How Plastic Bag Sealing Types Affect Strength and Performance

Introduction: It’s Not Just About Thickness

When it comes to plastic bags, most people focus on material (HDPE vs LDPE) or thickness (microns and mils).
But there’s another factor that often goes unnoticed — and it can make or break the bag’s durability: the sealing type.

A plastic bag’s bottom seal determines how it handles stress, weight, and leakage.
From flatseal grocery bags to starseal trash liners and drawstring bags, the way a bag is sealed directly impacts its strength, flexibility, and application.

In this guide, TP Plastic USA breaks down the most common plastic bag sealing types, how they’re made, and how to choose the right one for your business or packaging needs.

1. What Is a Plastic Bag Seal?

The seal is where two layers of plastic film are bonded together to form the bag’s bottom or closure.
This seal is typically created through heat sealing, where controlled pressure and temperature fuse the film edges into a durable joint.

A strong, consistent seal ensures that:

• The bag holds weight without tearing.

• Liquids or waste don’t leak.

• The bag maintains shape and balance when filled.

Even the thickest plastic bag will fail if its seal is poorly designed or unevenly welded.

2. The Three Most Common Plastic Bag Sealing Types

2.1. Flatseal (Straight Seal)

Overview:
The most traditional sealing method — also known as a “bottom seal.”
Here, the two sides of the film are sealed straight across the bottom edge.

Key Features:

• Simple, cost-effective, and easy to manufacture.

• Clean rectangular shape.

• Can hold light to medium weight.

Best Used For:

• Grocery and T-shirt bags (HDPE)

• Lightweight trash liners

• Retail and food packaging

Advantages:

• Economical for mass production.
• Smooth, flat bottom that maximizes internal space.
• Ideal for automated packing.

Limitations:

• May leak if used for wet waste.
• Weaker corners since the seal bears full load stress.

Ideal Thickness: 10–25 microns for HDPE, 20–40 microns for LDPE

2.2. Gusset Seal (Side Gusset or Pleated Seal)

Overview:
A gusseted bag has extra folds (pleats) on both sides that expand when filled.
The seal is made at the sides, not just at the bottom.

Key Features:

• Creates more volume and flexibility.

• Allows the bag to stand upright or expand evenly.

Best Used For:

• Bulk packaging and industrial liners

• Clothing or courier bags

• Food storage or frozen goods

Advantages:

• Expands to fit irregular shapes.
• Better balance and stability when filled.
• Works well with thicker LDPE or LLDPE films.

Limitations:

• Slightly more expensive to produce.
• Requires higher-quality sealing to avoid weak folds.

Ideal Thickness: 25–80 microns

2.3. Starseal (X-Seal or Hex-Seal)

Overview:
The starseal (also called X-seal) is one of the strongest and most advanced sealing types.
Instead of a flat weld, the bottom of the bag is gathered and sealed in a star-shaped pattern.

Key Features:

• Distributes weight evenly across the seal.

• Eliminates sharp corners where waste could collect.

• Allows the bag to conform naturally to container shapes.

Best Used For:

• Trash and garbage bags (industrial or household)

• Wet waste or liquids

• Large liners for bins and drums

Advantages:

• Leak-proof and durable.
• Ideal for heavy, irregular loads.
• Uses material efficiently — less wasted film.

Limitations:

•  Slightly higher cost per unit.
•  Not suitable for bags that require flat bottoms.

Ideal Thickness: 20–80 microns depending on load type.

3. Other Specialty Sealing Types

3.1. Side Seal Bags

Instead of being sealed at the bottom, these bags are sealed along both sides, with the bottom left open for filling.
Common in packaging film rolls, zipper bags, and medical packaging.

3.2. Drawstring Bags

Include a heat-sealed top hem containing a string or tape closure.
Popular in household trash bags, laundry packaging, and pet waste bags.

Advantages:

• Easy closure and carrying.
• Premium look for consumer packaging.

Ideal Thickness: 25–60 microns (depending on bag size).

3.3. Perforated Roll Bags (On Roll Seal)

Heat-sealed between each unit with perforation lines for easy tearing.
Common in supermarkets, food service, and produce sections.

4. How Sealing Affects Bag Strength and Performance

The type of seal determines how load stress travels through the bag.