Medical Plastics Tubing: Applications, Quality, & Extrusion Process Challenges

Braiding and coiling can improve performance in medical device applications by increasing torque force or the bend radius of a tube. Designing and developing complex multiple lumen tubing can spur innovation, provided that the designer understands the latest technology and how best to work with a tubing manufacturer.

Medical Tube Extrusion: Managing the Challenges

Some of the major problems that can impact quality and resulting in rejections are :

• Melt fracture,
• Pressure control,
• Gels,
• Downstream considerations

MELT FRACTURE

Melt fracture is surface roughness (sharkskin appearance), a common issue when small tooling gaps are used with some polymers. Usually, the higher the viscosity of the polymer and the lower the melt temperature, the greater the probability of melt fracture.

Melt fracture is caused by a disruption of uniform flow along the metal surfaces when high shear is present. Typically, melt fracture occurs in the final tooling gaps and is a result of the shear rate on the polymer, which is a function of the tooling gap, melt viscosity, and output rate. Some of the resins that are most susceptible to melt fracture are HDPE, LLDPE, polycarbonate, fluoropolymers, and higher-viscosity thermoplastic urethanes.

The possible common approach to avoid melt fracture is to utilize larger tooling gaps, which means making the tubing with a larger drawdown.

GELs

Gels are seen as bumps on the tubing surface. Their size depends on the source. The thinner the tubing wall, the more obvious the gels typically become. Some of the olymers that seem to be gel-prone are flexible PVC, TPU, and some other TPEs.

Gels can come from a number of sources, including material inconsistencies, degradation, crosslinked particles, and contamination. Gels in flexible PVC are typically the result of PVC resin particles that have not absorbed enough plasticizer, causing them to “float” along with the melt. They are typically impossible to break down along the screw, and are also difficult to trap in screen packs or filters. If a particle can pass through a screen of 250 to 300 mesh, there is no way a screw would have any chance of breaking it down. There are some claims of singlescrew extruders removing these gels, but a trial should be run to prove the point.

Thermoplastic urethanes can have issues with durometer differences throughout the extrudate, especially where the material has been made from different durometer stocks. The resulting viscosity differences can be reduced via shear mixing along the screw. The mixing level would need to be tested on a given screw to see if acceptable tubing can be obtained for a given output rate.

Gels should be investigated to try to discover their source and how they can be minimized. Attempting to melt them can sometimes be done on a heated, temperature-controlled surface, to determine their melting point versus that of the bulk of the material. The gels may not melt at all, which indicates the severity of the problem. Another clue is whether the gels are a different color from the main material, suggesting degradation or contamination.

To solve the problem, first try a high-shear screw with elevated barrel temperature settings to see if the gel level can be reduced by shear and temperature. If the gels are not reduced noticeably—unfortunately the usual case—you can assume the screw won’t solve the problem.

The options then are to either find a way to filter out the gels with a fine-mesh and large-area filter, or have the gels removed prior to extrusion.

Challenges Of Meeting Standards For Medical Tubing

One important standard for medical tubing is non-adhesiveness. Non-adhesiveness is achieved by making the surface of the tube’s inner wall as smooth as possible. While some level of roughness is needed to prevent air bubbles from forming in the tube, the roughness must not interfere with the tube’s transparency.

Manufacturers must quantify the roughness of their tube’s inner surface to ensure they meet the standards. Yet, measuring roughness can be difficult since some tubes have a special coating on their inner surface or have a small diameter.

Wall thickness is another critical specification for medical tubing products, and there are often tight tolerances that need to be met. However, measuring wall thickness can be difficult as tubing becomes increasingly miniaturised for non-invasive procedures.

3D Laser Scanning Confocal Microscope : To Measure Surface Roughness

3D laser scanning microscopes can precisely measure the surface roughness of a medical tube’s inner wall. The microscope scans use a laser to acquire detailed, accurate, and repeatable surface data. It does so without damaging the tubes, even if their inner surface is coated. This non-destructive method minimises the risk of inaccurate roughness data caused by damage to the tube’s inner wall.

Ultrasonic Testing (UT) To Measure Wall Thickness

Another helpful technology for meeting strict medical tubing requirements is UT. This can precisely measure the wall thickness of medical tubing with tight tolerances, such as catheter tubes and extruded tubing.

References :

https://www.ptonline.com/articles/four-keys-to-consistent-tubing https://www.medicalplasticsnews.com/news/meetingexpectations-for-medical-tubing/

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