Retrofit package to enhance injection molding performance.
The spiral-shaped hopper is said to improve on conventional conical designs by providing consistent feed rate regardless of the amount of material in the hopper or mixture of virgin and regrind. Consistent flow, without bridging or "rat-holing" reportedly is a result of the hopper's spiral curve, which allows all material to descend evenly through the spiral hopper. IMS v.p. Kenneth Berger claims that the spiral hopper completely eliminates the need for vibrators to prevent bridging even of hard-to-feed materials.
A 'GENTLE' SCREW
The primary advantage of the Fliteless screw is minimized front-end shear; it is claimed to bring the temperature of the material fully under the control of the computer. Deep flights in the feed zone pump material forward to the flightless section, in which melt channels have been eliminated. This flightless section is tapered, gradually compressing the material and maximizing contact of unmelted pellets with the surface area of barrel and screw.
The internal heat pipe redistributes heat from hot to cool spots on the screw, thereby balancing temperature of the melt. Although this device is said to be new to Fliteless screws, it has been used successfully on general-purpose screws, of which IMS has 30-40 in commercial installations, says Berger.
AFTER THE SCREW
Molecular orientation of the melt is said to be the major benefit of the spiral end-cap system, consisting of the special screw tip, end cap, and nozzle. The spiral screw tip is said to push all material out of the barrel with every shot without fracturing the melt. A "Brody-ring" seal on the check valve reportedly eliminates blowback around the valve, which may be of sliding-ring or ball-check type. The nose cone uses a spiral taper that realigns the melt as it leaves the barrel, essentially relieving the causes of internal melt shear. It's also said to reduce the stress waves that the pressure pulses of injection build into successive layers of material ahead of the screw tip. Like the nose cone, the nozzle also uses a spiral curve to achieve polymer orientation. The spiral form leads the meld toward the sprue bushing opening with minimum shear stress. Berger claims that the reduced stress in the melt is evident during purging, when the material comes out in a relaxed, straight flow.
DOES IT WORK?
One molder that was involved in developing some aspects of this system reports producing improved parts, particularly with heat-sensitive materials. Roger Holtslander, maintenance supervisor at Wolf Engineering, Dearborn, Mich., has seen incremental, although noticeable, improvements in various stages of the molding cycle, and has documented some of these with a computer program comparing performance of various components.
Holtslander claims that use of the spiral-curved front-end components on the barrel solved a burning problem when molding acetal. "In a conventional funnel-shaped approach, material flows down the middle first; near the front end, material can flow backward as well, developing a shear line," he says. He believes the spiral curve was effective in unrolling the molecular chains, allowing them to come out straight. The reduction of melt stress led to slightly (up to 4-5%) stronger parts, testing showed. Holtslander says he also observed that less melt "stirring" improved the flow of polyethylene.
Similarly, Holtslander believes that the Fliteless screw--because it doesn't produce as much shear that can break molecular chains--also helps make stronger parts. Holtslander thinks that the combination of Fliteless screw and spiral end cap was one factor in steady increases of part quality that allowed Wolf Engineering to eventually achieve reject rates of one part in 22 million, an improvement by a factor of 10. He adds that the heat pipe, by redistributing the heat at the front of the screw where the melt conducts heat more rapidly, is especially effectiv and produces almost instantaneous results.
Perhaps most surprising of all, tangible improvements in part dimensional stability were demonstrated with the spiral hopper at Wolf Engineering. "Roll tests" compared the surface contact area of curved molded parts and a matching piece of curved metal. Parts molded on a machine with a straightwall hopper showed an 85% contact rate; the same part molded on a machine with a spiral hopper had 100% contact. Holtslander theorizes that temporary bridging in the conventional hopper caused uneven feeding into the screw flights. This caused a variation in the volume conveyed forward along the screw, and a consequent oscillation in melt temperature. "Even a couple of degrees either way can make a difference, and a 5[degrees] defference can be significant," he remarks. The uniform feeding of the spiral hopper reportedly stabilized that aspect of the process.
An added benefit of the retrofit package is standardization of end cap, screw, and hopper. In addition to making various components interchangeable between different molding machines, this will also help eliminate process variations between one machine and another, says Berger.
Price of a retrofit package on a Van Dorn machine with 50-mm diam. screw, including spiral hopper, Fliteless screw with heat pipe, and front-end system of spiral end cap, screw tip, and nozzle, is $4932.
This same spiral-curve technology may have future applications in other areas of the injection molding process as well. Some potential areas include the runner system or gate designs to better control melt temperature inside the mold by eliminating shear-produced heat. (CIRCLE 18)
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|Title Annotation:||Injection Molding|
|Author:||De Gaspari, John|
|Date:||Oct 1, 1991|
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