Tube Rack for the Cedex Bio HT
The Cedex Bio HT has a carrier for the samples it is running. It has nothing for the twenty tubes waiting their turn. Thaw a week of timecourse supernatant and they end up in a foam block, a beaker, or lying on the bench in whatever order you pulled them out of the box — and one tube loaded out of order turns a whole timecourse into a puzzle. This is a printed rack that sits beside the analyser and holds those tubes upright, in the order you sampled them, until they go on.
A rack is drawn around one tube. Cedex sample tubes and cups come in more than one size and labs do not all stock the same ones, so measure yours before you print. The section below is the difference between a rack you use every day and a bin full of plastic.
Before you print
Put a caliper across the widest part of a capped, labelled tube — on most tubes that is the cap flange, not the barrel — and write the number down. Then measure the tube’s height, because well depth is the other half of the design: deep enough that a full tube can’t tip, shallow enough that you can still pinch a cap out with gloves on.
Two things catch people here.
Labels are thicker than they look. A wrapped thermal label adds a few tenths of a millimetre to a tube’s diameter, and one that overlaps itself adds twice that. Size the wells for a labelled tube. A rack that only takes bare tubes is a rack you stop using on the first day of a real run.
FDM holes print undersize. Almost every printer lays a vertical hole a little tight, by an amount that belongs to your machine and your filament rather than to the model. Print a two-well test coupon, push a real tube into it, and size the wells from what the coupon tells you, not from what the caliper told you. Change the well diameter itself — scaling the whole rack to fix a hole takes the wall thickness and the position pitch with it.
Parts
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- PETG filament, 1.75 mm — print the rack in this. The material section below is why
- PLA+ filament, 1.75 mm — a 4 × 1 kg pack, and fine if the rack never leaves a climate-controlled bench
- Digital caliper — the well diameter comes off a tube you measure yourself, never off a spec sheet
- Deburring tool — one pass takes the burr off a well mouth
- Needle file set — the round one opens the wells that still come out tight
- Self-adhesive rubber bumpers — a 200-pack; you need four, under the base
- CA glue with activator — only if the rack is longer than your build plate and you split it
- Brother P-touch label maker and laminated TZe tape — for the position numbers. Laminated tape survives a solvent wipe; paper tape lifts on the second one
- 99% isopropyl alcohol — 16 oz × 12, for wiping the rack down. Cut it to 70% first, roughly seven parts alcohol to three parts water; neat 99% flashes off the plastic before it has done anything
Material
Print it in PETG.
PLA is easier, and it is what most bench racks get printed in, and it is the wrong answer here for three reasons. It softens around 60 °C, which is close enough to the top of a warm instrument or a sunlit bench to matter. It is brittle to begin with and more so cold, so a loaded rack knocked against a shelf in a 4 °C cold room snaps at a well wall instead of flexing. And it hydrolyses, so the media that wicks into the layer lines and stays there is slowly working on the plastic.
PETG holds its shape to about 80 °C, stays tough cold, and takes an ethanol or IPA wipe without crazing. It costs you a slower first layer and a brim. Print the PLA+ instead only if the rack stays on one cool bench — it is tougher than plain PLA and it prints more accurately, so the wells land closer to the number in the model, but it is still PLA at 60 °C and still PLA in spilled media.
Nothing you print on an FDM machine is autoclavable. PETG is soft well below 121 °C and PLA is a puddle. This rack gets wiped down, not sterilised. Keep it on the bench side of the workflow and keep it out of anything that has to come back sterile.
If your lab wipes with something more aggressive than ethanol, test it before you commit a spool. Print a coupon, leave it under a soaked wipe for an hour, and check whether it has gone cloudy, tacky or soft.
Print settings
| Setting | Value |
|---|---|
| Layer height | 0.2 mm |
| Perimeters | 3 |
| Infill | 20%, gyroid |
| Supports | None |
| Material | PETG |
| Orientation | Base flat on the plate, wells opening upward |
| Brim | Yes in PETG, optional in PLA |
Wells opening upward is the whole orientation argument. Printed that way every well is a plain vertical hole — nothing to bridge, nothing to support — and the layer lines run around each well rather than across it, which is also the direction that resists a tube being shoved in.
Three perimeters rather than more infill. What wears on a rack is the wall between two wells, and that wall is perimeters. Pushing infill past the 20% in the table buys print time and nothing else.
A rack is a wide flat footprint and PETG lifts at the corners. Brim it. If it still lifts, the fix is a hotter plate, not more brim.
Finishing
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Deburr every well mouth. A tube that catches on a stray strand on the way in is a tube you drop on the way out. One pass with the deburring tool per well, and do the underside too if the wells go all the way through.
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Test-fit a real tube in every well. Not one well — every well. A printer that under-extrudes on a single wall gives you exactly one tight position, and that is the one you will find at eight in the morning holding a rack of thawed samples.
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Open the tight ones with the round needle file. A few strokes, test, repeat. Do not reach for a drill: it grabs the plastic and takes the whole well wall with it.
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Stick a rubber bumper under each corner of the base. A freshly wiped bench is wet, and a loaded rack that slides is a loaded rack on the floor.
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Number the positions. Run a strip of laminated tape along the front edge with the numbers on it, printed rather than written — a Sharpie number comes off with the first ethanol wipe.
Number the rack, not the tubes, and load it in sample order. The rack then is the run list: a tube in the wrong position is visible from across the bench, which is not true of twenty tubes in a foam block.
Files & downloads
The model files aren’t published yet. Everything above is what you’ll want ready when they land.
The source files for this project aren't in the repo yet — there's nothing to download
beyond the notes. They land in projects/cedex-tube-rack/ when they do, and this
section fills itself in.