By Jan H. Schut
Low-shear, low pressure “extrusion” molding machines developed by Extrude to Fill LLC, Loveland, CO (www.extrudetofill.com), a 2014 start-up, have been commercially molding tiny acoustic parts since 2015. Extrude to Fill’s process melts plastic without motion using electric heater bands in a thin-walled extruder barrel using conduction, not compression or friction between screw and barrel. The screw turns only to fill molds.
Melt temperature is controlled with sensors in the barrel and torque on the screw. A PID control loop uses those measurements to determine viscosity each time the cavity is filled. “You tell the machine what you want the output to be in terms of mold pack pressure, and let it run its algorithm to assure accurate plastic temperature and viscosity to produce the desired part,” explains Extrude to Fill CTO and co-founder Richard Fitzpatrick.
“Cycle time is the same as for conventional high-pressure molding, dictated by part cooling,” says Fitzpatrick, who worked almost 20 years on the process. “Fill time will last a second or half a second, similar to traditional injection molding. But where traditional molding pressure starts high and drops down; we start at 500 pounds and build up. Either way 500-3000 pounds is the final pressure in the mold cavity. Because of this build-up to the final pack pressure, we require less clamp tonnage than traditional machines—usually ½ to 1 ½ tons of clamp pressure per square inch (of mold surface).”
FIRST COMMERCIAL “EXTRUSION” MOLDING
Extrude to Fill’s first commercial customer, Aurisonics Inc., Nashville, TN (fender.com), a maker of high-tech audio equipment, wanted to mold tips for ear buds out of temperature-sensitive Class 6 medical grade thermoplastic elastomer instead of typical two-part silicone. Plunger-shaped tips are thick at the center, thin around the edge and couldn’t be molded out of TPE conventionally, Aurisonics’ president, Dale Lott says. The advantage of TPE tips is that they “deform in response to heat from the user’s ear,” says Aurisonics’ patent application (U.S. Pat. Applic. # 20160173971), so they’re more comfortable than silicone tips, which itch.
Aurisonics bought its first machine in 2015, molding 30 and 50 durometer TPE audio tips in up to four cavities and 13-second cycles. “The machine really enabled my business to grow. Our needs went from 100s of tips a month to 1000s a week,” Lott says. Aurisonics, which was acquired in 2015 by acoustic guitar maker Fender Musical Instruments Corp., added a second machine in 2016 for up to eight cavities and ordered two more machines this year to mold larger audio components.
Extrude to Fill offers machines in two sizes with either vertical or horizontal mold opening. Both sizes were originally designed for aluminum molds. The small machine is for 6 x 9 inch molds with up to 30-gram shots and a 3/4-inch screw 20 inches long (26.6 L/D). Aurisonics uses the small machine with vertical mold opening and a customized shuttle table and mold cavity. The operator inserts parts of the ear bud device before molding. After molding, the operator removes the core with parts, removes tiny undercuts by hand, and puts a clean core in. Aurisonics builds its own aluminum molds with inserts and undercuts.
A year ago Extrude to Fill built the first prototypes of the larger machine with both horizontal and vertical opening platens for aluminum molds to mold parts for another customer and do additional development. The large machine is for 15 x 15 inch molds with up to 200 gram shots and a 1-inch screw 28 inches long (28 L/D).
Potential customers for the large size, however, wanted to be able to use steel molds as well as aluminum, so Extrude to Fill redesigned the platens, integrating them into the machine frame to support heavier molds. The company has just built the first commercial large machine for horizontally opening steel molds, which will be delivered in November, and has four more on order for wire and cable and electronics to be delivered in the first quarter of 2017.
HOW ‘MELTING WITHOUT MOTION’ WORKS
Extrude to Fill machines have four temperature zones: a chiller on the initial feed zone followed by three heat zones: preheat, melting, and a heater band on the nozzle before the mold. The 2-3-inch long feed zone is chilled to control the start of melting. Then in the 3-4-inch preheat transition zone, pellets soften and merge, but don’t melt, forming a soft seal that moves forward and keeps melted plastic from flowing backward when the screw stops. Depending on polymer, plastic can be held in the barrel at melt temperature with the screw stopped. PP and PE don’t degrade if held at melt temperature, where ABS, nylon and some other resins do over time, Extrude to Fill’s Fitzpatrick says.
Next comes a 9-inch melting zone where screw geometry changes to short flights that roll melted plastic to get uniform temperature. The screw can be made of brass alloys or coated tool steel and can be heated. It’s driven by a three-phase motor: 1 h.p. for the small machine, which plugs into a 110-volt wall socket; 3 h.p. for the larger one, which needs 208 volts single-phase. The last zone has a patent-pending nozzle which closes with a needle-type valve, preventing formation of a cold slug of material. When the carriage moves forward, the screw retracts, opens the valve, and fills the mold. Instead of an injection or metering zone determining shot size like conventional injection molding, Extrude to Fill machines pump plastic continuously and on demand. There are runners and gates, but less mold cooling is required than for conventional injection molding.
Extrude to Fill describes lots of advantages to this process. Because of low pressure and sheer, it doesn’t degrade plastics. Because it doesn’t compress plastic under high force, it uses 85-90% less energy than conventional molding. Because of low friction, it can mold materials that are hard or impossible to mold conventionally like 50% glass-filled nylon or 75% stainless-steel-powder filled metal injection molding feedstock. Temperature control is more precise. It also has more control of part weight and can mold 100% regrind or recycle with consistent part weight despite variations in melt flow index.
Extrude to Fill isn’t the only electrically heated injection molding process. Hot melt adhesive and lab injection molding machines also melt with electric heat, not friction. But hot melt potting machines are limited to low temperature polymers, and lab machines are limited to very tiny shots. Extrude to Fill’s machine isn’t limited in either way.
Conventional injection molding machines can also use low pressure and resemble extrusion, like “intrusion molding,” in which plastic is first continuously extruded at low pressure into a mold, then packed with a final injection shot. (See an ANTEC 2010 paper on “Improvement of Injection Molding Machine Capacity by Intrusion Technique” by Abbas Mokhtarzadeh and Avraham Benatar of SABIC Innovative Plastics.) There is even licensed “Exjection” technology (U.S. Pat. # 7910044) from IB Steiner, a plastics engineering firm in Spielberg, Austria (www.ibsteiner.com), introduced in 2008, which can be used with conventional injection molding machines. Exjection combines extrusion with injection to mold long or continuous parts. But these processes still melt with compression and friction.
So Extrude to Fill is genuinely different, and more developments are coming. Extrude to Fill’s machines have tiebars now, but will eventually be tiebarless. Air-actuated clamp and carriage drives will be replaced by servo motors. For larger parts and higher cavitation, multiple extrusion molding machines can be used together in a patent-pending multi-port array. Extrude to Fill hasn’t built one yet, but that will be fun to watch!