Summary

Another zinc casting in an ABS mold.

See my other metal casting projects for more info.

Instructions

[Update 5-31-16
Dang... The casting broke at the top of the stem, where a hole has been drilled (See photo). I pushed maybe 500 pellets through, and it eventually let go. Too much stress.
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Metal solidifies in crystals. Zinc has large crystals. Crystal boundaries are weak points. Large crystals boundaries in small sections like this stem-hole are a definite weakness. I think a casting like this might be heat treated after casting, to re-crystallize things, and to make it stronger.
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But anyway, too bad it broke. This piston was far superior to the OEM part. I may cast another one, or cut one from steel or aluminum... this was disappointing...
End.]
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This is a zinc casting in an ABS mold. It may be the first really practical application I've found for 3D printed molds.
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I have a pellet rifle (Daisy 880) and am attempting some modifications. It has an integrated air pump. The piston on the end of the pump has an inefficient design, and might be improved.
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(Update: Photo of a typical, melted mold added for illustration. Also, my chronometer says there was a nice improvement of at least 70 FPS with 10 pumps and a 7g pellet. A properly fitted piston reduces air space at the end of the stroke, and forces more of the highest pressure air into the compression chamber.)
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I intended to cut a new piston from stock aluminum or steel rod, but had no raw material close to the required size. So I got to thinking about the possibility of casting an oversized blank, and machining that blank.
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The rifle's original piston is zinc. The pump tube is aluminum. A steel or aluminum piston might damage the tube. So I decided to follow the manufacturer's lead, make it from zinc, and minimizing unforeseen complications.
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The Featured photo shows the original piston on the left, and the cast replacement on the right. They are approximately 1.3" tall (33mm) and 0.75 diameter (19mm).
The mold design is a simple two sided box. Printing a set took 90 minutes with Afinia defaults, except for 0.35mm layers to save some time. 72 grams ABS.
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The cast part is somewhat larger than the original piston, and was lathe turned down to the proper dimensions. I had never turned zinc and didn't know what to expect. It turns quite well.
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Zinc was melted in a 400W Palmer Hot Pot electric ladle, capable of about 950 F, commonly used for casting lead fishing weights, bullets, and the like.
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This zinc solidified (seriously lost fluidity) at about 710 F, and was gravity poured slightly above that, near 725 F to 730 F. I use a multimeter with a temperature function and its submersible wire thermocouple to accurately measure the melt temperature.
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There was only a wisp of smoke before removing the cast part from the softened mold.
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Casting metal has many dangers. Do not cast metal unless you have the knowledge and proper equipment, and know and follow all the safety precautions. It's a fun and useful hobby but it's not one to be taken lightly.
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See my other metal casting projects for various information. To resize these SAE files to Metric, scale up by a factor of 25.4.

Zinc piston cast in ABS