How the Differing Structures of Metals and Plastics Lead to Their Different Feels

Why does metal vibrate differently when compared to materials like plastic? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Miranda Marcus, Applications Engineer at EWI specializing in Plastic Joining, on Quora:

Essentially, metal is elastic and transmits vibrations easily while plastic is viscoelastic and does not transmit vibrations nearly as well.

The typical analogy used to think about this is the spring and dashpot model. The spring represents the elastic portion of a material, and the dashpot represents the viscous portion.

When force is applied to the spring, it compresses to some point and then transfers that force. When force is removed, it returns to it’s original shape and length.

When force is applied to a dashpot, the force is absorbed and not transferred. You can think of a dashpot like a hydraulic cylinder, or for a more generic comparison, like a cup of silly putty. When you press on it, the material resists movement and slows momentum.

Metals are like springs.

Plastics are like springs and dashpots together.

[image: two common combinations of springs and dashpots used to model the motion response of polymers]

So, you must be wondering why metal and plastic react so differently to applied forces, right?

Well, it has to do with molecular structure, of course!

Metals have a well organized crystal lattice structure.

[image: metal crystal lattice structure model]

When you push on one side of this type of structure, the force is neatly transferred from molecule to molecule as there is no room for any compression or reorganization internally. There is simply nowhere for the material to move, so force is transferred through it.

With polymers, on the other hand, there is little internal organization. Even the most organized polymers are still only semi-crystalline, with many distinct amorphous areas.

[image: model of semi-crystalline structure]

As you can see, the molecules in the amorphous areas are not very well packed. They can certainly internally compress when force is applied and do not directly transfer load through the material.

Thus, when a load or vibration is applied to a polymer, a significant portion is absorbed. That energy is not lost, however, it turns to heat. If enough loading is applied rapidly enough, if can generate enough heat to melt the polymer, such as occurs during ultrasonic welding of plastics.

Now, in the case of tuning forks, and other acoustical instruments, the metal is carefully designed to naturally vibrate at certain frequencies. Sound is a wave, it has a frequency, amplitude, and wavelength. Each of these properties varies depending on the shape of the material and the modulus of the material.

[image: sound wave with amplitude and wavelength labeled]

Because metals transfer vibrations so well, they can be designed to vibrate at a certain frequency by creating prongs that match the wavelength of the desired frequency for the material being used.

Because plastics transfer vibrations very poorly, it would not be very possible to make a tuning fork out of them.

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