Plastic pressed parts made of PTFE
Handling a high-quality material – Progress due to professionalism
Due to its high melt viscosity, PTFE – unlike most thermoplastics –cannot be processed by casting, injection moulding or transfer moulding into semi-finished or finished products. Due to its high molecular mass of approx. 10 million g/mol, PTFE forms extremely long molecular chains, which mean that the material remains a solid even when the melting temperature of approximately 327°C is reached. For this reason, different methods are necessary here than for other thermoplastics.
Manufacturers supply the raw material in the form of fine or free-flowing powder. The bulk density for fine powder is about 300 – 500 g/l and about 700 – 950 g/l for free-flowing powder. Homogeneous PTFE material is achieved by compression to around 2,100 g/l. Depending on the type of powder, this corresponds to a compression ratio of 2.2:1 to 7:1. Compression must be carried out very slowly so that the air contained between the powder particles can completely escape. This is achieved with high pressures of 12-130 MPa. Special tools are required to withstand these enormous pressures.
These tool moulds are usually made of high-quality steel. Even a relatively small expansion of the mould during the pressing process can lead to shearing and cracks in the material. Often the result is a partial or total loss of the part. At Beichler + Grünenwald, such failures are minimised to the greatest possible extent by means of the special geometry and design of the tool moulds.
In static pressing, PTFE granulate is compressed into press moulds under hydraulic pressure to form simple moulded parts. These blanks are sintered in the furnace with precisely defined sintering curves. The blanks are also sintered in the same way during isostatic pressing. Beforehand, however, the PTFE granulate is placed in custom-cut tools – very close to the finished part contour - and pressurised on all sides. This is the most efficient way to process PTFE for finished parts with dimensions that vary greatly.