ShopWhy PTFE Cannot Be Injection Molded
Jun
- Introduction of PTFE
Polytetrafluoroethylene (PTFE) is a polymer of tetrafluoroethylene. Its wide range of applications extends to the chemical, mechanical, electronic, medical, textile and other sectors. The relative molecular mass of PTFE is relatively large, ranging from several hundred thousand to over ten million. Its overall crystallinity is between 90 and 95%, with a melting temperature ranging from 327 to 342°C.
- Molecular structureof PTFE
In the PTFE molecule, the CF2 units are arranged in a zigzag shape. Due to the slightly larger radius of fluorine atoms compared to hydrogen atoms, adjacent CF2 units cannot be fully oriented in a trans-cross pattern. Instead, they form a helical, twisted chain structure. Fluorine atoms almost cover the entire surface of the polymer chain, and the shielding formed makes it difficult for even the smallest hydrogen atoms to enter the C-F bond. Fluorine atoms also exhibit high electronegativity and a small atomic radius, while the bond length of C-F is relatively short. Therefore, C-F is difficult to break. These characteristics determine the various properties of PTFE.
- Why PTFE cannot be injection molded?
The main reasons why PTFE cannot be injection molded include its high melting point and the shape stability it maintains in the molten state.
- Highmelting
The melting point of PTFE is approximately 327℃, and its melt viscosity is several orders of magnitude higher than that of ordinary thermoplastics. This means PTFE becomes less fluid at high temperatures, which makes it difficult to inject into a mold using an injection molding machine to produce the desired shape.
- Shape stability in the molten state
In a molten state, PTFE can maintain its original shape, resembling a jelly-like state that cannot flow. This characteristic means that PTFE cannot be injection molded like other thermoplastics.
In addition, the dimensional stability of PTFE during processing is not ideal. Its coefficient of linear expansion changes very irregularly with temperature variations, and the thermal and cold shrinkage changes significantly. This further limits its application in injection molding.
- The forming process of PTFE
The crystalline melting point of PTFE is 327℃, but the resin can only be in a molten state above 380℃. In addition, PTFE has extremely strong solvent resistance. It is evident that neither the melting method nor the dissolution method can be utilized in this instance. The manufacturing of these products generally follows the methods used for processing metals and ceramics. This involves initially compacting the powder and subsequently sintering and mechanical processing it. Alternative methods include extrusion moulding, isobaric moulding, coating moulding and calendering moulding.
- Compression molding
Compression molding is currently the most widely used molding processing method for PTFE. The compression moulding process involves the use of a metal mould to form raw materials under specific temperature and pressure conditions.
- Hydroform method
The hydraulic method is a process that involves evenly applying PTFE resin to the interface between the rubber bag and the mold wall, followed by the injection of liquid (typically water) into the rubber bag. The pressurized rubber bag expansion towards the mold wall compacts the resin.
- Pushing molding
Pushing molding, also known as paste extrusion molding, involves mixing sieved dispersion resins with organic additives to form a paste, pre-pressing it into thick-walled cylindrical blanks, and then putting them into the press cylinder. The components are formed into their desired shape using a heating process involving pressure and a plunger mechanism. Following the processes of drying, sintering and cooling, strong and resilient products, including extruded tubes and rods, are manufactured.