Powder Coatings Scale-up: Is it science or a black art?

 

 

It is not always clear why scaling-up a formulation that works perfectly well on a lab extruder does not perform the same in production. Xtrutech offer some insight into the issues of achieving a reliable scale upShot 31.jpg

One question asked regularly is 'what operating parameters would you keep the same?'

The simple answer is, as many as possible. Primarily RPM, torque and barrel temperature should be set so that the machine is in theory adiabatic, i.e. no heat into the product from the barrel and no heat taken out of the product to the barrel. The reasoning for this is that this does not scale up in the same way as throughput, which is the function of the ratio of the diameters cubed, while heat transfer is only a function of the square of the diameter ratios. 

In contrast we are also asked 'what operating parameters would you change?' In theory, they should be the same for the same formulation operating at the same screw speed/torque condition.

One of the problems that can be encountered when evaluating a scale-up is the colour as there will be an increase in colour development on a smaller machine in comparison to a larger machine operating on the exact same conditions. 

This is partly due to the particle size of the product being the same, but the machine clearances being clearly different. You will also never get a true adiabatic ideal operation, and you will proportionally get a larger heat transfer acting on the product on the smaller machine than on a larger one. This is also true when scaling from a 40 or 50mm extruder to a larger 90 or 100mm extruder. 

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Clearances are maintained on machines ranges generally as a function of the diameter, e.g. D/100, particle size of the raw material remains constant.

You will always get more of a grinding effect on the smaller machine due to this despite what machine you decide to invest in. This therefore means that you will generally find better colour development on a smaller extruder than on a large production sized machine. Customers generally, quickly learn the difference in colour developement between the machine, sizes and thus know that if colour X is achieved on a lab machine, then colour Y will be produced from the production machine.

Additionally for similar reasons, you will find there is less of a 'matting effect,' on smaller machines than on production machines. This is because the matting agent finite particle size is acted upon slightly less in the production machines than in the lab machines, this is due to a law of physics and true of all twin screw extruders, not just the XTS range produced by us here at Xtrutech.

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We also get asked how well the XTS19 scales up to a XTS24 and onto larger extruders. In theory the throughput doubles while the heat transfer area only increases by 1.5X.

However, moving from a 19 to a 56, let's say, throughput would be increased by around 25X whilst the heat transfer area only increases by 8.68X. From these numbers you can see why it is important on any manufacturer's machine not to run too much cooling, or heating for that matter, on the smaller machines as it will not scale up.

There are many questions to be asked when scaling up from lab scale to a production machine from any manufacturer however we can promise to achieve the closest replication of the outputs and quality of the original machines. To see how we can assist your needs with a real understanding of powder coating machines send us an email at sales@xtrutech.com or call us on +44 (0) 1782 621122

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