JMC XL – Pressure-Activated Silicone Cap System for XL-format coffee press

The JMC XL is an experimental, open-hardware cap system designed for compatibility with the large XL-format cylindrical coffee press used by many manual brewing fans. It pairs a printed PCTG frame with cast-silicone components that handle the internal flow and pressure behavior. Strong structure on the outside, flexible silicone where the hot brew travels.

This is a pre-alpha release. It’s stable enough to brew with, strange enough to invite tinkering, and aimed squarely at makers who enjoy modifying, hacking, and redesigning their gear.

How the system works

The PCTG carrier cap

• Compatible with the locking and diameter geometry of popular XL coffee press systems
• Six large radial flow holes for high-volume extraction
• Central hole anchors the silicone plug assembly
• Angled tabs print cleanly without supports
• Best printed at 100% infill for rigidity

The silicone inner interface layer

• Cast using the included mold
• Six matching flow holes align with the carrier
• Small silicone standoffs lift the paper or metal filter slightly
• Forms a silicone-only inner brew pathway

The silicone plug array

• Six cast silicone plugs seat into the underside of the cap
• A central silicone shaft with a flared end ties the whole system together
• Plugs remain sealed until internal pressure builds during the pressing phase
• Once pressure rises, the plugs open and release the brew in a controlled flow

Why silicone?

Silicone offers consistent flex, heat resistance, and smooth sealing behavior. It also lets you experiment with flow characteristics by trying different hardness levels or reshaping the mold geometry.

Status

Very early and very experimental. If you enjoy dialing in prototypes, adjusting silicone thickness, or discovering unexpected pressure physics before breakfast, you’ll be right at home.

Printing & Casting

PCTG Cap

• 0.4 mm nozzle
• 0.2 mm layers
• 4 walls
• 100% infill
• No supports

Quick PCTG note:
At 100% infill, PCTG can show the infamous “crumbs of doom” on printers with heavy toolheads and high acceleration. I hit this myself — reducing acceleration fixed it. Zach Freedman explains the behavior well here (timestamped):
https://www.youtube.com/watch?v=S6_xnzt9YPU&t=756s

Printed Silicone Molds

These are just the molds for casting the silicone parts — not the parts themselves.

• PLA or PETG both work well
• 0.2 mm layers
• 2–3 walls
• ~15% infill is fine
• A slightly lower print temperature helps keep details crisp
• No supports needed unless your slicer insists
• Let the molds cool fully before casting to prevent warping

Silicone parts

• Any food-grade RTV or platinum-cure silicone
• Different durometers will change when the plugs open under pressure
• Casting is straightforward: fill mold, cure, demold

License

Released under the CERN Open Hardware Licence v2 – Strongly Reciprocal (CERN-OHL-S).

• Free to make, remix, manufacture, or sell
• Modified versions must publish their source under the same license
• Attribution requested: Jason J McPheron (JasonMakesCoffee.com)