This build is quite complex but should be within the capabilities of makers who are interested in 3D printing, metal casting, metal machining, and general fabrication. It makes use of a re-purposed microwave oven transformer as a power source and some ancillary electrical parts such as a DC power supply for switching the transformer via a solid state relay, a digital timer for setting the weld interval, a salvaged DC cooling fan from a PC and a few switches. I have included ready made STL files for all the casting patterns as well as STEP files of the complete design. Both 2D and 3D drawings are available for the engineering drawings. I have tested the machine and it is capable of welding steel and stainless steel although the spot size has to be kept small (approx <2mm) to maintain the current density. It is worth checking out Youtube to research the methods used to rewind the transformer. Be warned though, this machine uses mains voltage and should only be attempted by those who are confident working with high voltage/high current electricity.
It is worth printing 1 to 2% oversize to allow for material shrinkage. I printed in PLA which is fine given that the patterns are only likely to be used once. If you intend to do batches of castings it is worth printing in ABS or nylon is even better
Preparation of the casting patterns
I will usually sand the prints as smooth as I can and then fill the surface with spray putty which can be purchased in a rattle can. The surfaces need to be smooth enough to hide the print layers. I then paint the spray putty with two coats of an automotive acrylic lacquer - doesn't matter what colour. This provides a hard slippery coating so that the green sand doesn't stick to the pattern.
The complete machine was modelled using Autodesk Inventor. The mechanism for the over centre clamp was exported as 2D DXF files and laser cut from 3mm acrylic. These parts were assembled using screws so that the geometry of the handles and clamp could be validated. Any parts that needed to be cast were then modified to create the machining allowances and draft angle so that the patterns could be withdrawn from the sand moulds. Where necessary the parts were split on an appropriate centre line and holes added for sprue pins to align the parts for moulding.
Part 1 - How to 3D print a casting pattern
Part 2 - Assembling the metal components
Part 3 - Electrical fit out and testing the welder