Surface roughness

Surface roughness plays an important role in the production and use of rubber and plastic parts:

• Adhesion: roughness affects adhesion between components or coatings.

• Friction: a smooth surface reduces friction, while a rough surface increases abrasion.

• Sealing function: for seals, low roughness is crucial to prevent leaks.

• Appearance: roughness affects the look of surfaces, particularly on visible components.

• Cleanability: smooth surfaces are easier to clean and more resistant to dirt deposits.

Surface roughness is described by various measurement parameters that quantify the geometry of the surface:

Arithmetic mean roughness (Ra)

Ra is the most commonly used value and describes the average distance of the irregularities from the centre line. A low Ra value stands for a smoother surface.

Maximum roughness depth (Rz)

Rz measures the distance between the highest peak and the deepest valley within a defined measuring section.

Root mean square roughness (Rq)

Rq is the quadratic mean of all irregularities and provides an alternative description of the surface structure.

Profile curve parameters

Parameters such as the material ratio curve (Rmr) provide information about the area distribution of the surface profiles.

Various methods are used in the rubber and plastics industry to determine surface roughness:

Tactile measuring methods

• Measuring principle: a stylus is moved over the surface and the differences in height are recorded.

• Examples: stylus roughness testers.

Optical measuring methods

• Measuring principle: the surface is scanned with lasers or microscopes to analyse its structure.

• Examples: confocal microscopy, white light interferometry.

Non-contact measuring methods

• Measuring principle: use of light reflection or scanning methods.

• Examples: 3D laser scanners, optical profilometers.

In the rubber and plastics industry, controlling surface roughness is crucial for numerous applications:

Seals

• Function: ensuring effective sealing through low roughness.

• Typical applications: flat seals, O-rings.

Conveyor belts, sheet material

• Function: roughness for grip and low-friction movement.

• Typical applications: conveyor technology in the food and automotive industries.

Medical technology

• Function: smooth surfaces for hygienic applications.

• Typical applications: medical tubing, implants.

Optical components

• Function: mirror-smooth surfaces for light-guiding components.

• Typical applications: lenses, housings for optical devices.

Surface roughness is influenced by various factors:

• Tool surface: the roughness of the tools is transferred to the end product.

• Material: rubber and plastic compounds with different flow properties result in different roughness.

• Manufacturing process: processes such as extrusion, injection moulding or calendering produce different surface structures.

• Post-processing: grinding, polishing or coating affect roughness.

The requirements for surface roughness differ between rubber and plastic:

Property	Rubber	Plastic
Typical roughness values (Ra)	0.4 to 1.6 µm	0.05 to 1.2 µm
Manufacturing process	Extrusion, vulcanisation	Injection moulding, calendering
Applications	Seals, conveyor belts	Optical components, housings
Surface treatment	Grinding, grooved structures	Polishing, coating

• Why is surface roughness important? It affects the function, appearance and performance of a component.

• How is surface roughness controlled? Through manufacturing parameters, tool quality and post-processing.

• Which roughness values are ideal? That depends on the application; seals, for example, require low roughness values.

• Can roughness be changed afterwards? Yes, by grinding, polishing or coating.

Surface roughness is a decisive quality factor in the rubber and plastics industry. It affects not only the function and appearance of products but also their durability and range of applications. Precise control of roughness through modern measuring methods and manufacturing processes ensures optimum results in a wide variety of industrial applications.