Wednesday, February 03, 2010

Modified PTFE Can Be Processed As A Thermoplastic

A structurally modified PTFE can now be processed as a thermoplastic. This 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 high-performance material with a 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 opens up further applications due to the 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. Due to the thermoplastic process ability 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 overmoulding individual parts that is now available can simplify product design.

Lean manufacturing replaces the previous complicated multicomponent solutions. For manufacturing PTFE coverings, processors have so far had two methods available. In isostatic pressing, 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. 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, 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 outer skin from Moldflon not only requires a great deal of experience with the material behaviour, but also detailed technical knowledge about the production technology. Since Moldflon has a melting point of about 320C, melt temperatures of 360C and mould temperatures of about 260C are necessary.

The parts to be overmoulded must consequently pass through a pre-heating station before they are transferred to the cavity. If required, retaining the centring 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 the injection, the pins are retracted, requiring special mould equipment. For top part quality, it is essential to coordinate all the relevant process parameters, from injection to demoulding. 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 minimise 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 operations reduces the maintenance outlay and system costs. What must be the composition of a material that can optimally meet these requirements? The structure of this semi-crystalline material is composed of lamellar crystallites and the amorphous zones between them. To achieve good mechanical strength of the material, the crystallites must be joined by sufficient numbers of tie molecules.

The tie molecules are anchored in various lamellae and tied together. The main difference from standard PTFE is that the lamellae in Moldflon are about a factor of 10 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 thermoplastic, such as injection moulding, extrusion or transfer moulding.

Even melt spinning can be used to produce very thin fibres with an extremely smooth surface. The close-meshed crosslinking of the very small crystallites make the material extremely compression resistant compared to conventional PTFE and it therefore features low cold flow. Because of the molecular displacement within the crystalline 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 material. 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 and Inconel 625, which are familiar for processing PFA (perfluoroalkoxy polymer) and FEP (fluorinated ethylene-propylene). The moulds 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 for the cold-runner system is often larger than the weight of the parts. Due to Moldflon's good recyclability, the sprue scrap can be easily returned to the product stream.

PTFE's properties combined with the possibility of thermoplastic processing open up a variety of new applications, which it has not been possible to cover in this way before. Although the product is still in the launch phase, a 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. Moldflon components are primarily accessible via extrusion and injection moulding.

However, secondary processing methods such as blow moulding or thermoforming, as well as automatic machining of simple extruded profiles, are used here. The combination of traditional properties of PTFE - with the new process techniques of this material, such as injection moulding, transfer moulding 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 in Moldflon. This will allow the applications of this material to be expanded significantly beyond the existing limits of fluoropolymers.

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