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Twin-sheet thermoforming achieves durability with economy

29 July 2011

A US based thermoforming specialist has used a novel twin-sheet process to reduce cost and weight, as well as improving the aesthetic appearance of an enclosure component for a proprietary medical device.

Family-owned Kintz Plastics, based in Howes Cave, New York, is a leading US manufacturer of heavy-gauge thermoformed products. In a recent application for client Siemens Healthcare Diagnostics, the company turned to twin-sheet thermoforming with a proprietary thermoplastic alloy to produce heavy-duty enclosures for the latter's Immulite immunoassay medical diagnostic testing instrument

The two side doors of this device, previously moulded from glass fibre reinforced plastics, were redesigned in order to take advantage of the twin-sheet process’s manufacturing and cost advantages; namely: the ability to make lighter parts with better performance using less expensive capital equipment, less expensive tooling, and more flexible production. Kintz Plastics’ engineering manager, Michael Righi takes up the story:

“These parts offer excellent stiffness, lighter weight, and more detailed geometry than the labour-intensive fibreglass versions, which were heavy, and required long lead times. The twin-sheet process gave us greater control in manufacturing and more consistent shape and size. Unlike fibreglass, there was virtually no variation and we realised a significant weight reduction.”

The smooth and finely detailed surface finish demanded by the redesign also results in a highly attractive appearance, adds Mr Righi.

The process
Twin-sheet thermoforming involves two simultaneous vacuum forming operations, which produce an integrally-welded, hollow part, similar to one that has been blow moulded. The two halves of a twin-sheet part can be of the same or dissimilar materials and thickness, with front and back finished surfaces. Effectively replacing two processes with one, twin-sheet thermoforming saves labour, results in reduced weight and yields greater strength compared with separately formed parts that are either mechanically fastened or bonded with adhesives.

The two plastic sheets are heated in a double framework set-up and then transported to the forming station where, as the two moulds are brought together, air is evacuated and the sheets are pressed and fused together at certain pre-determined weld points.

The side door enclosures were originally designed in flame-retardant ABS/PVC sheet but Kintz opted for a rigid proprietary sheet from Boltaron, of Newcomerstown, Ohio, because of its improved properties and cost/performance benefits. Boltaron 4335 is a fire-retardant, extruded alloy sheet offering a UL 94 V-0 rating, impact resistance of 2.1-2.5 kg/m, and broad chemical resistance, allowing cleaning using concentrated cleansers. The company uses 4mm thick sheet front side and 3.2mm back side, and reports extreme formability with minimal thin-out in deep recesses and on outside corners.

Once moulding is complete, several secondary operations are performed including machining and trimming with a three-axis CNC router. Backside mounting blocks for hinges are incorporated, a screw block is bonded to the sheet with an epoxy system, and brass inserts are ultrasonically pressed into the part. Both side doors are painted for a two-tone colour, after which Kintz applies a metal-based EMI/RFI spray-shielding component on the interior side for anti-static protection.

Along with the front doors, Kintz also manufactures a large front bumper enclosure which is pressure formed from Boltaron 4335 sheet.

Twin sheet thermoforming not only lowered the cost of the doors by between 30 and 50%, but is also achieved an overall weight reduction, lowering shipping costs to customers, and making the unit that little bit easier to move around in the hospital.

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