Fiberglass mold core

The goal of this project was to reproduce a unique contoured shape, originally designed in 3D on the computer, using the CNC. The final shape is designed to be fabricated out of composite material, so a high tolerance machining foam was used to create the positive shape, and was used to cast a fiberglass mold . For the prototype, the finished part will be cast in fiberglass as well.

This is essentially a shell housing intended to protect some sensitive electronics. The designed started as a series of hand drawn conceptual sketches, which, over time a corroboration with a professional artist and designer, evolved into the shape seen here. The part is kept intentionally simple so as to make it easy to fabricate a fiberglass lay up.

Shapes of this size and proportion are generally designed with gentle curves to improve the final surface finish. Large flat surfaces have a tendency to buckle and form surface wobbles and irregularities and generally have a poor overall appearance if they are to be given a hight quality painted finish. Gentle curves, on the other hand, break up the surface and make it more difficult to pick up on surface irregularities. Curved surfaces also improve the geometrical stiffness of a surface, and make the part more robust.

I broke up what would otherwise be a flat surface on the side of the housing by adding a gentler louvre, which added a bit of interest, and improved the overall quality of the part. The curves were kept gentle to accomodate the limitations of fiberglass.

The part was then transferred to a second software package to generate the CNC toolpaths used to carve the shape. A series of toolpaths were applied to the surface of the part, to define the path of the CNC cutting tip. I used a 3/4″ diameter cutting tool with a spherical cutting tip, made a pass every 0.010″ to create a smooth surface finish . Because of the size of the part, and the detail on three unique surfaces, I needed to generate separate toolpaths and set the part up in three different orientations.

I machined the two sides of the mold first. The first side was machined to match the contours of the final part. The resulting face now had a high quality machined finish and nice flat areas which I could clamp down in my mill.

I carefully measured the set up in the mill, and calibrated the top surface to line up exactly with my computer design. I then machined the opposite face. I now had two machined surfaces of the exact width of my final part. The top surface was still unfinished.

The part was then positioned with the top surface up. Another toolpath was defined to remove the remaining material from the top surface.

The foam core was then sanded by hand, using 80 grit sandpaper to remove the machining marks left over from the 0.010″ overlap intervals. The results was a final shape, true to scale, of my intended fiberglass part.

Once the foam was cut to shape, I then coated everything with an automotive grade water borne urethane primer, to build up the surface and smooth it for casting the mold. I used a water borne primer rather than a solvent based because some foams have a tendancy to melt under harsh solvents; I believe this particular foam is pretty stable, but it wasn’t worth the risk.

I applied 4 coats of primer, and sanded smoothly with 220grit followed by 320 grit. I used a contrasting paint, sprayed lightly over the entire surface to serve as a guide for my sanding. Once all of the contrasting paint was removed, I could be sure that I had sanded all of the surfaces smooth and flat. The contrasting paint (guide coat) also shows low spots and flaws in my surface that I could fill and smooth with a filler material. The surface is now buttery smooth and ready to cast a fiberglass mold.

More to come…