Flat wire, often referred to as ribbon wire, is commonly used in devices designed to reduce catheter profiles or increase available lumen size. Typical applications include safety wires in a catheter guidewire, helical coils in a catheter guidewire and braiding wire. Fort Wayne Metals uses two manufacturing techniques to yield different types of radius edged flat wire: rolled flat wire and drawn flat wire (see illustrations).
Flat Wire Comparisons
Both products exhibit a smooth bright surface finish and tight size tolerances. However, each has its advantages. Rolled flat wire has a larger cast, less camber (see Straightness), less stress induced in the wire and a lower cost versus drawn
flat wire as the width/thickness ratio increases. While drawn flat wire has the advantage of improved size tolerances (see Standard Tolerances), it’s often specified for applications requiring more consistent and tighter dimensions.
The maximum available width for both types of flat wire depends on the thickness and the alloy. The maximum width/ thickness ratio of rolled flat wire is approximately ten to one, alloy allowing. Rolled flat wire is available as thin as .0003". Drawn flat wire is available as thin as .0015". Standard tolerances for each wire type are described below. Depending on width, thickness, alloy and width/thickness ratio, both drawn flat wire and rolled flat wire may be offered.
The tensile strength of flat wire is determined by manufacturing techniques. Tensile strength ranges from annealed to spring temper in most alloys. The maximum
tensile strength is a function of both the alloy itself and other requirements of the specified wire, such as cast.
Cross-Sectional Area Calculation
When determining tensile strength, it’s necessary to properly calculate the cross-sectional area using the flat wire conversion factors (see chart on the back of this insert). Because both rolled and drawn flat wire have full radius edges (see illustration), necessary adjustments to remove the corners of the rectangle from the area calculation must be determined. Accurate calculation is vital because minute
differences in cross-sectional area can make significant differences in tensile strength.
Flat Wire Conversion Factors
The first column is width divided by thickness.
The factor is to be used to calculate cross-sectional area
(e.g. 0.010" ÷ 0.003" =3.3; look up 3.3 to get 0.984; 0.003" x 0.010" x 0.984 = 0.0000295; this is the cross-sectional area).
Secondary Cleaning Capabilities
Fort Wayne Metals uses various techniques to improve the surface cleanliness of flat wire. These include heat cleaning, solvent wipes and hot alkaline or ultrasonic-cleaning, used on their own or in various combinations.
Straightness: Cast and Camber
If straightness is critical to the flat wire application, then a minimum cast and/or maximum camber may be specified. Cast is measured by cutting a three foot piece off the spool and laying it on its edge on a flat surface so it forms a circle or an arc. The size of the circle or arc is the cast. To determine camber, a short length of flat wire is cut. Next it is placed on its width rather than its edge. Then, by holding the wire in the middle against a straight line, the distance that the free ends extend from the line is measured as camber.