Theory

THEORY

Some squeaks and rattles theory in case you are geeky like us

Why does noise occur in automotive interior trim?

This is information is just intended as an introduction.

Some materials may be not be compatible when they come in to contact with each other. Other materials maybe smooth against other surfaces but will be rough when rubbed against another surface of the same type, e.g. ABS or Glass.

When two surfaces rub, the true area of contact is usually only a tiny fraction of the whole surface, where peaks and troughs of materials meet.

 

One of the keys to understanding the concept of noise generation is understanding the surface grain frequency of the materials you are using.

Grain frequency is the pattern of the microscopic surface structure formed during the manufacture of the material. The relationship between two grain patterns, such as size (depth and height) of grain is very important in their potential to generate noise when dry sliding.

Surface Topography

The microscopic physical shape of the surface has a big say on the compatibility between two connecting surfaces.

Like a jigsaw puzzle, some surfaces have a grain frequency that results in the interlocking of ridges under pressure (green colour), the outcome of which is a possible noise. Some surfaces have low surface affinity under pressure (blue colour) due to their low grain frequency – not as much chance of noise resulting from this scenario.

The microscopic physical shape effects the compatibility between two connecting surfaces.Unwanted noise generated by two surfaces can be due to the accumulation of multiple factors such as:

  • Generation of stick slip
  • Bending of surface peaks
  • Compression of spring over peak
  • Squeezing out of air to cause a partial vacuum
  • Stretching of polymer strands which cause potential energy rises at 1000’s of points. At a certain point the energy is released and causes a noise

One way to gauge how materials interact is finding out the coefficient of friction (COF) between two surfaces. Coefficient of friction (aka the frictional coefficient) is a physics value that describes the ratio force of friction between two materials and factoring in the amount of force pressing them together.

The coefficient of friction obviously depends on the materials used; an example of this would an ice skater. Metal glides smoothly and easily over ice. The COF between ice and metal would be very low.

There are other factors contributing to a COF figure:

  • Compression to spring over peaks
  • Squeezing out of air to cause a partial vacuum
  • Stretching of polymer strands which cause potential energy rises at 1000’s of points. At a certain point the energy is released and causes a noise

So, unless problem surfaces are separated or a specialist physical barrier is introduced between the two materials (e.g. felts or foams), there three main factors that must be overcome when trying to solve a noise issue between to two trim materials with an antifriction coatings or dry film lubricant:

  1. Surface grain frequency
  2. Shear strength of the microscopic grain
  3. Yield pressure of the microscopic grain

Would you like to know more about the products? Have a specific problem in mind? Get in contact with us for assistance.

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