A structurally modified PTFE can now be processed as a thermoplastic. That gives designers a new range of freedom, and enables hitherto impossible applications for this fluoropolymer. At the same time, part production is more efficient.
Polytetrafluoroethylene (PTFE) is a well-known high-performance material with a unique range of properties that have made it particularly suitable for challenging applications. By modifying the polymer, it has become possible to obtain a product, the newly developed Moldflon, that can be processed by thermoplastic techniques. This material thus opens up further applications thanks to the new freedom offered by thermoplastic shaping. It can overcome the conventional disadvantages of PTFE processing methods, particularly high material waste as a result of machining and poor surface quality.
Thanks to the thermoplastic processability of Moldflon, PTFE parts can be produced in a single operation. With traditional PTFE, at least three steps – pressing, sintering and machining – were necessary. The possibility of overmolding individual parts that is now available can greatly simplify product design. Lean manufacturing replaces the previous complicated multicomponent solutions For manufacturing PTFE coverings, processors have so far had two methods available:
Isostatic pressing: in this process, the insert is embedded in the PTFE raw material powder, which is then sintered. The surface usually only has to be finished by machining to meet the tight tolerance specifications. It is difficult to achieve smooth surfaces and challenging contours.
Part prefabrication: another way of producing the PTFE skin consists in prefabricating the individual parts from PTFE. Then the insert part is embedded in further shaping steps, or the PTFE components are welded to form the end shell contour. Because Moldflon can be processed as a thermoplastic, the inserts can now be overmolded, thus greatly shortening the process chain. This results in a huge savings potential, particularly for large scale production. Typical parts that are difficult to produce by conventional PTFE processes include joint capsules of ball-and-socket joints.
Generating this new ‘outer skin’ from Moldflon not only requires a great deal of experience with the material behavior, but also detailed technical knowledge about the production technology. Since Moldflon has a melting point of about 320°C, melt temperatures of 360°C and mold temperatures of about 260°C are necessary. The parts to be overmolded must consequently pass through a preheating station before they are transferred to the cavity. If required, retaining and centering pins can be used to reduce the part tolerances. To prevent the plastic melt solidifying prematurely in the region of the pins, they are heated together with the surrounding components. During injection, the pins are retracted. This requires special mold equipment. For top part quality, it is essential to coordinate all the relevant process parameters, from injection to demolding.
Polymer structure and material properties
The sliding layers in the ball joints undergo steady as well as intermittent compressive loading during use and, in cars and trucks, they also experience high temperatures in the region of the drive components. Fluctuating pressures and temperatures both require the material to have high compression resistance, but it should not have relaxation properties. This property combination is essential to minimize play during use. Frictional loads require the capsule material to have a low coefficient of friction and high abrasion resistance. The possibility of dry running during continuous operation reduces the maintenance outlay and system costs.
What must be the composition of a material that can optimally meet these requirements? The polymer structure of Moldflon in the solid state: the structure of this semicrystalline material is composed of lamellar crystallites (shown dark) and the amorphous zones between them. To achieve good mechanical strength of the material, the crystallites must be joined by sufficient numbers of ‘so-called ‘tie molecules’. These tie molecules are anchored in various lamellae, and thereby tie them together.
The main difference from standard PTFE is that the lamellae in Moldflon are about a factor of ten smaller than in conventional PTFE. As a result, comparatively small molecular chains can act as tie molecules. Short molecular chains in turn reduce the viscosity of the polymer melt, which is the prerequisite for thermoplastic processing. It is therefore possible to process Moldflon by the traditional methods used for thermoplastics such as injection molding, extrusion or transfer molding. Even melt spinning can be used to produce very thin fibers with an extremely smooth surface.
The close-meshed crosslinking of the extremely small crystallites makes the material extremely compression resistant compared to conventional PTFE, and it therefore features low cold flow. Because of the molecular displacement within the crystaliine regions, it can act as a dry lubricant similar to graphite, molybdenum disulfide or PTFE micropowders. This is the guarantee for the low friction coefficient and the consequent low abrasion of this innovative material. Table t shows the Molflon’s position in the family of ‘perfluorinated plastic products’.
Challenge and new opportunities
At normal processing temperatures, Moldflon has a corrosive effect on steel. All the melt-contacting parts should therefore be made of corrosion-resistant metals.
The screw and cylinder are made of materials such as Hastelloy C4 (manufacturer: Uddeholm Deutschland) and Inconel 625 (manufacturer: Uddelholm), which are familiar for processing PFA (perfluoroalkoxy polymer) and FEP (fluorinated ethylene-propylene).
The molds and dies are made from nickel, nickel alloys and specially coated tool steels. Large runners are used because of the melt’s shear sensitivity. For small injection moldings, the weight of the coldrunner system is often larger than the weight of the parts. Thanks to Moldflon’s good recyclability, the sprue scrap can be easily returned to the product stream. A balanced runner system such as that used to produce extremely small seals for microoptical systems: the pressure and injection velocity are the same for all cavities. A balanced manifold system ensures a wider processing window and more constant part quality.
Unlimited applications diversity
PTFE’s properties combined with the possibility of thermoplastic processing open up a variety of new applications for this innovative, thermo-plastically processable high-performance material, which it has
not been possible to cover in this way before.
Although the product is still in the launch phase, an extensive range of new system solutions are already emerging. The possibility of producing a large number of compounds additionally extends the range of the ‘natural’ material. The possibility can be illustrated with reference to automotive applications. The Moldflon applications and compounds produced from these are primarily accessible via extrusion and injection molding. However, secondary processing methods such as blow molding or thermoforming, as well as automatic machining of simple extruded profiles are used here.
Summary
The combination of traditional properties of PTFE, with the new process techniques for this material, such as injection molding, transfer molding and extrusion, but also thermoforming, blow molding or melt spinning, will allow completely new products to be produced economically on a large scale. It will also be easier to produce compounds based on Moldflon. This will allow the applications of this material to be expanded significantly beyond the existing limits for fluoropolymers.
Authors:
Katja Widmann
Michael Schlipf
