Plastic bags are made from polyethylene film that is blown as a thin tube, printed, and then sealed, cut and punched into bags. When two quotes for the same printed carrier bag differ by 20 %, the gap usually sits somewhere in that route: a different resin blend, a thinner film with a wider thickness spread, a cheaper handle construction or less testing. This guide follows a polyethylene bag from resin pellets to a counted, palletised carton, explains the settings that decide strength and appearance, and lists the tests and defects worth agreeing before an order. It sits within our overview of how custom printed packaging is made; for formats and ordering, see the custom printed plastic bags hub.
How are plastic bags made, in short?
Most printed plastic bags are made in five to seven stages: polyethylene resin and masterbatch are melted and blown into a thin tube of film, the film is corona treated so ink will stick, printed by flexography, then sealed, cut and punched into bags on a bag-making machine, tested and packed. Mailing bags add an adhesive closure strip; patch and loop handle bags add a separate handle-welding step.
The process line for a typical printed retail bag:
- Resin and masterbatch
- blown film extrusion (tube, lay-flat, gussets)
- corona treatment
- flexo printing
- bag making (sealing, cutting, handle punching or welding)
- in-process testing and inspection
- counting, packing and palletising
| Stage | What happens | What it decides for the buyer |
|---|---|---|
| 1. Raw materials | PE grades blended with recycled resin and masterbatches | Strength, feel, colour, recycled content, cost |
| 2. Blown film extrusion | Melt is forced through a ring die and inflated into a bubble | Thickness, width, gussets, opacity, toughness |
| 3. Corona treatment | Electrical discharge raises surface energy | Whether ink adheres |
| 4. Printing | Flexo prints one to several colours on the film web | Brand appearance, registration, rub resistance |
| 5. Bag making | Seals, cuts, punches, adds patches, loops or adhesive strips | Bag size, seal strength, handle strength |
| 6. Quality control | Thickness, mechanical, seal, print and load tests | Consistency with the approved sample |
| 7. Packing | Counted stacks, cartons, pallets | Arrival condition, handling at store level |
Many converters run extrusion and printing in separate halls, and some buy in film from a specialist extruder. Printing can also happen in line with extrusion for simple one- or two-colour work. The order of stages stays the same.
Raw materials: resin, recycled content and masterbatch
A plastic bag is usually a blend rather than a single polymer. The converter mixes one or more polyethylene grades, often a share of recycled polyethylene, and small amounts of masterbatch (concentrated colour or additive pellets) to reach the strength, feel and appearance the bag needs at the lowest practical thickness.
Polyethylene grades
Three families of polyethylene cover almost all carrier and mailing bags. Their differences in feel, clarity and strength are explained for buyers in LDPE vs HDPE vs recycled PE carrier bags; here is what matters at the extruder.
| Resin | Typical role in bag film | Processing behaviour |
|---|---|---|
| LDPE (low-density PE) | Base of soft, glossy retail and mailing bags | Long-chain branching gives a stable bubble and easy processing; good clarity; lower strength per micron |
| LLDPE (linear low-density PE), incl. metallocene grades | Blended into LDPE (or HDPE) for puncture, tear and dart impact strength | Tougher but harder to process alone; higher motor load and pressure; prone to melt fracture without process aids |
| HDPE (high-density PE), usually high-molecular-weight film grades | Thin vest bags and produce bags | Runs on a "high-stalk" bubble; high tensile strength at very low thickness; crinkly, translucent film |
Film-grade resins are chosen by melt flow index (MFI, measured to ISO 1133 or ASTM D1238) and density. Blown film grades typically have a low, often fractional, melt index, because a stiffer melt holds a stable bubble. Resin suppliers provide a certificate of analysis per lot and, for food-contact use, the information the converter needs for its own Declaration of Compliance.
Recycled polyethylene (PIR and PCR)
Recycled resin arrives as pellets made by a recycler who washes, melts, filters and re-pelletises collected film. Post-industrial recycled material (PIR) comes from production scrap and is usually clean and consistent. Post-consumer recycled material (PCR) comes from used film collected after use and varies more in colour, odour, gel content and melt flow. Converters typically blend recycled resin with virgin resin, or place it in the core layer of a co-extruded film, and adjust the recipe per recyclate batch. The handling issues PCR creates in production are covered in a separate section below.
Some markets also use mineral filler masterbatch (usually calcium carbonate) in bag film to cut resin use or change the feel. Filler raises film density, so a filled bag weighs more per micron than an unfilled one. Ask whether filler is used, because it affects bag weight, recycling behaviour and how the bag counts in weight-based reporting.
Masterbatch and additives
Masterbatches are added at a let-down ratio, typically a few percent of the blend, through a gravimetric dosing unit.
| Masterbatch / additive | Purpose | Points to watch |
|---|---|---|
| White (titanium dioxide) | Opacity and a clean print base | Higher loadings needed for opaque white film and to mask grey PCR; affects cost |
| Colour masterbatch | Coloured film, matched to a Pantone or sample | Colour shifts with film thickness and with the base colour of recycled resin |
| Black / grey | Opaque inner layer of mailing bags | Used in co-extrusion so a white outside hides contents |
| Slip (typically fatty acid amides such as erucamide) | Lowers coefficient of friction (COF) so film slides on machines and bags open easily | Migrates to the surface over days; too much reduces print adhesion and seal strength |
| Antiblock (typically silica or talc) | Stops film layers sticking together | Too much increases haze and wears machine parts |
| UV stabiliser | Slows degradation for bags exposed to sunlight, such as outdoor or garden-centre use | Specify only where needed |
| Polymer processing aid (PPA) | Reduces melt fracture and die build-up, especially with LLDPE | Many traditional PPAs are fluoropolymer based; PFAS-free alternatives exist and food-contact buyers increasingly ask which type is used |
| Desiccant (typically calcium oxide based) | Absorbs moisture in recycled resin to prevent voids and lensing | Common in high-PCR film |
One additive category is deliberately absent from this list: oxo-degradable additives, which make film fragment under heat or UV. Oxo-degradable plastics have been banned in the EU since 3 July 2021 under the Single-Use Plastics Directive (EU) 2019/904, and "degradable" claims on bags carry legal risk in the UK and US as well. For the PFAS question around processing aids, see PFAS-free food packaging.
Incoming inspection
Typical incoming checks are: resin grade and lot number against the certificate of analysis, melt flow index and density spot checks, moisture in recycled resin, colour of masterbatch against a standard, and visual checks for contamination and pellet uniformity. For bags intended for food contact, converters also collect supplier compliance documents for every resin and additive. Material terms used here are defined in our packaging materials glossary.
How does blown film extrusion work?
In blown film extrusion (Blasfolienextrusion in German), molten polyethylene is pushed through a ring-shaped die, inflated with air into a tall bubble, cooled, then flattened by nip rolls into a two-layer tube called lay-flat film. The ratio of bubble to die diameter, the speed of the haul-off and the die gap together set the film width and thickness.
The line, station by station
| Station | What it does | Key parameters |
|---|---|---|
| Hopper and gravimetric dosing | Weighs and blends resin, recycled pellets and masterbatch in the set proportions | Blend ratio accuracy, moisture in recycled resin |
| Extruder (screw and barrel) | Melts, mixes and pressurises the blend | Screw design, barrel temperature zones, melt temperature, melt pressure, output (kg/h) |
| Screen changer / melt filter | Removes contamination and unmelted particles | Mesh fineness, pressure rise as screens load (more important with PCR) |
| Annular die | Shapes the melt into a thin tube | Die diameter, die gap, temperature uniformity, spiral mandrel design |
| Air ring (often with internal bubble cooling, IBC) | Cools and stabilises the bubble | Air volume and temperature, lip design, frost line height |
| Bubble and cage | Stretches the film in both directions | Blow-up ratio, bubble stability |
| Collapsing frame and gusseting boards | Flattens the bubble; boards fold in side gussets | Frame angle and friction, gusset depth |
| Nip rolls / haul-off (often oscillating) | Pulls the film, sets draw-down and seals off the bubble air | Line speed, nip pressure, oscillation to spread gauge bands |
| Gauge measurement | Measures thickness around the bubble or across the lay-flat | Sensor type, profile control loop |
| Corona treater (in line) | Treats one or both surfaces | Power, treatment width, dyne level |
| Slitting and winding | Edge-trims or slits the tube; winds reels | Winding tension, reel diameter, core size |
Extruder and melt temperature
The extruder screw conveys pellets forward, melts them through friction and barrel heat, and homogenises the melt. Melt temperature for polyethylene film typically sits somewhere around 180-240 °C, depending on the resin, line and output: LDPE tends to run at the lower end, LLDPE blends higher, and HDPE high-stalk lines have their own settings. Too cold a melt causes poor mixing, unmelted gels and high pressure; too hot a melt weakens the bubble, can degrade the polymer and, with recycled resin, increases odour. Line output ranges widely, from under 100 kg/h on small single-layer lines to several hundred kg/h or more on large co-extrusion lines, depending on machine, format and material.
Die, air ring and frost line
The melt leaves the die through a narrow circular gap, typically around 1 mm or wider for LLDPE-rich blends, and is immediately cooled by the air ring. As it cools, the polyethylene crystallises and the film turns from clear melt to slightly hazy solid at a visible band called the frost line. Frost line height affects clarity, strength balance and output.
LDPE and LLDPE blends usually run a "pocket" bubble that expands close to the die with a low to moderate frost line. HDPE film runs a high-stalk bubble: the melt rises as a narrow neck several die diameters tall before expanding, which lets the melt relax before it is stretched. The result is more balanced orientation in both directions, which gives thin HDPE vest bags better impact strength and less tendency to split along the machine direction.
Blow-up ratio, draw-down and lay-flat width
The blow-up ratio (BUR) is the final bubble diameter divided by the die diameter. It sets the stretch in the transverse direction (TD). The haul-off speed relative to the melt speed sets the stretch in the machine direction (MD), and the draw-down ratio (DDR) relates die gap to final thickness.
| Term | Formula (simplified) | Typical range | Effect |
|---|---|---|---|
| Blow-up ratio (BUR) | Bubble diameter / die diameter | Typically about 2:1 to 3:1 for LDPE/LLDPE bag film; about 3:1 to 5:1 for HDPE high-stalk film | Higher BUR balances MD and TD strength and widens the film from the same die |
| Lay-flat width | π × die diameter × BUR / 2 (about 1.57 × die diameter × BUR) | Set by bag width plus gussets | Decides which bag sizes a given die can make efficiently |
| Draw-down ratio (DDR) | Die gap / (film thickness × BUR) | Varies by resin and line | High draw-down orients film along MD; tear resistance in MD can drop |
| Thickness | Set by output, haul-off speed and BUR | Per specification | Thicker film at the same output means slower haul-off |
Worked example: a 200 mm die at a BUR of 2.5 gives a bubble about 500 mm in diameter and a lay-flat width of about 785 mm. That tube could be converted into a bag around 750-780 mm wide on a bottom-seal machine, or slit, folded or gusseted to suit other formats (on a side-weld machine the lay-flat width sets the bag height rather than its width).
Why it matters to a buyer: film blown with poorly balanced orientation tears easily in one direction. A bag that splits from the handle downwards in a load test may have the right micron value but the wrong bubble settings. Balanced BUR and draw-down are one reason why a well-made 50 µm bag can outperform a poorly made 60 µm one, as discussed in plastic bag thickness.
Nip rolls, gusseting boards and the tube
At the top of the tower, a collapsing frame flattens the bubble and nip rolls close it. If the bag needs side gussets (vest bags, some retail and garment bags), gusseting boards fold the edges inwards before the nip. Bottom gussets are normally formed later on the bag-making machine. The film leaves as a lay-flat tube, which may be wound as tubing, slit on one edge to give J-fold or C-fold film, or slit into single sheets for other bag constructions.
Gauge control and thickness profile
No blown film is perfectly even. Thickness varies around the bubble because of small differences in die gap, melt temperature and cooling air. Modern lines measure the thickness profile continuously, using capacitive, infrared or other non-contact sensors, and correct it automatically, most often with segmented air rings that cool thick zones more, or with thermally adjusted die segments.
Even a corrected profile has thick and thin zones. If they always land at the same position on the reel, they build up as ridges called gauge bands, which cause wrinkles, poor printing and uneven seals. Lines therefore rotate or oscillate the die or, more commonly today, oscillate the haul-off, so that thickness variations are spread across the reel width.
A thickness tolerance (often in the order of ±10 % of nominal, agreed between buyer and supplier) should be in the specification together with the measurement method, because individual readings will vary even when the average is right.
Co-extrusion
Co-extrusion lines have two or more extruders feeding one multi-layer die, so each layer can have its own recipe. Three-layer film (often called A/B/A or A/B/C) is common for quality bag film; five or more layers are used for specialised films and are more typical of the barrier films covered in how flexible pouches are made.
| Co-ex use | Typical layer logic |
|---|---|
| Recycled-content bag | Virgin or white outer skins for print quality and sealing; recycled core for content share |
| Mailing bag | White (or grey) outer layer, black inner layer for opacity |
| Tough retail bag | LLDPE-rich core for impact strength; LDPE skins for gloss and processability |
| Sealing performance | Skin layer with a lower seal initiation temperature |
Why is corona treatment needed before printing?
Polyethylene has a low surface energy, typically around 30-31 dyn/cm (mN/m), so liquid ink does not wet it or bond to it. Corona treatment passes the film under a high-voltage discharge that oxidises the surface and raises its surface energy. Printers typically aim for about 38-42 dyn/cm on the side to be printed, with water-based inks usually needing the upper end or more.
Treatment is usually done in line at the extruder, then topped up ("bumped") at the printing press because the effect decays over time. Slip additives migrating to the surface lower the dyne level, so film stored for weeks before printing may need re-treatment.
| Treatment point | Why |
|---|---|
| At the extruder, in line | Fresh film treats most effectively; sets a baseline |
| At the press, before the first print unit | Restores surface energy that has decayed in storage |
| Both sides | Only when both faces will be printed; untreated inner faces seal and open more predictably |
Surface energy is checked with test inks or dyne pens (ISO 8296, ASTM D2578). Over-treatment can cause blocking and weaken heat seals, so more is not automatically better. Treater electrodes also need maintenance: a worn or contaminated station can give a treated band with untreated strips, which shows later as ink lifting along the reel.
How are plastic bags printed?
Printed plastic bags are produced mainly by flexography, either on a central impression (CI) press, where all colour units print around one large temperature-controlled drum, or on a stack press, where units are stacked and both sides of the web can be printed in one pass. Printing is done on the film reel before bag making, so the print repeat has to match the bag pitch.
What the press side looks like in this family (the buyer-facing view of colours, artwork and adhesion is in printing on plastic bags; flexo is compared with other processes in flexo vs offset vs gravure vs digital printing):
| Element | Typical practice on PE bag film |
|---|---|
| Press type | CI flexo for quality multi-colour work and stable register; stack flexo for simpler and two-sided work; in-line print units on some extrusion lines for one or two colours |
| Colours | Commonly one to six; CI presses for film are often built with six to ten units |
| Plates | Photopolymer plates, mounted on sleeves or cylinders with cushion tape matched to the artwork (solids vs fine line) |
| Anilox rolls | Ceramic engraved rolls; higher cell volume and coarser screen for solids and opaque white, lower volume and finer screen for line work and halftones. As an indication, solids often use around 4-7 cm³/m² and fine work around 2-4 cm³/m², varying with ink and press |
| Halftone screen | Coarser than on paper; typically roughly 85-133 lpi on bag film, finer on quality CI work |
| Inks | Solvent-based (alcohol/ester systems), water-based, or UV-curable on some presses |
| Drying | Hot air between colours and a main drying tunnel; temperature limited because PE softens and stretches |
Solvent-based vs water-based inks
Solvent-based flexo inks dry fast and wet low-energy film easily, so they are tolerant of slightly lower dyne levels and high speeds. They emit volatile organic compounds (VOCs), which plants control with extraction and, typically, thermal oxidation or solvent recovery under local emissions permits. Water-based inks reduce VOC emissions and odour but dry more slowly, need higher surface energy and more drying capacity, and can be harder to run at high speed on thin film. Many bag converters use solvent-based inks for film; water-based systems are growing where emissions or retained solvent odour are a concern.
Drying and retained solvent
Ink that is not fully dried causes blocking in the reel, set-off onto the reverse side of the film and odour in the finished bag. Converters control drying with air temperature and volume per unit, web speed and tunnel length. For bags that may contact food, retained solvent and set-off are both relevant: Regulation (EC) 2023/2006 on good manufacturing practice requires that printing inks on the non-food side do not transfer to the food-contact side through the stack or reel in amounts that would breach the safety requirements of Regulation (EC) 1935/2004. Germany's Printing Ink Ordinance (Druckfarbenverordnung) applies from 1 January 2027 to inks on food-contact materials, and EuPIA guidance and the Swiss ink ordinance are common references for low-migration ink selection. The documents involved are covered in food contact packaging compliance.
Registration on stretchy film
Thin polyethylene stretches under web tension and softens with dryer heat, so each colour's position can drift. Presses counter this with closed-loop tension control in separate zones (unwind, print, rewind), a temperature-controlled impression drum on CI presses, short web paths between units and automatic register control with cameras or sensors. The bag maker also has to find the print: a printed eye mark (register mark) tells the bag machine where to seal and cut, so the bag pitch follows the print rather than a fixed length.
Practical consequences: thin film registers less precisely than paper, front-to-back alignment on two-sided print is looser than colour-to-colour alignment on one side, and fine keylines show misregistration first. The design rules that follow from this are in our packaging artwork guide.
Prepress and tooling
Prepress for bag film covers distortion compensation for the plate (the plate stretches round the sleeve), trapping between adjacent colours, minimum line widths and dot sizes for film, and step-and-repeat across the web width. The print repeat must equal the bag pitch (or a multiple of it), so it is tied to a plate cylinder or sleeve circumference. If a new bag size needs a repeat that no existing sleeve or cylinder matches, new tooling is needed. Terms like anilox, trapping and distortion are defined in the printing and prepress glossary; colour approval and proofing are in Pantone vs CMYK and the artwork guide.
How does a bag-making machine turn film into bags?
A bag-making machine unwinds printed film, aligns it to the print, forms any bottom gusset or fold, seals it with heated bars or a hot knife, cuts each bag, punches or attaches the handle and stacks counted bags. The two basic seal geometries are the side weld and the bottom seal, and the choice decides how the bag is laid out on the film.
Side weld vs bottom seal
| Side weld | Bottom seal | |
|---|---|---|
| Starting film | Folded film or slit tube; the fold becomes the bag bottom (or a W-fold forms a bottom gusset) | Lay-flat tube, plain or side-gusseted |
| Sealing tool | Heated knife that cuts and seals both side edges in one stroke | Heated sealing bar across the tube, followed by a separate cut |
| Seal appearance | Narrow bead along each side | Flat seal band across the bottom |
| Print repeat runs | Across the bag width | Along the bag length |
| Typical products | Die-cut retail bags, patch and loop handle bags, mailing bags | Vest bags, produce and roll bags, some gusseted retail bags |
| Strength character | Strength depends on bead quality; the bottom is a fold with no seal | Seal width and temperature critical; the bottom seal carries the load |
The hot knife in a side-weld machine runs much hotter than a sealing bar because it has to melt through the film and form a bead in a very short contact time. Bottom sealers work at lower bar temperatures with longer dwell time and controlled pressure. On both, the three seal variables are temperature, dwell time and pressure, plus the cooling time before the seal is loaded. Machine speeds range from tens to a few hundred bags per minute per lane, depending on bag size, film, format and the number of extra operations.
Die-cut (punched) handles
For die-cut bags, a punch unit on the bag machine cuts the kidney or oval handle through both walls as each bag is formed, or the handle is punched in stacks afterwards. A fold-over top, where the mouth is folded inwards before punching, doubles the film around the hole. Punch alignment with the print is part of the register system, and the punched-out slugs are collected for regrind.
Patch handle sealing
For patch handle bags, a narrow strip of patch film is fed from a separate reel, placed inside the bag mouth on both walls and heat sealed to the bag body before the handle is punched through all layers. The patch is usually the same PE family as the bag, which keeps the bag mono-material. The patch seal needs the same temperature, dwell and pressure control as the main seals; a patch that peels away under load is a common failure.
Soft loop handle welding
Soft loop handles are made from a separate film strip, often co-extruded or doubled for strength, which is formed into a loop and heat sealed to the inside of each bag face at the mouth. This is either done in line on a loop-handle bag machine or on a separate handle unit. It adds stations, slows output and adds film, which is why loop bags cost more per unit. The performance trade-offs between handle types are in plastic bag handle types.
Vest (T-shirt) bag cutting
Vest bags start as side-gusseted HDPE tube. The bottom-seal machine seals and cuts each bag, stacks a counted block, and a punching press then cuts the handle shape (and often a central hanging tab for dispensing) through the whole stack in one stroke. Some blocks are lightly welded together at the header so that bags tear off one by one from a rack.
Perforation and roll bags
Perforation knives cut a tear line between bags, so bags can be supplied on rolls (produce bags, bin liners) or as tear-off pads. Perforation depth and pattern are balanced so bags separate cleanly without breaking on the roll.
Mailing bags with peel-and-seal strip
Mailing bags are typically made from co-extruded film on a side-weld machine with a lip or flap. A strip of pressure-sensitive adhesive (commonly hot-melt) is applied to the flap and covered with a silicone-coated release liner. A second adhesive strip and tear line can be added for returns. The adhesive strip has a shelf life and is sensitive to heat in storage, so it is worth asking about the adhesive type and recommended storage conditions. Bag-making and handle terms are explained in the converting and manufacturing glossary.
What changes when a bag contains recycled PE?
Recycled polyethylene runs on the same lines as virgin resin, but it brings more gels, a grey or off-white tint, possible odour, moisture and batch-to-batch variation in melt flow. Converters manage this with finer melt filtration, blending, co-extrusion with virgin skins, desiccant masterbatch and more frequent testing.
| PCR issue | Where it comes from | How production manages it |
|---|---|---|
| Gels and specks | Cross-linked polymer, unmelted particles, paper fibres, ink residues | Finer screen packs or continuous melt filters at the recycler and the extruder; recyclate specifications with gel limits |
| Odour | Residues from previous contents, degraded polymer, inks | Washed and degassed recyclate grades; moderate melt temperature; limiting PCR share where odour matters |
| Colour variation | Mixed source film | Blending recyclate lots; white or colour masterbatch; recycled core in a co-ex film |
| Moisture | Washed recyclate, hygroscopic contaminants | Drying or desiccant masterbatch to avoid voids and lensing |
| Melt flow variation | Mixed grades in the feedstock | Incoming MFI checks; adjusting blend and line settings per lot |
| Lower and more variable strength | Shorter chains, contamination | Adding LLDPE; slightly higher thickness; more frequent dart and load tests |
The buyer-side effects on appearance and strength, including indicative recycled shares, are in LDPE vs HDPE vs recycled PE carrier bags. Recycled content now has financial and legal weight: UK Plastic Packaging Tax applies to plastic packaging with less than 30 % recycled plastic, and the EU PPWR sets recycled-content minimums from 2030 at the earliest. Those rules are summarised in plastic carrier bag regulations and, for the UK, in UK packaging regulations. Production's part is traceability: recycled-content claims need recyclate supplier documentation, batch records linking recyclate lots to bag lots, and a calculation method that matches the scheme or law being reported against.
How is plastic bag quality controlled in production?
Plastic bag quality is controlled at three points: on the extrusion line (thickness profile, width, dyne level, visual gels), at the press (colour, register, adhesion) and at the bag machine (dimensions, seal strength, handle position, count). Mechanical tests on film and load tests on finished bags are run at agreed intervals and recorded per lot.
In-process checks and test methods
| Test | What it measures | Common method references | Where it is used |
|---|---|---|---|
| Thickness and profile | Average thickness and spread across the web | ISO 4593 (mechanical scanning); on-line gauge data | Every reel or at set intervals |
| Lay-flat width, gusset, bag size | Dimensions against specification | Steel rule or gauge | Start of run and periodically |
| Dart impact | Resistance to a falling dart | ASTM D1709; ISO 7765-1 | Per lot or recipe change |
| Tensile strength and elongation | Strength and stretch in MD and TD | ISO 527-3; ASTM D882 | Per lot or recipe change |
| Tear resistance | Elmendorf tear in MD and TD | ISO 6383-2; ASTM D1922 | Per lot, especially HDPE |
| Seal strength | Force to pull a seal apart | ASTM F88 | Start of run and periodically |
| Load (hang) test and drop test | Finished bag carrying a defined weight, sometimes lifted repeatedly or dropped | Agreed protocol per bag type | Start of run and periodically |
| Coefficient of friction (COF) | Slip of film on film or metal | ISO 8295; ASTM D1894 | Per lot; affects opening and stacking |
| Surface energy (dyne) | Readiness for printing | ISO 8296; ASTM D2578 | Before printing |
| Print adhesion tape test | Ink removal by pressure-sensitive tape | ASTM F2252 (commonly used for flexible packaging) | Each print run |
| Rub and crinkle | Ink resistance to scuffing and flexing | Agreed method | Each print run |
| Visual: gels, specks, colour | Appearance against approved sample | Light table; approved standard | Continuous |
| Count and weight | Bags per pack; bag weight against nominal | Counting machine; check weighing | During packing |
General sampling plans (AQL), golden samples and pre-production approval are covered in packaging sampling and quality control; test names and defect terms are in the packaging testing and quality glossary.
Typical defects in plastic bag production
| Defect | Typical cause | Prevention in production | How the buyer can detect it |
|---|---|---|---|
| Gels and fish-eyes | Unmelted or cross-linked polymer, contamination, high PCR share | Melt filtration, correct melt temperature, recyclate gel limits | Hold film against light; small clear or dark lenses, sometimes with a "tail" |
| Gauge bands and thin spots | Uneven die gap or cooling; no oscillation | Automatic profile control, oscillating haul-off, die maintenance | Thickness readings across the bag with a micrometer; ridges on reels |
| Wrinkles and creases | Gauge bands, misaligned collapsing frame or rollers, wrong tension | Profile control, roller alignment, tension settings | Creased print, wrinkles through seals |
| Weak or leaking seals | Low temperature, short dwell, contamination, slip bloom, ink in the seal zone, over-treatment | Seal parameter control, seal-strength tests, ink-free seal zones | Pull seals apart by hand; load test filled bags; seals separating rather than film tearing |
| Burnt-through or brittle seals | Too much heat or pressure | Parameter windows, knife maintenance | Seals that crack or tear right beside the seal line |
| Blocking | Too little antiblock, over-treatment, hot winding, undried ink, heavy ink coverage | Additive balance, cooling, drying control | Bags stick together and are hard to separate or open |
| Curl | Unbalanced co-ex layers, uneven cooling | Symmetric layer design, cooling control | Bags will not lie flat in stacks |
| Print rub-off or tape-test failure | Low dyne level, wrong ink, poor drying, slip migration | Corona treatment checks, ink selection, drying | Tape test with standard tape; rub with a thumb |
| Misregister | Web stretch, tension variation | Tension control, register systems | Colour gaps, shifted outlines, print off-centre to handles |
| Handle tear-out | Thin film, poor patch seal, punch misalignment, unbalanced orientation | Patch or fold-over top, punch set-up, BUR control | Load test with real contents; tears starting from handle corners |
| Odour | Retained solvent, degraded or contaminated recycled resin | Drying control, recyclate selection | Open a carton after transit; sniff test, or a formal odour test where required |
| Colour variation between lots | Masterbatch dosing, recyclate base colour | Gravimetric dosing, lot blending, colour checks | Compare against the retained approved sample |
How are plastic bags packed and shipped?
Finished bags are counted into stacks or packs (by machine count and check weighing), packed into corrugated cartons or bundled in film, and stacked onto pallets that are stretch-wrapped. The pack count, carton labelling and pallet pattern are part of the specification and should match how bags are used at store or fulfilment level.
- Packs and cartons: retail bags are commonly packed in counted stacks (for example 50 or 100 per pack) inside cartons; vest bags in blocks or header packs for hook dispensers; mailing bags in counted packs or boxes; roll bags on cores.
- Labelling: carton labels typically show product code, size, thickness, quantity, lot or production date and the buyer's references. Lot traceability on cartons connects any complaint back to the extrusion and print runs.
- Pallets: stable column or interlocked stacking, corner protection where cartons are heavy, stretch wrap and top sheets against dust. Pallet sizes differ by market (EUR pallet in the EU, UK standard pallet, 48 × 40 inch in the US), which is covered in sourcing packaging from Türkiye.
- Storage: cool, dry and out of direct sunlight. Heat in containers or warehouses can cause blocking, weaken adhesive strips and shift COF as slip continues to migrate. Film bags are light and pack flat, so freight cost per thousand bags is usually low compared with paper alternatives.
How does the production route drive MOQ, cost and lead time?
Minimum order quantity for printed plastic bags is driven mostly by the extrusion run: starting a line, reaching stable thickness and colour, and purging afterwards produces waste that only makes sense over a minimum film tonnage. Colour changes, plate and sleeve tooling and the number of conversion operations add the rest.
- Extrusion minimum run: each start-up and recipe or colour change produces scrap film. Non-standard film colours, special blends and high recycled shares are typically run as dedicated batches, which raises the minimum.
- Colour changes: changing film colour or masterbatch means purging the extruder; changing ink colours means washing up print units. Fewer colours and standard film colours lower both cost and minimums.
- Plates and sleeves: a new set of photopolymer plates is needed for each colour and design; an unusual print repeat can require new sleeves or cylinders.
- Conversion route: die-cut bags convert fastest; patch and especially loop handle bags take more stations and more time per bag.
- Film weight: resin is the largest cost item, so bag size, thickness and recycled-resin pricing dominate unit cost. The weight formula is in plastic bag thickness.
- Lead time: depends on artwork approval, plate making, resin and recyclate availability, extrusion and print scheduling, conversion and transport.
The detail is in our general guides on packaging MOQ, packaging cost drivers and packaging lead times. For any particular bag, the minimum and timing depend on material, size, print and run configuration and are confirmed at quotation.
What hygiene standards apply when bags are made for food use?
If the bag or film will contact food, the manufacturing site works under good manufacturing practice (Regulation (EC) 2023/2006 in the EU), and the plastic itself must comply with Regulation (EU) 10/2011, supported by a Declaration of Compliance. Many food businesses also ask converters about certification to an industry hygiene scheme.
In production terms, food-contact hygiene means: controlled raw materials with compliance documents for every resin, masterbatch and ink; segregation of food-contact and non-food production or clear line clearance between them; clean, covered storage of reels and finished goods; pest control; glass and brittle-plastic policies; blade and sharp-tool control at bag machines; protective clothing and hand hygiene rules; and lot traceability from resin to carton. Recycled resin needs particular care: post-consumer recycled PE is generally not suitable for direct contact with unwrapped food unless it comes from a recycling process authorised for food contact.
Schemes buyers commonly ask about include BRCGS Packaging Materials, FSSC 22000 and ISO 22000-based systems. They differ in scope and audit model, and certification belongs to a specific site and scope, so ask for the certificate and its scope rather than a general statement. Many bags only carry pre-packed food, in which case food-contact requirements for the bag are usually less demanding. The broader legal context for plastic bags (EU lightweight bag rules, PPWR, UK carrier bag charge and Plastic Packaging Tax, US state bag laws) is covered in plastic carrier bag regulations and UK packaging regulations; obligations depend on your role and market and should be confirmed with the relevant authority or compliance scheme.
What should you ask a plastic bag manufacturer about production?
Ask questions that show how the specification will be controlled, not only what it says. The list below covers the production route described in this guide; general supplier evaluation is in how to choose a custom packaging manufacturer and RFQ structure in the packaging RFQ guide.
- Film recipe: which PE grades, what LLDPE share, any filler, any recycled share (PIR or PCR) and where it sits in a co-ex structure.
- Additives: slip and antiblock levels, UV stabiliser if needed, type of processing aid (fluoropolymer or PFAS-free), confirmation that no oxo-degradable additives are used.
- Thickness: nominal value, agreed tolerance, measurement method and number of points averaged.
- Extrusion: mono-layer or co-extruded, film colour, whether gussets are formed at the extruder.
- Surface treatment: target dyne level and whether film is re-treated at the press.
- Printing: press type, ink system (solvent-based, water-based or UV), number of colours, front and back print, colour approval method on the actual film.
- Bag making: side weld or bottom seal, handle construction (die-cut, fold-over, patch, loop, vest), patch or loop film thickness, adhesive type for mailing bags.
- Testing: which tests are run per lot (thickness, dart, tensile, seal, COF, tape test, load test) and whether results can be shared.
- Recycled content evidence: recyclate supplier documents, batch traceability, calculation method.
- Food contact (if relevant): Declaration of Compliance, ink compliance statements, hygiene scheme certificate and scope.
- Packing: count per pack and carton, carton labelling with lot numbers, pallet type and pattern, storage advice.
- Market of use, so thickness and recycled content can be checked against local thresholds.
Next step: send a production-ready specification
To get a quotation that reflects the real production route, send bag format and size, gussets, film type or "advise", thickness with tolerance, film colour, recycled-content target, handle type, print colours per side with Pantone references, artwork on a dieline, quantities per design and the market where the bags will be used. Use the checklist on the custom printed plastic bags hub, or send it through our contact page. 9 Packaging, a B2B printed packaging manufacturer and exporter in Istanbul, reviews each specification against the intended market at quotation and technical review. For the other packaging families, return to how packaging is made, or compare with how paper bags are made if a paper alternative is on the table.