FOREWORD - Medical Extrusion 101



Multi-Lumen Tubing Design Considerations

When designing the tubing for the application, keep these basics in mind.

1. The ID cannot be larger than the OD

a. Ensure that all features fit within the circumference of the cross-section.

b. By design, all cross-sectional NOMINAL values need to equal the NOMINAL OD….not the OD Upper Specification Limit!

c. Ensure that drawings to be submitted for quote are scaled properly; if a drawing is not to scale, clearly note that it is not to scale.

d. It is understandable that a designer wishes to do what has never been done before, but physics are physics. Please note, Pauli’s Principle:“No two objects can occupy the same place at one time".

2. Symmetry

a. Symmetry often provides the best chance of delivering an extrusion that resembles the print.

b. Mixing very thin and very thick concentrations of mass within the distribution of walls and septums will often result in a grossly out-of-round structure.

i. Where possible, use ‘shaped’ lumens to work the cross-section so that material distribution is reasonably even. Yes, fluid dynamics are extremely important for many devices….but if you don’t have a tube to dynamically test, where are you?!

ii. Adding extra lumens to reduce or eliminate heavy wall masses within the crossection can actually be helpful in delivering an extrusion that more closely resembles the print.

c. When working with cross-sections that are not extrusion-friendly, creative tolerancing may need to be applied.

3. What to expect from the initial multi-lumen extrusion prototype run

a. Typically, a run done on a ‘best effort’ basis.

b. Less than perfection.

c. That the supplier will put forth their full understanding of the processing of the material and that they will apply all their knowledge and experience in the extrusion of multi-lumen tubings when they perform the run.

i. For this reason, if no other, it is critical to use a supplier that has vast multi-lumen AND processing experience.

ii. Going with a less experienced extrusion supplier to save a couple hundred, or even a thousand dollars or more….can be a questionalbe decision when it comes to meeting the ‘extrusion’ needs for your project. Use Proven Professionals!

4. With multi-lumen and profile extrusions, multiple iterations are often required before the tooling is able to deliver a production-ready component

a. The development of the process for specialty, custom extrusions often requires significant process characterization. With some materials and some material/structure combinations the characterization process can require multiple setups with multiple tooling iterations.

b. Try to be patient with the development process; as it is understood that the extrusion is one of the first things needed to generate concept, and or prototopye, devices.

c. In most cases, plan for 2-3 full-cost prototype efforts. There are times that more will be needed. There are times that follow-up runs can be less costly. Too, there will be times that the first effort will produce components that function well, and the downstream processes can be established to effectively utilze the extrusion.

i. If it is evident in early prototyping that good downstream yields will be elusive with the components produced with the first and or second run results, ensure that the need for continuing efforts to refine the extrusion are passed along to the process development team so that followup with the extrusion supplier will be proactive. Reactive efforts in production when you are trying to ramp up for release can be extremely costly!

5. An understanding of the most critical needs of the extrusion have to be communicated

a. What are the functional requirements of the extrusion? Perfection would be great, but expect that concessions on an early design may be required to move the extrusion, and the product, into production.

b. It is imperative that the extrusion supplier knows what the most critical features are as early in the process as possible.

i. If it is stated that everything is critical it means that nothing is truly no more critical than any other feature; and provides no assurance that your device needs will be met.

c. “Everything is critical” needs to be applied to the functional requirements; not on the need to make the tubing exactly as it is drawn.

d. A good practice is to formally prioritize every feature of the extrusion.

6. When it comes to downstream process validations

a. Base downstream processes, tooling, fixtures, etc. on actual parts using the full range of parts received.

b. ‘Cherry picking’ most ideal parts from a run for concept work is fine, but when this mode of use is applied in downstream development phases it can set the supplier, and one’s own downstream operations, up for a difficult journey.

i. Remember that the ultimate goal is maufacturability; if you have a product and can’t produce it effectively it is impossible to realize its full fiscal opportunity.

c. It is suggested that downstream processes not be validated until a minimum of three production-sized lots of the multi-lumen are received. Procure them early for process development!

7. Don’t assume that the quality (adherence to spec) will improve as time goes on

a. If the extrusion doesn’t meet specification when you receive components used to launch a product, quality often can’t get better because of the most basic of control requirements.

i. Remember the agreement you ask your supplier to sign?: the one that states that no changes to process can/will be made….. think about it.

ii. If you are using an ISO approved supplier, the supplier’s own control system makes it ethically and fiscally difficult for improvements to be made to the process.

b. Process and Tooling have a very intimate relationship. It is a relationship that can be very high maintenance without a ‘prenuptual agreement’ in place.

c. If it is clear that improvement is needed after parts have been used in product for release, proactively establish a plan with internal and external engineering AND quality personnel attached to the component that provides direction for quality improvement efforts.

i. Keep the development team attached to the production efforts until full resolution for quality issues is released; whether that be as a result of working with the supplier to improve the quality of the extrusions or if the result be that downstream processes are modified to accommodate current quality that can be warranted through agreedupon, working specifications.

Specification Guidance

1. Variables tolerancing must accommodate a reasonable degree of process variation.

2. Be willing to specify non-critical features on the print as references.

3. Common Practice for Specification and Tolerances for Multi-Lumen Tubing

a. OD

i. Average OD (most applications)

1. ((Max OD +Min OD)/2)

2. Best-fit circle (8 points or more)

ii. Max Axis Measurement

1. Applied when the extrusion is required to pass through a rigid orifice, fixture, or other extrusion.

b. Lumens

i. Lumen sizing

1. Are solid elements passing through lumens? 

a. If so, Minimum Pin Fit can often be used.

b. Using actual downstream tools for attribute testing is better.

i. Use a customer supplied device for lumen inspection.

2. Is fluid flow critical?  If so, a minimum dimension for best-fit circle may be a reasonable method for measurement.

ii. Minimum Pin Fit

1. Pin gages

a. Most medical extrusion houses use + pin gage sets for measurement; .025 pin gage would actually measure .0252. This helps to ensure that minimum pin fit lumens will adhere to specification

b. NOTE: with multi-lumens, it is rare to have an issue with a USL being exceeded as the normal task of getting everything within the limits of the circumference usually requires setup to Min-Nom pin fits.

iii. Best-fit Circle (8 points or more)

c. Septums

i. Often specified as a minimum acceptable value.

ii. For many vascular applications where the tubing cross-section is not going to be modified with secondary processes, a minimum value of .002” or .003” is often applied.

4. Measurement Method

a. It is critical that features of the extrusion be measured the same way, with the same equipment, for supply and receipt.

b. Sample preparation can affect the outcome of measurement, regardless of method used; don’t ignore sample prep when it comes to aligning inspection procedures.

c. It is necessary to identify and correct any discrepancy in measurements as alignment of measurement methods during development phases is critical for longterm success.

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NOTE: Specifications and tolerancing should be based on actual process capability.

NOTE: Out-of-roundness (OOR) is a master thief of tolerances. Designs are typically based on round crossections. Actual crossections are usually not perfectly round, therefore some sufferage of internal features automatically occurs. It can be helpful, where roundness is a critical need, that an OOR (out-of-roundness) requirement is combined with an OD requirement to ensure function.

Final Note:

With multi-lumen extrusions, what everything boils down to is usually: what can be accomplished to cover the ‘most critical’ elements of the tubing for use in the application. 

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