How to choose the right polythene bag

What should I decide before choosing a polythene bag?

Choosing a polythene bag comes down to six questions, best answered in order: what is going in the bag, what it weighs, how sharp or abrasive it is, if anyone needs to see the contents, how the bag is closed, and how many you get through. Answer those and the bag type, the gauge and the supply format follow on their own.

The order matters because each answer narrows the next. A 2kg item with hard corners rules out thin film before you have thought about print. A bag that runs through an automatic packing machine rules out formats the machine cannot feed, however well the film performs by hand.

Take the measurements while you are at it. Polythene bags are specified by layflat width and length, plus gusset depth if the bag has one, and those are the numbers a manufacturer needs to quote. Our bag and mailing size finder works a bag size back from the product dimensions, and our polythene product selector narrows the range if you want a starting point before you enquire.

What gauge of polythene bag do I need?

Gauge is set by the weight of the contents and the handling the pack takes, not by how big the bag is. A large, light item such as a duvet travels safely in film that would split under a small box of fixings. Work from the load and the journey: a bag that sits on a stockroom shelf needs less film than the same bag going through a courier network where parcels are thrown, stacked and sorted.

Polythene gauge in the UK is quoted in microns, one thousandth of a millimetre. As a rough map of the range we manufacture in Blackburn, light picking and packing bags sit around 15 to 50 micron, general purpose bags 30 to 200 micron, mailing bags 55 to 90 micron, and heavy duty bags for sharp or dense contents 100 to 250 micron.

Going thicker is not automatically safer. Heavier film costs more per bag, adds weight to the parcel and puts more material into the market. The right gauge is the lightest one that survives the journey without failures. Our polythene thickness converter moves between microns, gauge and mil if a supplier has quoted you in different units.

How do the shape and sharpness of the product change the bag?

Sharp corners, hard edges and abrasive surfaces cause most split polythene bags, and they are why a bag that works on one product fails on another of the same weight. A boxed item concentrates the load on four small points of film, and metal components, timber offcuts or anything with an exposed fixing will work through thin film given enough handling.

There are three ways to deal with it. Step the gauge up, which is what a high strength polybag is for: film from 100 to 250 micron with strength and stiffness set for the job. Change the shape of the bag, because a gusset lets a boxy product sit square inside instead of straining the seams. Or change the film, since co-extruded film is built from two or more layers extruded together, so one layer carries the strength while another handles opacity or slip.

Bulk and shape also decide the format. A product that stands upright and fills the bag suits a pre-made pillow bag. A product whose length varies from order to order suits layflat tubing, which you cut and heat seal to whatever length that item needs.

Should the bag be clear, coloured or printed?

Clear film is the default for anything checked, picked or scanned through the bag. A picker identifies contents without opening the pack, a barcode reads through the film, and a quality check does not need the bag cutting off.

Opaque and coloured film is for privacy and presentation. Mailing bags are opaque so nobody can see what has been ordered, with grey and white the usual choices. Colour is also a sorting tool: coloured refuse sacks separate waste streams on a site without anyone reading a label, and anti-static bags are commonly tinted so static-sensitive stock is obvious on a shelf.

Print is applied during manufacture, so the ink sits on the film rather than on a label that can peel off in a sorting office. A logo, return address, recycling wording, handling instruction or warning text all print to your artwork. Extruding to a plain colour with no print is the cheaper route to a consistent look, so it is worth asking for a quote both ways.

How does the sealing method change the bag you order?

How a bag is closed decides its format, so settle it before you fix size and gauge. A permanent peel and seal strip suits despatch, which is how mailing bags work: the packer peels the liner, presses the flap and the bag is shut for good, and an attempt to open it in transit shows on the film. A heat sealer suits a bench packing varied products, because you cut from layflat tubing and seal to the length each item needs. Tape or a tie suits open topped bags that get opened and reclosed.

Filling speed matters as much as the seal. Perforation on reel bags come on a continuous reel with a tear line between each bag, so a packer takes one at a time single handed instead of picking through a flat pack. Pillow bags are pre-made and open at one end, which suits high volume hand packing and machine filling where every bag has to present the same way.

If a machine is involved, say which one before anything is made. Slip and stiffness are film properties we tune at extrusion, and film that is too grippy or too floppy will jam a machine that runs a similar bag perfectly well.

How do volume, minimum order and lead time affect the decision?

Volume decides how far it is worth moving from a standard specification. A one-off job or low annual usage is usually best served by a standard product in a size close enough to work. Steady, repeating volume justifies a bag made to your exact dimensions, because the film you stop paying for on every bag adds up and a bag that fits is quicker to pack.

Minimum order quantity varies by product and gauge, because the film is extruded before it is converted into bags, so a heavy gauge in a narrow width behaves differently to a light gauge in a wide one. We support one-off bespoke trials, so a specification can be proved on a small run before it goes into a standing order.

Plan the timing. Bespoke runs are typically 2 to 3 weeks, quicker on repeats and standard specifications, so a new bag needs specifying before existing stock runs low rather than after. Decide the supply format at the same time: flat packed in cases, on a perforated roll or on a reel, packed to counts that suit how your packing bench is restocked.

Should I specify recycled or biodegradable film?

Recycled polythene is the usual starting point, because recycled content changes the film rather than how the bag behaves on your line. Recycled grades are available across much of what we manufacture, and refuse sacks can be produced from post-consumer waste. The practical questions are how much recycled content the specification can carry while still meeting the strength you need, and how the film has to look, since recycled material can affect appearance.

Biodegradable film is a separate decision, not a stronger version of the same one. Biodegradable grades are designed to break down by biological action at end of life, which suits some applications and is wrong for anything that has to sit in stock for a long time or feed back into a recycling stream. We manufacture biodegradable options, so tell us the shelf life and the disposal route before choosing.

Recycled content also interacts with the UK Plastic Packaging Tax, which applies to plastic packaging that does not contain enough recycled material. Rates, thresholds and registration rules change, so check the current figures on gov.uk rather than relying on any supplier's summary, including ours.

Frequently asked questions

How do I measure a polythene bag?

Measure a polythene bag by layflat width first, then length, and add gusset depth if the bag has one. Layflat width is the bag laid flat and measured across, which is half the circumference rather than the width of the product going in it. For a boxed item, allow for the depth of the box as well as its width, plus a little clearance so the film is not under tension when the bag is full. Our bag and mailing size finder works these numbers out from the product dimensions.

Is a thicker polythene bag always better?

No. A thicker bag costs more per unit, adds weight to the parcel and puts more material into the market for no gain if the bag was not failing in the first place. The right gauge is the lightest film that survives the journey without splits or punctures. If you are getting failures, look at where the bag is giving way before you increase gauge, because a seam failure and a puncture from a sharp corner call for different changes.

Can I get polythene bags made in a size that is not standard?

Yes. We extrude the film and convert the bags at our Blackburn factory, so width, length, gusset depth, gauge, colour and print are all specified by you rather than picked off a stock list. A bag made around the product normally uses less film than the nearest standard size that fits it. Send the dimensions of the item with your enquiry and we will work the bag size back from it.

What is the difference between a mailing bag and a general polythene bag?

A mailing bag is opaque, has a permanent peel and seal strip along the flap, and is built to survive a postal or courier network on its own. A general polythene bag is usually clear and open topped, made for storing, lining, packing or protecting an item inside another pack. If the bag is travelling through the post with nothing around it, you want a mailing bag.

How long does it take to get bespoke polythene bags manufactured?

Bespoke runs are typically 2 to 3 weeks, and quicker on repeat orders and standard specifications. The film has to be extruded before it is converted into bags, which is why a brand new specification takes longer than a repeat of one we have already made for you. Order a new specification while you still have stock of the old one rather than after it has run out. Call 01254 691354 or email sales@morepolythene.co.uk to check timing against a launch date.