posted September 15, 2005 08:51 PM               

There are many aspects to getting the most economical screw deal. These are the components that go into the calculation;

Uptime production rate is typically the most important economic aspect of a screw purchase. Extrusion lines are usually worth more than $100 per hour. If a line runs just 10% faster over a year because a an improved design, that screw can generate an extra $20,000 minimum.

The next consideration is the ability of the screw-barrel combination not to wear over time. Again, 10% output rate is easy to loose with wear. Premium flight tip wear coatings are amoung the easiest thing to justify based on longevity.

Can the screw be rebuilt? Nitrided screws are generally impossible to rebuild, although they can be purchased cheaply.

You should also have a spare screw, in good condition, ready to go. Check out the value of one or two weeks downtime due to a broken screw.

All of these items can be estimated and combinations can be compared in order to figure of the best course of action.

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Best Regards,

Tom Cunningham

Extrusion Technical Services

posted September 20, 2005 10:53 PM               

There is no disadvantage on rebuilding a feed screw unless it is not done right,

Screws should be checked prior to rebuilding to make sure the base material is not fatigued, flight widths are not too thin and to make sure there are no cracks in the root diameter. Feed screws must be rebuild concentric for obvious reasons. The screw also need to be straightened a number of times through the rebuilding process. The screws have to be heated to a specific temperature prior to welding. The old hardfacing needs to be totally removed. A buffer layer may need to be applied to compensate for the dissimilar metals (screw base material and the flight hardfacing). A nitrided screw can be rebuild this way, CPM screws can also be rebuilt with very special procedures which may be cost prohibited. The correct hardfacing and thickness has to be selected depending on the process, temperature, polymer and additives the screw will be exposed to and if it is an abrasive or corrosive environment or both. Once welded the screw has to be slow cooled over a number of days to avoid the hard face cracking and pealing off. Chrome plating can also cause problems if not done correctly. Hydrogen embrittlement can be created which can cause the screws to break in the future. Pins holes throughout the screw root diameter will need to be repaired. Seal areas should be rebuilt if required to make sure material does not flow out the back end of the extruder. Cooling holes should be checked to make sure they are unobstructed. Rockwell hardness of the hardfacing should be within certain limits and platings should be of a certain thickness. Feed screws should not be over polished as flight depths can increase and flight radii can change. Properly polishing a feed screw is a real art. The distance between the flights must remain parallel to the screw axis as much as possible. The outside diameter of the flights should be ground to extremely close tolerances.