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Updated: Sep 2


Coiled Catheter

Want to save time when ordering extrusions for your next catheter project? This article will help you calculate the diameter of your next coiled catheter.

Background

Many neuro and peripheral catheters are made by a process of reflowing or laminating polymers and reinforcement materials over a mandrel. The reinforcement material we'll consider in this article is a coil. A coil winder is used to wind a wire over a catheter mandrel. Then a polymer jacket is sized to slide over the coil during assembly. Heat shrink tubing is then slid over the jacket. During the catheter reflow process, the jacket melts and is compressed by the heat-shrink. This causes the jacket to reduce in diameter and increase in wall thickness.


Conservation of Area

For starters, let's think of the cross section of a reflowed catheter without a coil. There's a mandrel, liner, and polymer jacket. During the catheter reflow process the jacket melts and is compressed by the heat shrink tubing. This reduces inner and outer diameter of the jacket. Since no material is lost during the catheter reflowing process, the cross-sectional area of the jacket must be the same before and after reflowing. We can use this to calculate the relationship between the pre and post reflowed jacket diameters.

Catheter diameters before and after reflow lamination

Keep in mind that the final ID of the jacket would be the liner OD, or the diameter of the mandrel plus twice the liner thickness.


Enter the coil

Coiled catheter segments

Now let's add a coil. The conservation of

area method is invalidated because the coil is not a uniform tube. In fact, the jacket is no longer a uniform tube.

The reflowed jacket has a helical void in it because of the space the coil takes up inside of it. Instead of thinking of a conservation of area, we can use a conservation of volume to equate the pre and post lamination conditions. Again, we assume that we're not losing any material during catheter reflow. Let's cut a section of coiled catheter into a unit length and work out the volumes of each component. We'll use 1 inch for the length unit and use inches for all the diameter and volume calculations too.

Finding the volume of the jacket is straightforward. We just multiply the cross-sectional area of the jacket (first equation) by the length. Since the length is 1, it's unchanged. For the coil volume it's the same plan. We'll multiply the area of the coil wire by the length of the coil wire in a unit length of catheter. Visualize un-wrapping the coil wire along the coiled length. The result forms a right triangle.

Calculating length of coiled wire around catheter

The height of the triangle (a) is 1 unit (the same unit as the rest of your calculations). For the base of the triangle (b), think of how many times the cylinder would turn if you unwrapped the coil. The number of turns can be represented by the length of the cylinder (1 unit) divided by the pitch of the coil. We multiply the number of turns by the coil mean diameter and Pi to find this leg of the triangle.

Great, we have the length of each leg, now we just apply the Pythagorean theorem to get the wire length. Note that this number expresses how much coil wire is required per length of catheter. So, if you wanted to know how much total wire a catheter would take, you just multiply this number by the length of the catheter.


To find the volume of coil wire in a unit length, we now multiply the cross-sectional area of the wire by the length of the wire. If using a round wire you'd use the equation for the area of a circle. If you're using flat wire, you'd multiply the wire width by the thickness. Add the volume of the coil to the volume of the jacket and that gets us the total volume before and after reflowing! Let's remember the equation for the area of a pipe and assume that the area is equal to the volume since the length is 1. Solving for OD we get:

Conclusion

Using this method, you can calculate the OD and wall thickness of polymer jackets for reflow. It should save you a few prototype iterations when designing a new catheter. Additionally, you can fine-tune the OD of the device by varying the coil pitch. If reflowing a braided shaft, the volume of the braid can be calculated similarly based on the braid angle, number of wires, and cross section of the wire. Of course, these calculations should only be used as a starting point. Fine-tune your catheter OD with prototypes and testing. Good luck!






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Updated: Jul 20


Clear acrylic dry box with hydrometer showing 10% humidity. Carbon filled nylon filament in dry box being 3D printed in the background
Dry box with Carbon Fiber Nylon Filament

You've probably heard that 3D printing carbon fiber-filled nylon is difficult. You may have heard it requires special hardware. Well that's half true. You will need a few essential components to be successful at it, but it's not that difficult. And this filament is worth the effort. Nylon is tough and has excellent layer adhesion. The addition of carbon fiber (CF) to the filament makes it stiffer and reduces the material's tendency to shrink during cooling. This means you can print it at room temperature and get amazing prints. The finished parts are strong, tough, and temperature resistant! Here are a few tips that will ensure success.


 

If at first you don't succeed, DRY DRY DRY!

This may as well be the title of the blog post. I can't emphasize enough how important this is. Nylon is hygroscopic, meaning that it readily absorbs moisture from the air. When printing, this moisture turns to steam and expands inside the filament as it's extruded. This causes several issues including a rough surface, blobs, and stringing. So how do you ensure your filament is dry?

Dry in a convection oven at 90° C for 4+ hours

There are several filament dryers on the market. They range from re-branded food dehydrators to purpose-built devices. But these units have a limited temperature range. We dry our nylon filament in a convection oven with a tightly controlled PID temperature controller at 90 C for 4 to 8 hours. Weigh your spool first on a gram scale. Write the value on the spool before it goes into the oven and then repeat when it comes out. You may be surprised to see it lose several grams of water. Don't forget to dry your desiccant material at the same time and weigh it too. Understanding the weight loss will help you understand your drying process.

Dry that brand new spool too

Just because you opened up a fresh spool of filament doesn't mean you can skip this step. We typically extract about 10 grams of water from new 1Kg spools (that's 1% by weight!). The image below shows a part printed with two spools of carbon fiber filled nylon from the same brand. The lower part was printed with dried filament and the upper 2 mm with a spool of fresh filament straight out of a vacuum sealed bag. You can clearly see a color change and blobs of material on the upper 2mm of the part. The same spool lost 1% of moisture after drying.

3D printed nylon part showing differences between dried and undried filament. There is a color change and blobs on the layers made with the un-dired carbon fiber filled nylon filament
Old dry filament vs new un-dired filament (top 2mm)
Print out of a dry box

This may seem overkill if you're used to printing the same spool of PLA for weeks with no conditioning. But nylon filament will absorb enough moisture to ruin your print if it lasts more than a couple of hours (not to mention needing to be dried again for your next print). The solution is to store and use the filament in a sealed, dry box.

The filament should exit the box through a tube that runs directly into the extruder to minimize moist air contact. What else should you keep in your dry box? Dried desiccant and a digital hydrometer. You can find hydrometers on Amazon and other online vendors. If the relative humidity rises above 10%, you should re-dry the filament and the desiccant. Please note that cheaper hydrometers don't display values below 10%. This is the range that you are most interested in.


Turn up the heat, Turn off the cooling fan

You know that nylon needs to be printed hotter than PLA or ABS. The exact temperature will vary by material and printer setup. We run between 280°C and 295°C. This will require an all-metal hotend.

Not all hotends are created equally

Now that your all-metal hotend can reach a searing 300°C, you may start to find new problems. One new thing to consider is the efficiency of the hotend's heat break. The heat break is the part between the hot nozzle, and the cold tube leading to it, usually surrounded by a heat sink. When the nozzle is running hotter, more heat will be transferred through the heat break, into the cold end of the hotened. If the filament melts in this zone, it will get stuck and be quite difficult to remove. You may want to add a bigger fan or better ducting to the heat sink (as I did). Instead, consider buying a better hotened that performs better at higher temperature.

Speaking of fans

You might think that printing at such high temperatures means you need to run the part cooling fan. But nylon does not need part cooling. In fact, you will lose layer adhesion strength if you do. Running the fan slowly over bridges only is acceptable, but leave it off for the print. If small islands are being printed, you can experiment with slowing down the print speed. You can then add minimal fan speed if needed. But for general printing, it should be off.

Should I buy a super-hard, diamond-tipped nozzle?

No. But you do need something harder than brass. The reason is that carbon fiber filled nylon will erode the nozzle. The carbon filler is abrasive and will destroy a brass nozzle in a short time. Hardened steel nozzles from quality vendors are an excellent alternative. But heat transfer through the steel into the filament is worse. Increase the print temperatures and slow down the maximum speeds a bit when using steel. Another option would be to use a ruby-tipped brass nozzle. You'd get the heat transfer of the brass, and the abrasion resistance of a ruby.

close up of a 3D printer making a carbon fiber nylon part
A CF-filled Nylon part being made
What about build surfaces, heated chambers, etc?
Build surface and adhesion

When it comes to build surface, the secret sauce is PVA glue or regular glue stick. PEX is a favorite build surface at Pilot Line but garolite works well too. Either surface has to be coated with glue stick. The first layer parameters must also be correct (speed, height, etc).

Chamber Temperature

For chamber temperature, you're in luck. Carbon fiber filled nylon doesn't require a heated chamber. Room temperature or higher will work well. The carbon fiber helps reduce shrinking and warping in the nylon which helps stabilize it.

Build plate Temperature

The build surface does not have to be heated. We run a PEX build surface at 45°C out of tradition. Markforged printers do very well with an unheated garolite bed (and glue stick).

Print settings

Are you wondering how much retraction nylon needs to avoid oozing and stringing? Scroll back up and read the section on drying the filament. Seriously, there are no special settings needed to print nylon. Of course you'll have to experiment with the usual print settings to see what works best. If you find yourself running twice the normal retraction to avoid oozing, the problem is moisture in the filament. Dry it and try again.

Why are my nylon parts getting softer over time?

Remember that nylon is hygroscopic? Well your super-dry freshly 3D printed part is now absorbing moisture from the air. This moisture acts as a plasticizer. A plasticizer is something that is added to a thermoplastic to make it softer. This softening of the material after it's exposed to real-world conditions is something that must be considered when designing a nylon part. If you need a very stiff part, try PLA. If you want something amazingly tough with some decent stiffness that can take some heat, it's hard to beat CF nylon.

 

I hope this guide helps clarify the topic of 3D printing CF filled nylon. It might be your new favorite filament. Let us know what your experience is like, or if you have other tips for printing nylon. Good luck!

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What if you needed very precise heat control for your catheter reflow process and commercial solutions weren't cutting it? Maybe you need to reflow different polymers at different temperatures along the length of your device. What do you do? Maybe creating your own PID controller and tuning it is the answer. This article will walk you through a tuning methodology.

The Basics: We'll assume that you're familiar with PID control but let's go over some basics. The purpose of a PID controller is to reduce the error between a process set point and the actual process parameter. This difference is called the error or "e" in the equation. This error is what drives the whole control loop.

The "P" coefficient is multiplied by the error to affect the output. The higher the error, the more the proportional term reacts. The "I" coefficient is multiplied by the integral of the error over time. Simply put, this term grows over time the longer there is error in the system. Finally, the "D" coefficient is multiplied by the derivative of the error over time. How fast the error is changing affects this term.


Setting a goal. It's important to define what an acceptable outcome will be. If you don't define the goal, you may never finish the tuning process. In this case we want the process temperature to reach the target temperature in 10 seconds or less. We want to limit overshoot to less than 10º C and we want the process temperature to stay within 5º C of the target.


Seeing the Process. It can be daunting to think of where to start tuning a PID system. The first thing you need is a way to visualize the process parameters and change the process setpoint. Here we've programmed the heater to alternate the target temperature between 300º C and 400º C (red line). We're plotting the actual air temperature in blue. The heater output is shown in green and varies between 0-100% power.

Is the Process Controllable? This may sound like a stupid question but answering this upfront can save you countless hours and frustration. If you were to turn on the heater element, could the temperature reach and surpass the target? Does it come down with the heater off? If the heater doesn't have enough power to reach the target, or has way too much power and blows past the target, you'll never be able to control the system. (I've learned this the hard way). The heat should also react proportional to the power applied to the heater. Say the first 80% of power commanded did nothing, and then suddenly the heater turns full on at 81% commanded power. This would make controlling the system impossible.

 
P=10, I=0, D=0
P=20, I=0, D=0

Adjusting the P coefficient. The first thing to do is set the I and D coefficients to zero and start adjusting the P. The images above shows a P at 10 and 20. When P is set to 20, there are large oscillations in the temperature. With P set to 10, the Oscillations are dissipating quickly. We could pick a P term that doesn't produce any Oscillations or overshoot, but we will take care of a little overshoot later with the D coefficient. Notice that in either case, the temperature is consistently under the target. This will be resolved with the I coefficient.

 
P=10, I=1, D=0

Adding I coefficient. Now that we have the P coefficient producing some output, we can add I to the controller. With I set to 1 in this case, you can see that already the temperature is running much closer to the target. Some of the oscillations from the P term are reduced as a result of the smaller error too.

 
P=10, I=3, D=4

Adjusting the D coefficient. Now that P and I are set up, we can increase the D coefficient and attempt to remove overshoot and oscillations. The image above shows the response after adjusting the D coefficient. Notice the extra I term keeps the temperature very close to the target. But there is too much overshoot and there are still some oscillations.

 
P=5, I=1, D=5

Refining. In this image, we see a much more predictable response. Overshoot is reduced and the process quickly finds the target without oscillating. The overshoot and oscillations were reduced by reducing the P and I coefficients. As we saw from the initial P adjustment, this term was producing some oscillations. It was too much to fix with the D term, so we backed off of the P.

 
P=4.5, I=0.7, D=5

Finishing up. With a little more refinement, we have a process that quickly reaches the target temperature without overshoot. This type of controlled response is critical when changing temperatures along the length of a catheter. In this extreme example, the temperature setpoint changes 100º C and the process temperature follows in less than 10 seconds

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