Need a REALLY strong injection moldable material

[ClintOConner]

I'm project manager on development of a new fitness product. We have an electronic module embedded inside the fitness product, and we need to protect the electronics from the use/abuse the product experiences in normal use. Basically, the product is a big ball filled with sand, and with a housing inside that holds the embedded electronics. The ball is slammed against walls and floors with as much force as really strong people can apply to it.

Here's a description of the structure, starting with the electronic module at the center of the ball:

The electronic module is surrounded by a cushioning layer of PU foam, and the PU foam is surrounded by a rigid plastic case. (I'm looking for material suggestions for the rigid plastic case.) The plastic case is suspended inside the ball (which is made of really tough TPU). The ball is slightly larger than a basketball.

The ball is filled with a mix of sand and sawdust to make it heavy. The weight of the ball will vary between 8 and 40 pounds depending on the ratio of sand to sawdust that we put into the ball.

We surround the rigid plastic case with a skin of TPU. The TPU skin protects the rigid plastic case from abrasion from the sand. The product needs to survive the force of many thousands of slams. (I'm simplifying the description of the design a bit, but this is the gist of it.)

The product will not be subject to extreme temperatures - it is designed for indoor use. And the product is not intended to be exposed to water or chemicals. It is just intended to be slammed really hard against walls and floors.

We need a material for the rigid plastic case that is very strong. The case will be formed in two parts that need to be joined together with a very strong joint.

My initial thought was glass-filled (or glass-reinforced) nylon or ABS, and then ultrasonically welding the two halves together. However I suspect there may be better choices of materials and/or methods to join the two halves together. I would greatly appreciate any suggestions from members of this forum.

Thanks
Clint

 

[CoryPad]

Poyamides (also known as nylons) are a good starting point. If your budget can afford it, there are stronger materials like PPS, PEEK, LCP, PARA, etc. The joining may become more difficult if you look at the other materials.

 

[Pud]

Have a look at this stuff:

http://www.emsgrivory.com/en/products-markets/products/grivory/grivory-gv/

Similar materials are made by others (partially aromatic nylon blends)

Note that glass filled materials are not the easiest to ultrasonic weld.

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Demon3]

Interesting. From the description you do not need a strong box, you need one with very good impact resistance. As this will not be exposed to any chemicals, the best choice would be polycarbonate (used to make motorcycle helmets).

Nice idea to mix two powders to vary the density. Have you heard of magnetite? It's an iron oxide and looks like black sand but twice as dense / heavy. So you could make your ball heavier, or, make it smaller but keep the weight the same.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com

 

[ClintOConner]

CoryPad, Pud and Demon3 (Chris): Thank you for your replies. Based on this input, I think we'll start by doing tests with glass-reinforced PC and glass-reinforced Nylon. If those don't have high enough impact resistance we'll try more expensive/exotic materials.

Thanks again,
Clint

 

[MikeHalloran]

How do you replace or recharge the battery that powers the electronics? Or does all the electronic power come from impacts? Just curious.

I'd start with neat nylon and thick-ish walls (2 to 4 mm), sealed with Permatex #2, and held together by screws and captive nuts in external bosses around the periphery for development, and remove the bosses and glue the housing with something more permanent after the design of the electronics is stable.

Polycarbonate is wonderful for impact, but that magical property goes away, completely, in the presence of, and after the presence of, a lot of common chemicals and adhesives. Select your adhesive carefully if you go with PC.





Mike Halloran
Pembroke Pines, FL, USA

 

[ClintOConner]

Hi Mike. Thank you for the reply.

By "neat nylon" do you mean nylon with no glass? Just straight nylon?

If we use PC, we would probably use ultrasonic welding or vibration welding to close the case. (Not solvents or glues.)

We have an access point to allow us to recharge the battery.

 

[Demon3]

You could perhaps induction charge the battery and skip the access point.

Nylon has very good impact resistance too but that varies a lot with humidity (low humidity means much lower impact resistance).

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com

 

[ClintOConner]

Induction charging is a good idea, but not feasible in this product. (Too much distance and material between the charging pad and the electronic module; and it would push the cost of the electronics above budget.)

Thanks for the reminder about humidity and impact resistance in nylon.

The impact forces inside this product are unusual. It's not like a hammer strike. It's a pile of sand that compresses very abruptly (in about 0.002 seconds) then decompresses almost as quickly. It's pretty hard to model. We've had to rely on a lot of empirical testing to work out the design details of this product.

 

[Demon3]

BTW you can compare impact and other data for all polymers, filled and unfilled here:

http://www.matweb.com

(it's free)

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com