Philips Medical Systems of Milpitas, CA, developed a nuclear diagnostic imaging system whose innovative design not only offers advanced medical imaging capabilities, but simplified installation, ease of use and pleasing aesthetics. Named SKYLight®, the system is the industry's first and only gantry-free nuclear camera, creating an open-floor design that allows imaging of a patient in any position, on any bed, and imaging of two patients simultaneously.
The instrument's state-of-the-art performance and practical advantages are complemented by the aesthetics of a newly engineered enclosure with sleek lines, reinforcing the high-tech capabilities of the system.
To improve upon the original housing, which consisted of painted and textured fiberglass panels, Philips worked with Specialty Manufacturing Inc. (www.smi-mfg.com) (SMI), a pressure former in San Diego, for design assistance, material recommendation and production of the thermoplastic enclosure system.
SMI evaluated several thermoplastic sheet materials, recounts Haydn Forward, SMI's Vice President, including ABS, an ABS/polycarbonate alloy and an ABS/PVC. A proprietary alloy from Kleerdex Company called Kydex® T was ultimately specified.
"The material carries a UL Std. 94 V-0/5V fire rating and, and based on our history with it, provides the formability, toughness and color qualities required by the SKYLight application," says Forward. "With impact resistance of 15 ft-lbs/in (801 J/m), it also offers significantly better protection against impact from wheelchairs and gurneys than other thermoplastic materials we considered," he says.
Formability and repeatability of the sheet were critical because the enclosure panels were redesigned with precise tongue-and-groove connections to simplify assembly. The snap-fit capability is so effective that only 16 fasteners are used to secure all 36 parts comprising the enclosure. The tolerance of the tongue-and-groove connections is measured in thousandths of an inch, explains Forward. The tabs and receiving slots of the connections are formed into the panels during processing and maintain tolerances within 0.015" (0.51mm).
The integrity of the tongue-and-groove connections also relies on memory of the polymer to maintain the shape of the part after the tabs and slots engage. "A key to this design is the natural memory of the thermoplastic sheet in achieving a snap-fit," says Jack Schrieffer, engineering manager at SMI, "and the stiffness and predictability of Kydex T sheet in this regard are crucial to the success of the novel design."
Because the SKYLight instrument is produced in limited quantities, with part production typically in the hundreds per year, color consistency is also of special concern. "Philips, must be able to mix and match components as necessary, whether for new units or as replacements, with no variation in color," says Schrieffer, "and consistency of the Kydex T sheet enables us to offer the repeatability associated with a precision color house."
Forward explains that appearance also hinges on the ability of the thermoplastic sheet to accurately reproduce the acid-etched texture of the aluminum pressure forming molds, and that the sheet reproduces mold finish accurately, with no part-to-part variation, even among deep-drawn parts.
The parts that SMI pressure forms range in size to 30" (194 cm) square, with draws up to 9" (229 mm). Sheet thicknesses range from .187 to .250 inch (4.8 mm to 6.4 mm) depending on part size, shape, and rigidity required.
Schrieffer says that pressure forming is an effective way to produce tight-tolerance parts in low volumes. "The sheer size of these parts would make injection molding a difficult and cost-prohibitive option," he explains, "especially considering the large number of different parts and relatively low annual volume. We are able to achieve the appearance and fit of an injection molded part with shorter lead times and lower tooling costs by using pressure forming."
Schrieffer adds that pressure forming readily allows the use of undercuts, which are critical to the tongue-and-groove design. Since there are no restrictions on core and cavity interface with the female cavity only of a pressure forming tool, it is easier to incorporate undercuts than it is in injection molding tools, which utilize costly collapsible cores to achieve the same result. With single sided aluminum pressure forming tools, design changes can also be made faster and at significantly lower cost.
SMI uses multi-axis CNC equipment to trim pressure formed parts. "A design change can often be accomplished by a simple CNC programming change—such as adjusting a height cut or an opening dimension. This makes pressure forming even more attractive from an overall cost and time to market viewpoint."
SMI precisely controls heating of the sheet as an important element of the overall process parameters. "We know we can count on the consistency of each sheet of Kydex to react the same to our heating profile to yield repeatable results. The material is not only predictable, it provides an excellent balance of physical properties and formability required for this application" says Schrieffer.
SMI received the "Multi-Part Assembly Award" in 2004 for the Philips SKYLight pressure formed enclosure from The Society of Plastics Engineers Thermoforming Division.
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Formed components used to enclose the Philips Skylight Gamma Camera System. Over 36 individual sections make up the final assembly. Kydex-T® was the material of choice to pressure form the product, based on aesthetics, physical properties and forming characteristics.
An innovative tongue-in-groove design utilizes the thermoplastic alloy’s memory to achieve a snap-fit. Pressure form ability to precisely duplicate intricate detail is shown here.
Cut away section. Kydex-T® facilitates complex geometry for mating parts. Also, note pressure form textured surfaces (from tool) and incorporates style line.
Undercut section on left part creates a tongue for assembly with mating part (right), without additional fasteners. Note style line at mating seam.
Undercut on part (left) creates a receiving groove for assembly with mating component.
Haydn Forward (left) and Jack Schrieffer (right) holding the Rear Bonnet next to a 4 station pressure form rotary machine, 1 of SMI’s 7 presses.
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