The heater design is based on this machine.
The heater unit will be 6" long and 1/2" OD. It will be heated by Nichrome wire ~95" long to provide approx 10 turns of wire per inch of heater. Using a calculator I have come up with 38 gauge wire.
The pipe will be black pipe from home depot. It will be wrapped in kapton tape then NiChrome wire, then insulation (suggestions?), then a wood block housing.
I plan to use a thermometer to check the temperature at the pipe's tip.
The power is controlled with a lamp dimmer (suggestions?), the Nichrome will be attached to the lamp dimmer terminals with a screw and washers or with screw terminals if available.
The pneumatic ram which injects the molten plastic is based on this design on thingiverse. http://www.thingiverse.com/thing:42589
The injection molder mentioned earlier appears to use a 6:1 ratio and have an ID of 1". Judging by the struggle the user puts up with I'd guess he is applying about 150 lbs of force. 6*150= 900 lbs force. Approx 900 PSI inside the injector/ram.
Shop air is about 90 PSI. The burst strength of PVC is below that so picking a diameter is just about choosing force. I chose 2" schedule 40 diameter pipe. 3.14*2^2= 12.56 in^2. 12.56*90psi = 1130 lbs force.
I do not know if the pvc pipe will buckle under 900lbs of force. I plan to build a lexan shield for if it buckles and bursts explosively. Intuition in working with PVC pipe tells me it won't buckle but better safe than sorry. I'd be appreciative of anyone smarter than I who can do the calculations.
I have sourced an electronic 3 way valve that costs less than a manual valve. It uses a 1/4" threaded fitting, which coincided nicely with standard shop air fitting sizes.
At the end of the ram will be a PTFE bushing screwed into the ram's shaft. It should be slightly large that the heater pipe.
The test frame will be made of 2x4 lumber. The PVC pipe will be supported by lumber on all sides. The heater will also be supported. The base of the frame will include a bench vice.
The Plan (Subject to change)
So it's been a year and the injection molder isn't complete!
The new plan is to...
1. Mill out aluminium blocks to make the mold halves and heating block.
2. Beg someone to lathe the plunger since I can't use a lathe (red tape).
3. Test it! Hopefully something cool will happen.