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August 3, 1998

4 Min Read
Systems approach to thin-wall and DVD molding

Resin suppliers don't get much larger than GE Plastics, which earned $1.5 billion last year on sales of $6.7 billion. But even at this magnitude, it is unusual to find any among "the majors" that investigate not only material performance and part design, but processing technology, mold design, and tool qualification as well. GE now believes what most OEMs and custom molders know--making high-quality products at the lowest cost means coordinating every step in the manufacturing process.

At a recent press tour to mark the 10th anniversary of the Polymer Processing Development Center (PPDC), GE displayed several significant investments in this "systems" approach, primarily focusing on specialty markets. Most notable of the myriad of projects taking place in the 96,000-sq-ft facility were a thin-wall computer monitor housing program and optical media development center for CD-R and DVD manufacturing.

During a panel discussion at Antec, Blair Souder, manager of GE's injection molding programs, described how the R&D efforts of the materials suppliers have changed. They are now "focusing on the game changers," he said, instead of doing basic process research. Molders who lament the disappearance of general molding R&D from the materials suppliers will have to understand that suppliers are focusing on specialty areas with the greatest promise--ones where they can both make a difference and speed market growth for their materials.

Thinner Walls for Larger Parts

According to Kurt Weiss, GE's program director for thin-wall molding, cell phone housings led the way to thinner wall thickness in small, relatively flat injection molded parts. "Today, OEMs want to translate that technology into large, deep parts with molded-in features," says Weiss, "using more robust materials while remaining cost competitive." The newest program at the PPDC--molding computer monitor housings from Cycoloy 2800 (PC/ABS) with 2-mm thickness rather than 3 mm--targets that goal.

In April, GE acquired a 1760-ton Mega HCS press--the first in the U.S.--from Sandretto USA designed specifically for this project. Tony Firth, vice president and general manager for Sandretto, explains the design process, "Together with GE, Sandretto looked at all of the variables for the monitor housing project--part design, wall section to thickness ratios, material, part weight, flow length, and target cycle time. Then we calculated residence time so that the injection unit would be optimized for the specific application."

Firth notes that residence time depends not only on barrel capacity, but also on screw profile, L/D ratio, and cycle time. "Next, we addressed specific issues of injection rate and pressure on the material," he says. "With PC/ABS, to achieve a fast filling rate, you need higher injection pressures, for example."

As a result of this consultation, GE technicians now compare results using the HCS and sequential valve gated mold designs to standard gating on a conventional machine. During the press tour, operators were testing a single gated mold on the HCS. Even with this restrictive mold design, the machine was running at only 30 percent of the available injection pressure and producing acceptable parts. According to Weiss, however, the typical scenario will involve sequential valve gating to minimize knit lines.

Emulating DVD Production

At the Optical Media Development Center, another major commitment at the PPDC in terms of floor space and capital investment, manufacturing cells are pumping out future versions of DVDs and recordable CDs (CD-Rs).

"By emulating a production environment," says John O'Sullivan, program leader, "we can develop the right combination--Lexan formulas that are manufacturing worthy given the current equipment and processing technologies." For example, he adds that in DVD production, bonding is critical to good yields. "GE can test various PC formulas for their bonding performance to address this issue."

Current processes for DVDs involve precision molding the disks (1.2-mm thick) using a metal master that contains recorded information in the form of pits. The PC disks are then sputter coated with metal and lacquer sealed. GE is also working on future products, such as two-sided DVDs with the ability to read through both sides.

Quick Tool Qualification

To shorten the time for mold trials, GE has developed a proprietary software package called QTip (quick tool introduction program), available to its customers. The package defines the size of the tool process window using a Windows interface. "It displays the process window in one graph, which takes temperatures, pressures, cycle times, and speed into account," says Tom O'Connor, technical program leader. "We designed QTip to give molders a way to simulate part quality at various machine settings."

Current versions of the software can handle up to 11 processing conditions. Using a design-of-experiments matrix, QTip characterizes both the tool and process in one run, and can also determine necessary tool modifications, according to O'Connor. Once a process window is defined, transducers placed inside the tool measure the "fingerprint"--conditions for producing a good part--so that moving the tool to a different machine can be done quickly.

Contact information
GE Plastics
Phone: (800) 845-0600
Fax: (800) GE FAXBACK
Website: www.geplastics.com

Contact information
Sandretto USA
Middleburg Heights, OH
Phone: (440) 243-4673
Fax: (440) 243-4588
Website: www.sandretto.it

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