Balloon forming is a science-based, advanced manufacturing process in which every step must be performed with the highest precision to achieve the desired results. These steps include selecting and preparing raw materials, preformed parison sizing, balloon extrusion, and balloon forming.
|This balloon taper cone is a nonstandard balloon dimension.
Process parameters such as material chemistry and dimension, material preparation, applied force, heat flow, chamber pressure, and rate of cooling interact in specific ways during the balloon forming process. The engineering and design teams must fully understand the reactions and their variations to create the appropriate properties for a balloon’s intended clinical use. Depending on the material, balloon type, and balloon length, these variables must be held within tightly defined ranges throughout the entire process to achieve consistent and reproducible balloon properties. Every step in the process must be completed with care and vigilance to produce high-performance balloons. Sacrificing or shortening steps to speed up the process or reduce costs will only result in inferior production and high failure rates.
It is important to initially gather essential information from the client, such as intended application (for example, angioplasty or stent delivery); diameter and length, compliance characteristics, balloon wall thickness, rated burst pressure, and distal and proximal neck sizing. This knowledge helps determine which materials are the best candidates. Each material (polyethylene—including different blends, nylons, polyethylene terephthalate, and polyurethanes) has its own unique heat-forming properties, strengths, compliances, and compatibilities with adjacent components for bonding or welding, chemical resistance, and gas permeability. These factors must be considered before a final material is selected.
Based on the dimensional and performance requirements of the balloon as specified, the most suitable material and design is chosen for the client’s application. Companies that specialize in catheter manufacturing can offer assistance in selecting an appropriate material. They will likely have access to a database of scientific and process information on raw tubing and finished balloon materials, sizes, and dimensions. Such companies can also provide complete sets of process parameters and finished balloon test results for available balloon types and materials. The information contained in such a database can serve as a starting point for new balloon development projects. When used with a stress and strain software program, the information enables an accurate balloon design with a specific burst strength and compliance.
All applicable stress and strain formulas are arranged in a logical sequence in a program based on validation software, such as MathCAD-based. Design engineers input the required balloon design parameters into the program, which immediately determines minimum wall thickness and required raw tubing size, as well as the temperature, pressure, expansion, stretch characteristics, and other necessary parameters. Unlike the trial-and-error approach used by many companies, this method increases the overall speed of production and quality of the final product.
Before determining the tubing size needed to produce a balloon with a specific diameter and burst strength, the stresses in its weakest section must be determined to confirm that the pressure will not exceed the ultimate tensile strength of the material. Tensile strength values are greatly influenced by the biaxial orientation of the molecules and the level of material crystallization achieved during the forming process. The most efficient way to obtain realistic material tensile strength values is to perform a burst-pressure test on an existing balloon that is very similar in size and material grade to the client’s proposed balloon.
Some catheter manufacturers can access their own database to find an existing balloon of similar size, material, and performance. These balloon dimensions are carefully measured and recorded. The design team then tests the experimental balloon to determine burst pressure and tubing axial stretchability. The experimental balloon’s test results are entered into the company’s computerized balloon-sizing program, which calculates the ultimate tensile strength of the material and material-stretch ratios of the experimental balloon. Using the material properties from the experimental balloon, the program then calculates the size of the new balloon and the new balloon tubing. These calculations can be overriden, especially if the material’s properties fluctuate as a result of the tubing extrusion process or balloon forming process.
|Points of weakness in the material are called fisheyes. They can be reduced by increasing primary heat and heat time. Click image for larger view.
To produce high-quality balloons, manufacturers must start with high-quality materials. The engineering and design teams need to understand the chemistry of the polymers at a meaningful level, because the parameters for each react in different ways. They also require specific settings and controls during the extrusion process.
Using balloon material of a homogenous composition is critical to production success. When polymers degrade slightly, their key physical properties, such as tensile strength, brittleness, flexibility, crystallinity, and coloration, are altered, often creating points of weakness in the material (called fisheyes or gel spots).
Raw polymer pellets must meet material procurement specifications and demonstrate the highest purity. Balloon strength, compliance, and other performance requirements can be met by a variety of compatible materials such polyamides, polyurethanes, and polyesters of different grades and hardnesses. Blends or copolymers may be selected to obtain desired balloon characteristics. For example, very soft materials (60A Shore scale) are typically used for highly stretchable, low-pressure occlusion balloons. At the other end of the spectrum, hard compounds (110 D Shore) are used for noncompliant, high-pressure dilation balloons.
A parison is used prior to the forming process to better define cone angles and the body of the balloon. The configuration of the parison is determined by the stretch capacity limits on both the tubing stretching machine and the balloon forming machine. For balloons 210 mm or longer, the optimal parison bubble length is 60% of the mold length. For shorter balloon lengths, stretch capacity limits are not as much of a factor.
A parison is used to control the balloon-forming operation. The left and right sections of the tubing are necked down to make them harder than the nonnecked section of tubing. The nonnecked tubing is considered the parison. The length of the parison is determined by a certain percentage of the balloon working length depending on the material type and, more importantly, the outcome of the formed balloon. In this instance, the outcome refers to obtaining the desired wall thickness and dimensional requirements, including body length, cone, and neck. The hardened sections on both the proximal and distal side of the parison should allow for stretch and blow control during the primary and secondary stretch operation. If these sections are not drawn down, the force and distance required to displace the material during balloon blowing will be unpredictable and inconsistent.
Balloons can range from 0.5 to 50 mm in diameter and 1.0 to 320 mm in length. They can be produced in a variety of shapes, including cylindrical, spherical, oval, conical, stepped, dog bone, and offset. These shapes can be combined to achieve other unusual balloon profiles. For example, a cylindrical balloon may be tapered at one end and semispherical at the other, or it may have one cuff that is concentric with the balloon body and one that is offset.
For instance, an occlusion balloon that must take the shape of a certain cavity will need to be made of a compliant material and should be spherical, depending on the application. A thick-walled balloon may need to have long drawn out cones to decrease the crossing profile.
The cone is the transitional area between the body and the neck of a balloon; the cone angle is the angle from the centerline of the balloon to the surface of the cone section. Depending on the function of the balloon, cone angles can be as steep as 30°. Forming precise cone angles is essential to avoiding damage to healthy tissue, and is the result of a proper end-plug design and a stable balloon forming program.
Some catheter manufacturers use metal molds that are electrically heated and water cooled. These molds can be precision-machined from a heat-treated beryllium copper alloy, which provides excellent strength, durability, and high thermal conductivity. Various sizes and types of molds can accommodate a wide range of balloons dimensions. The molds are designed with modular interchangeability in mind and sized to minimize heating and cooling times. Switching these molds on the balloon-forming machines can take just a few minutes.
Catheter manufacturers must maintain an inventory of molds for standard balloon sizes. However, it is also important to accommodate highly-specific performance requirements, tighter tolerances, and unique requests with regard to balloon cone and neck dimensions. Custom molds may be required. In those instances, they can be manufactured in a tooling department to provide short turnaround time and enable the quick adjustment of mold size.All balloon molds produced for a customer are the property of the customer.
Strength and Compliance
Material strength is typically described as either stress at burst or as balloon burst pressure. Stress is most commonly defined as the unit of force applied per unit of a material’s cross-sectional area that is perpendicular to the force applied.
A material’s compliance is typically described as the change in balloon diameter as a function of the balloon’s internal pressure. Volume compliance is applied to mostly low-pressure, compliant balloons that are usually made from very soft stretchable polyurethanes. These balloons can double or even triple in size during inflation; inflation pressures are usually very low (from 5 to 10 psi). Volume compliance is measured as balloon diameter vs. the volume of injected liquid or gas.
Burst and Fatigue
Stretch blow-formed balloons are very strong. The nature of the balloon-forming process imparts a prominent biaxial orientation to the polymer’s molecular chains, resulting in uniform walls and extremely high tensile strength, often five times greater than that of the original raw material. Stretch-blow balloons offer superior strength yet can also achieve extremely small folded profiles. Balloon burst strength depends on the material type, balloon wall thickness, and diameter. Typical values for wall thickness range from 0.0002 to 0.007 in., with burst pressures that range from a few psi to as much as 450 psi.
|Nylon balloon tubing extrusion
The quality of balloon tubing is dependent on the level of in-house expertise, because the engineers and operators develop the extrusion process for each lot. The set-up and monitoring process is also important for quality output. Operators must follow a strict sequence of carefully monitored steps to create a final product that meets or exceeds the high-performance requirements of the end user. This method is the best way to reduce or eliminate the microscopic flaws in the balloon tubing that could result in underperformance or failure. Critical extrusion factors include forming temperature, primary stretch length, stretch speed, and fill rate. To preserve balloon forming process parameters, it is important to maintain consistent tubing characteristics. Any changes in the characteristics can change preprogrammed balloon forming parameters, as well as the final balloon production outcomes and yields.
Flaws and How to Avoid Them
The first step to minimizing flaws is to vigilantly prevent the contamination of the raw materials being used. For example, tubing should be extruded in a cleanroom environment. Contaminants can also be removed when the molten resin passes through an in-line filtration system, which also helps reduce gel spots, die lines, foreign particulates, and scratches. If, however, flaws still appear, they can be reduced or eliminated by adjusting key parameters, such as stretch, heat, and pressure. For example, fisheyes can be greatly reduced by increasing primary heat and heat time, and decreasing stretch distances or pressure. It takes practice to master these techniques and understand the underlying causes of flaws.
Balloon extrusion, balloon development and production should take place in Class 100,000 cleanrooms. Every balloon work order is subjected to a rigorous inspection process. First, the balloon forming equipment is set up and qualified under fully documented and controlled procedures. Throughout production, in-process destructive dimensional testing and strength testing is conducted. If at any time during testing a reject is encountered, the job is halted and a nonconformance investigation is initiated.
Inspection sample plans are based on ANSI standards Z1.4 and Z1.9 to ensure the highest level of quality. Computerized high pressure burst testers and leak testers can be used for balloon performance testing. An optical gauging system and laser micrometers inspect a balloon’s dimensional properties. Every balloon should be visually inspected by the operator for any flaws or imperfections.
Whether an OEM wants to increase yields, improve performance properties, develop new products, upgrade production capabilities, or engage a manufacturing partner, an experienced catheter manufacturer can be of great help. Balloon tubing must meet critical production standards to achieve important performance characteristics such as minimal wall thickness and improved burst pressures. These parameters must be within prescribed limits and must be consistently reproduced in all subsequent production runs.
The desired tolerances and mechanical properties of produced balloon tubing must be met with a carefully controlled extrusion process. This method is necessary to reliably design, calculate, and predict the behavior of balloons and maintain repeatable and consistent production processes. Each manufactured balloon has its own unique properties. A well-developed production process should be able to accommodate these properties and consistently produce high-quality results.
Kenny Mazzarese is product director of balloons and balloon extrusion at Interface Catheter Solutions (Laguna Niguel, CA).