Amorphous materials have no true melting point

They simply soften as they heat up. These materials (PC, PVC) can be shear heat sensitive so feedscrew meter depths are usually greater to reduce the shear rates in this area. Compression rates are also lower to avoid high shear.

Crystalline or semi-crystalline materials must go through a heat of fusion to relax the crystalline regions before they will melt. (Ice for example must melt entirely before the temperature will rise). These materials (Nylon, PP, PE) require shear heat energy to facilitate melting. Feedscrew meter depths should be shallower to increase the shear and contact with the barrel. Compression rate is also higher to facilitate shear. If these resins contain glass fiber filers then the compression rate will decrease depending on loading to avoid breaking the glass fibers.

The cost of wear is a real number

The cost of wear is a real number and should be understood. For example at constant rpm a 60 mm feedscrew with .020” (.5 mm) of diameter reduction (wear) will produce 10 lbs (5 Kg) LESS sellable product per hour of operation. A 90 mm feedscrew will experience double this output loss. To combat this output loss the rpm can be increased which increases the wear rate, the power usage, the melt temperature and the scrap rates. A clear understanding of the cost that results as screws and barrels wear should quickly focus a company’s resources on preventing wear or repairing worn equipment rather than compensating for this wear with higher screw rpm.

Tool steels used to make feedscrews are usually D2 (high chromium) or CPM-9V (high vanadium)

Due to the greater alloy content of these steels they provide good toughness and abrasion resistance. Such steels are termed through-hardened as they undergo specific heat treatments after machining to harden the matrix. The high alloy content makes weld repair once a tool steel screw is worn nearly impossible and for this reason worn screws are discarded. Tungsten carbide coating is an option to replace up to .025” (.60 mm) total wear by thermal spray buildup on the top of screw flights. This is a proven option that will result in two to three times more useful life from such a repaired tool steel feedscrew.

 

All plastics processing feedscrews suffer from adhesive wear

All plastics processing feedscrews suffer from adhesive wear. In fact, 95% of all plasticating feedscrews are repaired or replaced due to adhesive wear of the flight outside diameter. Incidental contact between the rotating screw and the stationary barrel results in eventual reduction in diameter of the screw. This leads to longer plasticating times (Inj Mold) or reduced throughput (Ext) and overall lower productivity of a screw/barrel system. Bi-metallic feedscrews are an attempt to counter this adhesive wear by weld application of a better wearing alloy material on the top of screw flights. Our XC1000 tungsten carbide has proven to offer as much as ten times more adhesive wear resistance than standard alloys in real world applications. An example – 4-1/2″ 24:1 extrusion feedscrew wearing .020″ (.5 mm overall) in 13 months. A layer of .020″ (.5 mm overall) of XC1000 on the screw flight had only .005″ (.125 mm overall) of wear after 14 months of operation. This is a ten-fold improvement compared to the previous alloy

Ultimate abrasive wear resistance in steels is determined by the volume percentage of hard carbides.

Standard alloy steels rely mostly on chromium carbide formation to impart more abrasion resistance. These carbide particles are microscopic in size, and constitute from less than 5% to over 20% of the total volume of the microstructure of the steel.  Chromium carbides are about 65/70 HRC, molybdenum and tungsten carbides are about 75 HRC.  Our thermal spray coating of XC1000 contains 88-90% by weight (80% by volume) of hard tungsten carbide.   This hard, dense high volume percentage carbide layer at the point of wear provides the ultimate abrasive wear resistance possible.  Harder carbides exist but compared to tungsten carbides none can be applied in a thick enough layer or with sufficient bond strength to offer any substantial value of the life of a feedscrew.

All plastics processing feedscrews suffer from adhesive wear.

In fact, 95% of all plasticating feedscrews are repaired or replaced due to adhesive wear of the flight outside diameter.  Incidental contact between the rotating screw and the stationary barrel results in eventual reduction in diameter of the screw.  This leads to longer plasticating times (Inj Mold) or reduced throughput (Ext) and overall lower productivity of a screw/barrel system.  Bi-metallic feedscrews are an attempt to counter this adhesive wear by weld application of a better wearing alloy material on the top of screw flights.  Our XC1000 tungsten carbide has proven to offer as much as ten times more adhesive wear resistance than standard alloys in real world applications.  An example – 4-1/2″ 24:1 extrusion feedscrew wearing .020″ (.5 mm overall) in 13 months.  A layer of .020″ (.5 mm overall) of XC1000 on the screw flight had only .005″ (.125 mm overall) of wear after 14 months of operation.  This is a ten-fold improvement compared to the previous alloy

Halogen-free resins are designed to meet RoHS – Restriction of Certain Hazardous Substances requirements.

Started in Europe as a way to reduce harmful residual material reaching landfills this requirement is making its way to the rest of the world.  This benefit to the public has a hidden cost to plastics processors – aggressive corrosion.  These flame retardant materials are extremely corrosive to steels at processing temperature.  Most processors new to this material are surprised when they discover a feedscrew that looks like a broomstick after only 4 to 5 weeks of molding these halogen-free compounds.  Extreme Coatings XC1000Ni is a nickel based tungsten carbide formulation that is completely inert.  Coating a feedscrew with XC1000Ni will stop this corrosive attack and ensure that feedscrews can last for years.

A mirror surface finish of our hard XC1000 carbide coating can reduce the requirements to remove and clean a feedscrew.

Most large diameter feedscrews have chrome plating surface treatment to the screw core.  On a new screw this is a smooth surface that resists material build up.  Eventually, with resins that are prone to sticking on the screw surface, a feedscrew must be removed for cleaning.  Even careful handling of chrome plating will leave small scratches on the surface.  These areas then foul more rapidly and the screw must again be removed and cleaned.  XC1000 is 5-10 points harder on the Rockwell scale them chrome plate and will not scratch during cleaning.  Our super surface finish .1 Ra µm reduces COF and ensures a long time between cleaning episodes.

Our XC1000 tungsten carbide has proven to offer as much as ten times more adhesive wear resistance than standard alloys

All plastics processing feedscrews suffer from adhesive wear.  In fact, 95% of all plasticating feedscrews are repaired or replaced due to adhesive wear of the flight outside diameter.  Incidental contact between the rotating screw and the stationary barrel results in eventual reduction in diameter of the screw.  This leads to longer plasticating times (Inj Mold) or reduced throughput (Ext) and overall lower productivity of a screw/barrel system.  Bi-metallic feedscrews are an attempt to counter this adhesive wear by weld application of a better wearing alloy material on the top of screw flights.  Our XC1000 tungsten carbide has proven to offer as much as ten times more adhesive wear resistance than standard alloys in real world applications.  An example – 4-1/2″ 24:1 extrusion feedscrew wearing .020″ (.5 mm overall) in 13 months.  A layer of .020″ (.5 mm overall) of XC1000 on the screw flight had only .002″ (.125 mm overall) of wear after 14 months of operation.  This is a ten-fold improvement compared to the previous alloy.