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Materials
Photo courtesy Cordis Neurovascular
ceramic: As people were taking steps, their joints were literally
squeaking—loudly!—and these are permanent implants, so
there’s no easy way to minimize the sound. The other challenge
with ceramics is that they’re comparatively brittle—if
they receive the wrong impact, they break, creating a problem
much worse than noisy joints.”
While the squeaking is a quality of life problem, it is not
much of a wear problem. Ceramics are essentially selflubricating.
But there are also serious machining issues with
ceramics. They are extremely hard, so shaping them is a
problem—especially when you need such a perfect fit.
Nitinol: Thanks for the Memory
A material that is growing in popularity for certain applications
is the titanium/nickel alloy nitinol, which has shapememory
capabilities that make it exceptional, MacNeal says,
and there are players in the industry that specialize in making
devices that take advantage of that ability.
“A common example would be nitinol stents,” she notes,
“which can be manufactured in a shape needed to rebuild a
blood vessel, then collapsed to a much narrower diameter for
easier insertion into the vessel, and finally allowed to resume
its ‘remembered’ original shape as a scaffold to support the
blood vessel.”
Cordis Enterprise stent, a self-expanding shape-memory
nitinol microstent.
Shape-memory nitinol is also used for filters deployed
in the aorta—if a blood clot gets through the aorta into the
heart, it can mean instant death for the patient. MacNeal is
impressed with the nitinol-based solution. “These filters are
amazing—shape memory allows them to be inserted in a
compact form, but when they deploy, they look like fishing
lures, with tiny prickers or barbs that extend out to catch clots
before they can enter the heart.”
Nitinol is also a metal popular in angioplasty applications:
“The cardiac sector of the medical device industry is huge—
Putting MIM in Gear at Parmatech
ATW about an articulation gear part in an instrument
subsidiary Parmatech Corp. (Petaluma, CA) was
approached by a medical instrument manufacturer
primarily used for minimally invasive surgical operations. The component
was designed to be polymer injection molded, but during trials and
development, the articulation gear would strip due to the forces involved.
Since it was already in late-stage development trials, the OEM had temporarily
switched to an aluminum machined part to be able to continue
production without delay. The machined part was then insert molded with
plastic. Machined aluminum was sufficiently strong to prevent stripping
of the gear teeth, but the subsequent cost to machine was a significant
departure from planned costs.
The OEM launched the product with the more expensive machined
component, and then sought to convert the machined component to a
MIM part. The part is insert molded, a process in which tolerances are
typically accurate to within 0.0005" (0.0127 mm). Historically, plastic
molders have been hesitant to use MIM parts for insert molding, because
the tolerances are so tight that if the MIM part’s dimensions miss
any tolerance, the insert molder’s tools could be destroyed.
One of the most commonly used alloys, MIM-stainless steel 17-4,
was chosen due its low material cost, robust operating parameters, and
extensive operational history in terms of as-sintered tolerance capability.
Parmatech made the 17-4 stainless steel articulation gear part with
its proprietary MIM process, and then sent the part to the insert molding
company, which put the part into the mold, sealed it and injected the
insert molding. The two pieces, now joined together, were sent to the
ultimate customer for assembly into the instrument.
MIM process variation can induce a ±0.003" (0.076-mm) tolerance
on a 1" (25.4-mm) dimension, so planning to put a MIM part into a hard
tool steel mold, and have it properly shut off to prevent flash or tool
damage, is a challenging exercise. Parmatech worked closely with the
insert mold tool builder to determine precisely how much room we had
and tolerances required. The critical portion of Parmatech’s sintering is
to make sure the part’s feet have a certain pocket or envelope that must
fit exactly: missing just one part risked crashing the insert molder’s tool.
At the end of the day, the customer received an insert-molded MIM
part that met their functional and cost requirements. The success of the
project demonstrates how MIM can be used to increase part strength
without the high cost of machining, even when insert molding operations
are involved. Cost savings were substantial over the machined
part, with no individual part handling occurring after the initial stack at
molding. In addition, there is a much higher production rate capability
with injection molding compared to machining. MIM material strength
meets application requirements, and MIM material surface finish on the
gear teeth was superior to that of machining. There was very little material
waste in fabricating the parts versus machining and no secondary
operations involved with burr removal like those needed in machining.
82 ManufacturingEngineeringMedia.com | May 2013