Lead kebabs

I am aware that I have often stated things like "HDPE needs more force than ABS to extrude" and a "short thermal transition is easier to push plastic through than a long one" but I have never produced any figures to back up these statements. In fact I don't think anybody on the RepRap project has published any extruder pressure figures. Odd because it is the key piece of information needed to design an extruder and it isn't too hard to measure.

I have put together a test rig to measure the rate of extrusion for a given pressure, which I can vary. That will allow me to evaluate different extruder barrel and nozzle designs quantitatively.

I designed most of the parts in CoCreate and printed them with HydraRaptor.



The boss on the far right has mounting holes which match the extruder pump and holds a PTFE cylinder over the filament entrance of the thermal break. I chose PTFE for its low friction. I place a 55mm sample of filament into the cylinder and then push it down with a piston laden with weights. The piston is just the end of a 6mm aluminium rod turned down to 3mm.



An M6 nut stops the green cylindrical saddle, which carries the weights, from sliding down the rod.

The top of the rod is held in line by a guide that it clips into and slides through. A flag 40mm long slides through an opto switch to allow me to measure how long it takes to extrude 40mm of the sample.



The 2mm thick green ABS allows a little IR through, not surprising as it lets some visible light through as well. It was not enough to give a bad logic level but I painted it with black paint to be on the safe side. I should have used black ABS!

The opto connects to the unused filament empty input of HydraRaptor's extruder controller and the heater and thermistor connect to their usual places. A simple Python script tells me how long it takes the flag to pass.

My first idea for weights was to use reels of solder and that is what I designed the rig to accommodate. I managed to muster this little lot, which weigh about 2.2 Kg.



That weight only managed to extrude HDPE at a rate about 1.1 mm³, which is only about 1/3 of the rate I normally extrude at, so I figured I needed about 6Kg to get realistic results.

I needed long thin weights with a hole in the middle, so I ordered some stackable lead sash window weights. I got 10lb, 5lb, 3lb and two 1lb. That allows me to add any weight between 1 and 20lbs in 1lbs increments. A shame they are not in kilograms but sash windows are rather traditional. They cost £50 including shipping so not a cheap solution but they should be handy for measuring motor torque, etc.



They were supposed to be next day delivery but I ordered on Sunday and got them Thursday. The two one pound weights were not the painted stackable ones I ordered and paid for. When I complained I was told they don't stock them any more. Why they let me order them and invoiced me for them I don't know. I shall not be using that company again!

I made a new saddle for the weights to ride on, a centralising collar for the top and two containers for the unpainted weights.



I also insulated the heater with ceramic wool. That reduced the heatsink temperature from 67°C to 57°C by stopping convected heat from the heater warming it. Unfortunately the boss that holds the PTFE cylinder covers a large area of the heatsink. When I make a new pump I will try to leave more of the aluminium exposed.



With this heater, which is a 20 x 20 x 12 mm block with the thermistor mounted halfway between the heater and the melt chamber, the simple bang-bang temperature control works extremely well. The temperature measured at the thermistor varies by less than 1°C. I have an LED which shows when the heater is on. With previous heater arrangements I see it go on and off at about 0.5 Hz. It does not switch cleanly on and off but fades in and out because of noise in the thermistor reading, i.e. I get PWM for free. With this heater the LED simply gets brighter and dimmer, so I have proportional control with just a single if statement! Who needs PID?

Here is the experimental set-up: -



So far the results are a bit disappointingly inconsistent. Six runs loading it with 55mm of 3.1mm HDPE filament and measuring the time to extrude 40mm of it at 240°C through a 0.5mm nozzle with a weight of 8.27Kg gives the following times: -

90, 95, 100, 114, 163 and 98 seconds.

I have no idea why there is such a big variation. 96s would correspond to 3.14 mm³/s, which is the normal rate I extrude at. So we are looking at a force of 81N. With a 5mm shaft that Adrian's pinch wheel design uses that would require a 0.2 Nm motor, I think. You need some margin so it would be the top end of what a Nema 17 can provide.

I don' think I counter bored my 0.5mm nozzle like I did my 0.3mm one, so I may be able to reduce the force somewhat. A lot more experimenting required I think.

Siamese twins

I designed a right angle bracket that I intend to use in pairs. Due to its triangular shape, and the fact that my software creates rectangular rafts, it would be quite wasteful to print them individually.

I am not sure if STL files are supposed to contain more than one object. CoCreate seems to think so but ArtOfIllusion not. However, if you have a set of parts that go together to make one item then it would more convenient to store them in one file and print them together.

A simple workaround is to join all your parts together with an impossibly thin rectangle at base level.



The slice software samples at the middle height of each layer so this 0.1mm base gets missed out completely.

The down side is a bit more stringing as the head moves between the two objects.



These were made with 0.5mm filament through a 0.3mm nozzle and highlighted a problem. As you can see the top surface of the lower triangular part is rippled. The reason is that the filament is not being stretched much, if at all. That means that the sparse infill sags because it is not pulled taught. Three solid layers over the top is not enough to recover to flat as they are not being pulled tight either.

So it appears that some stretch is definitely needed unless you are making a solid object. Here is the same thing made with 0.4mm filament and all is well again.



The upper limit on filament diameter that is usable from a given size of nozzle is somewhat less than the die swell as you need to stretch it a bit.

Top tip

I got the tip to use welding tips for an extruder nozzle from Andy. They come in packs of five from Halfords for £4 on-line and £5 in the shops.



They are made from copper and have a 0.6mm hole down the middle. The thread is M5.

I drilled out the one on the right to 3mm, almost to the end, to reduce the pressure needed to extrude. They drill easily if the drill is lubricated with a little paraffin. It's a shame they don't work as is, but all the same it is much quicker and easier than turning, drilling and tapping the standard design.

They also simplify my evolving extruder design because the heater block no longer needs a spout. I can simply drill and tap the bottom of the melt chamber M5 and screw these in. I can also change the M8 penny washer for an M6 one. That allows me to reduce the outside diameter of the PEEK collar to 8mm so it can be made from the same stock as the thermal transition. The area of the collar will be less so it will conduct less heat.

The 0.6mm orifice can be made smaller if necessary by filling it with high temperature solder and then drilling it with a fine drill. Solder is very easy to drill so less chance of breaking a fine bit. Also Vik Olliver suggested you can make a small hole by soldering in some fine Nichrome wire and then pulling it out again to leave the hole (solder does not stick to Nichrome).

I haven't tried one yet but I can't see any reason why they wont work well.