Filament diameter - differences between 1.75mm and 3.0mm

I can imagine that > 0.5mm you would get an interesting drop in head pressure.

But okay, I will load up on a 0.5mm J-head for the 1.75mm filament then. Even though I have my 3mm setup spanking at the moment, it's running super haha, you know that saying about not fixing something that ain't broke?

yeah haha its wierd with the larger nozzles on the 1.75, id still rather use 3mm, but the hot end i have and am getting in the future is only 1.75mm, unless they make a 3mm in the future, which would be nice.

Which hotend would that be?

Quote
T3P3
The diameter of the filament does not have a direct influence on the extrusion force at the nozzle as the plastic is molten. It can make a difference further up the path though. It varies by hotend design for example the length of the transition zone.

This information is completely false.

P = F / A

For the same pressure drop through the nozzle,

A 3mm diamter cross-sectional area requires more extrusion force.

Edited 1 time(s). Last edit at 01/13/2014 03:41PM by ShadowRam.

The pressure drop of a liquid flowing through a nozzle is proportional to the velocity squared. (Bernoulli's theorem and assuming molten plastic is a Newtonian fluid).

The velocity through the nozzle is the same in both cases (as is the downstream pressure) so the upstream pressure in the liquid plastic must also be the same for both 1.75 and 3 mm filament.

However to generate the same pressure in the molten plastic upstream of the nozzle the extrusion force on 3mm filament must be higher by the ratio of the cross sectional areas - about 3 x more force.

This conversation is getting interesting

I have decided to use a bowden extruder, direct drive, so with no gears. Because it's direct drive, I have to use 1.75mm, because the 3mm filament will kill the NEMA17, it cannot output enough force I think.

There are a few extruders out there, but they all use the kind of bearings I cannot get in my town and I can't find them with Dutch suppliers, such as the 5mm I.D. bearing to counter the strain on the steppers axle and a grooved bearing is nowhere to be found, except for one that costs 16 euros.. Outrageous.

So, I guess I will have to design my own.

Edited 1 time(s). Last edit at 01/13/2014 05:09PM by Ohmarinus.

Ohmarinus: I had bad experiences with a direct drive QU-BD extruder so I went for a bog standard Wade extruder. It is easy to get all the parts. The amount of force a Wade can push on a filamant is very high. Try Ebay or these guys:-

[reprapuniverse.com]

They are in Holland. I bought a Wade from them to get me started. I now I print my own bits. The bearings are just skate board bearings. Dirt cheap. I got a grooved bearing on Ebay along with hobbed bolts.

Edited 2 time(s). Last edit at 01/13/2014 05:43PM by dave3d.

Haha ohnoes, too late!

I already ordered everything for the direct drive extruder and will use triffid hunters' direct drive bowden kit for 1.75mm

Also, The MK8 direct drive hobbed wheel will be used and I ordered some of the 5mm ID bearings.

Only thing I couldn't get were affordable grooved bearings. Nevermind, I'll do without them.

Will update when all the parts are in!
- Marinus

How about grooving the bearings yourself? It's easy.

---

Yvan

Singularity Machine

In my mind it's the accuracy that's the thing if you think of steps per volume of plastic extruded not the length of plastic extruded as you set it up. Is where you gain with the 1.75 over 3.0 volume of extruded plastic is very critical and if you think of it is just like the rest your machine steps per distance but in the software when it's sliced it's actually the volume. So for a given extruder there is more steps per volume in other words extruder running faster. Which gives you finer resolution of the volume with the stepper motor

Edited 1 time(s). Last edit at 01/13/2014 08:47PM by cnc dick.

Quote
Yvan
How about grooving the bearings yourself? It's easy.

I tried with my Dremel, no luck. Those bearings are superhard!

There are quite a few YouTube videos showing how to do hobbed bolts. I think grooved bearings as well.
You need to construct a rig I think. Doing it with a Dremel by hand is difficult.

I am running my direct drive without a grooved bearing successfully. The groove in the hobbed bolt/wheel is holding the filament sufficiently. Only if you don't use a filament guide at the point where the filament enters the extruder, you need extra guidance that the grooved bearing provides or so I read. As my filament path is extremely constrained by guides, I thus never ran into problems.

Quote
Ohmarinus

Quote
Yvan
How about grooving the bearings yourself? It's easy.

I tried with my Dremel, no luck. Those bearings are superhard!

Bearings are super hard, but cut off disks are super harder!

I have used a Dremel/die grinder with one of those little heavy duty cut off disks. Position things so the die grinder is resting against something and the bearing can spin freely on a bolt held in a vice or drill press for example. The trick is to approach the bearing with the cut off disk at an angle so it is not just rolling against the bearing but "scrubbing" against it. I also hold a nylon brush against the bearing to limit its RPM.

I've also used a 4 1/2 inch angle grinder with a thin cut off wheel in the same kind of arrangement. You have to be more careful to get things set up right, because the disk has to be on the same axis as the bearing, so it will impart an insane amount of RPM to the bearing if not controlled!

As uGen points out, the groove in the bearing is not required in all cases. It does reduce deformation of the filament at the pinch point. Reduces jams and stripping, and allows more clamping force to be applied.

---

Yvan

Singularity Machine

with my new drive gear i actually need a very shallow but narrow groove to effectively guide the filament otherwise it doesnt push the filament all the way into the drive gear. btw i made the qu-bd drive gear my b**** by cutting a groove in the middle of the groove, it grips the filament on both sides .

I have been reading up a lot and decided to order the MK8 Direct Drive gear

Seems like the right choice, according to Airtripper, so we'll see!

I gave up with my QU-BD. Why anyone would make and market something that does not work straight out of the box beats me.

I bought the QU-BD MBE extruder. The filament is gripped inbetween the drive gear and a set screw. There is no spring to set the tension. It is very hit or miss trying to set the right amount of grip. Cheap and nasty. I abandoned it and went for a Wade.

In 1.75mm nozzles, the back pressure is lower than in 3mm nozzles. Which makes it easier for the extruder to push the filament through the tiny extrusion orifice.

So, because of all this, one advantages of 1.75mm is that you can make a direct drive extruder with a nema17 stepper only. For 3mm you will need to get a geared motor and that is bigger = more weight on your carriage.

I would be tempted to say that for the same reasons 1.75mm would be less prone to stripping but I dont actually use one right now... to much 3mm filament in stock.

I would also be tempted to say that 1.75mm may be easier on bowden setups but once again, I dont use one so confirm this first.

Well I realized the flaw in the QU-BD design is the motor actually has to use some torque to compress the filament into the drive gear. So you need at least a 5kgcm motor to do this effectively. The roller bearing helps but by adding the groove like I have done reduces the amount of force needed to grip the filament. The retraction is great though because its like a rack and pinion.