Make the walls thicker, add ribs or other features to strengthen the sidewalls, switch to PET, redesign the crates or cartons to bear more of the load, etc.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

You can add fillers to the HDPE. Calcium carbonate is very cheap and will increase the stiffness and lower the strength very slightly. A filler with high aspect ratio like talc or mica will improve both strength and stiffness.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Trusted adviser to leading companies around the world
www.phantomplastics.com

Quote:

on each other's

If that means 'stackable', you might need to add features to help.
E.g., some brands of bottled water sold in the US now come in 3l bottles that have an inverted conical bottom, providing a cavity to clear the neck and cap of a bottle below, and also a shoulder below the neck, where the outer part of the above bottle can rest.
They also include recesses for handholds, and ribs to provide strength in a vertical direction.

If you post a photo of your current bottle, suggestions may appear.

Mike Halloran
Pembroke Pines, FL, USA