Hybrid plaster/3D printed PLA mold demo

It leverages the advantages of both materials

The challenge: Create a 3D printed case mold that incorporates a plaster section just for the finished surface.
Top right: The secret is M3 brass threaded inserts in pyramid-shaped 3D-printed anchors (I have just pressed them into the 4.4mm dia, 10mm deep, -3 degree tapered holes using a soldering iron). These brass/plaster pyramids embed into the plaster to provide a threaded hole that M3 bolts can screw into.
Upper left: We made a cross-section CAD drawing of a three-piece demonstration mold (upper left). The top plate has holes for the M3 bolts, air escape and natch clips and recesses for clamps to hold a 3D shell, with flanges, in place (not shown).
Lower left: The anchors have been screwed onto the upper plate.
Center left: The plaster was poured, and over-filled, then the top plate, with anchors, pressed down on top of it. After set, the plate was unscrewed and removed.
Bottom right: The plaster section has been reattached and natch inserts and anchors put in place. The plaster was not sanded or prepared, this is a demo.

Related Pictures

Custom-printed rib smooths this plaster surface

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This is part of a test hybrid case mold for a Medalta Potteries ball pitcher. The bottom plate is 3D printed and the top form is solid plaster. Recessed holes in the back of the plate enable securely inserting screws into threaded anchors embedded into the back of the plaster form. The upper plaster surface has artifacts (stair-casing remnants) from the 3D printed shell used to cast it. While these could be sanded out, we find that a flexible metal rib works much better. Even better than that is this custom 3D printed rib that I made, the edges are sharp and precise. I designed it with contours to match the belly, neck and rim of this piece. Using this, it takes minutes to smooth out the surface.

v9 Hybrid 3D-Printed/Plaster Case Mold

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At v7 I adopted a hybrid plaster/PLA approach, enabling a smooth working surface. This involved the use of threaded anchors to hold the back-plate to the model. v8 was the discovery of how to use tsplines in Fusion 360 to enable an oval neck on a round body. The big advances at this stage are the use of magnets and inset flanges (to hold it together) and discovery of how to project the lip geometry to make a perfect-fitting spare. As before, the 1.2mm thick parts print quickly.
The Backplate: It has holes for small bolts (to hold the threaded anchors), airholes, holes for natch clips, recesses to press in the magnets and an outer ridge to hold the rails in.
The Railing: Printed in two parts for easy removal. The bottom flanges have magnet insets. They are inward-facing, enabling a bigger size to fit on the bed of the 3D printer.
The Model Mold: The center piece will be filled with plaster to the brim (from the back) and the back-plate (with anchors) will be pressed down onto it. It has an inherently strong shape to produce a precise plaster model (that will be surface smoothed later).

In-plaster Anchors With Threaded Inserts

Available on the Downloads page

M3 3D printed threaded inserts for plaster

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These brass/plaster pyramids embed into plaster to provide a threaded hole that M3 bolts can screw into. That enables attaching 3D printed elements to plaster elements when making hybrid molds. Narrow inserts permit placement in cramped spaces and nearer edges.

These are made possible using M3 brass knurled nuts and M3 bolts that can be purchased on Amazon. The brass nuts can be pressed in using a soldering iron. The pyramid-shaped 3D-printed anchors are 13mm high, they will accommodate 12mm, or less, inserts (the longest ones in the kits shown here bottom left). The holes are 4.4mm dia at the top and taper inward at -2 degrees. Of course, you can adjust sizes and angles as needed for your application.

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