One of the first questions a business ordering bags asks is about thickness. The question looks simple, but the answer is not a single number. Two bags of the same thickness can behave very differently depending on their raw material and how they were made. In this post we look, step by step, at what to consider when choosing thickness.
The aim is to move beyond the “the thicker, the better” approach. A bag that is thicker than necessary wastes material and raises cost; a bag that is thinner than necessary tears in use and fails to do its job. The right thickness lies between the two, and it depends on the use.
What thickness tells you, and what it doesn’t
Bag thickness is expressed in microns. One micron is one thousandth of a millimetre. Thickness directly shows how much material went into the bag, so it also largely determines its weight and cost.
Thickness alone, however, does not tell you how strong the bag is. Strength also depends on the type of raw material, the film structure, the production conditions, the quality of the seal seam and the shape of the handle cut. A well-designed thin bag can perform better than a carelessly made thick one.
Single-layer and double-layer thickness should not be confused either. A bag is made of two layers of film; quotations and specifications should state clearly whether the thickness is given for one layer or for the two layers combined. This small detail can easily make comparisons misleading.
Describe the use first
Choosing thickness should start from the use, not from a number. What will go into the bag, how it will be carried and how long it will be used set the frame for the right thickness. The following questions help build that description:
- Load: Is the product going into the bag light or heavy? Is the load spread over the base of the bag, or do sharp corners press against the film?
- Carrying: Will the bag be carried by hand, or will it go into a box or a courier shipment? When carried by hand, the load concentrates in the handle area.
- Duration: Will it be used once, or opened and closed again and again?
- Conditions: Are there demanding factors such as cold environments, sunlight, humidity or abrasion?
- Appearance: Do printing and transparency matter? The opacity of the film affects whether the product is visible or hidden.
Once these answers are clear, the conversation with the manufacturer changes too. Saying “we want a bag that will carry this product under these conditions” instead of “we want a bag of this thickness” leaves room for the manufacturer to bring their experience to the job.
Raw material is half of thickness
Members of the polyethylene family show different strength at the same thickness. That is why thickness and raw material cannot be considered separately.
HDPE gives a stiff film and offers high tensile strength even at a low thickness. This is why t-shirt bags can be thin. LDPE gives a softer, more flexible film and is usually made thicker to do the same job. LLDPE increases impact strength and puncture resistance while keeping flexibility; when added to a blend, the same job can often be done with a thinner film.
Films that use recycled material are a different case. Because the properties of regranulate content can vary from batch to batch, a small margin in thickness may be needed to secure the same strength. This should be assessed together with the manufacturer and on the basis of samples.
How do design details affect thickness?
The shape of the bag also determines how much thickness it needs. Where the load concentrates and where the film weakens vary with the design.
In a t-shirt bag, the load concentrates on the two handles. When the corners of the handle cut are smooth and clean, even a thin film can carry the load; a sharp notch, on the other hand, can start a tear even in a thick film. In reinforced bags, a second layer added at the mouth strengthens the handle area and can allow the body to be thinner.
In gusseted bags, the gusset lets the bag hold more volume; as volume increases, so does the load. The thickness of a gusseted bag should therefore be considered differently from that of a flat bag of the same size. The quality of the seal seam is decisive too: a seam made with too little or too much heat becomes the weakest point of the bag, regardless of film thickness.
Is increasing thickness always the solution?
When a bag tears, the first solution that comes to mind is to make it thicker. This sometimes works, but if thickness is not the real cause of the problem, it only raises cost. A bag that opens at the seal seam may open at the same place with a thicker film too; a thick film can even make the right sealing setting harder to achieve.
So when a problem arises, the first step is to look at where and how the damage occurs. Does the tear start at the handle corner, does the bag open at the seam, or is there a spot on the body where a sharp corner has punctured it? Each points to a different solution: correcting the cutting die, changing the sealing setting or reviewing the raw-material blend. Thickness is only one of these options.
Discuss thickness tolerance
Blown film is never exactly the same thickness at every point. Small differences in the die gap, and the cooling and drawing conditions, leave thinner and thicker zones in the film. A thickness value therefore only has meaning together with the accepted range of deviation.
Thickness tolerance should be discussed openly before ordering. Looking only at average thickness can be misleading; the thin zones of a film whose average looks sufficient may be where tearing starts. When checking incoming bags, taking measurements from different parts of the bag rather than from a single point also gives a more reliable picture.
Confirm with a sample
Every calculation on paper should be confirmed by real use. Requesting a sample before moving to series production, and testing that sample under real conditions, is the step that most reduces uncertainty in choosing thickness.
In the test, fill the bag with the real product, carry it over the real distance and, if needed, open and close it a few times. Check for stretching in the handle area, opening at the seal seam or punctures at the corners. Testing several samples of different thicknesses side by side also shows the point beyond which extra thickness makes no difference.
Short checklist
- Describe the use: load, way of carrying, duration and conditions.
- Choose the raw material together with the thickness; if the blend changes, reassess the thickness.
- Clarify in writing whether the thickness is single-layer or double-layer.
- Do not consider design details such as handles, gussets and seals separately from thickness.
- Discuss the accepted thickness range before ordering.
- Test the sample in real use before series production.
The right thickness is not a value read from a table but the result of a conversation between use and production. The more concrete that conversation, the better the bag does its job.