Short answer: 6D is common because bending simultaneously stretches the outer wall, compresses the inner wall and shears the adhesive interfaces. For a bend radius R, a small-deformation estimate of outer-wall strain is ε ≈ d/(2R + d), where d is the outside diameter: about 7.7% at R = 6d and 14.3% at R = 3d. That explains the trend. It is not an allowable strain and not a universal installation limit.
- Outside: tension — whitening, local thinning or visible cracking are warning signs.
- Inside: compression — wrinkles, buckling or flattening concentrate there.
- Aluminium and adhesive layers introduce interface shear and peeling risk, especially when a weld sits at an unfavourable position in the bend.
- A pipe that holds its shape well can be harder to correct after a wrong bend; repeated re-bending is not controlled forming.
- Field rejection differs from factory testing: pressure, tensile, longitudinal-shrinkage, wall-uniformity and oxygen-barrier checks cannot replace inspection of the installed bend.
Why radius is expressed as a multiple of diameter
A diameter multiple scales with the pipe and does not hide the geometry in one fixed length. The estimate above shows the trend: reducing the radius from 6D to 3D nearly doubles the outer-wall strain in this simple model. It is useful for explaining risk, not for inventing a strain threshold. Polymer behaviour, aluminium thickness, weld construction, adhesive formulation, temperature, tooling and bend direction all affect the actual result.
What happens through the wall
The polymer layers can respond differently, the aluminium layer resists shape change, and the adhesive carries relative movement between layers. On the outside of the arc, tension may cause whitening or local thinning. On the inside, compression may create a fold or an oval section. With poor support, the defect can remain even without a visible fracture. A clamp, spring former or suitable die supports the inner wall more consistently than foot pressure or a hard pry.
Field rejection in three steps
- Before work. Obtain the grade-specific technical data and record the stated minimum cold-bend radius and whether hot forming is allowed.
- After bending. Inspect the whole bend for outside whitening, visible wrinkles, flattening or ovality, and aluminium-layer lifting.
- On any sign. Reject it under the project procedure, do not bend it back, and record the batch and bend location in the concealed-work record.
| Observation | Where | Action |
|---|---|---|
| Whitening or local thinning | Outer arc | Reject; do not re-bend |
| Wrinkle, fold, flattening or ovality | Inner arc | Reject; check former or die support |
| Aluminium-layer lifting or separation | Bend or weld zone | Isolate; review with the manufacturer |
The boundary of 6D
6D sits inside a common manufacturer range of about 4D–8D. It is not a standard value. Heat forming is acceptable only when the grade documentation explicitly permits it and defines the method. Do not infer permission because another multilayer product can be heated.
Where the supplier fits
SANLI PIPE supplies multilayer structures including butt-welded and overlap-welded aluminium composite pipe, silane double-crosslinked composite pipe, aluminium-composite PP-R and PE-RT alloy-aluminium oxygen-barrier pipe; its stated sample includes 20 extrusion and composite lines and 26 outgoing inspection procedures. These are supply-side facts: the bend radius and rejection criteria for a named grade must come from that grade’s technical data. For the installation sequence, see the eight-stage guide; for shape retention, see the STABIL explainer.
Scope and limits
This page explains the mechanics and the field decision. The strain values are a small-deformation geometric illustration, not a material limit or a measured result. EN ISO 21003 evaluates the system; a pipe certificate alone does not prove system conformity.
Sources
- EN ISO 21003 Parts 2, 3 and 5 — current official editions (ISO).
- ISO 15875, ISO 22391, ISO 15874 — material-family standards (ISO).
- ASTM F1281 and F1282 — separate PEX-AL-PEX and PE-AL-PE routes (ASTM).
- DIN 4726:2024-12 — oxygen-barrier requirements and test methods (DIN Media, retrieved 30 September 2026).
Frequently asked questions
Is 6D a mandatory value?
No. It is a common manufacturer reference within a typical 4D–8D range, not a universal standard value.
Does the 7.7% figure prove the pipe is safe?
No. It is only a small-deformation geometric estimate at R = 6d; it supplies no allowable strain or material limit.
Can a fold be corrected by bending back?
That is not a safe default. A fold, whitening, flattening or aluminium-layer lifting should be rejected under the project procedure.
Does a pressure test replace a bend inspection?
No. Factory tests and field pressure checks address different risks; bend geometry and workmanship are verified by inspection.
When is heat forming acceptable?
Only when the technical documentation for that grade explicitly permits it and defines the method.
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