30 May 2017
30 May 2017
Thermwood has unveiled a new design for the print head on its Large Scale Additive Manufacturing (LSAM) systems.
This new Universal design print head can be equipped with any of three interchangeable 'melt cores.' The print head is large by industry standards, at over 10 ft long and weighing 1.5 tons, but still moves at speeds up to 5 ft per second, Thermwood reports. The print head is designed so that the melt core, which consists of a feed housing, extruder and polymer melt pump, can be changed should higher or lower print rates be required.
The print head is a critical element in Thermwood’s additive manufacturing process which functions differently than other FDM thermoplastic 3D printers. Most thermoplastic additive manufacturing systems print with a relatively small print bead onto a heated table in a heated environment. The heated environment is needed to keep newly printed layers from getting too cool to properly fuse with subsequent layers. In Thermwood’s approach the only heat source is the print head itself and a heated environment is not required. The process prints a large bead at such high output rates that the printed layer must be cooled rather than heated to achieve the proper layer to layer fusing temperature, the company reports, and produces large, high quality, virtually void free structures.
Each layer is printed at a rate that allows it to cool to the ideal temperature before the next layer is applied. If the layer becomes too hot, print speed is reduced to allow more cooling time. If it becomes too cool, print speed is increased to reduce cooling time. A built-in thermographic imaging system displays a real time thermal image on the CNC control screen which aids the operator in achieving the ideal print temperature.
The LSAM Universal Print Head can process material at temperatures up to 450°C. It uses a Siemens temperature control module integrated within the print gantry CNC control. This allows full integration of temperature and pressure control with Thermwood’s print gantry CNC, better supporting processes unique to 3D printing.
The three melt cores offer a different maximum print rate. The maximum print rate determines the longest bead that can be printed during the available cooling time between layers. This cooling time varies depending on material, amount of fan cooling and geometric shape of the layer. The faster the print rate the more material that can be laid down within the cooling time between layers, so faster print heads allow larger parts to be printed, but do not really print parts faster. Even the standard 40 mm LSAM melt core is generally so fast that it must be slowed on most parts to keep from printing a layer so fast that it does not have sufficient time to cool properly between layers. In this case, often multiple parts can be printed in the same time it takes to print just one.
The LSAM machine is equipped with a standard 40 mm melt core which includes a patented 40 mm high speed extrusion screw coupled to a corresponding melt pump and deposition head. This standard configuration processes over 150 lbs of material an hour and is suitable for parts that have a print layer lap length of up to 175 ft while printing a standard bead that is 0.200 inch thick and 0.830 inch wide. This configuration has proven more than adequate for virtually all large parts today, Thermwood reports.
If longer layer bead lengths are required, higher output melt cores are available. A 60 mm melt core can process 50% more and a 70 mm melt core has operated at rates of over 500 lbs per hour.
The fastest speed at which a part can be printed is determined by the cooling time required to reach the proper bonding temperature between layers and not by the output of the print head. Larger print head outputs simply allow larger parts to be printed within the required cooling time between layers. High output melt cores do, however, have a minimum operating speed so may not be suitable for smaller parts. If both small and really large parts are required on the same machine, the melt core can be changed from one size to another in less than a shift.
Thermwood has installed a universal print head on its current 10’ x 10’ LSAM development machine with a 40 mm melt core and has printed 20% carbon fibre reinforced ABS plus 40% and 50% carbon fibre reinforced PPS in operational tests. This print head will be installed on a new 10’ x 20’ demonstration machine currently under construction.
Thermwood plans to have all three melt cores available for this demonstration machine. Production machines come standard with the 40 mm melt core. Thermwood’s LSAM machines both print and trim on the same machine using separate gantries.
Photo provided by Thermwood
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