Hybrid additive/subtractive manufacturing machine produces large-scale parts

The standard Thermwood LSAM has a 10-ft.-wide by 20-ft.-long work envelope. Lengths up to 100 ft. are available.

Thermwood Corp. first made a name for itself by manufacturing big, high-speed CNC routers. It remains well-known today for building 3-, 4-, and 5-axis routers used to make products ranging from furniture to boats. In recent years, the Dale, Ind., firm has also gotten into additive manufacturing (AM)—in a big way.

The company’s LSAM (large-scale additive manufacturing) machines have work envelopes that measure 10 feet wide by 20 to 100 ft. long. They are the largest polymer printers in the world, according to Thermwood Vice President of Marketing Jason Susnjara. Capable of both 3D printing and trimming, the machines are used to make thermoplastic composite molds, tooling, patterns, and parts. They are marketed to companies in the aerospace, marine, automotive, and foundry industries, as well as the military.

A Different Approach

LSAMs are made with steel plate using slot-and-tab construction. A patent-pending design featuring special steel alloys can handle processing temperatures of 450 degrees C, allowing the printing of high-temperature polymers for autoclave tooling.While other efforts to print large thermoplastic structures have amounted to scaling up small, filament-fed desktop printer technology, the machine is the result of a fundamentally different approach.

“We knew that because we were going to a very large scale that we couldn’t just go to filament and make it a bigger process with a bigger machine,” Susnjara said. “We’d have to adjust everything.”

Instead of using filament-type thermoplastic feedstock, for example, Thermwood worked with Techmer PM LLC, Clinton, Tenn., to develop pellet-form thermoplastic materials reinforced with carbon fiber. At the beginning of the process, the pellets are dried in hoppers and then vacuumed into another hopper that releases them into the “melt core” below.

This patented extrusion-screw-and-barrel combination does not work like a traditional plastic extruder, which relies on the shear action of the rotating screw to generate most of the heat to melt the material. Since screw speed is constantly changing in an AM operation, the conventional approach produces uneven heating. With the LSAM extruder, by contrast, more than 60 percent of the heat comes from barrel heaters rather than the screw, resulting in the polymer being heated more evenly.

The standard melt core is 40 millimeters in diameter. Thermwood also offers a 60-mm melt core designed to increase print head throughput for the production of very large parts.High-quality printed structures require print bead dimensions that are accurate to thousandths of an inch. Thermwood claims such print bead precision is not possible when using conventional extruders, which are often plagued by an uneven output flow known as surging.

The LSAM’s design prevents this by directing material from the extruder into a polymer melt pump. The job of this servo-driven, fixed-displacement pump is to precisely meter heated polymer to the nozzle to produce a precise and consistent print bead. The control automatically synchronizes melt-pump output to machine speed so that changing speeds will not have an adverse effect on bead dimensions.

With its ½-inch-diameter nozzle, the machine prints a large bead at room temperature. Beads are laid down next to each other, one after another, to form a layer. Layers are printed on a patented bead board designed to minimize part warping and cooling stresses.Right behind the nozzle is a patented compression wheel that presses each bead onto the layer below. This action collapses any air pockets between layers. Fully fused and free of voids, parts produced by the machine can maintain vacuum in an autoclave without the application of a seal coating, according to Thermwood.

The LSAM’s ½-in.-dia. nozzle lays down one bead upon another at room temperature. A compression wheel follows the nozzle and presses each freshly laid bead onto the layer below. This action collapses any air pockets that form between layers.

LSAM prints near-net-shape parts that are a little larger than the required size. “With such a large bead, there are bigger ridges than normal on the outside that need to be machined away for a smooth surface,” Susnjara explained. Following the printing operation, the machine’s trimming head removes excess material.

Printing Versus Machining

Why print a large part instead of machining it? Machining processes often start with a large piece of material, most of which has to be cut away to attain the desired shape. To compare the waste generated by subtractive and additive processes, Thermwood conducted a test with a company that uses traditional machining methods to make an oil drip-pan mold for a Chinook helicopter. Thermwood used an LSAM to print the same mold.

The result? “We were able to save 34 percent on material using the additive method and reduce labor by 69 percent,” Susnjara reported. In addition, “it only took us three days to complete that project, while it took them eight.”

Clearly, large-scale printing can yield large-scale benefits.

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