Armor snail This is how extruder screws become more wear-resistant

Wafo, renowned for high-quality extruder screws and cylinders, excels in wear technology. Their performance is further enhanced by Oerlikon Balzers' Balinit Croma Plus coating.

When an extruder screw pushes the melted, relatively tough plastic melt through the guide cylinder, wear is almost inevitable— especially if the plastic is also reinforced with glass fibers or mineral fillers. The press pressure reaches values between 250 to 800 bar. The friction of the screw in contact with the cylinder as well as the often highly abrasive fillers of the material mixture along with chemical processes due to the heat during processing, put strain on the steel screw material also on the insides and thrust flanks. Local overheating during extrusion can also lead to corrosion. Deposits can also form on the systems.

Plastic processors struggle with wear

"The demands on technical plastics are increasing. And so is the wear," says Andreas Wagner. The graduate engineer is the owner and managing director of Wafo Klaus Wagner GmbH, founded in 1967. The company supplies clients from the plastics industry in Europe, but also in Asia with extruder screws and cylinders. To save costs, some users use more fillers for less demanding end products, as far as this is justifiable and cheaper than the plastic itself. These, however, also attack the screw surface through abrasive effects. Anyone who wants to be on the safe side should therefore take a closer look at the Balinit Croma Plus coating system, as Oerlikon Balzers recommends. Wagner has also experienced this.

Everything is well protected under the "rainbow"

The rainbow-like shimmering PVD coating is extremely hard, smooth and protects well against abrasion, scratches and scale formation. As a chrome-nickel based multi-layer coating including oxide top layer, the system is also very ductile, highly oxidation-resistant and adheres very well, it is said. The coating has therefore proven its worth in extrusion, but also in injection molding of numerous plastics, because it also increases the service life of the injection molding tools. Maintenance and cleaning efforts are also reduced. According to the provider, the layer also leads to a better material flow, which for example promotes the throughput in extrusion. If color changes are pending, one benefits from short rinsing times.

Hard chrome plating sometimes works, but it has its disadvantages...

Wagner always recommends the PVD layer (PVD = physical vapor deposition) when it becomes corrosive and abrasive for the screw core and flanks and when there are often adhesions and therefore a need for cleaning. Examples are the processing of tough elastomers of all kinds, when flame retardants are in the polymer, color pigments or generally when there is a high proportion of fillers. In many cases, however, the Wafo boss still resorts to classic hard chrome plating. However, in order to be able to chrome poorly accessible places in complex screw geometries, a similarly complicated, tailor-made anode concept for charge exchange and precise metal deposition is needed. This is relatively expensive and not everyone has the know-how. And the use of Chrome-6 is also reduced as a result of the REACH regulation on hazardous substances.