We'd already created an MDF laser cut fingerboard, from which we wanted to make a mould. But after sanding the board smooth and leaving it overnight, we noticed, the next day, that some of the mdf "grain" was visible on the surface. Not only this, but the textured surface of the mdf meant that it wouldn't release fully from the plastic once it had set hard.
Since we'd already had experience with using Polymorph for making moulds, we knew how much detail it could pick up on (it once transferred a fingerprint off a master, because it captures so much detail!). So it is important that our fingerboard masters are as smooth as possible.
And if mdf isn't smooth enough, it simply means repeating the whole laser-cut fingerboard process - only this time in acrylic.
Since we already had the arwork from the mdf version, and our acrylic sheet is 3mm (same as the original mdf top layer) cutting an acrylic version was quick and easy. Gluing it together, however, was not!
With PVA glue, onto a relatively porous surface like MDF and when spread thinly enough, it creates quite a "grab" effect when you press the two surfaces together. Plastic glue, however, creates a chemical bond between the surfaces, meaning they both get very slippery and slimy before the glue starts to go off.
This means having to glue just one or two sections and leaving them to dry - at least partically - before attempting to glue any more on. It only takes one tiny slip and you can end up having to re-align about six or seven fret sections; each one slipping and sliding about and refusing to lie just where you put it!
Eventually, however, we managed to get all the pieces laid up roughly where they needed to be. Where the cutting kerf on the acrylic was particularly noticeable, we had to do a little sanding (on the sides, for example) before rounding off using our "radiused" sanding blocks. Unlike mdf, acrylic takes a lot of sanding to get the rounded effect on the surface. The coarse-grit sandpaper also left a rough texture on the surface of the acrylic. It was actually quite a pleasing texture, but given the trouble we'd had with demoulding last time, we used finer and finer sandpaper to create a smoother surface.
This time we went all the way down to 1200 wet-n-dry paper, to get a really smooth, almost polished surface. Having learned from our previous mould-making lessons, this time we rubbed the acrylic masters lightly with some Vaseline jelly, to act as a crude mould release.
We fixed our acrylic blanks to a wooden table (so that any shrinkage in the plastic as it cools could be resisted to avoid the mould from curling slightly as the polymorph sets). Then we repeated the heat-and-mould-by-hand process to create our moulds.
(of course, it's Thursday night, it's BuildBrighton nerd night, so that's pizza - and vaseline?!)
After cooling, the acrylic master popped out of the mould. Some of the panels even popped off the acrylic base - it looks like we didn't quite glue them down well enough either! Inspecting the inside of the mould showed disappointing results:
While the polymorph is great for capturing lots of detail in small moulds, it's not great for larger moulds. As a large volume of goop is hot and sliming about, it needs constant re-shaping until it can support it's own weight. During this time, we've obviously introduced a few air bubbles, which can be seen in the final mould.
The divisions where the fret wires will go are not entirely fully defined, but would be suitable to act as markers, should we need to make them deeper with a fretsaw in the final casting. But it's looking like the polymorph is either too coarse, or just too plain difficult to work with, to create a decent mould just yet!