An engineer’s guide to designing and specifying custom tubing
Robert LaDuca, C.E.O., Duke Empirical, explains what engineers need to know about the extrusion process when designing and specifying custom tubing.
What are the key things for engineers to consider within the extrusion process?
The importance of the relationship between draw down ratio and tubing mechanical properties is a factor that can greatly affect the mechanical properties of the tubing. The same exact dimensions and materials can produce dramatically different performance results depending on this important processing factor. In general, a higher draw down will impart more molecular chain alignment within the polymer producing a higher tensile modulus product in the axial direction with greater ultimate tensile strength than a tubing produced with a lower draw down ratio. Products which have higher degrees of molecular alignment axially due to higher draw down ratios will also have a greater degree of resultant stress in the tubing wall which may require annealing to relieve in order to avoid dimensional changes from occurring, not only during additional thermal processing steps such as bonding and reflow lamination, but also during transit where elevated temperatures can occur.
Another important factor is the effect of die temperature on the surface finish. A colder die temperature can impart a ‘frosted’ finish on the surfaces of the tubing which reduces the tackiness at the cost of reduced clarity and imparted surface texture. Similarly, a hot die temperature can improve the clarity of the material and will have an impact on subsequent thermal bonding and lamination operations. When performing extrusion process validations, the range of parameters should be wide enough to understand the impact on subsequent processes as well as the impact that variation in the molecular weight of the raw material coming into the extrusion process can cause in both the extrusion process and also the downstream processes.
How to choose the right polymer system for medical tubing applications
A wide variety of materials are extruded into tubing products, therefore it’s important to know what to consider when it comes to specifying materials for extrusion in order to ensure a material is a suitable choice for a specific application. The starting point is to fully define the user needs and proceed iteratively to a qualified specification.
Following ISO compliant design controls, once the design outputs are measured and meet the design inputs, the verified design is then validated using objective evidence to demonstrate the design works for the user as intended. The product designer is responsible for identifying a complete set of user needs for the product’s duration of intended use, as well as ensuring the manufacturing process required to produce the design is suitable.
For medical applications Foster Corporation launched its HLS brand of Heat, light stabilizer package that can be added to a wide variety of resins such as nylons, polyurethanes, polyethylenes etc. If the product is to be coated with a hydrophilic coating solution which has a UV cure, then adding a UV stabilizer will help prevent degradation of the polymer during subsequent processing. Similarly, if the extruded tubing is to undergo a subsequent lamination reflow process using removable heat shrink to add reinforcements into the wall of the tubing such as braids or coils, the use of heat stabilizer in the additive package can reduce processing variations encountered during extrusion and post process reflow steps. This allows for greater precision in the final product and translates into tighter tolerances, improved manufacturability, and even enables new catheter technologies when new designs require pushing existing limits.
Other physical properties will change with the addition of radiopaque additives to provide visibility during use under fluoroscopy or ultrasound imaging. These changes include increased surface hardness, decreased tensile strength and burst pressure, and less surface tackiness. This is particularly the case with softer materials such as 40A Chronoprene by AdvanSource Biomaterials, soft urethanes such as PolyOne’s NEUsoft UR842A, and low durometer SEBS such as Kraton’s SEBS G1642. The loading percentage will vary depending upon the degree of visibility required as well as the wall thickness where a thicker wall can use less radiopacifier. Typical loading for wall thicknesses of .005” through .050” may be 20-25% barium sulfate by weight whereas glass microspheres may be added in loading percentages of 50% or higher by weight to improve the echogenicity of polymers.
Since barium sulfate is whitish in colour, additional pigments such as TiO2 can be added to bring the colour to a bright white or alterative colours can be matched to pantone colors. It should be noted that fillers have a distribution of particle sizes leading to the potential for defects in the tubing as agglomerates can create surface roughness on the ID or OD, create drag marks, pinholes, lumps, necks, and dents. Therefore, if specifying constituent materials at the compounding phase, it is important to identify the wall thickness of the tubing to be extruded so that it can be compared with the particle size distribution of the filler. This allows a fine enough powder to be selected which would avoid or reduce the prevalence of these types of defects.
Nonetheless and independent of the effect additives may have on the quality of the tubing produced, gels may be present in natural grades of materials such as those which are found in amides, HDPE and urethanes. When specifying tubing made from materials where gels are unacceptable such as balloon tubing, specifications may include the requirement for the processor to use a candle filter at the single screw extruder output which will help break down gels into smaller sizes and reduce the number of gels that end up in the tubing. This inline single screw filtering is preferred to running a separate melt process on a twin screw compounding line with an inline candle filter as the additional heat history and mechanical shearing of the additional pass will reduce molecular polymer chain length and the resultant tubing will have degraded performance in tensile and burst strength which are critical in balloon catheter applications.
Besides the aesthetic aspects, the addition of pigments can also help improve the tackiness of very soft materials and can make the difference in being able to perform subsequent assembly steps such as insertion of mandrels used in catheter construction. One potential pitfall to be aware of is the possibility of incomplete colour constituent breakdown during the single screw extrusion process. This is typically seen when using colour concentrates which are blended with natural grade pellets at the feed throat of the extruder. These unbroken-down colour concentrates can appear to be embedded foreign matter and can cause non-conformances to visual criteria requirements. A similar defect can be observed when extruding thin walled material of less than .005” where streaks of color can be seen next to clearer areas where the colourant was not well dispersed by the shearing of the material in the screw and barrel. These issues can be avoided with the use of precompounded coloured material produced in masterbatches.
A related issue is the specification of packaging so that the tubing product is not rendered unusable by shipping and handling. This is particularly important with regards to soft tacky materials which may have the tendency to weld to itself when compressed during shipping. To avoid loss due to damage, soft tubing may be supplied inflated to prevent the ID from collapse.
The use of specialised packaging configurations such as individual chambers can also prevent the tubing from being in contact with other tubing. Alternative packaging designs include providing the tubing on a mandrel to allow subsequent operations to be performed without needing to insert a reflow mandrel into tacky tight-fitting lumens which can be problematic and time consuming.
Conclusion
In conclusion, these are a few of the important aspects that design engineers should be aware of when specifying materials and processes to optimise product development outcomes. Working closely with extrusion processing experts who are collaborative in their approach to providing application specific customer solutions can be highly beneficial towards achieving design goals in the development of new medical tubing applications.