September 7, 1998

8 Min Read
Engineering Productivity Kit - Plastics 9-7-98

September 7, 1998 Design News


Plastics paint medical montage

Polymers give engineers advantage in medical-device designs

Gary Chamberlain, Senior Editor

Newton, MA--No one enjoys getting a shot with a hypodermic needle, but was a new design really needed? Absolutely, says Danny Patel, a plastics engineer who started his own custom injection-molding company, Computerized Plastics Molding (Irvine, CA). So Patel came up with a needleless system that is virtually painless and never breaks the skin.

Patel went to India to bring his wife and young son to the U.S. His family needed inoculations before they could leave the country. "Afterwards, my son cried for hours," Patel sadly recalls.

In addition to the general discomfort, repeatedly puncturing the epidermis with a conventional hypo leaves marks and can invite infections. "Most injectors are spring loaded and have metal parts: They must be sterilized between shots, are cumbersome and heavy, and quite costly. We knew we could do better," Patel adds.

His J-Tipr injector, a disposable syringe, incorporates its own pressurized gas power source. The all-plastic, molded unit measures about four inches long and weighs about nine grams. Patients load the J-Tip with the desired medicine dose, place it against the skin, and trigger its action with their thumb. The trigger breaks a seal releasing pressurized gas that drives a plunger forcing medication into subcutaneous tissue. It happens in a split second. The small syringe is convenient to carry and easy to use; there is no high initial cost.

Patel worked on the project with Tooling Manager Jack Kay. Finding the right material for the gas container proved the biggest challenge.

National Medical Products successfully launched the injector around the world. Having already received clearance for insulin, Xylocaine, and Calcimar/Calcitonin, the company has applied for general-use clearance with the FDA.

Needle-free IVs. Another product designed around plastics takes the ouch out of IVs. A plastic resin that resists lipid emulsions for about the same cost as similar resins, while offering easier processing, made the IV design possible.

B. Braun Medical Inc. implemented the lipid-resistant polycarbonate on its SAFSITEr needle-free IV system. Bayer Corp.'s Polymers Div. (Pittsburgh) supplied the Makrolonr DP1-1805 PC resin.

The SAFSITE system requires only a few components to eliminate the risk of needle sticks in IV care. Its simple, yet ingenious valve system allows for injection, aspiration, or infusion of fluids and medications--without needles.

"Offering our IV system in a lipid-resistant material was a breakthrough, and the increased chemical resistance in a polycarbonate (PC) material is an added value we can offer customers," reports James P. Roma, senior materials engineer for Braun. "Moreover, it adds these features without a major cost increase over other PCs."

Braun molds the two-component valve system using multi-cavity tooling. A hollow plunger goes over the disk, then the valve housing is ultrasonically welded. An end cap protects the valve when not in use.

The normally closed SAFSITE reflux valve opens at the insertion of any standard Luer taper, such as a syringe or IV tubing. Once the Luer taper is removed, the valve closes automatically, preventing backflow.

"Bayer supplied all of the FDA-required compliance data we needed for the resin," says Roma. "The material is also easier to process than other materials we've worked with, from drying to its flow in the mold."

Airing out bellows design. When a domestic maker of highly technical medical rehabilitation devices sought to enter competitive overseas markets, it faced prohibitive taxes and tariffs on its existing equipment, largely due to the devices' internal hydraulic systems. The company evaluated redesigning the hydraulic portion of the equipment and found that a pneumatic design would not only offer reliable performance, but help minimize tariffs.

The new rehabilitation equipment, used to build muscle strength and mobility for patients recovering from strokes, broken limbs, and other injuries, now features a pneumatic bellows made with a thermoplastic polyurethane (TPU) sheet. Produced by Sealtech Inc. (Athens, TN), the bellows keeps the equipment operating smoothly and reliably over extended periods of time.

Filled with air and constantly flexing, the bellows requires a material with high puncture and abrasion resistance, good memory, high flex fatigue resistance, and durability. SealTech recommended Stevens Urethane (Holyoke, MA) for the project.

With a minimum working life of two to three years, the bellows faces a variety of end-use environments. The equipment could be used almost constantly, or remain idle for long periods. Also, the operator's skill level may vary tremendously, meaning the material must withstand possible abuse or improper adjustment.

"Urethane is very tough," notes Sealtech's Jim Clare. "Its strength gives it puncture and abrasion resistance. At the same time, it contains no plasticizers, which translates into a long working life."

The bellows consists of pieces of black urethane sheet that are radio-frequency sealed. RF uses both pressure and radio frequencies to realign the molecular structure of the materials being bonded in order to create a strong, reliable weld.

"The material always meets our performance specifications, as well as any certifications we may need for medical applications," Clare adds. "In addition, Stevens' technical support makes a real difference in the success of a product."

Less color, no pre-drying. A new series of thermoplastic vulcanizate (TPV) elastomers enables makers of medical devices and health-care products to boost quality and reduce costs--while obtaining the same levels of strength and elasticity provided by more conventional TPVs. Tradenamed the Uniprener 7010 series, the compounds, produced by the Teknor Apex Plastics Div. (Pawtucket, RI), conform to 21 CFR for medical and food-contact uses. The materials can be injection molded or extruded.

"Manufacturers are evaluating the new TPVs for overmolded plugs in surgical cables and for plunger tips in syringes," says Peter M. Galland, Teknor Apex industry manager. Citing new data from tests comparing the compounds with conventional TPVs, Galland points out that the elastomers reduce or eliminate pre-drying because they absorb lower levels of moisture. "They also can be colored more readily because they exhibit less yellowness in natural uncolored form," he adds.

"When processors are forced to pre-dry materials, they incur extra costs for equipment, energy, and labor," Galland explains. "Yet there is no guarantee that the drying step will eliminate all traces of moisture, which can cause defects like splay in part surfaces or foaming in extruded components."

Coated tools produce more powder-metal parts

Ridgway, PA--Add long-term wear resistance to tooling made from a steel that combines high strength and fracture toughness, and what do you get? In the case of Alpine Pressed Metals, you get 100 times more productivity.

Alpine faced the usual severity challenge of powder-metal parts manufacturing--the effect on tooling of high press loads and the abrasive wear encountered in high-speed powder-metal compaction. The solution: a switch from conventional tool steels to AerMetTM-for-Tooling alloy, a specialty grade developed by Carpenter Technology Corp. (Reading, PA).

To obtain longer wear, Jim Paladino, Alpine engineering manager, turned to Metalife Industries (Reno, PA), a specialist in industrial chrome plating. Metalife applied a thin coating to Alpine's tools in two stages. First, a chrome film was bonded molecularly to the metal substrate. Then, in what's called a polyseal process, a melted polymer compound was added for lower friction. "The results have been spectacular," says Paladino.

Alpine's problem started with an inner bottom punch used to make iron-base powder-metal bushings for the seat adjustment in automobiles. The bushing measures 0.900 inch long, and has a 0.650-inch OD, a 0.400 ID, a key on the OD, and a 25-degree taper across the face. Two 30-ton compacting presses turn the parts out at the rate of 700 to 800 per hr. Pressure applied during the procedure averages 32 tons psi during each compacting stroke.

Initially, the punches consisted of AISI Type S7, AISI Type D2, and 4% vanadium powder metallurgy cold-work tool steels. The punches warped in heat treatment, making it impossible to grind relief in the punch ID. Worse, they would break near the top end, sometimes after making as few as 1,000 pieces.

"We couldn't get any repeatability," Paladino recalls. "It was like a crapshoot. We wanted to produce 150,000 to 250,000 parts a week, but never knew what to expect."

Alpine asked Dietech Tool & Die Co. (St. Mary's, PA) to make its punches from the AerMet alloy. Working from 11/4-inch bar stock, Dietech lathed the tools close to size, then heat-treated them in accordance with Carpenter's specifications. Next, the punches were double-aged, finish-ground, and face-detail-burned on a CNC/EDM machine; polished; and then delivered to Alpine.

With the new punches, Alpine eliminated all breaking problems and distortion. However, to make certain they could withstand all service conditions, Alpine had them coated by Metalife with a chrome solution held at 136F, resulting in an initial plating hardness of Rockwell C72 to 74. At last count, Paladino reports the company has produced more than one million bushings with the tools--and they have yet to show any signs of wear.

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