Injection Molder/Research Development
Narrow Steel Rod, Drilled Aluminium Block design
See Rick Sparber's modified Gingery system. Steel rods can easily be salvaged from (sadly abundant) broken-down inkjet printers from scrap yards. If a thick plate/ingot of aluminium cannot be found, a barrel/chamber can be cast out of small scraps of aluminium (pending a 3D-printed form for ideal shape).
Thick Steel Rod and Pipe design
Dave Hakkens has put together an initial prototype for an all-steel injection moulder. A solid design using a wide square-tube frame and a long piece of 30mm steel rod for the piston, this thing would not be easy to transport.
Drill Stand Accessory
Having recently got hold of a cheap power drill stand from LIDL (£13, brand 'Powerfix') that could be used to kickstart development of this, by focusing on designing a modular piston & chamber system that could be mounted in any such drill press, instead of one integrated into a frame like the Gingery design, I have been looking into options: 4ndy (talk) 21:57, 3 December 2014 (CET)
Limitation: the drill only traverses up to 60mm vertically when the lever is swung, though the whole carriage can be positioned anywhere on the top half-or-so of a 500mm post. Consequently, the piston should only need to press up to 60mm in order to inject the plastic, so a short & wide barrel is needed, rather than the long, narrow option of Gingery's steel rod in a drilled-out aluminium block.
Possible resulting problem: the piston will need a much higher force on it in order to deliver the same pressure out the nozzle, and hoop stress on a pipe used for the barrel may be an issue (need verification from those who have tested this before). Will the 4mmx20mm steel bar lever in this stand provide enough leverage to press such a piston down without warping? Moments: the press-bar mounting point is 80mm from the pivot/fulcrum, the handle extends 260mm beyond that, for a total of up to 340mm leverage distance.
Alternative solution: instead of using the drill stand's lever, a leadscrew could be mounted in a hammer drill in order to drive a piston into the injection barrel. For instance, LNS Technologies' PIM drill-press accessory appears to use such a system. This requires an appropriate (e.g. ACME thread) nut to be fastened onto the piston somehow, and for the drill's bearing to be capable of withstanding the corresponding pressure without damage (not to mention that the clamp holding it must not slip). So that the leadscrew does not need to pass through the piston itself, the corresponding nut needs to be fastened well above the piston, ideally to the internal length of the barrel.
Standardised Parts design
Drive an aluminium compressor piston down a 1½ inch steel pipe, with a bespoke con-rod made out of whatever (a wooden board should do here for compressive strength, but steel plate or rod is preferable), use a 1½" to ½" reducer nipple and tap a corresponding threaded hole into the mould gate for a perfect seal between nozzle & plates.
Problem: a 1½" BSP pipe is supposed to have a 'nominal' diameter of 40mm. Does this mean inside diameter, and how close do they actually come? Will a 40mm piston have a loose fit or interference fit, and so do we need a larger/smaller piston with silicone o-rings to seal it properly? Why do we still use imperial pipe sizes when everything else is already metric??
Reusing Unintended Parts design
Just get an old stainless steel medical syringe and stick a heat-sink on it. Done. Piston is guaranteed to fit the barrel perfectly. I am seriously considering getting a cheap second-hand one of these right now to start adapting.
Problem: will the wall thickness be too thin and cause a split? We need to know necessary injection pressure for small moulds for this.
Afterthought: In order to be used with a leadscrew, perhaps an ACME threaded nut could be welded or clamped onto the side of the shaft of one of these syringes, near the thumb-ring at the top. However, this would leave the driving force offset from the piston's longitudinal axis, and could result in the driving rod being bent through buckling force. Alternatively, a nut could be held inside something that fits into the thumb-ring itself, and the guiding plate left in place at the top.
Using a leadscrew to drive the piston in a drill stand/press instead of its leverage allows a longer, narrower barrel to be used, such as a 1" pipe/nipple. It should be feasible to turn a piston to fit the barrel out of aluminium cast into the rough shape needed. A nut holder / guide for the leadscrew could then be bolted onto the piston via some cast shape, holes drilled into the piston, or welded onto a steel rod piston if one could be found that fit the barrel.
Though cylinder pressure is unknown, clamping force for plates in commercial machines has been given (reliable?) as requiring a few tons. For lost-PLA cast aluminium plates, the top end of the plates could have smooth/threaded holes through each side to clamp them together with a pair of hexagonal-cap bolts, while the work-piece vice of the drill stand can hold the bottom end. This itself may be unnecessary, as this video shows a drill stand vice providing sufficient clamping force for small moulds. The bolts would make good locating pins however, making it easier to align the plates.