Why You Shouldn't Pull the Handles of an Insulated Bag in the Opposite Direction
The handles of insulated bags are usually attached using ultrasonic sealing, a process that creates directional strength. Here's a detailed explanation of why this matters, how ultrasonic sealing works, and some everyday examples to help you understand.
🧵 1. Why shouldn't you pull the handles in the opposite direction?
The handles of insulated bags are typically fixed to the bag body using ultrasonic welding (sealing).
When you pull the handle along the seam direction, the stress is evenly distributed, and the welded joint can withstand the force.
But when you pull in the opposite direction or sideways, the force acts directly on the edge or weakest point of the weld, making it easy to tear or break.
🔧 2. How is this related to ultrasonic sealing?
- Basic Principle of Ultrasonic Welding
Ultrasonic welding uses high-frequency vibrations to generate heat at the contact surfaces. This melts the material at the joint, and pressure is applied to form a firm bond.
A strong weld depends on the direction of the force being aligned with the welded surface — it works best when the load is even and symmetrical.
- Directional Strength
Ultrasonic welds do not have uniform strength in all directions.
They are strongest under vertical tension.
But under lateral tearing or twisting force, the welded area is more likely to fail.
🧺 3. Everyday Examples
📦 1. Courier Bag Seals
Some courier bags use heat-sealed closures (similar to ultrasonic sealing).
If you try to open it along the seal, it’s hard to tear.
But if you pull it from the side, it opens easily.
👕 2. Heat-Pressed Clothing Labels
Some sportswear has labels that are heat-pressed onto the fabric.
Normal wear is fine.
But if you lift a corner and pull, the label peels off easily.
👛 3. Edge Taped to a Table
If you press a piece of tape flat on a table:
Pulling along the tape won't break it.
But if you lift the edge and peel sideways, the whole strip comes off quickly.
Ultrasonic sealing behaves like this kind of tape — it's bonded with high-frequency energy, and its strength depends on the direction of the pulling force.
