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June 8, 2001

4 Min Read
Lower-tonnage molding strategies

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When trying to fit its mold for a polypropylene thin-wall tube plate, Robbins Scientific found that a 165-ton toggle was too big and an 83-ton press was too small. Eventually, the company settled on a 100-ton hybrid machine.

Molders have traditionally used an equation that determines the tonnage requirements for a given molded part. This rule of thumb has prevailed in the industry for years, but recently molders and machinery suppliers have begun to tweak this well-known calculation to bring new flexibility to the molding process. 

Take, for instance, Robbins Scientific Corp. in Sunnyvale, CA, a captive molder of laboratory disposables such as tubes, trays, and pipette tips. According to Jesse Cohen, senior processing engineer, Robbins was having trouble molding a thin-wall tube plate made from general purpose PP. The part weighs in at a mere 18g. While technically the mold is single cavity, it's composed of several cores that make it unusually complex and challenging. 

"There are 96 cores in this mold and 96 pin gates," explains Cohen. "We're injecting the plastic through these pin gates into the part's thin wall. It then flows around the cores and into the skirt, which is a thicker wall." This design creates core deflection and venting problems, he says. Because the tool was already built Robbins was limited in how it could approach a solution to these processing challenges. In fact, just finding the right machine to fit the mold proved difficult. 

First, Robbins tried to fit the mold in an 83-ton press, but the mold was too large. It was next moved to a 165-ton toggle machine. While the mold ran in the toggle press, ultimately the machine was too large. Residence time became an issue, and problems such as gas burns, long gates, and core deflection surfaced. 

What Robbins wanted was a smaller-tonnage press with larger tiebar spacing. This is where the company's needs coincided with Husky's development of a new 100-ton hybrid injection machine. 

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In conjunction with the moving platen, the stationary platen helps to distribute pressure evenly across the mold, as this analysis shows.

Small is Big 
The machine is the Hylectric hybrid press that features an electric screw drive, a hydromechanical clamp, large tiebar spacing, and a specially designed platen, which helps evenly distribute pressure. 

"It allows for more versatility—we can run larger hot runner molds that have small shot sizes without overclamping or degrading our material in the barrel," Cohen says. This versatility helped Robbins open up its processing window and address the problems it was having with the tube plate mold. 

"We were able to loosen the clamp and increase the venting," Cohen says. The Reflex platen, which uses a hydraulic and mechanical clamp, applies pressure evenly across the entire mold surface. This design helped Robbins eliminate the increased core deflection, he adds. Cohen also cites other improvements such as increased production yields and a cycle time reduction of 25 percent. In addition, there was no material degradation within the smaller barrel. 

The ability to use a smaller barrel is a plus, says Cohen. "For me, when molding smaller parts with high cavitation, a smaller barrel is advantageous," he says. The larger tiebar spacing also helps. "Our molds are getting bigger," Cohen reports. "We do hot runner molds as well, so the size of the mold is large, but the shot size is small." 

Platen Lineage 
The Reflex platen was born four years ago in Husky's large-tonnage business unit. "We were changing the stationary platen from a plate design to a casting design in an effort to reduce deflection across the mold face," says Bruce Coxhead, gm of Hylectric machines. 

Clamp tonnage was added through the center of the press, producing a uniformly distributed load of moving and stationary platens. "There's minimal deflection, and all cavities see the same clamp tonnage," says Coxhead. "We secured the tiebars to the back side of the platen, cast out material, and added stiffening ribs for even force distribution. This allowed the mold face to stay relatively flat. 

"The mold open and close stroke is fast because it is separate from the clamp piston and requires minimal oil," says Coxhead. "There's very little oil compressibility, and dry cycle time and press speed also improve." Because so little oil is used, Coxhead says any application where contamination is an issue, such as medical molding, would benefit from the Hylectric. 

The Hylectric Series will range from 55 to 1100 tons. Two, three, and possibly four injection units will be offered for multimaterial molding. In addition, rotary tables, stack mold carriers, and robots will be available. 

"The general consensus is yes, you can mold on lower tonnage," says Coxhead. Robbins demonstrated this at its technical centers, molding side by side with customers running their own tools. "In almost all cases we can mold at lower tonnages, and in some cases up to half their tonnage," Coxhead says. 

Contact information
Robbins Scientific Corp.
Sunnyvale, CA
Jesse Cohen
Phone: (408) 734-8500
Fax: (408) 734-0300
Web: www.robsci.com 
E-mail: [email protected]

Husky Injection Molding Systems Ltd.
Bolton, ON
Karl-Heinz Hoefert
Phone: (905) 951-5171
Fax: (905) 951-5385
Web: www.husky.ca
E-mail: [email protected]

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