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Medical Extruded Tubing: The Role of Process Parameters and Equipment in Catheter Performance Characterization


Medical Extruded Catheters: The Role of Process Parameters and Equipment in Catheter Performance Characterization

I. Introduction

Most medical tubing specifications contain tubing drawings with materials, dimensions, and tolerances. Specifications rarely contain other tubing properties or process parameters related to tubing production. A common misconception is that as long as a batch of tubing is made from the correct material and meets the dimensional requirements, it will be the same or equivalent to another batch of tubing made by the same supplier or a different supplier. While this may be true, it is quite possible that the two batches of tubing may be different. These differences are not always obvious or easy to recognize, even when checked by incoming QC. The process parameters and equipment used to extrude the tubing are often as important, if not more important, than the actual size of the tubing.

II. Extrusion process and degradation

The processes used to produce medical catheters are important in high-end diagnostic and therapeutic catheters. Market pressures are driving catheter manufacturers to design smaller and smaller, thinner-walled devices. Examples of such applications include high-pressure catheters; tubing used in the manufacture of angioplasty and stent-delivery catheters; balloon tubing used in the manufacture of medical balloons, particularly high-pressure angioplasty and stent-delivery balloons; tubing that is implanted or inserted into the body over time; and other applications in which mechanical, physical, chemical, electrical, or thermal properties are critical to the function of the finished medical device.

Degradation during extrusion can greatly affect the properties of end-use medical tubing. Polymers are very large molecules that derive their unique and useful properties from their size (molecular weight). Degradation is the breakdown of these large molecules and can result in changes in properties such as tensile strength, brittleness, flexibility, and discoloration. To understand degradation, it is important to understand the various interactions that occur during the extrusion process. The following diagram provides an overview:

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Various interactions occur during the extrusion process. The combination of these interactions can lead to material degradation.

Degradation during extrusion is usually due to the following reasons:

  • Inappropriate drying
  • Material overheating (running polymer at too high a temperature)
  • Excessive shearing of material (running polymer at too high a screw speed or using the wrong screw design)
  • Holding the polymer in the molten state for too long (excessive residence time)

This variation is mainly due to the effect of these factors on the chemical composition of the polymer. Some polymers, such as PET, are very sensitive to process parameters and degrade readily. Other polymers, such as polyethylene, are very forgiving.

Another cause of degradation during extrusion is multiple melting process steps. For example, some materials used in the manufacture of medical tubing must be pre-mixed, where the base material is melted and mixed with other materials, such as colorants, radiopaque fillers, stabilizers, processing aids, and the like.

This is usually done in a separate extrusion operation to ensure proper dispersion and distribution of the components. Compounding is usually carried out in twin-screw or single-screw extrusion processes. In addition to those generated by the pipe extrusion process, this process step also generates heat and shear. The combination or sum of these processes results in an overall loss of molecular weight and polymer degradation. If any of these steps are not performed correctly, results may be compromised.

III. Extrusion overview

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An extrusion line combines several pieces of equipment, including a resin drying system, an extruder, a die, a cooling bath, a take-up unit (wire puller) and a cutter or winder.

IV. Drying

Typically, the first step in the process is drying the polymer, which is a critical process in extrusion. Many polymers used in the medical device industry are hygroscopic, which means they readily absorb moisture from the environment. Hygroscopic polymers must be carefully dried prior to melt extrusion or compounding.

Not all materials dry under the same parameters. Some materials require high temperatures for long periods of time, while others require low temperatures for shorter periods of time. Some materials are extremely sensitive to moisture content and must be dried very carefully, while others are easier to dry and less critical. For example, drying PET is critical to the extrusion process. Even a very small amount of moisture can cause PET to degrade and become unusable.

Drying the material for too short a time and/or at too low a temperature can lead to under-drying. This can leave residual moisture in the polymer, which can lead to hydrolysis during the extrusion process. Hydrolysis is a degradation process that results in a significant decrease in molecular weight. Under-drying of polymers typically occurs in medical extrusions where run times can be very short and a lot of material conversion is required. Customers often require the same size tubing in multiple material grades – for example, 3 different hardnesses of the same material – to optimize flexibility for a particular application. If the processor does not have 3 dryers available to pre-dry all 3 materials, the second and third materials may not be dried properly before extrusion. The result could be that the engineer evaluated partially degraded materials and made the wrong choice for the application.

Because many medical extrusion lines operate at very low throughputs (low pounds per hour), overdrying can also occur. Most commercial resin dryers are oversized for medical extruders. As a result, residence time in the dryer can be very long. If not properly monitored, this can lead to over-drying, which can result in thermal degradation of certain materials. Many polymers, such as nylon and polycarbonate, are sensitive to over-drying. Most resin manufacturers specify minimum drying times and temperatures for their materials. These recommendations must be followed very carefully to ensure that the material is properly dried prior to extrusion. Typically, desiccant type dryers are used in the medical extrusion industry to ensure proper drying. These dryers must be well maintained and regularly cleaned, tested and calibrated to ensure they function properly.

V. Extruders

An extruder is a melting and pumping machine. It converts solid particles into a uniform molten state and forces the material through the die at a constant rate. Melting is accomplished by frictional heat generated by the mechanical work of the screw and heat transfer from the heated barrel of the extruder. The design of the extrusion screw is critical to achieving uniform melting and pumping of the polymer without overworking (over shearing) the material. Different materials require different screw designs to optimize the extrusion process. Many pipe manufacturers use generic screw designs and try to run all materials with the same screw. This can lead to over shearing and degradation of some materials and improper melting and gelling of others.

VI. Extrusion molds

The extrusion die is located at the end of the extruder and is where the polymer enters the cooling bath. The die forms the initial shape of the tube. A pipe die typically consists of 2 main components: a mandrel or tip that forms the inner diameter of the pipe; and a die or ring that forms the outer diameter of the pipe. The die and mandrel are usually contained within the extrusion “head”. Dozens of extrusion head and die manufacturers, as well as many extrusion companies, have developed proprietary head, die and mandrel designs. The design of these components plays a key role in the extrusion process and the ability of the extruder to produce accurate dimensions and maintain proper physical properties of the material. The relationship between the size of the die and mandrel and the size of the finished pipe is called the “stretch ratio”.

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Figure 3. Extrusion processes that produce stretch ratios

Very small diameter medical tubing with very thin walls can be difficult to extrude through a standard extrusion head/die. Often, the viscosity of these materials in the die is so high and the die clearance is so small that the extrusion operator must raise the temperature of the polymer to reduce the viscosity of the material to obtain sufficient flow through the die. This practice can dramatically change the properties of the material. When extruding thin-walled pipe, specially designed heads are often required to produce high-quality pipe without degradation, gels, black spots or undesirable residual stresses.

Many custom extruders overcome these problems by using high stretch ratios to produce thin-walled tubing with tight tolerances and small diameters. This significantly improves dimensional tolerances, increases line speeds, and makes tooling (dies and mandrels) easier to manufacture. Unfortunately, running high tensile ratios can also lead to significant orientation and residual stress/strain in the finished tube. This orientation can significantly increase the tensile strength and reduce the elongation of the tube in the machine direction. It also reduces tube burst pressure due to loss of strength in the circumferential direction. Residual stresses from high tensile ratios can cause serious damage during subsequent heat treatment, sterilization or aging (natural or accelerated). These thermal processes can release the stresses generated during extrusion, resulting in significant tube length shrinkage and increases in diameter and wall thickness.

VII. Cooling

The extrusion cooling process is the next critical step. Cooling is critical for many polymers, and different cooling conditions can lead to significant changes in physical properties and morphological structure. For example, many polymers are semi-crystalline, meaning they contain amorphous and crystalline regions. As the polymer leaves the mold and cools, rapid cooling/quenching tends to delay crystallization or eliminate it altogether, while slow cooling results in higher crystallinity and/or very large crystal formation.

In some medical applications, such as balloon manufacturing, it is critical that the extruded tubing is amorphous prior to the balloon formation process. Therefore, cooling parameters are critical to ensure that crystallization does not occur in the tubing during the extrusion process. In other applications, such as the extrusion of PEEK tubing, it is critical that the tubing achieves a relatively high degree of crystallinity during extrusion to ensure that the tubing has the outstanding thermal, physical and mechanical properties of PEEK. In materials such as polyethylene and polypropylene, in some applications it is desirable to minimize crystallinity in the tube to improve clarity and softness. In other applications, however, increased crystallinity is required to improve stiffness and lubricity.

Most processors cool the polymer in a water-filled cooling tank as it leaves the mold. This is usually accomplished either in free extrusion or through a vacuum sizing tank. However, in both methods, the polymer is cooled by contact with the water in the cooling tank. Variables that affect the cooling process include water temperature, circulation of water in the tank, length of the cooling tank, and line speed. All of these variables affect the physical characteristics of the pipe.

In many applications, controlling the temperature of the water in the cooling tank is critical. However, many processors do not use temperature controllers or have very cursory control of their water temperature. This can result in significant changes in the cooling rate of the polymer from batch/lot to batch and from the beginning to the end of the batch. Processors that use tap water for cooling can see the incoming water temperature vary 30°F or more from summer to winter. In addition, hot spots can develop in the cooling tank, especially in areas where the polymer enters first. That’s why it’s important to circulate the water in the cooling tank, even with an accurate temperature controller.

Many medical extrusion lines have very small, undersized cooling tanks that may not be well suited for long production runs, extruding large diameter and/or thick walled tubing, or extruding small thin walled tubing at higher line speeds without enough time in the tank to properly cool the tubing. Higher line speeds or shorter cooling tanks can result in insufficient residence time in the cooling tank. This can further result in the tube exiting the extrusion process with the interior still warm or hot and inadequately set. Once the tube exits the cooling tank, the cooling process reverses itself and the tube can begin to rewarm from the inside out because the center of the tube has not cooled sufficiently. This can produce different physical properties in the tube.

VIII. Extrusion equipment and its importance to pipe success

It is important that medical device designers ensure that their tubing manufacturers have the expertise and equipment to manufacture high-end tubing for use in the medical device industry. In recent years, many industrial extrusion companies have entered the medical device extrusion business because they see higher profit margins than industrial applications. However, these manufacturers often use oversized extruders to produce medical-grade tubing, which can lead to long dwell times. In many polymers, excessive dwell times can lead to thermal degradation.

In addition, some tubing manufacturers use older equipment or equipment that may not meet the high standards of the medical device industry. Many older extrusion lines do not have state-of-the-art controls, resulting in wide variations in processing temperatures and other parameters. This can lead to inconsistent thermal histories and inconsistent properties during or between runs. This also applies to equipment that is well designed but poorly maintained or calibrated. For example, a thermostat on an extrusion line can operate over a temperature range of 300° to 600°F or higher. If this thermostat shuts down 1%, it shuts down 5° at 500°F. If it is off by 5%, it is off by 25° at 500°F. For some materials, a 10° process variation can result in a dramatic difference in tube performance.

Medical tubing manufacturers typically have very small extruders. When medical devices require larger diameter tubing than these small extruders are designed to produce, processors may run their extrusion lines at maximum output and high screw speeds. This can be harmful to many shear-sensitive polymers. Shear sensitive polymers running at high screw speeds may suffer the same type of degradation that occurs when polymers are heated too long or too hot. It is important to recognize and understand that many interactions occur during the extrusion process.


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