Thermwood Corp. first made a name for itself making large, 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, Indiana company has also moved into additive manufacturing (AM) – in a big way.
The company’s large-scale additive manufacturing (LSAM) machines have work envelopes that measure 10 feet wide by 20 to 100 feet long. They are the largest polymer printers in the world, according to Jason Susnjara, vice president of marketing at Thermwood. Capable of both 3D printing and trimming, the machines are used to manufacture molds, tooling, models and thermoplastic composite parts. They are marketed to companies in the aerospace, marine, automotive and foundry sectors, as well as the military.
A different approach
LSAMs are fabricated with a steel plate using a slot and tab construction. A patent pending design featuring special steel alloys can withstand processing temperatures of 450 degrees C, allowing high temperature polymer printing for autoclave tooling. While other efforts to print large thermoplastic structures have consisted of augmenting the small desktop printer powered by filament technology, the machine is the result of a fundamentally different approach.
“We knew that because we were going on a really large scale, we couldn’t just go filament and make it a bigger process with a bigger machine,” Susnjara said. “Everything should be adjusted.
Instead of using a filament-like thermoplastic raw material, for example, Thermwood worked with Techmer PM LLC, Clinton, Tennessee, to develop thermoplastic materials in the form of carbon fiber reinforced granules. At the start of the process, the granules are dried in hoppers and then sucked into another hopper which releases them into the “fusion core” below.
This patented combination of extrusion-screw and barrel does not work like a traditional plastic extruder, which relies on the shearing action of the rotating screw to generate most of the heat required to melt the material. Since the screw speed is constantly changing in AM operation, the conventional approach produces uneven heating. With the LSAM extruder, on the other hand, more than 60% of the heat comes from the cylinder heaters rather than the screw, which allows the polymer to be heated more evenly.
The standard cast iron core has a diameter of 40 millimeters. Thermwood also offers a 60mm molten core designed to increase print head throughput for the production of very large parts. High quality printed structures require print bead dimensions accurate to thousandths of an inch. Thermwood claims that such print bead precision is not possible when using conventional extruders, which often experience uneven output flow known as pumping.
The design of the LSAM prevents this by directing the material from the extruder into a molten polymer pump. The job of this servo driven fixed displacement pump is to precisely dose the heated polymer up to the nozzle to produce an accurate and uniform print bead. The control automatically synchronizes the output of the fuser pump to machine speed so that speed changes do not adversely affect bead dimensions.
With its ½ inch diameter nozzle, the machine prints a large bead at room temperature. The beads are placed next to each other, one after the other, to form a layer. The layers are printed on a patented ball board designed to minimize part warping and cooling stress. Right behind the nozzle is a patented compression wheel that presses each ball onto the layer below. This action reduces any air pockets between layers. Fully fused and void-free, the parts produced by the machine can maintain a vacuum in an autoclave without the application of a sealant coating, according to Thermwood.
LSAM prints near-mesh shaped parts that are a little larger than the size required. “With such a big bead, there are larger than normal ridges on the outside that need to be machined for a smooth surface,” explained Susnjara. After the printing operation, the trimming head of the machine removes the excess material.
Printing versus machining
Why print a large part instead of machining it? Machining processes often begin with a large piece of material, most of which must be cut to achieve 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 pan mold for a Chinook helicopter. Thermwood used an LSAM to print the same mold.
The result? “We were able to save 34% on material using the additive method and reduce manpower by 69%,” said Susnjara. In addition, “it only took us three days to complete this project, whereas it took them eight.”
Obviously, large-scale printing can generate large-scale benefits.