Post-Use Capsule Separation Mechanism

Six required sub-functions were expanded into a morphological matrix of working principles (rail-and-gravity feeds, mandrels, guillotines, brushes, piercing spikes, compression pistons), combined into eight candidate concepts, then screened down to three feasible directions:

Peeling: capsule held like a drill chuck and peeled open via combined rotation + linear screw motion (2 DOF); low force, but needs a motor and a tightly toleranced cutting path.
Compression: falling capsule crushed against a fixed wall by a user-driven piston; 1 DOF, no motor.
Mandrel: capsule clamped, rim sheared by a guillotine, contents excavated by a drill; best coffee removal, most parts and space.

3D-printed piston heads at four diameters were pressed into used capsules on an instrumented test rig (Fig. 5) to find the smallest force that still gave clean, symmetric compression.

Both 32 mm and 20 mm gave symmetric compression with 85% coffee removal; 20 mm was selected for needing far less force to do it (142 N vs. 245 N).

Published hand-pulling-strength data set a hard ceiling of 25 N (the low end for women at age 55). A simplified 2D slip model at the design weight (3.9 kg) then tightened that further: at the existing rubber feet's friction coefficient (≈ 0.3), the machine begins to slide above roughly 12 N of lever force.

Both limits became binding constraints on the mechanism (D6), not just the piston.

Six lever/gear layouts were screened for force-transmission linearity and synchronization between the dripping and compression compartments. A rack-and-gear layout with the compression compartment fixed in place won on both counts, but even so, a single 90° lever stroke could not bring user force under the 12 N ceiling from D5. Splitting the stroke into two 90° pulls (using a double-ratchet mechanism so the gear only advances on the power stroke) halved the peak force per pull. A second gear, engaged only on the return stroke, then resets the piston with a single lever lift.

The molded cover part must survive repeated compression-reaction loading around the coffee-ground discharge hole without cracking. Solved in Abaqus with a tetrahedral free mesh, global element size 1 mm, refined to 0.5 mm around the hole.

Even against the low end of the ABS yield range, the peak stress leaves better than a 4× margin, confirmed separately by a 3D-printed prototype that survived the same loading in testing (Fig. 9).

The high-load compression gear (25 mm dia., module 1, 25 teeth, 20° pressure angle) must clear both Hertz contact stress and bending stress at the compression mesh, with no interference risk. Six candidate steels were checked against the minimum gear thickness each would need.

CK45 was selected over the higher-grade alloys: it clears the requirement at a practical thickness without paying for hardening or case-carburizing that this load case doesn't need.

The shaft carrying gear torque during compression must stay comfortably below the CK45 yield strength in torsion, and the drive key (wedge) transmitting that torque must clear both shear and bearing (matting) stress limits.

A 10 mm shaft leaves roughly 25× margin against yield in torsion, more than the load needs, and flagged in Section 7 as a candidate for a lighter revision. The Type-B drive wedge (a = 2 mm, L = 13 mm) checks out at ≈ 50 MPa bearing stress, inside the 40–150 MPa window for a fixed steel key, with shear stress held under half the material's yield strength.

The piston needs a linear guide that can absorb the radial load and torque the gear/rack mesh feeds into it, and the ratchet shaft needs a bearing that carries radial load while still allowing it to slide axially between its two clutch positions.

An SKF LVZ7 linear slide (50 mm stroke) was selected off the shelf: the applied load uses only ≈ 3% of its rated capacity, leaving ample margin for the torque component not captured in the simplified radial check. A needle bearing was chosen for the ratchet shaft for its high radial capacity, low cost, and because it does not resist the shaft's axial sliding motion.

A rough should-cost estimate was built from the added mechanism's Bill of Materials to check the cost increase against the customer's ceiling, before recommending a full DFMA pass with a manufacturing partner.

The ≤40% cost-increase requirement is missed at prototype/unit volumes but closes at high production volume, where purchased-component pricing (bearing, linear slide, gears, springs) typically falls by more than half. This is flagged in Section 7 as needing a real supplier quote rather than the linear scaling assumption used here. Material selection followed directly from D7–D8: ABS for the molded housings (lightweight, low-cost, ample FEA margin) and CK45 carbon steel for the loaded gear-train components, chosen for its Hertz/bending-stress-to-cost ratio over the higher alloy grades considered.