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Packaging Supplies in the UK & Europe since 1953

Polythene Bags - Your Questions Answered

Safety in use

Of all the wide range of plastic films, polythene (PE) is by far the safest to use. Being chemically inert it does not impart any kind of residue or taste and is impervious to most chemicals, even acids. So it is suitable for every kind of application; all our polythene bags are made from “food safe” quality film.

Polythene bags and child safety

Light duty (especially) and medium duty polythene bags can both cause suffocation if the bag is pulled over the head. Subject to minimum quantities bags can be perforated to avoid this risk, but our normal stock bags are NOT perforated in this way.

Heavy duty bags are less conforming and, as such, are very unlikely to represent the same suffocation risk. As part of our range we do offer some self-seal clothing bags which are both perforated and printed with a standard warning notice. The link below will take you straight to them.

Garment bags

Are Polythene bags watertight and airtight?

The simple answer is NO. Polythene is "air-permeable" so it cannot be used for vacuum-bagging. There are special nylon coated polythene bags available for this purpose. Our polythene bags are not recommended for liquids as they are not designed for this, and although they may hold liquid for a short period of time, they will start to leak.  Similarly you won't be able to make our polythene tubing into bags for liquid. 

What "duty" should I use?

Firstly, remember that most polythene is much stronger than you think, and most users choose a heavier bag than they really need. But here is a guide to the duties:

Our "duties" usually conform to the following thicknesses:-

  • Light Duty - 100/120g = 25-30 microns
  • Medium Duty - 200 / 250g = 50-62 microns
  • Heavy Duty - 400 /500g = 100 – 125 microns


Light Duty - This is typically the film that is wrapped around a new electrical appliance inside it’s box. It is glossy and very clear. It has a very poor resistance to sharp corners and edges. Because it is so light and "floppy" it can be harder to pack with - especially if the item is nearly as big as it’s bag.
Light duty is mostly used as a means of keeping things clean and free of dust. However, if you are simply bagging up such items as clothing etc for storage then Light Duty bags are fine.

Medium Duty - is twice the thickness of the above. The clarity and gloss is still good but it will cope with some corners and edges (though, not metal). It is also much easier to pack with as it tends not to collapse as you’re filling it.
Medium duty is mostly used as a means of collating items together in one pack. If you want to pack such things as cables, card & flat wood modelling kits, or even plastics plumbing & drainage items our Medium duty bags are ideal.

Heavy duty - has 3 jobs; to carry weight, to cope with harder and sharper contents and to provide extra rigidity both when packing and when displayed. Builders use this weight of polythene when packing shingle or sand into 25kg bags.
If you are still in doubt, ask for a sample or two.

Polythene and the environment

Normal polythene is not biodegradable, though it IS "photodegradable". This means it will not break down when buried in a landfill, but bags will break down in sunlight. If you have ever wondered why our countryside isn’t covered in them it is because most bags will break down into dust in less than 12 months.

As this subject has become a more global issue recently we have now written a whole article within these pages. See "Plastics and the Environment".

Sizing polythene bags

On our website most of our bag sizes are shown in millimetres. To convert roughly to inches divide by 100 and multiply by four. E.g. if the bag is 375 x 500 this converts to (3.75 X 4) x (5 x 4) = 15 x 20 inches. It’s not exact, but its very close. Alternatively, go to our "measurements" section.

Manufacturing Tolerances

The polythene film extrusion process is complex and affected by external conditions like heat, therefore factories are unable to guarantee the size of bags exactly, and a degree of manufacturing tolerance is required on every polythene product. It should be noted that our measurements are the internal dimensions.

Width - Plus or minus 3mm (0.125") or 2%, whichever is greater.

Length - Plus or minus 6mm (0.25") or 2%, whichever is greater. The external dimensions are cut to cut and will therefore include a bottom skirt of about 8mm on end weld bags (apart from "grip seal bags" which are measured from the grip seal to the bottom of the bag).

Gauge - Plus or minus 10%.

Quantity - BESPOKE MANUFACTURE ONLY, Plus or minus 10%.

Sealing & closing polythene bags

Some "polythene bags" come with their own closure method. This can be a "press-together" gripper (like our "Self Seal Again bags") or self-adhesive tape on a flap. The most common form of closure though, is heat-sealing. This is where a hot wire is pressed onto the top of the bag to weld it together along a thin line. For widths up to 400mm there are very inexpensive machines that do this very effectively and are only hot during the actual welding cycle. You can use this link to view our range of heat sealers.

Printed polythene bags

Plastic bags of most types are printed as film, before they are made into bags. This means that the minimum that can be printed runs into thousands, depending on the thickness of the material and the width of the ultimate bags. There is no hard and fast rule on what these quantities are; this depends on the machine to be used, the number of colours and the dimensions of the bags, etc. As a rule of thumb, and to assist your calculations consider the following guide...

Based upon an average bag of 300mm x 400mm printed 1 colour on 1 side the minimum quantity is likely to be...

  • Light duty - 15,000
  • Medium duty - 10,000
  • Heavy duty - 6,000

For smaller bags the minimum quantities get much higher. For multiple colours the minimums also get higher.

N.B. Polythene does not easily hold a print, there is a tendency for it to rub off. To prevent this happening any film that is going to be printed is specially treated as a preparation. Standard stock bags are not treated in this way; as a result please don’t consider buying a few and getting them screen printed. The print may fall off!

Poly Bags

In our extensive range of polythene bags you will find:

If you can't find what you are looking for, call or email us - we can quote you for bespoke bags including watertight bags for transporting fish - quick turnaround times

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