The task: to mold plastic parts with functional hollow sections for conveying liquid media or routing cables, for example, in a cost-effective manner. The proposed solution: a combination of gas injection technology (GIT) and blowmolding.

The process, GITBlow, is in development at the Institute of Polymer Engineering, Univ. of Paderborn (Paderborn, Germany), and will eventually be commercialized through 3 Pi Consulting and Management GmbH (Paderborn, Germany) and several corporate development partners. 3 Pi is a startup company spun out from the Institute of Polymer Engineering.

GIT is often employed to create cores in parts that reduce shrinkage and improve the cosmetic appearance and dimensional tolerance of parts. Generally speaking, however, the cross-section of the gas bubble cannot be shaped at will and for this reason a specific minimum residual wall thickness must be retained to ensure that the gas bubble does not break through the part wall (see February 2006 MPW for a comparison of GIT and water injection technology). Application of GIT is therefore challenging if large, hollow cross-sections are required.

The GITBlow process employs two steps to realize parts with large hollow cross-sections. Firstly, a standard GIT-molded part is produced. Then, the mold cavity is "made considerably larger" using moving cores. By re-applying the gas pressure after core pull, a thin-walled channel is realized by the resultant stretching (see photographs). Currently, core displacements of 5 mm are possible but the developers expect progress will be made and that eventually, 10-mm core displacements will be feasible.

"During the inflation process, the temperature of the molded part needs to be within the thermo-elastic or thermoplastic range of the plastic being processed," says project manager Helmut Ridder. This can be achieved either by rapidly expanding the mold cavity and immediately inflating the part so that the residual heat GIT process can still be used (direct process), or by re-heating the part in the open mold using a radiant heater system (two-stage process). "The direct process is fundamentally easier and faster to use, but exactly which process option is employed [direct or two-stage] depends on the initial blow-geometry," says Ridder. "There are limits in the direct process. The two-stage process can expand the cavity to a larger geometry."

A prototype part molded from high-impact polystyrene on a Battenfeld TM1300 injection machine using GITBlow incorporated a more uniform and larger channel compared a GIT-molded part (see Figure 3, left). This also contributed to faster cycle time than standard GIT.

Cell draws a crowd

An automated blowmolding cell running at the Uniloy Milacron stand during February''s Plast exhibit in Milan, Italy, attracted significant visitor attention.

The cell, making complex shaped automotive pipe lengths, included a suction blowmolding machine from Uniloy, combined with a Kuka robot to maneuver moldings to an automated post-processing unit. This unit, manufactured by GS Automazioni (Fontanelle, Italy), completing trimming of the moldings; separation of pipes by cutting the weld joint; milling of pipe ends; welding of a support; and unloading of the machined pieces.

A compressor from Bekoblizz completed the cell. The complete cell fit within about 10m2 floor space. MD

Stephen Moore . [email protected]

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