This technical information has been contributed by
Roembke Mfg. & Design

Collaboration is Key to Design and Production of Water Test Cartridge

OSSIAN, Ind.—The increased incidence of acute gastrointestinal illnesses in the U.S., often attributed to municipal water supplies contaminated by non-disinfected groundwater, has driven the need to develop new microbiological test methods that are faster and easier to use than traditional methods. Recently, a turnkey single-use water test cartridge and automated monitoring system has been developed that reportedly yields results 20-80 percent faster than traditional methods by monitoring water samples continuously, rather than at the end of a typical 24-hour incubation period. Its early alerting capability enables the system to provide results within 2 to 18 hours, depending on the level of contamination.

The water test cartridge incorporates overmolded rigid plastic and optical grade liquid silicone rubber (LSR) in a fully-automated production process that utilizes two-shot molding and subsequent robotic transfer into an integrated assembly cell to complete six processes outside of the molding area. It is then put in a box ready to ship.

The water test cartridge system is the result of an approximately three-year, dedicated collaboration involving three entities: Roembke Mfg. & Design, Inc. (Ossian, Ind.), a designer and manufacturer of inserted flashless rubber molds; ENDETEC^TM (Kingston, Ontario), a water sensor and monitoring company; and Engel Machinery North America (York, Pa.), a major injection molding machine manufacturer.

Early Considerations: Design, Material, and Production Process

ENDETEC'S TECTA^TM automated microbiological testing system for E. coli monitoring features an instrument with built-in incubation, a sophisticated optical system, integrated user interface with interpretive software, and a special test cartridge for the detection of E. coli and total coliforms. Key design elements in the cartridge included a plastic vial with a living hinge and cap, coupled with an optically clear LSR plug/nub at the bottom of the vial for testing the water via the optical system.

Initial discussions between the entities 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 in order to see the fluid inside the cartridge and pass gamma sterilization processing. There could be no inhibitors in the chosen plastic in order to achieve an optimal seal with the LSR. In addition, 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; the LSR was an optically clear 1003 grade silicone. The materials were finalized by tweaking the thermoplastic process to figure out estimated cycles, temperatures of the mold, and sealing.

The next vital decision in the early stages was whether to mold the materials separately and assemble them, or overmold them. Two-shot molding and overmolding, among the fastest growing sectors of LSR across multiple markets, are specialties that Roembke has been adapting and perfecting in recent years to produce finished products out of the press.

"This two-shot overmolding process was quickly adopted, since it allows new formulas to bond to different substrates (such as silicone and plastic) most cost-effectively," said Roembke President Greg Roembke.

Proving Out the Process: Prototyping and Testing

Roembke (www.roembke.com) and Engel worked together in developing the two-shot molding cell and LSR tooling prototypes. Testing of the prototypes was conducted at Roembke's 3,600 sq. ft. Application Center, which has seven injection molding presses ranging in clamp force from 35 tons to 200 tons. This allowed them to thoroughly test and validate their molds on-site to develop the mold prototype and the process that would be used to produce the end product. "Additionally, the co-molding / injection molding machines were needed to ensure the process worked before buying the actual co-molding press," said Roembke.

A series of prototype tools were modified for the plastic vial and LSR component to complete the two-shot mold/press process. One press was used to mold the plastic vials, which then moved to the second press to apply the LSR tool. Engel's Victory 200H/80L/160 Combi, a 160-ton single cavity machine with wide platens, was the primary machine used. It is equipped with two injection units—primary and secondary—that were used to run the two separate materials. The primary (200H) is for thermoplastics and the secondary (80L) is for LSR.

This set-up was for pre-production testing and was designed with the capability to eventually expand to higher cavities (4+4) once the production process was proven out. According to Engel's Elastomer Project Engineer, Steve Broadbent, "Modification was also done to accommodate the side-change injection of the screw and barrel on the thermoplastic side."

Additional focus was directed to the thermoplastic side to meet the challenge of temperatures required to achieve a good cycle time. This was also done to produce an effective seal that would not leak fluid between the plastic and LSR materials. Endetec worked on the design of the seal between the two materials and determined that there could be no chemical bonding agent between them—another reason for choosing the special optical grade LSR.

Ultimately, the Roembke mold was a single-drop cold deck with a standard LSR valve gate design, featuring side injection versus back injection for the plastic. The final Engel machine was a standard two-component machine that incorporated the smallest diameter screw possible (12 mm) on the injection unit. The wide-platen machine incorporated a special ejector pattern for the tool design, centered behind each application for the thermoplastic and LSR. Dedicated servo valves were used on both ejection units, and valve controls were used for optimal speeds and pressures.

The first stage of automation was also addressed in the prototyping by Engel, which enlisted Pro Systems (Churubusco, Indiana), the automated machine systems builder for the plastics industry, to integrate the turnkey automation into the machine. Originally, a three-axis linear robot was incorporated on the operator side; however, because of operating height restrictions, a six-axis robot was used for the final assembly.

Notably, the six-axis robotic system completed six processes outside of the molding area. This also included placing a water soluble pouch, containing pre-measured amounts of growth media that support the enrichment of any target bacteria present in the sample, for each water test cartridge.

Ultimately, ENDETEC'S TECTA automated microbiological testing system for E. coli is expected to improve public health and safety of drinking water systems and help municipalities mitigate the risk. By providing an early warning in the case of a positive contamination event, operators can react faster and better protect their customers from illness.

The potentially revolutionary product also represented a unique challenge in overmolding technology for all entities involved. The collaborative effort by ENDETEC, Roembke Mfg. & Design, and Engel Machinery North America proved out a difficult and intricate system to optimize the synergies of having low melt thermoplastics and LSR work together.

"The world-wide market potential for this new water monitoring technology is simply huge," said ENDETEC VP of Operations, Doug Wilton. "We're already selling systems in North America, Europe, and Asia with additional markets under development."

Greg Roembke noted the potential for this growth in stating, "Through our extensive prototyping and testing, the system was proven-out and set up for future expansion. The single cavity tool can be easily expanded to a four-cavity tool as sales ramp up to anticipated quadruple volume.

"We have a unique design to build molds that allows our customers to mold products with the least expense by eliminating much of the labor and secondary processes," Roembke concluded. "The goal is to decrease mold costs while providing the highest quality, service, and responsive deliveries for the U.S. and worldwide."

Roembke Mfg. & Design (www.roembke.com) has specialized in the design and fabrication of quality inserted flashless molds for rubber, LSR, and HCR applications for 35 years. The company, which operates a 62,000 square foot manufacturing facility in Ossian, Indiana, offers precision compression molds, injection style molds, and liquid injection molds with open nozzle or valve gate nozzle. Roembke, a worldwide supplier to the medical, automotive, consumer, and associated manufacturing industries, has also expanded its product line to include specialized products, such as press tables and heating or cooling tables.