Mug Handle Casting

Handles are the 'user interface' for your pottery mugs, make them something people will love to hold. Handle-making in pottery may be a place where 3D printing can find a real home in your process.

While manually pulling handles has its place, casting them has important advantages:
-Consistency of size, shape
-Handle can be made to fit perfectly to mug wall contours
-Precast handle can be stuck on in seconds with just slip
-They can be made off-site by other people
-Ready-to-apply handles can be stored in a damp box
-The procedure of applying them can be relegated to helpers
-The casting process produces no waste clay, everything is used
-Casting clays dry-shrink much less than plastic ones so handles don't crack even if dried quickly
-Because you can mix your own casting slip you have control of the working and firing properties
-Making the handles might be a first step in leading your to making the mugs themselves in holds (either casting or jiggering)

Learning 3D design is difficult but this type of project might be just the motivation you need to get started. We will be providing step-by-step tutorials here. You will make the design and the molds will duplicate it perfectly for each piece. Because only small amounts of material are needed this is a great onramp to learning about clay bodies and making one of your own. That experience may take you some good places!

Making a casting slip that matches your plastic clay fired appearance, thermal expansion and degree of vitrification sounds complicated but not really. I will give you a starting recipe and information on how to fine tune its properties and test it.

Related Information

Cast handle on thrown mug, pulled handle on cast mug

Left #1: A thrown mug with a slip-cast handle.
Right #2: A slip-cast mug with a pulled handle.
#1 is much less likely to crack on drying? Why? The much higher drying shrinkage of the plastic throwing clay is the main issue. A cast mug, stiff enough for handling, may have as little as 1% shrinkage left while the pulled handle being attached could have 5% or more. While #1 has survived, stresses lurk within seeking relief (e.g. when bumped). The situation is much more favourable with a cast handle on a thrown mug. At leather hard it may have 3% shrinkage left. A fresh-out-of-the-mold cast handle would have about the same - so they will dry happily together. Of course they also need to have similar firing shrinkages so the kiln does not put the join under stress.

My first 3D-printed handle case mold

These molds were my first effort using 3D design and 3D printing. They served well for making hundreds of pieces. They were part of a 2019 casting-jiggering project to reproduce a Medalta Potteries 1966 mug. Even with a thick handle like this and slip made using kaolin and ball clay not intended for casting, the molds split in less than an hour when casting solid (and in half an hour when casting hollow). As a parting agent on the 3D-printed surface I used Murphy's Oil Soap. The sidewalls had a draft of about 5 degrees and the handle cross-section was round rather than oval, so the plaster molds released without corner breaking.

This mold had spares - they did not provide a big enough reservoir and I later switched to using 3D printed spouts. These also had plaster matches (I later found that no matches worked better on this size and shape of mold). I went through multiple more versions perfecting the process to enable making handles of more difficult shapes and finally using 3D printed natches.

Poor plaster release from 3D printed mug handle case molds

My objective was to continue skipping the making of a rubber case mold and 3D print them directly. Since 3D printed surfaces naturally part well from plaster and the artifacts, although visible, do not show on the final fired pieces, I even wanted to do this whole process without any sanding or oiling. However, despite printing a dozen or more variations, carefully controlling plaster/water ratios and waiting/mixing the recommended time periods, few good plaster molds were extracted without corner-breaking. Even painting the inner surface, oiling over it and beveling corners did resolve this issues. It seems that a combination of the printing artifacts, sharp corners, the handle perpendicular (because of the oval cross-section) and the inside negative shape all enabled the plaster to get a very firm grip on the PLA print. Although I could have resorted to a heat gun to soften the PLA material enough to pull it away I relented and decided to switch to making a block mold (for rubber) rather than a case mold (for plaster).

Drawing the 3D printed shell for a mug handle block mold

This was done in Fusion 360.
1: A make a sketch of a box, around the handle, on the XY plane. Offset that outward by 1.2mm (my printer prints 0.4mm wide, three passes give good strength).
2: Extrude to create box 1: The base backward by 1mm and the sides forward by 20mm.
3: Use five sides of the box as cutting planes to slice it out of the mug.

At this point I could print this in PLA filament, pour plaster into and then use a hair drier to peel it off. But let’s make rubber molds instead.

4: Move the box-with-handle away from the mug. Pull the four sides out by 5mm to thicken them.
5 & 6: Create box 2 around the outside of it, as a new body, 1.2mm wider and taller, 1mm more frontward and 1mm less backward.
7: Use box 1 as a cutter to remove material from box 2 and then pull the outer 1.2mm sides 5mm backward.
8: Shell out the back side to 1.2 wall thickness and make two 9.4mm holes (to accommodate natch clips).

To make side 2 mirror-image a new body using the front or back as the reflexion plane. The back side is then filled with PMC-746 rubber to make the block mold. Plaster is poured into that to make each working mold.

3D printing case vs block mug handle molds

Top: Case molds (for pouring plaster into to make working molds) in the slicer about to be printed.
Bottom: Block molds (actually molds of block molds for pouring rubber into) about to be printed.
The top situation would have been a dream for a potter like me, the simplest possible way to introduce mold-making for slip casting into a process. However, my goal of print-and-pour was not met, this shape is not conducive to the extraction of the plaster without corner breaking. The block mold master (lower left) was made by pouring PMC-746 rubber into these molds. That works extremely well, there will be no problems making plaster molds with this. Notice how well it has preserved the printing artifacts, they look like wood grain. The white embeds enable inserting natches (that I make on the printer also).

By Tony Hansen
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