In order to learn a bit about silicone mold making and epoxy casting I consumed a solid number of product web pages and 'how to' videos. These were invaluable demonstrations of 'how to do it right', but I wanted to document my journey into this space, with special reference to the 'how not to do it' stuff I learned along the way...
In typical Chris Style, I decided to jump in a the deep end with a non-simple project. In this case I wanted to cast my interpretation of the Æsahættr (Subtle Knife) from the BBC's adaptation of the His Dark Materials book series by Philip Pullman. The prop used in the show has a twisted blade that I found æsthetically pleasing. I thought it would be a fun and non-trivial way to learn molding and casting. I won't dwell too much on how I got to the point of having a model to cast from (OnShape for the 3D design work; an SLA print service for the blade; a friend's better-than-mine FDM printer for the handle; laser-cut acrylic for the guard) - this page is about what happened next...
My model is quite a challenging shape in several ways: it's long and thin; the twist means there's always some kind of 'overhang', irrespective of orientation. It's relatively large as well, meaning large quantities of silicone unless I got clever about the mold housing.
I decided to use Smooth-On, Inc. products, as people that know an awful lot more about this than I do use them. They have an extensive range of both molding and casting products, and a comprehensive set of written and video product guides. I decided on Mold Star 30 silicone for the mold (a firm silicone that suited a 'long, thin' object, yet capable of handling a small amount of overhang). I decided on EpoxAcast 655 for the casting (for a high-density, rigid result).
Silicone Mold Making
I elected to build my mold housing out of a stack of laser-cut MDF plates, designed in such a way as to minimise the quantity of silicone required. The bulk of the stack is split lengthways to give a two-part mold, with a pair of plates on either side to make the stack 'liquid-tight'.
- The MDF stack led to a stable and rigid mold, that required minimal silicone (it still required most of the smallest Mold Star 30 kit, though).
- The outer plates, with bolts through the whole stack, provided a robust and reliable structure for liquid containment and half-mold alignment.
- I used Ease Release 200 on my model, and this worked perfectly.
- The Mold Star 30 is 'thin' enough to make post-mix degassing optional (in my limited experience). This was less true for the EpoxAcast 655.
- Sprue placement required a lot of thought, both for the (abnormally, fully-enclosed) mold itself, and for where in the mold the casting sprues needed to go - placement of the the latter, in particular, could have been better (more on that, below).
- I used hyper-accurate weighing scales to precisely measure a completely wrong proportion of the two-part silicone mix (far too much confidence in my ability to do ratio maths in my head, apparently). The resulting mix was missing roughly 20%(!) of the Part B. I didn't realise this until far, far too late. Amazingly, the resulting mold was still reasonably usable, but did have some defects: the result was quite 'greasy' (presumably the unreacted Part A slowly oozing out) - this makes the mold hard to keep clean (it picks up any/all cruft in the environment), and, presumably, the egress of the unreacted Part A will cause the mold to breakdown/change shape over time (it's still fine after a week-or-so of use, however). Smooth-On provide a comprehensive set of mix calculators, which means this mistake is utterly avoidable...
- The virgin EpoxAcast 655 resin required a mega amount of stirring before the first mix. You'll want a long-handled, steel spoon for this job - this stuff is way beyond the strength of a wooden or plastic spatula. It's also worth noting that even after a good, long mix, the resin is still viscous enough to recommend some kind of post-mix degassing (beyond my budget, unfortunately).
- I snapped the 'long, thin' section of my first cast object whilst trying to extract it from the mold. On subsequent casts I inserted a 'spine' of aluminium down the inside of my mold once about 75% of my epoxy was inside. The spine is completely invisible, and I experienced no further breakages. Two other suggestions in this space: wiggle the object to loosen it before trying to extract it (especially if it has overhangs); let it cure for at least 24-hours (I tried to de-mold my first cast 'next day', which probably undercooked the 24-hours by a non-trivial amount).
- As mentioned above, my sprue placement could have been better. With reference to the picture, right, the sprues were misplaced by only a few millimetres, which resulted in few-millimetre-sized voids in the cast. These sprues were also very difficult to clean out for subsequent casts. Good sprue placement is clearly a 'black magic' area...
- Pouring the resin/hardener mix in a thin stream, and from a height, 'tears' most of the bubbles from the mix - but if you see a 'lump' in your mix stream, that's actually a big bubble going in - avoid!
What About Using Regular Epoxy?
The bulk of my epoxy experience involves using West System epoxy and additives for surface-coating, fibreglassing, and gluing. I briefly wondered about trying to cast some of this epoxy. A quick bit of research suggested why this might not be a good idea: 'regular' epoxy has a relatively fast-and-hot cure, whereas mold casting epoxy has a relatively slow-and-cool cure - the former stands a good chance of cooking your mold (it also tends to form bubbles if it can't shed heat fast enough - say if it's in a mold).
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