📌 Key Takeaways

Pre-qualification testing prevents costly trial-order failures by verifying supplier claims with independent lab data before you commit resources.

• Evidence Beats Promises: Request standardized lab tests citing specific ISO or TAPPI method IDs (like ISO 536 for basis weight or ISO 1924-2 for tensile strength) to eliminate vendor bias and create clear accountability from the start.
  
• Sampling Integrity Determines Data Quality: Draw samples per ISO 186 from three rolls across the production lot—head, middle, and tail positions—then condition at 23 ± 1 °C and 50 ± 2% RH per ISO 187 to ensure results reflect real manufacturing variation, not cherry-picked material.
  
• Directional Balance Reveals Hidden Problems: Evaluate MD/CD balance in tensile and compression tests rather than just checking individual minimums; extreme imbalances signal process instability even when both values technically pass specification.
  
• Measurement Uncertainty Matters at Spec Limits: Demand that labs report measurement uncertainty alongside test results—when values fall near your tolerance limits, uncertainty data determines whether an apparent pass or fail is statistically meaningful.
  
• Complete Documentation Prevents Disputes: Require labs to deliver method versions, instrument calibration dates, statistical summaries (n, mean, SD, min/max), chain-of-custody logs, conformity statements, and raw data files in CSV or XLSX format for deeper engineering analysis.

Smart procurement creates clarity before commitment, not cleanup after problems emerge.

Procurement managers and quality assurance professionals in global packaging conversion will find immediate value here, preparing them for the detailed testing protocols and lab reporting frameworks that follow.

Pre-qualification testing stands between you and costly surprises. When a kraft paper supplier submits a proposal, their spec sheet might look flawless—but those numbers mean nothing without independent lab verification. By the time you’ve committed to a trial order and discovered the material doesn’t perform as promised, you’ve already invested time, money, and production capacity.

The solution is straightforward: request standardized lab tests before you commit to anything beyond initial discussions. This approach gives you verifiable data, eliminates vendor bias, and creates a clear foundation for negotiation. You’re not questioning the supplier’s integrity; you’re establishing a professional baseline that protects both parties.

This guide provides a ready-to-use testing request list, complete with specific ISO and TAPPI method references, sampling protocols, and lab reporting requirements. The goal is simple: equip you with a standardizable framework that you can issue to any supplier or third-party lab, ensuring consistent, comparable results across multiple pre-qualification exercises.

Why Test During Pre-Qualification

Most procurement teams test after placing a trial order. This backward approach creates risk. If the material fails to meet expectations during production trials, you’ve already committed resources to receiving, storing, and attempting to use substandard stock. The supplier relationship starts with friction, and you’re left negotiating remediation from a weak position.

Testing at the PQQ stage flips this dynamic. You establish performance expectations based on evidence, not claims. The lab data becomes part of the qualification record, creating clear accountability. When you eventually move to trial orders and full production, you have a baseline for ongoing quality verification.

Pre-qualification testing also reveals which suppliers understand quality control. A supplier who resists independent testing or provides vague sampling procedures signals future problems. Those who cooperate professionally and provide proper chain-of-custody documentation demonstrate they run a mature operation.

Essential Test Panel and Method IDs

The testing panel below covers the mechanical properties, moisture behavior, and surface characteristics that affect converting performance and end-use functionality. Each test must cite a specific method ID and version—generic descriptions like “tensile strength test” are insufficient for dispute resolution or audit purposes.

Core Mechanical and Physical Properties

These tests establish the fundamental strength characteristics of the kraft paper. They’re applicable across most grades, from sack kraft to converting grades, and form the baseline for any technical qualification.

Grammage (basis weight) determines the mass per unit area and directly affects cost calculations and performance expectations. Request testing per ISO 536 or TAPPI T410, with results reported in g/m². This measurement seems basic, but inconsistent grammage across a roll causes tension control problems during converting and affects the predictability of downstream processes.

Thickness (caliper) affects bulk properties, stiffness perception, and printability. Specify ISO 534 or TAPPI T411, with results in micrometers. Caliper variation within a lot signals process instability at the mill, which often correlates with other hidden quality issues.

Tensile strength and tensile energy absorption (TEA) in both machine direction (MD) and cross direction (CD) reveal how the paper performs under stress. Use ISO 1924-2 (constant rate of elongation method) or TAPPI T494. The TEA value is particularly valuable because it indicates how much energy the paper can absorb before failure—critical for applications involving shock loads or impact resistance. Report tensile in kN/m and TEA in J/m².

Burst strength (Mullen test) measures resistance to rupture under pressure, which matters for packaging applications and bag performance. Request ISO 2758, with results in kPa. While burst strength has lost some relevance in modern converting, it remains a useful indicator of fiber bonding quality.

Tear resistance (Elmendorf) shows how easily the paper propagates a tear once initiated. Specify ISO 1974 or TAPPI T414, reported in mN. High tear resistance in the CD is particularly important for applications where the paper experiences handling stresses perpendicular to the machine direction.

Short-span compression strength (SCT) predicts performance in applications involving compressive loads, particularly relevant for corrugated medium and linerboard. Request ISO 9895, reported in kN/m. This test provides insight into the fiber’s inherent strength characteristics, independent of formation effects.

Moisture, Sizing, and Porosity

These properties govern runability during converting and determine how the paper interacts with adhesives, inks, and moisture during use.

Moisture content affects dimensional stability, strength properties, and converting behavior. Specify ISO 287 or TAPPI T412, with results in percentage. Paper that arrives outside your target moisture range will behave differently than the tested sample, creating process headaches. The test must be performed after proper conditioning to provide meaningful results. Prioritize moisture alignment first—it drives the stability of nearly every other mechanical property.

Water absorptiveness (Cobb test) measures how much water the paper absorbs in a specified time, which directly affects print quality, adhesive penetration, and barrier performance. Request ISO 535, explicitly stating 60 seconds unless your application requires a different interval. The test can be run on either surface—specify which if your use case makes this critical. Report results in g/m².

Air permeance or porosity indicates how easily air flows through the paper structure. The ISO 5636 series covers multiple methods; the Bendtsen method (ISO 5636-3) and Gurley method (ISO 5636-5) are most common. Your choice depends on the expected porosity range and industry conventions in your sector. High porosity matters for breathable packaging; low porosity matters for air barriers.

Stiffness and Converting Behavior

These tests help predict how the paper handles during converting operations and how the finished product feels and performs.

Bending resistance (Taber stiffness) indicates how much force is required to bend the paper, which affects bag formation, folding behavior, and perceived quality. Request ISO 2493-1, reported in mN·m. Stiffness in both MD and CD provides insight into the fiber orientation and formation characteristics.

Surface roughness (Bendtsen method) affects print quality, coating adhesion, and tactile properties. Specify ISO 8791-2, with results in mL/min. Roughness variation across a roll can cause print defects and inconsistent lamination, making this test valuable even if surface characteristics aren’t the primary concern.

Optional Tests Based on Use Case

For applications involving printing, coating, or adhesive lamination, pH testing per ISO 6588-1 or ISO 6588-2 reveals potential compatibility issues. Acidic paper can degrade adhesive bonds or cause print problems over time.

If your application involves directional moisture exposure, request orientation-specific Cobb testing per ISO 535, explicitly stating whether you need edge (CD) or top/bottom (MD) measurements.

Sampling and Conditioning That Avoids Disputes

Poor sampling invalidates everything downstream. The lab might execute perfect test procedures, but if the sample doesn’t represent the lot, the data is meaningless. Specify these requirements clearly in your testing request to avoid arguments later.

Sample Draw and Representation

Request samples drawn per the ISO 186 standard to prevent cherry-picking and ensure lot representation. Specifically, request samples from three rolls distributed across the production lot, not three consecutive rolls from the same position in the production run. Within each roll, pull test sheets from the head (beginning), middle, and tail positions. This approach captures any variation that occurs during the production run and across the winding process.

Explicitly prohibit end-of-roll sampling unless you’re specifically qualifying end-of-roll material. The outer wraps of a roll often show different properties due to tension effects and environmental exposure during storage and shipping.

Conditioning Requirements

All testing must occur after conditioning per ISO 187: standard atmosphere of 23 ± 1 °C and 50 ± 2% relative humidity. The lab must report the equilibrium time—how long the samples remained in the conditioning room before testing. Paper properties, particularly moisture-dependent characteristics like tensile strength and stiffness, change dramatically with conditioning. Without proper equilibration, you’re comparing samples that exist in different physical states.

Paper typically requires 24 to 48 hours to reach equilibrium, depending on the initial moisture content and the thickness of the material. Rushing this step to meet a deadline produces unreliable data.

Replicates and Statistical Reporting

Specify a minimum of 10 test pieces per direction (MD and CD) for properties where directional differences matter. The lab must report not just the mean value but also the standard deviation, minimum, maximum, and the number of specimens tested. This statistical detail reveals consistency—or lack thereof.

A kraft paper sample might meet your target mean tensile strength but show a standard deviation so large that individual sheets across the lot will vary by 30%. That’s a production disaster waiting to happen, and you won’t catch it if the lab only reports averages.

Chain of Custody and Documentation

Every sample must carry a unique identifier that ties back to the specific roll and lot. Use tamper-evident seals to maintain sample integrity during transit to the lab. Record the date, time, and environmental conditions when samples were drawn.

Retain a buyer-side sample under proper storage conditions. If disputes arise or you need to run additional tests, you have material from the same lot to work with.

For critical first orders or high-value commitments, consider requiring remote video witnessing of the sample draw and testing process. Alternatively, for strategic purchases where long-term supply relationships are at stake, arrange a reference-lab cross-check on a small split sample before final onboarding. This independent verification confirms repeatability and catches systematic measurement issues early.

What to Demand in the Lab Report

A complete lab report removes ambiguity. The following elements must appear in every report you receive, with no exceptions:

• Method identification and version: “ISO 536:2019” is acceptable; “basis weight test” is not. Methods get updated, and the version matters for technical disputes.
  
• Instrument details and measurement uncertainty: The specific model of testing equipment, the most recent calibration date, and the stated measurement uncertainty for each property. Uncertainty data becomes critical when results fall near specification limits—it tells you whether an apparent pass or fail is statistically meaningful.
  
• Test atmosphere and conditioning time: Confirmation that testing occurred at 23 ± 1 °C and 50 ± 2% RH after the specified equilibration period.
  
• Operator identification and signatory: The person who ran the tests and the person who certifies the results. This creates accountability.
  
• Complete statistical summary: For every property, the report must show n (number of specimens), mean, standard deviation, unit of measurement, and the outlier rule applied. If the lab excluded any data points, they must explain why and show the raw data.
  
• Visual documentation: Photographs of the sample labels, roll markings, and any notable visual characteristics. Include the chain-of-custody log that tracks the sample from receipt through testing.
  
• Statement of conformity: An explicit statement comparing the test results to your specified acceptance criteria and declaring whether the lot passes or fails.
  
• Raw data files: Request CSV or XLSX format alongside the PDF report so your engineering team can perform distribution analysis, trend tracking, and deeper statistical evaluation.

Laboratory Accreditation

Prefer laboratories holding ISO/IEC 17025 accreditation, which demonstrates technical competence and management system quality. When requesting quotes or proposals, ask the lab to state whether paper testing falls within their accreditation scope. Some labs hold 17025 accreditation but only for certain test categories—testing paper outside their scope provides less assurance.

Non-accredited labs aren’t automatically disqualified, but accreditation reduces risk. The accrediting body conducts regular audits and proficiency testing, providing external validation that the lab produces reliable results.

Setting Acceptance Criteria Sensibly

Acceptance criteria transform test data into pass/fail decisions. The criteria must be tight enough to protect quality but realistic enough to reflect manufacturing variation. Over-specification drives up costs and eliminates capable suppliers; under-specification leads to quality problems.

Your acceptance criteria template should specify these elements for every tested property:

Target value and unit: The ideal specification, stated with precision. “90 g/m²” is clear; “around 90” is not.

Tolerance band: The acceptable deviation, expressed as either a percentage or absolute value. A ±5% tolerance on 90 g/m² means 85.5 to 94.5 g/m² is acceptable.

Lot acceptance rule: The decision logic for the entire lot. A common rule is “mean value within specification AND no more than 1 of 10 individual measurements outside specification by more than Y%.” This approach acknowledges that manufacturing processes have inherent variation while preventing the lot from containing too many outliers.

When evaluating directional properties like tensile strength or SCT, don’t just check whether each direction clears its minimum threshold. Examine the MD/CD balance—extreme imbalances signal process issues even when both values technically pass. A roll with 85 kN/m MD tensile and 35 kN/m CD tensile might meet individual specs but will cause directional tearing problems during converting.

Critical fail triggers: Conditions that automatically reject the lot regardless of mean performance. For example, “moisture content greater than 8% at 23°C/50% RH” or “any test conducted using an unreported or unapproved method” might be automatic disqualifiers for your application.

When results fall near your tolerance limits, evaluate them in context with the lab’s stated measurement uncertainty. A result reported as “89.5 g/m² ± 1.2 g/m²” against a specification of “90 g/m² minimum” might actually be a statistical pass when uncertainty is properly considered. Conversely, a supplier showing consistent high spread across multiple properties—even when means are acceptable—warrants a repeat test on fresh material before approval.

The acceptance criteria become part of your purchase order terms. During ongoing quality audits, they provide the standard for lot acceptance. If you later need to reject material, these criteria—established during PQQ—give you clear contractual grounds.

Strike a balance between precision and practicality. Setting tensile strength tolerance at ±2% might feel rigorous, but if that level of control is unusual in the industry or requires premium pricing, you’re over-engineering. Review your criteria against actual production requirements and, where possible, benchmark them against industry standards or similar supplier relationships.

Ready-to-Use Testing Request Template

The framework above translates into a concise request you can copy and adapt for your specific requirements. Here’s professional language you can issue directly to suppliers or third-party labs:

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Please test the provided kraft paper samples using the following protocols:

Sampling: Per ISO 186

Conditioning: 23 ± 1 °C and 50 ± 2% RH per ISO 187; report equilibration time

Required tests and methods:

• ISO 536 — Basis weight (g/m²)
• ISO 534 — Thickness/caliper (µm)
• ISO 1924-2 — Tensile strength MD/CD (kN/m) and TEA (J/m²)
• ISO 1974 — Tear resistance MD/CD (mN)
• ISO 2758 — Burst strength (kPa)
• ISO 9895 — Short-span compression MD/CD (kN/m)
• ISO 287 — Moisture content (%)
• ISO 535 — Water absorptiveness, 60s (g/m²)
• [ISO 5636-5 — Air permeance/Gurley, if applicable]

Report requirements: For each test, provide method version, instrument model, calibration date, measurement uncertainty, test atmosphere confirmation, n/mean/SD/min/max, and MD/CD orientation where applicable. Include sample photos, batch/lot ID, chain-of-custody documentation, and statement of conformity to provide acceptance criteria. Deliver signed PDF report plus raw data in CSV/XLSX format.

Lab credentials: State ISO/IEC 17025 accreditation status and scope, or provide equivalent competency verification.

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Adapt the acceptance criteria to match your grade datasheet and converting requirements. For high-stakes qualifications involving strategic suppliers or large-volume commitments, consider establishing an annual reference-lab program where a neutral third party periodically cross-checks results to maintain long-term data compatibility.

Download the Testing Request Sheet and Lab Report Template

The framework above translates into two practical tools: a Testing Request Sheet that you can issue to suppliers or third-party labs, and a Lab Report Template that specifies exactly what the lab must deliver.

The request sheet lists every test, references the specific ISO or TAPPI method, states the required number of replicates, and includes the sampling and conditioning protocols. The lab report template provides a structured format covering all required elements—method IDs, statistical data, measurement uncertainty, chain-of-custody documentation, and the conformity statement.

These templates create consistency across multiple PQQ exercises. When you’re evaluating three potential suppliers, identical testing protocols ensure you’re comparing equivalent data. The templates also serve as training tools for procurement staff who need to manage technical qualification processes without deep materials science expertise.

Share these tools internally with operations teams, quality managers, and procurement colleagues working on related materials. The standardization reduces friction and accelerates future qualification cycles.

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Related Resources:

• Kraft Paper Supplier Pre-Qualification & Documentation
• How to Interpret Lab Test Reports
• Remote vs. On-site Supplier Audits
• Kraft Paper Supplier Audit & Compliance Program

Explore more practical guides at PaperIndex Academy, discover qualified suppliers through our supplier directory, or join PaperIndex to access our global network of pulp and paper professionals.

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