This page contains a Flash digital edition of a book.
Materials


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 chal- lenge 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 self- lubricating. 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 applica-


tions is the titanium/nickel alloy nitinol, which has shape- memory 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.”


Putting MIM in Gear at Parmatech ATW


subsidiary Parmatech Corp. (Petaluma, CA) was approached by a medical instrument manufacturer about an articulation gear part in an instrument


primarily used for minimally invasive surgical operations. The compo- nent 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 tem- porarily 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, be- cause 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 capabil- ity. 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


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—


82 ManufacturingEngineeringMedia.com | May 2013


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 opera- tions 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 mate- rial waste in fabricating the parts versus machining and no secondary operations involved with burr removal like those needed in machining.


Photo courtesy Cordis Neurovascular


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184