How Are Plastic Bags Made? Step-By-Step Plastic Bag Production Process

Flexible, lightweight, and affordable. For billions of people plastic bags are an efficient and cheap way to transport goods. As the production of plastic bags continues to increase worldwide we must ask, what are the environmental drawbacks of our obsession with plastic bags?

How can we address consumption trends where plastic bags are often used only one time? How can we make the bags themselves from environmentally friendly materials? We first need to understand what plastic bags are made of and how are they made.

Let’s take a look at the most popular method of producing conventional plastic bags, a process called blown film.

What are plastic bags made of?

Petrochemicals dominate the raw material stock for producing plastic bags. The vast majority of single use plastic bags are made out of a few structural chains of the molecule polyethylene.

Approximately 6 percent of oil produced globally goes towards the plastic manufacturing industry, with plastic bags making up a considerable (35-40 percent) portion of plastics production.

The process of making plastic bags

#1 Extraction of raw material

The polyethylene that makes plastic bags is derived either from refined oil or cracked natural gas. These hydrocarbons are found mostly underground and must therefore be reached by drilling wells. This process disrupts the local ecosystem and can present disastrous consequences for human and environmental health if leaks occur.

The raw gas or oil is sent through pipes to a refinery, petroleum is separated at these sites into different densities so the right oil needed for plastics can be extracted.  

This oil or gas is superheated and pressurized to isolate pure polyethylene chains which can be combined (polymerized) to form resin pellets of pure plastic.  

Whether the product will be a grocery bag or a plastic shopping bag, different amounts of heat and pressure will be applied to create plastic resin pellets of different densities.

Grocery bags are often made from a high-density plastic (HDPE) which has a higher tensile strength compared to plastic films which are made from low-density plastic (LDPE or LLDPE).

#2 Extrusion of plastic film

To begin the manufacturing process, raw HDPE, LDPE, or LLPDE plastic pellets are again superheated and pressurized to form a uniform molten liquid-which air is pumped into from below, producing a long thin balloon of pliable plastic film that passes through a tall vertical corridor.

This plastic “bubble” cools as it expands upwards, encountering multiple rollers that stretch the plastic into thin sheets that will make the walls of the bag. These sheets are then rolled onto two separate flat beds and sent to the printing machine.

#3 Printing

Single use plastic bags are composed almost entirely of pure polyethylene molecules, however most plastic bags tend to have some form of branding printed onto them. The printing process for a sensitive and light material like a plastic bags requires a series of flexographic rollers that add color and type to the bags.

Printing requires additive dyes that can be toxic and could threaten marine organisms. However, safer alcohol-based inks can also be used and are starting to be introduced commercially.

Producing alcohols from sugary plant material helps lessen the negative environmental impact of dying plastic bags. Once the print has been applied to the bag it is sent to be converted into the finished product.

#4 Converting: Plastic bag making

The two printed sheets are then pressed at the edges to form the sides of the bag. The plastic can be wound and then cut to the desired shape and size, depending on the desired commercial intent for the bag.

At this point in the process custom options exist for altering the type of bag, for instance if the client wants to create a perforated bag for easy tearing (such as ones found in the produce section of grocery stores) they are able to do so.

The environmental footprint of plastic bag manufacturing process

Petrochemicals used to make plastic bags do not degrade quickly, in fact their very formation is due to millions of years of pressure and heat. Over the lifespan of a plastic bag it will continue to break down but not biodegrade.

The largest environmental impact of plastic bags is in the form of water pollution and, as a result, the detrimental impacts on aquatic wildlife.

Marine organisms often encounter these small pieces of plastic, sometimes in layers of seawater in a sort of “plastic soup” which animals can easily consume either unintentionally or mistaking for food.

Plastic has the ability to alter the biological composition of certain marine organisms, for instance changing the sex of fish, and can severely disrupt reproduction cycles for marine species. Over time UV radiation and heat will also cause the plastic to leach into the ground or waterways, altering the chemical composition of soils and aquatic ecosystems.

Reports of wildlife deaths related to plastic ingestion has continued to increase as plastic production has skyrocketed and future demand is expected to maintain that trajectory.

Opportunity for change

Various methods are being employed to capture plastic in world’s waterbodies and recycle the plastic in ways that will prevent future pollution. Where conventional means of recycling cannot address impurities and additives in post-consumer plastic, novel techniques can be used to maximize plastic recovery.

Take for instance pyrolysis-a process where a heated chamber devoid of oxygen separates the plastic back into its pure molecular form and capable of being repurposed as plastic resin pellets. The same as the ones used in making the original plastic bags.

It is the process of making plastic bags that will serve to benefit the emerging bioplastic industry most.

The efficiency of the blown film process makes plastics such an economic and environmentally sound method for producing bags in contrast to paper bags which require almost twice the energy to produce.

This process can easily use bioplastic resin pellets made out of cellulose or starch as a base material, allowing bioplastic bags to be readily scaled up for the market. Options to move towards bio-based plastics is increasing with the severity of our global plastic pollution.

Cellulose-based materials from sources like cornhusks or starches such as potatoes can be converted via fermentation into molecules that, when heated and mixed with additives such as glycerin, form sheets that can be heated and molded in the same way as conventional plastic bags.

Bioplastics are a necessary step forward to help address the dangers of plastic production, however more has to be done in terms of scaling-up existing bioplastic efforts.

One of emerging markets for bioplastics is 3d printing, where many of the plastic pellets or resins are derived from biological sources.

It is clear that we are now in the midst of a plastic pollution crisis, where current estimates show that the total weight of all plastics in the world’s oceans will exceed that of fish by 2050. Changes must be made to alter the impact from plastic bags, from replacing petroleum-based plastics with natural raw materials to altering consumer habits and enhancing recycling methods.  

The drastic effects of plastic pollution on the environment must be curtailed now before it’s too late, and change will begin at your local grocery store.