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The Manufacturing Process of Polythene Bags

As a UK polythene manufacturer, we make a lot of polythene bags – so we know a thing or two about the manufacturing process! If you’ve ever wondered “how are polythene bags made?”, then this is for you. Because, as dull as it might seem, the process of making polythene bags is science and engineering in action. And it’s pretty fascinating stuff… Well, it is to us at least!

Let’s guide you through the manufacturing process of polythene bags – starting with how we choose raw materials.


Photograph of a polythene bag on a black background. It appears to be a clear bottom weld polythene bag.


1. LDPE or MDPE?

First, we need to select the right base polymer. Most of the time, LDPE (low density polyethylene) is used for polythene bags. MDPE (medium density polyethylene) can be used instead for applications where greater strength at lower gauges is required.

Learn more about MDPE

The choice of material all comes down to the packaging design and specification phase – but generally speaking, LDPE is the most sought after material. After figuring out what the best material for the job will be, it’s time to devise a polymer blend – where we can introduce things like performance enhancing additives, colours, and more.

2. Making the masterbatch and polymer blend

This is where we start the customisation process, and where the raw materials are mixed to achieve the desired properties and characteristics of the final polythene bag. The blend of pre-melted solid materials is called the masterbatch – which can be tuned with a ton of variables for different outcomes.

How is polythene made?

For example, if the bags are going to package electronic components, anti static additives will be introduced to the blend – or if the bags are for outdoor use, ultraviolet inhibitors will be added. Colours and pigments are also added to the masterbatch, and these can be tuned to taste; anything from a slight tint to fully opaque (depending on the colour, gauge and intended use).

Learn more: How Is Black Polythene Made To Be Opaque?

The masterbatch is also where recycled material is blended in. A blend of 30% recycled material by mass will allow the final material to avoid the plastic packaging tax, so this is a common blend – but 100% recycled material is also possible, depending on use. Now we’ve got the blend sorted, it’s time to make some film…

3. Extrusion and blowing

The masterbatch of cold granules is placed in a film extruding machine, which heats up the masterbatch until it’s just melted. The jam-like masterbatch is forced through a die, that pops the material out vertically – and air pumps blow it into an elongated bubble.

With a careful balance of temperature, pressure and speed, the width and thickness of the film can be controlled. Rollers stretch the film in the other direction, for greater control. By doing a stretch in both directions, the polymer chains are layered across each other, making the polythene film stronger.

The blown film is then flattened between another series of rollers – and now, we have LFT, or layflat tubing. This is the starter material for all polythene packaging products, be they sheets, bags or sleeves. For making polythene bags, the flattened roll of LFT is left as-is before moving on to printing.

4. Printing

The LFT can be printed with branding, information, regulatory wording (like recycling info) or anything else – typically with a digital printing process that allows for detailed shapes and rich colours. The film is processed to create a surface that will accept inks better, to avoid smearing and running. Once printed, the film can be formed into bags – and there’s a lot of variety on offer!

5. Converting LFT to bags

The continuous film can be cut, folded and sealed in a multitude of ways to form individual bags. Gusseted bags can be created, using folds and welds that allow the bags to open out with greater volume, hold more weight and produce a reliable, self-standing shape after filling. But for simplicity’s sake, let’s focus on the main polythene bag types; side weld, bottom weld and skirt side (or mixed) weld.

Side weld bags

Side weld bags are produced by a machine using a hot knife that cuts and seals the LFT at the same time. The cut edges form the sides of the bag, and the bottom of the bag is the fold in the LFT.

Bottom weld bags

To make bottom weld bags, the LFT is pulled through the bag-making machine and a hot wire makes a seal across the tube. Just after the seal is made, a fast moving, super sharp blade cuts the film below the seal. This forms the bottom of the bag, and the folds of the LFT make the sides.

Skirt side (mixed) weld

For heavy duty usage, mixed weld bags are often specified. These bags have a bottom weld seal at both sides, which is typically stronger.

Bags on a roll

Using perforations, precise cuts and other methods, bags can be wound on a roll – to either be used for simplified manual packaging, or to be used as Autobags on a roll in automated packaging systems.

6. Quality control

Far from being a final checklist item, quality control is ongoing throughout the polythene bag manufacturing process – through visual inspections and mechanical testing. But at the end of the process, we take some extra steps to make sure the final product is exactly what the customer ordered, within trade tolerances.

Find out more about the trade tolerances we work to

What happens next?

Post manufacturing, we deliver the finished polythene bags to the customer, or add them to our stock packaging inventory for purchase as general packaging material. Our polythene bags go on to package everything from car parts to medical supplies – and anything else you can possibly think of, including transporting live koi!

After sale, the fate of these bags is out of our hands, but we do provide guidance on safe disposal and recycling of pre-consumer material to better manage polythene waste.

Create polythene bags to your exact specification

NPF Packaging creates your ideal packaging products – including polythene bags, made to spec and manufactured on-site. Get a quote now, or call us on 01773 820415 to start your order.