Bending and Flattening Heat Pipes: Design Rules
Short answer: Keep bend radii at 3× the pipe outer diameter or larger (2× is the practical floor for thin-wall copper), and limit flattening to 50–60% of the original diameter for sintered wick pipes and 30–40% for grooved wick. Bend and flatten in the same operation where possible, keep the flattened section short, and never deform within 5 mm of the sealed end caps. These limits preserve the vapor channel and wick contact that carry the heat. BQUQ machines and assembles heat pipe heat sinks in one ISO9001 Dongguan factory, holds ±0.005 mm on CNC features, and returns quotes in 12 working hours.
Why bending and flattening matter more than the pipe spec itself
A heat pipe datasheet quotes a maximum heat transport capacity — often 30 W to 150 W for a 6 mm copper pipe depending on length and orientation. That number assumes a straight, round pipe. The moment you bend it around a chassis wall or flatten it to fit under a 12 mm clearance, you change three things at once:
- Vapor channel cross-section shrinks, raising vapor velocity and pressure drop.
- Wick structure compresses on the inside of the bend and may separate from the wall on the outside.
- Thermal resistance rises locally at every deformation, creating a hot spot exactly where you least want one.
None of this means you should avoid bending or flattening. Almost every laptop, LED street light, telecom radio and inverter module uses deformed heat pipes. It means the deformation has to be designed, not improvised on the shop floor.
What is the minimum bend radius for a heat pipe?
The industry rule of thumb is 3× outer diameter (3D) for a safe, repeatable bend and 2D as the practical minimum for thin-wall copper with a sintered wick. Below 2D, the inner wall of the bend starts to wrinkle and the wick can delaminate.
| Pipe OD | Recommended radius (3D) | Practical minimum (2D) | Notes |
|---|---|---|---|
| 4 mm | 12 mm | 8 mm | Common in phones and small modules |
| 5 mm | 15 mm | 10 mm | Thin-wall, sintered wick preferred |
| 6 mm | 18 mm | 12 mm | Workhorse size for CPU/GPU coolers |
| 8 mm | 24 mm | 16 mm | High-power IGBT and server modules |
| 10 mm | 30 mm | 20 mm | Large vapor chambers, telecom |
Two additional constraints matter as much as the radius:
Wall thickness and bend direction
A 6 mm pipe with 0.3 mm wall bends far more gracefully than one with 0.2 mm wall. If your design pushes toward 2D, specify the thicker wall and accept the small weight penalty. Also note that bending against the seam of a grooved pipe is riskier than bending parallel to it — ask your supplier which orientation their tooling supports.
Distance between bends
Leave at least 2× the pipe diameter of straight section between two bends. Two bends placed back to back concentrate stress and make the mandrel hard to extract. If your layout demands a tight S-shape, consider two separate pipes joined at a common block instead of one aggressively formed pipe.
What is a safe flattening ratio?
Flattening converts a round pipe into an oval or flat profile so it can sit under a base plate or behind a display. The flattening ratio is the final thickness divided by the original diameter.
| Wick type | Safe flatten ratio | Aggressive limit | Effect on performance |
|---|---|---|---|
| Grooved | 40–50% | 30% | Grooves collapse first; capacity drops sharply |
| Sintered (powder) | 50–60% | 40% | Most tolerant; slight capacity loss |
| Mesh | 50–60% | 40% | Good tolerance, lower baseline capacity |
| Fiber/composite | 40–50% | 30% | Ask supplier; data is limited |
A 6 mm sintered pipe flattened to 3 mm (50%) typically retains roughly 70–85% of its straight-pipe capacity over a 200 mm length. Flatten the same pipe to 2.4 mm (40%) and you may keep only 50–60%. Those figures are indicative — always request a sample and test in your own orientation, because gravity-assisted and gravity-opposed performance diverge dramatically.
Flatten only where you must
The best-performing designs flatten the pipe over a short, defined length — usually the section that contacts the heat source or the fin stack — and leave the rest round. A 60 mm flattened contact zone on a 250 mm pipe costs far less capacity than flattening the full length.
Keep the bend and the flat apart
If a pipe must both bend and flatten, do them in separate zones with at least 3D of straight, round pipe between them. Forming both features at the same station collapses the vapor channel from two directions at once and is the single most common cause of a heat pipe that passes incoming inspection but fails in the field.
How does bending and flattening change thermal performance?
Deformation raises thermal resistance in three ways: reduced cross-section, wick compression, and increased fill-factor sensitivity. The table below shows typical indicative shifts for a 6 mm sintered copper pipe, 250 mm long, horizontal, at 40 W.
| Condition | Typical ΔT penalty vs straight round | Comment |
|---|---|---|
| 3D bend, single | +1 to 2 °C | Usually negligible |
| 2D bend, single | +3 to 5 °C | Acceptable if you have margin |
| Flattened to 50% | +4 to 7 °C | Depends on flattened length |
| Flattened to 40% | +8 to 15 °C | Design margin required |
| Bend + flatten combined | +10 to 20 °C | Test before committing |
Treat these as directional guidance, not specifications. The only reliable number comes from testing your actual geometry.
Design rules checklist
Work through this list before releasing a drawing:
1. Bend radius ≥ 3D, 2D only with supplier confirmation and thicker wall.
2. Straight run ≥ 2D between bends.
3. Flatten ratio ≥ 50% for sintered, ≥ 40% for grooved, unless testing proves otherwise.
4. Flattened length ≤ 40% of total pipe length where possible.
5. No deformation within 5 mm of either end cap.
6. Keep the evaporator section round if the pipe is long and the power is high.
7. Specify orientation in the test condition — gravity matters more than any bend.
8. Allow for springback — copper recovers 2–5% of the bend angle after the mandrel is removed.
Manufacturing tolerances you should put on the drawing
Deformed heat pipes are hard to hold to tight tolerances, because the forming process moves material. Realistic callouts for a 6 mm pipe:
| Feature | Realistic tolerance | Why |
|---|---|---|
| Bend angle | ±1.5° | Springback and mandrel wear |
| Bend position along pipe | ±0.5 mm | Fixture repeatability |
| Flattened thickness | ±0.10 mm | Die wear, wall variation |
| Flattened width | ±0.30 mm | Material spread is uncontrolled |
| Overall length | ±0.5 mm | Cut after forming |
| Flatness of contact face | 0.10 mm typical | Needs a coining or machining step |
If your assembly needs a flatter contact face than the forming process can deliver, the answer is usually to machine or coin the contact pad after forming — a secondary operation BQUQ runs on the same CNC machined heat sinks line, holding ±0.005 mm where the interface demands it.
Tooling, fixturing and process control
Bending and flattening are done with hardened steel mandrels, rotary draw benders, and matched flattening dies. Three process controls separate a good supplier from a marginal one:
- Mandrel sizing — too small and the pipe collapses; too large and you cannot extract it without scoring the wick.
- Annealing state — copper arrives in a temper that suits forming. Re-annealing after forming is sometimes needed to relieve stress, but it must not disturb the working fluid charge.
- Leak and performance test after forming — every deformed pipe should be helium leak-tested, and a sample from each lot should be thermally characterized. See our approach to heat sink quality inspection for how this fits into a production flow.
Because deformation happens before the pipe is charged and sealed in most processes, the sequence matters: form first, then clean, then charge, then seal, then test. Reversing that order risks contaminating the wick.
Integrating deformed pipes into a heat sink assembly
Once the pipe is formed, it has to bond to a base and a fin stack. Three integration choices dominate:
- Soldered into a grooved base plate — best thermal path, requires the flattened face to be flat and clean.
- Epoxy-bonded — cheaper, lower performance, sensitive to gap thickness.
- Clamped or press-fit — used where rework is needed; performance depends heavily on contact pressure.
If your design also uses a vapor chamber or an embedded pipe in a machined block, the rules overlap substantially — our article on embedded heat pipe heat sinks covers the interface side in more detail. And because the pipe contact face is a thermal interface in its own right, the guidance in thermal interface selection applies directly.
Common failure modes and how to avoid them
| Failure | Root cause | Fix |
|---|---|---|
| Dry-out at high power | Flatten ratio too aggressive | Reduce flattening, shorten flattened zone |
| Local hot spot at bend | Wick delamination | Increase radius to 3D, thicker wall |
| Leak after forming | Over-stressed end cap | Keep 5 mm clear of caps |
| Inconsistent ΔT between units | Bend angle variation | Tighten fixture, add angle inspection |
| Poor contact to base | Flattened face not flat | Add coining or machining step |
| Capacity loss in one orientation | Gravity-opposed mounting | Re-test in the actual mounting orientation |
When to reconsider the whole approach
If your layout demands a bend below 2D, a flatten ratio below 40%, or a deformation zone that overlaps an end cap, the heat pipe is probably the wrong geometry. Options include:
- Split the thermal path into two straight pipes with a common block.
- Switch to a vapor chamber, which handles spreading better in tight, flat envelopes.
- Use a machined or extruded base with a larger cross-section and no phase change at all.
BQUQ runs four production lines in one Dongguan factory — CNC machining, metal stamping, custom springs and heat sink assembly — so a design change from pipe-based to solid-metal cooling does not mean changing suppliers. Our heat sinks and extruded heat sinks ranges cover most of those alternatives, and flexible MOQ means you can prototype before committing to tooling.
Frequently Asked Questions
Q: What is the minimum bend radius for a copper heat pipe?
A: Three times the outer diameter is the standard recommendation, and twice the diameter is the practical floor for thin-wall sintered pipes. Below 2D the inner wall wrinkles and the wick can separate from the wall, which shows up later as a local hot spot. If your layout forces a tighter radius, specify a thicker wall and confirm the tooling with your supplier before releasing the drawing.
Q: How much can you flatten a heat pipe without losing performance?
A: Sintered wick pipes tolerate flattening to 50–60% of the original diameter; grooved pipes are closer to 40–50%. A 6 mm sintered pipe flattened to 3 mm typically retains roughly 70–85% of straight-pipe capacity. Treat these as indicative and validate with a thermal test in your real mounting orientation, because gravity-opposed performance differs sharply.
Q: Can a heat pipe be bent and flattened at the same location?
A: No — keep at least three diameters of straight, round pipe between the bend and the flattened zone. Forming both features at one station collapses the vapor channel from two directions simultaneously, which is the most common cause of pipes that pass leak testing but underperform in the field. Separate the two operations and inspect both zones.
Q: Does bending direction relative to the pipe seam matter?
A: Yes, for grooved pipes. Bending against the seam concentrates stress along the weakest line of the extrusion and increases the risk of collapse. Sintered pipes are less sensitive because the powder wick is isotropic, but the wall thickness still governs. Ask your supplier which bend orientation their mandrel and dies are set up for before finalizing the drawing.
Q: What tolerances should I expect on a formed heat pipe?
A: Bend angle ±1.5°, bend position ±0.5 mm, flattened thickness ±0.10 mm, flattened width ±0.30 mm, and overall length ±0.5 mm are realistic for a 6 mm pipe. Flatness of the contact face is typically 0.10 mm unless you add a coining or machining step. Tighter callouts drive cost quickly, so only tighten what the thermal interface actually requires.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Heat sink product range: /heat-sinks/
- Extruded heat sink profiles: /extruded-heat-sinks/
- CNC machined heat sinks: /cnc-machined-heat-sinks/
- Industry trends in thermal management: /industry-dynamics/
- Technical articles library: /bquq-blog/
- Frequently asked questions: /faq/
- Contact our engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
