Help with home made J-heads

I made a j-head last year and it is still printing flawlessly daily. Couple months ago i made some more and there seems to be a huge difference in the amount of pressure it takes to hand feed filament between the first one i made and the new batch. The first one feeds so easily it takes almost no pressure,the new ones take a lot of force and are having problems with filament jamming in the melt chamber. Ive tried a few different things to no avail. What am i missing? The only thing i think could be different is the drilled diameter of the melt chamber where it meets the nozzles orifice, but ive tried it drilled out to 2.3mm and also at 3.25mm, neither works well. To get it to hand feed easily i have to raise the temp to crazy levels for PLA like 210C.
.4mm nozzle, using 1.75mm PLA only.

I don't really understand. If you have made your latest hot-end identical to your first one then should it not perform the same?

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crazy levels for PLA like 210C

I use 220C for PLA. Is it too liquid at 210C?

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[www.hydraraptor.blogspot.com]

wild guess here, but are you using the same temperature sensor same heater block and same controller with the newer hot ends?
What I'm getting at is that the core melt chamber may actually be at different temperature compared to what you have had with the original hot end

is your setup identical?

Different thermistor would be my guess..

Thermistor and resistor are the same, the heater block is aluminum on the origional and brass on the new one i built. I wouldnt think that the temperatures would be any different but i dont know, the thermistor should read correctly whether its mounted in aluminum or brass wouldnt it? And 210C is 210C whether the metal is aluminum or brass, or am i wrong?

It doesnt blow steam out or anything at 210C. Should i simply just up the temp untill it starts steaming and burning the PLA and then back it down a hair? I will make an aluminum heater block for the new one so at least there will be nothing at all different that i know of. Then try readjusting temperature and see how it goes.

I guess i just cant understand why there would be such difference between one batch of hot end and the other unless the heater block itself can make that much of a difference.

Long before it starts burning I find PLA becomes like syrup and drips out of the nozzle rather than forming a filament. There is only about 20C between that and being too viscous to extrude and there is about +/20C difference between colours.

I calibrate with a thermocouple inside the brass part of the extruder and find 220C is about right for Fabberdashery village green. I met with the owner a couple of days ago and he agrees 220C is the right temperature. The odd thing is one of my kit customers insists it needs to be 185C with the same plastic, same hot end, same 1% thermistor and same firmware. I used to think the people who advocate 185C were measuring the temperature on the outside, which will read low, but now it is just a mystery to me.

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[www.hydraraptor.blogspot.com]

With black Ultimachine PLA i printed around 185C, with Ultimachine white PLA i ended up around 195C, then switched to some white PLA from ebay and now im at 200C+. Thats with the nozzle that feeds easily. I just ran a print on the newer nozzle at 230C, it printed fine and looks great. I dont know, i guess ill keep printing at that temp and see where it leads. I just cant help but feel that it shouldnt need to be that hot.

Thermistors are known to have inaccuracies, especially above 150º celsius. Though most thermistors of the same brand and type will be roughly about the same, there is always a chance there is a weird one in between.

If it prints on 230º celsius, maybe you have gotten a batch of ABS? Or it's still PLA and the thermistor is just off.. Who knows.

What about variances in the balancing resistor on the PCB itself that is used to form the voltage divider which feeds the ADC? These boards are made by the lowest bidder in many cases, I wouldn't be surprised to see some variance in the resistors. Or even just "That resistor is close enough" when they don't have the right part in stock. It doesn't matter how awesome your thermistor is if the other half of the divider has atrocious tolerance.

Also is there any inconsistency about how AREF is dealt with over boards/atmegas? I know the original arduinos with an ATMega8 had a different internal VRef to the later ATMega168/328 chips.

The Melzis I supply use 1% resistors and the thermistor inputs are tested with a known resistance.

I don't think any boards use an internal Vref. They use 5V as the reference and also the supply to the thermistor so any variation cancels out.

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[www.hydraraptor.blogspot.com]

I've been printing PLA for the last year at 220C and HBP @ 110C (stock MK6 Makerbot Hot End). A small fan on the upper Hot End to keep the PLA from swelling and jamming.

I have printed PLA at a normal lower temp of 180C and it was OK but, I made a mistake one day. I hadn't changed or adjusted from the ABS start gcode. I still manually edit the generated gcode before I print. One day I forgot to lower the temperature for the PLA and printed at the ABS 225C and the HBP @ 110C. The PLA printed very well. I since have lowered PLA to 220C printing. Both White and Black PLA. Swapping between ABS and PLA is easy. Simply pull out the PLA and insert the ABS or vice versa. No clean out.

[removed comment -- I realised that I confused brass for copper, so everything I wrote does not apply to this case]

Edited 1 time(s). Last edit at 07/17/2013 07:59AM by Alexey Kruglov.