Key selection points
- Coil inside diameter is the empty centre of the coil—not the pipe diameter or bending radius.
- Pipe OD, wall thickness or SDR, and coiling conditions must be reviewed together.
- The machine’s smallest reel setting is not automatically suitable for every pipe size.
1. Know which diameter you are choosing
Coil inside diameter (ID) is the diameter around which the first layer is wound. Coil outside diameter (OD) is the overall size of the finished package. Coil width is its axial thickness. Specify all three separately.
Bending radius is measured to the pipe centreline. For pipe outside diameter D and coil inside diameter ID, the centreline radius is (ID + D) / 2. A radius limit cannot be used directly as a coil-diameter limit. Installation bending rules also should not be treated as factory-coiling instructions.
Why it matters: the first layer has the tightest bend. Adding more layers increases the finished coil OD, but it does not relax the bend already formed at the core. Set the inside diameter before checking how much pipe the reel can hold.
One coil. Three dimensions.
Front view
ID sets the first-layer bend.
OD is the complete package diameter.
Side view
W is the package width.
It is different from radial coil build.
2. Use a calculation to understand the size
For an ideal circular bend, approximate outer-surface bending strain is D / (ID + D). For example, 63 mm pipe wound around a 1,200 mm core gives about 5% strain. Increasing that core to 1,800 mm reduces the calculated strain to about 3.38%: the same pipe follows a gentler curve.
The table shows the geometry if 5% strain is assumed: ID = 19 × D. This is an illustration, not an approved strain limit or a universal minimum-diameter recommendation. It does not check flattening, kinking or the effects of tension and temperature.
Larger core. Gentler bend.
1,200 mm core
1,800 mm core
| Pipe outside diameter | Calculated coil inside diameter |
|---|---|
| 32 mm | 608 mm |
| 40 mm | 760 mm |
| 63 mm | 1,197 mm |
| 75 mm | 1,425 mm |
| 90 mm | 1,710 mm |
| 110 mm | 2,090 mm |
Before selecting a reel, confirm the actual permitted coil ID with the pipe producer for the material grade, wall thickness or SDR, temperature and storage conditions.
Reference: PPI TN-6: Polyethylene Pressure Pipe Coil Dimensions. Industry practice guidance; actual coil dimensions must be verified for the pipe and conditions. The calculation examples above are not the PPI tables.
3. Check the pipe, not just the reel
A core that is too small can cause flattening or kinking. Thin-wall pipe, insufficient cooling, excessive pulling force and uneven winding all need attention. Increasing the reel diameter alone will not correct every winding problem.
Wall thickness still matters even though it is absent from the simple strain formula. Two pipes with the same OD but different walls need not behave alike when coiled. Review wall thickness or SDR with the pipe producer instead of applying one fixed correction percentage.
During a trial, distinguish a bend-related defect from a winding defect. A flattened first layer calls for a review of the core and pipe condition. Crossed turns, local pressure marks or a loose package also call for checks of guiding, tension and the winding pattern. Use the approved machine instructions for any adjustment.
- Use a representative pipe sample and the planned production conditions.
- Check the first layer and the pipe after unwinding for flattening, folds, surface marks and unacceptable residual curvature.
- Check dimensions against the pipe producer’s acceptance criteria; agree any additional testing before production.
4. Balance pipe quality with coil capacity
With the same finished coil OD and width, a larger core leaves less space for pipe. A wider coil or larger outside diameter may increase capacity, but also changes package weight, handling and transport requirements.
For a simple comparison, keep coil OD at 2,200 mm and width at 800 mm. Increasing ID from 1,200 to 1,600 mm reduces the annular winding space by about 33%. That is a geometry comparison—not a guaranteed reduction in pipe length or freight cost. Actual capacity depends on whole layers, pipe size and packing.
Confirm the required length together with all coil dimensions. Pipe diameter alone does not establish a 30 m, 50 m or 100 m limit. Check the actual transport opening, loading orientation and unloading equipment—not just the empty container dimensions.
A larger centre leaves less winding space.
1,200 mm core
1,600 mm core
5. Send three things to start selection
You do not need a long enquiry form. Send the information you already have; we can clarify the remaining details together.
The practical sequence is: confirm the pipe’s permitted coil ID, choose a reel that can reach it, then check the required length against maximum coil OD and width. Finally, agree a sample trial and the way the finished coil will be secured, unloaded and unwound.
- Pipe: material, outside diameter and wall thickness or SDR.
- Coil: required length and any approved inside diameter or package-size limit.
- Line: normal and maximum production speed.
For HDPE, we match the approved coil geometry to the machine’s adjustment range and review capacity with your pipe specification. A machine adjustment range is not a guarantee that every pipe can use its minimum setting.