Wildseyed simple hot end

I have been planning to build a Wildseyed hot end, and would be interested in the feedback of anyone else doing so. I have mentioned certain issues on the wiki discussion page.

I'm conscious that Adrian warns against using different metals for the nozzle and block, because of different degrees of thermal expansion. He says:

Quote

You could almost certainly make the nozzle and block from aluminium rather than brass. But don't use two different metals: if you do thermal expansion will cause the heater block to work loose.

This hot end, on the other hand, suggests a mixture of brass, copper, and aluminium. Is this likely to be an issue?

Edited 1 time(s). Last edit at 09/06/2011 01:52PM by dslc.

My hotend uses a brass heater barrel and an aluminium Heater Block and I have no issues. BUT mine is threaded and uses a jam nut to keep the heater block tight to the threads. Adrians has no threads and relies on the block being crimped to the brass barrel which has no means of keeping the block on if it expands more than the barrel.

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I have a push fit hot end (Adrian's design made by Cdnreprap). Unfortunately, since the aluminum block has a higher thermal expansion coefficient than brass, the block loosens from the brass nozzle when heated.

The Wildseyed (that's me) hot end uses an M3 set screw to keep things together. When I drill the hole in the aluminum block for the brass coupling, it's not a particularly tight fit. There's just some heat sink compound there to maintain good thermal contact.

I like the look of this variation [www.thingiverse.com]. Perhaps the barrel of the air hose connector could take a thread to hold the heater. Alternatively, if you want a push-fit heater with a split in it, make the flanges around the split a little longer and put a set screw through them for a tight hold on the barrel. I'd also probably make the thermal break/support rod out of PEEK rather than ptfe for temperature resistance, with a thin ptfe liner, or a threaded brass top barrel like the mendel-parts V9.

Thanks for the info and advice all. Will look into this more as soon as I have time.

@droftarts: I had wondered about the choice of PTFE over PEEK as well, but will probably work with the PEEK rod I have for the time being.

david

The difference in thermal expansion coeficients between Aluminum (outside), Brass (middle), and Copper(inside), is very small, and considering that on the Wildseyed hot end, these parts are bound in concentric rings insures that things are not gong to fall out of each other. The reason being that expansion occurs in both directions (the hole remains very close to their original dimensions.

The picture you show above has one fatal (well, in my oppinion) flaw, and that is the threading of metal on the inside of PEEK. I don't know the expansion coefficient of PEEK, but it should be MUCH higher than brass. This will cause the peek to loose it's grip on the brass screw. It' may not fall out, but it might wiggle as you print.

The only expansion-related issue that I have experienced with using the Widseyed hot end is that the Z-depth home position must be calibrated after the hot end is heated, as the PTFE does expand both in length and girth. This is one reason why it holds the brass coupling so well (it's threaded into the coupling).

Dunno if i am mistaking or not, but i think around 150-250C dilatations* 10^-6 in m/(m*K) are as follows: ptfe ~60; peek types from 100 to 65; aluminium 25; brass 19 (? question mark on brass).

Ok time to throw some theory at this discussion! Since I'm lazy I will be guestimating on geometry, alloys, and change in temperature. Feel free to correct me if I am horribly wrong.
First, www.matweb.com has thermal expansion coefficients.
Aluminum 6061-T6: 25.2E-6 1/K
free machining brass C36000: 20.5E-6 1/K
Oxygen free pure copper C10100: 17.7E-6 1/K
dT=250°C
D_CuBr=5.56mm
D_BrAl=7.94mm

When a ring of material is heated both its inside and outside diameters grow. Think of a strip with a length equal to the circumference of the ring. Now heat that strip up. It is pretty clear that the strip gets longer. Now wrap that strip into a circle so that the ends touch but are not bonded together. Once again imagine heating the material. One way to picture this is to imagine bending two strips of different length each into a circle. The longer strip makes a bigger circle. In this example the longer strip is the higher temperature case.

Some of you might point out that the strip doesn't just get longer as it is heated, it also gets fatter. However, the amount the strip gets fatter by is much smaller than the amount the strip gets longer by. This is why the ring inside diameter and outside diameter both grow.

So... how much does the fit between copper weld tip/nozzle and brass coupling change?
Cu growth = 5.56mm * 17.7E-6 1/K * 250 K = .024603mm
Br growth = 5.56mm * 20.5E-6 1/K * 250 K = .028495mm
So, the copper grows some, but the brass grows faster. In fact their relative change is .003892mm. For reference, a human hair is typically .05mm thick.

What about the aluminum block and brass coupling fit?
Change in fit = 7.94mm * 250K *(25.2-20.5)E-6 = .00933mm
Once again the fit gets looser.

Are these changes in fit significant? That is a more complex question which depends on the temper (or yield strength) of each of the materials. If the change in fit is less than the combined elastic deformation of both parts due to press fitting the joint then the joint will stay tight. The amount of elastic deformation depends on the yield strength. I can make a reasonable guess for the aluminum and brass yield strengths, but the copper could prove troublesome. Regardless, that is an exercise for another day.
Tyro