Md Plastics' system avoids problems common with reciprocating screws
Md Plastics is developing a new polymer plasticating and injection system that eliminates processing and melt-quality issues related to molding with a reciprocating screw.
The new plasticating unit, called Inject-Ex, has a modified plasticating screw and injection plunger configuration that produces a more homogenous melt and precise polymer mass displacement, President Michael Durina said. The company manufactures screws, nonreturn valves, nozzle tips and shut-off nozzles, as well as process analysis systems, for injection and extrusion processes.
Durina cited three deficiencies of the reciprocating screw injection molding process that lead to molded part variations:• Viscosity variations caused in part by low melt quality and poor homogenization;• Variations in the quantity of melt that is dispensed into the mold; • The inability to accurately measure mass density displacement.
Durina said poor melt quality is inherent in a design that has to utilize a considerably longer feed zone to accommodate the "stroke." Every pellet experiences a different level of shear and residence time, and the solids bed is disrupted when the screw moves forward while injecting the melt.
Polymer within the melting and mixing sections of the screw experiences unintended shearing as the reciprocating screw moves forward, which can impact the homogeneity of the polymer, while the design and operation of a nonreturn valve at the discharge end of the injection unit can impact the flow and volume of polymer delivered into the tool, Durina said.
The Inject-Ex unit employs a nonreciprocating, helical plasticating screw housed within a heating cylinder assembly, but what makes the design unique is that the screw is coupled in-line to a special plunger head assembly, drive housing and motor. The Inject-Ex system prepares and injects the molten polymer more precisely and efficiently than either a reciprocating screw injection unit or a two-stage injection system featuring a continuous extruder and shooting pot combination, Durina said.
The Inject-Ex unit is designed so that the entire in-line assembly reciprocates — including the screw, barrel and plunger — an approach that Durina said results in a more precise mass of polymer being injected into the mold. The screw is viewed as a stationary device as both the screw and barrel move together, hence the various sections of the screw are in the same, fixed location relative to the barrel.
The Inject-Ex plasticating unit is comprised of several patented plasticating components designed by Md Plastics, including a fixed or stationary version of its Posi-Melt screw design and a mechanical shut-off mechanism in the plunger that ensures precise, instantaneous shutoff of the polymer flow. Durina said the in-line configuration of the Inject-Ex system features less piping and fewer components than a conventional screw/shooting pot design, and requires less space.
Additional benefits of the Inject-Ex include:• reduced energy consumption and a decrease in system pressure due to the in-line design;• no disruption of solid bed compaction;• a high degree of consistency among pellets because they've all experienced the same level of shear;• more consistent output and recovery time.
The Inject-Ex system also incorporates the company's Temp-Tek nozzle sensor, which takes melt temperature readings at various points and helps determine mass displacement of the shot.
Md Plastics developed hydraulically operated versions of Inject-Ex initially and is now working on an electric version, Durina said. The hydraulic Inject-Ex model EL 14-24-HZ/TP features a 14mm-diameter screw and 24mm-diameter plunger head and is designed to process 66 pounds of PS an hour. This model can generate 26,256 pounds per square inch of injection pressure and provides 4,496 pounds of nozzle-touch force.
Durina said an Inject-Ex unit is being shipped to an overseas injection molding machine OEM where it is being tested as a replacement injection unit on a press with 20 tons of clamping force. The Inject-Ex unit can be programmed to behave like an injection blow or a low-pressure injection unit, Durina said.
Mikell Knights, senior staff reporter
Contact
Md Plastics Inc.,Columbiana, Ohio, 330-482-5100, www.mdplastics.com
