Semi-finished products

PTFE belongs to the group of thermoplastics but cannot be processed using the standard techniques for thermo-plastics such as e.g. injection molding. The very high molecular weight results in a too high melt viscosity and allows PTFE to be processed only with the special press and sinter methods as used in the manufacture of sintered ceramics. PTFE powder is compressed under very high pressure followed by a sintering procedure in which the PTFE particles fuse together. Please refer to the competence area for a detailed explanation of the manufacture of semi-finished products.

Our semi-finished products

Good to know

Find important information and guidance below on the manufacture and processing of semi-finished products.

Tolerances in semi-finished production

• Specifically the diameters often have very tight tolerances.
• Plastic materials follow other physical principles than metals. In particular, no guarantee for ensuring narrow dimen-sion tolerances as for metals can be provided due to an 8 to 10 times larger thermal expansion, volume changes due to moisture absorption as well as subsequent dimensional variations owing to slow reduction of residual tensions in long-term behavior.
• This circumstance naturally is the cause of undesirable discussions and annoyance and results not least in goods being rejected. Such predestined “scrapped” parts are inevitable even with the most reliable machines and skilled personnel.
• We therefore recommend our customers to prudently determine the tolerance ranges for turned and milled plastic parts.
• Normally, the functionality of the plastic part is guaranteed even under such enlarged tolerances.
• Accordingly, for plastic parts used in mechanical engineering, specifications of the ISO tolerance series IT 6.7 and 8 have to be avoided.
• For turned PTFE parts, the ISO tolerance series IT11 / IT12 are recommended (see also the leaflet “Tolerances for the machining of plastics” by the Fluoropolymergroup, an expert group of the “pro-K Industrieverband Halbzeuge und Konsumprodukte aus Kunststoff”, Germany.

Machining guidance

Machining tools made of HSS are sufficient for processing unfilled PTFE material. For filled materials (PTFE com-pounds), carbide cutting tools should be used to extend tool life, and also the rotational speed should be somewhat reduced. Cutting speeds of 1 to 2.5 m/s are best suited for precision turning and at these speeds the use of cooling liquids need is not required. The feed rate should be between 0.05 and 0.25 mm per revolution.

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