering, really large structures can be built, even on smaller, lower-cost additive manufacturing systems.