09/16/2005

Clamason Industries Ltd

Designers! Why not metal?

A look back over the 20th Century shows that the vast majority medical devices were invariably made from some type of metal and fabricated using one of the common manufacturing methods available at the time. In the present day, these methods have largely been left aside in favour of stereolithography in the case of low-volume items or highly complex injection-moulding techniques because the current material of choice is plastic. There is a danger that materials and methods will be chosen merely because they are the most modern and older processes will be ignored even though they may be a better option if they are given more serious consideration. There are three basic means of producing device components from metal and all three have a place in the modern medical device industry, if only the designer will examine their benefits.

Machining centres, together with wire and spark erosion, have gained in popularity in recent years, and even if they are not used for actual components, their applications are well known for the manufacture of mould tools. However, pressing and casting options have had far less exposure and it is these processes that are examined here.

Pressing.

Most people associate pressing with the manufacture of cars and their bodywork. It’s perceived to be labour intensive, and old fashioned. Indeed, much of the pressing industry does have some involvement with the automotive market place but this has positive effects. Detailed quality systems and the safety critical nature of many components have resulted in an industry that is cost-effective and well placed to satisfy the exacting demands of medical device protocols.

Components can be produced from virtually any metal, from stainless steels and copper base for conductivity through to aluminium or magnesium if weight is an issue. Metals also have a unique advantage in that they possess perceived value. The touch, weight, and general appearance is pleasing to the eye, in almost any application we appreciate the quality that metal conveys. We expect to see stainless in our kitchens because it suggests a clean environment, our TV’s are silver & our hand held devices although made from plastic are coated to appear to be metal.
For example, one of the Worlds’ leading companies, Nokia who are recognised as a major design innovator in addition to manufacturer have recently introduced & and are marketing the fact that their 6170 model has a stainless steel case, a perfect example of using metal in what would normally be a first choice application for plastic.






All metals have the following characteristics.

Accuracy and precision. Components can be extremely thin (25 microns) with tolerance spreads of less than 10 microns, and unlike plastics, there is no shrinkage to allow for, & no split lines.

Speed. Precision presses can run at speeds in excess of 2500 cycles per minute, making them very cost effective when compared with injection moulding machines.

Consistency. Materials are commonly supplied from single master batches and as they are not hydroscopic or light sensitive they’re unaffected by their environment.

Conductivity. Metal components can possess heat and electrical conductivity features if the product needs it.

Longevity. Metallic safety-critical automotive components such as airbags can remain unused in the vehicle for up to fifteen years, yet remain in perfect condition ready to react if needed. Medical devices need the same guarantee that their function will not impaired by the passage of time.

Cosmetics/Finishing. It is possible to apply virtually any deposit or conversion to the metallic surface for appearance or protection.

Cleanliness. Metal parts are obviously easy to clean and sterilise.

Waste. There is normally less waste, part for part, with pressings compared with a plastic moulding and any waste is recyclable. In 2003, 380 million tonnes of stainless steel were recycled, 40% of the total manufactured. (Source: Scott MacDonald, Divisional Director Corus, speech to Institute of Materials, Birmingham November 2004.

In the device field pressings can be, and are employed in a range of applications from asthmatic inhalers through to advanced drug delivery systems including needle-less injections, as well as the more obvious implants and surgical instrumentation. Their dexterity demands further consideration.

Castings

Castings also have automotive connotations and are often viewed as old fashioned and suitable only for engine components; however, have significant uses outside the arena of the motor car. They enjoy the same manufacturing parameters as plastic moulding and their weight and/or strength offer advantages over other options. For example, wristwatch bodies are cast for strength, protection and weight because in this application weight has a perceived value. This is also the case in the cosmetics industry where zinc castings are used in lipsticks and compacts to add weight and thus value; or they are applied as complex highly decorated packaging because plastics are thought to be unsuitable.

Designers often automatically turn to metals when they need strength, but this feature can also be coupled with lightness if aluminium or even magnesium is employed. Magnesium is the world’s most abundant metal (1 metric tonne for every cubic Km of sea water) and today it is used in applications as diverse as mobile phones and football boot studs for its lightness, (1.8g/cc v 1.4 for a glass filled nylon) which is comparable with plastic. It provides the advantages of weight with the characteristics of strength, rigidity, and wear that only a metal can offer.


Mechanical Properties
Tensile Strength MPa Yield Strength MPa Impact Strength J Elongation% in 50mm Shear Strength MPa Hardness Brinell 500Kg Fatigue Strength MPa
301 stainless steel
862 517 150 25 80 up to 255 303
Zinc ZA8 374 290 42 8 275 up to 103 103
Magnesium AZ91D 234 165 3.7 3 140 up to 63 97
Aluminum 5052 138 117 - 11 138 up to 60 117
Brass CZ1221 370-460 160 20 15-25 320 up to 140 -
Acetal POM 58 60 0.75 45 52 too soft 24.8
Polypropylene - -
Physical Properties
Melting Range Centigrade Density g/cm³ Coef. of ThermalExpansionµm/m°C ThermalConductivity coefficient
Cal/cm/sec degrees C/cm ElectricalConductivity%IACS
(Institute Annealed Copper Standard)
301 Stainless steel 1399-1421 8.03 16.6 0.039 2.4
Magnesium AZ1D 468-596 1.8 25.2 0.38 40
Aluminum 5052 632-654 2.7 23.8 0.49 64
Brass CZ1221 875-890 8.5 21 0.38 28
Acetal POM 160-171 1.4 110 0.25 0











The medical device industry turns to plastics because it understand them and their method of manufacture. Yet, metals can be, and often are better suited to the rigors of the medical device environment; they can and should be so much more than merely needles, pins and scalpel blades.

For more information, contact Timothy D. Jones, Business Development Manager, Clamason Industries Ltd, Gibbons Industrial park, Dudley road, Kingswinford, UK, tel +44 1384 408513, e-mail: timj@clamason.co.uk
www.clamason.co.uk