There have been many impressive 3D printing projects across the world which test the limits of what can be 3D printed, but a new 3D printed pedestrian footbridge from the minds at Shanghai Construction Group and Polymaker proudly stands out among the rest. SCG and Polymaker are no stranger to ambitious projects; SCG they are responsible for boring the tunnels in Shanghai’s underground metro system and constructing the Shanghai tower, the second largest building in the world while Polymaker have helped develop materials for a range of industries and products including the worlds first mass produced 3d printed electric car.
An incredible achievement in engineering, this pedestrian footbridge is most likely the single largest plastic 3D printed object to date, measuring 15.25 meters by 3 meters wide with a height of 1.2 meters. To bring this project to reality Shanghai Construction Group collaborated with leading filament manufacturer Polymaker to develop suitable materials and Shenyang Machine Group and Coin Robotic to design and build a custom 3D printer totaling a $2.8 million investment.
Engineered to meet the demanding mechanical and printing requirements, Polymakers industrial division developed a unique ASA plastic AS100GF for the project. To determine the best material, 5 meter sections of the bridge were printed in various plastics with AS100GF ultimately chosen due to its favourable weather and chemical resistance, thermal stability, and toughness. The ASA is 12.5% glass fibers by weight, adding strength and also reducing the warping effect that plagues large 3D prints.
Of course printing at this scale brought some new challenges.
The printers bed consists of 16 18mm steel plates which have steel wedges driven underneath them, these steel wedges are then welded into place. To ensure the first layer adhered to the print bed, ASA pellets were glued to wooden planks which were then clamped to the steel bed.
Traditionally when printing large parts on a desktop printer, it is common for many plastics to buckle and warp, which can result in a print failure or impact the strength of the finished print. To combat this, many 3D printer manufacturers enclose their machines or add active heating to maintain a high environmental temperature, reducing internal stress in the print and reducing warping. With a build chamber 24m long, 4m wide and 1.5m tall with planned expansion, there was 144 cubic meters to keep heated. To maintain heat, a large actively heated bellowed tent was designed which moves with the gantry. The tent is heated to 38°C while blankets are placed on top of the print to slow the cooling process which relaxes the polymer chains and prevents warping. During the printing process, technicians worked inside the printer to monitor progress and move the blankets.
Additionally with a print time of 30 days, each layer has already cooled significantly by time the extruder returns to print the next layer. Both the blankets and glass fibers helped to slow the cooling, however reheating the print is vital. The print heat is equipped with equipped with four 600°C hot air guns aimed towards the extruder. These air guns help ensure the print is always hot around where the extruder is working for maximum layer adhesion.
With a round nozzle, the extrusion from a 3D printer is round. When the layer is pushed flat they have a tapered top, which is not ideal for layer adhesion. With desktop 3D printers, the taper is small enough that it isn’t noticeable but the SGC uses a nozzle 14 x the size of most desktop and industrial 3D printers. To ensure layer levelness and excellent layer adhesion, tamping the plastic right after the plastic is extruded was critical. With such a large print area, the printer is still accurate to 0.1mm so those differences in levelness are still important.
After printing, a CNC cutter is used to mill the surface in preparation for post processing. After post processing, the bridge will be transported in one piece to a park in downtown Shanghai for the public to interact. The SCG pedestrian bridge is an exciting project which truly displays the how far 3d printing has advancements over the times. There is no doubt the SCG 3D printer will certainly be used for more construction projects in the future, paving the way for new large scale 3D printing applications and projects.
To learn more about Polymakers materials for desktop and industrial 3D printing, click here.