[thedoc (OP)]

asked the Naked Scientists:
   Good Morning,

I have a simple question. I have a product that I have made in China. It has a 2 plastic base that contains a 2" round magnet that is glued in. I recently received a shipment and ;many of the magnets are cracked, I am hearing a bunch of theories but want a scientific answer. I ordered 1,000, 2 years ago, same time of year with no magnet issues. This shipment 50% of the base magnets are cracked. Attached is one of the worst cases. ;Most are hairline. What do you think caused it? I must add there is a matching magnet disc that attaches to this base of which I have yet to find a crack. The factory said they were not cracked when the left the facility. They are saying it must have been the weather making the plastic expand/contract. I believe it is pretty mild in August in China. From there it sails to LA, and then is trucked to Florida. I am wondering if they used a substandard magnet in this production run, or could it be the glue or or or... I would love a scientific answer. To confirm or dispute.

Thank you for your time

Amy Griffith
What do you think?

[chiralSPO]

My best guess is that the package was handled roughly or dropped.

[evan_au]

Many metal magnets are fairly brittle, and can crack if dropped.
The plastic-embedded "fridge magnets" seem more resilient and pliable.

It may not look pretty, and the magnetic field will be a bit weaker, but does the hairline crack affect the functionality of the product?

[alancalverd]

Sintered ferrite magnets are brittle, as are some "rare earth" magnets. I'd be inclined to look at the bulk packaging: were the parts jammed in to a box, or was the box inadequately padded and dropped? A photograph would help.

[PmbPhy]

asked the Naked Scientists:
   Good Morning,

I have a simple question. I have a product that I have made in China. It has a 2 plastic base that contains a 2" round magnet that is glued in. I recently received a shipment and ;many of the magnets are cracked, I am hearing a bunch of theories but want a scientific answer. I ordered 1,000, 2 years ago, same time of year with no magnet issues. This shipment 50% of the base magnets are cracked. Attached is one of the worst cases. ;Most are hairline. What do you think caused it? I must add there is a matching magnet disc that attaches to this base of which I have yet to find a crack. The factory said they were not cracked when the left the facility. They are saying it must have been the weather making the plastic expand/contract. I believe it is pretty mild in August in China. From there it sails to LA, and then is trucked to Florida. I am wondering if they used a substandard magnet in this production run, or could it be the glue or or or... I would love a scientific answer. To confirm or dispute.

Thank you for your time

Amy Griffith
What do you think?

You won't be able to get a "scientific answer" unless it  can be duplicated and further studied in a controlled environment, i.e. the lab. Otherwise it's guesswork. You haven't stated exactly what material the magnet is made of, exactly what kind of plastic it's made of and maybe the dimension and shape of the magnet (i.e. its size and geometry).

One possibility that I can think of is that the magnet and plastic are made of materials having different coefficients of expansion. That means that when there's a change of temperature there will be different changes in dimensions of the magnet as opposed to the plastic. That will cause stress in the material and possibly crack it.