Straining

Straining is typically carried out in a specially equipped machine known as a strainer. It consists of a feed screw, a heating element and a filter system that retains foreign matter. The process is divided into the following steps:

1. Material feed

The material, for example a rubber or plastic compound, is fed into the strainer’s hopper.

2. Conveying

A feed screw transports the material through the strainer. The material is plasticised by friction and external heaters.

3. Filtration

The plasticised material is pressed through a screen that retains foreign bodies such as metal particles, hard pieces of rubber or other contaminants.

4. Material discharge

The cleaned material leaves through the outlet and is now ready for further processing.

Straining offers a wide range of advantages:

• Improved material purity: unwanted particles are removed efficiently.

• Higher product quality: homogeneous, defect-free material leads to high-quality end products.

• Process reliability: contaminants that could damage machines are eliminated.

• Cost efficiency: reduces scrap and improves production efficiency.

Straining is used in various industries and for different materials:

Rubber industry

• Seal manufacturing: ensuring the homogeneity of rubber compounds for precise sealing.

• Tyre production: removing contaminants to avoid air inclusions or weak points.

• Conveyor belts: ensuring consistent material quality for heavy-duty applications.

Plastics industry

• Extruded products: filtration of thermoplastics for the production of pipes, films and profiles.

• Injection moulding: ensuring the purity of plastics for high-quality moulded parts.

Recycling

• Processing plastic granules: removing contaminants from recycled plastics.

• Rubber recycling: filtration of regranulates for reuse.

Various machines are used for straining:

Machine type	Properties	Typical applications
Extruder strainer	Integrated feed screw for continuous operation	Rubber and plastics processing
Batch strainer	For smaller quantities and special applications	Prototyping, small series
Automatic strainer	Automatic screen changes and minimal operator effort	Large-series production

The straining process also involves some challenges:

• Screen changes: clogged screens must be changed regularly, which can affect productivity.

• Energy consumption: plasticising the material requires a considerable amount of energy.

• Material losses: small material residues can remain in the filter system.

• Which materials is straining suitable for? It is used for rubber compounds, thermoplastics and recycled polymers.

• Which screen sizes are used? Screen sizes vary according to the application and typically range from 50 to 500 micrometres.

• Can straining affect material properties? If the process is run correctly, the material properties are preserved; however, excessively high temperatures can cause thermal damage.

• How often do screens need to be changed? This depends on the material quality and the amount of contamination, but is typically after several operating hours.

Straining is an indispensable process for ensuring the purity and quality of materials in the rubber and plastics industry. By removing contaminants efficiently, it improves product quality, extends machine life and reduces scrap. Despite some challenges, straining remains a central part of modern production processes.