Finding the perfect balance with tubing, catheters and stents
Most medical tube extrusion lines begin with very basic concerns: How do I process this material into tubing with enough speed and efficiency to make a good product and a good return? Concerns get translated into requirements, and ultimately into an extrusion system.
Sometimes, processors develop and purchase an entire line at once - perfectly matched to current production requirements. Yet at other times, processors may also wonder if they can successfully want to react to business and competitive needs by adapting existing extrusion equipment and tooling. However, there are risks and difficulties in adapting, and results may produce problems rather than cost savings. Below we explain a number of situations that we have found are common in the experience of tube extruders, and suggestions for resolving them with a minimum of difficulty.
To see what can go wrong when you attempt to adapt a line to new requirements, it’s important to understand what needs to be right for that line to work properly. Ideally, any new extrusion line is designed to process a specific material at a specific speed, through a specific die at a specific temperature, and calibrate it to a controlled size through properly designed tooling with an adequate cooling process. Everything is in balance:
- Extruder is appropriate for material mix, production rate.
- Die dimensions are suited to material characteristics and product size.
- Drawdown rate is calculated, so predicted size of extrudate cone matches inlet of sizing tooling.
- Proper sizing method selected, calibration tooling prepared, with cooling adequate to product requirements.
- Production runs smoothly at a selected line speed, and line is stable.
When a few “real world” challenges arise, we discuss whether it is possible for processors to adapt using the equipment they have, or whether they must retool to deal with the change.
1. Change in tubing dimensions
Frequently, processors of the specialty pipe or tube need to make small dimensional changes or produce a slightly different size using the same material. For example, a tubing maker who makes 3/8 inch tubing (0.3750 inch diameter), might get an order for 1/4 inch tubing (0.25 inch diameter). Rather than invest right away in a new die, the processor decides to see if the existing 0.3750 die can do the job with the help of new calibration tooling. In some cases, it is possible to use a die designed for a larger diameter product to produce a smaller diameter product, but the ability to do so depends on the material - materials that exhibit a high degree of drawdown are very forgiving of size reductions.
For example, many medical tube makers are familiar with Pebax (polyether block amide), a highly viscous material with a high level of drawdown. A small, thin wall tube (0.125in OD x 0.008in wall) will use a drawdown of 50% or more since the material is more viscous and pulls down quickly as it leaves the hot die. So, it is certainly possible to use a 0.375 in die to produce a ¼ in tube product using that material.
One case where it would not be possible involves a material like rigid PolyVinyl Chloride (PVC). The drawdown rate of that material is so low that you may not be able to achieve the needed size reduction, so you would need a new die and new tooling.
Increase production rate and/or speed
When a line is working well, you might wonder: “Can it continue to produce - but at a higher rate?
The answer is that it depends. Line speed increases of up to 20% may be possible if you incrementally increase both puller and extruder speed - and if you’re running materials that will tolerate the change e.g. polyethylene, polypropylene or other polyolefins.
If you elect to boost line speed and extruder output, you are raising the risk of process instability. Higher extruder revolutions per minute will tend to increase material shear in essentially the same way as if you raised the temperature profile of the material. Boosting line speed may also require you to adjust the amount of material going into the extruded pipe or tube or adapt the “adjusted size” of the contact tooling, since higher line speeds are associated with greater shrinkage of the end product.
2. Change in material supply
Within “identical” types and grades of material, small variances in key characteristics can be sufficient to upset extrusion processes and require time-consuming adjustments. For example, a processor who is making nylon tubing for a new customer might select a better grade of material, and, based on their experience with that material, the processor’s extrusion team sources tooling and “dials in” the line to perfection.
Later, to take advantage of a great price, the processor sources “identical” material from a different supplier, however, extrusion efficiency suffers because the “identical material” has a wider molecular weight distribution - it tends to swell upon exiting the die, causing periodic overpacking of the calibration tool and tube wall inconsistencies whenever the excess extrudate “folds in” and goes through. Finally, after numerous adjustments and plenty of scrap, the drawdown problem is corrected, and the line is stabilized.
The lesson here is pretty simple and has nothing to do with equipment: To extrude with consistency, you have to buy materials with consistency. Develop a window of tolerance for the molecular weight distribution of materials you buy and stick to it.
To many processors, tube extrusion still looks and feels a lot like an art, since there seem to be so many judgments to be made, so many factors - large and small - to “get right” before a process will work properly. Yet, if you organize and leverage design, materials, and production experience, you may find it possible not only to master this process for the medical tube products you’re currently making, but to cost-effectively stretch the capabilities of your equipment and processes to achieve even greater flexibility, productivity, and output. Sometimes you’ll need new equipment, new dies or tooling, or improved controls to get the job done - but not always. In our experience, many processors are surprised at just how adaptable the extrusion process can be.