Martin SchÃ¤fers, a doctoral student at the Institute of Polymer Technology at the University of Paderborn, Germany, described the GITBlow process, which combines aspects of gas-assisted injection molding and blow molding.
Actually, GITBlow uses two gas-injection steps, the first to make a hollow section, and the second to expand that hollow section even more. The finished parts have thin walls and extremely large hollow areas — so GITBlow competes favorably with water-assisted injection molding and extrusion blow molding, SchÃ¤fers said.
Applications include conduit for automotive fluids, air conditioning ducts and intake manifolds. For appliances, the process can make conduits and ducts for air and water, and other hollow parts.
The first step is making what SchÃ¤fers called a preform by using gas-assisted molding with nitrogen gas. To finish the part, a core pull is moved, and gas is pumped in again.
Researchers in Paderborn molded parts on an Arburg press with 120 metric tons of clamping force to run the trials. They tested two methods, a one-stage proc¼ess and a two-stage process.
One-stage GITBlow does both gas-injection steps in the same station, in a closed mold. For the two-stage method, the preform is made in one station, then is moved to another mold, then reheated for the second shot of gas.
Two-stage GITBlow requires a rotating mold and a handling unit for the radiant heater, so it's more expensive. But it has several advantages. Selective heating of the area to be inflated the second time means that no areas that have already cooled get stretched. Also, the gas-conveying zone of the preform and the finished part can have different external contours, providing for more freedom of design. Finally, the preform and part are molded at the same time on the rotating mold, cutting the cycle time.