Combined Screw Design and Heating Mechanism for Low Shear Resins
The apparatus and method that best results in low shear to melt shear-sensitive materials, such as polyethylene terephthalate, polyvinylchloride, acrylonitrile butadiene styrene, acrylics and/or resins with fiber fillers, without sacrificing process speed and/or cycle time, comprises the use of a barrel, screw and induction heating coil. The screw, in this case, preferably includes a low volumetric compression ratio, a relatively short feed section, an extended transition section, and a typical metering section.
1 . An apparatus for plasticizing low-shear resinous material, comprising:
an electrically conductive barrel having a longitudinal axis, along which material moves axially from an inlet to an outlet;
a rotatable screw disposed within and cooperating with an inner wall of said barrel, said screw adapted for plasticating low-shear resinous material fed into said barrel through said inlet as a solid, the screw having a longitudinal axis and a main flight having a pitch arranged helically on, and extending radially from a core of the screw forming a channel having a root depth in the axial core in reference to the inner wall of said barrel, the screw including a length having a feed section, a transition section and a metering section disposed sequentially downstream along said screw axis, the transition section being between about 45% to 60% of the screw length, wherein the core depth is progressively reduced; and,
an induction heater having a longitudinal length along the longitudinal axis of the barrel and a layer of thermal insulation interposed between an induction winding of the induction heater and an outer wall of the barrel, the longitudinal length of the induction heater arranged over the feed section between 20% and 40% of the screw length.
2 . The plasticating apparatus of claim 1 , wherein the feed section is between 20% and 30% of the screw length.
3 . The plasticating apparatus of claim 1 , wherein the induction heater operates in a range of 10 to 40 kHz, and the shear sensitive material is selected from the group comprising PET, PVC, ABS, acrylic and resin with fiber filler.
4 . The plasticating apparatus of claim 3 , wherein the feed section is between 20% and 25% of said barrel length.
5 . The plasticating apparatus of claim 1 , wherein 80% to 95% of external heat added to the low-shear resinous material is provided to said feed section by the induction heater.
6 . The plasticating apparatus of claim 4 , wherein the channel of the transition section includes a barrier flight.
7 . The plasticating apparatus of claim 3 , further comprising a plurality of heating bands with insulated covers over the outer wall of the barrel and along the length of the longitudinal axis of the barrel arranged over the transition and metering sections of the screw.
8 . The plasticating apparatus of claim 3 , wherein said screw has a volumetric compression ratio of between 1.2 and 3.
9 . The plasticating apparatus of claim 3 , wherein said screw has a volumetric compression ratio of between 1.2 and 1.8.
10 . A process of plasticating low-shear solid plastic material into a molten state under pressure, the process comprising the steps of:
(a) feeding solid low-shear plastic material with a rotating screw in a barrel having a cylindrical inner surface along a longitudinal axis, said screw having a helical flight with said flight cooperating with said inner surface to form a helical channel having a varied root depth in reference to the inner wall of said barrel to move said material toward an outlet port, the screw including a feed section, a transition section and a metering section disposed sequentially downstream along said screw longitudinal axis, the transition section is between about 45% to 60% of the screw length;
(b) applying induction heat along the feed section of said barrel using an induction heater to convert the solid plastic material to a solid-molten combination state while moving the material along said helical channel in the feed section, the induction heater having a length, and a layer of thermal insulation interposed between an induction winding of the induction heater and an outer surface of the barrel;
(c) mixing said solid-molten combination in the transition section arranged to reduce shear damage to said solid-molten combination, wherein the average root depth is reduced progressively downstream to form a substantially homogeneous molten material having substantially uniform temperature, viscosity, color and composition; and,
(d) metering said substantially homogeneous molten material through said outlet port.
11 . The plasticating process of claim 10 , further comprising the step of:
(e) applying resistant heat along the transition and metering sections using a plurality of heating bands in heating zones over the outer wall and along the length of the longitudinal axis of the barrel.
12 . The plasticating process of claim 10 , wherein the induction heater operates in a range of 10 to 40 kHz, and the low-shear plastic material is selected from the group comprising PET, PVC, ABS, acrylic and resin with fiber filler.
13 . The plasticating process of claim 11 , wherein said screw has a compression ratio of between 1.2 and 3.
14 . The plasticating process of claim 12 , wherein said screw has a compression ratio of between 1.2 and 1.8.
15 . The plasticating process of claim 12 , wherein 80% to 95% of the external heat added to the low-shear resinous material is provided in said feed section by the induction heater.
16 . The plasticating process of claim 15 , wherein the feed section is between 20% and 30% of the screw length.
17 . The plasticating process of claim 16 , wherein the metering section is between 20% and 30% of the screw length.
18 . The plasticating process of claim 16 , wherein the channel in the transition section includes a barrier flight.