Decision pillar · from problem to specification

How to choose a plastic core: 7 data points and a decision tree

ROLL Lab · updated 24 October 2026 · sources at the bottom of the page

The method in brief

Many enquiries for plastic cores contain two numbers: inside diameter and length. That is not enough. Two cores of the same 76 × 1,000 mm can have very different walls, materials and prices, and only one of them will carry the roll. The method we suggest has three steps:

1. Define the seven data points of the application, even when they look obvious.
2. Choose the polymer with the decision tree: temperature, hygiene, environment and reuse almost always rule out some options.
3. Size the wall on the pressure of the roll, with a margin for creep and temperature, and confirm it with a test.

1. What you wind and how heavy the roll is

The wound material decides the radial pressure on the core. Every wrap wound under tension squeezes the ones below, and the core collects the sum. In the literature the pressure on the core ranges from about 5 bar to over 100 bar. Elastic films (cast and blown stretch film, shrink film) are the most demanding, because they keep shrinking after winding. Paper and fabrics squeeze less, but the rolls are heavier.

So you need:

• the type of material (for example LLDPE cast film, BOPP, PET, paper, nonwoven, geotextile, adhesive tape);
• the material thickness and winding tension, if known;
• the maximum outside diameter of the roll and the roll weight;
• whether there is a lay-on (nip) roller, and the storage time before unwinding.

Time matters: according to data published by a core manufacturer, with cast film radial failure usually happens within 72 hours, with blown film between 7 and 14 days. A plastic core that survives the first day can close up after two weeks if the wall is marginal.

2. Inside diameter and shaft type

The inside diameter (ID) must slide onto the shaft without forcing and without too much play. The most common sizes are 76 mm (3") and 152 mm (6"), but there are many sizes in between. Before choosing you need:

• nominal ID and accepted tolerance (usually plus-only, as in ISO 13542);
• shaft type: lug or bladder air shaft, friction (differential) shaft, expanding chucks, plain shaft without expansion;
• torque to transmit and winding speed;
• any notches or drive dogs.

The bore surface is chosen with the shaft. A smooth bore suits most air shafts. A ribbed bore grips better with high torque, slippery films or friction shafts with little expansion. On a plain shaft without expansion, a tight tolerance matters most. Diameters, tolerances and shafts in detail.

3. Length, tolerances and straightness

The core length almost always equals the roll width, plus any overhang for plugs or handling. Three questions:

• Length tolerance: on machines with several cores side by side on one shaft, one extra millimetre per core adds up. With ten cores it becomes a centimetre.
• Straightness (bow) and concentricity: these matter on long cores and fast winding, where a bent core causes vibration and an uneven roll.
• Ends: as cut, deburred or chamfered. A deburred end avoids chips and scratches on the material.

Asking for tighter tolerances than necessary raises the price with no benefit. Start from the machine: how much play it accepts, and which deviations have already caused problems.

4. Temperature: process, storage and transit

This is the data point most often missing, and the one that best separates the polymers. Plastics lose stiffness with heat and creep more under load. Three temperatures count:

• in process: film can reach the winder still warm from extrusion; some heat treatments pass through the core;
• in storage: unheated warehouses in summer easily exceed 35 °C under the roof;
• in transit: a container in the sun exceeds 50 °C inside.

As an order of magnitude, the heat deflection temperature under load (HDT at 1.8 MPa) is about 54–75 °C for rigid PVC, 50–60 °C for PP and 45–60 °C for HDPE (typical literature values). Above 40 °C in continuous service, choose with a calculation rather than by feel. Temperatures and moduli by material.

5. Environment: humidity, outdoors, UV and cold

Plastic does not mind humidity, but outdoor storage brings other enemies:

• UV: without stabilisers, PP degrades in sunlight within a few months; HDPE with carbon black resists far better.
• Cold: homopolymer PP becomes brittle around 0 °C; rigid PVC loses impact strength below freezing; HDPE stays tough even at −30 °C.
• Direct sun: a dark surface in the sun can get much hotter than the air, and heat brings deformation.

Plastic cores for outdoor use: moisture, UV, cold.

6. Hygiene and contact

If the roll goes into a food, pharmaceutical or medical plant, or into a cleanroom, the questions change:

• does the area accept dust and fibres? If not, cardboard is often ruled out;
• does the core touch the food, or only the film? Usually only the film: food contact compliance is assessed case by case;
• do you need a declaration of compliance with Regulation (EU) 10/2011? Then you need virgin polymers, or recycled polymers from authorised processes;
• do the cores need washing, or bagging?

Hygiene, food contact and cleanrooms.

7. Life cycle: one-way or returnable

The last data point decides whether plastic really pays. A one-way core makes a single trip and ends up in the customer’s waste: there, cardboard almost always wins on price. A returnable core comes back, is inspected and goes out again: there, plastic can cost less per use. You need:

• a realistic return rate (not the hoped-for one);
• the cost of one return: collection, transport, inspection, washing;
• the end of life: who takes back scrapped cores and which stream they go into;
• the customer’s regulatory obligations: minimum recycled content under the PPWR from 2030 (and the 30% threshold of the UK Plastic Packaging Tax today) and, if the core is declared reusable, the requirements of PPWR Art. 11.

Reuse and return loops · PPWR and plastic cores.

The decision tree

With the seven data points in hand, choosing the polymer comes down to a few questions in sequence. The tree below is a ROLL Lab simplification: it points a direction, it does not replace a test.

1. Do you really need plastic? If the core does not come back, the area accepts fibres and the environment is dry, look at cardboard first.
2. Continuous temperature above 50–60 °C? Rule out PVC and standard HDPE; consider PP or, above that, engineering materials and metal cores.
3. Declared food contact or cleanroom? Rule out unauthorised post-consumer recyclates; prefer virgin PP or HDPE, or virgin PVC with a declaration.
4. Outdoors, sun and frost? Prefer HDPE with carbon black or UV-stabilised copolymer PP; avoid homopolymer PP in frost.
5. Heavy roll or high pressure, many reuses? Rigid PVC gives the highest stiffness for a given wall and the highest reported cycles; calcium carbonate-filled PP is a lighter compromise.
6. Recycled content a priority? Recycled PVC and HDPE are widely available; check that the recyclate is post-consumer if you need it for the PPWR.

Try it with your own data below; method, assumptions and sources are on the dedicated page: the PVC, PP or HDPE selector.

From polymer to wall thickness

Once the polymer is chosen, the wall is sized against two possible failures:

• material crushing: the wall yields under the roll pressure. It is estimated with the thick-walled tube formula (Lamé);
• buckling: a thin ring under external pressure goes oval and collapses well before it yields. The critical pressure grows with the cube of the wall-to-diameter ratio.

That is why, at the same pressure, going from 3" to 6" needs much thicker walls: doubling the diameter divides the critical pressure by eight for the same wall. Both checks include a factor for creep and one for temperature. The formulas explained.

The calculation is below. For the full version, with the formulas explained, open the minimum wall estimate with creep.

Colour and marking: details that work

Colour is decided last, but it is not just about looks. One colour per customer, per polymer or per return loop makes sorting returned cores immediate and reduces recycling errors. Recycled PVC is usually grey or black because it comes from mixed-colour material: asking for a light colour may force virgin material. In the sun, dark colours heat up far more than light ones, and heat speeds up creep: black protects against UV but needs more wall. Marking the polymer and its recycling code (for example PVC 3, PP 5, HDPE 2 under Decision 97/129/EC) helps whoever has to sort at end of life.

The most common mistakes

• Copying the cardboard wall. A plastic core and a cardboard core with the same wall do not have the same long-term strength. Resize it.
• Trusting a five-minute test. The 3% strength measured in a few minutes says nothing about creep over weeks. Ask for long-term data or run a storage trial.
• Forgetting the container. The core holds in the factory and closes up after a month at sea in summer.
• Counting on reuse without counting it. If nobody measures how many cores come back, the reuse calculation is a guess.
• Mixing up recycled and recycled. For the PPWR only post-consumer plastic counts; reground industrial scrap does not.
• Ignoring plugs. A heavy roll resting on a core without plugs can ovalise it in storage. Core plugs and end walls.

Samples and line trials

Every estimate must be confirmed. A serious sampling plan includes:

1. incoming measurement of ID, outside diameter, wall and length on a sample per pallet;
2. a line trial with the most critical roll (largest diameter, most elastic material);
3. measuring the bore at 24 hours, 7 days and 14 days of storage, ideally in the warm season;
4. an unwinding test: the core must come off the shaft without forcing;
5. for reuse, an inspection after the first return: ovality, cracks, ends.

The calculators in this guide are ROLL Lab estimates: they help you start from the right place, they do not replace a test on the real core. What to ask the manufacturer.

Frequently asked questions

Further reading

Sources

1. ISO 13542:2006, internal diameters and tolerances of reel cores (used as practice reference)
2. ISO 9969:2007, ring stiffness of thermoplastic pipes
3. D. Van Hove, “Radial approach for the design of cores in film applications” (radial pressure 5–100+ bar; failure times for cast and blown film)
4. SpecialChem, Heat Deflection Temperature of Plastics, specialchem.com (typical HDT values)
5. Fiaptech technical documentation, Smart Book “Which core?” (a company with corporate ties to ROLL)
6. Regulation (EU) 2025/40 (PPWR), Arts. 7 and 11, EUR-Lex
7. ROLL Lab analysis (decision tree, sizing method)
8. Decision 97/129/EC, identification system for packaging materials