Rebuilding the Original Heater

I've rebored the PTFE to remove the notches I previously made on the inside of the barrel. It now has a diameter of 5.5mm and is completely smooth again. I've remounted it on the extruder base and attached the nozzle.


I've also rebuilt the original nichrome wire heater wrapped around the nozzle, just in case the new heater block I've made is causing the problem with molten PLA backing up into the PTFE barrel.


The heater part works fine, unfortunately the nozzle is still filling with molten PLA. Since it never used to do this before the original catastrophic failure of the PTFE barrel that this is the cause. I'll need to make a new barrel.


The problem is I don't have a lathe, so do I spend $95 on the techshop lathe class so I can use their lathes and then pay $125 for the one month membership, or pay ~$500 for a mini-lathe for my workshop. 


I'd like a lathe, but I doubt I'd use it again after this. Perhaps Santa will bring me one?

Extruder Problems Continue

After rebuilding the extruder and running some test prints the extruder is still having problems. After about 10-minutes of printing the filament stalled again. The PTFE of the nozzle had backed up with molten PLA again.


I think I'll try two more things tomorrow. Firstly, I'll rebuild the extruder with the original nichrome wire heater - perhaps the block heater is more suited to the new style extruder where the brass doesn't get inserted as far into the PTFE. I think that perhaps because the brass nozzle extends into the PTFE 15mm or so, it is transferring heat further up the insulator than with the new style extruder.


Secondly, I'll try boring out the PTFE insulator. There is a slight notch on the inside where the drill slipped when I was drilling out molten PLA previously. This notch appears to be catching the molten PLA, which could be causing the feed problems. This will increase the internal bore diameter slightly which hopefully won't cause any problems.


If none of this works, I'll be rdering some PTFE and joining TechShop and using their lathes.

Building the Block Heater

As part of rebuilding the extruder I thought it would be a good idea to build the block style heater, rather than than the original heater. I made a couple of modifications to the build, we'll see how they work out.

The basic components of a block heater.

I followed the mechanical drawings and plans on the RepRap Site. It did require me to buy a drill press, metric drills and a metric tap and die set, but I do enjoy having new power tools!


I used a 3/8" ultra-machinable brass bar from McMaster. Cutting and machining it was fairly straightforward. The new drills and drill press did their job well.


I haven't tapped anything since I was at school, but the technique came back quite quickly. A quick test fit showed that everything went together easily.


I used 1mm PTFE tubing I found at Frys to insulate the leads for the thermistor.


To increase the thermal conduction between the thermistor and the resistor and the block I used Biostar Nano Diamond thermal grease. It is good up to 240c and is an electrical insulator. I filled the thermistor hole with the grease before pushing it home. The resistor was coated with the grease before sliding it in to its hole. 

With all the parts inserted.

While I was shopping for the fire cement I found a Rutland caulk good to 600F at Ace. It doesn't require heat curing and is flexible. I'll give it a try and see how it works out. It was a little messy to apply but should be better than fire cement.


After wiring it up and waiting for it to cure first tests appeared to be promising. It heated up quickly to the specified temperatures.


First tests with filament did not go so well. The filament did not feed very well and the output from the nozzle curled up! - Not sure why?


Then catastrophe struck again, the nozzle appeared to seize up. It appeared that molten PLA had backed up the PTFE and solidified.


It was another late night dismantling the nozzle and cleaning it out and drilling out the solidified PLA out of the PTFE. After it was all clean I checked the nozzle opening in case there was something stuck in there causing the curling.

Complete and back on the extruder. Note the new sprung loaded mech.

Using another thermistor as a temperature probe inside the nozzle it I checked the temperature inside the nozzle. There appears to be a significant difference between the block temperature and the nozzle temperature. The block has to register between 250 - 260c for the internal temperature of the nozzle to register between 205 - 220c. This may have been the cause of the feed problems. At a block temperature of 220c the internal nozzle temperature was only at 185c.


Cranking the temperature up to 260c and trying some manual filament feed tests shows this temperature setting was much better. Tomorrow I'll try rebuilding it completely and testing it with the motor feed.

Fixing the Extruder

It looks like it might be possible to salvage my current extruder. I have successfully managed to clean out the brass nozzle by heating it up with a butane torch and pushing the stuck filament through with a drill. It's currently going through a final cleaning by spending the night in a glass full of acetone.


The PTFE insulator was a bit tougher. Due to my earlier problems with the hot-end, it is well glued into the extruder base and it is half-full of PLA that followed the nozzle down as it was pushed out. I managed to get this clear by screwing the nozzle back in as far as it would go then heating it with the torch, then screwing it in some more. This pushed the excess into the nozzle and I could pull it all out with the nozzle.


The nozzle appears to fit still, so I'm not sure why it decided to drop out during the print process, unless the PTFE deformed. I plan to wrap the nozzle in a few layers of PTFE tape and screw it back in, then re-build the whole extruder again. But first, I need a new thermistor, it got destroyed whilst trying to clean the nozzle up. I'll check Fry's tomorrow or place an order with Digikey.


However, I doubt the extruder will last much longer after it is rebuilt so a complete new extruder is on order from Mendel-Parts with the PEEK style nozzle. However, judging by the latest update message from Camiel on his site, I doubt I'll see it before February. Hopefully his assistant is alright, it sounds like they had quite a week!


I also plan on building the brass block style heater instead of the wound wire version. It will give me something to do while I can't print. It's also a great excuse to buy a drill-press. I found that the kapton tape on my current extruder would melt gunk into the objects that I was printing.


I'm wondering if this is an indication the extruder was getting too hot? When I rebuild it I'll put the thermistor slightly further up the nozzle, rather than close to the tip.


Fingers crossed I can at least print something for the next few weeks, otherwise I'll be writing applications or playing with CAD packages.

Extruder Problems

The most frustrating thing about my Mendel is my extruder. I've had lots of problems with it since I completed the build. It's Adrian's Geared extruder with the hot-end epoxied into the base rather than held in by a PEEK block.


The first problem that I had was caused by my wimpy stepper motors getting too hot. The excess heat caused both the extruder base and the X-carriage to distort. This has since caused all sorts of meshing problems for the extruder gears as they no longer line up. 


The second problem that I had was the hot-end dropped out completely while working on a print. Frustrating, but easily fixed by liberal application of more epoxy.


Today I resolved to fix the meshing issue which causes the gears to skip, resulting in poor extrusion. I completely dismantled the extruder body to fix the alignment between the motor mounting plate and the extruder base. After they had become distorted they developed a slight kink in both parts, and whenever I tried to align the gears the parts would prefer to slip back to align with the kink. 

Note the kink in both parts and the buried washer.

The solution was to file the kink flat on both parts and make up for the missing material with an M4 washer. I also found that the excess heat had partially closed the bolt slots on the base, preventing the bolts from sliding as needed. After opening these back up again I could position the motor so that both gears engaged successfully.


Several tests showed that everything was working perfectly, so back to printing. In this case some new frame vertex with feet, since my Z-axis pulleys stand a little proud for the current feet to handle.

Previously the wires were flush with the PTFE.

Unfortunately, half-way through the print another extruder problem developed. The threaded brass insert that makes up the extruder was pushed out of the PTFE block. Shifting it down about 5mm and leaking molten PLA out over the nozzle.


I'm not sure if I can salvage this extruder hot end, I might need to build a new one using the PEEK block to attach the nozzle to the PTFE. This should solve at least two of the extruder problems I've experienced in the past two weeks. It wouldn't be so bad if I've been churning out hundreds of Mendel parts, but I've only managed to print a couple of small objects and two vertex's. Reading other RepRappers blogs, I can see that others have far more success with their extruder than I do with mine.


Oh well, time to research new hot-ends!

What Causes the Warts?

Printing with Warts

On all of my prints there are little warts in the layers. These are caused by excess plastic being deposited at various points during the print.


At first I thought these were caused by the oozy nature of PLA. Just turn on the extruder to heat and without running the motor molten PLA will ooze out of the extruder, seemingly forever.


However, I was determined to stop it from happening. I started playing with both Oozebane and retraction speeds and distances in Skeinforge to no avail. These settings only affect the end of a thread, these warts appear in the middle of a thread run.


I started to watch my printer more closely to see what was causing them. It became apparent that the warts are caused when the printer stops for a fraction of a second and PLA oozes out on to the print. Why was my printer pausing?


I reviewed the firmware code to see if I could spot the cause. It's complex as there are so lines of code for other printer configurations in the firmware that you get lead down a particular path only to discover that piece of code isn't compiled into your specific printer. Nothing obvious jumped out.


Then I wondered if the GCode buffer was running dry so I started to review the RepSnapper log files. The timing information in them wasn't sufficient to spot buffer under-run, so I switched to PortMon, my favorite serial monitor. It didn't appear that there was any noticeable delays in getting the GCodes on chip at a baud rate of 19,200. A few quick calculations confirmed this. 


I did note two things in the log file though. The first was there were loads of retransmissions. Mainly caused by Skeinforge generating M101 and M103 extruder commands, which aren't used by Mendel. I'm not sure how to prevent Skeinforge from generating these, if anyone knows let me know. When the Gen6 electronics receives these commands it responds with a 'Huh - I don't know what this is' and then it asks for the last packet before that to be resent. Perhaps this was causing the buffers to run dry as the firmware flushed the previous commands? So I wrote a quick Python script to remove the duff M10x commands and retried the prints. They were still warty.

Wart-less Test Print

The other thing I noticed were lots of M105 commands, these are temperature requests generated by RepSnapper. The firmware takes sometime to process these commands as it has to take multiple temperature readings and average them. Switching this feature off in RepSnapper instantly removed the warts on the test prints. The Atmega should be capable of performing these measurements while still printing, but without a real scheduler perhaps the timing is too complex - I need to spend more time with the firmware.


From now on, I'll switch off the temperature monitoring in RepSnapper, but I'll still continue to run my pre-processor script as it prevents thousands of re-transmissions per print. I've also added the ability to substitute the Skeinforge start-up code with my preferred start-up GCodes.

Before and After

Tuning Skeinforge

Skeinforge has *lots* of parameters to set and not much information on what they all do. To help me set my version up I found the following page very useful (although some of the parameters have changed name or no longer exist) Skeinforge Wiki.


In the end I went with the default parameters for PLA with the following changes.


Feed Rate = 40mm/s
Flow Rate = 57.6 
Retraction Speed = 250
Distance = 30mm
Extra = -10mm


The last 3 settings help with the blobbing that you get at the end of each thread by sucking the excess filament back into the nozzle by momentarily retracting the filament. I've found PLA to be very oozy. 


I found the best page to help me set up the feed rate and flow rate was this one from Tony @rapman3. Since I'm using a stepper based extruder the flow rate setting, which appears to be a PWM control value was a bit meaningless, however, I followed Tony's instructions to set up the deposited filament width to 0.72mm with a height of 0.4mm by varying the Flow Rate setting for the specified Feed Rate setting. By trial and error I found that the setting for a feed rate of 16mm/s (bed speed) required a flow rate of 23.

Final Test Print, 0.4mm Layer Height plus Warts

At 16mm/s, this printed too slowly and I know that the Mendel can print much faster, so I upped it to 40mm/s and found that the Flow Rate scaled in a near linear fashion. To test the various parameters, I used Tony's approach of printing a single filament walled object, a 3cm diameter, 1cm high cylinder. The end results were quite pleasing, except for the warts, which is the subject of a future investigation.