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.