Corrugated Board Liner & Medium: Kraft vs Testliner GSM & Burst Strength Data

Overview #

Selecting the right liner and medium combination for a corrugated transit carton is one of the most consequential specification decisions a brand partner makes — it directly determines whether your product survives a 1.2-metre drop test, a 72-hour humidity cycle, or a 10-day ocean freight journey. This guide is most relevant to brands shipping consumer goods, electronics, food products, or industrial components in B2C or B2B corrugated cases. The single most important insight we can share from our production floor: burst strength is not just a paper grade number — it is a system property that depends on the liner GSM, medium flute profile, and adhesive bond quality together, and we test all three before releasing any production run.

Liner & Medium Paper Grades: Kraft vs Testliner — What the Numbers Actually Mean #

When a brand partner asks us to quote a corrugated transit carton, the first question we ask is: what is the gross weight of the packed case, and what is the shipping route? Those two inputs determine whether we specify virgin kraft liner or recycled testliner — and at what GSM.

Virgin kraft liner is produced from long-fibre softwood pulp. On our production line, we source kraft liner in 125 gsm, 150 gsm, and 175 gsm grades. Ring crush test (RCT) values for 150 gsm kraft liner typically run 220–260 N/m per ISO 12192. Burst strength for a finished 3-ply B-flute board using 150 gsm kraft outer liner and 120 gsm semi-chemical medium will typically reach 1,200–1,400 kPa when tested per ISO 2759.

Recycled testliner (TL2 or TL3 grade) uses short-fibre recycled furnish. At equivalent GSM, testliner delivers 15–25% lower RCT and burst values than kraft. However, for domestic or short-haul shipments with packed case weights under 8 kg, 175 gsm testliner outer / 127 gsm semi-chemical medium in B-flute is a cost-effective and structurally adequate specification — and it supports our FSC-certified recycled content offering.

The medium (fluting) grade also matters. We use 112 gsm or 127 gsm semi-chemical medium (SCM) as standard. Flat crush test (FCT) values per ISO 3035 for 127 gsm SCM run 220–260 kPa — below 200 kPa we flag the reel and reject it before it enters the corrugator.

Quality Control Parameters, Measurement Methods & Acceptable Ranges #

Our QC process for corrugated board covers five primary parameters, each with a defined test method and an internal acceptance threshold that is tighter than the GB/T 6544 national standard we work to.

1. Burst Strength (ISO 2759 / GB/T 6545)
We test every board grade at goods-in and again on finished blanks. Our internal minimum for export-grade B-flute (150 gsm kraft / 127 gsm SCM / 150 gsm kraft) is 1,100 kPa. GB/T 6544 specifies 980 kPa for this construction — we hold a 12% tighter threshold.

2. Edge Crush Test — ECT (ISO 3037)
ECT is the most reliable predictor of stacking strength. For a standard B-flute export case, we require ECT ≥ 6.0 kN/m. For heavy-duty double-wall (BC-flute) cases carrying packed weights above 20 kg, we specify ECT ≥ 10.5 kN/m.

3. Moisture Content (ISO 287)
Board moisture at the time of case erection must be 7–12%. Above 12%, the board loses up to 30% of its ECT value — a critical failure mode in humid warehouse environments. We measure moisture on every production lot using a calibrated pin-type meter.

4. Adhesive Bond Strength (GB/T 6548)
We perform a peel test on corrugated board samples every 2 hours during a production run. Acceptable bond: no fibre-free delamination zones larger than 3 mm² per 100 cm² of board area.

5. Box Compression Test — BCT (ISO 12048)
For finished cases, we run BCT on a sample of 5 units per production lot. Minimum BCT for a standard 0201 RSC case (400 × 300 × 300 mm, B-flute, 150 gsm kraft) is 2,800 N. We use the McKee formula as a design check before committing to production, then validate with physical BCT.

Regulatory Compliance: Food-Contact, FSC, REACH & PPWR #

This is where we see the most gaps in brand partner briefs — and where getting it wrong creates real liability.

Food-Contact Compliance
If your corrugated case will be in direct or indirect contact with food (e.g., a retail-ready tray or a case with no inner barrier), the liner and adhesive must comply with FDA 21 CFR 176.170 (components of paper and paperboard in contact with aqueous and fatty foods) for the US market, and EU Regulation 1935/2004 plus the specific paper and board guidelines for the EU market. We maintain a Declaration of Compliance (DoC) for all food-contact board grades we stock, and we can provide full migration test data on request. Mineral oil aromatic hydrocarbons (MOAH) and mineral oil saturated hydrocarbons (MOSH) are a specific concern with recycled testliner — we source only MOSH/MOAH-screened recycled grades for any food-adjacent application.

FSC Chain of Custody
We hold FSC Chain of Custody certification (FSC-C[our cert number]). For brand partners who need FSC-certified corrugated cases — increasingly required under EU PPWR (Packaging and Packaging Waste Regulation) sustainability claims — we can supply FSC Mix or FSC Recycled certified board and provide the on-pack FSC licence number. PPWR, which entered into force in 2024 and sets mandatory recycled content targets for paper-based packaging by 2030, is already shaping how we specify testliner grades for EU-bound shipments.

REACH Compliance
All inks, adhesives, and coatings used on our corrugated lines are REACH-compliant. We maintain Safety Data Sheets (SDS) and substance declarations for all chemical inputs, and we can provide a REACH declaration confirming no SVHC (Substances of Very High Concern) above 0.1% w/w threshold.

AQL Inspection System & Defect Classification #

We apply ANSI/ASQ Z1.4 (equivalent to ISO 2859-1) sampling procedures to all finished corrugated case production. Our standard inspection level is General Inspection Level II.

For a production lot of 5,000 cases (Inspection Level II, AQL 1.0 Major), the sample size is 200 units. If 6 or more major defects are found in the sample, the lot is rejected and 100% inspection is triggered. We document all inspection results on our internal QC report, which is shared with the brand partner as part of the shipment documentation package.

Print registration on our flexo corrugated lines holds to ±1.0 mm under normal run conditions. For brand partners requiring tighter register — for example, QR codes or fine-line brand graphics — we recommend pre-print litho-laminate construction, which achieves ±0.3 mm register on our offset pre-print line.

Specification Notes for Brand Partners #

When you brief us on a corrugated transit carton project, we need the following to develop an accurate quote and structural recommendation: packed case gross weight (kg), product fragility rating if known, primary dimensions (L × W × H in mm), shipping route and mode (air, sea, domestic road), and whether food-contact compliance or FSC certification is required.

The most common brief mistake we see is specifying board grade by flute type alone — “B-flute” or “C-flute” — without specifying liner and medium GSM. Flute profile determines cushioning and stacking geometry; liner GSM determines burst and ECT. We always confirm both before sampling.

Our typical process: structural design and digital proof in 3–5 working days, physical sample (unprinted or printed) in 8–12 working days, production lead time 15–20 working days after sample approval for standard RSC cases. For complex die-cut or litho-laminate constructions, add 5–7 working days. We provide a full compliance documentation pack — including board test reports, FSC certificate copy, food-contact DoC where applicable, and AQL inspection report — with every production shipment.

Frequently Asked Questions #

Q1: What burst strength should I specify for a corrugated case shipping 10 kg products by sea freight?
A: For a packed case weight of 10 kg on an ocean freight route, we recommend a minimum burst strength of 1,200 kPa, which corresponds to a B-flute or C-flute construction using 150–175 gsm kraft outer liner and 127 gsm semi-chemical medium. We test burst strength per ISO 2759 on every production lot and hold an internal minimum 12% above the GB/T 6544 baseline.

Q2: What is your MOQ and lead time for a standard corrugated transit carton?
A: Our standard MOQ for a plain or single-colour flexo RSC case is 1,000 units. For full-colour litho-laminate cases, MOQ is typically 2,000 units due to plate and lamination setup costs. Production lead time is 15–20 working days after sample approval for standard constructions.

Q3: Can you supply FSC-certified corrugated cases for our EU market shipments?
A: Yes — we hold FSC Chain of Custody certification and can supply FSC Mix or FSC Recycled certified board. This is increasingly relevant under the EU PPWR regulation, which sets mandatory recycled content targets for paper-based packaging phasing in through 2030. We provide the FSC licence number for on-pack use and include the FSC certificate copy in your compliance documentation pack.

Q4: Can you print brand graphics directly on the corrugated case, and what register tolerance should I expect?
A: Direct flexo print on corrugated holds ±1.0 mm register on our lines, which is suitable for logos, barcodes, and bold brand graphics. If your design includes fine-line elements or QR codes requiring tighter tolerance, we recommend litho-laminate construction, where our offset pre-print line achieves ±0.3 mm register. We always request a print-ready PDF with 3 mm bleed and Pantone colour references before sampling.

Q5: How do you handle moisture-related failures in corrugated cases during humid season shipping?
A: Moisture content above 12% in the board at the time of case erection can reduce ECT by up to 30%, which is the leading cause of stacking failures in humid transit conditions. We measure moisture on every production lot using a calibrated pin-type meter and target 7–12% at point of packing. For shipments to high-humidity destinations (Southeast Asia, coastal US in summer), we recommend a water-resistant adhesive specification and PE-coated inner liner — we can quote both options.

Planning a corrugated transit carton project? Contact our team to request a complimentary specification review and sample quote.