First real design project for the H2C: replace the acrylic pencil holder James uses for syringes with a printed caddy that also solves an actual problem — holding a syringe hands-free while it draws testosterone through a 30-gauge needle.
Here's what it's replacing:
It works, but the syringes lean everywhere, the swabs have no home, and it does nothing for the tedious part of the routine: drawing oil through a tiny needle takes a while, and you have to stand there holding the syringe with the vial balanced on top.
The interesting problem: vacuum suck-back
The one-liner spec was "a place on the side where I can lock one of the larger syringes with tip pointed up in the fully open position." James's first idea was tabs to capture the syringe's flange wings plus a guide to keep it balanced.
Tabs alone don't survive the physics. With the tip up, plunger fully drawn, and an inverted vial sitting on the needle, the vacuum in the barrel is actively pulling the plunger closed the whole time the oil creeps through the 30 ga needle. Wing tabs hold the barrel; nothing holds the plunger. The fixture needs two capture points working against each other, plus support for the vial so the needle carries no load (30 ga needles bend if you look at them wrong).
The v0 answer, bottom to top:
| Feature | What it does |
|---|---|
| Plunger fork (bottom) | Rigid open-front fork at exactly the drawn-plunger height; catches the top face of the thumb pad and directly opposes suck-back. No springs, no flexures — PLA creeps under sustained load, rigid geometry doesn't. |
| Bayonet wing capture (middle) | James's tabs, evolved: drop the syringe through a keyhole slot, rotate ~60° so the flange wings ride under tabs. Locked against vertical pull in both directions. |
| Vial cradle (top) | Open-front ring that takes the 10 mL vial's weight. Slides on a rail mast with the height tunable after printing, because that dimension is a stack-up of three measurements and will be the thing that's wrong. |
Insertion is one motion, one hand: thumb pad into the fork, wings drop through the keyhole, twist to lock, tip the vial on. Walk away.
v0 renders
The model is parametric OpenSCAD — every syringe and vial dimension is a named variable keyed to a measurement checklist. Calipers arrive midweek; until then the numbers are published EasyTouch and serum-vial specs, which is why nothing prints yet.
Design decisions so far
- Bins angled 15° so the orange caps present toward the user instead of leaning randomly like the acrylic box.
- Open tray for swabs, not a dispenser slot — James wants loose packets at hand.
- Rigid capture geometry everywhere. Considered and dropped: friction clips on the plunger shaft (print-tolerance roulette) and flex clips on the thumb pad (PLA creep under a minutes-long sustained load).
- Tower and cradle are separate parts from the bin body. The capture slots get 0.3–0.4 mm clearance in v1 and will almost certainly need one tuning pass; reprinting a 30-gram tower beats reprinting the whole caddy.
- PLA to start. Nothing here needs a living hinge, and the H2C's textured PEI plate handles it without drama.
What's next
- Calipers land midweek: ~16 measurements replace the placeholder variables (drawn-plunger length and flange-to-thumb-pad distance are the two that set the whole tower).
- v1 render pass, then first print on the H2C.
- Expected iteration: one tower reprint to dial the bayonet and fork clearances.
Design notes and the measurement checklist live in the Obsidian project; the .scad is the source of truth. Print results will follow here.