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.

 

[alphy]

for reaching out and sharing your project with me. Designing a plastic scrap grinder can be a complex task, but incorporating impact energy can certainly help determine the force needed for shearing. Impact energy is a measure of the energy required to break a material, and it can be determined through tests such as the Izod or Charpy test.

To use these values in your calculations, you will need to first determine the impact energy of your specific plastic material. This can be done by conducting an Izod or Charpy test on a sample of the plastic. Once you have this value, you can use it in the equation for impact energy: E = (mgh)/A, where E is the impact energy, m is the mass of the hammer, g is the acceleration due to gravity, h is the height of the hammer, and A is the cross-sectional area of the sample.

Next, you will need to determine the energy required to shear a certain area of the plastic. This can be done by calculating the shear strain energy, which is equal to the product of the shear stress and the shear strain. Once you have this value, you can divide it by the impact energy to determine the force required for shearing.

It's important to note that these calculations may not provide an exact force needed for shearing, as there are other factors that can affect the process, such as the design of the grinder and the properties of the plastic material. However, incorporating impact energy can certainly help in determining a more realistic force for your design. I recommend consulting with a materials engineer or conducting further research to ensure the accuracy of your calculations. Good luck with your project!