Syringe Pump Metal Extruder
Release status: Experimental
There have been many RepRap-based projects on extruding low-temperature-melting alloys. This is one of them. This article is a mirror of the one here. The "See Also" section lists some of the early projects that inspired this work as well as some of the later projects that were done independently.
The idea of operation is that a standard RepRap prints a layer of plastic (HDPE, ABS, etc). Then the standard toolhead is switched out for the metal extruder, and the metal toolhead deposits molten material in the channels or cavities formed by the plastic. If necessary, the process can be repeated for constructing thicker and more complex parts.
It is possible that this extruder may work also for extruding wax, which may be useful for doing automated wax casting or for using wax as a support material (see also Waxuum). It is also possible that this extruder, without the heaters, may be useful as a paste extruder, but there have been more successful efforts in that area (for example see Ceramic_Extrusion)
We have had good success using the nozzle as a hand-held tool for transferring metal. For example, we used it to make this thing: electric motor .
We have had somewhat less success using it when mounted to a RepRap, as seen in the pictures. The main problem is poor control of flow-rate (see photo of pattern on hot plate). With some modification (such as a smaller nozzle hole diameter) and tuning of parameters (feed rate, height of nozzle above plate, etc) it might work much better. We RepRapped a simple test piece, manually filled it with metal while it was still on the heated bed, then let it cool (see photo of widget with 25-cent piece). This yielded fairly good results, indicating that an automated system may be able to produce usable parts.
The main components of the system are
1) Heated cup - this is a heated reservoir that contains the molten work material.
2) Heated copper nozzle - the nozzle slurps up material from the cup, then moves to a desired location and deposits the material.
3) Syringe pump - a motor-driven syringe controls air flow in and out of the nozzle, which in turn causes the nozzle to slurp or deposit material.
4) Heated bed - this keeps the work hot to improve the flow of the heated material. It also melts the base of the deposited part, creating a good seal between part and plate, so that molten material does not leak under the part. In general, the heated plate seems to help avoid warping, but the downside is that the bottom of the part starts to ooze outwards after a while.
The standard safety warnings apply. Low-melt alloys are toxic and hot enough to cause burns and fires. Use them only if you are experienced in a lab environment.
In addition: the nozzle, cup, and plate get dangerously hot. These can cause fires and burns. Operate with good ventilation, never operate the system unattended, and know where your fire extinguishers and escape routes are.
The cup heaters were purchased from American Science and Surplus which seems to no longer have them in stock. They were probably intended for coffee makers. By chance, the cups were the right size and resistance (8 ohms) for what was needed. You can probably find something similar at one of the many surplus outfits. For temperature control, a thermistor was JB-welded to the bottom of the cup.
This is a derivative of the old RepRap Thermoplast Extruder V1.1. The copper part of the nozzle is treated with JB-weld, wound with nichrome wire, and equipped with a thermistor as described here.
The copper nozzle and PTFE adapter have to be machined using a lathe. A drill press or small milling machine, in addition to simple hand tools (files, etc) will come in handy. A standard disposable syringe is cut up and glued to the PTFE adapter with silicone caulk. The syringe top is used as a plumbing fitting to connect to the syringe in the pump.
The little safety pin is important. Without the pin, if the nozzle were to get blocked during use, the syringe pump could generate enough pressure to pop the copper nozzle out of the PTFE adapter, spraying molten metal everywhere. The pin prevents this. It can be tricky to get a good seal between the PTFE and copper nozzle. If needed, you can leak-check the whole assembly underwater using another syringe to provide pressure.
The nozzle mount is made from laser-cut parts and is intended to fit the old-school carriage of an early BitsFromBytes kit. It is a good idea to make a little stand that will safely hold the hot nozzle while it is not in use.
The syringe pump uses a standard disposable 12cc plastic syringe (0.7 inch outside diameter). It is made mostly from laser-cut parts, except for the motor coupling. The motor coupling design could certainly be improved, but we made it with bits we had sitting around and so that is what's uploaded.
You will need assorted nuts and bolts to put the thing together, in addition to 2 microswitches, a Solarbotics gear motor, a 6-32 threaded rod (3.5 inches long), and a 6-32 nut. The syringes have a standard Luer-lock fitting, which allows quick connect/disconnect of the hose that goes from pump to nozzle.
If you bypass each limit switch with a diode, you can create a 2-wire device that acts like a DC motor, but has the limit function built-in (see schematic in gallery).
This is a 6 by 6 by 1/4 inch aluminum plate with five 40 ohm, 25 watt power resistors (wired in parallel) JB-welded underneath. The plate has some tapped holes at the corners so you can clamp stuff down to it. In the photo you can see some heat damage to the wiring underneath, indicating that this thing gets HOT when running. Be careful...
Electrical Interface to RepRap
This system is designed for the older single-Arduino setup. The heated plate and heated cup are controlled by a separate Arduino with PWM Driver running off a 24V supply. The nozzle is a pin-for-pin replacement for the old RepRap Thermoplast extruder. The interface is: 3 leads for thermistor board (+5, ground, signal), 2 leads for motor, and 2 leads for nichrome heater. We use a big rotary switch to select between different toolheads.