分子式Published: September 1, 2008
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the girl is mineExtruded Tubing Designed for Balloon Production
Understanding the balloon-forming process is critical to producing angioplasty catheters.
By: Eric Mabry
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Manufacturers must understand balloon processing requirements. Shown here is a balloon coming out of a mold.
Producing balloons ud for angioplasty catheters calls for tubing that has more stringent requirements than that ud for other us, such as catheter body or strain relief. To ensure that the proper tubing is lected, it is esntial to understand the requirements for balloons as well as the balloon-forming process.
Angioplasty Balloon Catheters
exchangerateBalloon catheters are ud for both plain old balloon angioplasty (POBA) as well for stent delivery and deployment. The catheters are typically inrted through a narrow-gauge introducer (e.g., 5 or 6 Fr) into the femoral artery, and the balloons placed in a constricted passage within the stenosis. As a result, the balloon must have very thin walls, typically on the order of 0.001 in. (25 µm). However, becau calcified stenosis can be difficult to open, the balloons must be tear-resistant and have high burst pressures, with nominal pressures typically ranging from 6 to 8 atm and rated burst pressures in the range of 12–20 atm.
Multiple deployments may be necessary, so balloon fatigue is also a factor. Finally, to prevent damage to vesl walls caud by overinflation, balloons must have reliable diameters along the length of the balloon at nominal pressure. They must also be only micompliant (e.g., limited increa in size past the nominal pressure on the order of 5–10% from nominal to rated burst pressure). To create balloons with the characteristics, high-quality tubing with uniform properties is a key requirement.
The Balloon-Forming Processanchor
Figure 1. During the stretch blow molding process, the polymers are stretched as shown.
Balloon forming is done through a process called stretch blow molding in which polymer-bad tubing is stretched under pressure and at an elevated temperature in a biaxial fashion both longitudinally and radially. The temperature and pressure vary by material and balloon diameter. For example, nylon 12 balloons are formed at 170° to 200°F, and a 3-mm nylon 12 balloon may be formed at 35 bar while a 10-mm balloon is formed at 15 bar. The intent is to mechanically stretch the polymer chains so that they provide maximum strength to the balloon as well as resist further growth. The result is a well-defined diameter.复仇者联盟经典台词
Figure 2. (click to enlarge) As blow molding stretches the polymer, the strain remains relatively constant.
成人高考辅导班The polymer orientation in the tubing should be randomized. During the blow molding pro
cess, the polymers are stretched as shown in Figure 1. During the stretching process, the strain will be relatively constant as shown in Figure 2. Once the polymer strands become maximally stretched, the strain increas dramatically. At this point, the material has its greatest strength and will resist further growth. Typically, there is an expected stretch ratio for a given axis and material (e.g., approximately 6× radial and 4.7× axial stretch for nylon 12).
In the balloon-forming process, a parison is placed into a mold as shown in Figure 3. A parison is a piece of tubing in which both ends have been necked down in a controlled process in order to achieve three functions:
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Control where the balloon forms on the tube.
Improve formation of the cone ction.
Allow for smaller neck outside diameters (ODs) needed for low-profile catheters.
Figure 3. (click to enlarge) In the balloon-forming process, a parison is placed into a mold
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as shown (a). A condary stretch at lower pressures is often ud after the main balloon body is formed in order to create thinner cones and neck wall thickness (b). The formed balloon is cooled using chilled circulating water in a surrounding jacket while maintaining a high internal pressure to t the dimensions (c).
The parison is then stretched while pressurized internally with clean, dry nitrogen and under a controlled elevated temperature from surrounding heating elements. This process promotes balloon formation. Note that the temperature is lower than the melting point (nylon 12 Grilamid has a melting point of 352°F, for example), which would cau material flow and randomization of the polymer chains. The temperature is typically in the range of the glass transition temperature for the tubing material. A condary stretch at lower pressures—typically one-third or less of the forming pressure—is often ud after the main balloon body is formed in order to create thinner cones and neck wall thickness.
Finally, the formed balloon is cooled using chilled circulating water in a surrounding jacket while maintaining a high internal pressure to t the dimensions.
Quality Issues for Balloons
Typical quality issues faced in the production of balloons include gel spots, fish-eyes, impurities, drag lines, zipper lines, bowing, and visual imperfections. Quality is critical. The issues can lead to failure such as early burst, fatigue, or incorrect dimensions. Such failures can compromi patient safety as well as increa procedure time, both primary concerns for clinicians.
Gel spots are typically the result of impurities in the tubing or of broken polymer chains caud by shear stress within the extrusion system. The former can be handled by proper filtering within the extrusion system. The latter is a function of the extruder design. In the reduction of pellets to melt to tubing, right angles or extreme transitions can cau shear forces that will break the polymer strands. Once broken, the resulting material has different physical properties and, in effect, constitutes an impurity that may show up in the balloon wall and increa its propensity to burst at lower pressures or fatigue early. One situation in which this can happen is when a larger-bore, 1-in. extruder is ud to make the smaller microbore tubing (typically 0.15 in. OD or less) needed for balloons.
