Above: Davis-Standard says various treatments can be used to lengthen the life of screws

Each type of wear is best handled in a slightly

different way. Abrasive wear is caused by hard particles – sus- pended in a filled polymer – or small projections from a surface roll or slide under pressure against another surface. The most effective corrective action is to use protective coatings and harder construction materials. Adhesive wear occurs when there is metal-to- metal contact of two surfaces in relative motion to each other – such as a screw flight and barrel. It is best handled by using borescoping equipment and performing regular maintenance. Corrosive wear happens when a corrosive

material attacks the surface metal, causing pitting and increased surface roughness. It can be caused by corrosive flame-retardants and some grades of PVC. It is dealt with by selecting correct construc- tion materials – or adding coatings – that do not react with the corrosive substance.

Torsional strength The material used to make the screw needs enough torsional strength to transmit power to the process, resist wear and offer good machinability. “It would be nice if one material would provide

adequate physical properties, wear resistance and deliver a long life, but there is not,” said Christiano. “That’s why most high-performance screws consist of a base material for the body, a wear-resistant alloy applied to the flights, and a surface treatment.” There are several methods to improve the wear

properties of screw flights. These include: heat treatment by flame hardening, for minimum protection; applying a wear-resistant alloy, for moderate to high protection. A popular method for applying alloys is to use the PTA welding process, to add either cobalt- or nickel-based surfacing alloys. “Almost all of our high-performance screws use this method to minimise wear and maximise screw life,” said Christiano. Wear- and corrosion-resistance of a screw body

26 PIPE & PROFILE EXTRUSION | November/December 2019

can be raised with surface coatings, which range from thin metal coatings to high-performance materials – using techniques such as HVOF thermal spraying. This uses the combustion of oxygen and fuel to produce a high-velocity stream that propels powders onto the surface at supersonic speeds. A range of materials can be used including thermal sprays of stainless steel and nickel-based alloys. Coatings containing tungsten carbide have also shown good results. “These processes are needed when traditional methods have been exhausted due to process limitations and expense,” said Christiano. “Other technologies such as vapour deposition and titanium nitriding have been used in a limited number of applications.”

Tailored service CA Picard has developed tailor-made products and services for preventative maintenance for production improvement. These include a barrel measurement device (BMD) for providing accurate barrel wear information and analytical reports. In addition, the company has developed the

flexible dismantling system (FDS) with PLC control, to dismantle screw elements from the shafts efficiently – with full protection of the screw elements and shafts, without the risk of injury to workers. The company also offers a barrel repair service, giving customers an overhauled barrel without the need to manufacture a new external body. The used, worn barrel is examined and the flow

through the cooling channels is tested. At this point, a decision is made as to how to proceed with the general overhaul. On receipt of the order, the worn liner is re-

moved, and cooling channels are flushed and undergo pressure testing. As the repair progresses, bores and threads are re-machined and the entire external body is cleaned using a suitable blast medium. Once the external body has been restored, the new wear liner is inserted into the heated external body and the sealing faces are ground. The final stage is the outgoing inspection, which is performed using a 3D measuring instrument in a climate-controlled room. At this point, all geomet- ric data is checked. Before delivery, the refurbished barrel is checked for leaks in the cooling channels in another pressure test.


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