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Plastic core wall thickness calculator

ROLL Lab · app 6 of 7 · Prof: instant verdict, full report by email · updated 24 October 2026

In shortThe minimum wall is the larger of two checks: material crushing (Lamé) and wall buckling, which grows with the cube of wall/diameter. Creep and temperature reduce stiffness and strength: a 6" core needs much thicker walls than a 3".

Minimum plastic core wallPlastic doesn’t break: it gives in slowly, with heat and with time. Tell me film, roll and temperature and I’ll estimate the minimum wall with the right margin.

minimum wall · PP

What you wind Roll Ø Storage temperature

Wall by material	mm

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How the calculation works

1. Pressure on the core. From the tension stress σ in the wound material (estimated from the film type or entered by the user) and the ratio of roll radius R to core outside radius r: P = k × σ × ln(R/r). The simple elastic model (k = 1) is an upper bound; the ROLL Lab applies k = 0.6 to account for the compressibility of the wraps. It is a calibration without a public source and may be optimistic for very elastic films or films with strong shrinkage: for these the user can set k = 1.

2. Design pressure. P × safety factor (default 1.5).

3. Crushing check (Lamé). p = σ_all × (r_o² − r_i²) ÷ (2 r_o²), with σ_all = limit stress × long-term factor × temperature factor.

4. Buckling check. p_cr = 2E ÷ (1 − ν²) × (t ÷ D_m)³, with E = modulus × long-term factor × temperature factor.

5. Minimum wall: the smallest wall that satisfies both checks.

Default parameter	PVC	PP	HDPE
Short-term modulus, 23 °C (MPa)	3,000	1,500	1,000
Limit stress (MPa)	50	32	25
Poisson’s ratio	0.38	0.42	0.42
Long-term factor, weeks at 23 °C	0.65	0.45	0.45
Temperature factor at 50 °C	0.8 (virgin) · 0.7 (recycled)	0.6	0.55

References: moduli and stresses from polymer technical literature; long-term factors consistent with the EN 13476-2 creep ratios (PVC-U ≤ 2.5; PP and PE ≤ 4 at two years); PVC factor at 50 °C from ROLL Lab estimates; PP and HDPE factors are ROLL Lab assumptions.

Limits: free ring, without shaft support; homogeneous, isotropic material; no effect of concentrated loads from supports or plugs. The creep and temperature factors are assumptions to be calibrated with constant-load tests. ROLL Lab estimate, to be confirmed by a line and storage trial.

Frequently asked questions

Why does a 6" core need much thicker walls?

Because the critical buckling pressure depends on the cube of the wall-to-diameter ratio: for the same wall, doubling the diameter divides it by eight.

What are long-term factors?

Reductions in stiffness and strength that account for creep under constant load. For weeks at 23 °C the ROLL Lab uses 0.65 for PVC and 0.45 for PP and HDPE, consistent with the EN 13476 creep ratios.

Does the calculation account for the shaft?

No: it treats the core as a free ring, the most severe case after it comes off the shaft. On an air shaft the core is supported from inside.

Can I enter the measured strength of my core?

Yes: if you have an ISO 9969 ring stiffness or a measured failure pressure, you can use it instead of the estimate. It is the best way to make the result reliable.

Sources

ISO 9969:2007
EN 13476-2 and EN ISO 9967
Z. Hakiel, “Nonlinear model for wound roll stresses”, Tappi Journal 70(5), 1987
Lamé formulas and buckling of a ring under external pressure
ROLL Lab analysis

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