Sunday, November 30, 2008
Granule Extruder - the sequel
I've rabbited on about granule extruders before. I think it's important to have a design for one as:
Here's my latest attempt held in a vice under test. It's a PTFE tube with a 10 mm hole down the middle containing the granules. At the left end a brass plate is (very securely) attached by being screwed in and then held with extra screws, which you can see projecting to the left. (Also note the the solid-state force transducer near the top of the picture...)
A length of M10 threaded rod is pushing against the granules in the PTFE tube and being dragged to the right by the string tied to it.
At the right end is a heated brass chamber where the granules melt and are then extruded. This has a nichrome heater embedded in fire cement with a copper tube round that for mechanical strength. The nichrome is 6 ohms, which is a bit high - it'll just about get to full temperature at 12v, but there's no leeway; the next one I do I'll make 4 ohms. The green and white wires are a K-type thermocouple.
Here it is extruding:
This is running at 243 oC. To get the same flow rate as on my conventional RepRap Extruder at the same temperature I had to hang 17 kg on the string - a force of 167 N. The flow seemed very smooth and even under constant load and - equally importantly - seemed to stop pretty quickly when I lifted the load off. As you can see, I've run the extrude nozzle off sideways so that the main axis of the device is horizontal. This reduces heat convection from the hot end to the cold end, thus keeping the granules at the cold end from melting.
Here's a close up of the device with some of the extrudate it made. The test tube (10 mm internal diameter) contains the granules that didn't melt ahead of the screw. I tipped these out at the end of the test. They form a buffer between the melt and the screw, and mean that it should be easy to withdraw the screw and recharge the device with more granules when the screw has moved inwards to the end of its travel.
Now to design a hopper, a motor drive for the screw, and a force transducer for it to push on so we can control the flow rate...
- Granules are the standard form that plastics are supplied in,
- It's easy to make granules yourself by shredding, thus RepRap becomes a home plastic recycling machine, and
- If you have a hopper full of granules as the feed, it's much more compact than a plastic filament, and also very easy to refill in mid-build.
Here's my latest attempt held in a vice under test. It's a PTFE tube with a 10 mm hole down the middle containing the granules. At the left end a brass plate is (very securely) attached by being screwed in and then held with extra screws, which you can see projecting to the left. (Also note the the solid-state force transducer near the top of the picture...)
A length of M10 threaded rod is pushing against the granules in the PTFE tube and being dragged to the right by the string tied to it.
At the right end is a heated brass chamber where the granules melt and are then extruded. This has a nichrome heater embedded in fire cement with a copper tube round that for mechanical strength. The nichrome is 6 ohms, which is a bit high - it'll just about get to full temperature at 12v, but there's no leeway; the next one I do I'll make 4 ohms. The green and white wires are a K-type thermocouple.
Here it is extruding:
This is running at 243 oC. To get the same flow rate as on my conventional RepRap Extruder at the same temperature I had to hang 17 kg on the string - a force of 167 N. The flow seemed very smooth and even under constant load and - equally importantly - seemed to stop pretty quickly when I lifted the load off. As you can see, I've run the extrude nozzle off sideways so that the main axis of the device is horizontal. This reduces heat convection from the hot end to the cold end, thus keeping the granules at the cold end from melting.
Here's a close up of the device with some of the extrudate it made. The test tube (10 mm internal diameter) contains the granules that didn't melt ahead of the screw. I tipped these out at the end of the test. They form a buffer between the melt and the screw, and mean that it should be easy to withdraw the screw and recharge the device with more granules when the screw has moved inwards to the end of its travel.
Now to design a hopper, a motor drive for the screw, and a force transducer for it to push on so we can control the flow rate...
Labels: extruder, granules, polymers