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From overmolding to water testing in record time

Experience, creativity and cooperation were essential to developing the automated production process and machinery for a unique microbiological test vial that certifies water free of E. coli in hours instead of weeks.

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Verifying that drinking water is free of harmful E. coli and Total Coliform bacteria contamination is essential and often time-consuming. Typical water vial testing samples can take up to two weeks of processing to achieve a declaration of β€œclean water.” A new portable, single-use water test cartridge and automated test system from Endetec called Tecta reduces the process to 2 to 18 hours depending on the level of contamination. But Tecta might never have happened without three years of collaboration among three main entities: Endetec, its mold manufacturer and its injection-molding machine maker. Both the test vial and the process of producing it required extensive creativity and cooperation.

The Tecta system features an instrument with built-in incubation, a sophisticated optical system, integrated user interface with interpretive software, and a unique test cartridge. Key design specifications of the cartridge included a plastic vial with a living hinge and cap, coupled with an optically clear liquid silicone rubber (LSR) plug/nub at the bottom. The LSR nub holds a water-soluble patch of growth media and enables testing via the optical system.

Discussions between the three companies focused on determining what materials could be used for the product and what process would most efficiently produce it.  The materials had to be clear to see the fluid inside the cartridge and pass gamma sterilization processing. Also, there could be no inhibitors in the chosen plastic in order to get an optimal seal with the LSR. Lastly, the materials could not interfere with the microbiological growth that takes place during the test process.

Given these parameters, the plastic selected was a modified polypropylene and the LSR was an optically clear 1003-grade silicone. The materials were finalized by tweaking the thermoplastic process. The next vital decision in the early stages was whether to mold the materials separately and assemble them, or to overmold them.

β€œProduction of the cartridge represented a unique challenge for overmolding technologyβ€”Roembke’s specialtyβ€”in that it combined low-melt thermoplastics and LSR,” explains Greg Roembke, CEO of mold manufacturer Roembke Manufacturing & Design in Ossian, Ind.

β€œRoembke has been adapting and perfecting two-shot molding and overmolding in thermoplastic injection molding to produce finished products out of the press,” he says. β€œIt’s one of the fastest growing sectors of LSR across multiple markets because it allows new formulas to bond to different substrates, such as silicone and plastic, most cost-effectively.”

The Tecta cartridge was particularly challenging because although LSR molds are heated to 320Β° to 400Β° F, the plastic has lower melting temperatures. Temperature also has direct impact on the cycle times, as well as the effectiveness of the seal between the plastic and LSR.

β€œWe have developed a unique process for increasing the melting temperature of the plastic to prevent it from deforming as the LSR molds are heated,” says Roembke. "We're the only toolmaker in the U.S. that has completed a two-shot LSR project successfully.”

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