By Jan H. Schut
Ultrasonics, previously limited in plastic’s processing to welding, have been under development for a decade as researchers tried to use high frequency sound waves to enhance injection molding and extrusion. Basic research in ultrasonics isn’t cheap, so even technical papers and patents on the subject were limited chiefly to work at universities and research institutes. Interestingly, despite a global recession, four papers on ultrasonic processing were given at the Society of Plastics Engineers’ recent ANTEC 2010 conference last May in Orlando (see May blog selections). They highlight recent developments in commercializable ultrasonic injection molding and extrusion processes, citing compelling advantages like 5-10% higher output from single-screw extruders, improved mixing of additives, better exfoliation and dispersion of nano materials, and more efficient molding of micro parts.
These reports provide a road map to state of art of ultrasonic processing. Two papers, one from ASCAMM Technologogical Centre in Barcelona, Spain (www.ascamm.com) and another from the Institute of Plastics Processing at RWTH Aachen University (IKV) (www.ikv-aachen.de) in Germany, describe separate processes for micro injection molding with ultrasonics. ASCAMM is part of a consortium introducing the first commercial molding machines later this year.
The other two papers by graduate students at the Institute of Polymer Engineering at the University of Akron (www.poly-eng.uakron.edu) in Ohio describe advantages of ultrasonic modification to single-screw extruders. The University of Akron sold its first commercial license and built and shipped its first commercial ultrasonic extruder this summer to a recycler of rubber elastomer in Viet Nam. Ultrasonics will be used to break vulcanized bonds in the material so it can be reprocessed. What is even more exciting, though no paper was give on it, a privately held injection molder in Denmark is believed to be the first in the world to commercialize ultrasonic injection molding.
FIRST COMMERCIAL MACHINE FOR ULTRASONIC MICRO MOLDING
Mar Folgueral, a project manager at ASCAMM, presented SONOPLAST New Process and Machinery for Micropart Moulding based on Ultrasound Excitation [W6]. ASCAMM holds international patents on the use of ultrasound to melt plastic for micro molding and on the device for ultrasonic molding of micro and mini parts (EP2189264). It uses a sonotrode or ultrasonic horn to melt a small amount of micro pellets in a cylindrical chamber. The horn pushes melting plastic out of the cylinder directly into the mold. The process uses no plastification barrel, reciprocating screw or electrical heaters in the tooling, but takes only a few seconds from melting through filling.
Because plastification time and injection pressures are much lower than in conventional microinjection molding, there is less degradation of costly resins, ASCAMM says. “Tensile testing gave better mechanical performance retention, compared with traditional microinjection processes,” Folgueral writes. The process also reduces sprue volume by over 50% for considerable savings in expensive engineering resins, for example for small medical parts. Eliminating the plasticizing barrel and reducing sprue size reduces the machine size and saves energy, ASCAMM notes. They also found that the technology could replicate very tiny mold details (around 70 microns) without using high injection pressures. ASCAMM uses 30 kHz and vibration frequency or amplitude of 20-40 microns for amorphous polymers and 40-60 microns for crystalline polymers. (Below 20 kHz, noise becomes a problem, says one supplier of ultrasonics.) ASCAMM uses micro pellets of about 2.2 mm for ultrasonic processing.
The two-year R&D phase was completed last June, after which a new company, Ultrasion, was set up in Barcelona to commercialize the machines, called Sonorus. The first are being built now by Mateu y Sole S.A. in Barcelona (www.mateusole.net). Called Sonorus, they will be shown for the first time by ASCAMM at the Kunststoff Show in Dusseldorf in October.
Professor Walter Michaeli at the IKV and Thomas Kamps, an engineer at the institute, also gave a paper on Heating and Plasticizing Thermoplastics with Ultrasound for Micro Injection Molding [session M13], showing another process with production potential using ultra sound to melt small amounts of plastic for micro molding. Melted plastic flows in front of a piston and is then pushed into micro molds. The IKV uses an ultrasonic frequency of 20 kHz and 30 microns of vibration amplitude.
The IKV notes that controlling and measuring melt temperature with ultrasonic heating is challenging given the high speed of ultrasonic melting. A 2 mm thick plate of PC melts in as little as 0.4 seconds. “Energy released during plasticization drops with higher amplitudes, so if amplitude is doubled, plasticizing times shortens by 10 or 15 times,” the IKV found in these trials Using a melt probe to measure surface temperature of the melt isn’t always a good indicator because as heat dissipates into the material, the temperature can rise. So inside the material, it may be hotter than on the surface. This spike at the end of the cycle is difficult to measure with a surface probe. The IKV also notes that the form in which the polymer is used, i.e., pellet, powder or solid blank, affects melt temperature. Using a semi-finished blank allows the most control of melt temperature since it eliminates interfacial friction among pellets or grains under pressure from the sonic horn. The IKV so far isn’t working with commercial partners or applications for its ultrasonic molding trials.
FIRST COMMERCIAL ULTRASONIC SINGLE-SCREW EXTRUDER
It’s not extruding plastic, but it’s still the first commercial application of ultrasonics in a commercial single-screw extruder after years of R&D and numerous patents at the University of Akron. The 3-in. diameter single-screw extruder was built and shipped this summer to a Viet Namese company, which also took a license. It will be collecting data on devulcanizing rubber for recycling. Setareh Niknezhad and Rishi Kumar, both graduate students under Dr. Avraam Isayev at the university, gave two separate papers at ANTEC highlighting other uses of ultrasonics in single-screw extrusion. Niknezhad’s paper on Ultrasonic Extrusion and Film Blowing of PA6/Clay Nanocomposites [W13] describes successfully blowing nylon 6/clay nanocomposite film with an ultrasonically modified single-screw extruder. “Ultrasonic waves were used to exfoliate nanoclays, causing high-energy mixing and dispersion due to acoustic cavitation: the formation, growth and implosive collapse of bubbles in the melt,” she reports. “Ultrasonic waves also break down the molecular chains reducing viscosity of the melt.”
The University of Akron’s extruder has a conventional extruder barrel with a modified screw and applies ultrasonics directly to the barrel and to the melt pipe before the die to increase flow and throughput. Other advantages may be material specific. Niknezhad’s research on dispersion of nano clay in nylon 6 used barrel temperatures of 210 C in the feeding zone and 230 C thereafter with ultrasonic frequency of 20 kHz and amplitudes of 5, 7.5 and 10 microns, much shorter frequencies than other researchers. She also reports that die pressure with ultrasonics in conjunction with clay is complex.
Kumar’s paper on Processing and Characterization of PEEK/MWNT Nanocomposites Prepared by High Power Ultrasonic Waves [W13] also describes single-screw extrusion with an ultrasonic barrel attachment to enhance dispersion. In the case of multi-wall carbon nanotubes in PEEK, the research is aimed at avoiding a secondary step of prolonged ultrasonic treatment of CNT in solution, which then has to be removed, a process which can break the CNTs. Ultrasound also reduces die pressure and increases throughput, he reports, due to molecular chain scission of the polymer.
WORLD’S FIRST COMMERCIAL ULTRASONIC MOLDING
DKI plast A.S. in Denmark (www.dkiplast.dk), a molder and extruder of highly technical parts for electronics, automotive and lighting industries, is believed to be the first in the world to mold plastics commercially using ultrasonics. DKI reportedly uses it to improve flow, allow thinner walls in large parts and shorten cycle time. The technology was developed by a European Union sponsored collaboration of four plastics industry associations between 2002 and 2006, which grew out of a previous project, involving the Pera Group, a private innovation firm in Melton Mowbray, U.K. (www.pera.com), and Brunel University in London (www.brunel.ac.uk). DKI plast was one of the industrial partners and became the patent assignee for the technology (WO/2004/024415 and U.S. Pat. Applic. 2006016532), which applies ultrasonics to both runners and injection molds in order to make it retrofittable onto existing molding machines.
DKI plast commercialized the process two years ago to improve resin flow in large thin-walled PC and acrylic parts. An ultrasonic horn extends directly into the melt in the runners. Ultrasonics are also used in the tooling to enhance flow. “Our focus is mainly for large parts, where you can reduce wall thickness or cycle time with ultrasonics,” the company says. DKI plast also uses ultrasonics for its patent-pending MicroPrism process, proprietary embossing process that creates highly accurate prismic surfaces for light control. MicroPrism was announced at the Light and Building show in Frankfurt-Main, Germany, in 2008.