The halves fit together well. Even with a thick handle like this, the mold splits for me in less than an hour when casting solid (and in half an hour when cast hollow). The cast handle is very sturdy and easy to cut and glue on to leather-hard mugs. As a parting-agent I use Murphy's Oil Soap on the 3D-printed PLA mold, this makes it fairly easy to extract the freshly-cast plaster molds (the sidewalls have a draft of about 5 degrees). The PLA mold is very durable so it is practical to use the end of a knife to get soap residue out of the recesses (when cleaning). If I do not remove the soap bubbles sufficiently before pouring in the plaster, corners are poor quality (so the mold produces a more visible seam on the cast handles). Notwithstanding that, the open plaster mold shown produces excellent quality handles.
These molds are 3D-printed from PLA filament. They are part of my 2019 year-long casting-jiggering project. A quick soaping, 164g water, 236g plaster and a fifteen minute set produced this plaster mold. It takes time to learn how to soap the masters properly to get optimum quality, but these molds seem to work well regardless. The two halves mate with a tiny amount of play, but it is easy to line them up perfectly (the play actually enables lateral movement that aids in releasing the handle). It is actually easier to cast handles solid rather than pour the slip out, they can be ready to apply in an hour after pouring. The ease of making these molds puts slip casting within much easier reach for potters and small companies.
2019 Jiggering-Casting Project
A method of forming ceramics where a deflocculated (low water content) slurry is poured into absorbent plaster molds, forming a layer against mold walls, then poured out.