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Weld lines, also called knit lines, are a common problem encountered by injection molders. Beyond aesthetic issues, weld lines can also lead to functional failure. During a press conference at K 2022 in Düsseldorf, Germany, compounder Sirmax Group introduced Gas-Assisted Push-Pull (GAPP) technology, which, it claims, can prevent weld lines in injection molded plastic parts.

Professor Giovanni Lucchetta of the University of Padua, founder of Smart Mold, a university spin off that developed and patented GAPP, explained the technology at the Sirmax stand. Italy-based Sirmax has a 50% stake in Smart Mold.

Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Weld lines are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. In a weld line, reinforcement fibers adopt an orthogonal orientation with respect to the filling direction, which results in a significant reduction in the strength and rigidity of the molded part. During the presentation at the K, Lucchetta explained how GAPP promotes the interpenetration of weld line flow fronts by changing the shape of the interface, promoting inter-diffusion between flow fronts, and realigning reinforcing fibers.

“Others have tried unsuccessfully to eliminate the criticality of weld lines in structural parts,” said Lucchetta in a prepared statement. Dynamic packing during the initial phase of the “holding” process allows for improved matrix interdiffusion at the interface and reorientation of the fibers in the direction of flow. However, according to Smart Mold, this typically requires two independent injection molding units. By contrast, GAPP allows for the dynamic packing of weld lines using a single injection unit and miniaturized gas injectors. Moreover, the small size of the gas injectors makes the technology compatible with existing molds, according to the company. “I am convinced that the strength of GAPP technology lies in the ability to implement it in existing molds with noninvasive and inexpensive modifications,” stressed Lucchetta.

Experimental tests cited by Lucchetta reportedly showed that GAPP can increase weld line strength by 240% in polypropylene reinforced with 35% short-glass fibers, almost reaching the nominal strength of the compound.

The technology can play a key role in advancing the use of recycled plastic in glass-fiber-reinforced compounds, he added.

GAPP technology can solve weld line strength issues in all parts made of fiber-reinforced thermoplastics that require high mechanical performance, said Smart Mold. Applications include supports, brackets, cooling fans, pulleys, and other structural parts.

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