A collaborative effort between Spirit AeroSystems, Inc., Techmer PM, and Thermwood Corporation, has produced what is believed to be the largest stretch form tool ever to be 3D printed. At 6.5 feet wide by 15 feet long, the tool project has stretched the capabilities of additive technology in several directions.
It was determined that a carbon fiber-filled polycarbonate would provide sufficient strength to withstand the forces used in the stretch form process. However, unlike traditional lubrication methods, Techmer PM blended a new polycarbonate formulation that contained an internal lubricant in addition to the carbon fiber – eliminating the need for any additional lubrication.
The tool was printed using Thermwood Corporation’s LSAM 1020 dual gantry print and trim system. While this machine is equipped with Vertical Layer Printing and could have printed the tool in one piece, because it is required to wait for each printed layer to cool enough before adding the next layer, it would have required 58 hours of continuous print time to produce a one-piece tool. Instead, they decided to print the part in four sections, two at a time for a total print time of 29 hours and 20 minutes – cutting the print time in half. The printing of these four parts required 3,613 pounds of Techmer PM’s polycarbonate material.
The four parts then needed to be machined and assembled. All surfaces of the parts, except for the front working surface, were then machined in place. The holes in the center of each side are for center-of-gravity brackets used for part handling during assembly.
The matching faces were then machined flat – except for the slightly raised bosses which insured proper gapping for the adhesive. The adhesive is only one part of a multi-part approach used to permanently and securely attach highly accurate, precision surfaces to each other.
In addition to the adhesive, draw bolt holes and slots were machined into the back of the stretch form tool parts, which allow the parts to be bolted together. Alignment holes and countersink holes were machined into the center sections of the tool.
The 6-inch long alignment pins for these holes were machined with adhesive channels – providing not only alignment between parts but also an additional level of permanent attachment.
The next stage of attachment used brackets, positioned inside the structure near the front surface, which were attached using tensioned aircraft steel cables – securely holding the front surface of the four parts of the large stretch form tool together.
The parts were then assembled. Each part is relatively heavy, and because of the relatively short open time for the adhesive, there was a limited amount of time available during assembly to apply the adhesive and attach the parts securely. The parts needed to be pushed together, while aligned within a few thousandths of an inch, and needed to be matched absolutely evenly. This was fairly easy using the Vertical Layer Print table mechanism installed on Thermwood Corporation’s LSAM machine.
The parts were then carefully hand-fitted. After this, one part was attached to the machine table and the other to the mechanism that moves the vertical table. The vertical table drive then moved the parts apart by about 20 inches. The adhesive was applied, and the vertical table mechanism pushed the parts back together again – square and perfectly aligned.
Although this is only a first step, and additional tests need to be performed and more data needs to be collected – it does prove that large, 3D printed, composite stretch form tools can be produced using currently-available materials and equipment.