Designing using Impact Energy of Plastics

[raniero]

Hi, I am attempting to design a plastic scrap grinder. To begin my calculations I need to know the force needed to shear a certain area of plastic. After noticing that by using the shear stress equation would result in a huge, unrealistic force, it came to my mind that the plastic will be cut by impact since the grinder is composed of a rotor, rotating at 1200 rpm.

How can I use values of impact energies (Izod / Charpy etc.) to determine how much energy is dissipated in shearing a certain area of this material and thus deriving the required force?

Thanks

 

[Danger]

You know that it won't be homogenous, right? You have to figure for the worst-case scenario, which means the toughest plastic that you'll ever encounter. Anything less is a walk in the park. As to the math part, I have no idea.

 

[Stephen Tashi]

After noticing that by using the shear stress equation would result in a huge, unrealistic force

How are you calculating the area over which the force is applied? Is the grinder like a sanding drum that applies roughly a uniform force over its area of contact with the plastic?

 

[raniero]

I will not be actually building this machine, but my task is to design it. It is intended to granulate sprues from injection moulding machines and other small plastic parts. Yes, I considered the toughest plastic that a typical injection moulding machine uses.

I considered the worst case scenario as a slab of plastic 1cm thick, having a roughly 4cm length per cut. I got a length of 4cm from a calculation which takes into consideration a throughput of 300 kg/hr and a screen mesh having 6mm in diameter holes.