Rear printer: Head side view

Published on May 22, 2014

Rear printer: Head side view

A general view is how to use them, native analogies. Modest reality. Only FDM, only home, live - but from the inside and in detail. Hotend? In a section, with zones and the schedule of temperatures. Advantages and disadvantages, properties of various solutions in the design of printheads. Features of printing with trimmer fishing line, and experiments on obtaining tables of optimal settings for high-speed printing with nylon (nylon). Conclusions from them. On the organization of a good supply of nylon rod in the extruder. Everything is very exciting and dramatic.

About 3D printers now write a lot, write with enthusiasm, describes a lot of models and technologies, as well as a lot of their skills. And now, I am standing in front of you, a simple Russian man from Lithuania (C), with a 3D printer. Of course, he assembled it himself. Not from a set - a year and a half ago there were few sets, and they were expensive. I planned and chose everything myself, focusing exclusively on the Internet. But this is not what I want to tell, I want to talk about what happened afterwards - after its construction, and questions are being asked - Why is this? - What can he do? - Why buy it? - ask themselves many people passionate about technology.

Let's take it in order: first I built it. It was very interesting, in the sense that I had to constantly solve a lot of technical issues, find the reasons for abnormal work - by calculating, often by indirect signs. A kind of game in technical puzzles. Earned. Established. I saw decently possible improvements. Did a few - with great pleasure. Then he developed a really convenient end sensor assembly for the printer. I even sold it once. Well, a set of sensors - you won’t get rich on this, but what a pleasure I received from the sincere gratitude of the buyer. Tinkering with the print head - I was convinced that, despite the good workmanship (branded MK IV), it also has several disadvantages. Yes, and there was only one - and buying more is expensive. Developed his - you will laugh, the first time something happened, although not perfect. True, I didn’t so blindly, but carefully studied what was available, read what they wrote, and only then did it. Then he began to understand. To make experiments. There was a long break - disappointment and depression, but this is not connected with printers, for example, a straw that broke the back of an elephant.Now I have 4 heads, of different diameters, and the printer prints three times faster than normal and prints with a layer of 150 microns. And it prints not with expensive imported filament, but with cheap fishing line for trimmers. It turns out very, very durable, although not without tricks.



We’ll talk further about the picture, and now:

So, definitely, I got a convenient way to make a little thing out of plastic and a lot of positive emotions from technical creativity, I improved my skills in solving technical problems. Something like this. But do not forget that they trained me as a scientist and chemist-technologist. that is, I already had certain skills and mastered the methodology of finding the result, moreover, also an amateur electronic engineer and computer specialist.

So the massiveness of this method of acquisition is in question. Now I read another article on Habrir about Them, and something reminded me of it ... I have old scientific and technical journals, I can remember. So, the 60s, 70s, 80s - it was written everywhere, more and more about the remarkable capabilities of computers, they became more and more, they became smaller, and appeared in life a little and from afar. That printed sheet with the calculation of salary will bring - so, after all, also with errors! That train tickets, in Moscow, it suddenly becomes unusually good to buy. Then there appeared Household Computers - BK-0010, Radio-86, Mikrosha - they were mainly used for their own technical creativity, it was difficult and inconvenient to really apply them. They came up with thousands of flimsy and inconvenient applications, they were upgraded - I had Radio-86 with 56K (!) memory and 64K ROM (!) - it contained all the necessary programs, and at that time there were areas in the industry where a computer could no longer be dispensed with. But how far they were from the people! (C).It was very similar to the current situation with 3D printing now: - many “ham enthusiasts” are messing around with their small FDM printers (that is, those who print a trickle of something), and at this time, dentists and jewelers already use them in full. Doctors, so far in single copies, but use it where it would cost too much because of the single manufacture

, It turns out that we got another tool for the development of technological progress. So far, only that. But medicine is at a low start, it is preparing to start using them actively, and this is understandable, medicine is a high-tech industry. Probably you can find quite a lot of such places, but it would be dishonest to spread to the readers of Habr only a bunch of bare reasoning - even if the topic, even honest.
So, I want to tell you today about how one of the nodes of our home printer - the print head - works. You say - why is it interesting! Plastic melts and flows through a hole. If in three words - then it is, but dozens of different models of heads fall under this definition, among which are completely unsuccessful - people constantly clean them and curse dirty plastic. Oh, I doubt something about this reason. There are heads that will only work normally with PLA-polylactide. Although very environmentally friendly, it is far from good everywhere. Rather, it is good because it is easy to make a cheap and stable printer for it. There are heads that degrade quickly enough from work, especially when working at high speeds and with not too fusible plastics.I’ll try to tell you interestingly, the general patterns that I partly dug up on the Internet, partly found myself - I had to put a lot of experience and think a lot, it happened to get confused, so if my opinion does not coincide with yours, I ask you not to beat me hard, and take into account what I write according to the results of measurements of my system, so they may differ from the usual ones. I tried to check everything - to one degree or another. Think, try, maybe you will benefit from this information.

So, let's talk about the heads of the most common type of printers so far - printers, where a fishing line from one or another plastic turns into a printable object. Fishing line is usually called filament or welding rod. Its material is usually ABS plastic, which means - Acrylonitrile-Butadiene-Styrene - is quite ordinary plastic, PLA is also common, Polylactide, this name in Russian is correct, despite the direct translation from English, which is contrary to it, this polymer is specially produced for 3D printers. However, in theory, almost any thermoplastic can be used. Practically used - nylon / nylon, polyvinyl alcohol - it can dissolve in water and others. PLA plastic is loved by printer manufacturers because it has the least problems with it; parts are obtained from it in almost any environment.

So - the part of the printer in which the filament melts and extrudes a molten stream of polymer is called the print head. An important part of it is the extruder, a knot that grabs the filament and pushes it into another, heated part of the head - the hot end. Above it is in the picture. Let's look at the picture of the hot end in the context, and how the temperature is distributed. I must thank colleagues from reprapology.info/archive/1422/rheology
- from this page I got an initial idea of ​​how the melt and extrusion of the melt thread from the head occurs. A little according to the scheme: the purple outside the head is a heating spiral made of nichrome or cantal, however, a resistor is often used as a heater in the heads - this is convenient in production, but reduces the reliability and uniformity of the heating field, since the resistor is located locally, therefore it experiences local overheating . However, there is no fundamental difference here - resistor or winding.
The red in the lower part of the head is the zone of molten polymer, it is heterogeneous, as the polymer changes viscosity in the temperature range, and the orange zone is the area where the polymer has already warmed up to plasticity, but not yet flowing. This zone is very important - in it, the plastic rod expands slightly, fits snugly against the walls and turns into a piston, which pushes the melt through the outlet - the die. Green is a zone where significant changes in ductility have not yet occurred and the bar transfers pressure further without creasing. Different shading depicts a brass head and an aluminum radiator. The fluoroplastic guide tube is highlighted in white, because of very little friction, the bar along it passes with minimal resistance. Gray - stainless steel tube, bearing head and radiator, as well as a ring insert - it allows you to avoid mechanical stress on the Teflon tube in the hottest part. This is important - Teflon, in such conditions, is extremely mechanically unstable, so here it abuts steel everywhere - it can leak a little, but not soon, and this piece of Teflon tube is very easy to replace. The stainless steel tube is soldered to the brass head with silver solder - so there are no questions about thermal stability and strength. The ring (polished from the inside, to reduce friction!) Is simply inserted inside and pressed down by a Teflon tube. Please note - stainless steel must be with low thermal conductivity, for this it was chosen. Usually this is something like X18H10. therefore, here it abuts steel everywhere - it can leak a little, but not soon, and this piece of a Teflon tube is very easy to replace. The stainless steel tube is soldered to the brass head with silver solder - so there are no questions about thermal stability and strength. The ring (polished from the inside, to reduce friction!) Is simply inserted inside and pressed down by a Teflon tube. Please note - stainless steel must be with low thermal conductivity, for this it was chosen. Usually this is something like X18H10. therefore, here it abuts steel everywhere - it can leak a little, but not soon, and this piece of a Teflon tube is very easy to replace. The stainless steel tube is soldered to the brass head with silver solder - so there are no questions about thermal stability and strength. The ring (polished from the inside, to reduce friction!) Is simply inserted inside and pressed down by a Teflon tube. Please note - stainless steel must be with low thermal conductivity, for this it was chosen. Usually this is something like X18H10. ) is simply inserted inside and pressed down on top by a teflon tube. Please note - stainless steel must be with low thermal conductivity, for this it was chosen. Usually this is something like X18H10. ) is simply inserted inside and pressed down on top by a teflon tube. Please note - stainless steel must be with low thermal conductivity, for this it was chosen. Usually this is something like X18H10.

On the left, in the picture, you see a temperature plot (approximate graph) along the axis of the tube. I can not vouch for the accuracy of the scale, but the directions must be correct. The farther from the axis with the marking of zones, the higher the temperature. So we will describe by zones, starting with the last : Top - everything is cool.
Zone 6 - the temperature increases evenly down the tube, since stainless steel has low thermal conductivity, so growth is quite fast. Nothing special here. The Teflon insert works in good conditions, the filament warms up slowly, the heat passing through the tube is dissipated by the upper radiator or holder.
Zone 5Due to the fact that in it, a small aluminum radiator is put on the tube, the temperature is almost constant and lies in the region not higher than the beginning of softening of the plastic! The heat flowing through the tube from zone 4 has time to be dissipated by the radiator - since the tube in zone 4 is a thermal barrier between the hot and cold parts.
Zone 4- zone of active softening of plastic, zone of thermal barrier. Its length is not great. Then the plastic begins to warm up to form a cork, which serves as a piston pushing further and further. As colleagues from reprapology.info showed, if the plug-piston increases in length, this will stop printing due to increased friction and painful cleaning problems. In order to prevent this cork from growing, a small radiator is used higher in the 5th zone. With the influx of heat along the thermal bridge from the stainless steel, it heats up, which means it begins to dissipate more heat. Thus, the process is self-stabilizing and getting into the area of ​​sustainable work is not at all difficult. Heat dissipation by the radiator is large, and the thermal resistance of the bridge between the head and the radiator is large, therefore, the temperature increase with distance is high, which means on this segment, it is very likely that the point of optimal working temperature is laid to create a short, effective piston. I didn’t even have to bore a stainless steel tube on a cone, as they had to, only slightly wiped it so that the Teflon insert would fit in and out. The insert ring polished well - from the inside, of course. The size of the lower heatsink must be selected so that its temperature is in the region of 100-110 degrees, probably (this is the usual temperature of the printer desktop). This is done so that as much as possible of the thermal barrier zone is in the probable working temperature range (corking!). So in short - for the stable operation of the head, you need the existence of a narrow zone with high thermal resistance (it is called a thermal barrier), in which for a small extent the temperature would change from the working one (we consider 235), to 100 degrees - approximately the temperature of the working table. I consider it inexpedient to lower the lower radiator below this temperature - it becomes more difficult to control the actual width of the cork zone, and a lot of heat does not leak up through the stainless steel tube, and even extra energy is lost. Yes! Important! Most heads are designed or work better with continuous blowing with a small fan!
Zone 3 - I believe that it is in this that the final formation of the cork occurs, since there is quite strong heating, this is the junction of the brass head and the steel tube. However, this heat somehow works to melt the filament.
Zone 2 - active melting zone. When pulling the bar out of the working head (experiments), it was evident that the central part at some distance from the walls remained unmelted, that is, as the theory promises, melting occurs from the walls to the center of the bar. The active heating zone was selected quite extensive - a centimeter and a half. For a rod 3 mm in diameter, it matters because it warms up more slowly, moreover, it is more convenient to wind the heater - there is where to turn around.
Zone 1The temperature is almost stable, gradually decreases towards the nozzle, here it is good to place the temperature sensor - it will show the temperature closest to the actual temperature of the flowing melt. The total length of the brass part is the same as in the popular J-Head Mk 5-VB.



It should be said that the absence of threaded connections at the junction of the heated part and the stainless steel tube - a thermal insulator, justifies itself, since the melt does not leak out, and it is still difficult to untwist the thread with plastic that has got inside. It was not difficult to clean the hot end in the event of plastic change and sometimes hardening occurrences - I turned the head on for heating, waited, took out the filament with the extruder reverse, helping with my hand, squeezing out liquid residues with a bamboo barbecue stick through the die - the diameter just suits. If it is very necessary, the melt can be removed with a drill - just let it warm up with the plastic until the latter melts. Ftoroplast, too, in hot, is pulled out easily.

More about why all-metal hotends are becoming more popular.Yes, plastic gives significantly less friction than metal, especially if you take fluoroplastic. The only trouble is that the fluoroplastic (Teflon) under mechanical stress flows a little and bends, even at ordinary temperatures. This property of it led to the almost complete disappearance of structures where Teflon played the role of a thermal barrier. It turned out badly. It remains to use it as internal liners - where there is no place for it to leak, and a slight curvature of the channel does not lead to anything unpleasant, and the tubular liner can be easily removed and a new one inserted. True, I have not yet had to change them. Take out to see - took out, but to change - no. Here he is out of competition - very slippery. For a thermal barrier, a different type of plastic is often used (as in the photo above) - PEEK. This is a significantly stronger, more heat-resistant and less slippery plastic. By the way - and expensive, and scarce. The problem of the heads, where it is used, is that the maximum working temperature there is about 250ºС. Above is the risk of leakage through the threaded connection. However, usually these temperatures are enough.



Now about the print itself - look at the drawing, at the end of the hot end there is such a patch around the die, it serves to smooth out the protruding stream of plastic. When printing, the height of the layer is indicated in the settings - it is always less than the diameter of the die. For example, for 0.35 mm, the height can be 0.2 mm and 0.25 mm, it depends on what we need. Thus, a trickle of plastic extruding in the form of a cylinder is smeared with a patch on the end of the head. With a little smearing, the worst connection between the layers is obtained, with a large smearing, the line width grows, which means lower print resolution. On the squeezed fishing line, even with its complete evenness, you can see the bubbles - this is the water with which the filament could accumulate. ABS - captures moisture from the air weaker, kapron - stronger. And printing with wet capron the product turns out to be noticeably softer, and even slightly lighter.

To be able to somehow evaluate the capabilities of the heads, I undertook a series of experiments. The main parameter that interests me was speed.The program Pronterface, which controls printing, allows you to manually set all the print parameters: temperature of the head, desktop, feed speed of the extruder. The thread was squeezed into the air - the complete absence of other obstacles. The yield of the plastic thread was estimated by weight - due to the considerable heterogeneity of the thread along the length. This is due to the heterogeneity of the weight of the extruded thread acting on the thread at the exit of the nozzle, where it is most easily stretched. To avoid the effect of plastic leakage during a simple operation, before the measurement, a command was given to extrude a small piece, when the extruder stopped, the extruder broke off very quickly and the measuring section was already started to extrude (quite large - 10 cm of filament). I also had to take measures against the revealed phenomenon, appearing at very high feed speeds - the hobbol began to gnaw out part of the material due to slipping. This could introduce distortions of up to 20% by weight. Therefore, after measurements at high speeds, 10 cm of the filament had to be simply drained at medium speed - or rather, I was not guided by the speed - it may differ depending on the diameter of the nozzle, but focused on the difference in theoretical weight of the filed piece and actually squeezed out.
If the difference is 2 or more times, then the damage to the filament was great. I'm talking about nylon printing - ABS is not very interesting to me, there is nothing particularly exotic. Maybe later. If I get a small, precise lathe - you can try to do even faster printing, it will be possible and ABS.

And here’s the other head of my work, let's call it based on BASS (Brass-Aluminum-Stainless Steel) materials. Having



taken all these measures to avoid mistakes, I began to press the plastic at different speeds, temperatures and heads, and here’s what turned out:
1. Head J -Head Mk 5-VB - native, USA, 0.4mm nozzle.
I led to the speed of the plastic thread at the nozzle exit - along its diameter.
Tables of experiments, I do not give here, only conclusions. At a filament feed rate of 130-200 mm / min, the leakage rate at the nozzle exit reached a maximum of 90 or 120 mm / sec. For kapron and ABS, respectively. A further increase in feed only led to a decrease in extrusion. Slipping increased, spalling increased, speed did not increase. However, raising the temperature from 240ºС to 245 increased the leakage rate by 25%.
It is clearly seen that here insufficient plastic heating limits performance.
I did not significantly increase the temperature for this model - because it is close to the recommended limit.
2. Now the BASS-0.6 head - with a nozzle diameter of 0.6 mm, and at a temperature of 230 ° C and at 240 ° C,
with a filament supply of 264 mm / min, the limit was not reached. In this case, the shear speed was about 90 mm / s, which, due to the larger nozzle cross-section, corresponds to a plastic consumption of about 200 mm / s for the Mk 5 head! At the same time, slipping was only about 20%, that is, the speed could be increased! The melting efficiency turned out to be much better, probably due to ring winding and a design with a mini-radiator (a shorter plug-piston).
3. Now the BASS-0.6 head - 0.3 mm diameter - is the thinnest nozzle I could make.
It is clear that here the performance rests on the resistance to flow in a narrow nozzle.
For this, I did a series of experiments at temperatures of 240ºС, 250ºС and 260ºС. With each temperature increase, the maximum speed grew and rested at 95 - 110 - 140.
Then I did one more thing. I laid out on the desktop a strip of fine sanding paper (600), lowered the head to a slight pressure (in the cold, of course) and began to drive back and forth. I lowered it three times by 0.1 mm, when it became easy to grind. This gave strict parallelism and reduced the narrow channel of the head by 0.3 mm. This action gave a speed of up to 170 mm / sec.

So I got for myself, a fairly accurate knowledge of the maximum speeds, at different temperatures, as well as a feed defect for these conditions. That is, how much less plastic will actually come at this speed. After that, he introduced a plastic feed correction of 1.25. The temperature was set at 260 ° C and made all speeds in the settings close from 90 to 140 mm / s.

With a simple hot head, without any pressure, quite a lot of melt can flow out of it, then by the start command, extrusion will begin only when all the void in the head is filled. Sometimes because of this, I had problems with the adhesion of the first layer - and this is critical! Now, before printing, I warm up both the table and the head to the operating temperature, give a command to squeeze a small amount of the bar at medium speed, and after that I give a command to print the model. In this case, the thread begins to go immediately (more precisely, it is not interrupted) and this cause of poor adhesion is eliminated. Also, an important point - I took the operating temperature of the printing table at 50ºС, I tried 100 and 110, it was worse sticking, but better sticking. It is very, very difficult to peel off a nylon product from a kapton adhesive tape on a cold one. Kapton is a sticky film it is glued onto glass and is a common and convenient substrate for printing - almost everything adheres well to it, and when it is needed it is peeled off. So, now I print at 50 ° C, and when the print is over, I turn on the table heating at 100-110 ° C, I wait until it warms up and begin to separate. Significantly better.

It is also important that with a solid product made of kapron with dimensions greater than a few centimeters, its edges begin to bend upward, to lag behind the substrate. A skirt (available function to counter this phenomenon) helps a little. That is, it helps, but the plastic is so tough that if it does not separate the skirt from the kapton, then it locally separates the kapton from the glass substrate, it’s true, it’s not so bad, but ... It facilitates the situation, low filling density, thin walls and the absence of solid arrays - holes, cuts, bends. The decision has to be made at the product design stage. ABS - does not have such a degree of deformation, although it does exist, but less. But PLA, they say, does not have this property at all. ABS plastic, moreover, breaks and cracks beautifully - it is quite fragile, but it also sticks together perfectly. A drop of acetone in the crack, after 10 minutes everything is fine again. Kapron, however, does not stick, though it does not break, but it is flexible enough.

So - everything worked out. Here and there, in the first layer, the unevenness of spreading is visible - but the first layer ... Of course, for precision printing, you can and should reduce both speed and temperature, but I really liked the result that I got. Now the main thing is to learn how to properly design products. So that they are not bent and will be very good. I did not succeed in raising the temperature to 270 ° C due to a limitation in the printer firmware - it will be necessary to redo it. An attempt to apply 265ºС - did not give any increase - the minimum viscosity was reached? We must try to make the head with a different configuration of the inner part to further increase the speed.
Thus, I got tables according to which I can tell at what maximum speed (and, accordingly, set it in the slicer settings), at a given temperature, a capron will be pressed through a nozzle of a given diameter (0.3 and 0.6 are tested well) . In doing so, I will know the magnitude of the extrusion multiplier correction. With a nozzle diameter of 0.4 mm, the maximum speed can reach 200 mm / s. This is estimated, you can check if there is a BASS working head by 0.4 mm, but you need to shorten the thermal barrier tube.

An important point in the print head is the feed mechanism of the extruder. The effectiveness of its work determines the quality of printing. Particularly high demands on this mechanism are made by capron printing, because the capron is rigid and slippery, in order to effectively push the hobbolt bar, it must crash into it.This is only possible if the pressure springs are strong enough and the hobbolt notch must be very sharp. More precisely two points. A hobbolt is a bolt with risks applied around the circumference, which, being pressed to the filament, grab and propel it in the right direction. If you use a filament with a diameter of 1.75 mm, this is probably not so important, but if you are 3 mm in diameter and you are trying to print with nylon / nylon, it is very likely that your hobbol will begin to slip on the hard nylon. Therefore, I use a hobby bolt with a “diamond” notch - I got her idea from Prof Braino's Prusa Build Log



In general, it captures well, everything in a row, only on soft ABS, with excessive pressure, can crumble a bar - like a grater, if there is a plug in the head - well, here anyway, the problem needs to be solved, and it is not in the hobbolt.
For a filament diameter of 3 mm and a Wade extruder, the total spring force should be around 7-20 kg. This, of course, may depend on the design of the extruder.

In conclusion, it should be added that the extremely important parameter of the print head is weight, and this is understandable, it is difficult to move a heavy head at high speed, these are extra loads, chassis vibrations. One of the solutions -bowden extruder, is to separate the extruder with the motor and the hot end, and unfortunately, has its drawbacks, so I did not use it, as I focused only on the simplest and most reliable solutions.

On this I stop the permitted speech (C). It would be necessary to build tables for the heads of 0.4 and 0.5 mm, for ABS, we should try to change the internal profile of the head - there are such notes that should help.