How Printed Plastic Bags Are Made: Extrusion, Printing, Sealing

Printed plastic bags are made in three main steps. First, polyethylene resin is melted and blown into a thin tube of film. Second, the film is surface-treated and printed on a gravure or flexographic press. Third, the printed roll goes to a bag-making machine that seals, cuts and punches it into finished bags.

That is the short version. Below is what actually happens at each stage on a factory floor like ours, where quality problems come from, and what you need to send to get a printed order right the first time.

Step 1: Film extrusion

Almost every printed carrier bag, mailer and food bag starts as blown film. Resin pellets go into an extruder, where a heated screw melts them. The melt is pushed through a ring-shaped die and comes out as a thick tube. Air blown into the tube inflates it into a bubble many times wider than the die, which stretches the plastic thin. The bubble is cooled by an air ring, flattened between rollers and wound onto a roll.

A few choices are made here that you will feel in the finished bag:

• Resin. HDPE gives a crisp, strong, economical bag that rustles in the hand. It is the standard for grocery T-shirt bags. LDPE and LLDPE make a softer, glossier film that stretches before it tears. That suits die-cut boutique bags, mailers and heavier gauge bags.
• Thickness. Set by how fast the film is pulled and how much the bubble is blown up. US buyers usually spec in mil, Europe and Asia in microns (1 mil = 25.4 µm).
• Color. White, black or colored film is made by adding masterbatch at the extruder. Many printed bags are white film with colored ink on top, because white gives the brightest base for print.
• Layers. Co-extrusion lines run two or three layers at once, for example a white outside and a black inside for an opaque mailer.

Corona treatment

Untreated polyethylene has a low surface energy, so ink beads up and rubs off. Before printing, the film passes through a corona treater, a high-voltage discharge that raises the surface energy of the side that will be printed. Printers check it with dyne pens. Most solvent and water-based inks need the film in roughly the 38–42 dyne/cm range. Treatment fades with time, so film should be printed reasonably soon after it is treated. When a customer complains that the logo scratches off with a fingernail, poor treatment is the first thing we check.

Step 2: Printing

Two press types do almost all volume bag printing: rotogravure and flexography. Both print from a roll at high speed, one color station after another, with a drying section between colors.

Rotogravure printing

In gravure, the image is engraved into a metal cylinder as millions of tiny cells. The cylinder turns in an ink bath, a doctor blade wipes the surface clean, and the ink left in the cells transfers to the film under pressure. Deeper cells carry more ink and print darker tones.

Gravure gives sharp detail, smooth gradients and very consistent color from the first bag to the last. Engraved cylinders are also durable and can be stored and reused for repeat orders. The cost is upfront: each color needs its own cylinder, so gravure makes most sense for long runs and artwork that does not change often.

This is the process we use at Hanpak. Our presses print up to 8 colors, on one side or both. We match your Pantone references, lay down a white base layer when you print on clear or dark film, and keep your cylinders for up to 2 years so repeat orders print the same as the first.

Flexographic printing

Flexo uses flexible raised plates, like a very fast rubber stamp. An anilox roller meters a thin film of ink onto the plate, and the plate prints onto the film. Plates are cheaper and quicker to make than gravure cylinders, so flexo suits shorter runs and frequent design changes. Modern flexo is good, but fine gradients and photographic images are still easier to hold on gravure.

Factor	Rotogravure	Flexography
Image carrier	Engraved metal cylinder	Flexible raised plate
Setup cost	Higher per color	Lower per color
Best run length	Long runs, repeat orders	Short to medium runs
Detail and gradients	Excellent, very consistent	Good; fine gradients are harder
Reuse for repeats	Cylinders store and reuse well	Plates wear and may need remaking
Artwork changes	Costly; new cylinder per changed color	Cheaper to change

Other methods you may hear about

Screen printing pushes ink through a mesh screen onto individual finished bags. It needs little equipment and suits a few hundred bags, but it is slow and usually limited to one or two colors. Heat transfer presses a printed transfer onto thicker bags with heat and pressure. It is common on fabric and some reusable bags. Neither method is used for export volumes of film bags.

Step 3: Bag making

The printed roll goes to a bag-making machine. A photo-eye reads a printed registration mark so every cut lands in the same place relative to the logo. Then, depending on the bag:

• Bottom-seal bags (T-shirt bags, trash bags, produce rolls): the tube is sealed across and cut or perforated.
• Side-seal bags (flat bags, mailers, many die-cut bags): the film is sealed and cut along both sides at once.
• Handles: a die punches the T-shirt handle shape or the die-cut oval or kidney handle. Patch handles are welded on separately.
• Extras: gussets are folded in before sealing, zippers or adhesive strips are applied, and wicket holes are punched for bags sold on wire wickets.

Finished bags are counted, packed into your cartons with your labels, barcodes and shipping marks, and palletized for loading.

Quality checks that matter

• Color: printed sheets checked against the approved proof and Pantone references.
• Registration: colors line up with each other and the print sits in the right place on the bag.
• Ink adhesion: tape test on printed film.
• Seal strength and thickness: checked during the run against the spec.

Our plants work to Japan’s JIS standard and ISO 9001. For bags that touch food, we supply FDA-compliant food-contact material (21 CFR 177.1520).

How a printed order works at Hanpak

1. You send the spec: bag style, size, gusset, thickness, film color, quantity and destination market.
2. You send artwork: vector files (AI or PDF), with fonts outlined and Pantone references.
3. We send a proof: a digital layout at real size, with any issues flagged, for example a color that needs a white base or a logo too close to a seal.
4. You approve, we make cylinders. After approval the cylinders are engraved. Changes after this point mean new cylinders, so check the proof carefully.
5. Production and shipping. Typical lead time is 4–6 weeks for a first printed order and 3–4 weeks for repeats.

The full list of print options is on our private label and custom printing page. The most common printed items we ship are T-shirt bags, die-cut handle bags and poly mailers.

FAQ

How many colors can be printed on a plastic bag?

On our gravure presses, up to 8 colors on one side or both sides. Each color needs its own cylinder, so a one- or two-color logo is the cheapest to set up.

What file format do I need for printed bag artwork?

Vector artwork in AI or PDF, with fonts converted to outlines and Pantone colors named. Photos should be high resolution. We will tell you if anything needs to change before we make cylinders.

Why does the print on some plastic bags rub off?

Usually because the film was not corona-treated enough before printing, the treatment faded before the film was printed, or the ink was wrong for the film. It is a factory process problem, not a design problem.

Can you print on clear or black film?

Yes. On clear or dark film we print a white base layer under the colors so they stay bright and true to the Pantone reference.

Is flexo or gravure better for printed bags?

For long runs and repeat orders with stable artwork, gravure usually gives more consistent color and lower cost over time. For short runs or artwork that changes often, flexo setup is cheaper.

If you have a printed bag in mind, send us your spec and artwork. We will check it, suggest the right film and thickness for the load, and send a free stock sample with the quote.