How Are Plastic Bags Made? A Step-by-Step Guide

Plastic bags may appear simple, but their production involves a detailed, multi-stage process.

A plastic bag is created by melting raw plastic pellets¹, extruding them into a thin film, printing designs onto the film, and then cutting and sealing the film into bags using specialized machinery.

The process blends chemistry with mechanical engineering, beginning with raw materials extracted from the earth and ending with high-speed manufacturing. This guide outlines each step, from raw material extraction to the production of finished bags.

From Oil to Raw Material: Extracting Petroleum

The creation of a plastic bag begins with crude oil or natural gas, extracted from underground and processed into key components for plastic production.

The initial stage involves extracting crude oil or natural gas, transporting it to a refinery, and separating it into various fractions, including naphtha, the primary feedstock for many plastics.

Drilling and Extraction

Crude oil and natural gas are drilled from underground reservoirs. The raw material is then transported to a refinery. The consistency of the raw material is crucial, as it affects the quality of the final plastic.

Refining Crude Oil

Fractional distillation at the refinery separates crude oil into different components:

1. The oil is heated until it vaporizes.
2. The vapor rises in a fractionating tower.
3. Cooling at different levels condenses fractions into liquids.
4. Lighter fractions rise to the top, while heavier ones remain at the bottom. Naphtha, a critical feedstock for plastic, is among the fractions collected.

Creating Monomers

Naphtha undergoes a cracking process, breaking large hydrocarbon molecules into smaller monomers such as ethylene and propylene. These monomers serve as the chemical building blocks for plastics. While most plastic bags originate from fossil-based feedstocks, alternatives like PLA (derived from plant-based sources) can also be used.

Turning Petroleum into Plastic Pellets

Once monomers are obtained, they are polymerized into solid plastic pellets.

Polymerization links monomers into long polymer chains, producing polyethylene or polypropylene. These polymers are cooled, solidified, and chopped into pellets for ease of transport and processing.

The Polymerization Process

Thousands of monomers join together under controlled temperature and pressure, forming strong, durable plastics. The resulting polymer type determines the properties of the final product.

Pellet Types and Uses

Different pellet types yield plastics with distinct properties:

Pellet Type	Full Name	Key Properties	Common Bag Types
HDPE	High-Density Polyethylene	Strong, stiff, lightweight	T-shirt bags, grocery bags, garbage liners
LDPE	Low-Density Polyethylene	Flexible, transparent, good sealing	Food packaging, bread bags, produce bags
PP	Polypropylene	High clarity, tough, heat resistant	Laminated pouches, snack food bags
PLA	Polylactic Acid	Biodegradable, compostable	Eco-friendly shopping bags, organic food packaging

The molten polymer is extruded into strands, cooled, and chopped into pellets ready for use by manufacturers.

Feeding Pellets into the Blown Film Machine

Plastic pellets are processed into thin film using blown film extrusion².

Pellets are loaded into a hopper, which feeds them into the extruder barrel by gravity.

Hopper Function

The hopper ensures a continuous flow of pellets into the extruder, preventing thickness and quality variations.

Additives

Colorants, UV stabilizers, and other additives can be blended with the pellets at this stage to achieve desired properties.

Automation

Precision feeding systems maintain a constant pellet flow rate, ensuring uniform film production and minimizing waste.

Heating and Melting for Film Production

Inside the extruder, pellets are heated and melted into a uniform molten state.

A rotating screw pushes pellets through a heated barrel, where they melt through applied heat and friction.

Extruder and Screw

The screw mixes and homogenizes the melt to ensure uniformity.

Blown Film Formation

Molten plastic is pushed through a circular die, forming a tube. Air inflates the tube, thinning the film to the desired thickness.

Quality Factors

Key parameters include:

• Temperature – set precisely for the polymer type.
• Screw speed – controls extrusion rate.
• Air pressure – determines bubble size and film thickness.

The cooled bubble is flattened into a film and wound onto rolls.

Printing Designs on the Plastic Film

Most bags feature printed graphics or branding applied through flexographic printing³.

Film is unwound and fed through a printing press, where flexible plates transfer quick-drying ink onto its surface.

Flexographic Printing Steps

1. Artwork is etched onto flexible plates (one per color).
2. Plates are mounted on rotating cylinders.
3. Ink is applied to raised areas of the plates.
4. Film passes through each color station until the design is complete.

Tension Control

Consistent film tension ensures accurate color registration and prevents smearing.

After printing, the film is dried and rewound.

Cutting and Sealing in the Bag-Making Machine

Printed film is converted into finished bags.

Film is cut to length and sealed by heat or pressure to form the bag's structure.

Process

Film is pulled through rollers, registered for print alignment, sealed with heated bars, and cut into individual bags.

Machine Types

Different bags require different machines:

Bag Type	Machine Type	Function
Shopping Bag	T-Shirt Bag Machine	Cuts handles and seals sides
Courier Mailer	Courier Bag Machine	Creates side seals, applies adhesive strips
Food Pouch	Lamination Pouch Machine	Handles multi-layer films, adds zippers
Produce Bag	Bottom Seal Machine	Creates a simple bottom seal
Boutique Bag	Loop/Patch Handle Machine	Adds separate handles

Precision

Seal temperature, pressure, and dwell time are tightly controlled to ensure strong, consistent seals.

Final Quality Check and Packaging

Before shipment, bags undergo inspection.

Operators or automated systems check seal strength⁴, print quality, dimensions, and material condition.

Quality Checks

• Seal integrity – tested for strength.
• Print alignment – checked for accuracy.
• Dimensions – verified against specifications.
• Material quality – inspected for defects.

Approved bags are stacked, packaged, and shipped.

Conclusion

Plastic bag manufacturing transforms raw fossil or plant-based feedstocks into a finished product through multiple precise steps. Each stage—from raw material processing to film production, printing, sealing, and quality control—relies on specialized machinery and exact process control to ensure a consistent, high-quality result.

---