Additive Manufacturing

MAY 2018

ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.

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Page 28 of 43

AM / Precast Concrete, Meet 3D Printing 27 president. The "as-printed" model incorporates sufficient stock for the machining process and is fed into the printer as an STL file. Each form weighs between 450 and 750 lbs and takes 8 to 11 hours to print, Schmidt says. Following printing, a large five-axis router machines each form to the "as-machined" model. Light sanding and inspection follow before the forms are shipped to Kentucky. The precast concrete windows are poured on top of a large platform in Gate Precast's Winchester facility. The 3D- printed forms actually represent the front and inner sides of the window cavities, with the outer edges formed by removable plywood walls. A typical pour uses three of the forms to make a three-panel precast window. After the forms are treated with a release oil and rebar and inserts have been placed, the pour begins. It takes about three buckets of concrete (see the photo on page 25) to fill one of the three-window pours, with each concrete cast weighing 20,000 to 30,000 lbs depending on the profile. Curing takes 12-14 hours, and then the forms are stripped and the concrete pieces lifted from the platform. Each window frame is manual- ly cleaned with an acid wash spray that exposes the sand in the mix. Then, some of the front faces are polished. The resulting finish sparkles—quite intentionally—like sugar cubes. Finally, Gate Precast also installs the glass in each pane. This is a new assembly step for the company, says Schweitzer, but a good example of how precast concrete can provide sav- ings in cost and labor by shifting assembly tasks off-site. Lessons Learned Getting to the point of producing windows with these forms took a bit of R&D. Pours on early 3D-printed forms resulted in what Schwietzer calls "the corduroy effect"—visible bead lines from the 3D print that transferred into the concrete. ORNL solved this problem by increasing the bead size in the BAAM and overprinting the forms so that the excess could be ma- chined down to a smooth surface. Gate Precast also discovered that the bottom faces of the forms were not necessarily square as-printed, so the manufacturers have to flip each form after milling the walls to then CNC the bottom to a flat surface. AES also cites thermal distortion as a challenge in work- ing with parts of this size. "Even though we are using carbon fiber-filled ABS, the parts still have a tendency to warp slightly," Schmidt says, "and that must be accounted for during the design process." (Like Gate Precast, AES typically deals in small quan- tities but has benefited from the repetitive nature of this job. Manufacturing multiple pieces that are very similar "has given us the ability to really hone in our process," Schmidt says. "This was a welcomed bonus as we are usually making one-off parts.") There were also challenges with some specific features. For instance, one of the window profiles has an indentation in the frame which must be supported by the 3D-printed form. Early forms lacked reinforcement under this feature, and the weight of the concrete caused sagging in the form and final product. It took about half a dozen design iterations to achieve a workable solution, Schweitzer says. The forms for this profile now include ribs under the indentation to support the concrete during the pour and curing process. Maintenance has also been an ongoing process of discovery. While the 3D-printed molds are far more durable than their wooden counterparts, they still require regular maintenance as they age. "We know we're going to ding it up," Schwietzer says, so "How do we repair it?" is a necessary question to answer. The most common maintenance issue is separation of the 3D-printed layers at the bead line, which Gate Precast mitigates with Bondo (commonly used on the wooden forms as well) and the use of a heat gun to deposit polymer pellets (which are then sanded down to a smooth finish). A Piece of the Pie Gate Precast estimated that each wood form necessary for the Domino Sugar building would cost between $1,500 and $1,800, and require substantial skill and labor to produce. With the sheer volume of forms needed, this approach would have been costly and time consuming. In addition, a wooden form would be good for about 15-20 pours before requiring maintenance or scrapping. Each 3D-printed concrete form, by contrast, costs about $9,000—by no means a small investment. However, this project should show that a printed form can support as many as 200 con- crete pours in its lifetime. For the consistency and high volume required on the Domino Sugar project, the repeatability and durability of these forms makes sense. The 3D-printed forms also provide an aesthetic benefit: Compared side-by-side with con- crete casts made on wood forms, the 3D-printed precast window frames can be smoother, with sharper corners. The project has gone so well so far that PCI has been using Gate Precast as an example for its members. The Winchester plant has hosted several groups of precasters who are interested in see- ing how 3D printing can support their work. These tours include representatives from companies that could be considered Gate Precast's competitors—but that doesn't worry them. "Our competitors are not other precasters, but cast-in-place concrete and other materials," Schweitzer explains. For Gate Precast, "It's about getting a bigger piece of the building pie, not just the precast pie." The 3D-printed forms for the Domino Sugar windows are providing proof of concept that could win more of that pie for precast concrete.

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