50 Years of High Free Volume

Back in the mid 1950's, whilst working for an American company, Readco, two engineers, Loomans and Brennan, developed a novel mixing machine. The design was similar in some ways to the earlier patented designs by Erdmenger at Bayer in Germany: both had two parallel extruder shafts. The flights of both designs were intermeshing and the extruder screw threads were self wiping as originally conceived by an Italian designer Columbo in 1939. However, the Readco design was patentable in two important respects, namely a significantly higher degree of intermeshing, and an openable barrel. The Erdmenger design was licensed by Bayer to Werner and Pfleiderer in Germany and celebrated its 50th anniversary at the K2007 show. Meanwhile Baker Perkins, in the USA, acquired the parent rights to the Readco design and began manufacturing the first machines in 1960.

Thus began 50 years of continuous development, without any change in the basic cross section geometry. In the 1980's, responsibility for the twin screw range transferred to the European arm of Baker Perkins, and the design was refined, to increase the power, reliability and ease of operation, whilst minimising the cost of manufacturing and ownership. The design team went through several changes of company name and ownership. The latest version of the design forms the Xtrutech range of twin screw extruders for powder coating.

8.5% annual productivity increase

The power of the extruder drive equates in most cases to its productivity. Compared to the extruder design as it was in 1980, the present design (after allowing for changes in machine size) has increased by a factor of almost 12, which is equivalent to an 8.5% annual increase in productivity. Everyone involved in mixing with twin screw extruders is aware that the maximum operating speed of the extruders has increased over recent years, so every manufacturer would be able to show a productivity improvement purely as a result of changing the motor speed of the extruder. For most manufacturers speed increases would probably turn out to provide the majority of a productivity improvement. What is significant and most probably unique, about the Xtrutech range is that of this 8.5% figure, only 2% is attributable to the speed of the extruder. The remaining 6.5% is a result of design improvements in the torque handling capability of the extruder drive train. 

New Extruder- Decision making made easier

In the past, when making a decision on a new extruder, one had to compare the processing efficiencies of different twin screw designs. The most important design parameter for twin screw extuders is the ratio of the centreline distance between the two shafts (Marked a in the figure below) to the Diameter (D.) To maintain fully intermeshing screws with self wiping and co-rotating operation, the cross-section shape of the screw components is fixed by the C/r value. For a constant screw outside diameter, increasing the centreline distance increases the screw root diameter. 

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The 1950's Readco extruder range had a CentreLine distance/ Radius (CL/R) ratio value of 1.55. This translates to a Do/Di value of 1.81 (simply calculated by 1/ (CL/R-1.) This was the geometry used by Baker Perkins throughout for powder coatings, and is the basis of the Xtrutech machine. Other extruder designs used for production of powder paints employed a geometries ranging with a Do/Di of 1.5 to 1.6. This gave quite different performance characteristics, from screw filling to heat transfer. Normally extensive trials were required to establish how each competitor's geometry performed. 

More recently, Coperion (new owners of the Werner and Pfleiderer marque) elected to develop a high free volume geometry. Their MegaVolume and, since 2007, MegaVolume Plus ranges, use a Do/Di ration of 1.80. This is so close to the original Readco design that screw geometry no longer makes a difference (although screw configuration design obviously will.) No doubt other manufacturers in Europe and the Far East will, in time, follow this change. 

This leaves the single most significant feature of the extruder design as the barrel itself. The choice being to buy an extruder with a barrel that opens, or one with a solid barrel. Back in the 1980's a Baker Perkins salesman would have simply said "what happens after you've had a power cut" powder coatings being thermosetting, and material left in the barrel being quite hot enough to cure. The argument doesn't hold together so well these days- design improvements in the drive train help the extruder to restart much more reliably, and in general power supplies are less subject to interruption than they were 30 years ago. However simply increasing barrel temperatures to "burn" or degrade the resin mix to free off the screws is a none starter due to the emission of toxic fumes in the production area! 

The reason to have an opening barrel is quite simply to provide access for cleaning and maintenance. However it may provide a fresh perspective to look at some reasons for NOT choosing an opening barrel.....

Clamshell Barrel 2

It's going to be more expensive...

To go into a discussion of unit manufacturing cost for different barrel components would be somewhat futile, but a lot of it is down to inertia of the design, and of the machine tools that are used for producing the extruder components. A manufacturer that only produces solid barrels will struggle, economically, to produce an extruder with an opening barrel. One, as Xtrutech does regularly, that supplies spare parts for opening barrels, and that offers retrofit opening barrels for existing solid barrel extruders, has the design expertise, and the manufacturing technology and tooling readily available. 

The barrel might leak....

The barrel developed by Xtrutech has been designed with single-piece, robust backing blocks that eliminate any misalignment of barrel parts. Further to this, the pressures built up with the powder coating mixing process are minor. There is no pressure built up at the discharge of the extruder; the extruder has no die fitted. The chill knife design is virtually pressure free, and simply a way of controlling the flow of the melt from the flight of the discharge screws into the nip gap of the cooling rolls. 

Over feeding, a power failure, a Shaft breakage or a piece of metal from the pre-mix making its way into the extruder, whilst not so common these days still can happen at any time, can result in a major incident that requires the removal of each barrel section piece by piece until the blockage or shafts are free. Unless, you have a clamshell barrel that can be opened in minutes allowing for a quick chance of shafts and the resuming of that important production run! Additionally, the Xtrutech Clamshell Barrel can stay closed until you need to open it!

Clamshell Barrel Open

The barrel will take a long time to open and you'll never use it....

Twin screw extruders are designed to be self-wiping and have minimum dead spots, and, at least in theory, should purge and clean leaving no residues from previous production runs. However the success of purging depends on the extent of the colour change (black to while will take a little longer.) Cleaning between different resin types also places high demands, and after all, undoing 14 barrel bolts with an air operated wrench is not going to take more than a couple of minutes

There are no maintenance problems with a solid barrel....

As long as you are not planning to actively monitor extruder wear this is quite correct. More commonly extruder wear is an issue that requires careful management, and specialist equipment is needed...

The measuring device for the barrel bore detects wear in the processing section by measuring the inside diameter of each bore. The sensor measures the actual bore diameter over the entire processing section length. During the measuring process the bore is measured every 5mm. The degree of wear is then calculated by the evaluation software on the basis of the individual diameter values. Additionally, in order to precisely analyse the condition of the material surface, the measuring head is equipped with an endoscopic camera after the diameter has been measured. The camera provides pin sharp images and videos of the surface condition of the figure of eight bore. 

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(Photograph courtesy Micro-Epsilon)

With an opening barrel you can see the onset of any wear issues, and need simply a straight edge and feeler gauges to measure and monitor.

Whichever barrel type, the progress of wear of the screw components, and barrel liners should 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.

Summary

Many design improvements have been made in twin screw extruder technology for production and if anything, extruder designs seem to be converging rather than diverging - certainly this is the case where the screw geometry is concerned. Xtrutech's designer Engineers have also focused on efficient ease of operation and maintenance. Use our contact form if you would like to discuss our solutions to all your processing questions or alternatively call +44 1782 621122 to see what other progress has been made.