Wear Issues in Powder Coating Extruders

12 Jan 2015

The Forth Rail Bridge is the oldest major steel bridge. Famed in the coatings industry for its 100 year continuous painting programme, it now benefits from a state of the art coating with a life predicted to exceed 20 years. Just as it is no surprise to anybody that exterior coatings weather, and need replacing: should anybody be surprised that extruders screw and barrel components used in the production of powder coating become worn?

 

Xtrutech are a company that are experts in finding solutions to issues with worn extruder components. Since the company was founded in 2002, Xtrutech has grown to become a leading supplier of spares and service for twin screw extrusion equipment. From its roots as a independent supplier of high performance screw parts, Xtrutech’s activities have increased in scope to include machine upgrades with new extruder barrel systems, gearbox repairs, through to complete removal, rebuilding and re-installation of older extruders.

Just as in the case of the Forth Rail Bridge, state of the art material solution can be applied to a wear problem in an extruder. Across all the extruder Original Equipment Manufacturers (OEMs), various different metallurgy solutions are promoted, and in total there are over 75 different alloys, hardening processes, surface treatments, or coatings that are available – but all at a premium over the standard component cost. The most exotic wear materials, whilst offering considerably longer productive life, have in virtually all cases a disproportionately high cost.

There are three distinct types of wear: Abrasive, Corrosive and Adhesive with abrasive wear being the principle type encountered.

Corrosive wear: wear appears due to chemical reactions between the processing materials and the base metal. This appears as pitting of the screw and/or barrel surface. Normally powder coating ingredients do not cause any significant corrosive problems. However, if condensation is present on the internal surfaces of the chilled barrel sections this can become an issue.

 

Adhesive: wear results from metal rubbing against metal and can manifest itself in several different ways; for example when the screw shafts are forced against the barrel, the outer diameter of the screw flights can become flattened and scoring of the barrel wall occurs.

 

Abrasive: wear results in the erosion of machined screw and barrel components. The scale of abrasion depends not only on the material the screw and barrel are constructed from, but also on the screw speed and the make-up of the formulation being processed. Different fillers and pigments have vastly different abrasiveness however the intensity of the wear is more dependent on the overall tonnage of powder produced, rather than the length of time that the extruder has been in operation.

The three figures below illustrate the progress of abrasive wear.

 

Here in Figure 1 we can see the onset of abrasive wear at the beginning of the kneading section

Here in Figure 2, the position of the maximum abrasion also moves further down the barrel

The same process is repeated and results in even more extreme wear shown here in Figure 3, which means mechanical damage  is now possible.

What is the impact?

  • Reduces output
  • Increased screw to barrel clearances will reduce the self wiping benefits of the twin screw design  
  • Cleaning times are longer and the extruder requires more purging material 
  • Possibility of melt stagnation in the worn area, which may result in the formation of cured “gel bits”
  • When wear becomes extreme the extruder shafts are unrestrained in the worn sections, and may be subject to strong bending forces, and ultimately fail prematurely 

The way to manage these wear issues is to regularly inspect the extruder barrel and screws. The barrel design of the extruder has an important part to play in this.  An opening barrel design makes inspection of product contact parts very simple. The progress of wear of the screw components, and barrel liners can be measured and monitored, and plans put into place for replacement of worn screws and barrel sections before productivity and quality are too badly affected. The decision to continue with worn parts, or replace with new is a complex one, and depends not only on the mix of formulations being processed, but also on the application in which the powder coating is being used, and what tolerance there is for wear induced shifts in product quality.