Medical Extrusion Technology Q&A
Laminar flow plays a big role in extruded tubing whether it’s used for balloons or not. With balloon tubing, precise concentricity is critical to blowing quality parts and laminar flow is a dynamic in the extrusion process that also needs to be precisely uniform. Uniform flow of the polymer through the die and extrusion tip is adversely affected by a variety factors in the process such as uneven surface finish of the die and tip, a non-uniformly heated die and head, inconsistent backpressure, unevenly melted polymer and extrudate not drawn evenly from the die to the coolant. The angles used in the die and tip probably vary from one extruder to another. At MicroSpec and other extruders I know, that information is treated as proprietary knowledge.
Device makers and medical tube extruders both strive for the tightest tolerances possible. Determining what practical tolerances are and defining how to verify that the tolerances have been met is key to successfully launching a new device.
The method of verifying dimensional acceptability can get in the way of moving a new product forward to commercialisation if all the parties are not using the same method in assessing quality and process capability. The root of this problem often stems from the producer (extruder) of a part using AQL (average quality level) procedures in determining what is possible and the user of the extruded part applying statistical methods and Cpk to the same results and obtaining unacceptable findings. This happens frequently and can be avoided with effective communication.
For example, in the development of an extruded 7 French (0.092 in.) tube, an extruder finds that he or she can repeatedly produce that tube holding dimensional tolerances of ±0.002 in. (0.05 mm). The extruder produces a new lot of the tube. The inspection report indicates that although the process drifted during the extrusion run, the tube dimensions stayed within the specified dimensional tolerance. The tube is accepted by the quality assurance department and sent on to the next user, who performs an incoming inspection using Cpk as the criteria for acceptance. That user also finds no nonconforming parts, but the statistical result is that the Cpk value is only 0.98, and the incoming lot of tubing is rejected. What has happened here is that two methods will invariably yield different and often unacceptable results. Good communication can avoid this problem. How?
During development, the extruder and the user need to communicate and share data. In this case they both understand that ±0.002 in. yields an acceptable product. They must also develop inspection procedures that produce the same results. Having agreed that ±0.002 in. works but does not meet the Cpk criteria, a potential impasse exists. However, the parties find that by opening up the incoming inspection tolerance to ±0.003 in., the Cpk standard is met! So the extruder agrees to a new Cpk specification, understanding that they must continue to hold ±0.002 in. at extrusion. By working together, the extruder and user successfully move the product and their relationship forward.
Glass transition (Tg) only applies to amorphous and semicrystalline polymers. Simply speaking, when a polymer is heated, the glass transition point is the temperature at which the polymer begins to soften before reaching its melting point (Tm) and demonstrating melt flow characteristics. Conversely, in extrusion when cooling a polymer, Tg is reached when the polymer hardens to the point that it becomes brittle. The relationship of Tg and Tm is relevant to the extruder in that if the Tg and Tm are very close (only a few degrees apart, for instance), then the temperature range for a successful extrusion of the polymer probably will be very narrow, as well. In this case, control of the process temperatures becomes more exact.
I must first say that lot-to-lot MFI and viscosity variation with most melt-processible polyurethane is a universal issue for extrusion engineers, making validation of an extrusion process nearly impossible and impractical. To best approach this issue, I think we need to understand it as having three parts: lot-to-lot variability in the resin manufacturer’s process, reading the extrusion process and designing tools that work.
First, the resin manufacturer’s lot-to-lot consistency is dependent upon the consistency of the ingredients that make up the resins. It is my understanding that manufacturing the ingredients of PUR resins is inherently variable, so the resin manufacturer has the challenge of monitoring the variability and doing his best in using those materials to produce consistent resins. Supplying a technical analysis of the resin with each lot is extremely helpful to the extruder and should not be ignored.
Second, reading the extrusion process is more of an art than a science, and understanding what one is working with is fundamental to producing a good tube. Therefore, as stated above, the engineer or technician first needs to read the analysis sheet, which should indicate MFI and viscosity. With any medical extrusion there will be a variety of process variables to control. Documenting these variables and learning to read the extrusion process is what extrusion companies consider to be a large segment of their proprietary knowledge. Because there is generally a degree of artistry in producing a PUR medical tube, validating or defining exact process conditions to produce an exact tube is probably impractical. However, you can allow for artistry by specifying a range of extrusion heats as well as tolerances for the other process variables. With a good technician or engineer and proper documentation, everybody learns how to read the variability of the resin and the process; in the end, tubing gets made more consistently.
Finally, the die and pin design is critical to making a good tube. However, the design of the die and pin is also dependent upon the design of the extrusion head and the design of the screw. It is important to recognize that what works for one multilumen tube may not work for another. This area of extrusion technology is highly proprietary. Someone who intuitively understands the melt flow characteristics of the resin used in the extrusion process and how to design tooling for the process is worth his or her weight in gold.
Choosing the optimal material for a medical tube involves understanding the fundamental physical properties of each material under consideration for use. In this example, we are considering two materials: silicone and thermoplastic polyurethane (TPU). Silicone is used for a variety of medical tubing but it is not a thermoplastic. TPU, as its name states, is a thermoplastic and has gained wide acceptance as an alternative to silicone. In designing a medical tube, the engineer should start by considering four basic physical properties, which are critical to function. They are:
1. durometer (the hardness of the resin),
2. flex modulus (the flexibility of the resin),
3. ultimate tensile (the force it takes to break the tube) and
4. elongation (how much the material will stretch before breaking).
Two other important criteria are biocompatibility of the resin and postextrusion processability. Let’s take a general view of the physical properties of both materials and consider how they are related to see how tube design may be affected.
Silicone – natural
TPU – natural aliphatic
Durometer (Shore hardness)
Flex modulus (PSI)
Ultimate Tensile (PSI)
Elongation at break (%)
USP Class VI
USP Class VI
Durometer, flexibility, elongation and tensile strength are all functionally critical in a catheter tube. Typically, low-durometer (soft) polymers or silicones have lower flex modulus (higher flexibility), lower tensile strength, and higher elongation. If kink resistance is important to a tube, then a soft, low-durometer material would be a likely choice. However, what is gained in flexibility is usually lost in strength. If a tube needs high tensile strength but also must be very flexible, then the wall thickness of a tube and its outside diameter must be increased to compensate for the softer material’s lower tensile strength. Conversely, the harder, or higher durometer, materials for both silicones and TPUs have higher flex modulus (stiffness), higher tensile strength and lower elongation.
Both silicone and TPUs meet USP Class VI standards and both perform well inside the body. However, it is my experience that there is more variation with “in the body” performance among the different types of TPUs than there is among the different silicones. In this area, TPU technology has been advancing rapidly and one should stay abreast of the latest customised resins.
In terms of postextrusion processing and chemistry, silicone tubing and TPU tubing differ primarily in that silicone is not a thermoplastic, is inert, and will require special procedures for tipping, bonding and printing. TPUs are thermoplastics and, therefore, conventional heat forming methods can be used for tipping. And because they are not inert, they can be solvent bonded, RF welded and printed on, as the ink carriers readily bond to polymer chains.
In conclusion, material selection for medical tubing starts with consideration of the fundamental physical and chemical characteristics previously mentioned. It is important to understand how the physical characteristics and chemistry are functionally related—staying informed of the latest technology cannot be underestimated.
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