Composite Can Construction: Kraft Body, Metal End & Barrier Liner Specification

Overview #

Composite cans sit at the intersection of structural engineering and barrier science — getting the body wall, end component, and liner specification wrong in any one layer compromises the entire package. We see this most often with dry food, powdered supplements, tea, coffee, and personal care products where moisture ingress or oxygen transmission causes shelf-life failure long before the packaging shows visible damage. The critical insight most brands miss at brief stage: the barrier liner specification must be locked before the body wall caliper is finalised, because liner thickness and stiffness directly affect the winding tension and adhesive bond strength on our spiral-wound tube lines.

Body Wall Construction: Kraft Grades, Caliper and Winding Parameters #

The structural backbone of a composite can is the spiral-wound kraft body. We wind from 2 to 6 plies depending on the required wall thickness and the product’s drop-impact requirement. For most dry food and supplement applications, we target a finished wall thickness of 3.0–5.0mm, achieved by combining a 200–350 GSM outer kraft wrap with 150–250 GSM inner plies.

The outer wrap carries the print substrate or label laminate, so its surface must meet a minimum Sheffield Smoothness of 100–150 units for offset or flexo print adhesion. For direct gravure printing on the tube body, we specify a coated kraft at 250–300 GSM with a surface roughness below 2.5 µm Ra. The inner ply in contact with the barrier liner must have a Cobb60 water absorption value below 35 g/m² (tested per ISO 535) to prevent adhesive strike-through during winding.

Burst strength of the finished tube body is tested per TAPPI T807 — our minimum acceptance threshold for food-grade composite cans is 800 kPa for a 3.0mm wall and 1,100 kPa for a 4.5mm wall. Below these values, the body is at risk of panel deformation under vacuum-sealed metal end crimping.

On our winding lines, we maintain mandrel temperature at 40–55°C and adhesive open time at 8–12 seconds — deviating outside this window causes delamination at the ply interface that only becomes visible after 30–45 days in humid storage.

Barrier Liner Selection: WVTR, OTR and Food-Contact Compliance #

The liner is the most specification-critical component in a composite can. It must simultaneously provide the required moisture and oxygen barrier, survive the winding process without cracking or delaminating, and comply with food-contact regulations where applicable.

We work with four primary liner constructions depending on the product’s shelf-life target and regulatory market:

Aluminium foil laminate (ALU/PE or ALU/PET/PE): WVTR below 0.5 g/m²/day at 38°C/90% RH; OTR below 0.1 cc/m²/day at 23°C. This is our default specification for coffee, protein powder, and any product with a 12–24 month shelf-life target. Foil gauge is typically 7–9 µm; below 7 µm we see pinhole formation during winding that compromises barrier integrity.

Metallised PET/PE laminate: WVTR 1.0–3.0 g/m²/day; OTR 2.0–5.0 cc/m²/day. Suitable for tea, dried herbs, and products with 6–12 month shelf-life. Lower cost than foil but not appropriate for high-fat or oxygen-sensitive products.

PE/EVOH/PE co-extrusion: WVTR 2.0–4.0 g/m²/day; OTR 0.5–2.0 cc/m²/day. Good oxygen barrier with no foil, preferred for brands targeting recyclability. EVOH layer must be minimum 15 µm to maintain OTR performance after winding stress.

Uncoated kraft liner: No meaningful barrier. Used only for non-food applications — craft packaging, candles, cosmetic accessories — where aesthetics rather than protection drive the brief.

All food-contact liners we supply are compliant with FDA 21 CFR 176.170 (paper and paperboard in contact with aqueous and fatty foods) and EU Regulation 10/2011 (plastic materials in contact with food) where PE or PET layers are present. We require a migration test report from our liner supplier for each new food-contact SKU, and we retain these on file for customer audit.

Metal End Specification: Tinplate Grade, Seam Geometry and Crimp Integrity #

The metal end — whether a full-panel easy-open, a ring-pull, or a solid friction-fit lid — is the second most common source of field failures we see on composite cans. The two failure modes are: (1) seam leakage at the crimp interface between the metal end and the kraft body, and (2) end panel distortion under internal pressure or vacuum.

We source tinplate ends to GB/T 2520 (equivalent to ASTM A623) in T2–T4 temper grades. For vacuum-packed products, T3 or T4 temper (Rockwell hardness HR30T 57–65) is mandatory — T2 temper panels deflect visibly under 0.3 bar vacuum and consumers interpret this as product damage.

Crimp depth at the body-to-end interface is held at 3.2–3.8mm on our seaming equipment. Below 3.0mm crimp depth, we see seam separation rates above 0.8% in drop testing per ISTA 2A protocol. Above 4.0mm, the kraft body wall at the crimp zone can split if the wall caliper is below 3.5mm.

For easy-open ends, the score depth is set to achieve an opening force of 15–25 N — below 15 N the score is too shallow and the panel tears irregularly; above 30 N the end is not accessible to elderly consumers and fails EU accessibility guidance under EN 17049.

Specification Notes for Brand Partners #

When you brief us on a composite can project, the five things we need before we can develop an accurate quote and structural sample are: (1) product type and fill weight, (2) target shelf life and primary market (this determines liner specification and food-contact compliance requirements), (3) can diameter and height, (4) end type — friction lid, easy-open, or ring-pull — and (5) whether the outer surface will carry direct print or a paper label.

The most common brief mistake we see is brands specifying “standard kraft tube” without defining the barrier requirement. A 12-month shelf-life coffee product and a 3-month shelf-life bath salt use the same outer appearance but completely different liner constructions — quoting without this information produces a price that is either 20–35% too low (foil liner omitted) or unnecessarily high.

Our typical process: structural specification and digital proof in 5–7 working days, physical winding sample with metal ends in 12–18 working days, production lead time 25–35 working days after sample approval. MOQ on composite cans is typically 5,000 units for standard diameters (65mm, 73mm, 99mm) and 10,000 units for custom tooled diameters.

Frequently Asked Questions #

Q1: What wall thickness do you recommend for a 500g protein powder composite can?
A: For a 500g fill weight, we specify a 4.0–4.5mm finished wall thickness using 3–4 plies of 250–300 GSM kraft. This achieves a burst strength above 950 kPa (TAPPI T807), which is our minimum threshold for powder products that will be vacuum-sealed with a metal end.

Q2: What is your MOQ and lead time for a custom diameter composite can?
A: For custom tooled diameters outside our standard 65mm, 73mm, and 99mm range, our MOQ is 10,000 units. Lead time is 25–35 working days after sample approval, with physical winding samples available in 12–18 working days from brief confirmation.

Q3: Which food-contact regulations apply to the barrier liner in your composite cans?
A: All food-contact liners we supply comply with FDA 21 CFR 176.170 for paper and paperboard components, and EU Regulation 10/2011 for any PE or PET laminate layers. We retain migration test reports from our liner suppliers on file and can provide these for customer audits or retailer compliance submissions.

Q4: Can you print directly on the kraft body, or does it require a label?
A: We can print directly on the outer kraft wrap using flexo or offset, provided the substrate is a coated kraft at 250–300 GSM with surface roughness below 2.5 µm Ra. For photographic imagery or Pantone-matched brand colours requiring Delta-E below 2.0, we recommend a pre-printed paper label laminated to the outer wrap, which gives us tighter colour control than direct tube printing.

Q5: What causes seam leakage at the metal end crimp, and how do you prevent it?
A: Seam leakage almost always traces back to crimp depth falling below 3.0mm or a mismatch between the metal end flange diameter and the wound tube OD tolerance. On our seaming equipment, we hold crimp depth at 3.2–3.8mm and maintain tube OD tolerance at ±0.3mm — we check both parameters at the start of every production run and at 500-unit intervals using go/no-go gauges.

Planning a composite can project? Contact our team to request a complimentary specification review and sample quote.