3-D printing is taking off as an attractive technology for engineering, prototyping, and other commercial uses, and demand for the plastic rod or filament that feeds the printers has never been greater. Yet, the extrusion lines producing filament today tend not to be as efficient as they could be, and there is room for quality improvement as well. Taking advantage of experience in the production of high-precision profiles and tubing, including heart and brain catheters, Conair extrusion experts assembled a demonstration line to show how using the latest equipment and a complete systems approach can result in significant improvement over the current state of the art.
As 3-D printing technology advances and as high-end printing equipment becomes more sophisticated, tight filament tolerances are becoming ever more critical. If the diameter is oversized, or the ovality (roundness) varies, filament can misfeed or even jam in the feeder. If it is undersized, the feeder rolls may slip, and the flow of polymer to the printer nozzle may be inconsistent.
Most printers are calibrated volumetrically; that is, the amount of material delivered through the nozzle is assumed to be a function of the filament diameter, along with the speed at which it is being fed. An inch of filament of a given diameter is assumed to deliver a specific volume of polymer. If the diameter varies even slightly, the volume of resin deposited on the part will also vary, causing voids and other defects.
The best producers in the 3-D-filament industry today are supplying product with tolerances within ±0.002 inch (0.058 mm) on diameter and also on ovality, although much of the product on the market is more like ±0.003 inch (0.075 mm).
Demonstrating Fast Throughputs
A demonstration filament line, which was run off at the Conair Extrusion Development & Testing Lab in Pinconning, Michigan, has achieved throughput rates of 400 to 600 ft/min. (122 to 183 m/min.), which is three or four times greater than the typical production rate in the industry. Even at these high extrusion rates, quality is also much better than industry norms. Conair has recorded just ±0.0005 inch (0.0127 mm) variation on diameter and less than 0.001 inch (0.0254 mm) on ovality.
The extrusion line starts with drying of the ABS resin in a Conair mobile drying and conveying system (model MDCW 100), which also loads the resin to the extruder hopper. A 2-inch (51-mm) Davis-Standard Super Blue extruder was used with a Conair GRH-1.0 extrusion die designed specifically for filament/rod production.
Sizing & Cooling the Filament
After emerging from the die, the filament passes into a Conair pre-skinner that begins the critical sizing and cooling process. From the pre-skinner, the rapidly moving extrusion enters a Conair HTMP-series multi-pass cooling and sizing tank. The filament makes three passes through the tank before exiting to a precision belt puller and a Conair servo-driven automatic cut-and-transfer coiler. A laser gauge supplied by Zumbach Electronic, positioned between the cooling tank and the puller, continuously monitors diameter and ovality and provides a feedback loop to the extrusion line control to maintain a consistent product.
The HTMP tank is a key component when it comes to both quality and productivity. Unlike a standard open water bath, which is still used in many filament installations, the Conair tank combines vacuum sizing and precision cooling in a very space-efficient configuration. Cooling temperature in the initial sizing section is controlled separately from the rest of the tank by a Conair temperature control unit. Using warmer water in the vacuum section can help to prevent voids that may form in the center of the filament if the plastic cools too quickly. Vacuum sizing is usually reserved for the production of hollow shapes, but Conair has found that the technique can also be used with solid filament to ensure the highest possible level of filament uniformity.
After exiting the vacuum section of the HTMP tank, the filament enters the primary cooling water bath, where water temperature is precisely controlled by a Conair chiller. When the filament reaches the end of the tank, however, it does not exit as it would in a conventional tank. Instead, it loops around an oversized servo-driven capstan and is directed back toward the extruder again before looping around another capstan and making a third pass through the cooling zone.
Taking up only about a third of the floor space that would be required by a standard open cooling tank, the Conair multi-pass tank still has plenty of cooling capacity so the filament can move at the speeds required to achieve high throughputs. Tight control over water temperature and pulling speed (the large driven capstan actually functions as the primary puller for enhanced precision), together with continuous laser gauging, ensures high-quality filament is produced even at up to 600 ft/min. (183 m/min.).
An air-wipe removes moisture, and a three-axis laser gage measures and confirms ovality and diameter. A relatively long belt puller is used to draw the filament out of the tank, providing a strong but delicate grip over a long length of the filament. Finally, a self-regulating loop control feeds an automatic cut-and-transfer coil winder that smoothly wraps the filament onto production-sized reels, ready for transfer to smaller spools in a separate operation.
The Conair Extrusion Development & Testing Lab is available to filament producers who conduct pre-production runoffs on new equipment or develop customized solutions that meet and even exceed the industry’s highest standards for quality and productivity.
About the author… Dave Czarnik, Engineering Manager, Downstream Extrusion, manages the Conair Extrusion Development & Testing Lab in Pinconning, Michigan. The Lab was created to help Conair and its customers “push the boundaries of extrusion productivity and profitability.” Czarnik is a tooling expert with 25+ years of plastics experience and 22 years working in downstream extrusion for Conair. He holds five extrusion-related patents.
Note: The original version of this article was published as a Technology Bulletin at www.conairgroup.com in 2015 as “Optimizing Precision and Productivity in Extrusion of 3D Printing Filament.”