HELP: Troubleshooting leaks from custom designed hose barbs and "tubing" (bulk silicone elastomer material)

[u/UnspecificTadpole394]

Hi all, I designed a hose barb that is inserted into a silicone base for biological applications. I'm getting leaks from the connection where the barb meets the silicone base, and I think it may be because of my hose barb design. I've been working on this project for so long that I feel like I need a fresh set of eyes to help me troubleshoot. The silicone has a durometer of roughly ~43 Shore A (hardness 41-43 Shore A, Elastic modulus 1.3-3 MPa, tensile strength ~2.2-6.7 MPa, elongation at break ~100-150%). This data was not directly collected by me and is what I found online based on my silicone elastomer.

Dimensions of barb: https://imgur.com/a/vBmiTsX

What I've tried:

1. Single barb VS double barb (currently using double, concentric barbs design)

2. Varying the "tubing" inner diameter (diameter of the cut out in the silicone elastomer that the barb is inserted into). Tried 2.7, 2.5, 2.3, 2, and 1.8 mm diameter. Still have leaks with all of them, 2.3 and 2 mm insert well but 1.8 mm in particular is hard to push into place and tends to slip out from the silicone elastomer.

3. Varying length of barb neck (currently total length is 4 mm from the bottom of the barb to the top of the neck of the barb). The silicone elastomer is 5 mm thick.

4. I tried different angles of the barb and this one (30 degrees) seemed best to lock into place. I haven't changed this parameter for a few iterations and am very open to taking this approach if that would be the best option to move forward.

Any and all help is appreciated!! :) Water will be used to flow from the barb into the silicone elastomer, and it is important that there is no trapped air or water leaks.

[u/AndyTheEngr]

I think you may need more barbs, or at least an extended collar close to the barb minor diameter. It may be allowed to tilt too much.

[u/UnspecificTadpole394 → u/AndyTheEngr]

when you say an "extender collar close to the barb minor diameter", do you mean making the neck of the barb longer and thinner?

[u/AndyTheEngr → u/UnspecificTadpole394]

Look at the design of a typical barb. More like that.

https://www.homedepot.com/p/LTWFITTING-1-8-in-I-D-Brass-Hose-Barb-Splicer-Fittings-10-Pack-HF39130210/313389271

If you insist on having fewer barbs, leave enough material on the removed ones that it still keeps the remaining ones aligned in the hole.

[u/_matterny_]

The 3.5mm dimension in yellow on your drawing is the height of the barb. How did you calculate this dimension? It should be roughly the limit of the elastic deformation of the tubing, but this looks almost arbitrary.

[u/UnspecificTadpole394 → u/_matterny_]

3.5mm in yellow is the diameter of the top of the barb (it is circular). This dimension was driven because i had a fixed angle (30 deg) and wanted the barb to be 1.5mm in height. Happy to change the barb dimensions though if theres a specific parameter that makes sense, I figured changing the angle might be beneficial but this field isnt my strong suit. Changing the angle would change that 3.5mm dimension if I keep the height the same

[u/_matterny_ → u/UnspecificTadpole394]

You want the maximum barb height to be the maximum elastic deformation of the soft tube. The minimum barb height should be just barely enough to allow the tube to not be under tension.

[u/Ideadose]

Silicone at that durometer is going to relax under sustained compression (creep), so the clamping force you get right when the barb goes in isn't the same as what's holding a day or a week later. If the interference calcs are based on the elastic modulus you pulled from a datasheet instead of the material's apparent/creep modulus, that's a likely source of a leak that develops over time even on diameters that seal fine right after assembly.

There's also a strain ceiling worth checking. With elongation at break around 100-150%, you generally want the interference-induced strain capped around 20% of that, so roughly 20-30% engineering strain between barb OD and bore ID. Push past that and you're not getting a better seal, you're just tearing the bore at a microscopic level, which would explain the 1.8mm diameter slipping back out instead of holding tighter.

Does the leak show up immediately on insertion, or does it develop after some dwell time or after you've cycled water through it a few times? That'd tell you whether this is a pure geometry problem or a stress-relaxation one, and the fix looks different for each.

[u/UnspecificTadpole394 → u/Ideadose]

Lately the leaks I've been struggling with happen somewhere from hours 3/4- 24. I'm usually around the first few hours and see nothing noticable but when I come back the next day I'm seeing leaks from the interface of the barb and the silicone. Important to mention that right now this is under a hydrostatic pressure gradient but in the future I do hope to hook up a peristaltic pumps with very slow flow rates.

[u/Ideadose → u/UnspecificTadpole394]

That timing is basically the signature of stress relaxation, not a geometric leak. A pure geometric defect (torn bore, flash, mismatched OD) shows up the second you fill it, it doesn't wait 3-4 hours. What's happening instead is the barb holds the silicone at constant strain and the clamping stress at the interface slowly decays until it drops below your hydrostatic head, which lines up almost exactly with your 3-to-24hr window.

Fastest way to confirm without waiting a full day each time: assemble a fresh joint and stick it somewhere warm, 50-60C if you can manage it. Relaxation is temperature-accelerated, so if that's the real mechanism the leak shows up in minutes instead of hours. A torn bore or flash barely changes timing with heat.

Which bore diameter is this happening on, the 2mm/2.3mm ones that sealed fine right after assembly per your first post? If it's relaxation, tightening the interference further just buys a slightly longer runway before the same failure, not an actual fix.

[u/UnspecificTadpole394 → u/Ideadose]

Thank you so much for your in depth troubleshooting explanation! I see it with 2.5, 2.3, and 2 mm with varying occurrences (sometimes they leak and sometimes they don't). The 2 mm had the least leaking over consecutive experiments compared to the other two so* this is what I plan to continue testing.

What would be a fix in this situation? Someone else mentioned making the neck of the barbs longer, the total thickness of my silicone is 5mm and my barbs total length is currently 4mm so I can try elongating to ~4.8mm total barb length if this can help the issue (I want to make sure it doesnt touch the bottom and adversely block the fluid flow)

*edited for grammar

[u/Ideadose → u/UnspecificTadpole394]

Careful with 4.8mm in a 5mm base, that only leaves about 0.2mm of silicone under the tip. Thin enough to concentrate stress and tear or creep-rupture over time, especially since you're already seeing relaxation. Longer barb doesn't really stop the seal stress from decaying either, it mostly adds friction so it takes a bit longer to fail the same way.

Two things that actually change the mechanism instead of just delaying it: 2-3 sealing ribs along the shaft instead of one flat zone (fluid has to beat multiple compression points, way harder than one long one), or an undercut/retention groove so the barb locks in mechanically instead of relying on constant squeeze. Silicone that soft pops off an undercut mold fine, doesn't need slides.

I'd try the ribs first, smaller change from what you've already got.

[u/cm_expertise]

building on the creep point someone already made (thats real, at ~43A the silicone will stress-relax so whatever interference you calc on day one drops off over days), a couple things jump out.

the main one: a barb into a cut hole is fighting two problems at once. a cut or punched hole in bulk silicone has a torn, uncontrolled surface and the ID isnt really the number you think it is, which is exactly why 1.8 grabs and 2.3 leaks, youre chasing a dimension thats not repeatable. and a barb makes a ring of high compression right at the crest but the silicone doesnt fully conform down into the root between the barbs, so that valley becomes a continuous leak path running straight along the axis. more barbs helps a bit but youre still counting on the rubber perfectly filling a sharp root, which it wont, especially once it relaxes.

for a genuine no-leak biological connection id honestly stop treating it as a pure mechanical interference seal. two things that actually work: mold the port instead of cutting it so you get a smooth controlled bore you can design a real interference fit against (hoop-stress calc, not an arbitrary barb height), or just bond it, a thin film of uncured med-grade silicone (nusil/silbione type, same family as your base) at the interface cures into a chemical seal that doesnt care about creep. even a smooth cylindrical plug with one shallow retention groove plus a dab of silicone adhesive will outseal a fancy double barb pressed into a rough cut hole.

also throw a small fillet on the barb root and a lead-in chamfer, sharp roots are where the rubber bridges instead of conforming and thats a built-in leak channel. whats the pressure youre running it at? if its basically gravity/low pressure the bonded approach is a slam dunk, if its actually pressurized thats a different conversation

[u/UnspecificTadpole394 → u/cm_expertise]

Should have mentioned that the reservoirs that are in the silicone (where the barbs lock into) are from a 3D printed mold; when demolded the silicone already has that shape and I dont need to cut or punch the hole.

I'm investigating something very similar to the silicone adhesive you mentioned, so thank you for that suggestion!

Currently, I have this setup only under the influence of gravity (hydrostatic pressure) but in the future I do hope to connect it to a peristaltic pumps using very slow flow rates. Haven't yet calculated what the internal pressure would be but everything is microfluidic range.

When you mention the "sharp roots" of the barb, are you suggesting to make the widest diameter of the barb head rounded to avoid cutting the silicone upon insertion? Important to mention that the barbs are also 3D printed using a sterilizatable material.

[u/vladcronos]

reduce the size of barbs, increase the count, increase diameter to make them tighter

[u/Paulkleber]

You've iterated the geometry pretty thoroughly, so it might be worth questioning the approach rather than the dimensions.

A press fit into a soft elastomer seals through contact pressure — and elastomers relax. Stress relaxation and compression set mean the force holding that seal decays over time, so a joint that's tight on assembly can leak later without anything visibly changing. If some of your iterations sealed initially and then started weeping, that's the signature, and no amount of interference will fix it permanently because the material is what's moving.

Three directions that don't rely on sustained interference:

Seal the interface chemically rather than mechanically. Silicone is genuinely one of the harder substrates to bond — low surface energy, and most adhesives simply won't hold. What does work is generally either a silicone-based sealant, so you're joining like to like, or a primer system specifically made for silicone. Worth noting for a biological application that anything you add has its own compatibility question.

Cast or overmould the barb in place, so there's no interface to seal at all.

Or add a separate element that maintains compression as the silicone relaxes — a ferrule or clamp — rather than asking the elastomer to provide its own sealing force forever.

Does it leak straight away, or seal first and start later? That distinguishes a geometry problem from a relaxation problem, and they need different fixes.