Multi-Ply Kraft Paper Bags with UN 5M2 certification

Multi-ply kraft paper bags (often called multiwall paper sacks) are robust, industrial-grade containers engineered to handle large quantities of dry bulk goods (typically 5–50 kg per bag). They consist of two or more layers of high-strength kraft paper, bonded together with adhesives or heat sealing. At first glance, the bag looks like ordinary paper, but its durability comes from the internal layered architecture. Each ply serves a purpose: the outer layers provide a tough, printable surface that resists abrasion, while the inner layers carry most of the load. The result is a container far more robust and adaptable than a single sheet of paper or a simple plastic bag.

These multi-ply sacks function as a highly tunable packaging system. By varying the number of plies, selecting specific paper grades (bleached, unbleached, coated, etc.), adding liners or barrier coatings, and choosing appropriate closure methods, engineers can meet precise requirements for strength, barrier protection, and filling efficiency. In the following sections, we explore these engineering choices in detail, covering everything from material selection to manufacturing processes. We also emphasize regulatory compliance and safety: for example, our bags meet the UN’s stringent 5M2 certification for transporting hazardous materials by sea, ensuring full compliance with international dangerous-goods regulations.

Structure and Material Composition

A multi-ply kraft paper bag is essentially a tube formed from two or more plies of kraft paper that is sealed or sewn into a pouch. The plies are typically sack-grade kraft paper, with basis weights ranging roughly from 70 to 120 grams per square meter (gsm) per ply. Commonly, an all-paper bag has two to four plies, while very heavy-duty or specialized bags may have five, six, or more plies – often including dedicated film liners or reinforcement layers. Each ply is slit and supplied from a paper reel, then run through a form-and-glue machine on high-speed equipment.

Ply Roles and Layering

• Outer Plies: The outermost layers usually have a smooth or clay-coated surface for high-impact flexographic printing, logos, and product information. They also must resist abrasion and scuffing during handling and palletizing. In dusty or humid environments, an outer ply can be water-repellent or sized to shed moisture and oil. Printing on these layers typically uses water-based or eco-friendly inks.
• Inner Plies: Inner layers are chosen primarily for strength and load-bearing capacity. They are often unbleached (natural brown) kraft for toughness, though bleached white kraft can be used on the interior for a clean look if needed. These plies may be multiple to achieve the desired burst and tear strength. If extra barrier is required, one inner ply might be a special barrier paper (denser or with coating) rather than pure kraft.
• Barrier Plies: When products are sensitive to moisture, aroma, or grease, the bag may include one or more functional plies. Options include a loose polyethylene (PE) film liner inserted inside the paper tube or a paper ply with a laminated plastic coating (e.g. PE or PP). These liners drastically reduce moisture vapor transmission and prevent dust leaks. For instance, a 50–60 µm thick PE liner inside a flour bag can cut moisture pickup from >1% to under 0.3% in a high-humidity test chamber.

Ply weights and counts are balanced for cost and performance. A typical all-paper bag might use three plies of 90–100 gsm each (total ~270–300 gsm), whereas a bag with a PE liner might use lighter papers (e.g. two 70–80 gsm papers + a 60 µm film). More plies mean higher weight and strength but also greater stiffness and cost. Engineers choose just enough layers to meet the application’s requirements without excess. In later sections we show how adding plies or liners affects the bag’s burst rating, drop resistance, and moisture barrier.

Assembly: Adhesives, Seams, and Seals

To turn the paper plies into a finished bag, several assembly steps are used:

• Forming the Tube: The kraft plies are overlaid and run through a tubular forming machine. Side seams are made by applying adhesive and folding or by in-line hot-melt. Commonly, a water-activated starch or dextrin glue is applied as bars along the overlap, then pressed and heated to form a rigid seam. Some modern lines use polyvinyl acetate (PVA) or hot-melt adhesives (nozzle-deposited) for faster curing.
• Bottom Closure: For open-mouth bags, the bottom end is formed after filling. In valve and pasted-end bags, the bottom is sealed during bag-making. A paper bottom patch is usually pasted across the interior of the bottom, reinforcing the seal and preventing sifting of powders. The bottom adhesive is often a hot starch glue that cures as the bag passes under heated pressure rolls. Square bottoms or satchel bottoms can be formed to help the bag stand upright.
• Seams and Overlaps: At each overlap of ply edges, the adhesive overlap must be sufficient (typically 15–30 mm) so the seam doesn’t tear out under stress. Paper grain direction is often alternated between layers (machine vs. cross direction) to balance tear and burst in both directions. Modern baggers precisely align and laminate the layers to avoid wrinkles, which can weaken the structure.
• Quality Gates: During assembly, many checks are in place. Lines often sample the basis weight of the tube and liner, check thickness, and use vision systems to detect misfolds or voids. After bagging, random samples go to lab tests. Basis weight (TAPPI T-410) and caliper (TAPPI T-411) confirm proper paper. Adhesion and seam strength are tested (e.g. ASTM D903 for bond). Holes or weak seams cause automatic rejection. On high-end lines, each bag’s side seam is monitored by inline infrared or ultrasound sensors for glue application.

Ply Configurations and Performance

The number of plies in a kraft bag dramatically affects its strength, durability, and weight. Here is how common ply counts are used and what they deliver:

• 2-Ply Bags (Entry-Level Strength): Typically built from two layers of 70–90 gsm kraft. These bags are suitable for light-duty products or gentle handling – for example, retail bags of sand, salt, or simple agricultural products. Strength parameters are modest (burst ~300–500 kPa, moderate tear resistance), and they may flex more under load. Two-ply sacks excel in print appeal and low cost, but they survive only a few 1.0 m drops (often 2–3 drops) before failing. They are not ideal for very abrasive or heavy powders unless extra care is taken (e.g. plastic lining or pallet corner protection). A real-world example: a small stone-sand supplier switched from woven sacks to 2-ply kraft for retail, banking on better graphics. They compensated for the slightly reduced drop endurance by lowering their pallet stack height and adding an anti-skid varnish.
• 3-Ply Bags (Workhorse Grade): A common construction is outer 90 gsm / inner 80 gsm / inner 80 gsm (all kraft), or variants with one coated inner ply for a little barrier. These are the workhorses of industry, used for flour, sugar blends, feed, pigments, and many mineral powders. They typically achieve burst of 450–700 kPa and can survive ~3–5 drops from 1.0 m when well-manufactured. 3-ply bags offer a good balance of strength and cost. If modest moisture protection is needed, a thin liner (e.g. 30–60 µm PE) can be added. For example, a flour mill using 3-ply sewn valve bags with a loose 60 µm liner found that the dusting at the filler was cut by about one-third and air quality improved, even though the stitch line had minor needle-hole leaks. Their keys are sealed enough for most dry goods while keeping the bag light.
• 4-Ply Bags (Heavy Duty / High Barrier): By adding a fourth ply, often a specialized barrier or reinforcement layer, these sacks target demanding products. A typical 4-ply might consist of three paper plies (80–110 gsm) plus an inner 40–80 µm polyethylene film or HDPE scrim ply. The mechanical strength jumps (burst ~600–900 kPa), and moisture resistance is greatly improved. These bags are used for export-grade powders (like flour or milk powder), chemical powders, or any cargo that needs strict hygiene. In testing, a 4-ply bag with a 60 µm liner saw moisture gain drop by ~70% versus a 3-ply control. A dairy co-packer, for instance, switched to 4-ply valve bags with 60 µm liners for infant lactose; moisture uptake over 8 weeks at 30°C/75% RH went from ~1% in 3-ply bags down to ~0.3% in 4-ply, cutting spoilage. The trade-off is added stiffness and weight (often 190–260 g per 25 kg bag), but for many high-value products the protection is worth it.
• 5-Ply Bags (High-Risk Logistics): Five layers, often including a reinforced inner ply, are chosen for the most punishing conditions. A typical 5-ply might have four kraft plies plus a scrim/HDPE reinforcing ply. These are used when bags face long sea voyages in monsoons, very high stacking loads, or abrasive materials. The extra structure prevents corner splitting and bag bursting. For example, a pigment exporter experiencing ~2–3% corner splits in 3-ply sacks switched to 5-ply pinch-bottom bags with an inner scrim layer. Post-change, corner ruptures fell to near 0.3% on a standard drop test, and pallets arrived uniformly square despite rough handling. The downside is that 5-ply bags are heavier (230–320 g per 25 kg bag) and cost more, but they can prevent the multi-thousand-dollar losses that a single shipment of ruptured chemicals would incur.
• 6–7 Ply Bags (Specialty / Ultra-Barrier): Beyond five plies, bags are usually built for niche needs (extreme barriers, precision environments, or branding). For instance, a very moisture-sensitive powdered chemical or a food with strict oxygen requirements might use a 6- or 7-ply bag that includes one or more aluminum-laminated paper plies or multiple film layers. These achieve only marginal additional strength (since four to five plies already covered that), but they greatly improve barrier, extend shelf life, or control odors. Such designs are rare and expensive; they demand careful handling (overlapping multiple layers can create stiff “hinge” lines if not well-designed). In general, the more plies, the smaller the incremental gains in burst/tear. The key is that every extra ply should serve a purpose—extra strength, extra barrier, or some specialty function.

In practice, bag engineers aim for the lightest construction that meets all the performance targets. This minimization keeps costs and weight down while still passing all tests. For example, in some cases a 3-ply bag with a thin PE liner can outperform a 5-ply all-paper bag in moisture-sensitive applications, at lower cost. The ideal design always depends on the product’s density, abrasiveness, sensitivity to moisture or grease, and the expected handling and climatic conditions during distribution.

Barrier Technologies: Moisture, Grease, and Venting

Multi-ply bags often need to protect sensitive contents from water, oils, oxygen, or trapped air. There are engineering solutions for each challenge:

• Loose Polyethylene (PE) Liners: A common approach is to insert a separate tubular liner inside the bag before filling. These liners (usually 20–100 µm thick) are typically high-density polyethylene (HDPE) or low-density PE. A loose liner gives the highest possible moisture and dust barrier, because it is effectively a complete plastic inner bag. However, it requires careful handling on the fill line (the operator must keep the liner unfolded). With a 60 µm PE liner, tests have shown flour sacks gaining only ~0.2–0.3% moisture after 96 hours at 38°C/90% RH, compared to >1% for plain paper sacks. The trade-off is extra material cost and the need to manage the empty liner after filling. We supply liners that slip easily to avoid jamming and use high-strength films to prevent tears during handling.
• Coated or Laminated Inner Plies: Instead of a loose liner, many bags use an inner paper ply that is pre-coated or laminated with plastic (for example, a thin layer of polyethylene or a metallized film). This creates a semi-integral liner: the bag can still be sewn or sealed normally (since the plastic is part of the paper layers), but benefits from much lower permeability. Coated inners are easier for high-speed baggers because you don’t have to feed a separate liner. They typically achieve 80–90% of the barrier of a loose film. Many food-grade bags use PE-coated inner plies for their clean properties. For instance, sugar and salt bags often use a white PE-coated paper inside; they look just like all-paper bags externally but do not cake in humid warehouses.
• Grease/Oil-Resistant Papers: For oily or fatty contents (pet food, snacks, olive pomace, etc.), the inner ply can be a grease-resistant kraft. This is kraft paper made with special sizing (often fluorine-free oil repellent) that prevents fats from soaking through. We qualify these using the TAPPI T-559 “Kit test” or specific oil-penetration tests. A 20–30 gsm grease-resistant inner ply can stop visible oil stains on the outside. Sometimes we combine a grease-resistant paper with a thin PE layer for double insurance. This way, oil-rich products like flavored biscuits or fatty feed show clean faces on store shelves.
• Oxygen/Aroma Barriers: Though rare in simple kraft bags, high-barrier laminates (e.g. aluminum foil or EVOH) can be included if needed. Because they ruin recyclability, we only use foil linings in very specialized situations (e.g. a 7-ply bag for light-sensitive chemicals). More often, an extra-dense polypropylene (PP) inner ply or a thicker PE laminate is used for aromatic products. Without foil, the main goal is usually to prevent moisture, not oxygen.
• Micro-Perforations for Venting: A unique issue is trapped air or gases during filling. If air cannot escape, a bag can inflate or clog the valve. Small perforation holes in one of the plies act as vents. We tune perforations carefully: too many holes and you start losing fine product (sifting), too few and the bag “balloons” on fill. In practice, a certain pattern of tiny holes in the inner ply (covering a few percent of the area) lets air out during valve filling without letting powder escape. This speeds filling and ensures valves seat flat. We often specify, for example, 1–2 perfs per 100 cm² for large-grain products, and up to 5–10 perfs per 100 cm² for very fine powders on air-fill machines. Each customer’s filling equipment (air pressure, fill spout geometry) helps determine the exact perforation scheme.
• Balance of Features: The art of barrier design is to juggle these elements. For instance, adding a liner solves both moisture and dust at once. In very dry climates or for inert products, we might omit liners altogether to keep the bag fully recyclable. Where needed, we prefer PE as the mainstream inner liner material because it is effective and food-safe (meeting FDA and EU rules for food contact). Heavy coatings or foil layers remain niche choices due to cost and end-of-life issues.

Closure and Fill Methods

The style of the bag’s mouth and closure directly affects line speed, cleanliness, and leak risk. There are four main mouth types for multiwall sacks:

Sewn Open Mouth (SOM)

This is the classic style. The bag is made with one end open (open mouth), filled via gravity or a simple fill cone, then the top is folded and stitched closed.

• How it works: After filling, the operator (or machine) folds and stitches the top. The stitching is usually done by a chainstitch (lockstitch) sewing machine with multi-wall capacity. The mouth is typically hemmed (folded in two or more times) before sewing.
• Thread and Sewing: Polyester threads are popular for strength and abrasion resistance (ideal for 25–50 kg bags). Polypropylene thread is cheaper and chemically inert, so it’s used for corrosive products. Cotton thread can be used on lower-speed lines; it naturally swells with humidity, offering a bit of self-sealing, and is traditional for some food bags. Threads are often treated with anti-wicking finishes (silicone, wax) to slow liquid seepage along the stitch holes. Stitch density matters: coarse bags use about 6–7 stitches per inch (SPI), while fine powders often require 8–9 SPI to prevent leak paths. Too low an SPI allows puffing, too high an SPI weakens the fold like a perforation line. A tape (crepe paper or laminated) is often placed under the stitches to help seal the needle holes.
• Pros: SOM is rugged, requires relatively low investment (sewing machines are cheaper than heat sealers), and bag capacity/length is easy to change. Sewing machines are widely available and can run extremely fast (some lines do >50 bags/min). Field maintenance is straightforward (changing needles or threads).
• Cons: Not airtight. Even with tape, needle holes and imperfect folds allow some dust or moisture entry. The sewn seam forms a thick “stool” of thread that slightly reduces fillable volume. Also, if operators are careless in folding, small gaps can form. Sewn closures are generally not considered drip-proof, so they’re less suitable for very fine or hazardous powders that require zero leakage.
• Use Cases: Good for moderate-quality aggregates, feed, raw cement (where a tiny speck of dust is tolerable), and many chemicals that have backup liners inside. In food, SOM is used for items where hermetic seal is not mandated (e.g. flour for local markets) or where speed and cost trump perfect sealing.

Pinch-Bottom Open Mouth (PBOM)

These bags have a hot-melt adhesive bead applied across the open mouth before filling. After filling, an automated sealer (or hotplate) pinches and heats the mouth shut.

• How it works: The bag is made with a strip of heat-seal adhesive on the mouth. During filling, once the product stops, the bag is folded (often with one automatic fold and one manual fold) and placed under heated jaws or hot air. The heat (usually 160–200°C for ~0.5–1.0 second under pressure) activates the adhesive, welding the paper layers into a solid fiber-to-fiber bond. When done correctly, the seal is as strong or stronger than the paper itself.
• Peel Strength: A good PBOM seal should have high peel strength (often >15 N/15 mm as measured by ASTM F88 peel tests). This makes the mouth essentially dust-proof. The resulting folded face is flat and neat – an advantage for packaging presentation and pallet stability.
• Pros: Nearly sift-proof and water-resistant if the glue lines are well-sealed. Creates a square, flat top that makes pallet building easier (each bag stacks uniformly). No thread means the closure area has no holes at all. It’s a clean look and eases automatic coding/printing on the closure panel. PBOM bags can often run faster than sewn bags if the equipment is set up correctly, because filling can continue smoothly into sealing.
• Cons: Requires specialized bottomer equipment and careful control of temperature. The hot-melt adhesive or hot-air unit must be precisely calibrated; cold glue or air will produce weak seals, overheating will burn paper or catch fire. The machines are also higher-capital than a simple sewing machine. If a bag is sealed incorrectly, it usually fails catastrophically (you get a split bag rather than a slow leak).
• Use Cases: Premium or high-containment products. Many food and pet food manufacturers prefer PBOM for powders like sugar, starches, dry mixes, and pet kibbles, because they want dust-tight packaging without sewing. Also used for goods that will be repackaged in supermarkets (the flat top is ideal for easy placement and label scanning). In minerals, any time cleanliness or speed is paramount, PBOM is often chosen.

Valve Bags (Pasted-End Bags)

valve bags have one corner valve and two glued ends. They are used on high-speed automated lines.

• How it works: The tube is closed on both ends (usually by pasting paper patches over the bottom and top ends). A small valve (a short paper or plastic tube) is glued or inserted into a corner. Filling is done by placing the bag over a fill spout. The spout opens the valve and product drops in. The valve is designed to let air escape easily (often with its own tiny perforations or by venting around the edges) and then collapse or self-seal when filling stops.
• Valve Design: Valves come in various styles – basic paper flaps, film-lined sleeves, or self-closing paper flaps. A film-sleeved valve (a small PE tube inside a paper gusset) is often used for dusty materials to ensure that powder doesn’t clog or leak from the valve. Some valves have hinged flaps or mechanical closers.
• Pros: Fastest filling speeds (no manual top handling, continuous flow) and very low dust escape if designed properly. Bags naturally come out with a square block shape, which is great for pallet packing. They also leave the main faces of the bag free of seams or folds, maximizing print area.
• Cons: More complex bag construction (two pasted ends plus valve assembly) and only compatible with special fillers. If the valve geometry or a hopper’s pusher plate is mismatched, filling can be inefficient or leaky. Valve lines tend to be less flexible in product range (e.g. extremely fine or very lumpy materials can jam valves).
• Use Cases: Bulk commodities and minerals. Cement, fly ash, gypsum, chemicals, and bulk food ingredients often use valve bags. In any facility where thousands of bags per hour are needed, valves are common. Cement companies worldwide commonly use 4–5 ply valve bags to maximize throughput and keep dust contained. Where pallet cube efficiency matters, valves win.

Heat-Seal and Ultrasonic Closures

When a bag has a plastic inner liner or a coated inner ply that reaches the mouth, it can be closed by heat sealing instead of sewing.

• Heat Sealing: After filling, the bag mouth is folded (or cut square) and placed between heated jaws. The heat (typically 145–175°C) and pressure weld the inner PE/PP layer, forming a strong plastic seal. The paper stays relatively intact since only the liner melts. This method requires a liner or inner coating at least through the flap to work.
• Ultrasonic Sealing: This uses high-frequency vibrations to melt the plastic without external heat. An ultrasonic horn presses on the folded mouth where the inner liner layers overlap. The friction melts the plastic layers together. Ultrasonic can precisely seal even thin liners (10–20 µm) without heating the outer paper.
• Pros: Completely clean, no needle holes or tapes. Seals are hermetic (good for highly hygroscopic or oily products). Packaging remains flat and looks neat. Useful for food powders that must not contact sewing thread or for any product where sterility is a concern.
• Cons: Requires specialized equipment (ultrasonic drivers or heated bars) and consistent liners. Capital cost is high. Not all fillers can handle a heat-seal step (some are purely mechanical). Also, bag length must be precise so the weld falls at the right point.
• Use Cases: Food-grade chemicals, specialty spices, pharmaceutical ingredients, and any fine powder where absolutely no leakage is allowed. For example, vitamin mixes or instant coffee may use heat-sealed kraft bags. Many modern flour packing lines use heat seals on PE-lined bags to achieve grocery-store cleanliness. Ultrasonic is popular in hygienic facilities where seam integrity is mission-critical.

Mechanical Properties and Testing

To ensure reliability, multi-ply bags are subjected to standardized mechanical tests. These quantify how the bag handles stress:

• Mullen Burst Strength (TAPPI T-810): Measures the pressure required to rupture the bag wall. Higher burst rating means greater resistance to internal pressure or point impacts. For bulk sacks, 300–500 kPa is typical for light 2-ply; 600–900 kPa for 4-ply; 700–1000+ kPa for reinforced 5-ply. A practical implication: if your conveyor chute presses on the bag walls, a higher burst ensures the bag won’t bulge or split at the surface.
• Elmendorf Tear Resistance (TAPPI T-414): This measures how easily a cut tear propagates, in both machine direction (MD) and cross direction (CD). Multiwall designs alternate grain direction so that both MD and CD tear values are respectable. Typical Elmendorf values might be ~800–1500 mN for 2-ply and ~2000–3000 mN for heavy 5-ply. High tear strength is important in packaging lines – for example, if a sharp pellet or sieve edge nicks the bag, a high tear resistance prevents a tiny hole from ripping open the whole side.
• Drop Tests (ISO 7765 / ASTM D5276): Filled bags are dropped from prescribed heights (commonly 1.0 or 1.2 meters) onto hard surfaces in various orientations (corner, edge, flat). The number of drops without failure (or the condition after a set sequence) is recorded. A well-made 25 kg 4-ply bag typically survives 5–7 corner drops at 1.0 m, for instance, whereas a 2-ply might only survive 2–3. On receipt of a batch, some purchasers will demand that a certification sheet show “passes 5 drops 1.0 m without bursting.” Drop tests simulate real-life warehouse drops and truck unloadings. Always evaluate bags with the actual product inside, because heavy or dense product transfers more energy on impact.
• Compression and Stack Stability: In transit, bags are stacked in large blocks. The bottom bags can be under tons of load. A typical test is to simulate a full pallet stack (e.g. several hundred kilograms on one bag) for 24 hours. Multi-ply sacks with square bottoms and anti-skid finish can support higher stacks. We often specify an edgewise compressive strength or run ASTM D642 compressions. In practice, applying an anti-slip coating (which raises the static friction coefficient) and rounding corners greatly increases how many layers tall you can safely stack. Customers sometimes request the “angle of repose” or slip threshold for their bag.
• Seam and Seal Strength: The bag is only as strong as its closure. For sewn bags, we use ASTM D2724 (seam strength test) on stitched seams. For sealed closures, we use an F88 peel test. Targets are usually 15–20% of the bag’s own strength. For example, if a bag’s sidewall has 800 N burst, we might want the seam to hold 120–160 N per 15 mm of width. Weak seams are often the first failure in a drop test, so we stress-test them.
• Moisture and Sifting Tests: Hygroscopic tests (sometimes a Cobb water-absorption test) measure how much moisture the paper wall absorbs. For dust, we run mock fill trials and measure airborne fine particles. For very strict applications, we use a dust containment chamber. Ideally, a well-designed bag shows negligible dust even if violently shaken.
• Coefficient of Friction (ASTM D1894): Especially for palletizing, we measure the bag surface friction. A factory might coat every bag top with an anti-slip varnish if needed. A higher COF (e.g. 0.4–0.6) can allow stacking one or two more layers without sliding, which effectively increases trucking efficiency.

Meeting these performance targets requires a combination of good materials and good manufacturing. After testing, every lot of bags is typically shipped with a certificate of analysis (COA) citing the standards (TAPPI or ISO) used for each test. This traceability means if a shipment of goods was damaged, we can compare the faulty bags to the original tested samples and identify any deviation. In our experience, customers who invest in these test reports have far fewer surprises – when a retailer or insurer asks “did the packaging meet the spec?”, the answer is a documented yes.

Regulatory Compliance and Certification

Industrial packaging must often meet regulatory standards, especially in food or hazardous goods sectors. With our multi-ply bags, compliance is a core part of the design:

• Food and Feed Safety: For edible and feed products, all materials must be food-grade. The kraft papers must comply with regulations (for example, FDA 21 CFR 176.170 for paper and paperboard used in contact with dry foods, or EU 1935/2004 standards in Europe). Inks and adhesives are chosen to be non-toxic and to meet migration limits (e.g. 21 CFR 175.105 for adhesives). Often customers request certificates or declarations of compliance for each component (paper, adhesive, liner, printing ink). We source papers with clear FDA/EU compliance statements, use food-grade starch adhesives, and ensure any coatings (wax, resin) are approved. For pharmaceuticals or specialized foods, we can provide ISO 22000 or HACCP-related documentation for the bag production process.
• Hazardous Materials (UN Packaging): A key differentiator is our UN 5M2 certification. In UN packing codes, “5M2” designates a multi-wall paper bag that is water-resistant (the “2” means the bag is moisture-repellent) and approved for certain dangerous solids. Achieving 5M2 means we have built bags that passed drop tests, stacking tests, vibration tests, and a water-penetration test at a level required for packing classified hazardous goods under the IMDG (sea) and ADR (road) regulations. Our bags carry the UN mark (showing “5M2 / X / 20 / S / 10 / country / our-code”) that specifies they are rated for Packing Group I, II, or III (depending on the test level), maximum mass (e.g. 20 kg), and solid contents. This is not a casual claim: to earn 5M2, a design must include at least one moisture-barrier ply (so the bag retains strength when wet) and pass a battery of tests with a dummy substance. We work with certified labs and our own auditors to ensure each production lot has the same properties as the tested prototype. For customers shipping chemicals, fertilisers, pigments, etc., supplying a bag with official UN 5M2 marking is often a legal requirement for export. We highlight this certification to show we meet rigorous safety standards.
• Environmental and Other Standards: On the sustainability side, many clients demand responsible sourcing and recycling information. We use papers from FSC- or PEFC-certified mills and can provide chain-of-custody documentation. We hold ISO 9001 and ISO 14001 registrations at our plant to show our quality and environmental management systems are in place. If a client’s industry requires certain audits (e.g. FSSC 22000 for food companies), we help comply by supplying required paperwork. Even our liners can be specified as FDA or EU-compliant for food contact (e.g. writing “LDPE per 21 CFR 177.1520, no solvents needed”).
• Third-Party Testing: For peace of mind, clients often want independent lab verification. We partner with global test houses (SGS, Intertek, TÜV, etc.) and can furnish their test reports for dimensions, strength, burst, and especially the UN specification. Having external reports on file (tied to bag lot numbers) means if a customer audit arrives, we have documentation ready. This is increasingly common in food supply chains, where every packaging lot might be traced.

In summary, every new bag design we sell comes with clear specs and certifications as needed. We work from the design phase to ensure compliance, not just retrofitting it. For example, if a flour bag is claimed to be “food grade” but the ink or adhesive is not certified, we won’t ship it. Customers relying on our multi-ply sacks rest assured that they meet both packaging performance and regulatory requirements – including the all-important 5M2 marking for any shipment of regulated or hazardous powders by sea.

Industry Applications

Multi-ply kraft bags find use across a wide range of industries and products. Here are typical applications and why these bags are chosen:

• Food and Beverage (Dry Goods): Products like flour, sugar, starches, grains, and powdered milk are often bagged in kraft sacks. These require sanitary construction (no excess fibers inside), consistent weights, and sometimes moisture barriers. For example, a bakery supplier packing 25 kg flour often chooses a 3-ply or 4-ply valve bag with a polyethylene liner. The liner prevents moisture pickup during humid storage and stops flour dusting. The outer plies allow high-quality printing of brand and nutrition info. A pet food manufacturer might use a 4-ply sewn bag with a grease-resistant inner ply to prevent oil stains from the meat content. In all these cases, the kraft bag’s natural paper surface also makes it easy to apply water-based inks for attractive graphics.
• Animal Feed and Agribusiness: Animal feeds, seeds, and fertilizers commonly use multi-ply bags. Seeds and grains are bulk solids that benefit from the strength and ventilation features (some seed bags have small perfs to allow gas exchange). Fertilizers (like urea or NPK) are moisture-sensitive and corrosive; often 3–4 ply bags with liners are used. The robust paper withstands the weight and the rough handling in barns or fields. Some agrochemical additives or feed supplements also require the UN-certified packaging if they are classified as hazardous materials; in those cases, the UN 5M2-rated bags we offer are essential.
• Minerals, Construction Materials, and Chemicals: Cement, gypsum, lime, and bentonite have long been packed in either woven plastic or multiwall paper. Today, many cement plants are converting to 4- or 5-ply valve paper sacks for faster fill and better stacking. Pigments (titanium dioxide, iron oxide) and chemical powders (resins, fertilizers, sugar additives) also ship in kraft bags. The high burst and tear strength of multi-ply protects against rough dumping and conveyor abrasion. For example, a cement producer in Asia switched to 5-ply valve bags for overseas shipments during monsoon season; the result was far fewer burst bags in port. Specialty chemicals that are packaged in these bags often specify an inner PE layer or wax coating to meet purity standards.
• Retail and Industrial Packing: Many retail-sized products use multiwall bags too. Think salt bags, charcoal sacks, feed pellets, or charcoal briquettes. These often use 3-4 ply sewn bags with water-resistant coatings (because retail goods are often stored outdoors). The kraft bag’s ease of labeling and printing is a plus. At the wholesale industrial level, products like powdered detergents, dry milk for foodservice, or even sand and gravel for home improvement are bagged in multi-ply sacks. The key is that the bag can protect the product during long distribution while also presenting a clean face for barcode scanning and brand messaging.

Overall, multi-ply kraft bags are chosen whenever the packaging needs a balance of strength, barrier, and presentation at reasonable cost. The examples above show the versatility: from a humble bag of flour to a heavy sack of pigment, the design is adjusted but the principle is the same. For a quick overview of formats and options, see our product guide on Multi-Ply Kraft Paper Bags.

Operational Fit: Filling Lines and Handling

Even the best-designed bag must perform on the customer’s equipment and in logistics. Key operational factors include:

• Compatibility with Filling Equipment: We always consider how a bag will interface with the filling machines. For valve fillers, the bag’s valve size, position, and perforations must match the fill nozzle. Too much valve area or too many holes can let product escape; too little and the bag inflates. For example, an automatic air-packing machine inflates a 20 kg bag with air. If air release holes are too few, the bag will expand and lean during fill. If too many holes, fine product (like cement dust) will be blown out. We work with the packing machine supplier to set the right vent area. For gravity fillers, the bag must sit neatly; our pinch-bottom (PBOM) bags are designed with square bases so they stand up on a filling cone without toppling. Screw or vibrator fillers exert downward force as they compact product, so the bottom seal must be especially strong (often we use an extra patch or a strong adhesive pattern). Weighing scales on the line need the bag to remain vertical and stable, so our bags use stiff, high-burst paper to avoid bulging.
• Filling Speed and Throughput: If a plant wants to run 200 bags/minute, the bag and closure must allow it. Valve and PBOM bags generally support the highest speeds because closing is automated. A sewn bag line might run slightly slower due to sewing steps. We often do trial runs at customer sites to measure actual cycle times. The bag’s mouth design can be optimized for speed: for instance, a longer crease (overlapped glue) at the pinch bottom closure can slightly slow sealing, but it can be narrowed if speed is more important. We balance these details so that the bag changeover on a line does not throttle production.
• Dust and Contamination Control: On many powder lines, dust control is critical. Our bag designs often incorporate features to minimize airborne dust. For example, valve bags for fine chemicals might have an extra film sleeve inside the valve and a peel-off adhesive strip that seals the valve after filling. Sewn bags for food often use a double tape (crepe paper plus a polymer tape) under the stitch. We also recommend certain line practices: for instance, using a dust extraction booth in conjunction with vented liners. The bag design complements these measures; in our experience, switching to the correct bag style can cut line dust by half or more.
• Bag Feeding and Presentation: Bags are usually supplied to fillers in bundles or bales. They must be stiff enough to hold their mouth open for easy placement, but not so rigid that operators struggle. We optimize the combination of ply stiffness and material handling. On many lines, operators stack empty bags in a magazine. If a bag is too limp, it collapses in the stack; too stiff, and it can become entangled. Our standard 3-ply and 4-ply constructions have proven to feed reliably on most machinery. We offer sample reels so customers can test run a few hundred bags on their own equipment.
• Coding and Traceability: Modern logistics often require barcodes, date codes, or product IDs printed on every bag. For this, the bag surface and closure matter. Our flat PBOM panels are excellent substrates for inkjet or thermal print (high contrast, no folds). Sewn seams can sometimes shadow a printed code, so we place the barcodes on a separate panel. Valve bags naturally present a large flat face. We also pre-print compliance marks (like “UN 5M2 / X / 20KG / S”) where required. If a customer needs QR codes on every bag, we make sure the chosen bag style has the resolution and flat area needed.
• Palletizing and Storage: Ultimately, bags must go on pallets, be wrapped, and survive transport. We often advise on additional measures here. For instance, applying a thin anti-skid varnish to the top ply increases friction between layers, letting them stack taller. Many customers now require a static slip coefficient above 0.4. We can produce an “anti-slip” paper or varnish coat to meet that. We also recommend rounding the four bottom corners of the bag (via a patch or an extra strip of paper) to reduce stress concentration. In one test we did, adding just 10 mm radius corner patches cut corner splits by ~50%. We assist customers in understanding how the bag properties translate to pallet behavior.

Sustainability and Environmental Considerations

Multi-material bags do present recycling challenges, but their overall impact can still be lower when designed well:

• Renewable and Recyclable Materials: Kraft paper is made from wood pulp, a renewable resource. We source paper from mills with sustainable forestry certifications (FSC/PEFC) whenever possible. The plain kraft plies are fully recyclable and biodegradable. Even with a PE liner, the paper can often be repulped in industrial recycling. We use water-based, starch, or EVA adhesives that pose no toxicity issues. For printing, we use soy- or water-based inks. Every component has recyclability in mind.
• Avoiding Product Waste: The biggest sustainability gain is often reducing waste of the product, not just the packaging. A well-sealed, moisture-resistant bag prevents rotting or caking of food and chemicals. For example, preventing just 1% spoilage of flour or fertilizer can save far more resources than a few extra grams of polyethylene. We work with clients to do shelf-life tests. If adding a thin PE liner (and making recycling slightly harder) means 5% less spoiled product, the net carbon footprint per unit sold usually goes down. In practice, we perform comparative studies: e.g. how much flour remains unsold or damaged in all-paper vs. lined sacks over six months. This data often justifies the small additional material use.
• Recycling Considerations: It is true that a bag with mixed materials (paper + PE) is not as easily curb-recycled as an all-paper bag. Where regulations or brand policies demand recyclable packaging, we sometimes offer mono-material alternatives (e.g. a 3-ply all-paper bag with a special water-resistant liner paper instead of plastic). Some customers specify a return program or on-site pulping for used bags. Additionally, certain PE liners are now recyclable in industrial stream (marked as compatible with polyethylene film recycling). We provide guidance on how to separate or process used bags. In many regions, industrial facilities will accept entire bags for pulping since the fibers dominate the mass. We keep abreast of advances like biodegradable plastics or coating technologies. For instance, we have trialed PLA-based liners; they are more costly but can be composted in industrial facilities.
• Operational Efficiency: Lighter bags mean lower freight emissions. A 25 kg all-paper 3-ply bag might weigh 130 g, whereas a 4-ply with liner could be 200 g. Reducing bag weight by 10% (through design optimization) can noticeably cut shipping weight over millions of units. We also ensure our plants minimize waste – trimmings and misruns are recycled back into papermaking where possible. From an energy standpoint, modern production lines (like W&H bottomers) are tuned to use less power per bag. In short, our philosophy is: design the packaging to protect the cargo effectively (reducing waste), and then make the packaging itself as resource-efficient as possible.

A practical guideline we encourage: start with the lightest bag that meets the use-case tests (drop, humidity, dust). Only add heavier liners or extra plies if needed. Document the reasoning so that the environmental costs of the additional material are justified by actual risk reduction. This balanced approach often results in greener outcomes than following a blanket “all-paper” or “all-PE” policy.

Case Studies: Applying the Right Bag

• Case A – Coastal Flour: A flour mill in a tropical coastal region was plagued by caked flour and souring during the rainy season, and operators complained about clouds of flour dust on the line. The original 3-ply sewn bags were not keeping moisture out. We recommended upgrading to a 4-ply valve sack with a 60 µm PE liner and a film-sleeved corner valve. We also adjusted the perforations on the valve to handle the line’s air pressure. The results were dramatic: after 48 hours at 40°C/90% RH, moisture gain dropped from ~0.8% (in the old bags) to ~0.2%. Dust on the filler dropped by roughly 30%. Most importantly, customer complaints about spoilage and pallet leaks virtually disappeared. The modest cost increase was offset by fewer returns and better product shelf life.
• Case B – Pigment Export: A chemical producer shipping iron oxide pigments overseas was seeing 2–3% of bags with corner splits or seam failures after long sea voyages. We investigated and found that the 25 kg valve bags (3-ply all-paper) were too light. The solution was to switch to a 5-ply pinch-bottom sack. We added an inner polypropylene scrim ply for reinforcement and rounded the bottom corners in the bag maker. Also, the hem overlap was widened slightly for strength. This new design withstood our lab’s drop tests (9 drops at 1.2 m) with no failures. In production, corner ruptures fell from ~2.4% of bags to ~0.3%. The extra bag cost was recouped by near-elimination of rework and claims. Pallets now arrived in square shape, greatly reducing handling time at the destination.
• Case C – Pet Food with Oils: A pet food manufacturer using 15 kg kraft sacks found oil stains (grease halos) on the outer bag after storage. They were using a 3-ply sewn bag with no special barrier. We solved this by specifying a 3-ply PBOM bag with a 20 µm grease-resistant inner ply plus a 20 µm PE-coated second ply. Sewing was replaced by hot-melt pinch sealing at the mouth. We also targeted an ASTM F88 peel seal strength above 15 N/15 mm on the closure. After switching, the bags came out completely free of oil marks, and inspectors noted crisper, cleaner bags. The retailer no longer rejected shipments for visual quality, and buyers reported that shelf appeal increased. The new pinched-top design also sealed dust better, improving line cleanliness.
• Case D – Seasonally Moist Additive: A chemical distributor had a fine additive (10 kg bags) that was somewhat oily and hygroscopic. In hot summer months, the product would clump and break bags during transit. They were using basic 3-ply sewn bags. We recommended a 4-ply PBOM sack with a 30 µm PE liner and increased fold overlap. We also ensured the bag met UN 5M2 X standards so it could be air-shipped if needed. In testing, bags sealed to a peel force of >18 N/15 mm (ASTM F88), and under 60% humidity, the stored additive remained free-flowing. In practice, clumping failures dropped to nearly zero. The improved seal meant we also saw an 80% reduction in breakage claims. Despite the thicker bag, the filling line speed only dipped slightly thanks to the efficient pinch-top.

These examples show our methodical approach: identify the failure mode (dust, moisture, split), select the bag modifications (add liner, change plies, alter closure), and validate performance (lab tests or on-line metrics). Each case involved setting clear targets (e.g. “moisture gain <0.5% in 48h at 40°C”) and measuring the outcome. The result is packaging that works reliably in the real world, not just on paper.

Key Specifications and Buyer Considerations

When procuring multi-ply kraft bags, it’s crucial to specify the details. Generic labels like “multiwall sack” are too vague. Key points to include in an RFQ or spec sheet are:

• Ply Composition: Insist on ply-by-ply details (e.g. “Ply1: 100 gsm natural kraft; Ply2: 80 gsm polyethylene-coated paper; Ply3: 80 gsm kraft”). Never accept “4-ply” without saying what each ply is. This ensures the right burst and barrier layers are quoted.
• Total Bag Weight (grammage): For a given bag size and ply makeup, specify the expected total paper weight (e.g. 180–200 g for a 25 kg 3-ply). This affects freight and empty handling.
• Burst and Tear: Ask for expected burst strength (TAPPI T-810, in kPa) and Elmendorf tear (TAPPI T-414, in mN) for both MD and CD. These should meet or exceed your drop/tear criteria. For example, require at least 500 kPa burst for a 25 kg bag if you need heavy-duty performance.
• Drop Endurance: Define the drop test requirement (ISO 7965-2 or ASTM D5276). For instance, you might specify “pass 5 corner drops and 5 edge drops at 1.0 m without rupture.” Request a test report. Don’t assume a bag will survive your drop height; get proof.
• Closure Type and Parameters: Clearly state the mouth style (SOM, PBOM, valve, heat-seal) and any specific needs (e.g. stitch density 8 SPI, or hot-melt peel ≥ 15 N/15mm). If using sewn bags for fine powders, specify a crepe tape underlay or target seam strength. If PBOM, require an F88 peel test value. For valves, note valve size and any film sleeves.
• Liner and Barrier: If you need moisture or grease protection, specify liner type and thickness (e.g. LDPE 60 µm loose liner, or 30 gsm PE-coated paper). For grease control, specify a greaseproof inner ply or finish. If no barrier is desired (all-paper bag), say so. For venting, quantify perforation density (e.g. holes of 0.5 mm diameter, 5 per 100 cm²).
• Surface Finish: If you want anti-slip bags, specify the coating or COF (e.g. “outer ply with anti-slip varnish, static COF ≥ 0.4”). For food bags, specify no wax or no fluorocarbon treatment if purity is required.
• Certifications: Require a list of standards on the COA: e.g. TAPPI T-410 (basis weight), T-414 (tear), T-810 (burst), ISO 7965-2 (drop), ASTM D1894 (friction). If for food, ask for FDA or EU compliance statements and, if needed, material test results (overall migration). If for hazardous goods, demand the UN mark (e.g. “5M2 X 20/S”) and a copy of the UN test report.
• Supply Quality: Multiwall sacks are batch products. Request that the supplier tie tests to lot numbers. Ask if they provide third-party lab certificates for key tests. It’s wise to order a pilot run (100–200 bags) first and measure actual line speed, fill weight accuracy, and dust emission. Check if they are willing to adjust the bag spec based on your trial results.
• Filling Compatibility: Confirm dimensions (bag width/length), valve type, and mouth design fit your machinery. For valve bags, state the filler brand and model so the valve can be matched. For PBOM, specify the glue type and sealing temperature range so the bottomer can be tuned.
• Traceability: Flat PBOM faces or valve surfaces are best for printing batch codes or barcodes. If required, specify where on the bag the code should appear (e.g. 50 mm below the top seam). Ensure that the bag design does not place critical information under a stitch or seam.

In sum, a successful purchase specifies the “recipe” of the bag and the test criteria, not just the general idea of a “multiwall sack.” Detailed specification prevents surprises. For example, telling a supplier “I need a 25 kg multi-wall bag for fine powder” is too vague – what you want to say is “25 kg, 4-ply kraft (90/80/80 gsm + 40 µm PE liner), valve top, water-resistant outer (UN 5M2), sealing performance X.” With precise specs, you get bids that are truly comparable. Our technical sales staff can help customers write these specifications or interpret the data sheets.

Conclusion

Multi-ply kraft paper bags are a highly engineered packaging solution that brings together strength, barrier protection, and operational efficiency in one system. By layering different paper plies (and optional liners), we create a container that can be tailored to exactly the demands of a product and its logistics. Throughout this article we have emphasized the importance of designing the bag around real requirements – from mechanical tests (burst, tear, drop) to shelf-life targets (moisture uptake, dust containment). The goal is always a validated bag performance, not guesswork.

We also take pride in the regulatory aspect: our multi-ply sacks carry the UN 5M2 certification, meaning they have been proven safe for shipping regulated substances by sea. This certification, along with food-contact approvals (FDA, EU) and quality management standards (ISO 9001, etc.), provides assurance to customers that every bag lot is fit for purpose and compliant with the toughest rules. In practice, this means fewer headaches in audits and a lower risk of cargo claims.

Ultimately, the right multi-ply bag can make or break a bulk packaging program. We accompany our bags with data – for example, actual drop test counts, peel strength values, and moisture tests – so customers know what to expect. If a drop from 1.0 m is critical, we demonstrate that our bag survives it. If a product must not gain more than 0.5% water in a tropical warehouse, we show the moisture analysis. This empirical approach closes the loop between design and reality.

Our company’s mission is to provide packaging that works in the field. Whether it’s 15 kg of pet food or 25 kg of powdered cement, our multi-layer kraft sacks are engineered to protect the product, optimize filling lines, and meet regulatory requirements. Customers benefit from both the modular flexibility of these bags and the assurance that comes from our quality control systems. With our proven designs and UN 5M2 certification, clients can focus on their core operations, assured that their product is packaged safely, efficiently, and in full compliance with global standards.

Contact details:

Email: Info@pp-wovenbags.com
Headquarters Address:No.57 Jinliang Rd, Pudong, Shanghai P.R.C 201323
Website: https://www.pp-wovenbags.com/

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About Us:
Established in 2008, VidePak is dedicated to producing high-quality PP woven bags, BOPP woven bags, valve bags, kraft paper bags, HDPE FFS PE bags, and FIBC ton bags.