Hello all

I have sourced a small sample of ABS to test before actually spending real money on a larger purchase.

I had thought that as an initial test I should be able to feed the plastic through my barrel by hand.

It turns out the plastic is not perfectly circular but slightly oval and I cannot feed it through without actually buckling the plastic rod.

All of this is without applying heat to my barrel.

My question is - will it go easier if I feed this plastic through a heated barrel... does the plastic soften enough to deform enough to fit the hole?

Or should I look for another source of plastic?

The plastic is from ReprapKit and the barrel is from reifsnyderb

Thanks guys.

Oval filament is more trouble than it is worth in my opinion. The barrel has to be overly large and then you get more backflow. The feed rate tends to be a bit uneven as it depends how it sits into the pinchwheel. This is why I don't buy my ABS from the UK.

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[www.hydraraptor.blogspot.com]

Hi Chris.

Where do you get your ABS?

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Regards

Ian

reprapsource.com in Germany. The carriage is no more than I have paid in the UK and, being EU, there is no hassle with customs. The plastic is perfectly round and very consistent diameter.

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[www.hydraraptor.blogspot.com]

... you can test with the material at GRRF if it's in stock again - at start both (reprapsource and GRRF) used filament from Orbitech, but AFAIK GRRF is changing the supplyer for quality issues and different materials ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Hey Nop

you mention US sources are generally 2.8mm or so - Is this a problem with a 3mm barrel? Does it cause problems with the molten plastic backing up or does the filament soften and expand to fill the 3mm as it is pressed into the barrel?

Cheers

I made the barrel in HydraRaptor's extruder 3.6mm because my oval ABS was 3.5mm worst case. It doesn't work with the second reel of 2.8mm filament ABS I got from Makerbot, but it worked very well with the first and Makerbot said they hadn't changed anything but a year or two had passed. I couldn't spot anything different, but the backflow makes it hard to push and the plastic buckles in the barrel and strips in the pinchwheel. So it seems 2.8mm ABS is marginal in a 3.6mm barrel.

The closer the fit the easier it is to push up to the point where the filament is bigger than the cold part of the barrel and then it jams. I have used about 80kg of the reprapsource ABS and it has been in the range 2.98 to 3.14 (individual reels are closer than that). So I would advocate 3.2mm as a good size. However, I will probably switch to 1.7mm in the long term. I think my Mendel barrel is 3.3mm IIRC.

I have encountered "3mm" filament between 2.75 and 3.5, which is too big a range for a single barrel. There is also a massive difference in the bend radius, i.e. how tight you can coil it. That is because stiffness is the fourth power so the 3.5 is ~ 2.6 times stiffer than 2.75mm.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> There is also a massive difference in the bend
> radius, i.e. how tight you can coil it. That is
> because stiffness is the fourth power so the 3.5
> is ~ 2.6 times stiffer than 2.75mm.


... Which possibly suggests a very sensitive method for detecting small changes in filament diameter, much smaller than direct measurement. Hm.


Anyway, although this thread is about ABS, I have to echo that my experience has been similar with PLA. The first roll of PLA I got (which was donated from Wade, so I don't know the source) was ovoid and with a rough surface. The diameter was extremely inconsistent, and I was never able to print successfully with it at all. In a pinchwheel the ovoid tries to twist to the most stable configuration, and there's backflow, and it's just a huge amount of hassle.

Later I switched to PLA from Ultimachine (also donated by Wade - an exceptionally generous man!). The step up in quality was immediately apparent. The PLA cross-section was a very uniform, smooth circle, and printing became so much easier.

I don't have a machine yet.... but with inconsistent filament diameters could that issue be solved with a combination of a "soft" pinch wheel pressing up against a knurled hard one?

What pressure is the melted plastic coming out at? Perhaps a silicone "gasket/ o-ring type thing around the top of the cold end of the extruder barrel so the extruder actually fills a reservoir before exiting the nozzle? That way there could be quite a bit of "slop" to work with as the gasket would conform to the shape of the filament ... to a point anyway...

And to continue the "self replicating machine" philosophy, maybe a Reprap could actually print the mould for the silicone/urethane part to be cast in...

Sorry if this should be posted in another forum section but reading this sort of just sparked the idea..

I'm using barrels from reifsnyderb and I measure the ID to be 3.03mm. Presently, I am using filament that is out of round. The smallest OD is 2.74mm and the largest is 3.03mm. This seems to be about the limit of how much variation is usable for this size barrel. I have gotten it to work, but it was pretty difficult to get things set right.

Based on comparing this stuff with makerbot and ultimachine plastic, I'm thinking your target OD needs to be somewhere between 2.7mm and 2.9mm for a 3.03mm barrel.

Grant:
My experience has been that gripping oval filament is not as big of a problem as pushing it through the heater. If your plastic is so out of round that the pinch wheel can't grip it then it probably won't flow through the heater correctly anyhow. If your plastic is not flowing correctly, any attempt to grip the filament harder is just going to cause leaks, broken extruder parts, or missed steps and inconsistent extrusion.

I think the real problem with oval filament is that it leaves an air gap between two sides of the filament and the barrel. Molten plastic flows backwards in this gap and pushes a few mm into the insulator segment. Here it builds a plug that the heater can't melt. Eventually the friction from pushing filament through the half-melted plug stops the filament from feeding. You need a circular cross section of filament that fills the barrel so that it will act as a plunger and prevent the molten plastic from flowing backwards.

dazed.dnc Wrote:
-------------------------------------------------------
> I think the real problem with oval filament is
> that it leaves an air gap between two sides of the
> filament and the barrel. Molten plastic flows
> backwards in this gap and pushes a few mm into the
> insulator segment. Here it builds a plug that the
> heater can't melt. Eventually the friction from
> pushing filament through the half-melted plug
> stops the filament from feeding. You need a
> circular cross section of filament that fills the
> barrel so that it will act as a plunger and
> prevent the molten plastic from flowing backwards.


So would a Tapered barrel help this?

I guess I should really get machine and see for myself before thowing out ideas that others have already thought of...

I use the 3d ink stuff $36 for 5-6lb of the stuff. It works great as long as you have a spring tensioned idler..

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repraplogphase.blogspot.com

Most of the plastic I have printed with is 3D ink's white abs. The black ABS and white HDPE were too large to push through my barrel, but I got a refund without hassle and he has been working on fixing the filament dimensions. I received a sample of the new black ABS and it worked great. The new stuff should be available for purchase soon if it isn't already. I don't know if he is also going to change the other plastics at the same time or wait until his existing inventory sells.

Grant:
Sorry if I came off a bit harsh. I did not mean to kill your enthusiasm. The reprap can really benefit from people trying new extruder ideas since this is one of the most problematic parts of the machine. I don't want to steer the discussion off-topic, so I'll abruptly end my post by recommending that you read Nophead's blogs. Here is one about tapered barrels that you might like to start with.

Edited 1 time(s). Last edit at 12/04/2010 09:39PM by dazed.dnc.