How to manufacture medical tubing extrusion?
Most medical tubing extrusion specifications include tubing drawings with materials, dimensions and tolerances. Specifications rarely include other tubing properties or process parameters associated with tubing production. A common misconception is that as long as a batch of tubing is made from the correct material and meets dimensional requirements, it will be the same or equivalent to another batch of tubing made by the same 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 identify, even when checked by the incoming QC. The process parameters and equipment used to medical tubing extrusion are usually as important, if not more important, than the actual size of the tubing.
What is the 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 devices with thinner and thinner walls. Examples of such applications include high-pressure catheters; tubing for the manufacture of angioplasty and stent delivery catheters; balloon tubing for the manufacture of medical balloons, particularly high-pressure angioplasty and stent delivery balloons; tubing for long-term implantation or insertion into the body; and other applications where mechanical, physical, chemical, electrical, or thermal properties are critical to the function of the finished medical device.Degradation during the extrusion process can greatly affect the performance 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 lead to 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:
Various interactions can occur during the extrusion process. The combination of these interactions can lead to material degradation.
Degradation during extrusion is usually due to
Improper drying
Overheating of the material (running the polymer at too high a temperature)
Over-shearing of the material (running the polymer at too high a screw speed or using the wrong screw design)
Keeping the polymer in the molten state for too long (too much dwell 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 very easily. Other polymers, such as polyethylene, are very forgiving.
Another cause of degradation during extrusion is the multiple melting process steps. For example, some materials used to make 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, etc on plastic extrusion process steps
This is usually done in a separate extrusion operation to ensure proper dispersion and distribution of the components. Compounding is usually performed in twin-screw or single-screw extrusion processes. In addition to those produced by the pipe extrusion process, this process step also produces 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 performed incorrectly, the results may be compromised.
This is usually done in a separate extrusion operation to ensure proper dispersion and distribution of the components. Compounding is usually performed in twin-screw or single-screw extrusion processes. In addition to those produced by the pipe extrusion process, this process step also produces 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 performed incorrectly, the results may be compromised.
What does an extrusion line look like?
An extrusion line combines several pieces of equipment, including a resin drying system, an extruder, a die, a cooling tank, a take-up unit (wire puller) and a cutter or winder.
Typically, the first step in the process is to dry 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 are dried under the same parameters. Some materials require long periods of high temperatures, while others require shorter periods of low temperatures. 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 PTFE pipe is critical to the extrusion process. Even very small amounts of moisture can degrade PTFE pipe and make it unusable.
Drying the material for too short a time and/or at too low a temperature can result in insufficient 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 reduction in molecular weight. Underdrying of polymers typically occurs in medical extrusions where run times can be very short and significant material conversions are required. Customers often require the use of multiple material grades of the same size tubing – 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 properly dried prior to extrusion. The result may be that engineers evaluate partially degraded materials and make the wrong choice for the application.
Over-drying can also occur as many medical extrusion lines operate at very low throughputs (low pounds per hour). Most commercial resin dryers are oversized for medical extruders. As a result, the 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 will 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 are operating properly.
extruder clicking
The extruder clicking 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 achieved by frictional heat generated by the mechanical work of the screw and heat transfer from the heated barrel of the extruder clicking. The design of the extrusion screw is critical to achieve uniform melting of the polymer and pumping without over processing (over shearing) the material. Different materials require different screw designs to optimize the extrusion process. Many tube manufacturers use a generic screw design 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 gelation of others.
medical tubing extruder tool
The medical tubing extruder tool is located at the end of the extruder and is where the polymer enters the cooling tank. The die forms the initial shape of the tube. the medical tubing extruder tool typically consists of 2 main components: a mandrel or tip that forms the inner diameter of the tube; and a tool or ring that forms the outer diameter of the tube. 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 tool and mandrel size and finished tube size is known as the “stretch ratio”.
Very small diameter medical tubing with very thin walls may be difficult to extrude thin walls tube through a standard extrusion head/die. Often, the viscosity of these materials in the die is so high and the die gap so small that the extrusion operator must raise the temperature of the polymer to lower the viscosity of the material in order to obtain sufficient flow through the die. This practice can dramatically change the properties of the material. When extruding thin-walls tubing, specially designed heads are often required to produce high quality thin tube without degradation, gels, black spots or undesirable residual stresses.
Extrusion 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 both 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 leads to higher crystallinity and/or very large crystal formation.
Many medical extrusion lines come with 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. High line speeds or shorter cooling tanks can result in insufficient dwell time in the cooling tank. This can further result in the tube still being warm or hot inside and not adequately set when exiting the extrusion process. Once the tube leaves the cooling tank, the cooling process reverses itself and the tube can begin to reheat from the inside out because the center of the tube is not sufficiently cooled. This can create different physical properties in the tube.
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